Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on October 30, 2019, 08:33:27 am

Title: School me on cathode sharing in different stages?
Post by: Diverted on October 30, 2019, 08:33:27 am

Hi,

I'm trying to puzzle out a single channel AB763 build, with no tremolo just reverb. Hoffman has a great schematic and layout for that here on this site; Rob Robinette also has something similar, his "Blackvibe," though he provides just a layout and there is the second channel.

However there is one difference between the two I'm wondering about:

In Hoffman's version, V3A and V3B use separate bypass caps/resistors for each cathode. In the Robinette version, the same tube (V4 in his layout as there's an extra channel) use a shared cap/resistor on the two cathodes.

Is either acceptable? Or would I be better going with Hoffman's version?

Thank you.

Title: Re: School me on cathode sharing in different stages?
Post by: sluckey on October 30, 2019, 09:13:57 am

Rob stayed true to the Fender AB763 design. I did likewise in my TDR. Using separate cathode R/C is generally consider an improvement. Either method works just fine.

If you're short on cash use the shared R/C.    :icon_biggrin:

Title: Re: School me on cathode sharing in different stages?
Post by: Diverted on October 30, 2019, 09:24:19 am

Thanks. It's not so much the cash, it's the space. I went with the shared cap/resistor to save space. This layout is extremely tight and it allowed me to save half an inch on the board ... also pulled the load resistors off the board to save more space, ran them above the sockets etc. .... since I laid it out I started doubting whether or not I should have done it. Thanks for the information!

Title: Re: School me on cathode sharing in different stages?
Post by: shooter on October 30, 2019, 10:18:15 am

Quote

Either method works just fine.

+1

any problems tend to show when you combine the shared parts with too many gain stages fed by a common PS tap, combined they act as a gremlin help wanted sign  :icon_biggrin:

Title: Re: School me on cathode sharing in different stages?
Post by: pdf64 on October 30, 2019, 02:13:25 pm

As the bypass cap used for shared, cascaded stages ages and its esr increases, eventually higher reverb settings will result in oscillation, due to a positive feedback loop via the by now inadequately bypassed cathodes.

Title: Re: School me on cathode sharing in different stages?
Post by: Diverted on October 30, 2019, 02:35:21 pm

Decided to finally fire up the amp this afternoon for the first time with tubes in. Man, it's loud enough to kill me. Wow.

There's some background fuzz/hiss that I'm chop-sticking and trying to find, but even full out it's not that that bad. I imagine the noise floor on a big amp like this is someshat higher than the 14w amps I usually build :laugh: