Next Amp Considerations

First, the 5F6A / JTM that I recently completed building, includes a two-tube power/output section that is a class AB "push-pull" design, known for stability and power efficiency. A class A power section is a simpler design that can work with just one tube (and therefore doesn't require a phase inverter stage driving it), and although it is less efficient, this circuit is known to produce different audio/tonal characteristics than AB designs. So I'm interested in building something that uses a class A power section design, which would be lower power/volume than the 5F6A / JTM, but perfectly fine for my purposes. 

Next, I've always liked the depth of a stereo guitar sound, I used a Roland JC-120 (head version) for years, then later, a Fender Princeton Chorus, both stereo amps that allowed for time-based and textural effects - reverb, delay, tremolo, vibrato, chorus, etc. - that sound so much better in stereo. I've been intrigued by vintage Magnatone amps (especially the 96-10041 from the early 60's), and the recently resurrected company does have a new-ish stereo amp that utilizes their special versions of tremolo and vibrato. I considered building an imitation of that, but I think the analog tremolo, vibrato and reverb circuits would not be worth the trouble, considering the variety of great sounding digital versions of those and other effects (as external floor boxes or rack mounted gear) that one could just plug into an effects loop (like on my pervious amps). 

Finally, I've been intrigued by the Alexander Dumble sound, and have read a lot about the architecture of his original amps and amp mods (probably won't be buying one anytime soon), and the many designs that folks have created (Sebago Sound, Custom) to imitate the tone, so am interested in implementing some of that in the preamp section of my next amp. 

So that's the general idea: 

• Class A power section.
• Stereo for richer effects, which means:
• two-channel effects loop (buffered) between the preamp and power sections
• two single-tube output sections in parallel, to drive two speakers independently.

• Dumble-inspired preamp design.

Considering it will be a stereo amp, I also plan to build a 2-speaker cabinet to match, that can be run as stereo, with the new amp, or mono with the first amp. 

Calling the modded 5F6A done

I've spent a lot of time joyously playing this amp in the past 2 months, exploring the wide array of cleans, crunchy and overdriven sounds. The amp works well for the variety of styles from which I draw to get my own sound, from jazz, to roots-country, to pop and funk. I'm still trying to figure out whether there is anything for me in the the "stock" channel dirty sounds of the Tweed Bassman - it's just a bit ragged sounding for my tastes when cranked up. For output tubes I've tried 5881's, 6L6's and KT66's and so far prefer the 6L6's, but I have some other sets of 5881's and 6L6's still to try. 

Link to final schematic and layout.

For the past few weeks I have focused exclusively on better understanding the cascaded input mode and Cold Clipper bias setting of the V1A triode. With the MC5, I located the sweet spot where the amp maintains a full dynamic range, single notes ring and shimmer, double-stops crunch a bit but aren't fizzy, and when I switch the pickup from single coil to humbucker, the guitar output pushes the whole pre amp section into a nice, heavier but still articulate, fairly smooth overdrive. 

In the cascaded input mode, the Telecaster (both guitars picture below) with its slightly lower output single coil pickups also has its own sweet spot, where it maintains its Tele voice, but has a thickness and shimmeriness I like. To get a thicker overdriven sound here I use the ChaseTone Secret Pre pedal to boost the signal from guitar to amp input. 

To control overall volume, the amp's speaker output is routed to a Fryette Power Station, which drives a single Celestion G12H speaker, mic'd with a Shure SM57. The Dirty Tele and MC5 clips were recorded dry and a little reverb applied in the rough mix down, and the High Gain clip utilized the Fryette effects loop into a Line 6 effects unit for reverb and delay during recording. 

This short clip of the Tele uses cascaded mode, and progresses through light comping and a little lead playing on both pickups, then I switch to a clean boost and bridge pickup for the third half of the clip.  

