Fender Champ 5f1 clone squeal

Exla13

Member
2013-02-21 2:35 am
Hey! I just joined the forums a couple minutes ago because I have some questions on an amp I am building for the science fair. It's a Champ 5f1 clone, build to the schematics by triode electronics. Got everything hooked up and plugged it in for the first time a couple of hours ago.

All three of the tubes 12AX7, 6V6GT, and the 5Y3GT glow, but about 15-30 seconds of heating up, I get an ear piercing screech through my speaker.

I've looked around and found a couple of things ranging from tubes going microphonic to the output transformer (TF103-ULH from triode) secondaries needing to be switched. I have pictures, but I'm not sure how to put them in here yet. Ill put them up as soon as I can.

Anyway, I tried switching all the tubes out with new ones, and started it up again. Same thing happened. The output transformer had a 8ohm and a 4 ohm secondary, so i tried switching them. nothing happened besides the squeal being a higher pitch. It happens no matter what the volume is at too, by the way. any help?

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I looked at the layout on http://site.triodestore.com/5F140-18027120VREV2.pdf

I'm sure many of these have been built with no problems. I would suggest that you check the wiring from the 68Ks to pin 2 of the 12AX7. For that matter double and triple check all of the connections. It could be something as simple as a cold solder joint.

It most probably is a compromised wire connection.

Hey! I just joined the forums a couple minutes ago because I have some questions on an amp I am building for the science fair. It's a Champ 5f1 clone, build to the schematics by triode electronics. Got everything hooked up and plugged it in for the first time a couple of hours ago.

All three of the tubes 12AX7, 6V6GT, and the 5Y3GT glow, but about 15-30 seconds of heating up, I get an ear piercing screech through my speaker.

I've looked around and found a couple of things ranging from tubes going microphonic to the output transformer (TF103-ULH from triode) secondaries needing to be switched. I have pictures, but I'm not sure how to put them in here yet. Ill put them up as soon as I can.

Anyway, I tried switching all the tubes out with new ones, and started it up again. Same thing happened. The output transformer had a 8ohm and a 4 ohm secondary, so i tried switching them. nothing happened besides the squeal being a higher pitch. It happens no matter what the volume is at too, by the way. any help?

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It may well be the output transformer being connected in reverse phase. If you lift the 22k feedback connection and the squeal goes away then that is most likely the problem. Reverse the output transformer primary wires.

I looked at the layout on http://site.triodestore.com/5F140-18027120VREV2.pdf

I'm sure many of these have been built with no problems. I would suggest that you check the wiring from the 68Ks to pin 2 of the 12AX7. For that matter double and triple check all of the connections. It could be something as simple as a cold solder joint.

It most probably is a compromised wire connection.
 
I just built a 5F1 and am having squeal problems also. Goes away at full volume, which is appreciable. Tried lifting 22K feedback, reversing OT primary leads to no avail..... Plan to adjust preamp B+ voltages (are a bit low), and looking for bad connections. Also will put a dummy load on it and look at it on my 'scope. Any other ideas? Thanks!
 
I just looked up the schematic and was appalled to find no grid stop on the 6V6. Every single ended 6V6 circuit I've built required one to supress parasitic oscillation.
Lift the wire from pin 5 on the 6V6 and fit a 15K resistor between the wire and its old pin 5 connection. The resistor body should be as close to the tube socket pin as you can make it.
Cheers,
Ian
 
ADDED THE GRID STOPPER - STILL SQUEALS

Really cut the treble as well. I have bigger problems I'm finding out. Thought an old OT from a Hammond organ, for a p-p 6V6 amp would work.... Primary DC resistance only 535 ohms... Need 5-8K right? and it has no air gap. Will buy the right OT and go from there. Thanks for the tip! Had heard about grid stoppers (have notes on it somewhere - lots of great information out there, and not enough organizing on my part!). I'm a newbie. Need to build an amp with the proper parts, and THEN start substituting.....
 
You do want a transformer with a gap for the single ended amp like this.

BUT

Don't confuse DC resistance of the transformer wire with the impedance of the circuit. They are unrelated. 5k-8k refers to impedance. a 5k transformer wound with #28 wire will have a higher resistance than a 5k transformer wound with #24 wire. Same turns ratio means same impedance, but thicker wire will have lower resistance.
 
Currently waiting for new Champ PT and OT to arrive....Wanted to post a pic of my breadboarded 5F1. Could I have wire lacing/routing issues causing the squeal? Was using a p-p OT that used to drive two 6V6s, and B+ voltages were a bit off (both off an old Hammond S-6 organ). Will address those when I hook up the new xfmrs. Any comments appreciated. Thanks!
 

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Well those long parallel runs are not the way it was done in the old days. You have to watch that inputs (signal grids) don't have long parallel runs to high current outputs. Putting the components right on the back of the socket looks messy,but makes for short runs. Heater wires were usually twisted pair in a Hammond organ, and return currents of input signals are run explicitly to the metal ring around the tube amplifying the signal, instead of using the generic chassis ground. Lots of cheap guitar amps used the mix-em-up in the chassis ground scheme, though. Hammond were coping with 25 mv tones coming in, I don't know what guitar pickup signals are voltage or current.
On your breadboard, I don't see any ceramic disk capacitors either. Hammond sprinkled those around to keep out oscillations from the grids by RF sources - like cell phones or switching power supplies of computers, TV's etc. There is much more RF around a home or bar these days than there was in 1960.
 
FIXED MY CHAMP SQUEAL PROBLEM!!

Just needed to shield the input signal from the jack to the 2nd stage grid (pin 7)...... learning the basics of amp building, not the hard way, but by finding what one can get away with, and what is imperitive..... Oh, and a known correct OT helped I think. Now I can start fiddling with voltages, tubes and components to learn how they affect gain and tone. Can't wait! Thanks for all your inputs too!