Tube Amps / Music Electronics
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|8/14/1999 4:30 PM|
|Pat F||AC-30 Ghost notes - still....|
This is an old question revisted. My AC-30 clone is still giving me ghost notes when pushed into OD. Last time I posed this problem, I got recommendations to check filter caps, ground loops, different speakers, pretty much the gamut of normal things to look for. I've got the plate supply filter up to 44uf now. The amp is completely star grounded except for the speaker output, and an adjustable line out jack that are not isolated from the chassis. I've checked all the other obvious things, the output current is dead on. I swapped out the entire set of tubes, including a full set of matched EL84s. I've tried several different speaker cabinets/types, different guitars. The only thing that moderates the ghosting a is adjusting the tone, backing off on the bass a little.
I'm getting ready to carefully ream out the speaker and line out jack holes (alot more difficult to do on a fully assembled chassis) to isolate them from the chassis as my last effort to illiminate them as the source of the problem. But before I do, I'd like to hear from anyone who might have an idea of something I've overlooked. I have a guy who is interested in buying this amp, but I don't want to sell something that I personally am less than satisfied with. Any pointers are appreciated.
|8/15/1999 5:25 AM|
Sounds like you've tried all the normal stuff.
I'm assuming you aren't hearing and exagerated version of speaker cone cry....Celstion V30s and all most all Kendrick speakers do this to some degree in many amps.
Do you have an O'scope to watch the preamp stage waveforms while trying to duplicate the ghost notes?
Can you get it to do it with a single tone generator?
Do you have a two tone generator?
Do you have the screen supply cap(s) connected directly to the star ground with their own dedicated ground wire?
Check to make sure the volume control pot(s) shells are well grounded if you are not using the backs as part of the ground system. You should not have the audio low leads connected to chassis ground at the same point if you are trying to eliminate ground loops anyhow.
You could strap all the control pot's shell backs together and run a sperate ground wire too. (old Marshall style).
The pots are usually connected right to the grid of tube and a bad or poor ground here can wreck a half-way decent shielding effect of the shell.
Are you using a negative feedback loop with this one Pat?
If not, lifting the speaker jack from the chassis might do something but I'd still run a homerun from the audio low lead to the star ground so you can check it.
|8/16/1999 2:23 AM|
Another question made me think of this. It's pretty far fetched, but you've tried a lot of simpler stuff.
Where did you get your OT?
There is a weird form of distortion in Class AB amps where there is a discontinuity when one power tube turns off because the two half primaries are not well coupled to the entire secondary and the energy in the leakage inductance is dumped into the secondary.
It's like a form of crossover, but it cannot be adjusted out, because it's not a discontinuity of bias, but the transformer itself that causes it.
To find it you'd need a scope and enough drive to see the waveform near peaks on the AC30 circuit (which does go into AB from bias shift at high drives) I would use a triangle wave to help see it.
It's far fetched, but maybe...
|8/16/1999 12:58 PM|
The OP tranny was custom made for me by Stephen Delft (Moonlight Amp). I made all the changes Bruce recommended, and I don't know if it is wishful thinking, but it seems there is less ghosting than there was before, but there is a hint of it still there. Tonight I'm going to change out the B+ filter caps. They don't show leakage when testing them, but maybe they are under load? Don't know, won't cost much to try. If all else fails, I'll put a scope on it and take a look. I may at some point reduce the gain just a little in the preamp as the ghosting is only detectable when it starts to break up. There is plenty of crunch in this amp, it starts breaking up around 11 - 12 o'clock depending on what guitar I'm using.
|8/16/1999 1:34 PM|
My thought is the speakers. Are they doped?
If not, try a set of speakers that ARE doped. I'll bet the "cry" goes away.
|8/17/1999 1:51 PM|
What are doped speakers?
|8/17/1999 2:24 PM|
All together now,
... don't bogart that woofer, mah frin, pass it over to me...
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