Dirty Tele Clip on Soundcloud

This short clip of the MC5 uses the same amp settings, though I dialed the gain back slightly just to clean it up a bit; it also progresses through comping and lead playing just on the neck SC pickup, then switched over to humbucker, then through the boost pedal, and finally a few seconds playing just through the bridge humbucker.  

Dirty MC5 Clip on Soundcloud

I can get higher gain out of it, but the character of the guitar and amp both tend to diminish as gain increases, so I haven't spent a lot of time yet in that realm... but here's a sample of a high gain sound, using the Tele with boost peal and/or DOD 250 (I don't remember). 

High Gain Tele on SoundCloud

So, tone-wise I think I have a grasp on it and am usually able to dial up a specific sound for a specific application. A lot of folks-in-the-know say good amps are usually just really good at doing one thing, and don't expect it sound great at a clean setting as well as overdriven...but I'm pretty delighted with the versatility of this amp.

The tolex finally arrived and I spent the better part of a day learning and doing that bit.  

Left - Fender Telecaster (American Elite, 2014)
Right - Ibanez Musician (MC500, 1980)

 

  

Nearly complete

So, electronically, I think it's complete and where I'd like it to be in terms of tone. 

There has been a delay in receiving the tolex vinyl for covering the cabinet, so it's still bare wood, but the faceplate is done and in place. I read on a forum somewhere that someone had reverse printed their labels and such on a piece of clear plastic, so I used that approach. Generally, it went like this:

Use the layout from hole drilling template to locate the places for text, etc.

My partner is handy with a pen, and was generous enough to swirl out a bunch of variations of letters and labels and so forth. 

I scanned the calligraphy into images, blackened them and clean them up and converted to SVG, then placed those images onto the hole drilling template image and had Staples reverse print that final image onto a transparent sheet (could only get 8 x 11, so had to do across two pages and join the parts)

Cut holes for switches and spindles and and spray paint over the reverse image

Adhere to the front of amp with silicone

And voila!

(The star and tree graphic is taken from the old 1901 version of the Maine state flag.)

Tone Analysis III

I've done a variety of testing: So interesting, yet so tedious to produce a comprehensive enough collection of data to make definitive pronouncements. But here's what I think I can say. 

The TINA modeler does a great job showing what is likely to happen with my tube-driven circuit, especially when it's operating in linear territory, and it's even pretty good at showing saturation or cut-off conditions, when the inputs to tubes are driving them beyond their design limits. 

For example, here's a comparison of a modeled vs. actual 1kHz/200mv signal after is passes through channel I (full wide-open gain setting), cascaded through channel II set up as a "cold clipper" (very cool bias to force the clipping off of one lobe of the waveform, set to 25% gain), and measured at the grid of the second pre-amp tube:

Modeled using TINA:

Measured using a tone generator and capturing the signal in Visual Analyzer (VA):

Phase differences aside, they're pretty close in shape and proportion, and I think in actual measured signal voltage compared to modeled (I did some testing with volt meter and guitar generated signals, which conformed to expectations of signal voltage levels - no pics - but I haven't followed up using the tone generator). VA also shows the actual higher order harmonics accentuated by the circuit up to this point at these settings, in the lower panel. 

I also set up a suite of tone testing, but all the variations are too involved to capture and explain in detail, plus I changed some things as I went, and didn't want to go back and regenerate prior results to match up... Never the less, here are some examples of what I did and some key results. 

Using channel I and the stock TMB tone stack, I plugged in my MC500, set to neck humbucker PU only, and dialed in a sound that like - a full, round, straight-ahead jazz comping and lead tone. Then I replaced the guitar input with the white noise generator, and captured the plot in VA (the left side of figure below, green on black). Then I recreated the very same amplifier settings in TINA and plotted the modeled result (the right side of figure below), where the green curve is the signal prior to entering the tone control part of the circuit and the dark blue curve is the signal after being shaped by the tone controls. Here they are side by side:

Comparing the green curve on the left to the dark blue curve the right, you can see the similarities in overall shape, but the actual output appears to amplify the higher frequencies a bit more and extends them another 1k or so before beginning to fall off. There are also some spikes around 100 Hz which I'm not sure I actually hear.

