Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bassden on October 23, 2023, 09:42:00 am

Title: Twin reverb without bias pot measuring bias low vdrop on blue wire side
Post by: Bassden on October 23, 2023, 09:42:00 am

Hello, first timer on a twin reverb.bias

Output trans blue wire side  38 ohm  .06 vdrop   438vdc  69/34w
Output trans brown wire side   35 ohm   2.0 vdrop  411vdc 21/10w

What would cause a low voltage drop on 2 different sets of all new 6L6GC's rated at 30w?
Or what could be causing such a low voltage drop?
All bias components new, original OT and balance 10k pot, have not yet replaced an OT.

Thank you for any suggestions, Bob

Title: Re: Twin reverb without bias pot measuring bias low vdrop on blue wire side
Post by: shooter on October 23, 2023, 09:45:27 am

with the PA tubes OUT do you have the same Neg voltage on the grid pins?

Title: Re: Twin reverb without bias pot measuring bias low vdrop on blue wire side
Post by: Bassden on October 23, 2023, 10:14:49 am

YES

Title: Re: Twin reverb without bias pot measuring bias low vdrop on blue wire side
Post by: sluckey on October 23, 2023, 11:26:16 am

Is the amp working properly? If not, what are the symptoms?

I suggest putting a 1Ω resistor between cathode and ground. One resistor for each tube. Check the current by measuring the mV across each resistor.

Sometimes connecting a meter to the plate of an output tube can upset the circuit and cause crazy readings.

Title: Re: Twin reverb without bias pot measuring bias low vdrop on blue wire side
Post by: Bassden on October 23, 2023, 11:58:02 am

Thanks for the reply
I put the one ohm resistors on originally when I replaced the old original resistors on the 4 OP tubes and was getting this result. When I turn the balance pot little changes so I began to do the real bias.
One ohm mv values go like this V10 1.2mv, V9 0.8, V8 27.1, V7 27.5
I get a normal signal out, Originally the reverb was not working with a signal out. Boss says just clean the pots and it should be good to go HA HA right.I got it working with new tubes and had to replace the Roach was bad for tremolo. Had some hum and replaced all the output tubes and all the preamp tubes.Replaced all the bias components except the balance pot and replaced the tremolo caps before the Roach. I still need to replace some of the older blue caps and cathode caps on the preamps. Thought I would try to bias and ran into this.

Title: Re: Twin reverb without bias pot measuring bias low vdrop on blue wire side
Post by: Bassden on October 23, 2023, 04:30:30 pm

Update
Was able to get the balance pot to work varying voltage now. I don't know how that started to read properly but it is now. Thanks all for reading.