Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bruno on November 07, 2011, 07:03:58 am
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I've got a super reverb build here, that uses a 50k pot in the mids to get virtually lift the ground reference of the tonestack. I've got three problems that I'll have to tweak, and need some sugestions.
-First problem is related to the mids knob, when cranked all the way up (virtually lifting the ground), the amp gets a nasty, and not in a good way, buzzy distortion, but only when the mid is turned to it's fullest point, almost sounds like there is a bad solder joint making for a bad connection. Any advises regarding the best way to debug this?
-The reverb is too strong, and sometimes gets a little out of hand regarding feedback, I've tried using shielded cables for the reverb leads and even dropped a 12au7 in the reverb driver, to drop the gain, it improved but the problem is still there. Any tips? Should I mess with the plate resistor or cathode values of the reverb driver?
-Last problem is related to feedback at fuller volumes, again on full volume settings the amp seems to "runaway" at go into feedback, I know full settings are a little to extreme, but the guy is complaining and asked if I could do anything about it. Other than attenuating the signal a bit, I'm not really sure of what to do to address this.
Thanks
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I've got a super reverb build here, that uses a 50k pot in the mids to get virtually lift the ground reference of the tonestack. I've got three problems that I'll have to tweak, and need some sugestions.
-First problem is related to the mids knob, when cranked all the way up (virtually lifting the ground), the amp gets a nasty, and not in a good way, buzzy distortion, but only when the mid is turned to it's fullest point, almost sounds like there is a bad solder joint making for a bad connection. Any advises regarding the best way to debug this?
what's the circuit? a AB763 ?
fender used a 10k one channel as the mids pot. besides resolder/chopstick debugging, you could replace the pot with a 50k (47k) R to see if it behaves the same.
regarding the reverb: what reverb transformer and tank did you use?
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Putting 50K under the tone stack may just be too much gain for the rest of the circuit downstream. Personally, I like a 25K audio taper Mid pot but YMMV.
Have you tried alternate tubes downstream? Your description of the distortion might be a phase inverter being overdriven.
How hot is the power tube bias? Maybe a hotter bias would generate warmer distortion when the power tubes are overdriven (and maybe not).
Some reverb tanks are more sensitive than others. Do you have a substitute to try? I've found pretty big inconsistencies. Also, a Dwell pot or voltage divider before the reverb driver is an easy solution. If you do not have a shared cathode cap/resistor network for the reverb recovery triode and the gain stage right before the PI, you could simply cut the cathode bypass cap and see if that reduces the reverb enough.
About feedback, do you have a thick piece of cardboard taped over the open side of the reverb tank? Have you tried double bags?
Kind of random ideas - I hope something is helpful.
Chip
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I don't have the part number for the reverb tank, but I'll check it, as for the other ideas, they are great and they are good to start trying out.
thanks guys
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Hoffman:
http://www.hoffmanamps.com/MyStore/catalog/parts16.htm (http://www.hoffmanamps.com/MyStore/catalog/parts16.htm)
2-spring tank, 4AB3C1B
You can go to the Accutronics site to get the code break down.
BTW Antique Electronics is the only place I know of that carries the reverb tank liners (at bottom):
http://www.tubesandmore.com/scripts/foxweb.dll/catalog@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?PAGE=SUBCAT&SEARCH_TREE01=REVERBTANKS&SEARCH_TREE02=06_BAGS (http://www.tubesandmore.com/scripts/foxweb.dll/catalog@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?PAGE=SUBCAT&SEARCH_TREE01=REVERBTANKS&SEARCH_TREE02=06_BAGS)
Cheers,
Chip
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-First problem is related to the mids knob, when cranked all the way up (virtually lifting the ground), the amp gets a nasty, and not in a good way, buzzy distortion, but only when the mid is turned to it's fullest point, almost sounds like there is a bad solder joint making for a bad connection. Any advises regarding the best way to debug this?
That should not happen with a 50K pot. I have a 250K pot for mid/raw control on one of my AB763 amps. It could just be a naughty pot. If you wired it like the typical Fender, (ie, wire on wiper, wire on one end, other end open) then put a jumper between the wiper and the open lug. See pic below.