This set shows use of channel II with the TMB tone stack, single coil on the neck PU, with a much different, very nice acoustic tone for comping or finger style playing - different visual orientation to widen the scale a bit. 

Here, the modeled vs. actual plots appear quite similar. with a pronounced notch at 600Hz. 

And, as below with the James tone stack set up with maximum mid frequency notch (close to 300Hz, nearly as predicted), we can see it's similar with regard to frequency response but doesn't represent the levels quite right.

Also (and this is important) I'm just getting my feet wet with this. I'm a complete novice and I just pick up what I can from reading and asking questions of people who actually know things. So, I'm sure I have based some of my findings and conclusions on inaccurate assumptions, misuse of complex software tools I have just learned for this application, and just plain incorrect interpretation of facts, all due to my limited knowledge and experience. 

Results 

Overall, TINA the modeler did a GREAT job guiding choices with regard to both tone and gain management from one stage to the next, and predicting actual outcomes. Bravo Texas Instruments! Now that I'm playing around with the amp in high gain territory, listening to the tonal qualities and overtones of just-breaking-up and highly distorted signals, I think all bets are off in terms of accurate modeling. But TINA got me 95% of the way there, and that's pretty impressive. 

The real result, the range of "clean" and "on the verge of breakup" tones that the amp creates with my guitars, is really quite impressive and everything I hoped for - and it's damn loud, which is just so satisfying in ways that are hard to convey in words (although neighbors have come up with a few!). It's sonically vast, and while I've got a handful of distinct and delicious tones I can use right now, it also provides so many avenues to explore and a lot more potential, as I swap out tubes (I have to try 5881 power tubes again and also some 6L6's, and a GZ34 rectifier I haven't tested out yet), and try new combinations of built-in settings.

I'd say I'm nearly done at this point. I will probably modify one last resistor value in the V2GL switch, to help manage signal level going into V2 in Cascade mode. The cabinet for it is basically done and in use, but I need to do the related upholstery tasks. And the art and layout for the faceplate is being finalized. 

 

Tone Analysis II

I've mainly been playing through the amp with with two guitars: an old Ibanez MC-500 with Super 88 humbuckers, and a relatively recent vintage Fender Telecaster with Seymour Duncan BG1400 stacked humbuckers. Both produce a relatively strong signal, but I also have an echoplex style preamp boost pedal I've experimented with for high gain settings. 

The recently updated schematic of selectable tone sections now look like this:

It's worth noting that a James tone stack doesn’t usually have a mid control, but since I had the lower half of the mid duplex pot available, I did some testing to see what happens when one modifies the value of the (usually around ~100k) “balancing” resistor between the Bass pot and the signal out. It turns out that raising the resistance shifts the notch between bass and treble controls progressively down in frequency, and also changes the shape of the overall curve, so I’m just calling it a mid-shift control in this application. The side-by-side plots below show how the Duncan Amps tone stack calculator, set up with my James circuit, predicts the frequency response characteristics across the range of bass and treble knob positions, for each extreme end of the balancing resistor range: the left (yellow) plot shows 277k and the right (green) plot shows 27k, representing my mid pot in full-open and full-closed positions.

You can see how the curves in the yellow plot generally dip a little deeper than their green counterparts, and that the location of those dips are a little further left, in a lower frequency band, than those on the right. Aside from the audible tonal difference, this also means more of the signal is being removed from the circuit, resulting in less signal strength getting pass into the next stage (the phase inverter). At any setting, the James stack passes less signal on to the next stage, so selecting the James stack means lowering the overall gain of the signal going to the PI and power section of the amp. 

