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Hiwatt custom 20 tube hum problem

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

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Took this amp in for repair and it has 60 hz hum.
I've been told the rectifier diodes might be the source of the problem,
but without a schematic the cost is through the roof.
Any ideas what the exact cause is?

J M Fahey

Hi, tell us some things first.
Does it practically dissapear if putting the volume or any other control (or all) in "0" or unplugging the guitar/turning its volume into "0"?
Does it change if you move the guitar or if you touch the chassis with your bare hands?
Is it constant or rises with time?
Is the amplifier grounded?
Good luck.


Had some trouble figuring out how to post!
Anyway, the problem is that turning up the master volume even with the gain low will cause a dramatic increase in noise and hum. If we attempt to put the gain high and master low, we can't go much higher with the master because again we get more noise and hum.
This is with no guitar plugged in.
The amp was restored to the original values on the board(it's pcb) and 2 traces were reconnected. Supposedly the amp is grounded. It wasn't before.
Someone tried to get around the issue by killing the gain, but the problem was there.
Is this amp fixable without a schematic?
They're saying it'll be very expensive.
So if we turn everything off there is no hum.
There is also a buzzy static as well.

J M Fahey

Just a hunch. On your 20VAC scale, check the voltage from pin 9 to 4/5 (they're usually joined) on any 12AX7 socket and then from any of them to ground.
Post what you measure.
It sounds as if the center tap of the filament winding is ungrounded.
If not, we'll go on.
Any experienced tech can service it without schematic, because most (99%) tube amps use the same basic "building blocks" and you can check one-by-one.
That's not so on SS gear.
The problem is that we need both to have access to the same schematic, to be able to ask you to check certain things on "remote control".
EMail Hiwatt, adding the model and  serial number and explaining the problem, they might send you a copy.
Take some sharp, well illuminated pictures if possible, specially around the "repaired tracks".


Hiwatt doesn't have a schematic, so that's out.
I'll ask them to do what you said but I think they're telling me it's too expensive to fix.
Not sure if they want to do any more on it.
Would it be a good idea to just take the amp back at this point on Monday and try some other tech? I do have a picture of the components on the board.
One other tech told me this was unfixable. It was badly designed.
These guys think it's the power supply.
Would the fact that the transformers are not 90 degrees to each other be a factor?
I have apicture of the components don't know how to post it.

J M Fahey

Your post amazes me, let's see it part by part:
QuoteHiwatt doesn't have a schematic
Maybe not, if it's old.
Quotethey're telling me it's too expensive to fix
That means "I have no idea, take it away"
QuoteNot sure if they want to do any more on it
Quotejust take the amp back and try some other tech?
Try to find a good one, ┬┐Where do you live?
QuoteOne other tech told me this was unfixable. It was badly designed.
That means "I have no idea, take it away"[2]
QuoteWould the fact that the transformers are not 90 degrees to each other be a factor?
At worst it would have a very slight 60Hz hum, not affected by any control.
QuoteI have apicture of the components don't know how to post it.
Post it in some images server (see what the other guys use) and post the links here. Avoid "Rapid"share and any, such as Flickr, that forces you to register.
On Monday take some other sharp pictures.
Do you have some technician/nerdy friend who can solder, measure and read a schematic properly?


I'm amazed too by my situation but not in a good way.
Hiwatt or Fernandes Guitars the manufacturer of this amp told me they have no schematic for this amp and that's for sure.This in itself I can't believe.It's from 2004 this amp,
but that's the situation.
I have sent the tech people some emails with suggestions from here,but tomorrow I thinkI'll phone them and pick it up.
The amp was restored to the stock configuration,
which has increased the gain. I was told if it's a design flaw then forget it.
I just can't understand why this amp seems unfixable!
I'm in Lasalle Canada near Montreal. As for soldering, I certainly can do that.
But the voltages in this amp I don't think I want to mess with.
The tubes don't appear to be the source of the problem although one tube was bad.


Link to photo of components before repair.
Also They seem to think the problem is in the power supply section.
Without a schematic it will take 4 or 5 hours.

J M Fahey

Now I know there are Three (maybe more) Custom Hiwatt 20's.
There never was a "real" Hiwatt 20, for the very good reason that they always made Professional, stage rated amplifiers, so they went from 50W ones to 400W beasts, the 100W, 2x4x12" being the most popular.
The first and the third were made by "something" brothers in UK, hand made, PTP wired, and *very* expensive.
The one you have is Korean made, which not only uses a PCB (nothing wrong with that, if well made) but connectors for everything, specially the tubes (what were they thinking?)
Even if you get a schematic, probably it will be from the British one.
I distinctly remember having seen a Custom 20 innards and they were absolutely different, just two long thin strips of turret connectors, chassis mounted sockets , *VERY* clean wiring and a simpler circuit overall.
Maybe it had 3 12AX7's or even 2; definitely not four, and was single-channel, like its big brothers.
What irks me is that I can repair it in an hour or two, no schematic needed, but we are half a world apart, and don't know how to explain you how to do it.
Oh well.
As a last chance, when you get it back check those filament voltages I told you to.
I've seen you asked everywhere with not much luck.


