OT: Champ 5F1 clone bad distortion...

[marrstians]

i built a 5f1 champ clone, used a fender transformer and an eyelet board and it worked upon first fire up... there's a slight problem... when the volume is set low it sounds good, when it's turned all the way up i get the nice distortion tone.... if i put the volume pot towards the middle i get a very blatty distortion out of my amp... it makes a sputtery mis-biased type of sound but i have no idea why... any clues?

[Dave Z]

Well, as you roll off the volume the ration of highs to lows changes..more lows in relation to highs. Low freqs are going to sound ugly when distorted. Did you use a bypass cap on the preamp stages? If so, try lifting that (the original champ did not have them). If that fixes it, try using a smaller value bypass cap (or none). (1 or mF instead of 20 mFd)

[puretube]

maybe a dirty or spoiled (2nd hand/desoldered) pot?

[marrstians]

well it's not a bad part, i bought everything new... i used the original champ layout, i didn't do any mods or anything that's why i'm a little flustered....  it doesn't have the bypass cap in the preamp stage... any more clues?

[puretube]

interstage coupling caps are hi-volt types?
(don`t know the schem... :wink:  :wink: )

[marrstians]

it's basically this design withoug the first 25-50 cap in the preamp section...               http://www.tubegarden.com/champ/schematic.JPG

[puretube]

temporarily solder a 1M resistor from grid of 2nd triode to ground,
and rotate the pot again, listening if the sound at the middle setting has changed (improved).

before you do this: does the amp sound equally bad at medium (acoustical) output-volume, when you turn the amp all the way up,
and control the "loudness" with your guitar-knob?
If it does, you need not do the gridresistor thing...

[marrstians]

tried the guitar volume and it didn't improve anything untill it was  very very quiet... i tried the grid resistor too with no imporvement... i've switched out tubes, tripple check the parts values, tried it through different speakers... the only other thing i can think of that seems kindof odd is that the large caps don't really store anything after i shut it down... they seem to discharge them selves somehow but i don't know how... i dont know if that could be a problem or if it's just something they do...

[puretube]

seems I have misinterpreted the description of the distortion...
maybe s.o. else got better imagination?

[Paul Marossy]

Have you checked the tubes? Maybe you have a funky tube...

[marrstians]

yeah... i've checked the tubes... one thing i noticed is that my transformer is putting out 420v instead of the usual 380v in the champ amps... i notice a spike in the power when i turn it on, it goes over 500v... my power caps are 475v... could the spike damage my caps so where they are still working but sound crappy and discharge themselves? are they one of those things that either work or don't?  i'm at a loss...

[aron]

Yes, always try a different tube. But how about setting your meter to DC and checking the grids of each tube as you turn the pot?

I wonder if somehow something is leaky?

[Paul Marossy]

Your filter caps are probably fine. When I first built my Firefly, I connected the center tapped PT up wrong and had 585V instead of the ~300V that it was supposed to be. My caps are rated for 500V. Everything was fine, nothing blew up. I only had my amp on for a short time, though.

It sounds to me like the starting B+ voltage is a little too high. I think that could be a large part of the problem. I'm not really familiar with the Champ, but it very well could be that an additional 40V could be too much for the circuit as designed.

Aron has a point, too. Maybe you have a coupling cap that is leaky...

[aron]

My Champ has a high voltage on it too. I think it's close to 400V as well but I can't remember. That's how mine was stock - it's an old silver face.

[csj]

1.)Something's screwing up your V1b grid driving voltage. That's what puretube was getting at with the 1 meg grid resistor to ground...(I think).
The signal needs to swing fully back and forth otherwise the interplay between the cathode current and the grid voltage gets goofed up and that new waveform shows up at the plate. If all else fails come straight off the coupling cap into the grid with the 1 meg resisitor from grid to ground just to see if the pot's messed up.