Setup for testing theoretical vs. actual tone stack characteristics

There are just so many different variables associated with the shape and timbre of the sound coming out of the speakers (Celestion G12H, and EVM 15B) or as direct signal. Of course there is the signal being generated from the guitar pickups, which will vary from guitar to guitar and among different pickup types/configurations on a single guitar. And, as we know from an earlier post and this doc, each triode’s cathode components (resistor R_k and capacitor C_k), coupling capacitor (C_G) and gain bypass (C_BP) configurations of each channel produce signals with significantly different frequency response curves long before the signal even reaches the tone controls. In this figure below, the teal curve is the frequency response of the circuit measured at the channel II triode output, the blue is for channel I, and the grey is a blend of both channels (jumpered mode).

 

So, it helps to keep this in mind when comparing outputs of the two tone-stacks for each channel.  I’ll look at a variety of combinations, but not all of them, just to get a sense of things.

Also, up to now, the tone controls have not exactly behaved as I expected – they work, but they didn’t alter the sound in the way I expected through their ranges. So before testing and documenting how it all came out, I went over them again with the original Fender schematic, the Hoffman modified schematic, the Duncan Amps James tone circuit design explanation, my own schematic, and my layout to make sure I had designed and implemented what I intended… and of course, it turns out I had not.

For the stock Fender TMB tone stack, I mis-wired the Mid pot somehow, both in its connection to the bass pot and in the “knock-down” resistor I used to cut the resistance to ground seen by the bass pot from 250k (which is the actual R value of the mid pot) down to 25k which is what should be measured at the bass pot’s terminal 3. And for the James stack, I wired it as designed, but after playing with the circuit in TINA, I realized I could increase the “slope” resistor by a lot and deepen the available mid scoop – that will cause some loss of gain, but I can live with that since I have a second tone stack that loses much less. I had initially installed a 27k slope resistor, and now with a 200k I get a deeper mid notch at a lower frequency (~ 250Hhz). One reason I wanted to do a second tone stack in the first place, was to get something more like a Fender Blackface Twin sound, which I think comes from a mid notch at roughly that frequency. So, I made the corrections and modification.

But why bother?

I think partly, I just like to feel that I have some idea what is going to happen.... in life: If the situation is X and these are the options for influencing it, what affect is each likely to have? If I have no prior experience on which to base some prediction, then a model or metaphor that looks similar to the situation at hand is really useful. I just really like conceptual models and I love testing them against observable phenomena, both to learn and also to make them more useful predictors to use in the future. And that idea applies to lots I do in life, including I suppose, building my first amp. 

From testing, I'd like to know two things: (1) how close the result is to the modeled design, and (2) what does good sounding (to me) look like in a model and as some quantified expression?

To test overall response at all frequencies (not just guitar sounds), I'll pass a signal from a white noise generator on my iPad into each input channel and examine the audio output of the amp with a variety of tone control settings, by running the amp’s direct output signal (captured by my attenuator) to my computer, and examine it using a tool called Visual Analyzer. To keep it simple I'll just set up a few sonic scenarios for each case:

• Notch: the deepest mid notch that can be dialed in, keeping the Bass and treble pretty close to balanced (the upper and lower bounds of a managed response curve).
• Flat Output: dials adjusted to create the curve that appears most similar to the modeled response curve coming into the tone stack.

To test good-sounding-ness, I'll use each of my two guitars on two typical guitar settings, one on each channel and also one on each tone stack, for a total of eight. Then I'll just pass a white noise signal through after I've set the tone controls where I like them.

It’s worth noting that all of the tone pots have a “log” taper, which means the sweep on the dial from positions 0 – 5 transits only the first 10% of the resistance of the pot, and the remaining 90% of the pot’s resistance value is transited from positions 5 – 10 on the dial. The potentiometers in the TINA circuit simulator require setting a value as a percent of total pot resistance. I'll use the blue curve on the graph below (from eepower.com) to guide my TINA settings when generating the simulated results.

 stay tuned for results...

