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Traynor YBA200-2 tubes and maintenance

#21
BlacknWhite said:
This question is for @BassmanPaul or @beans-on-toast. Feel free to jump in if your know as well though! I opened up my YBA200-2 to check the bias. Just because it's new and so I would have a reference for down the road when I need to change a tube or whatever. Anyways my results were as follows
View attachment 2756536

The service manual says set to 75mv +/- 10mv. The V4/V7 pair seem ok just over 70mv.. But the V6/V5 pair seem low the V6 almost 13mv. The bias control pot is as high as it goes (was already set that way) and I did lower it and raise it back up and watched the values move. Is this normal? My last head was a SVT-cl so it was much easier to set the bias lol.
Click to expand...


Looking at the service manual, the power tubes are grouped V4-V5 and V6-V7. V4-V5 are connected in parallel and work together and V6-V7 are connected in parallel and that pair works together. For the push pull sides to be balanced, you want the currents on each side to be the same or as close as you can get them. If they are balanced THD will be minimized.

Based on your measurements, V4+V5= 138.4 and V6+V7=135.2. Not bad, a 3.2mA difference. Tubes are never perfectly matched. Some are closer than others. The four tubes are mixed and matched to provide as close a match and balance.

Now divide each sum by two. 135.2/2=67.6, 138.4/2=69.2. This is within the 75mV +/- 10% spec in the service manual. The target range is 67.5-82.5. A bit on the low side. If you find that at some point you can't bias a set of tubes within the spec, then it is time to change them.

THERE ARE A COUPLE OF IMPORTANT BUTS:
1) What you didn't measure was the line power AC voltage. Their spec is for 120VAC. If the voltage at home is not 120VAC, your readings are going to be off. Even over the period of taking the readings, the line voltage can drift a bit. So measure it at the beginning, and then at the end just to be sure. A tech would normally use a variac to set and maintain the line voltage at 120VAC.

2) When taking bias measurements, the amp has to be in a thermal steady state. That means wamed up so that components are stable and not drifting. To do this, let the amp warm up in playing mode, off standby, for at least half an hour. Playing through the amp helps it to warm up faster. Then take your readings.

If these points were not taken into account, retake your readings under the proper conditions. If you took the readings properly, then since the amp is new and V6 is marginal, it wouldn't hurt to ask Traynor for a better matched set or at least replace V6. If they say that it is acceptable, then you might need to buy a new set of tubes sooner than whn you normally would have to.
 
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#23
@beans-on-toast thank you very much for taking the time to look into this for me it is very much appreciated. When I took those readings I did make sure the unit was warmed up and the measurements were stable and not drifting (which they were in the beginning). I did not however check the line power AC voltage. Where would be a proper spot to grab this reading? This is all new to me. I'm basically just trying to learn my equipment enough to do readings and testings like this without going to a tech unless necessary for repairs obviously. Also one more thing the test points for the Tubes themselves are right in front to of the sockets on the board facing the back of the amp marked TP 1,2,3, and 4. There is also a test point right behind V5/6 and in front of two large filter caps (TP 5). What would that test point be for?
20170917_144257.jpg
 
#24
BlacknWhite said:
@beans-on-toast thank you very much for taking the time to look into this for me it is very much appreciated. When I took those readings I did make sure the unit was warmed up and the measurements were stable and not drifting (which they were in the beginning). I did not however check the line power AC voltage. Where would be a proper spot to grab this reading? This is all new to me. I'm basically just trying to learn my equipment enough to do readings and testings like this without going to a tech unless necessary for repairs obviously. Also one more thing the test points for the Tubes themselves are right in front to of the sockets on the board facing the back of the amp marked TP 1,2,3, and 4. There is also a test point right behind V5/6 and in front of two large filter caps (TP 5). What would that test point be for?
View attachment 2756813
Click to expand...

It helps to know your amp and be able to set the bias on your own.

Take the line voltage reading at the wall socket. Ensure that the meter is set to AC volts. Be careful.

Most of your our other readings are taken with the meter set up to read DC volts. TP5 is a circuit ground point. You can see it on the schematic to the left of V6. You can use it as a ground reference for your black meter probe when taking the TP1-4 readings.
 
#25
beans-on-toast said:
It helps to know your amp and be able to set the bias on your own.

Take the line voltage reading at the wall socket. Ensure that the meter is set to AC volts. Be careful.

Most of your our other readings are taken with the meter set up to read DC volts. TP5 is a circuit ground point. You can see it on the schematic to the left of V6. You can use it as a ground reference for your black meter probe when taking the TP1-4 readings.
Click to expand...

Perfect. Thanks again for your help!
 
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#27
This might help.

Invalid Link Removed

It’s a balance between sound and tube life. Optimal bias is a range, not necessarily a golden bias voltage. It depends on the power tubes and the amp. Some tubes can be run cooler than 70% maximum plate dissipation and still sound good. This extends tube life.

I like to set the bias as per the specs, then listen. Push the amp and set the bias for optimum sound. Then ensure that the bias chosen is within the safe operating range.
 

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