Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SourCream23 on February 28, 2024, 11:29:24 pm

Title: Im confuse about 1MA dual pot Lar/Mar Master
Post by: SourCream23 on February 28, 2024, 11:29:24 pm: Hey guys, how are you doing? Im looking lar/mar2 type master volume and I saw two variants of it. One of the dual gang pot 250ka and 2.2M safety resistors (most used on ab763 amps) and another one with 1MA dual pot with 270K safety resistors. Ok.

So, It seems to me that 1MA pot is wrong because at the minimum ou half way, for example, the loads of PI changes dramatically. So, lets suppose the pots on half, the PI will see a 500k load, what is huge compared to 220K.
Im wrong to think in that way? I have attached the schematic. Thank you guys!
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: PRR on February 29, 2024, 01:22:53 am: YOU do not say what is driving this. (Draw the WHOLE circuit!)

Typical is a 60k plate and a 100k plate resistor (on each side), so 38k. 38k driving 500k or 220k, not much difference.
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: SourCream23 on February 29, 2024, 11:07:59 am: Hi! Sorry about that, its a deluxe reverb ab763. I have attached the schematic. Thank you.
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: SourCream23 on March 03, 2024, 02:13:53 pm: Hey Guys! Im still trying to understand it. I will try the both pots but anyone has an idea what is better?
Thank you!
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: sluckey on March 03, 2024, 02:32:37 pm: You're gonna replace the original grid leak resistors with a dual pot. So, let the original grid leak resistor decide which pot to use. If the grid leak is 220K use a 250K pot. If the grid leak is 470K use a 500K pot. And use a 2.2M safety resistor for either case. Don't overthink it.
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: Brownie on March 03, 2024, 04:24:34 pm: What if you wanted to add Lar-Mar to an amp with 100K resistors at the bias junction? Would you use a 100KA dual pot or just switch to 250KA?
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: sluckey on March 03, 2024, 04:27:45 pm: Quote from: Brownie on March 03, 2024, 04:24:34 pm
What if you wanted to add Lar-Mar to an amp with 100K resistors at the bias junction? Would you use a 100KA dual pot or just switch to 250KA?

Quote from: sluckey on March 03, 2024, 02:32:37 pm
You're gonna replace the original grid leak resistors with a dual pot. So, let the original grid leak resistor decide which pot to use.
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: SourCream23 on March 03, 2024, 05:29:15 pm: Quote from: sluckey on March 03, 2024, 02:32:37 pm
You're gonna replace the original grid leak resistors with a dual pot. So, let the original grid leak resistor decide which pot to use. If the grid leak is 220K use a 250K pot. If the grid leak is 470K use a 500K pot. And use a 2.2M safety resistor for either case. Don't overthink it.

Hey Sluckey, you´re right. I will use the dual 250kA. Thanks!
Title: Re: Im confuse about 1MA dual pot Lar/Mar Master
Post by: dude on March 03, 2024, 05:48:10 pm: And Bownie, what Sluckey is saying, use a dual 100k bias pot if the grid leaks are 100k, ha. Just his way of saying, “ read what l said”. :laugh: