Traynor YGM-3 Bias Supply

[OneRoomStudio]

I'm fixing up an old Traynor YGM-3, which is a nice amp by the way - sort of like a strange cross between a Princeton Deluxe and a Marshall 18-Watt, with a Magnatone M10A-type reverb thrown in.  It's generally pretty easy to figure out, but the bias voltage scheme seems strange to me.

First, it doesn't use a center-tapped power transformer (the reissue does), but that's not that unusual, and the FWB rectifier mostly makes up for that. What's strange is both the standby switch arrangement, and the voltage feed to the trem circuit.

For the standby switch, there's a 47n cap connecting one side of the transformer secondary to the bias rectifier diode, along with a 330K resistor connected to the negative side of the FWB rectifier (connected to ground when the standby switch is switched to "run"). There's also an 18K resistor to ground before the rectifier diode. I'm assuming that this is to provide some small bias voltage that leaks through the 330K resistor while the amp is in standby, and then the full voltage through the 47n cap when the FWB is grounded? Seems like a strange arrangement either way. Is there any reason not to rewire it like the Princeton, with a (~100K) resistor in place of the 47n cap, remove the 18K to ground before the diode, and use the standby switch to switch the B+ instead of the bias?

Princeton Reverb for reference:
https://robrobinette.com/images/Guitar/AA1164/AA1164_Princeton_Reverb_Annotated_Schematic.png

Second, most bias-vary tremolo's I've seen (including the princeton reverb) only supply one side of the trem intensity pot  with bias voltage. The wiper is then fed to the power tubes as a mix between the bias voltage and the trem oscillator. For some reason, this amp does that, but also feeds bias voltage directly to the power tubes. What's the purpose of that? Is it to limit the throw of the trem intensity pot? Any reason not to remove R36 and disconnect the direct feed to the tubes?

I'm slowly wrapping my head around this amp, but it's definitely a little quirky.

 

[PRR]

> Is there any reason not to rewire it like

Is there any reason to think Pete was a fool?

Yeah, he followed a quirky path, but it usually turns out to make good sense.

 

[Tubetec]

Jack Darr's Electric guitar handbook , you should be able to find it online in pdf form for SFA.
Has great working descriptions  of all the old topologies found in tube amps , including trem/bias arrangments.
By all means do find out more about how your amp works , but if its working properly and your happy with the sound
probably no need to change anything .

High value carbon resistors will generally shift up over time , +50% on a 1 meg wouldnt be unheard of ,
if its just a grid resistor it may not make any difference , if its part of a tuned circuit like an oscillator/trem it could cause weirdness. Use your multimeter to find suspect devices but only replace if its likely to be an issue . 

 

[OneRoomStudio]

Both fair points. I'm certainly not questioning Pete Traynor's genius, nor do I want to change things just to change things. I wouldn't mind fixing up a few issues though. The tremolo is noisy (so is the reverb), so I started looking at the bias voltage. It seemed strange, hence the questions.

I think my main issue though is that the B+ is around 450Vdc. That's at least 50V too high for me to feel comfortable with 6BQ5s in there (and 50V higher than the schematic is marked).  Would it be a terrible idea to put an 820R resistor between the FWB and the feed to the OT? Each tube is drawing ~20mA, and the rest of the stages are drawing ~ 20mA total, so at 60mA, that should drop around 50V...it's need to be at least a 5W resistor though. Maybe things will start to behave more at a more reasonable voltage?

 

[PRR]

> put an 820R resistor between the FWB and the feed to the OT? Each tube is drawing ~20mA,

At idle. At full roar it may be over 130mA/pair, making a 100V drop, from 50V drop at idle. Did someone say Sag?

I carefully didn't say Pete was a genius. And frankly some of his operating conditions defy sensible warranty (even if tubes are not warranted). But his designs are idiosyncratic enough to be difficult to modify easily.

> tremolo is noisy (so is the reverb),

Noise? Hiss? Hum? Buzz? Canadian music? I doubt they were noisy when new. I think a tired part wants replacing.

 

[OneRoomStudio]

> put an 820R resistor between the FWB and the feed to the OT? Each tube is drawing ~20mA,

At idle. At full roar it may be over 130mA/pair, making a 100V drop, from 50V drop at idle. Did someone say Sag?

I carefully didn't say Pete was a genius. And frankly some of his operating conditions defy sensible warranty (even if tubes are not warranted). But his designs are idiosyncratic enough to be difficult to modify easily.

Very good point. Which is worse though, sag, or a constant 450V on the tubes (150% max operating voltage)? I worry that I might need to replace the PT.

> tremolo is noisy (so is the reverb),

Noise? Hiss? Hum? Buzz? Canadian music? I doubt they were noisy when new. I think a tired part wants replacing.

Buzz in the reverb, which I I thought I had fixed a while ago, but it's back. I suspect a grounding issue. Ticking and bleed from the tremolo, likely due to oscillation. All the electrolytics have been replaced and the resistors are all metal film. I think lead-dress, grounding, and/or the too-high voltage are more likely culprits that any tired parts at this point, but who knows.