Tone Analysis I

As I mentioned I might do, I looked into lowering the resistance on the "cold clipper" function, did some modeling and testing, and decided to make the change down to something more like 10k, which is typical for the Marshall JCM-800 and many others.  Now the total Rk (cathode resistance) is 11.7k when the switch is engaged (the sum of the regular resistor and the 10k addition), the output is a bit hotter, and it produces a lovely smooth distorted tone when driven by the channel I input at any setting over about 50% (7 or so on the dial). The updated pre-amp circuit now looks like this:

Before anything else, let's look at the switches that impact tone ahead of the second tube and the tone stack.

The input impedance switch (the “Z” switch) was included to give some options for signal management in front of the channel II triode. The 5F6-A and JTM-45 both include options for reducing the instrument signal here, and it seemed like a good idea to add a voltage divider in that part of the circuit for managing the high gain signal that would be coming from the channel I when in cascade mode, not unlike second stage circuit designs of higher gain amps from the 70’s. For similar reasons, I added a grid leak resistor on a switch just prior to the grid of V2 (the “V2GL” switch), which would be the 3^rd gain stage when in cascade mode. I later added by-pass cap to the resistor on the V2GL switch in the hopes of also draining off some of the upper frequencies from the signal at this point, if the switch is engaged.

Below are the relative frequency response curves at the V2 input grid, for these two cases. The purple curve shows the Z switch set to Lo (200k resistor to ground), with and without the V2GL switch engaged, and the teal trace shows the Z switch set to Hi, with and without V2GL engaged. 

(Cascade mode, channel I @ 80%, channel II @ 50%)

Other than simply cutting signal strength, there are some frequency response differences particularly in the low end, although they seem minor. However, what this plot can't show is the effect of the "grid leak" resistors provided by both switches, in preventing blocking distortion at high gain settings (tube behavior is a complicated thing, but Rob Robinette's explanation of distortion and grid current dynamics is about the most understandable I've come across). The by-pass cap on the V2GL didn’t quite work as intended, and modeling various different values doesn’t change the response in a way that I think makes sense to bother with, at least until I learn more. I’m glad to have both switches at this point, as they do provide a convenient way to manage breakup and some high end (via the channel I gain bypass cap) at V1B and V2 somewhat independently. They probably also help prevent blocking distortion, but I've never heard that from this amp, so it's hard to say.

Construction of the amp cabinet is underway....

 

Initial Testing and Tweaks

I've been playing it a lot. It's nice and quiet and behaves pretty much as expected. I made a few alterations/fixes:

- I had done the start-up and first week or so of playing with 5881 tubes (these came stock in the '59 Bassman) in the power section of the amp. They are/were a 23 watt version of the common 30 watt 6L6 tube. I changed over to KT66 tubes last week, and when I tried to adjust the bias for them, found I could not dial in a low enough voltage to get them into their optimal operating range. So, I reduced the bias circuit resistor value from 220k to 150k, and now the adjustment pot should cover a good range for both sets of tubes... I'll try some 6L6s tubes next, so hopefully the adjustment will work for them as well. In the process of doing that work, I made a mistake and blew one of the two 10uF capacitors in that part of the circuit. Luckily I had ordered extra of everything and had another cap to replace it. Live and learn - When I was organizing my resistor warehouse, I had mislabeled a resistor as 100k Ohms when it was just 100 Ohms. Eeeek! 

- added a 130pF capacitor bypass to the resistor on the V2GL switch (controlling grid leak and voltage divider just before the grid of the first V2 triode) the in the hopes of bleeding off some high frequencies when the switch is engaged, mainly for high gain configurations.

- reduced the 56k negative feedback (NFB) resistor value to 22k to increase the impact of the NFB when that switch is thrown. I left the 75k side of the switch as is, so now it provides selectable NFB of 22k, 75k, and none, which prevents any feedback at all from reentering the phase inverter. 

