The amp that lost its backbone...

[kalopsia]

Hey, what's up?

 

I have a 1969 Traynor YBA-1 that has suddenly started having power issues. This thing used to be so beefy, but all of the sudden it is extremely quiet and has zero headroom.

 

The electrolytics were all replaced about six months ago, so my first thought was faulty tubes. I swapped in a full set that I know are functional with no luck.

 

Then I checked the preamp cathode resister and it is still within tolerance. The only other thing I could think to do was replace the coupling caps. However, the problem hasn't gone away.

 

As far as I can tell, the circuit follows the stock '69 schematic.

 

I'm at my wits end here. Any one have any suggestions??

 

Thanks!

[steve_man]

I'd find a good amp tech and let them look at it. I know that's not exactly "help" but it might be the quickest solution to the problem.

[Grant Harding]

Here's a reference that you've probably already checked out: http://www.lynx.bc.ca/~jc/traynorSchmArchive.html

[onelife]

I always liked the fact that Traynor schematics are included with the amplifier and printed on the underside of the removable top cover. These amplifiers are built to be easily repaired when necessary.

 

 

Have you checked the voltages coming from the power supply?

 

The negative voltage (grid bias on the power tubes) impedes the flow of current through the power tubes. When a signal is applied to the grids it makes the grid more or less negative which has a direct effect on the flow of current. If the bias voltage is too high (negative) then it will impede the current flow more than it should which will result in reduced output.

 

I would start by (carefully) measuring the voltages on the power tubes and comparing them with what is noted on the schematic. If they turn out to be normal then I would look at the cathode and plate voltages of the preamp tubes.

 

The preamp tubes are biased differently than the power tubes. Instead of having the cathode at or near zero volts and a negative voltage applied to the grid, the preamp tubes have a positive voltage on the cathode with the grid at zero volts which makes the grid negative with respect to the cathode. This is accomplished with a cathode resistor and the voltage drop across the resistor is what provides the bias for the tube. These cathode resistors often have a bypass capacitor across them which normally does not affect the bias but provides some extra gain for the signal. If one of these capacitors becomes a short circuit or leaky then it can turn off the tube or greatly reduce its ability to amplify the signal.

 

If you get unusual voltage readings then they will be a clue to what is ailing your amplifier. Perhaps you can (again, carefully) take some measurements and report anything unusual here and we may be able to point you in the direction of your problem.