Input Jacks and front panel controls

I'm in the midst of pre-soldering a bunch of components and leads onto the input jacks and switches before installing them, so I don't have to do as much work in a tight space. This includes the "top" of the pre-amp ground buss - the thick bare wire running over top of the jacks, that will connect to the pre-amp ground on the circuit board. 

Unfortunately, after taking this picture and admiring/examining it, I realized I soldered the wrong terminals of the two input jacks onto the ground, so will have to un-solder and re-do it. Oh, well... it isn't the first time I've had to do that, and won't be the last I'm sure.

To prevent more of that, and just to be extra sure I wasn't missing anything critical in my thinking about how to set up the Mode switch, I re-checked my map of signal paths for all cases of channel used and switch position. The diagram excludes two of them: selecting the Cascade mode when plugged into the Normal Ch is not something I'll ever do since (1) the signal flow is designed purposely to go the other way, with tone shaping, voltage dividers and bias options on V1B that assume it will be the second gain stage in this mode, and (2) though it will work technically, it will probably result in a very low-power signal and an ungrounded grid on the Lead Ch, which may introduce noise. And only one Jumpered mode is shown - the second is identical to the one shown, just with the signal coming in via a different jack, so will be electronically identical.

Although I did this a few months back, it might be interesting for you to see where I worked out how the mechanical switch would address the various logical combinations/requirements of the mode switching. I wanted a 3 position toggle switch to do the job, and the one I found has twelve terminals (the round dots) with blades that connect them (the thick dark vertical lines) in various combinations. When the switch down, four blades connect each pair of terminals in the top and middle rows. When the switch is centered, the first and last blades slide down to connect first and last terminals of the bottom row with their middle row partners. And when the switch is up, The four blades connect each pair of terminals in the bottom and middle rows. Given that system, and knowing the logical requirements of my circuit, it was fairly straight forward to figure out whether and how the switch could work. 


 

Rectifier and heaters

I wired up the rectifier socket, complete with safety diodes to protect the PT in case of shorted tube, and a 2-phase solid state rectifier (which is just a pair of additional diodes), connected to a switch so that I can easily switch between tube-rectified and diode-rectified voltage. The rectifier takes high voltage AC current and converts it to high voltage DC that can be used by the power tubes and pre-amp tubes to amplify the signal at various stages in the signal path.  In my case, the PT is generating 350VAC, which will be converted to something between 400 and 450VDC for tube rectification (with different tube types generating different output volts) and 455 to 465VDC for solid state rectification. The layout snippet will put the second picture in context. 

(Rectifier layout)

(Rectifier all wired, and power tubes with heater wires complete.)

I also wired up the heaters to all the tube sockets (tubes use filament-type heaters, like an incandescent lightbulb, to free up electrons that then flow from cathode to plate). For a number of reasons:

• because we don't want heavy AC current traveling in wires near to the delicate low voltage signal 
• wires, which can pick up the noise
• and because of the closeness of the pre-amp tubes to each other and to the circuit board, 
• and the way the signal wires come from underneath the board, right down against the chassis 
• and also because I just wanted to try different things

...  I used two different methods to wire the heaters, both common practice though some folks claim one or the other method to be superior. In the case of the power tubes, I laid the twisted heater red and black wires (stranded #18) down on the chassis at the back and came forward to the tube sockets in linear fashion. For the pre-amp tube heaters I used blue solid core wire (which keeps its shape and stays where you put it space), following the same route at first, but then went skyward and arced up and down into each socket. Both methods keep the heater wires as far away from signal wires as possible, and/or on dissimilar axes, to prevent noise induction from the electromagnetic field created in the wires.

(Pre-twisting the power tube heater wires.)

(Final position of pre-amp tube heater wires.)

(It's a lot wire. But all tucked back as far as possible away from signal circuits.)

Layout adjustment 1

Doghouse for some of the filter caps

Recently, when I was dry-fitting the large components to get a sense of the space in the chassis, I realized my plan for laying out the three 22uF filter caps along the front edge of the circuit board would not work because the backsides of all the potentiometers (tone and volume dials, which in my design are duplex pots, twice the depth of the normal ones on the standard Bassman) would trap the relatively large caps in place, preventing easy maintenance. And filter caps are one of those things that sometimes need replacing. The advantage of putting them there would have been that they would be physically close to the portions of the circuit they serve, while not taking up space on the circuit board, which I wanted to keep as small as possible. This was an idea Doug Hoffman showed in some process photos of a 60's era Fender he was renovating.     

