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Hiwatt custom 20 tube amp hum problem

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frank57

New Member
Took this amp in for repair.
I've been told it has 60 hz hum and static issues.
The rectifier diodes are good but they seem to think it might have to do with the power supply.
I can't find a schematic anywhere.
Any ideas?
 

frank57

New Member
More info:
The noise increases with the master volume being turned up.
The tubes are good.
The gain low and master up: increase in noise and some buzz.
Gain high and master low: putting master up increases hum and noise.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
This amp is a pcb board valve amp.
I was told without the schematic the cost is very high.

Many modern repair agents don't understand valve amps, but they are dead easy as they are so simple.

A friend of mine bought a fairly new (only a couple of years old) Fender 60W combo off a guy he knew for £5. The original owner had taken it to three different repair shops, and been told it wasn't worth repairing.

I told my mate it couldn't be much, and even told him EXACTLY what was wrong with it before he brought it me. I was dead right, it wanted anode load resistors, as valve amps always have.
 

frank57

New Member
So what do you think is causing the problem?
They seem to think it has something to do with the power supply.
The diodes check good.
 

mneary

New Member
The electrolytic cap that filters the B+ for the preamp stage is probably dried out. The preamp stage probably has a B+ power that's behind a second RC filter.
 

bountyhunter

Well-Known Member
+1 Anytime you have an old piece of equipment, suspect the electrolytic caps. They have very short life spans compared to most electronics. If you want to keep and use the piece, replace all the power supply filter caps.
 

frank57

New Member
Here's a board drawing.
How do I make a schematic from this?
Anybody notice any problems?

[MOD-EDIT: IMAGE REMOVED DUE TO ADULT CONTENT.]
 
Last edited by a moderator:

tcmtech

Banned
Most Helpful Member
Well I see that when ever I click on the picture I am routed to a 'dirty girls' site! :eek:

Thats a problem sort of. :D

Yep, I keep trying and they keep coming back! :D:D:D
 

frank57

New Member
Thanks Nigel!
Here it is. I drew it as best I could.
 

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mel8030

Member
is the hum 60 or 120 hertz? Is the power supply a full wave rectifier type? If so, it seems the hum should have a 120 hertz fundamental if the filter caps are bad.
Might have some heater to cathode leakage that many basic tube tester don't catch. If the preamp tubes have the same number as later tubes, you can try interchanging them to see if the hum level changes.
 
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frank57

New Member
I was told it's 60 hz hum with some high end static.
The hum increases as the master volume is turned up.
Static shows up more with distortion. Not there at low volume but turn the gain up more static.
I'm waiting for it to be looked at but it's a month already.
The tubes were replaced and the hum I think is coming from the first 2 tubes.
The wires on the outside also were not tied down to the chassis.
Not sure about the power supply. Can you tell anything from the Layout?
One suggestion was to ground the input jack straight to the ground bolt.
But I think the jack might be defective. There still appears to be signal with nothing plugged in. The hum is still there.
 

frank57

New Member
Okay, the tubes and caps and resistors are all good.
Tube 4 has a bit too high voltage 350 instead of 285.
The hum seems to be coming from the power supply.
Anybody got any ideas?
My tech was looking at the output transformer,it's kind of small,
but it looks like it's the right part.
 

HiTech

Well-Known Member
Well I see that when ever I click on the picture I am routed to a 'dirty girls' site! :eek:

Thats a problem sort of. :D

Yep, I keep trying and they keep coming back! :D:D:D
They want you .... go to them!
 
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