Homebuilt amp question

[Gord K.]

One of you knowledgeable amp gurus will probably know the answer to this.

I built a simple tube amp (12AX7, 6V6, 5Y3) as point to point as you can get, things are hanging off tube sockets and jacks everywhere! The amp has two inputs, a 1 Meg volume control and a 1 Meg tone control. Similar to several different early Fender Champs and Princetons and probably other amps as well.

When I power the amp on I get a high-pitched squeal that does not vary with a change on either the volume or tone controls. Just for the heck of it, this evening I cut the wire for the 22K feedback resistor that goes from the output transformer to the second stage of the 12AX7. The squeal is gone and the amp actually works not too badly (except for a little fartiness when the volume is turned up on either the amp or the guitar.

My questions:
What will I be missing by not having the feedback resistor connected? Do I really need it? And if I do need it, how do I connect it so that I don't get the squeal?

I guess the follow-up question would be, how do I get rid of the flubbiness when the volume is cranked?

I appreciate any suggestions you folks can offer.

Gord.

[guitar_paul1]

Did you try hooking the feedback resistor to the other side of the output transformer? (Assuming it's tied to the speaker side, that is). One of the things Gerald Weber always talks about is the indeterminacy of phasing on output trannys. You want to feed back out of phase signal rather than in phase signal.
As far as what you are missing, the amp should sound tighter and cleaner with the feedback resistor in. It will sound more out of control without it.
One of the things I did to my sf champ was to greatly increase the value of the feedback resistor. Brightened up and opened up the tone considerably.
Good luck and congrats on making your own amp.
-paul

[MJ Harnish]

Watch your lead dress: My first thought is that wire carrying the NF is too close to another lead. Lead dress is critical especially in PTP wired amps b/c you can easily end up with a bird's nest.

If this doesn't fix it, check the primary & secondary leads on our OT to make sure you don't have one of the pair's reversed (i.e., the wrong lead connected to ground).

In terms of muddy bass, you might consider increasing the B+ filtration a little; bumping the first up to 20uF and the 2nd up to 10-16uF will help tighten the bottom end a bit though it won't exactly be a stock circuit anymore.

[Gord K.]

I implemented both suggestions...

I swapped the leads on the output side of the output transformer and I don't get the squealing.

I originally built the amp with three 10uf 450V caps instead of the 16, 8 and 8 because that's what I had. I have rewired the power supply filtering section with a 22uf and two 10uf and that took care of the muddy bass tones.

Can't do much about lead dress. I wired it with components connected directly to pots or tube sockets and where I have had to add wires I've kept them as short as possible. This definitely leads to wires criss-crossing each other.

I was never concerned about keeping it "stock". The amp is based on the 5F1 Champ circuit with some extra features. I added a single pot tone control as used on several Fender amps (I took it from the 5E2 Princeton schematic) and between the wiper of the tone control and the top of the volume control I added a pair of jacks for an effects loop (haven't tried that out yet).

It's been a fun project that now just needs a cabinet and a speaker. Then on to something a little bigger....

Gord.

[MJ Harnish]

Glad to be of help. Sounds like a cool little amp. :)