Squealing 5F2-A

[KingCameron]

I have recently constructed a 5F2-A circuit and it is squealing like crazy!
I have verified not signal lines are not crossing
I have verified that the tubes are not microphonic.

When I switch the amp on, and begin strumming the guitar, it will play for 1 second and then begin a high pitched squeal. (playing while squealing) This squeal is not affected by the volume control.

With the 12AX7 removed, voltage off of [Pin 8 Rectifier] is 397VAC. I know this is wayyy high.

Is this my problem? How can I fix it?

[Jag]

Try swapping the secondaries of your OT. My guess is that your negative feedback loop is currently positive. This would cause squealing.

[tubeswell]

yep same thing happened to me when I built my 5F2A - turned out to be the OT primaries round the wrong way. They are apparently not always how they are described in terms of colour codes. so try swapping them around and see.

[KingCameron]

I am currently using a TF103-48 from Triode. (see link for data)http://store.triodestore.com/tfchxfwi48oh.html

I have it wired Red to Fiberboard in the middle of the power capcitors
Blue to PIN 3 on the 6V6

On the other side I have Black to the RCA GROUND and Yellow (8ohm load) goes to the center of the RCA jack

[mikeboone]

Try lifting one end of the 22K FB resistor. If that helps, increase the value.

[Jag]

Before you do that, try switching the black and yellow leads.

[KingCameron]

I switched Black and Yellow and the squeal is gone!!

You are awesome...nailed it!

[Jag]

Good to hear! How does the build sound?

[KingCameron]

I built two of these amps together. I can say they both sound similar.

1. Tone is clean up to about 9 or 10 on the volume, then some break-up begins through 12.

2. Overall volume is not very loud...of course, I have built these amps using low-powered transformers, so they are most likely acting as they should...not sure.

I am testing them through a junky old speaker I have yanked out of some Silvertone somewhere. Also, I use a new EH 6V6 and a Chinese knock-off 12AX7 and a Sylvania 5Y3. I use these tubes because I beleive they are the most robust, not the best sounding.

I am looking to juice them up a tad...any recomendations on how to increase volume, or other cool ideas?

[Paul P]

A speaker can make a huge difference in the volume. Try plugging your
amp into a cabinet or some other amp's speaker and see what happens.

Paul P

[KingCameron]

I will be installing some Eminence 10inch (8ohm) speakers in my cabinets for the intial runs of these amps.

I know it will be hard to tell anything until I do this. I will choose better tubes and the speakers will be better suited than my junky test unit.

Attached is a shot of the custom cabinets my friend built. (unfinished)

Question:

What voltage should I be running on to optimize the amp tone?
I am using the P-TF22772 transformer which is supposed to put out 325VAC. I am getting much higher on both amps.
How much does this matter?

[tubeswell]

To lower the B+, you can lower the PT voltage using zener diodes in series with the center tap-to-ground (i.e. between the CT and the ground) of the High voltage secondary winding. Have the + end of the diodes going to ground. Try a few different voltage diodes there and see.

If you have got 3-pin wiring on your AC mains chord, I would take out the death cap (and make sure the chassis is grounded to the AC mains earth wire). The death cap was something they use in the old days before 3-wire AC Mains. It is dangerous to keep it in if you have a 3-wire AC Mains connection.

The death cap is the one going from the fuse to ground on the Mains side of the PT

[KingCameron]

Tubeswell,
Thanks for the tips.

I will try as you have suggested on one amp, then compare to the other.

You Rock!

[tubeswell]

I forgot to add - use 10W-20W zeners for lowering the B+ off the HT CT. They need to dissipate quite a bit of power - lower wattage ones might not hack the pace. Experiment with different voltage values (9V, 12V 15V etc) to achieve the closest to ideal B+. Make sure they are reverse biased.

[Bruce / Mission Amps]

I forgot to add - use 10W-20W zeners for lowering the B+ off the HT CT. They need to dissipate quite a bit of power - lower wattage ones might not hack the pace. Experiment with different voltage values (9V, 12V 15V etc) to achieve the closest to ideal B+. Make sure they are reverse biased.15v drop through a zener while drawing say, 100ma, is only 1.5watts. I think a two or three 9v to 12v @ 5 watt zener diodes will work fine and only cost a few dollars.

[KingCameron]

Thanks Tubeswell!
I will order some of these Zeners and give it a shot.