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Old 24th February 2008, 01:26 PM   #11
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Which amp is it that has the hum? How old? As I said, a lot of hum comes from bad elec caps or grounding issues.

Also is this with a guitar plugged in or not? If plugged in, what kind of guitar?
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Old 24th February 2008, 02:28 PM   #12
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It's the guitar amp that is humming. It is not that old (2 years) and always done so. Nevertheless i consider it a "normal" behaviour for a guitar amp.
The hum that bothers me is already apparent without guitar attached.
To keep the neighbours happy, i want to play at lower volumes and keep the hum at a minimum. At higher volumes the hum wouldnt't be such of an issue.
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Old 24th February 2008, 02:46 PM   #13
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Go to the forum at el34world.com and post your problem. That is where all the professional tube guitar repair techs hang out. I am sure they will be able to help you out quickly.

Make sure you tell them the correct frequency of the hum and indicate that the hum occurs with the guitar unplugged.
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Old 24th February 2008, 04:05 PM   #14
PRNDL is offline PRNDL  United States
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A quick look at the AC-30 schematic shows the culprit - minimal power supply filtering. The 1960 AC-30 has a choke and a single 8F cap.

In your amp, that cap most likely needs to be replaced.

Most guitar amps have three or more filtering caps, usually with larger values.

You could add some more caps before and after the choke to improve filtering, although the amp will stiffer and less reactive. That's the trade-off and most likely the reason for the AC-30 famous tone.
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Old 24th February 2008, 10:58 PM   #15
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It's a ac30cc2
22uF with 10nF in front and after 15mH Choke.
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Old 25th February 2008, 01:56 AM   #16
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Quote:
Originally posted by m6tt
Old guitar amps were commodity items, whereas old hifi amps were expensive. Thus, more time was spent in the old hifi amps to quiet them. Another factor is that most hifi poweramps have at most two gainstages before the power tube (excluding a phono preamp/RIAA)...many guitar amps have more gainstages, and also higher-gain gainstages. this tends to amplify a lot of hum unless careful precautions are taken.
All true, but design and implementation play a large part in it too.
Quote:
Originally posted by 1400WATTSRMS+
Well, for starters when you boost your signal to overdrive it so it can be clipped and generate distortion, you're also boosting the noise that's present in the input signal. Hi-Fi amps would in fact do the same thing only you'd never be boosting the signal into clipping like we do for instruments.

However, the more you refine your "distorting" set up, the less noise you'll have aside from the pesky grounding buzz which is really determined by the quality of your power system's ground.
All true, but almost all designs used are based upon 'classic' ccts which had little filtering and often poor design and layout. There are often 'better', 'alternate' ways of doing it, but then you lose a big part of the market; those for whom classic is god.

Simple PS, grounding, layout and other mods can clean up an old amp well, but most of my customers won't do it because it'll 'spoil the mojo'. Bollocks to that.
Quote:
Originally posted by Marcelinho
detaching the speakers is a good idea.

What about changing heater supply from ac to dc for the three 12ax7's
Have a load across the secondary at all times whilst it's on for safety's sake (OPT protection).

Hum doesn't come from speakers.

DC supply can pretty much gaurantee quiet, but it isn't neccessary.
Quote:
Originally posted by Geek
Hmmm, I've never designed a noisy guitar amp and also never needed DC filaments. All of them have headphone outputs and are quieter than any SS guitar amp I've tried.

Just decent layouts, proper tube selection (keep 12AX7's out of the V1 position) and bias the heaters +40V or so above ground and instant quiet

Cheers!
+1 Geek.
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Old 25th February 2008, 03:57 AM   #17
PRNDL is offline PRNDL  United States
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Quote:
Originally posted by Marcelinho
It's a ac30cc2
22uF with 10nF in front and after 15mH Choke.
It's important then to distinguish between power supply ripple (hum of 60/120 Hz) and hiss. The former would point to electrolytics beginning to fail, while the latter points to bad preamp tubes, although there are a lot of other causes.
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Old 25th February 2008, 09:01 AM   #18
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The his has to do with the top boost channel. I found some mods to improve that.

Have a load across the secondary at all times whilst it's on for safety's sake (OPT protection)..

For instance adding a bleeder resistor (like in borderpatrol psu)?? http://www.borderpatrol.net/psuind.htm

Wat does OPT protection stand for?
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Old 25th February 2008, 05:26 PM   #19
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High speed high voltage diodes in parallel with output tubes save output transformers.
Another option is, to use metal oxide varistors as a last resort: they may die saving your transformer and killing a plate fuse.
Speaking of hum and hiss, it was already said that guitar amps need more gain to add a character to electric guitar sound when tubes add harmonics, the more the louder you play, so hum and hiss is more amplified than in Hi-fi amps. And it is true that guitar amps had lack of B+ filtering and sometimes bad layout; for example I modified several SVT amps resoldering only few wires to silence them completely (both hum and RF oscillations).
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Old 26th February 2008, 06:38 PM   #20
m6tt is offline m6tt  United States
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Essentially, when there is no speaker attached, all the power put out by the tubes has nothing to do except break things. I have never tried using the diode/varistor method..I have an old lamp cord connected to a gloriously crappy speaker. When I'm working on an amp, I just connect the lamp cord (with a speaker plug on it when necessary) to the amp and toss the speaker into a closet or something. I've turned on amps without speakers before, and played through them, but I seem to have been able to figure that out quickly enough and yank power cords to prevent a problem.
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