Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on October 11, 2012, 02:13:17 pm

Title: two different PPIMV
Post by: Leevi on October 11, 2012, 02:13:17 pm

Which one of the following PPIMV do you recommend for a 50W Plexi?

1. Improved PPIMV

2. LAR-MAR

/Leevi

Title: Re: two different PPIMV
Post by: Ed_Chambley on October 11, 2012, 04:12:46 pm

I have tried multiple and ended up with the attached version for a plexi.  I did have to use shielded wire grounded at the pot as I could not get rid of a hum.  After I could not notice the mod.

Title: Re: two different PPIMV
Post by: basschops1528 on July 31, 2015, 12:38:15 am

Ed,

How could I incorporate your PPIMV Mod to a Hoffman 100W plexi, rather than the 50W? I attached the layout if that helps.

Title: Re: two different PPIMV
Post by: jojokeo on July 31, 2015, 01:30:07 am

Maybe this helps?

Title: Re: two different PPIMV
Post by: basschops1528 on July 31, 2015, 01:33:17 am

I believe it may! I just need to make sure I use shielded cable right, preferably the one Doug sells? AND I noticed that your schematic (fixed bias) has the grid resistors lower, and the 220k bias resistors were replaced with 2.2M. Does that I mean I wouldn't be able to use the ones already installed?

Title: Re: two different PPIMV
Post by: sluckey on July 31, 2015, 06:59:34 am

Those lower value grid stoppers are typical values for 6L6 tubes. It's more common to use 5.6K for EL34s, so keep your values.

Technically, the 250K pots replace the 220K bias resistors (AKA grid leak resistors). The 2.2M is a 'failsafe' resistor that will still supply bias voltage to the tube in the event that the pot wiper fails open.

Title: Re: two different PPIMV
Post by: basschops1528 on July 31, 2015, 01:15:29 pm

I see!/so in other words, I would need to replace the 220ks already on the board with the 2.2 Ms?

Title: Re: two different PPIMV
Post by: Willabe on July 31, 2015, 01:27:59 pm

Quote from: basschops1528 on July 31, 2015, 01:15:29 pm

I see!/so in other words, I would need to replace the 220ks already on the board with the 2.2 Ms?

No, look at the drawing Ed posted, reply #1. 220K's get taken off the board and the 2M2 R's are mounted on the MV pots.

Title: Re: two different PPIMV
Post by: sluckey on July 31, 2015, 01:28:33 pm

Quote from: basschops1528 on July 31, 2015, 01:15:29 pm

I see!/so in other words, I would need to replace the 220ks already on the board with the 2.2 Ms?

No. Just remove the 220Ks from the board. The 2.2Ms will mount on the pot between the wiper and end lugs. Putting the 2.2Ms on the board would be useless because they will no longer act as failsafe resistors in the event of pot wiper failure.

Title: Re: two different PPIMV
Post by: basschops1528 on July 31, 2015, 04:02:18 pm

OK great! I figured that drawing was different from my schematic and a different approach would be needed. Thanks guys