[_peter]

What? Are we talking about the same thing? I don't mean the standby switch.
Maybe you didn't built in a fx bypass switch. I attached a schematic where you
can find it before the tone stack. If you don't have it, disconnect the the tone
from V4s cathode follower.

[Guile]

Sorry, my bad

[Guile]

please explain what to disconnect exactly

[Stone]

Hi

Peter is thinking of disconnecting the whole tone stack from V4 - therefore you need to disconnect the 47k slope resistor of the tone stack and the 470pF of the tone stack as the connection between V4a cathode (pin 3) and the tone stack is hard wired on the PCB.

This will sort out if there is a broken capacitor in the tone stack which causes kind of short cut under some circumstances to V4a cathode.

Again … if we should sort out things we need some updated pictures and still the amp needs to be cleaned up a lot more - as far as I see we have a lot of faults on wiring - maybe some don't exist, but it is hard to figure out.

I will sum up, what I think I can see by the pictures:

- mains and standby switch "should" (must) be changed; you need to switch both "poles" of mains and of B+ … currently you are switching only one phase of mains and standby switch seems to be located after the rectifier

- Xicon resistors on the power tube sockets seem to touch one of the power tube sockets' screws (each one on each side) with one of the resistors lugs

- it is still not clear to me if fx jacks are grounded correctly and which one of the jacks is of switching type and how it is connected

- I am not able to see how (and if) the input jack is grounded (seems to be grounded by one of the lugs of one of the switches on front)

- there seems to be a white wire running from the 10µF 450 volts caps on the preamp pcb to one of the fuse holders which does not make much sense to me (if so)

- I am not sure about the wiring of the impedance switch and the OT, because it cannot be seen clearly

Overall I think the amp can be fixed within half an hour while being at the desk of Jürgen, Peter or me or nearly anyone else, because that 's the only chance to figure out where wires are running from and going to.

And I think we do have two mistakes / faults: one within preamp circuit and maybe one within the power amp circuit.

At least it can be a pretty simple fault in nearly everything … e.g. if fx send is not wired properly you won't get any good signal with any power amp connected to it, if a cathode resistors value is too high you won't have any gain at that point (every triode stage got its own gain … gain is not only the pot in front of many amps), if something is shortening or doing a crossover you won't get any useable signal, etc. etc.

Again - if you like to sell it, make an offer.

I am pretty sure it can be "healed", but it needs to be built a bit more properly.

Regards, Stone

[_peter]

Damn,

Stone is faster than me. I'll post it like this anyway:

Let's start over,

- the voltages at V3, as you posted them, make no sence - like this the cf would be biased at -20V in total cut off
- this may be
   -> a measuring error
   -> a faulty tube
   -> faulty component
   -> wrong values/wiring, unintended connections underneath the board
   -> because the voltages are jumping as you saw on V4; if so, why?
           * is the supply voltage jumping too? please let the amp warm up before measuring
           * is the stage oscillating? test: short the grid to ground via a capacitor (e.g. 47n 630V)
           * is the stage loaded with some failing circuitry? test: disconnect anything that follows
- as you don't have the fx bypass switch, the tone stack is never attached to V3 so it can't be responsible for V3 (as i thought first)
- regarding V4: now that the jumping stopped - did it? - what are the voltages now?

[Guile]

Do the dc voltages at V3 and V4 change when you toggle the fx bypass switch?
(Then you have a faulty capacitor in the tone stack.)

Cheers, Peter

Thanks for your patience so far.

I made a new picture of the FX send return. As you can see there is a switch (based on C3amps layout).
The voltages change in V4; They all increase about 10% when switched
No movement in V3

Please tell me if this is helpful or not before I proceed to the other suggestions.

Vielen dank
 

[Guile]

- mains and standby switch "should" (must) be changed; you need to switch both "poles" of mains and of B+ … currently you are switching only one phase of mains and standby switch seems to be located after the rectifier
OK

- Xicon resistors on the power tube sockets seem to touch one of the power tube sockets' screws (each one on each side) with one of the resistors lugs
IT'S NOT TOUCHING THE SCREW

- it is still not clear to me if fx jacks are grounded correctly and which one of the jacks is of switching type and how it is connected
FX JACKS ARE NOT GROUNDED BY THE CHASSIS (https://www.flickr.com/photos/115176149@N03/)

- I am not able to see how (and if) the input jack is grounded (seems to be grounded by one of the lugs of one of the switches on front)
INPUT JACK IS NOT GROUNDED BY THE CHASSIS BUT PER PREAMP PCB

- there seems to be a white wire running from the 10µF 450 volts caps on the preamp pcb to one of the fuse holders which does not make much sense to me (if so)
THAT'S THE CT

- I am not sure about the wiring of the impedance switch and the OT, because it cannot be seen clearly
I WILL CHECK

[Stone]

Hi

The fx jacks need to be grounded.