Yes, that's me everywhere trying to get help.
Maybe you should talk to my repair place, but I think I'm just going to get it back
and just get rid of it.I'm going to phone them today.

This is a one channel amp as well.
It drives me nuts that you can fix it while all I get here is:
it's a piece of crap and can't be fixed!

Would more photos help?
You can always try to explain what to do anyway.

J M Fahey

Hi Billyjoe.
Fact is, in USA
Quoteand just get rid of it
is normal, here it is usually not an option; getting schematics or factory parts neither, so we have to sharpen our skills.
As they say, "the fat cat doesn't catch the mice".
As a last chance, tell them to measure the filament voltage, from pins 4+5 to ground , from pin 9 to ground, in any 12AX7 and from pin 9 to 4+5.
They should have 6.3VAC in the third case and 3.15VAC in the first and second.
If not, your filament supply is not grounded, that causes terrible hum.
Some other guy suggested something similar to you.
Another possibility, is that the PCB grounding is missing or very poor, I'd need some extra pictures to tell you what to measure.
In the worst case, don't junk it, you can build a PTP wired "real Hiwatt" or a "Marshall 18W" or a "Tiny Terror/AC15" or an "Atomic16" clone using the original transformers and some of the filter capacitors (you'll need "real" chassis mounted sockets though) *or* if you don't want to work with high voltages you can house there some preamp plus a 15W TDA2050 amp or even an LM3886 50 W power amp.
Good luck.
Juan Manuel Fahey.


Just spoke to my guy.
He's worried that if we go ahead and repair it because there may be a design flaw and it will still hum. He can't guarantee it will work.He wants to know if it's a known issue with the amp.
He told me if there was a problem with the filament wires the tubes wouldn't light up.
If I want to check the caps and diodes possibly the filament wires they will charge another 60.00. He doesn't seem to think it's a component problem.
Could the ceriatone board fit in here? Their layout is free.
The thing is I would try to keep it at 20 watts.

J M Fahey

Hi billyjoe.
Just get your amp back.
As a side comment, an ungrounded filament winding will light them beautifully, but will hum like crazy too.
Anyway, stop wasting money.
I've found the British Hiwatt20.
Have a look:

try to get some greybeard technician.
Good luck.
Juan Manuel Fahey.


That's a beautiful amp.
Looking at it I can only I think I can wire a copy!
Famous last words.
But yes I will get the amp back,and figure out what to do.
I will post more pics.
If it's not the filaments or perhaps a dried out cap, what could it be?


Hi billyjoe,
              As *JMF* has already noted it's hard to work out as many things might be happening altogether.

My thoughts on this;
ANY Tube amp with that many preamp triodes is bound to hum, some worse than others. From the pics I can't tell if the preamp heaters are DC or AC powered?

If they are AC heaters then with so much gain you *WILL have Substantial HUMMMMMMMMMMMMM* :'(

I recently worked on a Laney 30 watt unit and it's 2nd channel Hum was so bad the owner wanted to sell it for similar reasons you mention here.

The gain on the laney was insane so I just turned it all down to an exceptable level and Now it gets played at all his gigs. 8|

So I can understand the chaps reluctance to fix it knowing that without some modification it will always hum.
Other comment is the yellow wire crossing the top of the 3rd Electro from left is Cut rather badly I'd get that fixed.

You could ask a good teck to pull one of the triodes out of circuit and rewire it to a more civilised circuit,, you will still have more gain than you will ever use,,, well if you know valve amps you know it can be done.

I get looks of confusion when I try to explain to the young players that Brian May used the *Normal* Ch on his Vox amps,,, not the *Bright Ch* as most would assume.
Now if you follow the signal in a Vox Amp the Normal Ch passes through one Triode (half an AX7) and then after passing through a Volume control goes Directly to the PI and power Amp.

With a good guitar and a simple treble booster you can have masses of distortion,,,,,,,and very little hum even though it's still using AC heaters.
Anyone who builds an all Valve Amp using more than 3 triode stages
(1 1/2 AX7's) does not understand valve teck.
So even if you get the potential ground issue fixed it may still hum quite a bit.
Sorry I can't be of more help. Phil.