2.) The other part of this is the cathode. Check your NF loop setup. It could be affecting the cathode/grid interplay. You might want to just  disconnect  the loop and listen... maybe the voltage drop across that loop resistor is screwing with your cathode's operation. If all else fails run the loop configuration of the AA764... 3 extra parts. This loop comes up through the lower end of the 1k5 instead of the top. (double check all your parts values). This route takes the cathode and feedback paths through roughly the same IR drop point which might better sync the degeneraton. I don't know... maybe puretube will speak to this.

3.) The filter caps are supposed to discharge when turned off. That's one of the reasons for bleeder resistors across the power rail... for quicker, steadier drain down.

4.) At 420 Ep you definitely want to check your current draw across the 6V6... any more than say, 20 ma (I don't know what tube you're using), you'll want to watch carefully for redplating. Increase your bias resistor for starters if so. If no redplating then go for it...you might have a short tube life but it'll sound great while it lasts.

[aron]

The other part of this is the cathode. Check your NF loop setup.

Good one! I missed that. Yeah, something could be happening there for sure.

I put a heavy-duty variable (clamp) power resistor for the cathode. Works well.

[R.G.]

(a) have you read the Tube Amp Debugging Page at GEO?

(b) blatty distortion comes from two sources, bias problems and oscillation.

(b1) Does your volume pot have DC on it? Check with a meter.
(b2) Does any grid have a positive voltage on it?
(b3) Are your coupling caps rated for 600V?
(b4) Are there grid stoppers on any grids?

[puretube]

csj: my Qs sure were about possible DC at the grid, or missing groundreturn thru a bad spot in the pot (possibly in turn burnt by leaky coupling cap - wanted to exclude this without measuring - which now seems to become neccessary.  

Wouldn`t start messing with the feedback at this point, except disconnecting the 22k from the OT for a moment, and see what happens...
(can get loud and hummy, but maybe that "halfway distortion vanishes").

Can we assume, that all values, esp. cathode resistor of 2nd triode are correct??

A scope would say more than 1000 words esp. regarding oscillations.

Mixed up OT secondary windings will be excluded by lifting the FB-resistor
(and would have caused loud, audible oscillations anyway...).

That leaves us to R.G.`s wise words (b4), concerning inaudible (HF or RF) oscillations (where a scope would tell us immediately...):
assuming your wiring/layouting is properly done (you noted having copied the original...), there`s still a chance the tubes and its close surrounding are swinging on an unhealthy frequency at large amplitudes, which screws up the idle settings, and interfere/modulate with the audio;

to exclude this, route a 1k to 22k resistor in series with the grid of 2nd triode,
by disconnecting whatever is running there (the pot`s wiper in this case), and inserting the resistor between what`s running there (the wiper) and grid.
Solder this resistor, with the lead connecting to the grid
cut really short, directly to the tube-socket`s appropiate lug.

If it hasn`t helped yet, leave it there & do the same on the 1st triode,
if it hasn`t helped yet, leave it there & do same on the 6V6.

If it still hasn`t helped, insert a 470 ohm 1W res. (short-cut lead) directly at the screengrid lug on the 6V6 socket.

If this helped, you can remove the first "gridstoppers", (which btw. form a lowpass with the tube-inherent capacities, to roll off high frequencies...)
one by one, until distortion re-appears. Put that last removed one back in.

Go.

oh yes: of course oscillations are being supported by long wires and improper layout - if there are lengthy loose wires messing around, you can wiggle and shove them around with a long isolated nonconducting (plastic) tool, while the amp is on, and listen for (positive) changes...

[csj]

hmmm... the consensus seems to be leaking dc...

well, whatever it is I hope he gets it fixed soon.

I can hardly wait for the group hug at the end...

[Paul Marossy]

Group hug at the end? Is that like "reach out and e-touch someone"?  :lol:

My thought is that with B+ being ~40V "too high" is that it could be messing with the bias some, possibly causing the nasty distortion, or at least contributing to it. I wonder if improper grounding could have anything to do with it? If that were the case, though, I would imagine that there would also be a nasty hum. A DC leak somewhere does seem like the most likely candidate...