JTM 45 Bias question

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dan_tramble
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JTM 45 Bias question

Post by dan_tramble » Wed Apr 27, 2016 8:37 am

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
MetroAmp addict!
JTM 50/45,1987's & 1986
1959 and 2203/2204 Amps
1974 amp

Can't stop building, very addictive!

coldengray
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Re: JTM 45 Bias question

Post by coldengray » Wed Apr 27, 2016 12:17 pm

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

dan_tramble
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Posts: 37
Joined: Fri Dec 19, 2014 11:34 am
Just the numbers in order: 13492

Re: JTM 45 Bias question

Post by dan_tramble » Wed Apr 27, 2016 2:19 pm

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

Thanks for your insights

Regards

Dan :rocker:
MetroAmp addict!
JTM 50/45,1987's & 1986
1959 and 2203/2204 Amps
1974 amp

Can't stop building, very addictive!

dan_tramble
New Member
Posts: 37
Joined: Fri Dec 19, 2014 11:34 am
Just the numbers in order: 13492

Re: JTM 45 Bias question

Post by dan_tramble » Thu Apr 28, 2016 1:41 pm

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
MetroAmp addict!
JTM 50/45,1987's & 1986
1959 and 2203/2204 Amps
1974 amp

Can't stop building, very addictive!

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