Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on September 25, 2010, 12:06:11 pm
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New build just started up yerst day.
Only motor boats at high volume settings but if I roll the tremble all the way up it goes away. Filtering is bridge rect. to a
47uf/choke/47uf/1k/47uf/15k/20uf
47s are F&T the 20 is an IC
BL
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Re: the verb, - is is a no-verb twin, or is the verb not working?
Re: the trem: double check the circuit component values, you may've put a wrong value resistor or cap in there (possibly?).
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Built it as a one channel, it has no trem/reverb built into it..........and it's cathode bias............
Wait that's not a Twin :laugh:
BL
Something like this........... thanks Topbrent............I'll post a pic.
http://www.el34world.com/Forum/index.php?action=dlattach;topic=9891.0;attach=17294;image
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Heres a couple,,
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From the looks of your build you may have some grounding issue's looks kind of random.
Main filter caps BR and PT center tap should be grounded together.
You can ground your heaters CT in the same spot as well. Make sure all your jacks are isolated from the chassis too.
When I build an amp I only two ground points one fore all the electronics the other is the green chassis ground for the AC.
Think of it as small streams flowing into bigger rivers then into the ocean.
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Check your v1 tube, and chopstick your v1 wiring.
I have read about rare cases of silver mica caps sometimes being faulty, unlikely, but perhaps your 250pF treble cap is.
Looks like you have a shared cathode cap/resistor arrangement for pins 3 & 8. 820/22uf (I presume)
- Have you tried splitting this and running individual R/C for V1a/V1b?
-- If you move the pin 1-100k plate resistor and connection over one eyelet, you could install the additional cathode resistor/cap in its place diagonally.
Where is your cathode resistor for the power section? Is it bolted to the back wall of the chassis?
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All good thoughts.This amp is quiet at low volumes, it's only at high volume, say 2/3rd and the bass in a normal setting, it starts put-put-put. If I turn the EQ to get rid of the lower Freq. the putt-putt goes away. It does not have the 60/120 Hz noise problems. I'll go through and chopstick it some more and move some grounds. The amp doesn't seem to be "picking up" outside noise as much as it's creating its own, like a loading and unloading.
When I did a search to read up on this, it seems as if a bad filter cap could have caused this problem so I change them and increase the gauge of the ground wire to lug.
Not sure what changing the cathode grounding scheme on VI would do for this?
Pair of cathode resisters bolted to the back wall for the power tubes.
"Main filter caps BR and PT center tap should be grounded together."
BR? The PT has no CT that's why I used a FWBR, are you considering the grnd from the bridge a PT CT?
Thanks
BL
DUH = BR = Bridge rect.
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Hi BL
In that gutshot, is that a shared Rk/Ck for V1a and b? If so, I bet that's your culprit (and I'm not normally a gambling man)
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In that gutshot, is that a shared Rk/Ck for V1a and b? If so, I bet that's your culprit
I agree. You don't usually see two consecutive gain stages sharing. Use separate 1500Ω/22µF for each stage.
You could also use separate decoupled B+ nodes for V1A and V1B, but this should not be necessary.
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I'll give it a shot.
Thanks Gents
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I just got back to this amp, Steve you hit it right on the head. I didn't do anything else other than separate the shared cathode grnd and cap on V1 and it got rid of the oscillation. Thank you Sir. Got a few more things to clean up and I'll post some more pic and impressions.
BL
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You're welcome (BTW You can call me Pete - Steve's my brother's name)