- reduced the value of the mode switch resistor from 470k to 100k to increase the signal strength from V1A to V1B in cascade mode (also a high gain improvement modification). It turns out, an un-bypassed 20k cathode resistor increases the clean input headroom on a 12AX7 (on the V1B triode) by quite a lot, and the V1A channel at full gain was just barely overdriving it. The lower resistor value has helped, but boosting the input signal strength is really the thing that pushes it to the right (sounding) place.  I'll play with lower cathode resistor values to address this, so that an input boost is not needed. 

- realized I had mis-wired the presence control, which, besides rendering the presence adjustment practically useless, resulted in a lower total resistance to ground (10.5k) for the phase inverter cathodes than designed (15.5k).  Changing this improved the functioning of the presence control and increased the overall gain of phase inverter. 

I also mocked up a paper faceplate so I can see where I am setting the controls as I play around with things. I now realize that any lettering below a knob is hard to see from above, so I'll have to reset the layout to improve the visibility and usability of it.

Here's a link to a deeper dive into analysis of the pre-amp section as built.

Operational!

The start up process worked fine - I plugged in a speaker, plugged in a guitar, switched it on, took a few voltage readings on the DC power supply rail and the tube plates. Then turned it up and voila! It seems to work as planned! I'm a little surprised, but on the other hand, I did a lot of planning, modeling, and unit testing, so that helped increase the probability of getting to an expected result at this point.

\

(front / top view)

(rear / top view)

Next is ordering and installing some knobs, making a faceplate, and doing a LOT more playing to see how it sounds in all its configurations. I'll record some rough audio clips and post them in the next week or so.

Here's the front panel face plate template, which I also used to center and drill the holes in the front of the chassis:

All wired in

I populated the circuit board and made all remaining connections (... except one resistor which I just noticed from the picture is missing in the bias circuit). 

(board populated and wired in - front view)

(board populated and wired in - rear view)

(input section detail)

Next, will be a lot of continuity testing, tightening up mechanical connections of the pots, switches, and other components. Then, I'll follow a power-up sequence and do some testing to ensure everything is in the right place. Rob Robinette has a sensible procedure I planned to review, but I also just found this one by one of the amp builders I like (retired FAA technician who claims to be a hobbyist but is clearly a master of his craft), and am relieved to discover that I have already followed all of his steps up to this point!

You may notice some of my components have a black mark on one end. These are capacitors, which are designed sort of like ... Drake's Cakes Yodels or home made jelly rolls: two sheets of paper or plastic stuff rolled up together with electrolytic cream filling between them. The two terminals sticking out are each connected to one of the two sheets. The sheet that ends up as the outer most layer acts as a shield in the same way cable shielding works, so you always want the terminal that is connected to the outer layer closer to ground in the circuit to bleed off any interfering radio signals. The thing is, not all capacitor manufacturers mark which terminal is connected to the outer layer, so one has to just listen to them to figure it out. By connecting test leads to each terminal of a capacitor and plugging them into an amp, you can almost always hear a slight hum just from ambient RF and nearby A/C circuits working. By reversing the connections of the test leads - switching them back and forth - you can hear which way is quieter and identify the terminal connected to the outer sheet. and mark it so it's easy to orient correctly when placing in the circuit.

Sockets wired

The plan for socket wiring looks like this:

 (pre-amp socket wiring plan)

The idea is to keep signal wires away from the high voltage and heater wires, or to cross them at 90 degree angles. 

Here's how they look, mostly complete:

(Pre-amp and phase inverter sockets wired)

The pre-amp sockets are closer together in reality than as shown on the plan, so a couple wires on V2 (a plate load wire and a cathode wire) are actually closer together than I'd like.  I may be able to separate them a bit, but will wait until I complete the wiring of the leads, then try to bend things around one final time.

