Troubleshooting a Traynor YBA-1

[Cloutier]

Hi,

 

I have my friend's Traynor YBA-1 that he uses for bass. It sounds really crappy, no headroom, distorting with volume as low as 1. A sound I would describe as "farty", for the lack of a better word. Same with a guitar plugged instead. No problem with the speakers as they play just fine plugged to my Fender Hot Rod Deluxe.

 

I have the schematic attached. Sorry for the poor quality, but that's the only one I could find that corresponded to what's inside the amp. The only difference is the amp I have has a 470K resistor between V2A grid and ground. Anyone knows what's the use, compared to none as per the schematic?

 

It came with GT valves, which where replaced with Electro Harmonix 6CA7EH. The problem worsened.

 

I checked every resistors and they are good. The voltages and resistances on the power transformer are good, no shorts or opens. Same for the primary side of OT. However, I measure only 1.8R between the secondary leads, that's 0.6 if I subtract the internal 1.2R of my DMM. I unsoldered them just to make sure I wasn't getting a false reading. So far that's the only thing I can see. Is this reading normal?

 

Any ideas appreciated.

 

Thanks.

[*BLEEP*]

 

If you happen to have a spare known-good ECC83 / 12AX7 on hand, you can try swapping out the current phase inverter valve (V3).

 

 

[Cloutier]

All the valves are new and tested good. I tried another good used 12AX7 and a new ECC81. Barely perceptible difference in distortion level and "quality", with the ECC81.

[Cloutier]

Anything else?

[WRGKMC]

Next step would be to check all your caps. If your meter has the ability to test caps, it sure makes that job easier. you do have to lift one leg when testing them so you don't read the rest of the circuit in parallel to them.

Caps can be off plus or minus 20% and still work fine, but any more can cause serious issues.

 

I'd of course check the power caps, change any that are leaking or bulging on the ends. Don't even need to check them if those signs are there because they re on the way out even if they are working. This will insure you have good clean DC power to supply everything up to specs.

 

Next I'd either check or just replace all your coupling caps. Coupling caps connect one stage to another. They pass the signal and block DC. If those caps are leaky or shorted you either don't get a good signal through them or they allow DC to leak through and alter the bias and therefore the amount of gain of the next stage. They can do crazy things like produce an asymmetrical waveform or simply cause the symptoms you describe.

 

I'd definitely change C12 & C13 because they are likely mounted near the power tubes and see allot of heat. C2, C4, C8 are all key coupling caps.

 

From there you can do the others.

 

The easiest way to narrow it down is to circuit trace the amp.

 

You pump an audio signal test tone into the front of the amp, then use a small amp to trace the signal stage by stage, component by component.

 

You can use a little battery operated amp, make yourself a ground lead from the shielded wire input, then on the probe end you need to put a capacitor between the probe and the battery amp. A .1us cap should be sufficient to pass the signal and block any DC voltage that might fry the amp.

 

 

You connect the ground probe to the chassis, then you pump the Traynor with a test tone, and begin at the input jacks, listen to the sound. Its should sound clean and flawless, Next compare pin 7 on the first 12AX7 tube and compare it to the output at pin 8. It should increase in volume at 8 but not have any detectable distortion.

 

Follow the schematic to your next item, C2. If the signal is clean going in and distorted coming out, you found your culprit, if not, move on to pun 7 of V2a. If the in is clean and output bad, you again, found your culprit,

 

If its dirty going in to pin 7 - we jumped over the tone stack, go back and check the input and output of those caps. The front controls will vary what you hear there so incorporate their operation when testing. The Traynor's I've worked on have horrible tone stacks, especially the bass amps. They seem to match their cabs OK but they aren't like a Fender Bassman amp's normal channel that sounds good for guitar or bass, Its muted and muffled like the Bass channel on a Bassman amp that sounds like crap for guitar. If you're using it for guitar, you may want to modify those caps.

 

Anyway, this is how you work through the amp, stage by stage. When you get something good going in and bad coming out, you found your issue.

Of course if its the input/output of a tube, it may not be the tube itself, it may be the caps, resistors, or DC bias of that tube so you have to confirm the support components that make that tube operate are all good.

 

Don't rule out things like bad ground either. More then once I was convinced I had a bad component only to find the ground leg had cracked, in fact its always best to inspect every solder contact and reflow any suspicious solder connections. I cant count the number of amps that were easily fixed that way. The last one was my Music Man amp. By buddy sold it to me for $50 because the sound was super low volume and distorted.

 

I went in there and found a grid resistor on the tube socket had cracked from heat and vibration. Then I also found one on the deep switch. Resoldered those and powered it up and it ran like it was bran new again. The amp is the 1976 version and is in mint condition, no rust and no dents. I can sell it for $700 and make myself $650. Pretty good profit for 10 minutes work, and most of that was heating up the soldering iron.

[onelife]

I have the schematic attached. Sorry for the poor quality' date=' but that's the only one I could find that corresponded to what's inside the amp...[/quote']

 

try this site...

 

http://www.lynx.bc.ca/~jc/traynorSchmArchive.html

[onelife]

V2A uses cathode bias which means the grid is negative with respect to the cathode by virtue of holding the grid at 0 Volts and applying a positive voltage to the cathode.

 

The only reason I can see for the 470K resistor you mention would be an attempt to give the grid amore direct path to ground than via the volume controls.

 

There should be no DC voltage on the grid of V2A and, if there is, that could be the source of your problem. If coupling capacitor C2 or C3 is leaking (suggested as a possibility by WRGKMC) then there could be some positive voltage on the grid (pin 7) which would compromise that stage of amplification.