Hiwatt custom 20 tube amp hum problem

The way the biassing works on your amp relies on the valves being matched equally...If cathode biassing were used whereby both cathodes had their own resistor/capacitor it wouldnt matter as the system is self regulating.....


The current you stated for the EL84s in class AB1 may be too low( borderin gon class B...)

Just dont exceed the 12watt dissipation limit on the anode...

Did this guy suck me in then?
So -24 is good or not?
Honestly I don't know.
But is the pot a good thing?
Better with it I think?

The pot is a good thing IF you have a way of measuring the current, and IF you know what you want it to be.

But it's not terribly critical on a guitar amp anyway, it's designed to sound bad, on a HiFi amp, or a PA amp, it would be more critical.

Just checked my 1958 HiFi amps book, and they all use cathode biasing - but they are looking for quality, not maximum power.

A bit of research revealed the following links...I suggest a bit of a read...

It would appear that the original biassing was set for class A...

Hiwatt Custom 20: Harmony Central User Reviews

Hiwatt Custom 20 and the Booker Valve Dynamics amps

**broken link removed**

I checked the amp again and it sounds pretty much the same as at the beginning:
Hum and buzz. Perhaps somewhat better. Tubes aren't the best.Mesa boogie sovtek I think.


Is the bias good at -24?
I'm not sure now.
The buzz goes away when I remove v2 the distortion tube.
What about metal resistors here for v2?
There is no hum or buzz or any sound at all with v3 or v4 removed .
This is with nothing connected to the amp for those tests.
When I put a guitar cable in not connected to a guitar the noise goes up somewhat.
The amp i suppose could be used at a low levels.
Any ideas?
Could the cut yellow wire or the wiring for tube 2 be part of the buzz problem?
I am going to put some heat shrink tubing and look at the wiring tomorrow.
Would a 68k resistor on tube 1 be a good idea for the hum?

With a 12ay7 in v1 hum goes down. Buzz stays the same though.
What about a lower value for the gain pot? But it looks like an engineering flaw.
The power tubes I was told might be arcing. Anyway to confirm that?If I can change the mounting brackets, I can move the transfomers further away.
Anyone know where to get them?

frank57 said:

With a 12ay7 in v1 hum goes down. Buzz stays the same though.
What about a lower value for the gain pot? But it looks like an engineering flaw.
The power tubes I was told might be arcing. Anyway to confirm that?If I can change the mounting brackets, I can move the transfomers further away.
Anyone know where to get them?

Click to expand...

Slow down (I know that's hard to do when you've waited so long to get this resolved).

I don't know how much repair/guitar amp experience you have, so I'll go through a couple simple ways to deal with this.

Nigel is right that the guitar amp circuits are often simple (very much so compared to some other electronics), but sometimes repair can be a pain. Your amp is not very old, so there shouldn't be a lot of problems happening simultaneously.

Anyhow, we want to know the frequency of that hum/buzz. It makes a difference. So do this: Play a B-flat on your low E string, 6th fret. Does the hum sound like roughly the same pitch, but out of tune? Or does it sound like it's an octave lower? Buzz in the sound may make it difficult to distinguish the two...

If it sounds like the same basic note, we're talking 120Hz hum/buzz. Probably power supply fault, but may be something else (the "something else" would be more likely if you built this thing yourself). If it's an octave lower, you're talking 60Hz hum, which has to be related to either tubes, tube filaments or wiring from the power cord to the power transformer.

The first dead-simple test, assuming no available test gear: pull tubes.

You might have already done this to an extent. Yank the phase inverter tube out (closest to the output tubes). Noise gone? If so, you replace this tube and working back towards the input, yank the next tube. Your goal is to find out which stage, when pulled, stops the noise and which stage when pulled does not stop the noise. The location of the noise *has* to be between the two.

You could probably also adjust the preamp gain knob and note whether this makes the noise louder/softer. If so, the noise is before the gain pot.

But let's also take a step back, and ask the dumb question. Does the amp make this noise when you have no guitar plugged in? What about effects? I see a loop in the schematic, does the use of effects change the noise, make it go away, etc?

I ask the dumb question because certain guitar wiring problems can result in noise, especially buzz. And most jacks used in guitar amps have a shorting contact which grounds the hot contact when nothing is plugged in. A sprung contact on the effects or input jacks could allow the jack to pick up all sorts of noise out of the air.

Anyway, we haven't done anything yet to pin down the thing to do to fix this; at this point, you're trying to localize the problem.

