Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bionic on December 18, 2020, 12:53:16 pm

Title: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: Bionic on December 18, 2020, 12:53:16 pm: Want to order the part for my tech. The amp is a Z 28 front end with 2X5881 cathode bias power
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: SILVERGUN on December 18, 2020, 01:10:22 pm: Alpha Dual amp pots - 1ma

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Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: Latole on December 18, 2020, 01:57:20 pm: Don't you find it strange, not to say worrying, that your "tech" doesn't know which pot to order?

I am not sure that I would give him my amp to repair.

Be careful......
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: 2deaf on December 18, 2020, 02:53:34 pm: What is the value of the current grid leak resistors?

What kind of phase inverter do you have?
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: Bionic on December 18, 2020, 04:08:20 pm: Thanks everybody, I figured I would order the parts and then have him install them but maybe I should just give it to him and let him source the parts. It was just a crossing off my list type of thing. It is a based on a Dr z Z28 so I will check to see what the values of the grid leak resistors. The phase inverter is a 12ax7
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: 2deaf on December 18, 2020, 04:42:28 pm: I think it has a LTP phase inverter and 100K grid leaks. If so, a Trainwreck Type 2 with a 100K audio dual pot might be more appropriate.
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: Bionic on December 18, 2020, 07:55:16 pm: So a Lar Mar ppivmv will do the trick? I think Lar Mar is type 2 but with protective resistors in case of failure
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: SILVERGUN on December 18, 2020, 09:01:24 pm: Here's a helpful link, and the info you are looking for is about halfway down the page:
https://robrobinette.com/The_Trainwreck_Pages.htm (https://robrobinette.com/The_Trainwreck_Pages.htm)
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: 2deaf on December 19, 2020, 06:51:57 pm: Quote from: Bionic on December 18, 2020, 04:08:20 pm
It is a based on a Dr z Z28 so I will check to see what the values of the grid leak resistors.

Any luck looking at those resistors yet?
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: MFowler on December 19, 2020, 07:19:04 pm: They should be 100k with 6v6 as Doc built the circuit, with 6L6 maybe he increased to 220k.
Title: Re: Which dual ganged pot for Ken Fisher type 1 ppivmv in a 2X5881 cathode bias amp
Post by: BrianS on December 21, 2020, 01:31:14 pm: Nothing to say about the circuit, but as a full time repair tech, I suggest letting them source the parts.