(Power tube sockets wired)

Speaking of leads, the thick looking wires in the layout  figure at top are the ones carrying the low voltage signal to the preamp tubes, or are otherwise connected to the sensitive tube grids (my guitar pickups typically generate up to about 300 millivolts AC). For these, it's good to encase the signal wire in a mesh wire shield, pull a sleeve of heat-shrinkable insulation over it (hence the added thickness), and leave the twisted mesh sticking out at one end so it can be connected to ground. You can buy shielded cable already made up, but since I have the materials, why not just make it. The net shield deflects radio frequency signals to ground before they can find their way into the amp via the signal wire. One could, and many do, add shielding to many more of the wires, but in this amp these are the ones most vulnerable to such interference since they are long in length and connect directly to the control grids of the tubes. Plus it's a pain to do, so I just did the four. Here are three of them:

(Pre measured Shielded leads)

These will be placed last, since they run in the space above the circuit board, and would get in the way of populating the board if I do them now.

Front panel wired

After one last validation of the schematic to the layout and the layout to what I've actually done so far, everything seems to be in order. I labeled the leads hanging off the circuit board and laid the board in where it goes, then sorted out pathways of the wires underneath board to untangle them, minimize crossing, and to make sure there was enough length in each wire before starting to solder the connections. 

(checking board alignment and organizing lead pathways)

The first connections to solder were the grounds running from each ground rail (see star grounding on Aiken's website), and the B+ leads to each filter capacitor for each power supply stage. Once I wired in the first stage filter caps (the big blue things), I tested them to confirm I had the full "unloaded" DC voltage I should have at the top of that circuit - a few weeks/steps ago, I had tested the rectifiers and found that without the rest of the filter circuit, they produced well under 400 volts, so was relieved to confirm that with the filter caps and choke in place, close to the right voltage was present. I also tested the bleeder resistor (you see it nestled between the two caps) and confirmed it drains the circuit of stored high voltage down to a safe level, about a minute after I switch the power off.

(testing the B+ "A" node)

("doghouse" wiring complete)

(power supply and heater circuit points tested to date)

Then I completed all the front panel connections I could. The junctions and wires you see still unconnected also involve wires that connect to the tube sockets or elsewhere, so I'll complete those when I have those wires ready to go. 

(front panel connections soldered)

I'm finding it tricky to get the soldering iron tip, the solder, AND a visual line of sight to all meet at the right spot, and do a good job. Some of my joints are messy, but they all feel solid and I tested them for expected continuity and resistances and all seem good.

Next: secure the board (which has been unfastened so I could peek under it and adjust leads as I went) and connect the leads from board to sockets and back panel...

Wiring the circuit board

I started attaching the wires that run from the circuit board to elsewhere in the amp. There are ongoing debates about the best wire to use for different situations. For example, some wires carry high voltage DC to the tubes' anodes (plates) while other wires carry low voltage AC signal to the tubes' grids, and one can understand how the type of wire could ultimately make some timbral difference. Some folks say they can actually hear the differences among wires of varying gauge, composition and construction (stranded vs. solid), and others say they can't. Some say selection choice is more about ease of laying the wires and making good quality connections, or whatever Leo or Jim's engineers decided back in the 50's and 60's. Having no real interest in historical accuracy nor any useful experience doing this, I lean toward the "ease of use" approach (lighter gauge solid wire), thinking I'll probably just do a better job overall since it's easier to put things just where I want... but in other projects I've done (from carpentry to coding) this approach has often worked against me in the end, coming also with limitations in durability or flexibility. So, I go with Michael's recommendation and use silver stranded #18 definitely for the strictly-signal wires and also for as much of the others as I can. I have a lot wire to choose from, but I don't think I have quite enough to be consistent throughout. so I'm using other #18 stranded, as well as some #20 solid in cases where I have multiple wires connecting on one terminal and need to conserve space. And, my predisposition to conserve and repurpose materials means I'll also use up scrap wire bits trimmed from the transformer (which are so pretty!). So, it's a hodgepodge with an aim to place the highest quality wire where it probably matters most, and my whims, within generally acceptable bounds, for the rest.

