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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: copland CTA 501 biasing posted by Kevin on May 9, 2001 at 22:25:47:
Kevin, I have a Copland CTA 401, which is basically the same as the CTA 501 except it has a volume control. Below are instructions on how to bias and some other suggestions on tube-rolling and tweaking this beautiful EL34 amplifier.PROCEDURE
1. Set P1-P2 in middle position. Turn P3 fully in clockwise direction.
2. Measure with + on the left side of R31 and - on the left side of R32. Adjust P1 to 0 mV. Repeat with R131-R132 and P2.
3. Measure mV over R32 and R132, choose the one with highest rating and adjust with P3 to 430mV
4. The channel difference should not be more than 50 mV. Example: When one side is 430mV the other should not be less than 380mV.
5. If all valves have the same rating and the difference is over 50mV between the channels, then set P1 P2 in middle position and measure the valves over R31-32-131-132. Take one low and one high and place each channel. Repeat bias adjustment.
6. Heat for 30 min and repeat 2 and 3.
TIPS
Now R31-32-131-132 are the resistors that are mounted immediately in front of each EL34 (nominal value 10 Ohm). The left side of each resistor is connected to the cathode of the EL34 and the right side connects to ground. Basically the voltage measured on these resistors divided by 10 equals the bias current through each EL34. E.g. 430mV equals 43mA and with a plate voltage of 450V each EL34 will dissipate around 20 Watt. The voltage on the middle lead of P1 and P2 is the grid voltage for each EL34. I recommend that you start the bias procedure with step 5 and set P3 at a value that will cause average 400mV over the resistors. This step basically measures the intrinsic bias current for each individual tube at fixed grid voltage and then by combining the high and low one to create an average that will make the difference measured in step 4 minimal. This difference is basically a function of how well the tubes are matched as a quartet. Depending on the quality of the tubes you have you'll notice that when you repeat the procedure a couple of days later that the amp will be more or less out of bias again. Especially with new tubes it will take a couple of weeks of burn-in before the tubes really become stable (you'll be surprised that the values really change during that period)
As for the bias current you can go up to 480mV max though it will not make much difference on sound quality. On a stable set of tubes that are matched, difference between channels should be less than 10mV and biasing between each set should be with in 1 to 2 mV measured over time (this I obtained with Svetlana and Mullard tubes only)
TUBE ROLLING
I experimented with a wide variety of tubes. My original amp came with TESLA EL34 and probably Russian signal tubes (Sovtek WXT's). The tubes I have tried are:
TESLA
JJ (same as TESLA but new production)
EI 6CA7
"Telefunken" (fake, east Germany NOS production)
Svetlana
Mullard (NOS, matched quad)
From the above I currently am using the Mullards. These are by all means the best of the lot and worth the money. Besides being extremely stable, the sound is most refined with very strong base and jaw dropping midrange and highs. However it has become almost impossible to find any matched quads these days. Second in line are the Svetlanas which come closest to the Mullard tubes, though kind of lack that mystical touch. Third are the TESLA's (good boogie, less refined than the Svetlanas). Fourth the EI's (shape is 'fat-tube'. these tubes have a 'darker' sound, very strong bass but are relatively unstable and hard to bias). The fake Telefunkens are basically junk.
For the 12AU7 and the 12AX7 I have tried many tubes as well. I never liked the Sovteks and when you compare the mechanical construction with NOS production it is clear the Sovteks have these tiny plates (there's a new Sovtek now with long plates, but have not tried these) For the 12AU7 I have settled for NOS Phillips (Made in Holland, my home country) E82CC SQ ("Super Quality", low noise). For the 12AX7 V3 tube I am intermittently using NOS Phillips, Mullard and Brimar. The sound varies slightly between each with each having its strong point. I am currently using the Brimar which sounds refined and well balanced. There's much room here to experiment!
TWEAKS
I'm not sure if you're interested in tweaking the amp, but I made a few modifications to squeeze more out of the 401. First the capacitors in the powersupply have been extended with roughly twice the capacity by using custom made capacitors I bought here in Tokyo. Next I have replaced the bias resistors R31-32-131-132 with 1% 2W models. In my amp there was a 'tiny fuse' soldered between each bias resistor. These fuses are there to protect the tube, but also are in the signal path so out they went (the amp has a fuse in the power supply for plate voltage and should be fine) Further I upgraded all capacitors in the signal path with Hovland Musicaps (the ones that come standard are not bad though) and replaced diodes in the power supply with soft recovery diodes. As I'm not using the phone stage, I took the 2 tubes V1 and V2 out to save on power/heat. Any fixes to the main PCB will require you to disassemble the entire amp...
Good luck!
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Follow Ups
- Re: copland CTA 501 biasing - TokyoTubeDude 23:32:49 05/19/01 (0)
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