Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on September 15, 2011, 06:29:43 pm

Title: The Twin Biasing Problem
Post by: plexi50 on September 15, 2011, 06:29:43 pm

I have a Fender (The Twin)
The schematic says to adjust the bias to 40m VDC between TP202 & TP203
I can do that. But it also says to adjust the balance for 0 VDC between TP201 & TP202

The balance trimmer will not adjust below 32m VDC. One pair of 6L6GC tubes are hot to the touch and the other pair are cold to the touch

Any toughts?

Title: Re: The Twin Biasing Problem
Post by: plexi50 on September 15, 2011, 07:05:11 pm

I just found R215 1 ohm cathode bias resistor cracked and open  :worthy1:

Are Diodes D202 & 203 cathodes even needed?

Could they have been the reason the 1 ohm resistor blew? Minus a power tubes itself

Title: Re: The Twin Biasing Problem
Post by: PRR on September 15, 2011, 09:58:55 pm

> Are Diodes D202 & 203 cathodes even needed?

No.

> Could they have been the reason the 1 ohm resistor blew?

No.

Title: Re: The Twin Biasing Problem
Post by: plexi50 on September 16, 2011, 10:40:51 am

Thanks PRR. Biased and balanced as the schematic says. Why did Fender use this type of biasing using the balance pot? I dont think it is like the DSL amps biasing. Those amps are a real mess

Title: Re: The Twin Biasing Problem
Post by: phsyconoodler on September 16, 2011, 10:46:44 am

I often wonder why some engineering decisions get made at all.Seems to me a dual bias system would be the easiest thing to implement.It's adding the extra circuitry to have LED's light up different colors that is silly to me.Good for dumb-dumbs to tell them that there is a tube problem but most people can hear that.
  Simple is always best in my book.

Title: Re: The Twin Biasing Problem
Post by: stratele52 on September 16, 2011, 02:31:39 pm

Fender use this bias circuit it is because they don't need to use matched pair tube, cost saving . Your tubes cold = doesn't working ; 1 ohms open ( ? ) or tube defect . That is why you can't have 0 volts between TP 201/202

Title: Re: The Twin Biasing Problem
Post by: sluckey on September 17, 2011, 11:04:06 am

Quote

Seems to me a dual bias system would be the easiest thing to implement.

If you look at the schematic you'll see that is exactly what Fender did in this amp. Who knows why they decided to call one pot a balance. They tell you to adjust for 40mv on one cathode then adjust for zero volts between cathodes. They could have said just adjust for 40mv on one cathode and adjust for 40mv on the other cathode. Just two ways of saying the same thing.

Title: Re: The Twin Biasing Problem
Post by: stratele52 on September 17, 2011, 03:51:36 pm

You are right phsyconoodler there is no real balance in this schematic . Only two bias pots , one for each pair of tube. This is a very good idea IMO