Dual Single-Ended TBM 5881

[CascoSieg]

Salvaged a dead Fender FM212 and used the cab and chassis as a home for a stereo Single-Ended amp, with TBM front end. After some corrections and tweaks, I'm pretty happy with it. It's a perfect complement to the 5F6A/JTM I built last year. Still working on speaker combinations for the 2SE.
Lessons learned:
- Next time I'll find a better amp to salvage - the MDF cabinet and flimsy chassis are harder to work with and probably won't hold up well gig after gig.
- Many of the parts - knobs, hardware, etc. - are not standard sizes so need to be fiddled with to make them work, or just replaced, which sort of diminishes the advantages of salvaging.
- The angled chassis needed a lot of modifications to accommodate transformers and tubes. It worked out, no regrets, but I wouldn't do that again.

link to a few process pics... https://photos.app.goo.gl/3QbcSNUCvUqrWD2R8

I'm really grateful for all the support and information here!   

[Latole]

Nice job !

[CascoSieg]

This amp works and sounds pretty good, but I'm stumped by an odd behavior: In short, at high gain settings a low volume tinny sounding version the overdriven signal seems to jump past the FX loop and get onto the grid of one of the power tube drivers (but not both) - it's like a shadow of the original signal. It's a problem because I really want to use a volume pedal as part of the FX loop to shut down the signal entirely and/or create swells, etc.  Here are the pot settings and some observations (see annotated pics and schematic for locations of things):
- The volume of the shadow signal at the speaker depends on the Preamp Gain (P04) and tone controls, OD Drive (P05), and L channel Master Volume (P12). 
- It's only audible when the OD circuit is active, and Gain, Drive and L channel Master Volume levels are all over 6 or 7, and when all three are dimed it's loud enough to hear it in the next room. 
- The OD Level control (P06) does NOT affect the volume of the shadow signal.
- It's audible when the Preamp Send pot (P10) is at zero (which I believe is the equivalent of an engaged FX loop and volume pedal at zero). When this control is turned up, the normal signal quickly overwhelms the shadow signal, but I'm pretty sure it's still there.
- When I first started trouble-shooting it, the movement (chopsticking) of wires between the L return jack and L Master Vol (P12) and the board connections, made the shadow signal slightly louder and softer as I pushed them nearer or farther from the input side of the board (they ran under the board). I have replaced these wires, plus the one connected to pin 2, with shielded wire running above the board, and now their movement does not affect the shadow signal volume but the shadow signal is still there and quite pronounced - I'm not sure if anything other than the sensitivity to movement changed as a result of changing over to shielded wire.
- The shadow signal is NOT audible in the right channel under any conditions I've tried.

Are the following deductions correct?:
- The source of the shadow signal is the yellow highlighted area of the schematic between red labels E and F. Since P05 increases the shadow signal volume and P06 does NOT reduce/alter it, this highlighted area must be the source.
- The receiver of the shadow signal seems like it must be the highlighted area around red label B, before the V3b grid (since P12 attenuates it) but after (right of) R51, since the wire left of R51 is grounded through P10.
- The tinniness of the shadow signal may imply it's coming through a high-pass filter, which could be a clue as to its path or source, but I'm not sure what to conclude from that.

I would be most grateful for any thoughts as to where the shadow signal may be coming from, given the observations and info so far, and what I might try next to get more info or fix.  Unfortunately I don't have a scope, but I can send a 100mV-pp 1kHz signal into the input and take voltage readings (have done some - see attached pic).

Many Thanks!!

[CascoSieg]

Greetings all,
I've spent many more hours on this, some changes, lots of new data, no solution yet.
To recap the main problem: a version of the input signal from the preamp, and especially from the OD section, is appearing on one grid (Left side, V3 pin 2) of the power section driver, and is loud enough at high gain settings to make FX loop volume pedal (as well as the Fx send pot) ineffective for shutting down the sound. The other grid (V3 pin 7) is almost dead quiet and seems to work as expected. The frustrating part is I don't understand why the difference between the two identically configured channels. The offending channel is a little closer to the preamp circuitry, but moving the wires around doesn't really confirm that is an issue. There are slight differences (< 5vdc) in the two plate voltages for V3, though I'm not sure how that could be significant. All of this is compounded by my lack of real understanding of capacitive vs. inductive coupling (presumed related to the problem), and my general inexperience with with circuit design.
Any thoughts, suggestions, questions would be most welcome!  Read on for details...

- Happens with OD on or off, but is only loud enough to be problem when using the OD channel.
- With OD on, Input Gain (P04) @10, OD Drive (P05) @10, OD Level (P06) @0, Fx Send (P10) @0, L MV (P12) @10, R MV (P11) @0:
   - very distorted signal, trebly version of what I'd hear if I turned the Fx Send (P10) up.
   - OD Trim pot (P07) increases/decreases distortion. Level pot P06, if turned up, has a slight modulating (?) effect on the signal... it doesn't make it louder but you can hear some kind of change when you swing the pot, slight change in amount of distortion or tone maybe but no change in volume.
   - Moving the wire that runs from the Cln/OD relay to R13 before the OD trim pot right up to the front of chassis slightly increases the volume, but mostly has no big impact on the signal.

