Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cboysen on May 01, 2018, 03:48:42 pm
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Hi!
I finally got around to building my very first amplifier! It worked fine, dead quiet, but the sound was a little "off". I changed a few components and removed the Type 2 PPIMV I had installed, and everything came to life, for a few minutes.. .. then it started being very noisy and now this; when I turn the volume all the way down, I get sound anyway. It now has a pre-phase master, as per sluckeys design, and when the master is down, and the volume is up, I also get sound.. Although it's very noisy either way.
If I turn up the master, the noise gets loud enough to make it ring/oscillate, also the volume knob scratches in this very typical 'oscillative' fashion.
I've tried grid-stopping every stage with 220kOhm resistors, without any luck
I've tried changing the volume pot, in case it was that simple, no luck
I've now installed shielded cable between V2 and V3, no change
I've checked and tried grounding/shorting (with 25uF cap) every preamp ground point, without any change.
Shorting (with cap) output of V1A and B to ground nearly stops the sound, but doens't change the hiss
I've swapped tubes, no luck
I've rearranged wires around V1 and V2, no change.
The Tone controls don't alter the sound, when the volume is down and master is up.
If I hover my hand around the resistors connecting to the reverb driver tube (v2), the noise increases tremendously (see picture)
No tubes are microphonic, no cables or components "scratches" when prodding them.
Changes:
V3A plate resistor (R22) is changed to 220kOhm for more non-linear clipping (no change when swapping back)
Reverb Mixing 10pF mixing cap (C8) removed
Reverb Driver V2 cathode bypass cap (C10) removed, no change when installed, when using 1uF, a loud low-mid squeal/oscillation appears when turning the reverb above 7
Master (R?) Changed to 50k log pot. Using a simple 47k resistor changes nothing.
150pF capacitor across phase inverter output, after coupling caps C16 and C17
100pF capacitor across NFB resistor, NFB resistor (R35) changed to 1.5k
Bias Capacitors are 2x22uF
Filter Capacitors are 5x 15uF
Increasing either changes nothing.
I'm lost for ideas of what to try.. So I seek out your help..
Kind regards, Christian
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Picture of amp just finished (forgot to wire the center tap, i did before turning it on)
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picture of amp after mods (with noise/volume problem)
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I'm a newbie I've only built like 5 small simple circuits. When I've had those type problems it was either a grounding problem like a broken solder joint or if I had it breadboarded and alligator clip wires jumping everywhere. I have also had that when the bias or balance of the power tube was off as in too high screen voltage or too many volts on the cathode so my dissipation was over spec. Recently I had an amp that I was tweaking the tone but it was still kinda 10% clipped together and I had only when I turn the master volume up to past 2/3 it squealed and sounded like some squelch sirenesque stuff as the knob rotated.it went away after I dropped the screen voltage below 250vdc . Gone now.
The pros will be along to probably tell you the right answer I just thpugh5 I would share my minimal experience as I have only built one off amps which were only loosely based on a mixture of schematics and such so I have had to do a lot of getting rid of mystery squeals
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squeal/oscillation
have you swapped the plate wires, (input side of the OT)?
Is your build now a clone of Sluckey's?
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I'm a newbie I've only built like 5 small simple circuits. When I've had those type problems it was either a grounding problem like a broken solder joint or if I had it breadboarded and alligator clip wires jumping everywhere. I have also had that when the bias or balance of the power tube was off as in too high screen voltage or too many volts on the cathode so my dissipation was over spec. Recently I had an amp that I was tweaking the tone but it was still kinda 10% clipped together and I had only when I turn the master volume up to past 2/3 it squealed and sounded like some squelch sirenesque stuff as the knob rotated.it went away after I dropped the screen voltage below 250vdc . Gone now.
The pros will be along to probably tell you the right answer I just thpugh5 I would share my minimal experience as I have only built one off amps which were only loosely based on a mixture of schematics and such so I have had to do a lot of getting rid of mystery squeals
Thanks for the answer, I appreciate it, and will definitely look into problems with regards to the power section. When I ground the signal right after the master volume, all problems disappear, so it seems like some kind of coupling that bypasses the tonestack/volume pot, maybe due to a grounding differential, where the signal is being fed through the ground or the B+ line.
I just measured my screen resistors, and they are dead on 470R, my bias resistors on the other hand is pretty off at 250k (instead of 220k) for both, but I increased the negative bias voltage to compensate. I have a few Carbon Films rating 215 that I'll try changing at some point though. I'll measure the voltages and report back.
Pulling V2 changes nothing btw.
kind regards
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squeal/oscillation
have you swapped the plate wires, (input side of the OT)?
Is your build now a clone of Sluckey's?
First time I fired the amp up, after testing, I had squealing when turning the master up, and swapping the plate wires took care of that problem. So I'm quite sure the OT wires are fine. The problems persist with the NFB lifted.
edit: And yes, besides the minor changes, it's an exact clone of sluckeys. Although I had to rearrange the layout slightly to fit my board, but the wiring is the same throughout. Only exception would be, that I've grounded the reverb return 220k ground resistor to the chassis, like the hoffman builds, instead of using isolated jacks and running the resistor to buss bar, as sluckey suggests.
kind regards