At least to ground of preamp PCB or directly to chassis. I would suggest to connect fx jacks ground to speaker jacks ground for testing purposes.

Regards, Stone

[Guile]

I did; but no difference

[_peter]

I made a new picture of the FX send return. As you can see there is a switch (based on C3amps layout).
The voltages change in V4; They all increase about 10% when switched
No movement in V3

I don't see no switch. If you mean the switching contact of the return jack... I posted the schematic some
posts ago and explained why - if it where there - it might help to find the problem. If you are unable to
distinguish between what the one in the schematic does and what the switching contanct of the return
jack does, I can't help you. You shouldn't be working on tube amps then.

The dc voltages shouldn't change when plugging sth. into the fx return.

And for gods sake, clean that jack. It's corroded like hell.

[Stone]

- there seems to be a white wire running from the 10µF 450 volts caps on the preamp pcb to one of the fuse holders which does not make much sense to me (if so)
THAT'S THE CT

Sorry to say, but that makes no sense … are speaking of HT? The high voltage wire? If so, it is more or less wrong wired. And I repeat Jürgen and Peter and myself: in order to sort things out it is mandatory to have a clear overview and also to make sure the wiring is correct - to me it sounds the wiring is incorrect.

I cannot follow how B+ is wired - standby needs to be BEFORE rectifier diodes, not after them; at the moment it is a one pole type and as the purple wires are connected to rectifier directly it seems to be installed somewhere after rectification … if there is a wire from HT fuse to preamp board (b2 or b4 labeled connection), how are the plates of the power tubes are sourced? How is choke wired? And so on ...

Sorry, but I started an explanation twice now and deleted it after a few sentences - you need to be familiar with wiring of PSU of an tube amp and a lot more; at the moment I am not sure, if you are able to follow the PSU schematic as it is provided by Joachim e.g. (slo-100_psu_0_9) and on the other hand side this does not help for all questions as the C3 PSU board is different in some things.

So, it is essential to have the schematics for the C3 boards at hand - if C3 did not send you any schematics, please ask them to send …

Without a deep knowledge you will not only get misbehavior and bad sound, but a lot of security risks which can cause serious injury by high voltages.

Regards, Stone

[Guile]

Thanks for your replies. They're not helpfull though.
Should I make some more pictures or should we call it quits?

[Guile]

It is completely built like the attached layout

[Stone]

Hi

I had a look to the layout - maybe I am wrong, but I have never noticed this wiring before … the layout shows OT and speaker jack connection labeled the right way, but not shown in the right way: impedance switch is connected (as labeled) to tip of speaker jacks, whereas sleeve must be connected to ground.

At least there is no ground connection drawn in the layout for speaker jacks - all what can be seen is a black or purple wire running from sleeves to OT. If it comes to the TT output transformer it must be wired "black" to ground and grey, brown, pink to impedance switch.

From the layout is becomes clear, why the white wire is running from fuse to the end of the board and I am now aware of what you meant by CT … the layout is different to those I know about (mostly the SLO clone layouts and that one Joachim described here at TT forum).

Is the OT's black wire tied to ground? Cannot see it on the pictures.

But at least I have run out of ideas … I am pretty sure, that the problem can be solved, but not "remotely" by me; there are too many possibilities which cause the unwanted sound, like e.g. the switch of the fx return may give no proper connection for the signal, etc.

Regards, Stone

[Guile]

Hi

I had a look to the layout - maybe I am wrong, but I have never noticed this wiring before … the layout shows OT and speaker jack connection labeled the right way, but not shown in the right way: impedance switch is connected (as labeled) to tip of speaker jacks, whereas sleeve must be connected to ground.

At least there is no ground connection drawn in the layout for speaker jacks - all what can be seen is a black or purple wire running from sleeves to OT. If it comes to the TT output transformer it must be wired "black" to ground and grey, brown, pink to impedance switch.

From the layout is becomes clear, why the white wire is running from fuse to the end of the board and I am now aware of what you meant by CT … the layout is different to those I know about (mostly the SLO clone layouts and that one Joachim described here at TT forum).

Is the OT's black wire tied to ground? Cannot see it on the pictures.

But at least I have run out of ideas … I am pretty sure, that the problem can be solved, but not "remotely" by me; there are too many possibilities which cause the unwanted sound, like e.g. the switch of the fx return may give no proper connection for the signal, etc.

Regards, Stone

- What is wrong with the impedance? Or is it fine the way it is?

- speakerjacks are grounded to the chassis

- what fx switch are we talking about? The jack switch? Is the picture clear enough?

- 'CT' is what is written on the notice with the output transformer (see attached)