I updated some long signal wire runs and grid leads to shielded cable as noted on the updated schematic, but that had no discernable effect (see updated schematic and circuit pic):
- C06 > relay
- Relay > R13
- P11 > R51/C50 > V3 pin 2
- P12 > R50/C51 > V3 pin 7
- L Fx Rtn jack tip > R50/C51
(notably I left the R Fx Rtn jack tip to R51/C50 un-shielded and it sounds fine)

Acquired a used Oscilloscope to get a closer look at things (input signal sine 1kHz @ 100mv-pp)...
First, baseline clean signal, OD off:
- Input gain @10, no PAB and all tone controls at noon (for all testing), signal out of second input stage, after C06 = 4.2v RMS (x10 with PAB!), no visible distortion on scope.
- Fx send (P10) @10, signal (@ wiper) = 360mv RMS
- With L and R MVs (P12, P11) @10, L and R V3 grids ea = 240mv RMS
- V3 cathodes are (R) 1.50vdc and (L) 1.45vdc, both show 1.49k ohms resistance.
- V3 Plates are (R) 201vdc and (L) 196vdc.
- With MVs @10, R (V5) grid = 14.9v RMS, L (V4) grid = 15.4v RMS (visible distortion starting at about 8 on the dial).
- Power tube Output
   - at L speaker jack = 6.0v RMS before distortion visible, @5 on the dial. Max = 8.0v RMS
   - at R speaker jack = 6.0v RMS before distortion visible, @5 on the dial. Max = 7.7v RMS

With OD on, and signal leaking through on L channel:
- OD Drive (P05) @10, OD Level (P06) @0, Fx Send (P10) @0, L MV (P12) @10, R MV (P11) @0.
- Junction at the "in" side of R50/C51 = 0v (2 Ohms, grounded through P10).
- L grid = 13mv RMS, signal (see pic)
- R grid = 8mv, no visible signal, flat line on scope)

Thanks!

[sluckey]

Your unwanted signal may be sneaking through your power supply. You have the preamp and OD circuits powered by the same node. I suggest to separate node D and use an additional node E to power the preamp. Also increase the decoupling resistors to 10K each. This will provide a lot better isolation between the preamp B+ and the OD B+. No guarantees this is your problem but it's easy to try. See attached...

[CascoSieg]

Great - I will give this a try!
Thanks for having a look.

[CascoSieg]

Hmmm....
Tried it, but no change.  First, I just up'd the D-node resistor to 10k - no audible change. Then went ahead and separated the B+ rails and ground busses, and added the second resistor and a temporary filter cap (22u) to additional parallel node. Still, no audible change, other than maybe slightly quieter, which might be due to lower plate voltages all around (?).
I'll keep poking at it...
Thanks!

[CascoSieg]

I also tried this power supply configuration (see pic): with each 12a_7's B+ coming dirctly from the B node, and each filter on  separate leads to ground. Still the phantom OD signal is coming through the left driver (but not the right). Then I disconnected the signal lead from pin 2 of Left driver triode, and disconnected the plate lead (pin 6) from the Right triode (just to take it totally out of the picture). No change. That does seem to point to the power supply (or the cathode?), but if the signal is coming through on the B+, I don't understand why I wouldn't be hearing the same thing in both of the driver triodes.
I assume I can use my scope to show the AC component of the B+ (lots to learn about this new tool), and try to see if that's really where it's coming from.
TBC...

[tubeswell]

In addition to Steve’s suggestions about better filtering in the PS nodes between the channels, are the signal paths between each channel and the ‘mixing stage’ separately coupled with individual coupling (AC blocking) capacitors? (This might seem to be a bit of a ‘belts and braces’ approach, but it may be worth checking/pursuing. I’ve encountered crosstalk difficulties previously in building 2-channel (6G6) circuits which don’t have separate channel coupling caps in the original factory configuration, and have been able to solve it by putting separate coupling caps in (alongside separate PS filtering). Not saying this is your problem, because I haven’t checked your schematic.


Another approach might be up up the filter capacitance to achieve more brute force filtering.

[CascoSieg]

Thanks Tubeswell.
Just to clarify: in the TBM preamp the OD path is sequential, so I don't think there is mixing the pre and OD circuits per se (see block diagram). The output section is stereo, left and right separated by a pair of 220k Rs before their respective drivers, which share a 12ax7 (on it's own PS node). It is one of the drivers that is picking up the OD signal, even with the signal lead disconnected from the grid.
I like the idea of playing with higher value PS filters to see whether that makes a difference.
Thanks!

[tubeswell]

Thanks Tubeswell.
Just to clarify: in the TBM preamp the OD path is sequential,… The output section is stereo, left and right separated by a pair of 220k Rs before their respective drivers, which share a 12ax7 (on it's own PS node). It is one of the drivers that is picking up the OD signal, even with the signal lead disconnected from the grid.

How does the clean channel switch go when the OD is on? Is the bypass taken completely out at both ends? Or only at one end?



You tried a tube swap?

[CascoSieg]

Oh, interesting question... the by-pass is entirely inside the switch - it's a jumper leading to an unconnected terminal.
However, your question leads me to wonder whether I properly translated my standard DPDT layout shown here, to the relay layout when made the connections - something tells me I didn't, but will have to check this evening.
Thanks!
[edit]  Oh, and yes, have tried tube swaps on the driver tube, but not the others... will try that as well.

[CascoSieg]

Just to tie this thread up...
This issue is beyond me.
I've updated the power supply nodes, checked and re-checked lead dress, added shielding to vulnerable leads. Thanks to your suggestions, I do think the rogue signal is coming in through the plate of the driver (V3b), but I'm out of ideas and energy for it. The truth is, it's more of an annoyance than a practical problem: it's significant in the left channel at very high gain only, and only when I'm trying to shut down the signal using a volume pedal in the Fx loop. it's just not a problem at all in the right channel (which is exactly the same as the left! ugh!  ), so if I need to, I can just mic up that speaker for recording and silence the other.
Thanks for all the help and suggestions!
On to the next...