Serious Voltage errors...

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J-Sty
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Re: Serious Voltage errors...

Post by J-Sty » Thu Jun 24, 2010 2:41 pm

Noise now gone! Should I just run it with no NFB?

Also, bias on the hottest tube is 50 ma with bias pot all the way down. Do I need to lower or raise the bias supply resistor to get it back down?
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Re: Serious Voltage errors...

Post by Roe » Thu Jun 24, 2010 4:22 pm

J-Sty wrote:Noise now gone! Should I just run it with no NFB?

Also, bias on the hottest tube is 50 ma with bias pot all the way down. Do I need to lower or raise the bias supply resistor to get it back down?
try reversing the OT primaries (pin 3s) and connect NFB again.

lower bias by increasing the bias resistor (from 47k to 68k or from 68k to 82k)
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Re: Serious Voltage errors...

Post by flemingmras » Fri Jun 25, 2010 3:26 am

J-Sty wrote:Noise now gone! Should I just run it with no NFB?

Also, bias on the hottest tube is 50 ma with bias pot all the way down. Do I need to lower or raise the bias supply resistor to get it back down?
Roe, BEFORE you go telling people what to change shit to in a bias circuit, find out what tubes they're running. Don't just assume that they're always running EL34s. Furthermore, you didn't even think to ask how and where he was measuring his bias at.

Put the shoe on the other foot. People spend WAY too much money on these kits to be subjected to "blind troubleshooting" from a hobbyist who only knows amps from the layout perspective and their own "blind troubleshooting", only to have something expensive fry in their newly built kit as a result of said "blind troubleshooting". This is EXACTLY why troubleshooting should be left to the techs who know how an amp works.

This should hopefully cover electronics lesson # 1....NEVER ASSUME SHIT!!! Lesson #2 would be that there are seemingly few absolutes and seemingly infinite exceptions.

This is why electronics theory is so important...it arms you with the knowledge required to know when and where the exceptions apply. Without that knowledge, you're going in blindfolded.

That being said...before you go changing anything ...is this an EL34 or a 6550/KT88 amp?

What do you mean by "bias pot all the way down"? Do you mean if you rotate it at all that it goes ABOVE 50mV? Or do you mean at the setting with the least negative voltage at pin 5? Also, where are you measuring your bias at?

+1 on the OT primary being out of phase...this is what is causing your howling sound when the NFB wire is hooked up. Reverse the wires on pin 3 of V5 and V6, reconnect your NFB wire and you should be good to go as far as the howling goes.
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Re: Serious Voltage errors...

Post by J-Sty » Fri Jun 25, 2010 9:32 am

By all the way down I mean that if I rotate th pot at all clockwise, the bias increases above the already hot bias of 50 ma at pins 1,8. And I am running EL34's and I did mention earlier to Roe that the amp was 100% stock except for a MV. Which I'm sure he took as still running EL34s.

That being said, should I still up the 47K bias resistor?
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Re: Serious Voltage errors...

Post by flemingmras » Fri Jun 25, 2010 11:03 am

J-Sty wrote:By all the way down I mean that if I rotate th pot at all clockwise, the bias increases above the already hot bias of 50 ma at pins 1,8. And I am running EL34's and I did mention earlier to Roe that the amp was 100% stock except for a MV. Which I'm sure he took as still running EL34s.

That being said, should I still up the 47K bias resistor?
Actually I would drop the 27K resistor just upstream of the bias rectifier. Try a 22K there and see what you end up with.
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Re: Serious Voltage errors...

Post by J-Sty » Fri Jun 25, 2010 12:12 pm

Will do.

Had to leave for work, but I'll be back Sundy night. I'll make the changes and let you know what I get.
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Re: Serious Voltage errors...

Post by J-Sty » Sun Jun 27, 2010 2:33 pm

So, magic bias number seems to be 22K right before the Diode and 56K in front of the trim pot.

Bias now perfect and I switched the OT primaries and the hum is gone as well.

Pulled amp off standby, turned up knobs and it made that really quite hiss as amps do whenever you turn them up really loud.

So, I plugged in a guitar to try it out with the Bright channel volume at about 1. (from 1-10) The moment I turn up my guitar and play a chord it makes the right sound for about a second immediately followed by a loud low buzzing noise that resembles the sound of a diesel truck. It was hard to tell, but it seems like the sound started when I hit a low note.

The low loud sound doesn't stop again until I turn the amp completely off. Then it's fine till you try to play again. In which the same thing happens again. It also happens on both the normal and bright channels.

Any suggestions at all would be awesome.
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Re: Serious Voltage errors...

Post by J-Sty » Sun Jun 27, 2010 11:40 pm

Roe, flemingmras, any ideas on what would cause this?
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Re: Serious Voltage errors...

Post by Roe » Mon Jun 28, 2010 3:54 pm

possibly oscillation. try a chopstick and post some pics
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

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