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Post subject: Super-Sonic 22 Bias
Posted: Thu Jul 27, 2017 4:42 pm
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So I finally decided to check my bias after owning my ss22 amp for over a year, and had some weird results. Most likely user error, but any input appreciated.

My plate voltage tested at D38 was 413, and D35 came in a little high at 32 ma. It's my understanding it should be around 24 ma with pv where mine is. The weird part is that D36 showed around 6.36 ma which is way out of where it should be. I checked this multiple times, and even switched tubes with only moderate changes in the readings.

So, am I checking the wrong place for bias check? Again checking D35 and D36. I lowered the bias to 24ma at D35, and all seems good, but wondering about the D36 reading.

Thanks, Dan


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Post subject: Re: Super-Sonic 22 Bias
Posted: Sun Jul 30, 2017 5:42 pm
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I don't have access to the schematic, but you can measure the Bias in this fashion if you feel comfortable.

With amp on, DC voltage @ pin 8 of the rectifier tube= xxx (example= 384)
With amp on, DC voltage @ pin 3 of each output tube = yyy1 & yyy2 (example= 375 & 372)
Subtract yyy from xxx= voltage drop (example= 9 & 12)

Turn OFF amplifier.

Measure resistance from pin 3 of each output tube to pin 8 of rectifier= zzz1 & zzz2 (example= 290.4 & 289.2)

Divide voltage drop of yyy1 by zzz1 and voltage drop of yyy2 by zzz2= current in milliamps. (example= .030 & .041)

Hope this helps! Edited to correct earlier error


Last edited by sfceric64 on Mon Jul 31, 2017 4:05 pm, edited 2 times in total.

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Post subject: Re: Super-Sonic 22 Bias
Posted: Sun Jul 30, 2017 9:17 pm
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I have s hematic at home but I will come back home august the 9 th with the answer


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Post subject: Re: Super-Sonic 22 Bias
Posted: Mon Jul 31, 2017 7:24 am
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shematic here:
Sorry I must leave.

http://www.thetubestore.com/lib/thetube ... ematic.pdf


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Post subject: Re: Super-Sonic 22 Bias
Posted: Mon Jul 31, 2017 8:06 am
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I appreciate all that info, but the amp is all buttoned up at this point, and I think I checked the right test points. Just wondering if anybody can confirm that D35 and D36 are the right points for bias check, and if so, why D36 was so off? Thanks!


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Post subject: Re: Super-Sonic 22 Bias
Posted: Mon Jul 31, 2017 12:33 pm
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Good test points for bias

Look tube V7 is dead if D35 and cathode resistor are good.
Put V6 in V7 socket and read bias. If bias is good, original V7 is bad.


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Post subject: Re: Super-Sonic 22 Bias
Posted: Mon Jul 31, 2017 2:42 pm
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N0, D35 & D36 are the diodes for the cathode bias test points to be accurate.
TP40 & TP41 are the test points for the cathode bias circuit.
R140 & R141 are the resistors for the cathode bias circuit.
By specs TP40/41 should both be 24vdc, if the bias voltage is -43.7vdc & R156 @ 12 o'clock.

Do as Stratele suggests, it will confirm a bad tube if the bad voltage follows the tube to the other socket. Or it will point you to a possibly bad resistor/diode in the previous socket if the voltage shows correct in the switched socket. Hopefully this makes sense to you.

Thanks for the schematic's.


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Post subject: Re: Super-Sonic 22 Bias
Posted: Mon Jul 31, 2017 6:22 pm
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Thanks for all the info!

It appears on the schematic that tp40 and tp41 are equivalent to D35 and D36. Is that not the case? It does look like maybe tp 41 should be checked on the side of D36 that is the far side from the D36 marking? Maybe that is why my reading was off?

There is no actual issue with the amp. It sounds fine, and I checked these readings with two sets of power tubes, so don't think there is a tube issue. May be no issue at all, just wondering why my D36 reading was so high. Thanks


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Post subject: Re: Super-Sonic 22 Bias
Posted: Tue Aug 01, 2017 11:13 am
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As long as you are on the right side of the diode ,yes you should get 24vdc. That's why if you measure from the test points vs the component parts there is no chance for error. If your troubleshooting the components then you should take readings as needed from the component points to id each possible problem.

Glad you have it working ok, as long as the tubes are matched pretty close just use the bias pot to tweak your bias to your desired tone. Very small increment turns and then re-measure bias and play through it until you find the sweet spot your looking for.


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