Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on December 14, 2011, 08:15:22 pm

Title: Ampeg B25 input question
Post by: firemedic on December 14, 2011, 08:15:22 pm

While wiring the inputs for my B25b build it occurred to me that I'm not real sure what's going on there with the grounding. The normal input is straightforward enough, 47k input w/ 5.6M to ground making a very high impedance input.

But the bright input is less obvious to me. Per the schematic, it looks like the normal channel 47k would become the grid leak when the bright jack is in use. Is this correct?
May be, since the 100k input resistor is bypassed with a 4.7nF cap.
But I just want to be sure I'm reading it right.

Title: Re: Ampeg B25 input question
Post by: PRR on December 15, 2011, 05:30:17 pm

This is what I see (dunno if that is what Ampeg did):

Title: Re: Ampeg B25 input question
Post by: firemedic on December 15, 2011, 06:06:09 pm

That's the one, except the bright jack is not shorted to ground when not in use. If it was it would short most of the "normal" signal to ground right at the input. I think.

Title: Re: Ampeg B25 input question
Post by: PRR on December 15, 2011, 06:35:36 pm

Then the Jschem file is wrong.

Title: Re: Ampeg B25 input question
Post by: sluckey on December 15, 2011, 08:08:25 pm

Quote

Per the schematic, it looks like the normal channel 47k would become the grid leak when the bright jack is in use. Is this correct?

yes

Title: Re: Ampeg B25 input question
Post by: firemedic on December 16, 2011, 06:18:39 pm

Isn't that kind of an excessive load on the bright input? This is why I wondered if I was reading it right. See what I'm saying?

Title: Re: Ampeg B25 input question
Post by: PRR on December 16, 2011, 09:35:44 pm

Heavy loading on gitar makes it dark. Treble-boost makes it bright. Put them together and what do you get? Not easy to say.

How does it SOUND? That's the only real answer.

Title: Re: Ampeg B25 input question
Post by: firemedic on December 16, 2011, 10:41:28 pm

Well, this build is a bass amp so as long as it doesn't get too badly attenuated, I'll trust the Ampeg designers on this one. Thx PRR.

Title: Re: Ampeg B25 input question
Post by: LooseChange on December 17, 2011, 07:24:19 am

Hey, I have an original one (head) on my bench right now.

Title: Re: Ampeg B25 input question
Post by: firemedic on December 17, 2011, 05:45:01 pm

Cool. I like my Reverberocket a LOT, but I don't know what to expect from this build, really.
Are the input jacks wired the same as the schematic or is there some trick I'm missing?