Trouble with Fender Champ 5F1 clone.

[jimmy74]

yes sorry for the error it is a 6L6GC

[martin manning]

OK so a 30W tube, which you are running at around 11W. Do you know what the OT primary impedance is, and what is the screen voltage (pin 4)?

[jimmy74]

OT primary impedance is around 100r and voltage at pin 4 is 275vDC

thanks
J.

[martin manning]

OT primary impedance is around 100r.

That looks like DC resistance. What I'm looking for is the AC impedance, which for the typical Champ with 6V6 is 5,000 to 8,000 ohms. Was there any information on the OT in the kit?

[jimmy74]

Oh ok well the OT was supposed to be correct for a 6L6 tube, custom made and bigger than the OT usually provided in the champ kit. There's no information about the OT in the kit but on the site it says 5000 ohms though I suppose that's for the general 5F1 OT.

[martin manning]

If it is 5k, then the operating point actually looks pretty good (well centered) right where it is. If you run it hotter you'll get higher peak voltages and screen current, which will put more stress on the tube and the transformers. You could experiment a bit, but go slow. Maybe try a 390 ohm Rk and see where it settles and what it sounds like.

[jimmy74]

Ok I will ask the seller what the actual electronic specs of the OT are and let you know. I've kicked the volume a little to hear what it sounds like but I've noticed that when getting over 3/4 of the volume, it starts to sqeal, what could be the cause?

thanks
J.

[martin manning]

Could be a microphonic tube in V1 (try swapping it for another), or a lead dress issue where you have some unwanted coupling and feedback going on inside the chassis.

[jimmy74]

I tried taking some readings off the pins of the tubes sockets (with all tubes in):
V1 Ecc83
Pin1 - 162vDC
Pin2 - 0
Pin3 - 1.25v
Pin6 - 160v
Pin7 - 0
Pin8 - 0

6L6GC
Pin3 - 295v
Pin4 - 265v
Pin8 - 19v

The chassis is very tight but I've tried moving around the wires specially between the output jack and the preamp tube trying to keep the wires as far as possible from eachother but that doesn't seem to tame the squeal. If I move a screwdriver near the various resistors in the preamp section noice seems to increase when goin near the resistors connect to pin 2 on the ECC83 and when getting close to the input jacks. I don't know if this has anything to do with the squeal. I've also tried various preamp tubes but nothing changes.

Any ideas?

[martin manning]

Something is not right with pin 8 of the ECC83. You should see about the same voltage as pin 3 there, so check the wiring for errors or unintended connections.

[jimmy74]

Yes you were right the soldering was a bit fidgety on that pin, resoldered it and I'm now getting 1.2v. I also tried reorganizing the wiring of the 2 input jacks, but while screwing one of them in, the sleeve part snapped off and now I'll have to wait to get a new one and continue when I receive it.
do you think a squeal can be caused by a faulty input jack?

thanks
J.

[martin manning]

...do you think a squeal can be caused by a faulty input jack?

Doubtful. Did you get to see if fixing the bad joint had any effect? If you did not try a different tube yet I'd say give that a shot. What kind of sound quality are you getting?

[jimmy74]

Well with the hanging wires of the broken input jack I suppose what I'm hearing now is ground hum due to those wires.... but turning the volume up all the way the amp doesn't squeal anymore... I did resolder the wire to pin 8 so that may have been the cause. Will wait till I get those jacks and see what happens.

thanks
J.

[martin manning]

You can see if its quiet by grounding the leads that were attached to the input jacks. I'm still not clear on whether you have gotten any guitar sounds to come out of the speaker yet.

[jimmy74]

I have tried playing through it, I noticed that there was basically no drive at all but that was probably because of the bad joint. I will try grounding the hanging leads and see what happens.