-The reverb is too strong, and sometimes gets a little out of hand regarding feedback, I've tried using shielded cables for the reverb leads and even dropped a 12au7 in the reverb driver, to drop the gain, it improved but the problem is still there. Any tips? Should I mess with the plate resistor or cathode values of the reverb driver?
Just remove the cap from the cathodes of the driver tube. If still too much reverb, add a dwell pot on the back of the amp. You'll love this!
-Last problem is related to feedback at fuller volumes, again on full volume settings the amp seems to "runaway" at go into feedback, I know full settings are a little to extreme, but the guy is complaining and asked if I could do anything about it. Other than attenuating the signal a bit, I'm not really sure of what to do to address this.
Well you could use lower gain tubes like 5751 or put a 100K on top of the volume control.
But, let me ask... Since all 3 problems relate to too much gain, have you ruled out the NFB loop as a potential source of all these symptoms?
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Hi Sluckey,
ruled out the NFL, how so? Decrease the resistor value to increase the signal being fed back? Or disconnect it and make sure that the primaries aren't wired wrong?
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I have one more question... would you install the dwell between 12at7 both triodes (the ones wired in parallel)? Or behind the first 12at7 triode?
thanks
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Dwell pot on left.
Chip
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Thanks chip
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Sluckey... I don't understand the use of a jumper on the mid/raw pot ? Fender did not connect those two lugs.
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I don't understand the use of a jumper on the mid/raw pot ? Fender did not connect those two lugs.
Sometimes the wiper of a pot will lose connection to the carbon track as it approaches the end of it's travel. This is the same as an open pot and is a 'possible' cause for the symptoms Bruno mentioned. ("the amp gets a nasty, and not in a good way, buzzy distortion, but only when the mid is turned to it's fullest point") Putting a jumper between the wiper and open lug will eliminate the mechanical contact of the wiper as a potential problem.
I've seen enough pots that lose wiper contact at extreme ends of travel that it just makes sense to me to take the extra 30 seconds to make the jumper connection. The jumper serves no other purpose. It's usually always there in commercial or military equipment that has to meet high reliability standards. Leo was just too cheap to do it. :icon_biggrin:
EDIT... While browsing thru some Fender schematics this morning I discovered that Fender did connect those two lugs on later amps such as the Twin II, Evil Twin, and all reissue amps.
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Bruno, any progress?
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Hi Sluckey,
I started by doing some wire re-routing, to improve lead dress, I then dropped the cathode cap from the reverb driver, this dropped the reverb gain a lot and improved most issues, including the mid pot one oddly. But I feel the first channel lost a lot of gain and the reverb also lost a lot.
Also some of the feedback noise (almost like a very subtle high pitch noise when the reverb knob is no max setting) is now present. So I'm going to put the cap back, and try to reduce the gain values some other way, and do the mid pot shut you sugested.
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Did you disconnect the NFB loop to make sure the primaries were connected properly?
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I did, everything seems ok in that department...
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Also some of the feedback noise (almost like a very subtle high pitch noise when the reverb knob is no max setting) is now present.
microphonic tube maybe?
Sounds almost like an issue I had building my TOS that had a microphonic tube.(built as a combo)
with knobs at 11 it almost sounded like a ring modulator when played, but with no signal it sounded like tinnitus.
Is my phone ringing? :l2:
any how give the tubes a tap and you'll know if its a microphonic tube real quick
Ray
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I redid the leaddress and managed to deal with the problems I was having.
I also installed a new dwell pot. Played it for 5 minutes, tried some extreme setting on the reverb, no noise or feedback, tried the dwell, it functions perfectly and adds versatility to the reverb.
Then, after 5 minutes, I get loads of noise. Turned the amp off. When I switch it off standby a get a big noise, not really a squeal but kind of. It gets worse whenever I switch the standby again to shut the amp down, it's like for the second I switch it off it momentarily gets louder. Then the amps goes into standby.
I can't even chopstick it, because the noise is too loud.