So, the current plan is to relocate just those three caps to the outside of the chassis, in their own compartment (doghouse) to prevent the high voltage leads from being accidentally shorted. The location will be very near to where they would have been, just on the other side of the chassis panel, maintaining the physical nearness to the circuit, yet getting them out of the way and in a location better for maintenance. 

I made and mounted a little circuit board just for them (circled in red).

No selectable speaker impedance 

When I was thinking of what I wanted in my design I thought it would be good to have flexibility in what speakers I could use with the amp. Other amps I've had offered multiple speaker jacks supporting speakers of various impedance, and I've found that useful. Many outputs transformers (OTs) have multiple secondary outputs taps to drive different speakers, often including 4, 8 and 16 ohm taps. I assumed mine offered such options as well, what with all those wires, so that's what I designed for. Actually the array of wires on my O45RS OT are on the primary side to better match various power tubes, and on the secondary side it has just one 16 ohm tap. Turns out, this is really just fine - it's slightly less efficient, but driving a single 8 ohm speaker works just fine with that tap, or I can drive two such speakers in series, both of which are options for me. The efficiency issue probably only produces subtle audible difference at high volumes.  Removal of the impedance selection switch actually creates a bit more distance between wires in that area of the chassis, which could help limit any interwire signal interference. It also requires a change in resistor values to drive the switchable negative feedback circuit, since that circuit will now be fed from a 16 ohm output tap instead an 8 ohm tap. 

That part of the layout plan now looks like this:

Early Wiring

I followed the advice of one of the amp gurus and hung paper copies of my layout and schematic behind the work table so I can easily check my work against design before I do it.

I worked on a number of controls I can do prior to their installation in the chassis - it's easier to solder components between the tiny lugs of a switch when the switch is held in a little clamp right at sternum-level, rather than mounted in the chassis, maybe at an odd angle or buried under other stuff. That included jumpers and resistors on some of the tube sockets, 

bypass capacitors on the volume pots, as well as resistors and common grounds on the set of 4 mode switches for the input channels. 

I installed the IEC connector (wall power input receptacle), main fuse, on/off switch and lamp, and went on to installed the main transformer, output transformer, and choke and pulled some wires around. This power transformer is truly a thing of beauty, and will power everything in the amp including a variety of heavier than usual (for the Bassman) power tubes - here's the wiring diagram: 

I grounded all the center taps (CTs) to the main PT ground (center of pic below), connected the line-in power to the PT and on/off switch, and did some testing to make sure all the voltages were as they should be. 

Then hooked up the lamp ...  the first bit of work my amp has done - yay! 

Chassis config

I did some testing with the power transformer, output transformer and choke, laid out on the chassis, to determine least noisy orientation. By design, the power transformer (PT) generates a 60 cycle hum that can be picked up by nearby wiring or the output transformer (OT, which ultimately sends amplified signal to the speakers). By connecting headphones to the otherwise disconnected OT  and powering up the PT, I can hear how they interact in close proximity and orient the OT to best avoid the unwanted hum. Same with the choke and OT. 

(This photo, taken later, shows the orientation of the transformers and choke, which seems to eliminate noise in the output transformer.)

Then I drilled holes and started populating the chassis with sockets, stand-offs, etc.

The tube spacing is tight, but I think oriented as best as can be for quiet wiring.

Tranny test

 I couldn't find a wiring diagram for the power transformer, so decided to do some testing to figure it out... 

I later wrote the manufacturer and the doc they sent based on the model # validated my findings and cleared up some confusion about a couple of leads from the secondary winding. This transformer is unusual  (at least for the size/class of amp I'm dealing with) in that it is tapped for two separate 6.3V circuits - I wasn't looking for that possibility as I tested, so having found one 6.3 V circuit, couldn't quite figure out what the additional leads were for.

The transformer is pretty beefy - easily meets my needs, and probably was meant to drive a 4-tube power section.