Traynor YGM-3 Guitar Amp

[OneRoomStudio]

I've got a Traynor "Guitar Mate" YGM-3 (an original 1973 model, not the reissue), and I'd like to make the fixed bias adjustable. Normally, this is a pretty simple proposition, but this amp is a little strange.

It has a solid state rectifier, which alone isn't that strange, but the power transformer doesn't have a center tap. That's not all that unusual, but the bias voltage is taken from one side of the main secondary (similar to Marshall 1987's, but without the center tap), plus, it has bias-vary tremolo, which further complicates the bias scheme. In the reissue, they added a center tap to the PT and changed the bias circuit up a bit, so obviously it wasn't perfect.

I've attached a picture of the schematic for my amp, and this doc includes the schematic for the remake: http://traynoramps.com/downloads/servman/sm_ygm3.pdf?v=1

Here are my questions:
1) In the original (the one I have), what's the reason for the 330K/18K/47nF network feeding D1 instead of just a series resistor like in the remake? I'm assuming the cap is there to block DC leakage and the 330K/18K is acting as a pull down (depending on stand-by switch state) to set the voltage?
2) Could I scrap those and replace them with just a series resistor, or does the lack of center tap preclude me from doing that?
3) To make the bias voltage adjustable, I would typically change R39 to a resistor/pot combo, but in this case, it seems like the 18K resistor before D1 would also have an effect on the voltage present at R36.
4) What should I do?

Thanks!

 

[PRR]

The 0.047u is needed because we want a negative voltage, and the main winding with FWB is always positive. The cap, diode, and resistor string is common in many Marshalls (both FWB and CT).

This amp is odd because it bias-shifts warm (-17V) for normal to cold (-29V) for tremolo. With tremmed 6V6 we often compromise bias to get decent trem. This is either a clever alternative, or a too-clever hack.

Make R39 a 15K fixed plus 25K adjustable, see where it gets you.

 

[OneRoomStudio]

The 0.047u is needed because we want a negative voltage, and the main winding with FWB is always positive. The cap, diode, and resistor string is common in many Marshalls (both FWB and CT).

This amp is odd because it bias-shifts warm (-17V) for normal to cold (-29V) for tremolo. With tremmed 6V6 we often compromise bias to get decent trem. This is either a clever alternative, or a too-clever hack.

Make R39 a 15K fixed plus 25K adjustable, see where it gets you.

Thank you, this made a lot of things finally click for me - the voltages had me confused. I'll try the R39 replacement tonight.

 

[OneRoomStudio]

That did the trick! I should have just gone with my gut and tried that to begin with.

Now I'm trying to sort out a strange issue with the reverb though. When hooked up normally, there's no reverb, just hum. At first I thought it might be the tank or the make-up stage, so I inserted a signal at the input of the tank - no problem, reverb-y signal can be heard. So then I figured it must be the driving stage, except that I measure plenty of signal on the tip of the RCA jack going to the tank if I plug a signal into the input of the amp. I replaced the cables thinking it could be a bad/intermittent connection, but issue remains. It seems the driving stage is fine, the cables are fine, the tank is fine and the makeup stage is fine, and yet, no reverb. I'm at a bit of a loss now.

 

[nashkato]

i´m sure you checked that several times or it even is colour coded (which isn´t at guitar amps most of the time )
and i feel stupid even suggesting it .
You´re sure in/out aren´t reversed at the RCA´s ?

 

[OneRoomStudio]

i´m sure you checked that several times or it even is colour coded (which isn´t at guitar amps most of the time )
and i feel stupid even suggesting it .
You´re sure in/out aren´t reversed at the RCA´s ?

I appreciate the suggestion - it's easy to miss stuff like that! But yes, the ins and outs are labelled and connected correctly.

 

[OneRoomStudio]

In case anyone is interested, I tracked down the issue - the previous owner had installed a new tank, which I had assumed was the same as the original. It wasn't. The input had an isolated ground. The trouble is that with no ground reference, the signal wasn't making it through the driving coil. Once I shorted the input RCA sleeve to ground (making sure not to connect the shield of the cable to the sleeve), the reverb came back to life.

All's well now.