Strange squealing from a just-built Francesca...

[DngrZne]

This isn't what I was hoping to have as my first post here, but I knew I wasn't lucky enough to complete this build without any problems. I'm getting an odd squealing out of the amp, that doesn't sound microphonic. With no input as I touch the amp chassis I can hear a little pop, and as I do anything near V1 I can hear the sound of it through the speakers, especially tapping the tube, but without the microphonic ring. Once a cable is plugged in, whether the guitar is attached or not depending on how I move the cable around I get a squeal that changes pitch with the movement, and at some places goes away completely. The presence knob seems to have the most effect out of all the controls on the sound of it, but it never completely goes away. I swapped tubes around to make sure that wasn't the problem. Also, I placed the chassis on a piece of aluminum foil to attempt to shield it. This also had no effect. I then disconnected the GNF lead to see if that took care of the problem and that changed the sound a little, but the squeal was still there. I'm not sure what the next step should be in locating and eliminating this problem. I've attached a picture of the amp in it's current state. Any suggestions?
Thanks,
Chris

[joe6v6]

Hello Chris - Try swaping the 2 output transformer secondarie wires that go to the powertube plates - pin 3 - just reverse them at the tube socketts.
JOE

[keithrick]

Looks like a great build!

You could also try moving the ground of the Presence Pot. It looks like it is grounded with the power supply?

Try shielded cable from V1 to the volume pot. It looks like you are using thin teflon wire? I have found that this is more suuceptable to becoming microphonic then the PVC equivalent, YMMV.

Also, take a close look at the Francesca pic and try to duplicate the wiring. All those close ups were taken for a reason. I agree that this amp can be made stable wiring it differently but he wired this amp that way for a reason. There are a couple of places with parallel runs that have an effect on tone.

Look at how the tone pots are wired. Try shortening the treble wire and route it like Ken did. Then look at the Mid and Bass wiring, he has them routed under the pots parallel, people have noticed a improvement in tone when recreating this. One other area is the V1b to the volume, this wire is extremely sensitve so make sure you try adjusting the wire in relation to the plate and cathodes.

After all that.... first things,,,make sure you seal the bottom of the chassis! or at least put a foil cover inside your cab and move it off your speaker cab. Try differenct preamp tubes as well. These two tips alone could solve your problem

[philmanatee]

Everything Keith said will help, you've got to remember these amps are more sensitive to everything when working correctly. I hear noise when my cord moves or when I step on it, and preamp tubes make such a huge difference. It took me quite a bit of swapping before I found the 12AX7's that would work without extra noise in this circuit. Once you get it squared away you should really enjoy what it'll do! Phil

[DngrZne]

Thanks for the replies guys! I'll definitely try the things you've suggested. So far I had already reversed the OT primaries to get it to the point it's at now. It was worse before. Also, I just bought an oscilloscope off eBay, so if I don't have any luck with the things suggested, that will maybe help in figuring it out once it gets here.

Thanks,
Chris

[DngrZne]

Here's an update. So far I redid the treble wire as recommended, making it shorter and routed very similarly to Francesca. Also changed the V1b wire from the volume to the tube to shielded. Lengthened the mid/base wires and ran them parallel under the pots. Also, I should add that I tried putting the chassis on a piece of foil to try to shield it. When playing the squeal isn't noticeable, but I'm not sure if it's because the amp is just overpowering it, but as soon as you stop it becomes very loud again. The squeal is a little different now I would say. Also, I've noticed the presence knob doesn't affect it much anymore, but the volume knob varies the pitch of the squeal. I would say the squeal sounds very similar to the sound you would hear with old radios being tuned between stations. I did attempt swapping tubes around, but this didn't seem to change anything.

[Allynmey]

What is the value of the tail resistor in the PI and the FB resistor? It should be 10K and 100K respectively. Also your Pres. Pot should be 5K. Check also your bright caps and first stage signal caps. It sounds like a microphonic cap and a PI problem.

[Allynmey]

Also, what is the two switches connected to in back? It's hard to see from the picture. I also noticed bias points that aren't hooked up to anything.

Allynmey

[DngrZne]

The tail resistor is 10k and the FB is 100k, like in the schematic. Presence pot is also 5k. What do I need to do to check the caps? Look for AC or DC voltage on one side where it shouldn't be?

The switch to the right switches the OT primaries between 5200/6600 ohms and the one to the left is for the impedance taps, just a toggle rather then a rotary. The bias test points are grounding through the center test point because I left out the plastic grommet on the back side.

Thanks,
Chris

[Allynmey]

It's hard to see in the picture but, it looks like you habe pin 1 + 8 grounded to the tube socket bolt...and....running it throught the 1 Ohm res. for the bias point. Is that right? If so the bias points won't work and you may be biasing your amp incorrectly. Hard to tell from the dark pictures.

Allynmey

[wsaraceni]

hey allyn. its bill. im over dngrzne's place working on amps now. we just changed my ax84 p1 into a high octane. anyways we just looked. pin 1 is grounded to the tube socket bolt. the diodes are on that pin. pin 8 is going to the bias points and grounding through the 1ohm resistors.

any of the parts in this amp look familiar?

thanks
bill

[DngrZne]

After some playing around inside, such as removing a ground from the input jack, adding lead weights to the bottom of the chassis, swapping tubes some more and moving a few wires around I think I've gotten rid of most of the squealing. I can still just barely hear a hint of a high frequency, almost microphonic squeal, but nothing like before. I've noticed that once the amp starts to feedback it sometimes takes a little work to get it to stop. Also, there is probably a bit more 60hz hum in there then it should have, but overall I would say it is getting very close. Once I get that oscilloscope I will really be able to work the kinks out of things.

Chris

[DngrZne]

After a little more trimming and moving things around in the amp I think I've gotten it to the point where I can consider it a success. Still a little 60Hz hum in there, but it's doubtful I will eliminate it all with the amount of gain this thing has. And as long as I stay more then about 2 feet from the head when all the volumes are cranked I'm not getting any more squealing. Now all I need to do is buy a set of NOS tubes and build a cabinet for it

Thanks for all the help guys!
Chris

[wsaraceni]

chris,

it has less noise than yesterday? yesterday had a lot of gain, and it sounded real nice, but was a little noisy like you said. i'll have to come by again and check it out. you at least need to make a sheetmetal cover for it until you get the cab ready.

tonight i will order the rest of the stuff for my build and we can start that next.

[Allynmey]

Hi Bill, just got home from 4 days away. I recognize quite a few things! I would try tying pin 1+8 together and going through the bias points or going directly to the ground lug on the socket on that one. Sounds like you built a theramin there!