Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Prento on September 19, 2021, 10:30:26 pm

Title: Ab763 lite c6, r13, r14 switch out
Post by: Prento on September 19, 2021, 10:30:26 pm

Hi, my first post here though I’ve been reading and learning for quite a while. I have used Sluckey’s ab763 lite circuit for the basis of a rebuild of an old Australian pa amp. I like the sound of that normal channel with the voltage divider before the PI bit I also like it without the divider. I know I could put a master volume in that spot but I want to keep the aesthetic of the old amp as much as possible and another pot is not an option. I also just want to be able to switch it in and out like a gain/cut switch. I plan to use a dpdt switch, using one pole to bridge the c6 r13 and the other pole to lift the ground on r14. I’ve been told that a large, as in 10meg, resistor across the terminals should stop the popping noise. Does this need to be connected directly to the terminals of the switch or can it be connected to the wires that I take from those terminals where they connect to the divider?

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: sluckey on September 19, 2021, 10:44:09 pm

I would just use a SPST to lift the ground to R14. Give it a try. You could even use a 1M-A pot with push/pull switch if you don't want to drill another hole. Put the 10M resistor directly across the switch terminals.

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: Prento on September 19, 2021, 11:10:20 pm

Thanks Steve. I didn’t expect a reply so soon as I think it’s still night time over your way.

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: Prento on September 20, 2021, 05:42:42 am

Just as an update, I took your advice and just lifted the ground on r14. There is very little difference that I could hear between cutting it out of the circuit all together and just lifting the ground. Thanks for your help.

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: sluckey on September 20, 2021, 07:36:27 am

Should make the switching simpler.

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: Prento on September 20, 2021, 04:24:17 pm

It definitely did. I thought that leaving r13 and c6 in the circuit would still attenuate it but it made little to no difference that I could hear. Thanks.

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: Prento on September 30, 2021, 08:32:14 pm

Am I right in thinking that even though c6 is 47nf, the amount of low frequency entering the phase inverter is governed by c8 at .001. So, if I wanted to allow more bass into the pi for whatever reason (this is purely academic) I would have to open the gates, for want of a better expression, and increase the value of c8?

I have implemented a similar type of voltage divider into an amp with a cathodyne pi. The coupling cap coming from the last triode plate before the pi is 22nf (let’s call it c6)  then the divider, just like r13 and r14 then another 22nf cap. (Let’s call it c8) into the grid of the pi. Is c6 actually necessary in this case as long as I have c8 entering the grid? In other words, can I connect r13 and r14 straight off the previous triode’s plate then into c8? Something tells me no as I am sending dc into the voltage divider.

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: sluckey on September 30, 2021, 09:05:06 pm

Quote

In other words, can I connect r13 and r14 straight off the previous triode’s plate then into c8?

no

Title: Re: Ab763 lite c6, r13, r14 switch out
Post by: Prento on September 30, 2021, 10:07:02 pm

Thanks Steve, I didn’t think so just needed the confirmation