Hughes And Kettner Statesman Dual EL34 channel switching noise.

[Shane]

Hi Folks

I have one of the above in that makes a loud thump when channel switching. Couldn't get a schematic from them but managed to get one for the EL84 version which is very similar.
From that I see it has a jfet mute transistor on the input to the Tube section that should mute it while the relays change so I think its not doing what it says on the tin!
Holding that Jfet on does infact stop the pop, but it doesnt work in normal operation.

Looking at the schematic the FET is turned on by a pnp transistor and is turned off by a RC pair charging to -33Volts (theres a 4M7 between this and the gate).
Looking back further I cant actually work out how the pnp is turned on! Its controlled by the outputs of a sort of flip-flop cct I think but I dont see how it does it.

I've gone throught all the components but can't find anything duff. I have no way to monitor the pulse since I dont have a storage scope.

Anyone familiar with these amps? If so I'd appreciate some thoughts!


The guys also asked me if I can reduce the amount of gain increase when the boost is engaged! Now theres another issue I gotta get my head round, another circuit I've not seen before!

Cheers
Shane

[g1]

Please post the schematic you have.

[Shane]

Ahh problem there, I only have a paper copy I'll see if I can draw up a bit of the relevent bits!

[Shane]

OK I drew this up quickly.
Some bits are omitted but they switch fx loop and boost and I dont think they will affect the bits in question.
The switches are momentary.H&K Statesman switching.pdf

Cheers
Shane

[nickb]

OK I drew this up quickly.
Some bits are omitted but they switch fx loop and boost and I dont think they will affect the bits in question.
The switches are momentary.[ATTACH]28143[/ATTACH]

Cheers
Shane

Nice drawing!

I suspect what I'm about to say isn't anything new to you, but just in case it's of any help:

If it thumps on both channels then if has to be something in common: R29, R30, Q4, R27, C15 ( you already checked the FET).
If an individual channel either C18/R31/D4 or C17/R33/D5

D4 and D5 form an OR gate. They OR the spike from the differentiators C18/R31 for lead and C17/R33 for clean. When an FF output goes low it pulls D4/D5 low via the caps C18/C7 which then charge via the resistors R31/R33

Something else to consider: You might get problems if the relay is going bad i.e taking longer than it should to change state. Another possibility is whatever is on the signal side of the relay is out of the normal voltage range meaning it takes longer than expected for the thump to go away.

Good luck!

[Shane]

Hi Nick
Thanks for the explaination! I've been messin' with valves too long and forgot my solid state stuff!!

I have considered the possibilty that the mute may not be long enough, got no way of monitoring it though. may try slowing it down, actually C15 which is 1uF in the amp and on my drawing is shown as 2u2 on the factory schem. might try uping it to 2u2.
Cheers
Shane

[nickb]

Hi Nick
Thanks for the explaination! I've been messin' with valves too long and forgot my solid state stuff!!

I have considered the possibilty that the mute may not be long enough, got no way of monitoring it though. may try slowing it down, actually C15 which is 1uF in the amp and on my drawing is shown as 2u2 on the factory schem. might try uping it to 2u2.
Cheers
Shane

It's your call of course, but rather than paper over the cracks I would try to find the real problem. I am assuming that it did work properly at one time! I would replace the relay since it's hard to measure and being mechanical it a target for failure. Maybe you have some in you odds and ends bin that could be tacked in to try?

[Shane]

Yes Nick you're probably right I'll see what I've got and try it.
They are little miniature low profile 24V ones so I doubt I'll have one knocking about, but they're not that expensive.

Cheers
Shane

[Shane]

OK right then, it's not the relay Changed it for a new one, no better, back to the drawing board!

Cheers
Shane

[Alf]

Hi,

Interesting info but does anyone have the entire schematic of the H&K dual 34 ?

Tia , Alf

[htrstew]

Any chance of getting the entire schematic you have?

OK I drew this up quickly.
Some bits are omitted but they switch fx loop and boost and I dont think they will affect the bits in question.
The switches are momentary.[ATTACH]28143[/ATTACH]

Cheers
Shane

Hello
Any chance of getting the entire schematic you have? Since it is on paper maybe a photo?

Thanks either way,
Stew