Cathode Resistor

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ky4bf
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Cathode Resistor

Post by ky4bf »

I have a Sano amp I am trying to restore for a friend. Probably model B18w or very close to that circuit. EL 84 Push-pull, cathode bias
At first I thought it was working but then noticed the OPT was running very hot. Indeed the winding ohms wer way off and it seemed partially shorted. I went ahead and replaced it, using a Hammond 1750PA.
I fired the amp up with new tubes and the new OPT.
Watching the scope, I failed to notice the new tubes were getting redder and redder.
Letting things cool down, I did a quick read on the cathode current which was close to 100 ma. And the new tubes were fried. shorted.
Now the mystery. Both tubes on this amp share a 125 ohm bias resistor, which was un damaged.
So I put in another set of EL84's and a bigger bias resistor. I tried several values of Rk till I got the no signal current down to about 25 ma.
I now have Rk=820 ohms, B+ is 410 volts bias on grid 1 is -18v.
All the similar circuits I look at have a much lower Rk. But this is the way the amp was built I believe.

Something is out of whack I'm sure but I cant find it. Sure apprecizte your help! Thanks,
Paul
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Bieworm
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Re: Cathode Resistor

Post by Bieworm »

What’s the plate voltage? B+ of 410DCV is very high for an EL84 tube amp. Mine are on the high side with 355V.. but 410?
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ky4bf
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Re: Cathode Resistor

Post by ky4bf »

Thanks for the response.
Yes that is high. The JJ spec sheet for their EL84 mentions 300v as a limiting value.
However the power supply appears un altered.
Matches the schematic except mine has a EZ81 rectifier tube instead of the 5U4.
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Bieworm
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Re: Cathode Resistor

Post by Bieworm »

Do you have spec voltages?
BTW an EZ81 has significantly less voltage drop than a 5U4, which drops about 50V
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ky4bf
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Re: Cathode Resistor

Post by ky4bf »

I should have said 5y3. My schematic shows a 5y3. The amp has the EZ81. All appears to be original except for what I did.

No, I don't have any spec voltages, but I agree that the 400v is high.

Thanks again for your help!
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Bieworm
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Re: Cathode Resistor

Post by Bieworm »

On the 18 watt amps we take around 345V B+ voltage as a target. With EL84 tubes we usually end up with 180Rk and 85% dissipation.
To drop the voltage check this :

viewtopic.php?f=4&t=25724
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ky4bf
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Re: Cathode Resistor

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Thanks very much, Bieworm. I will get that voltage under control and go from there.
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ky4bf
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Re: Cathode Resistor

Post by ky4bf »

Now I have 347v on the plate, -13.7 volts on the grid, 38.2 ma no-signal cathode current, using Rk=195 ohm.
Much better and thank you!
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KagliostroAmpwww
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Re: Cathode Resistor

Post by KagliostroAmpwww »

Ciao Paul

II'm very interested on your amp

Have I understand correctly, is it a Mixed Bias amp with Cathode resistor and a Fixed Bias Voltage on G1 ?

Please wich model is your Sano amp ?

Do you have a schematic that you can share ?

Many thanks

K
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