Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lespaul75 on May 25, 2020, 01:56:13 am
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Good morning all, I am new here so please feel free to redirect this question...
I have a Vox AC30 TB, it was misbehaving and was diagnosed with a faulty output transformer, this was replaced and the amp serviced, it recieved some new caps resistors and tubes, the work was do e by a competent engineer but he's not a vintage amp guy.
Now the amp has a horrible buzz on the Brilliant channel, it is totally unusable with the treble control turned up but okish with it turned down.
I've swapped the Ecc83 about and no difference.
Do you have any thoughts?
Thanks in advance.
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Ciao Lespaul75
Do you have a precise reference schematic for your amp ?
TB was standard or an addon mod ? Which year is your amp ? Is it a 6 input ?
Franco
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Has any ECC83 been changed or moved to a different socket? The one used for the input stage must checked to have low noise. Any heater-cathode leakage will result in a horrible buzz being prominent on the brilliant channel, as (except for the very early 60s versions) its cathode is only partially bypassed.
Note that a tube with such h-k leakage might be fine when fully bypassed; the buzz from the leakage may only manifest when it’s operated un or partially bypassed.
If this is a 60s model, then it’s heater circuit will be single ended, so very prone to buzz. Easy to mod it to balanced operation by removing the link wire and fitting a couple of balancing resistors, eg 100 ohm.
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Lespaul75: id check the wiring. Could be a polarized electrolytic put in backwards. Id look upstream from the treble pot first but id check the whole thing. If an amp has a problem from someone elses wiring I have to check everything myself.
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Wow, you guys are on the ball.
So... I do not have any schematic of any kind so I can't show any mods.
It is a '64, 6 input model with the TB integrated on the top panel, I've had it since '78 when it was deeply uncool.
The ECC83s are all new and were in the position the tech placed them, but it has buzzed since I got it back.
PDF64 comment about the buzzing sounds spot on, could my '64 example be late enough to be prone?
There's a lot of info there for me to work on, I must stress that I am NOT an experienced amp tech, but I will have a look.
Huge thanks guys, it's much appreciated.
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..... I must stress that I am NOT an experienced amp tech, but I will have a look.
The B+ filter caps can hold their charge and you can/will get zapped if you don't drain them!!!!!
Some amps have resistors across 1 of the B+ caps that bleed off the dcv, some do not.
If you open the amp up, measure for dcv on the B+ filter cap string.
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So has it had any mods, if so what do they do?
There’s a fair chance it just needs a low noise valve swapping in for the normal / brilliant input stages.
Offhand I think it’s the 4th one along from the input jacks. If you take that one out, all the buzzing (and signal) should stop.
It must have had a suitable one in before it went to the tech, so can it be retrieved?
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pdf64 - Sadly the tech dumped all of the old original valves before I got it back, I had asked for them to be kept...
Willabe- thanks, I will try and have a look when I get home (offshore just now).
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Disappointing but no big deal, just need a decent ECC83 / 12AX7, warantied for use in V1.
eg https://www.hotroxuk.com/tung-sol-re-issue-12ax7.html
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Hi PDF64, thanks again for your input.
The buzzing is a distortion to the input signal, it's not a buzz that is present without an input (its like having a distortion pedal on)
If i turn the amp up to max and the guitar down to almost nothing I can get a nice clean sound, so like the problematic input valve.
I'm off to open her up and tinker, as much as I can, which is not much).
Cheers.
P.
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it's not a buzz that is present without an input
even when you turn up the knobs?
IF knobs don't have an effect, I'd take it to the garage with different guitar and cable and re-test
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It may be (ultrasonic) oscillation. With these amps there’s a lot of gain and poor layout. Renewing tired old valves could fit with that, ie it was fine before. Try lifting the TB common cathode gain stage’s cathode bypass cap to calm things down a little.
My experience is that helps to get rid of gremlins, and has pretty much no downside.
It’s on the tagboard, 2nd from the end, under the tone ‘cut’ pot, 25 (or 22 if replaced) uF, wired to the TB valve at the end pin 3.
Many later versions of the ac30 dispensed with that cap.
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Hi Shooter.
I obviously get the usual hum and noise from the amp when the volume is turned way up, the tone controls do affect the natural hum as expected, what my problem is is a distortion on the guitar sound, so play a string and it sounds compressed and (depending on the treble pot setting) a nasty underlying distortion.
I'm.off to investigate as per info from PDF64.
Cheers.
P.
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You should probably ask that tech for your money back (and the original tubes).
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You should probably ask that tech for your money back (and the tubes).
Yeah, but no... too late for the tubes. And he is a really good guy.