Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jukelemon on March 22, 2016, 06:55:15 am

Title: Question on GA-25 RVT
Post by: jukelemon on March 22, 2016, 06:55:15 am: Hi all.

Can someone please explain the reason why C1 and C2 exist?

I see no point in C1 and figure C2 is serving as a HPF right off the bat?

Also, is R3 (the 470k right after C2) serving as a Grid load of some kind or?

Thanks all.
Title: Re: Question on GA-25 RVT
Post by: shooter on March 22, 2016, 08:22:19 am: I'll stab at it and say, you're correct, they are *not* needed, but you may want them. Suspect they are for High freq, cutoff, radios, computer gizmos, etc. R3 keep the grid referenced to ground, It could be removed, but the signal would probably be effected. Grid leak-grid stops are pretty common, so again, you might want it, even if you don't *need* it
Title: Re: Question on GA-25 RVT
Post by: jukelemon on March 22, 2016, 09:20:29 am: Thanks.

And R9/R10 and C4. What is the point of that?
Title: Re: Question on GA-25 RVT
Post by: shooter on March 22, 2016, 12:29:40 pm: Just another stab in the dark, tone control. Maybe in conjunction with R15-C7. Those should be labeled on the amp, I didn't bother looking up the amp.
Title: Re: Question on GA-25 RVT
Post by: jukelemon on March 22, 2016, 12:51:19 pm: It is off the Volume control as far as I can tell. I would not think the Bass or Treble is a 10meg pot?
Title: Re: Question on GA-25 RVT
Post by: shooter on March 22, 2016, 09:09:16 pm: Quote
It is off the Volume control
I just looked at one for sale, the normal channel only has 3 knobs and one's called "loudness", *love that nomenclature*
Title: Re: Question on GA-25 RVT
Post by: jukelemon on March 23, 2016, 04:59:08 am: Quote from: shooter on March 22, 2016, 09:09:16 pm
Quote
It is off the Volume control
I just looked at one for sale, the normal channel only has 3 knobs and one's called "loudness", *love that nomenclature*
Right. Vol/Bass/Treb on both channels. Reverb and Trem on Channel 2
Title: Re: Question on GA-25 RVT
Post by: HotBluePlates on March 23, 2016, 01:31:47 pm: Quote from: jukelemon on March 22, 2016, 09:20:29 am
... R9/R10 and C4. What is the point of that?

And the 10MΩ pot.

Input to this circuit is from after C5 (V1B's plate output), and flows back to V1B's grid through the pot. It's feedback, with a reduced feedback to highs, whose level is set by the pot. I suspect this is your Treble control.

R15 & C7 shunt highs to ground. Could be your Bass pot.

Or the names could be flip-flopped.

R18-20 and C10-12 is a Twin-T filter, probably to provide broad mid-cut. R17 limits the depth of the mid-cut (which theoretically could be infinite with a Twin-T filter).
Title: Re: Question on GA-25 RVT
Post by: jukelemon on March 23, 2016, 05:30:35 pm: Thanks HPB.

What is your thought on C1 and C2?