I use the scale drawing of the chassis floor to measure the length of each wire lead before soldering it on. Grey #18 stranded for the plates and cathodes, black mostly stranded some solid for grounds, red stranded mostly #18 some #20 for DC supply, white/blue #18 stranded for the signal wire)

(Circuit board with about half the wires in place.) 

I've completed the front panel controls as much as possible prior to making the connections with the board and mounted everything, though some front panel pots remain only lightly attached since they'll have to be pulled back to access the front terminals when I make the final connections to the board. Throughout the process of adding components, I was doing testing to make sure my switch and pot connections were good, and at one point got unexpected results. The connections were good, but I realized I had soldered the wrong value resistors into place from the jack (tip connector) to ground - should be 1M ohm, and I had 100 ohm resistors in there for some reason. I suppose it's partly because I haven't fully sorted and binned the gajillion resistors that got mixed up in transit, and some of them ended up in the wrong drawers. But mostly, it's because I have never bothered to learn how to interpret the resistor codes printed right on them (just laziness on my part).  So I test them before placement to confirm the values, but must have forgotten to do that in this case. So, I pulled the jacks once more, and put in the right resistors. 

(The mostly-completed front panel controls. (Left to right) presence (high cut), master volume, mid, bass, tone stack select switch, treble, normal channel gain, lead channel gain, (bottom 3 in a row) gain boost/cut switch, V1a cathode resistor select switch, normal channel impedance Hi/Lo switch, (above the three just noted) channel mode select switch, lead and normal input jacks.)

On the rear panel, I had trouble making a good solder joint with with the output transformer leads to the speaker jacks, the solder just kept balling up and looking wrong. Turns out, these leads, since they come directly from and are probably literally a continuation of the secondary winding, are coated with the same thin insulation material (think thin coat of polyurethane) that keeps the wires of the winding coil from shorting each other. Since I clipped these leads down in length, I clipped off the ends that were left as clean uninsulated wire, so I as was trying to solder insulated wire. I discovered that one must burn off the insulation using molten solder in order to clean the wire. Immersing the wire end in a glob of hot solder for 15 or 20 seconds seemed to do the trick. Normally I'd be able to put my ohmmeter on each end of the two wires soldered to the joint to test continuity, but in this case, the end of the transformer wire is buried somewhere inside the transformer, and I just haven't taken it apart to test it yet, so we'll see.

 

(rear panel connections complete: (left to right) speaker outputs, negative feedback selector switch, power tube bias test points,  rectifier select switch)

I've also updated my layout (below) to better reflect actual placement of things... updating actual wire colors will be the last thing I do since that' s a work in progress. 

You may notice there are some new resistors on the pots that are not shown on the schematic (which I'll update one of these days) - this is due to my not having the right value pots in some cases, and really wanting to use what I've got. The presence pot is 25k ohms (should be 5k), the TMB side of the treble pot is 500k ohms (should be 250k) and the TMB side of the mid pot is 250k ohms (should be 25k). It's mainly that I'm thrifty - I just can't see spending $10 - $20 on special duplex pots that have two different values on one spindle, as my dual tone stacks require. And in the case of the presence control, I've already got a PEC 25k pot (military grade high quality) so why not use it instead of doling out another $4 + shipping?  The extra resistors work in parallel with the pot resistances to make them function as if they were the spec'd values, so the electrons moving through the controls "feel" as if they're experiencing what's shown in the schematic. 

... And, here's the fully wired board. 

I still need to clean up the flux and solder splatter and and go over all the connections to be sure they're sound. Next will be going over the final schematic to be sure it's right, and final layout once more to be sure layout matches schematic matches what I've done to date, before I shoe-horn the board into the chassis, install components, and complete all the connections.