Hiwatt custom 20 tube amp hum problem

[frank57]

Don't worry about little goofs I can spot them quick
and fix them after in the computer and post it.

C2 is actually C3
and C2 is C3.
C3 has a value of 102.
R6 should be R1
R2 should be R4
R4 should be R5

I think that's right.
Fantastic work Nigel!

 

[Chippie]

here's my contribution to this thread....

ECC83 pinouts:

1 anode
2 grid
3 cathode
4 heater
5 heater
6 anode
7 grid
8 cathode
9 heater centre tap


EL84 pinouts

1 IC
2 g1
3 cathode/g3 (internally connected)
4 heater
5 heater
6 IC
7 anode
8 IC
9 g2

 

[Nigel Goodwin]

If you can finish it you'd be a real Christmas Angel!

If you have a look at:



It's a fairly similar design.

 

[frank57]

That's what I thought too!
But all I got back was "it's a Hiwatt in name only"type replies.
I think the tone stack might be based on this:
**broken link removed**

What do you think?
I just don't have the skill or knowledge to draw it out.
I hope you can complete the schematic,
you'd be giving me a great Christmas present!
It's great stuff!

 

[Nigel Goodwin]

That's what I thought too!
But all I got back was "it's a Hiwatt in name only"type replies.

Perhaps so, but it's a Hiwatt type circuit, and uses almost exactly the same phase-splitter and bias circuit as the one you posted.

There's more than enough there for any engineer to sort it out from.

I think the tone stack might be based on this:
**broken link removed**

No, the tone stack is the standard Fender type.

 

[frank57]

I think they were talking about the amp I posted, the Hiwatt custom 20,
when they said Hiwatt in name only.
But am I right to say this amp is based on Hiwatt?
It certainly seems that whoever designed it took a look at Hiwatt schematics
or has access to info we don't have.

 

[Nigel Goodwin]

I think they were talking about the amp I posted, the Hiwatt custom 20,
when they said Hiwatt in name only.
But am I right to say this amp is based on Hiwatt?
It certainly seems that whoever designed it took a look at Hiwatt schematics
or has access to info we don't have.

Like I've said all along, valve amps are crude and simple, and easy to repair - they all look somewhat alike. In this case the specific phase splitter design, with a presence control in the feedback, makes them obviously the same make.

 

[frank57]

So there is some Hiwatt magic in this amp?

It certainly sounds like something is going on sound-wise.
It does not sound like a typical low-priced tube amp.
They did cheap skate the speaker, and the pcb is cheap too.
I just don't get why these guys over here are having problems solving this amp.

I replaced the speaker with an Eminence ramrod, has a British made cone,
and nice Union Jack on it too!

 

[Nigel Goodwin]

So there is some Hiwatt magic in this amp?

It certainly sounds like something is going on sound-wise.
It does not sound like a typical low-priced tube amp.

I would have thought it sounds pretty similar to any other little valve amp, it's just a bog standard crude circuit (as they all are).

They did cheap skate the speaker, and the pcb is cheap too.

The pictures you posted on the other forum look absolutely horrible, probably the poorest looking construction of a valve amp I've ever seen?.

I just don't get why these guys over here are having problems solving this amp.

Likewise, I can only imagaine (as it's VERY common) that they don't have a clue about valve amps.

I replaced the speaker with an Eminence ramrod, has a British made cone,
and nice Union Jack on it too!

The speaker is the part liable to make the biggest difference.

 

[Nigel Goodwin]

OK, have a try with this, it's the full circuit now.

 

[Chippie]

I'd change the following caps, C32, C19 and C23..... for starters


and maybe C21 for good measure

 

[frank57]

Nigel, you're my hero!
A magnificent job and a wonderful christmas present!
I appreciate your work tremendously!
I wish I had the knowledge.
Sounds like some of the guys I showed it to don't have the knowledge either

If they can't figure it out now, with the great work you did,well I'm thinking maybe I can send you the chassis. I'll see what happens this week.

Yes, the construction of the PCB really is horrible!
The connectors, oh boy
Maybe I can get rid of them.

But have a Merry Christmas, you deserve it!
Thanks so much!

 

[gary350]

If the hum is something you never had before and now you have it and it gets louder and louder as you turn up the volume the problem is a bad electrolitic capacitor in the power supply.

If the hum is something you have always had and something you would like to get rid of then the filter capacitors in the power supply may not be large enough to filter out all the AC pulses in the DC. You want flat line DC.

Check C32 and C19

 

[frank57]

Thanks for the help!
I will tell my tech to do just as you suggest on Monday.

 

[Chippie]

Check C32 and C19

See my post further back.......

 

[frank57]

Don't worry, I saw it.
I'm going to talk to my tech on Monday.
If all goes well I'll let everybody know.

 

[mel8030]

See my post further back.......

in the power supply may not be large enough to filter out all the AC pulses in the DC. You want flat line DC.

Check C32 and C19

I thought the problem hum was mostly 60 hz? Full wave rectifiers have 120 hertz hum as their main harmonic. Half wave produce the 60 hertz hum. Am I thinking right on this?

 

[Chippie]

I thought the problem hum was mostly 60 hz? Full wave rectifiers have 120 hertz hum as their main harmonic. Half wave produce the 60 hertz hum. Am I thinking right on this?

Yes, trouble is the supply rail is so high that scoping isnt practical due to the input voltage limitations of the Y amp, although using an attenuator may help...

If it were me, I'd just change all the components in the power supply section as there arent that many...2 diodes and a couple of caps?

If the reservoir caps have dried out or are under capacity then this would have an adverse effect on the performance of the amp...easily remedied..

 

[Nigel Goodwin]

Yes, trouble is the supply rail is so high that scoping isnt practical due to the input voltage limitations of the Y amp, although using an attenuator may help...

Presumably you don't have a scope?, and aren't used to using one? - there are no problems whatsoever in scoping the supply in that valve amp, and presumably that's already long since been done?.

 

[Chippie]

Presumably you don't have a scope?, and aren't used to using one? - there are no problems whatsoever in scoping the supply in that valve amp, and presumably that's already long since been done?.

Yup, I have a scope, in fact three....What threw me was the comment made earlier...

Okay, the tubes and caps and resistors are all good.
Tube 4 has a bit too high voltage 350 instead of 285.

Although I'm not too sure what the max input voltage to the Y amp is on any of them...

That being the case, scoping the HT rail should have been one of the first checks, I would have thought...?