Help - Initial bias too high with DIYTube driver board?

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Help - Initial bias too high with DIYTube driver board?

Postby DerekVa » Thu May 16, 2013 10:21 am

I've just assembled a DIYTube Stereo 70 driver board and installed it in my 1963 Dynaco. Upon power up (no rectifier) the lowest I can set the bias with the wire-wound pots is -41.7VDC (both channels). Is this too high to move on to the next step of biasing with the rectifier in place? I don't want to screw up the amplifier

The amplifier is as follows:

- 1963 Factory-assembled Stereo 70 with original chassis / transformers / power & rectifier tubes
- DIYTube Stereo 70 driver board with 5751 and 12AU7s
- CE Manufacturing 80/40/30/20 can cap
- uf4007 mod on rectifier socket (rectifiers in line with HV on socket to protect the 5AR4)
- New uf4007 in bias supply
- New 47uF / 100V electrolytics and 10k / 2W caps in bias supply
- New 10 ohm / 1/4W cathode resistors
- New 1k resistors from driver board to pin 5 of EL34s
- New X1/Y2 caps on both legs of AC line + CL90 inline with fuse

Thanks in advance,

-D
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Re: Help - Initial bias too high with DIYTube driver board?

Postby DerekVa » Thu May 16, 2013 12:52 pm

I noticed that one of the EL34 tubes doesn't give a reading when checking bias - I'm guessing it has a bad internal connection (follows the tube when moved). Not sure if this would cause both sides to have a high bias, though.

-D
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Re: Help - Initial bias too high with DIYTube driver board?

Postby 20to20 » Thu May 16, 2013 1:39 pm

Without the amp running a real output load your bias supply will be higher. All the voltages you can read will be higher. Having the bias more negative is the best condition because that reduces the current through the output. But you should check out why you have no bias at one of the tubes though. The voltage must be at the socket connection before you try to run the tube otherwise with no bias on one tube that baby will go red in two seconds.

HTH

20
Headed for Tishomingo to sing in a can...
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Re: Help - Initial bias too high with DIYTube driver board?

Postby DerekVa » Thu May 16, 2013 5:22 pm

20to20 wrote:Without the amp running a real output load your bias supply will be higher. All the voltages you can read will be higher. Having the bias more negative is the best condition because that reduces the current through the output. But you should check out why you have no bias at one of the tubes though. The voltage must be at the socket connection before you try to run the tube otherwise with no bias on one tube that baby will go red in two seconds.

HTH

20


Tube was faulty inside the glass (crimp failure). Tried a replacement but it was drawing way too much current (Winged C). Evidently even with the balancing circuit, you still need to be reasonably close with regard to paired tubes. I'm going back to the gentleman who gave (sold) me the Winged C tomorrow and we'll see if we can find something a little closer to the OE Dynaco (Mullard) tube. I'm bringing the amp as well so we can test in-situ.

-D
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Re: Help - Initial bias too high with DIYTube driver board?

Postby Shannon Parks » Thu May 16, 2013 5:32 pm

The original manual specs -65V at the cathode of the selenium rectifier, then a 10K resistor, a 10K pot and another 10K resistor. So the lowest bias voltage one could adjust to (with the wiper cranked to one side) is -43V. So your voltages seem OK. Good luck sorting out the dodgy tube!

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Re: Help - Initial bias too high with DIYTube driver board?

Postby DerekVa » Thu May 16, 2013 10:53 pm

separks wrote:The original manual specs -65V at the cathode of the selenium rectifier, then a 10K resistor, a 10K pot and another 10K resistor. So the lowest bias voltage one could adjust to (with the wiper cranked to one side) is -43V. So your voltages seem OK. Good luck sorting out the dodgy tube!

Shannon



Good to know. I'm taking the amp with me (Roger wants to try it on the bench as well as see the driver board in person). I have never seen so much EICO gear in my life, BTW...

-D
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