As an aside, for the moment, forget tubes, forget biasing, forget the output transformer, etc. These are red herrings. There is possible tube involvement in this, but it is not probable at this point. So knock out the things I mentioned and report back.

Cheers!

[By the way, there's several of guitar amp forums out there. I'm normally at the Hoffman Amps forum, but stumbled over here looking up stuff on vintage test equipment.]

Thanks for the advice, I will report back soon! I want to put a shorting jack in there to better judge.
So far, I believe that V1 stops the hum. I have to do the gain test though.
When I use the effects loop for a reverb,, if I put up the gain, I tend to get some distortion in the reverb.
The jack,and it's a stereo cliff style jack I was told is not a shorting jack for some reason. So yes the hum is there with nothing plugged in going up with the volume control as we turn up. The noise also goes up by plugging in a cord only.I'm going to replace it with a shorting jack, if I can find one.
One weird thing is the 100 k Mid pot. I get a reading of 46. Even the MV is 206 instead of 250. Sort of borderline.
I can test this on the board or does it have to be removed? I have been able to move the OT a little further away by opening up the mounting slot.
The amp has been off for a week.
The jacks are suspicious I think. They were "reconditioned" by one tech whatever that means.

Okay... Let me try to recap some of what I've read, so you don't have to do everything.


So yes the hum is there with nothing plugged in going up with the volume control as we turn up. The noise also goes up by plugging in a cord only.

Hum/noise with just a cord plugged in is normal. The cord (mainly the plug at the end) acts as an antenna, and picks up random noise. But noise with nothing plugged in is abnormal. Every guitar amp I can think of, from the late 40's on, had shorting-type jacks or the input to kill noise when nothing is plugged into the jack.

So we have one clue: noise with nothing plugged in. That eliminates the guitar and cord as a noise source.

The buzz goes away when I remove v2 the distortion tube.

V2 is the 3rd and 4th gain stages, as shown on your schematic, right? Remove V1 and see if noise goes away or if it stays.

The concept again is that if you pull a tube, you break the signal chain. Knowing that pulling V2 kills the noise is good, but knowing whether yanking V1 kills the noise or not is the next needed step.

When I use the effects loop for a reverb,, if I put up the gain, I tend to get some distortion in the reverb.
The jack,and it's a stereo cliff style jack I was told is not a shorting jack for some reason. So yes the hum is there with nothing plugged in going up with the volume control as we turn up. The noise also goes up by plugging in a cord only.I'm going to replace it with a shorting jack, if I can find one.

"Cliff jack" is just a nickname to refer to a certain style of jack, after a well-known US manufacturer. Cliff jacks, just like Switchcraft jacks, could be stereo, mono, with or without switched contacts, etc. I also have a sneaking suspicion you're talking about the effects loop jack, because a common technique is to have a send and return (both mono) by using a single stereo jack. Not a good plan, as it requires you to have a special Y-cord to use. If you have a pair of jacks for the effects loop, then you likely have no stereo jacks in your entire amp. They're not common in guitar amps.

One weird thing is the 100 k Mid pot. I get a reading of 46. Even the MV is 206 instead of 250. Sort of borderline.
I can test this on the board or does it have to be removed?

Neither of those are an issue. The mid pot is wired as a variable resistor, and if you did not have it turned full up when you measured, you unknowingly fooled yourself (there is a parallel path from 1 leg to the wiper to short-out and lower the total resistance when not turned full-up). Regardless, 10k is plenty to get a lot of midrange; 100k is used for a different effect (crank it up, and you get the impression of more gain). As for the MV, 206k, 250k... more or less the same.

Taking voltage/resistance measurements are good. Just remember that nothing is critical in this or almost any guitar amp. +/-20% on anything is "the same".

I want to put a shorting jack in there to better judge.
So far, I believe that V1 stops the hum. I have to do the gain test though.

Gain test is unnecessary. You can hear the amp work when you play through it, and can hear hum. Gain is therefore "enough". As an aside to illustrate the point, you can plug a 12AU7 or a 12AX7 in the same socket, and you amp works regardless. 12AX7 amplification factor is 100, and an actual circuit gain of 50-60 is typical. 12Au7 amplification factor is 20, and typical stage gain is 12-16. So cutting gain by 80% just means less distortion and not a failure to operate.

So let's keep our eyes on the prize. Does pulling V1 stop the noise? Does "V1" mean just the 1st triode stage in your posted schematic, or is it the first 2 stages? What is the preseumed frequency of the noise, from a double-check with the guitar's B-flat?

