Metropoulos Forum

Hi All

Building 2 JTM 45 right now, I want to ask away

I use Gold Lion Tube depot tubes. Anyway I got "38" bias point.

With a 220K drop resistor and the 68K flying resistor at idle the bias is 1.5 amps over the recommended 38 and that's with the pot fully cold. I would like to have the pot in the middle point to have more control over the BIAS range.

I tried a 150 K drop resistor, got me there (lower) but the bias was only 33 mA I ran a ****** OT and the plate reads 465 VDC

I don't have a 180K to try, I wanted to know if I increased the flying resistors value to 75K or 82K would that get me where I need to be?

I see all the original schematics and they say 150K drop and 56K flying.

The kit calls for 68k / 220k

What would be my next logical step

Thanks in advance

Dan

Try 82. Use whatever value gives you the sweep you want. Yes, change the flying resistor that is attached to the bias pot.

Ok Thank You I was thinking this after I posted however wanted to confirm

Thanks for your insights

Regards

Dan

I got it addressed. What I did, I ordered two 38 hardness sets from TD and anyway here's what happened

I put in set A and v4 was 36 ma and v5 was 39-40. What a shitty match set. So before swapping the flying resistor I took out set A and installed set B. Let the amp warm up for 30 minutes on max cold bias. Checked the plates and they were 455 VDC. I got similar numbers to set A and then split the sets. I took the pair that were 36 and put them in and now the amp is at 65-70% and the pot is at 1/3 travel.

Not too happy about the tubes. Maybe their machines aren't accurate or better yet maybe the operator made a mistake.

Anyways the "hot set" will go in the other amp I'm building "it's replacement" as I built the other one for a customer and I think I'll have to up the flying resistor to 82k to get it where it should be

Thanks for the replies

Regards

Dan