Hiwatt Custom 20?

CJ · Nov 28, 2023

I had a feedback switch on a diy twin reverb. Only problem was I had the output xfmr wired wrong so that the switched actually connected positive feedback. So if we were doing a freakout in the middle of a song I could hit that switch and solo over the squealing which did not really go over that well, jus saying...

The amp did have a pot to change the bias as needed for the room.

absrec · Jul 11, 2024

Hey everybody. It’s been a while. Since my last post, I ended up building this circuit into that same chassis.

Triwatt

Although not all that (apparently) different from what I had going, it’s certainly a lot cleaner and more controlled gain wise. I'm learning that there's a lot of subtlety in tube amp design. I didn’t use their bias switch. I just used 5881s as I felt they sounded the best to my ears.

Anyway, after looking at the schematic below some more, I think it might be fun to revisit for a separate project. It was posted in one of the first couple replies.

SA210 / Custom20

Does anyone here have a clue what they were trying to do with the 4th triode stage? There is a resistor going from grid to cathode with no value marking. At first I thought it was some sort of local negative feedback but that isn't how it's usually done. Then, I thought perhaps they started drawing the phase inverter but got distracted and ended up drawing the first part of it twice. This is just going by the fact that the cathode resistor is 27K (which is pretty atypical) and the tail resistor of the phase inverter is also 27K. What I'm thinking is, this is just supposed to be a buffer stage between the master volume and PI without that extra resistor and probably a 1K or 1K5 on the cathode. I've been wrong so many times before that I'm not going to assume I'm correct about this. Hopefully somebody here might have some insight.

Thanks!
-Aaron

Tubetec · Jul 11, 2024

Forth stage acts like a recovery from the loss in tone stage and adds master volume ,

The only thing is the gain is total overkill for clean sounds , Id probably leave out the first stage entirely ,
or arrange a way to bypass it .

absrec · Jul 12, 2024

Doesn't it look like a mistake? 27K cathode resistor, no value on the other one? That was my question.

baldrich · Aug 23, 2024

Hi, I came across this thread because I am also trying to build a similar project to the custom 20 and I am also running into a few different schematics with things that don’t necessarily make sense. One big thing I’d like to add to the conversation is this analysis from AmpBooks regarding the PI biasing. Very interand informative read.
https://www.ampbooks.com/mobile/classic-circuits/hiwatt-phase-inverter/

Also, I have a schematic and I am not sure where I originally downloaded it but it claims that Hiwatt replaced the second and third stage cathode resistors from 1K5 to 15K despite what is printed on the PCB. I can’t find anywhere to validate that change and, as I have not yet constructed the amp, I haven’t tried it.