If you want to check the jack, don't just replace it. The jack (cliff style) has 4 solder lugs. Take a small screwdriver and touch it to a couple of contacts at a time to short them, with nothing plugged in. Does the noise stop? If so, then any shorting contact within the jack is "sprung" and not closing tightly when there is nothing plugged in.

Don't bother testing the jack unless pulling V1 stops the noise (meaning noise is at or before V1). The only sensible way to proceed is in a specific order, which rules things out and points you to the next thing to test.

I have a bit of a problem because the ground wire on one of the molex connectors has broken off,
so I have to recrimp it.
Maybe part of the problem, it was on pretty flimsily on tube 2.
When I checked the pot everything was on 10. I put the multimeter on the lugs of the pot and got a reading of 46.

These jacks are very odd. They have 3 L connectors on each side, but unlike a fender jack, the leg is on the back of the shaft. Maybe you can see it in this picture.It's a horror show of connectors.This is an old photo and the two piggyback resistors were replaced.
There are 5 similar jacks on the little pcb behind the tube pcb.
Maybe a Marshall PC mount will work as a substitute.
Also shot of back of amp. The OT is now twice the distance seen in photo.
Little more breathing room anyhow.
The last tech said the input was not a shorting jack but I wouldn't put too much into that.

So I found a diagram for Cliff jacks that might help, however it is for mono jacks with a shorting switch. You can find that here.

Anyway, the Molex connector will need to be recrimped, and yes, that could have something to do with the problem. I dislike Molex connectors for things that might see physical abuse, as they will fail. Also, what is the messed-with resistor near the gain pot? I didn't see a corresponding number on the schematic you and Nigel worked out.

Like I said, focus on the symptoms. Did you get a chance to check if the noise stayed or went away with V1 pulled? Did you get a chance to check what the frequency is?

Don't worry about that piggy back resistor it was replaced with one 1k5 resistor.
I'm going to try a marshall PC mount stereo switching jack with 6 lugs and hopefully it will fit.
The molex connector is a problem, because I'm having trouble finding a new crimp terminal like it.
I'll have to rejig the old one. I think it would be good idea to get rid of these connectors all together.
Not sure how to do it though.
As soon as I fix the connector, I'll proceed to the tests you mentioned.
Hopefully soon!

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?

It looks like someone attempted to repair these by soldering but they have come loose.
Don't know if theyy were working properly.

Does this mean there is no ground on tube 2 possible source of hum?

Those gray wires are shielded cables. You should have 1 or maybe 2 conductors that connect at both ends, and an extra conductor that is grounded at 1 end only. You won't see that wire on the other side. That wire grounded on 1 side only is the shield for that cable.

There is a black wire and a white wire coming from the grey sleeve. at the board on tube 2. The black goes to G and the white to
7.
At the tube pcb there is only the black wire.
So it's working properly then? If I touch the multimeter on both black wires I don't get a beep.
If I touch the tube pcb black wire and the white wire I get a beep.
Here's a more recent photo.

The pot I managed to take out and it's good.
This board has very thin traces though.
I'm waiting for a Marshall shorting jack to come in.

Here's a shot of the odd jack Hiwatt used.
It was not easy getting it out.
It's a reverse L-leg of some kind. Stereo Jack?
I've never seen this anywhere.
Anyone ever see this jack anywhere?

The Fender L-leg I don't believe will fit.
It ends up being too short for the screw.
Hopefully the Marshall PC Mount will fit.
As soon as I get it I will finish the tests off.

That's a standard stereo 1/4 inch jack socket pcb mount type

I haven't been able to find one exactly like it though.
The legs are a little odd with the l in the back.
I was told this isn't a shorting jack.
I have a Marshall s-h504 pcb mount jack coming in which I think will fit.
Is a cliff switching jack the same as a shorting jack?

What would be involved in eliminating the connectors?
Desoldering the molex pins and then soldering the wires to the pcb and tube pcb?

Here's an updated drawing of the board.
I added the tube pcb.
I'm still waiting for some parts.
If I wanted to try a 68k grid resistor, where would I put it?
On the tube pcb , tube 1 pin 6 and 7?

frank57 said:

If I wanted to try a 68k grid resistor, where would I put it?
On the tube pcb , tube 1 pin 6 and 7?

Click to expand...

Why do you want to add a 68k resistor?