Silly Doubt JCM900 Dual Reverb 2x12 Combo 100W (5881)

[Lee_ranaldo]

Hey i have a silly doubt

Everytime i hit power (with the standby off) i have voltage on pin 3 of each power tube and pre-amp tubes
just like when u activate stand by!. The switch is OK and is working properly.
This happened before and after the recapping

Before the recapping i got -70 volts on the bias section without a change if i adjusted the trimmer (in this case it was 22K instead of 47K).

But now i hit the power on, Standby off and the same thing happens i see the voltage raising on the pin 3 of every power tube and pre-amp tube (HT section)

Now the Bias voltage can be adjustable but goes as high as -35 and i can't get that lower. i know i can adjust the bias range resistor to adjust this but i wanna find out why this happens!. I know that some amps have the standby switch after the filter caps but usually i don't get voltage on each powertube without hitting standby.

So it doesn't make sense.

Is this normal ? it had the same behaviour before.

Thanks in Advance! and i'm sorry if this question is a bit silly. for the experienced forum members!

[booj]

It's normal for the bias to be hot when the amp is on standby. It prevents the output tubes from drawing excess current from having to wait for the bias voltage to get to normal after switching the amp out of standby mode.
Why your getting voltage on pin 3 ?????

[Steve Conner]

Did you put the bias filter cap in backwards when you recapped?

[Lee_ranaldo]

thanks for your replies

haha no steve i know the risk of caps with reverse polarity and i always double check caps when i install them.

it is weird

I'm gonna replace the 22K bias trimmer for a 50K bias trimmer that is closer to the factory value (47K).

The tubes are not installed and there are no shorts between bias voltage and HT

just wondering if this amp does this normally or something is wrong
because it worked before and had the same issue.

[Lee_ranaldo]

Something is wrong
everytime i hit standby the bias voltage raises and it's less adjustable..
Before any intervention on the amp, the bias the range was imposible to adjust and was -70 volts all the time.(with the 22K trimmer).

Well this might be caused by the standby switch..

it's not like a DPDT like the common ones it's an SPST and it only switches one winding from the secondary tap instead of 2 of them like in common amps

so one winding is still giving ac voltage to a pair or diodes in the Bridge rectifier that allows the JCM900 power supply to charge the filter caps.

I dunno why the design engineer did this.

[Lee_ranaldo]

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