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Hiwatt custom 20 tube hum problem

Started by billyjoe24, November 07, 2009, 01:00:20 PM

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Hello billyjoe, Sigh :'(

This is the very reason you go in circles:
I quote
"What about a 470k resistor on tube 2 where the trace was cut?
I'm going to try this.The solder is there.
I see it on a lot of Hiwatt amps including the sa412.
This should cut down the gain somewhat?
I'm going to try it tomorrow just to see what happens.
Dumb idea maybe but what the hell.Why not?
What about a 68 k resistor on tube one? A grid stopper. What exactly is that?"

Everything you just mentioned will be solved via ****DELETION OF V1****
So If you don't get the JOKE now you probably never will.

Here is a way to quickly delete V1 from the signal path with minimum work.
BUT ***READ THIS***First;

I am only going on a rough drawing you posted of this PCB layout so I have no way of knowing if it is *Accurate*

If you have no idea what I'm talking about don't do it!!!
If you don't know why the WARNING is there then YOU DOn'T HAVE THE Knowledge and therefore I will *ASSUME* you are inteligent enough not to touch it, RIGHT!!
So get someone to help you with it.
If this works well and the amp has much less hum problem then you just might be
interested in persuing my first comment and suggestions.

If you persist in making it complicated then YOU  *Not I*  will have to endure the
frustration and pain of such pointless complexity.

Get V1 OUT and work from there.

Till that happens you are just shooting arrows with no target.
Have fun and don't do something you may not live to regret. Wink Phil.


The board is accurate.Don't worry about that. I should repost it with the cap numbers finalized.
I can take out the cap and resistor, but I will get it done to avoid getting fried.
And that's no laughing matter.
But I am pretty sure the hum will still be there.
Can this amp be modified to be even more like the sa412? Dumb question?

Is it a waste of time messing with the transformers then?
I think you mentioned putting a metal piece there with an alligator clip to see if it does anything.
I am having a devil of a time getting two of the bolts loose on the PT.
I can move the OT way back using two of the existing holes, but the laminate will touch the baffle. Or I can drill 4 new holes.

Can this be of any use,like you said before:
QuoteThe only other way is to lift the CT of heaters to a reference voltage but I doubt this is done on this amp.
Definitely no resistors under tube board. This is proving very difficult to unscrew though.
Of all the things they did wrong, this they had to do right!

One thing I would like to ask is , is there any info you need regarding plate voltages and so on? Let me know.
I could get it checked for a very small fee and get some written answers once and for all.
So you , and myself included,know exactly what is what with the bias and everything else.

The only odd things  that have come up was too much voltage on one of the tubes.Tube 3 I think.
I think tech  2 mentioned something about the power tubes.
One wasn't dissipating as much as the other?
Can't remember.I asked him twice the next day about it but I didn't get an an answer.
I made him do the pin to ground test on the sockets and there was something he mentioned that didn't make sense. He said the noise on the a tube was 1 point something instead of 3 point something.Can't remember.
And do caps have to be removed from the board to be checked?
Certainly, these were not.
One thing the last guy said was the tubes felt hot.
"Imagine if they were running at 220 like fender." ???


Just on a side note. I have been able to move the OT a little further sideways away
from v1.  Probably can go a little further too.If I could find a mounting bracket with longer slots I could get at least 1/2 inch of separation. Gonna have to look around.


There is a problem with the wiring on tube 2.
There is a grey wire which has a blackwire on one end and a black and white wire on the other. The black wire goes to the Ground on tube 2 and the white to 7.
The ground was on very flimsily. I managed to recrimp it, but I am not getting a
continuity beep when I check the black wires. When I check the black and white I get a beep. Is that normal?  I think someone mentioned an open electro as a possible source of hum.
Is that what this is? What kind of wire did they use here?
It looks like someone attempted to repair these by soldering but they have come loose.