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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: bias on 6l6gc posted by Dave Z on February 5, 2006 at 15:27:26:
Thanks for the information. I'm going to try and interpret, but you correct me if I'm wrong.From what you have said, you have a modified Dynaco ST-70 amp with a 60/40/40/40uf@525WV aftermarket filter cap (which probably cost you a fortune, I'm guessing.) The driver board has been modified or replaced to utilize 2 6SN7 tubes per channel into the output tubes which you now want to change to 6L6GC tubes. You modified the stock circuit from the ST-70 per a 'Glass Audio' article to use a 22uf@100WV filter cap and a 15K resistor instead of a 10K resistor which is stock, I think. I'm going to guess and say that you still kept the 10K linear pot and that the center tap from it still provides the actual -VDC voltage to both output tubes per channel. In addition, you have changed the stock resistor at the cathode of the output tubes (pin 8) to 10 ohms. Finally, you are reading a V+ of 307VDC.
Okay, assuming that all of the above is close to what you have, the answer is that you are biasing the output tubes much too hot. Here's the deal. When Dynaco required or invited kit builders to bias their own amps, they didn't know what kind of Volt/Ohm meter the kit builders were going to use and they didn't trust it or believe that you could get an accurate reading from it. So, to guarantee that an accurate reference source was available to the kit builders, they used a precise and specific cathode resistor on the output tubes of all of their amps which mandated that the bias voltage be 1.56VDC. If you go up to your local drugstore and by a D,C,AA or AAA battery and put it on a volt/ohm or any other meter, it will read 1.56VDC. That's the voltage of a single cell battery. So now, Dynaco could tell the kit builders to measure the battery first, and use whatever the reading was on their own meter as a reference which would enable them to bias their own Dynaco amp correctly. Good idea, right? The problem with your amp is that you replaced the specific resistor Dynaco had in mind (15.6ohm?) with a 10 ohm resistor and that's screwing you up.
It's not a fatal problem but you now have to unplug your amp and measure the 10 ohm resistor at the cathode(pin 8) of your output tube's socket. I want you to do that on a Digital Volt Ohm meter, if you have one, to make absolutely sure you know what the resistance is because it is from that resistance reading that you can figure out the standing current in your amp. I'm going to assume that it is still 10.0 ohms, but as I said, your measurement may be different.
Now you measured 1.52-1.30VDC from the biasing pin on the external socket. I'm going to assume that you have only one bias pot per channel and that there is only one 10 ohm resistor connected at pin 8 and that both pin 8's of both output sockets are connected. Assuming the least destructive measurement, 1.30VDC, you divide that voltage by the 10 ohm resistance to learn the standing current of BOTH output tubes, 130mA. So, assuming that the current divides equally, each output tube will have a standing current of 65mA, half of the 130mA. I would say that the maximum current you should run through your 6L6GC tubes would be 50ma, but let's finish with the measurements that you have taken so far. If you want to find out how much standing wattage your output tubes are dissipating, take the standing current that you derive from the voltage measurement, 130ma, and multiply that number by the V+ voltage, 307VDC. 307x0.130= 39.91watts. That's for both output tubes. For each tube divide that for 19.96 watts per tube.
But that doesn't sound too bad for 30 watt tubes, right? Wrong. The other things you haven't told us is that your power transformer is getting hot as a pistol and your output tubes are turning cherry red. That is what's going on, isn't it? That's because Dynaco ST-70 amps are supposed to be running with a V+ of about 420VDC-440VDC. If you back off the bias voltage to 1.0VDC your output tubes will be drawing 100mA for both tubes, 50mA for each tube. The wattage should be (assuming V+ is 425VDC) 42.5 watts for both output tubes and 21.25 watts per tube. Under those conditions, your power transformer should be running substantially cooler, and your output tubes should stop turning cherry red. IF the V+ does not rise up above 400VDC after you lower the bias voltage to 1.0VDC or if your power transformer continues to run really, really hot, you may have damaged it. I think you should consider replacing it. Hopefully, that won't happen.
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Follow Ups
- Re: bias on 6l6gc - corerosin 00:49:48 02/06/06 (1)
- Re: bias on 6l6gc - Dave Z 15:57:23 02/06/06 (0)
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