800C adequate bias low cathode current

reeferman

reeferman

Active Member
I'm sure this particular question has been asked before, but I couldn't find it.

I have performed many upgrades including adding 100 ohm screen resistors, 10 ohm cathode resistors, changing 330K to 220K resistors and .047 to .1 caps, and re-stuffing cans/replacing all electrolytics. This included replacing the bias bridge, .1, and both electrolytic caps, and lowering resistor to 2.7K.

The unit sounds great. I measured cathode current on all 7591s with the intent of adding the individual bias circuits (didn’t) and found all four running below 30ma.

V10 16.5 ma V12 11.0 ma

V11 14.0 ma V13 9.8 ma

Voltage at the grids of the 4591s is -25.63 VDC. The bias supply is -26.28 VDC.

The 4591 tubes (four RCA, code CY) test good on my recently calibrated B&K 707 tube tester.

Should I leave well enough alone? If I were to take the next step in raising the cathode current what would you recommend?

Thanks in advance,

Phil
 
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What resistor did you lower to 2.7K -- the one supplying bias to the output stage that was originally 5.6K? If so, then that value is clearly too low. Usually, it is best to replace that resistor with a 10K mini pot, which will let you then adjust the output stage current.

The quiescent current your tubes are operating at is much too low, generating significant cross-over distortion. The tubes should be idling at about 33-34 mA each for optimum peroformance.

Dave
 
With a resistance of 15.6K I get the following: .34, .31, .27, .246. I'll install Drew's IBAM on all 4 tubes, increase the resistance in the P/S as needed, etc to even the tubes. I assume a 1/2 watt 10K trimmer will work in Drew's circuit. I plan on using the Bourns 3299 series (10 turn) mini-pot. (I really had a duh moment when I read your reply)
Thank you very much, Dave.
Phil
 
Phil -- If you had to increase the resistance that much to bring the output tube current up to more normal levels then something would seem to be amiss. With 15.6K resistance you determined installed:

Please let us know what the plate voltage is on the output tubes (pin #3), and the screen voltage is (pin #4), as well as the bias voltage at the point it is being supplied to the four (now) 220K grid return resistors.

For you to have to adjust the resistor that high -- basically triple the original value -- then either the screen grid voltage is significantly low, or your tubes are all well worn.

Let us know!

Dave
 
I installed Drew's individual bias adjustments an all the 7591s. With the 15.6k resistor connected to the bias supply I was unable to get the recommended current. So I jumped the 15.6k resistor and dialed in the correct current, with spare room on both the up and down side. I wanted to reinstall the 2.7k resistor but I couldn't find it, so I left as is. The unit ran for several hours and the current draw was rock solid.
Here are the individual adjusters. The mini-pots are 10K, and having 10 turns is a nice feature. They are rated at 1/2 watt.
Thanks again for your help.
Phil
 

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Phil. The IBAM replaces all Factory stock bias resistors so they must be removed when installing the IBAM or IBBA as you've found out. I suppose someone should have picked up on it for you, but we've gotten so used to doing them and it's noted on the IBAM instructions, that we didn't re-inforce it.
 
Wear unplugged earphones all the time.:D Keeps the nagging down to a dull roar too, especially with Earplugs added. :bigok:
 
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