Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: johnheath on May 31, 2011, 12:42:51 am
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Hi to you all...
I am a new guy here but I am a long time guitar player that have turned some interest towards building my own amps... due to the fact that I couldn't find the right amp for a reasonable price...
Anyhow...
I have built a couple of amps using parts mainly from Tube Amp Doctor from Germany. Recently one of my "AB763 Fender Super" clones went crazy... actually it was after my bass player used it as a bass-amp. He uses active pickups in his bass-guitar and I wonder what kind of problems this might cause... if any? First I must say that the following described problem occured a few days after a "successful" rehearsal at which time the amp worked just fine and had no troubles. A few days after that we came back and the amp first started to "wobble" very loud when turned on and after a short time it entered the current "state of mind".
The amp cannot be driven hard. Somewhere around "five" on the volume control after that it starts to squel like a very high-pitched tone...at "seven to eight" it stop and turns into some hizzing hum. The "guitar sound" also alters from clean fine sound to a"unnatrual" distorted to pure distorted, no tone just buzz, when i in a normal way is hitting a string. The overall power is also way less than expected in a "Super".
I have tried to find the problem ... both with "your" stetoscope and by totally rebuilding it... I can't find the problem
Now I need an advice... I have chanched all parts (except the main transformer)... checked and resoldered all joints... checked with other tubes..working tubes and new tubes... checked all, and redone some, groundings...and the problem remains.
As I said the main transformer is the only thing not being changed for a new one. Now I just wonder if this could be the problem... the main transformer? Or What? I am clueless here...please help me :)
All of my other amps builds...six of them in total and one standalone reverb works just perfect and I often use the typical AB763 circuit on most occasions so I "know" how that one works...
Regards
/John
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Pull out the tubes one at a time and see if the noise stops for any particular one. If it does, then try swapping that tube first-off. Then if that doesn't fix it, my guess would be bad lead dress (causing unwanted coupling at a certain signal strength) or something like an intermittent socket pin or dodgy input jack tip switch etc.
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Well... the tubes has been checked many times... actually I just took the tubes from a perfectly working amp...same model and same circuit...still the noises and problems.
I must specify that the build is "just" the normal channel i.e no reverb or tremolo... one channel only.
Every singel part have been replaced with a new one... tubes, capacitors and resistors...even the tube sockets themselves.
The only thing not replaced is the standby switch and the PT...
I have built three of these amps before and they work perfectly when rehersing, playing live and in at livingroom level.
So please... Is it even possible that it can be the PT?
/John
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Also...
the noises are actually detectable even when the amp is running without speakers... plugged guitar... just the amp itself
The sound or noises, when turning the volume knob, sounds and changes almost like an old radio when trying to dial in the right frequency ... meaning some odd squeals, hizzes and sparkles... sort of. It is hard to describe in words (for me being swedish anyway :))
Thanks
/John
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I just had that issue on an amp I changed some coupling caps on.I reversed my steps and changed the 'last' cap I installed and that was the issue.You need to check voltages and start moving some wires around,usually near the phase inverter tube.An oscillation can spoil your whole day.
a one channel super is about as simple as they get.
A picture is worth a thousand words.
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Yes, pictures and voltage chart would be helpful:
http://www.el34world.com/charts/valve/ValveData.php (http://www.el34world.com/charts/valve/ValveData.php)
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Yes... trying to add a pic.
Maybe the bias pot is too close to the phase inverter???
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You have some long(ish) runs of unshielded signal wires there (e.g.: from input jack to V1 and from the PI to the output tube grids). Things like that can cause oscillation. Also its hard to tell what the rear-inside of the amp looks like from that one gut shot, but it does appear like you have a lot of wires running next to each other there (- could be a similar problem).
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Well what you see is basically what's there but I hear you...
I tend to have the same length on the mentioned wires in other amps and there is realy no problemos there... I'll check it up tomorrow and moving those cables around a bit to see if that is the problem... no one would be happier than if it is hahaha :laugh:
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Some signal could be feeding back from those OT secondary wires that go to the impedance switch. Try if moving them away from the first preamp tube helps with the noise.
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The OT wires ara rising straight up from the hole but it is not visible on the picture.
Anyhow I must ask... the amp worked just fine for about six months and suddenly stopped working. It is after that that I have changed components and OT and so on... My question is: If it is the wires conducting and oscillicating why was this not an issue in the first place?
Regards
/John
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So you have re-tensioned the tube socket pins?
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"re-tensioned"? You mean if they little or no contact with the pins on the tubes? No I haven't... I replaced them sockets...
It is beginning to be a very costly project :dontknow:
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A simple question , have you turned this amp on without the speaker connected between the time your bass player , played thru it and the time it started malfunctioning??
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No... the amp was just left in the rehearsal studio for two weeks and when we came back it just was crazy... unplayable.
And there are no other people who have access to the studio but us.
/J
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Do we understand that you passed signal through the amp without a speaker plugged in?
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No... but when you turn on the amp ...no speaker and NO guitar ... just the amp you can still hear the noises. BUT if you plug a guitar and a speaker these noises are just louder... sort of
/John
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No... but when you turn on the amp ...no speaker and NO guitar ... just the amp you can still hear the noises.
But with a resistive dummy load in place of the speaker?
Brad :icon_biggrin:
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Problem solved!!!! I am now a happy man hahaha
I turned out that I had to get sheilded cables from input to pin 2 and from volume control wiper to pin 7 ... both on the first preamp tube.
I had to do this on a tweed super that I made two years ago but then removed without any changes back then... in this case it was the solution though.
Thanks for the support
/John