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I built a 5e3 last winter using components I had and transformers and chassis from an early 60's Filmosound amp. Only mod I had to make was using a string of zeners to drop the b+. It was all point-to-point with no board. Worked great, sounded better. Well I just got a 5e3 chassis to finally build it into amp and did that yesterday. Everything worked great and it's very quiet. I took my board with my zener string (b+ currently sits @390v with 24v/44 ma on the cathodes so I'm testing with JJ 6v6's) and put it in and no change in b+. I've rechecked the ground, center tap and all of the zener connections and even rebuilt it twice and stll nothing. I began to think that maybe it was the HV center tap so I checked the taps: 215R on the high voltage and 110R on the ct. So it doesn't appear to be the ct. Any ideas? I'm running out of zeners.
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Is what you think is a center tap actually a low voltage bias tap?
Usually the DCR from center tap to the outside legs of the high voltage leads is pretty close to the same value....
What rectifier are you using... is it for FW ?
Are you sure the zener diodes are connected correctly with their cathodes pointed to ground? -
Assuming the zeners are between the center tap and ground, did you put the board in with the correct polarity? The banded end of the diodes should go to ground. Another possibility is that the zeners have shorted out. Check the resistance with your meter. If the diodes measure below 50 ohms or so and the resistance doesn't change when you reverse the meter's leads, the diodes are blown.Originally posted by Zer09 View Post
It's not unusual for there to be a slight imbalance in the resistance on each side of the centertap.Originally posted by Zer09 View PostWARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school !Comment
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Zeners incorrectly oriented. Talk about banging your head for nothing. B+ now at 350. Should have marked the string for ground when I pulled it out of the old chassis. Just assumed they were oriented like a bias diode. Thank you Bruce. I really appreciate the help you provide on here....especially that fast.Comment
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Hey anyone here that hasn't made a simple mistake like that, well, is a liar!Comment
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The worst part is that I was so convinced that I was right that I didn't even consider that I could be wrong. As soon as I saw your response I thought "Oh sh$t, I can't can't believe I didn't try that!"Comment
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Hi,
I need to lower the voltage with ~50VDC in an amp where I have ~510VDC from the PT (at the first cap after rectification, 4 tubes idle at 35mA).
I searched the forum and read about the zener trick but my PT doesn't have a center tap and uses a bridge rectifier. I added 5x9V1/5W zeners after the choke thus lowering the screens voltage but I need to lower the anode voltage as well.
Since in my case the zener/CT trick is not applicable I was wondering if I can use some zeners at each power tube plate to lower the voltage?Comment
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You are using FW bridge rectification? No problem. Put the zeners (reverse biased - with cathode/banded end pointing to ground) on the ground side of the bridge.Originally posted by GainFreak View PostBuilding a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- EnzoComment
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Thanks for the quick reply. Did you mean the amplified zener as used in the CT power supplies because I don't think only the zeners are able to handle the whole current?You are using FW bridge rectification? No problem. Put the zeners (reverse biased - with cathode/banded end pointing to ground) on the ground side of the bridge.
Also I'm getting my bias from the AC tap as in JCM900 series of power amps. I guess this will affect the bias as well?Comment
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