Negative bias voltage problem - 50w 1986 bass spec
Moderator: VelvetGeorge
- leonard.gulyas
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- Location: Budapest, Hungary
Negative bias voltage problem - 50w 1986 bass spec
Dear Members,
I would like to ask your help in my new build.
The specs are the following:
50W
1986 bass spec
Marstran 1202-118 PT
Marstran 784-139 OT
Marstran C1999 Choke
Larry star grounding
everything else is according to the kit instructions
The problem is I can't get the required -20 ~ -50 VDC interval for the negative bias supply. I can only get -14VDC when the bias pot is on max and -12VDC when is on minimal. The wall voltage is around 236V so the voltage selector is set to 240V.
Here are the initial voltage reading for Testing Part1: Powering up:
Here are some more pictures for troubleshooting.
Thanks in advance.
Regards
I would like to ask your help in my new build.
The specs are the following:
50W
1986 bass spec
Marstran 1202-118 PT
Marstran 784-139 OT
Marstran C1999 Choke
Larry star grounding
everything else is according to the kit instructions
The problem is I can't get the required -20 ~ -50 VDC interval for the negative bias supply. I can only get -14VDC when the bias pot is on max and -12VDC when is on minimal. The wall voltage is around 236V so the voltage selector is set to 240V.
Here are the initial voltage reading for Testing Part1: Powering up:
Here are some more pictures for troubleshooting.
Thanks in advance.
Regards
- neikeel
- Senior Member
- Posts: 7231
- Joined: Tue Dec 06, 2005 8:31 am
- Location: Suffolk, England
Re: Negative bias voltage problem - 50w 1986 bass spec
Replace the 150k feed resistor (as used in JTM45RIs) with the more correct 220k for a JMP50 as that is what Brian's PT is
Otherwise looks pretty neat, but you might find that particular bright cap a bit spiky?
Otherwise looks pretty neat, but you might find that particular bright cap a bit spiky?
Neil
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Re: Negative bias voltage problem - 50w 1986 bass spec
Nice looking amp, what is that yellow wire connected to the left bias cap/15k/47k junction ?
David
David
?
- leonard.gulyas
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- Joined: Tue May 15, 2012 4:57 pm
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Re: Negative bias voltage problem - 50w 1986 bass spec
Dear neikeel,neikeel wrote:Replace the 150k feed resistor (as used in JTM45RIs) with the more correct 220k for a JMP50 as that is what Brian's PT is
Otherwise looks pretty neat, but you might find that particular bright cap a bit spiky?
I changed the 150k (bias supply dropping resistor) with a 220k one but nothings changed. Am I missing something?
Regards
- leonard.gulyas
- Senior Member
- Posts: 52
- Joined: Tue May 15, 2012 4:57 pm
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- Location: Budapest, Hungary
Re: Negative bias voltage problem - 50w 1986 bass spec
Dear David,V8 wrote:Nice looking amp, what is that yellow wire connected to the left bias cap/15k/47k junction ?
David
You are right the yellow wire (screen grid) along with the choke wire should be connected to the 4th terminal from the right. So I corrected it and right now the bias voltage reads -29 ~ -38 VDC. I hope this is right.
Neikeel, what bright cap do you recommend instead of the 250K?
Regards
- leonard.gulyas
- Senior Member
- Posts: 52
- Joined: Tue May 15, 2012 4:57 pm
- Just the numbers in order: 7
- Location: Budapest, Hungary
- neikeel
- Senior Member
- Posts: 7231
- Joined: Tue Dec 06, 2005 8:31 am
- Location: Suffolk, England
Re: Negative bias voltage problem - 50w 1986 bass spec
Well spotted V8!
- did you relocate the thin black choke wire to the B+ line with the yellow wire as well?
- 220k is correct for that PT but
- a 100pF cap is sweetest with a 1meg volume pot, 250pF to 1000pF is spiky (to my ears) and 5000pF is just a sonic assault after the volume passes 2.5 on the dial (I prefer that in a lead amp).
- did you relocate the thin black choke wire to the B+ line with the yellow wire as well?
- 220k is correct for that PT but
- a 100pF cap is sweetest with a 1meg volume pot, 250pF to 1000pF is spiky (to my ears) and 5000pF is just a sonic assault after the volume passes 2.5 on the dial (I prefer that in a lead amp).
Neil
- leonard.gulyas
- Senior Member
- Posts: 52
- Joined: Tue May 15, 2012 4:57 pm
- Just the numbers in order: 7
- Location: Budapest, Hungary
Re: Negative bias voltage problem - 50w 1986 bass spec
Yeah I did the relocation of the cables. I will definitely try a 100pF cap and let you know of the results but right now I have to find a decent cab for my amp.neikeel wrote:Well spotted V8!
- did you relocate the thin black choke wire to the B+ line with the yellow wire as well?
- 220k is correct for that PT but
- a 100pF cap is sweetest with a 1meg volume pot, 250pF to 1000pF is spiky (to my ears) and 5000pF is just a sonic assault after the volume passes 2.5 on the dial (I prefer that in a lead amp).
Regards
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Re: Negative bias voltage problem - 50w 1986 bass spec
I also have mustards in my two builds for tonal reasons but in the cathode position you may not notice much difference between brands. I am running several different brands of caps in that position and cannot tell much difference in tone between them. Changing the value will make a noticeable change though, so you may want to only do one cathode at a time and try the amp first before changing both. Let us know how you like those values in that spot.
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