5F6-A / JTM45 copy squeals

[txstrat]

Hello my fellow solder fume breathers.

My latest build is a copy of a 5F6-A or JTM45. I've used Ace Peppers schematic (you can find it here: http://www.sonicdeli.com/ThunderTwea...o/P-Matic2.pdf with a few tweaks (actually I tried to keep the circuit close to the 5F6-A in the clean channel).
The first triode has a 1.5k cathode resistor and a 22uF bypass cap. The second triode has a 4.7k cathode resistor and no bypass cap so far.
For the power stage I use two JJ 6V6s with 470R/2W screen resistors. I changed the 220k series resistor before the cap to the PI to a 100k and inserted a reverb circuit here (see attached schematic).
When I turn the amp on, it's dead quiet but as soon as I touch the plates of the PI with a meter it starts to squeal like crazy and won't stop (guess how I found out ).

When I put a guitar cord into the input and touch the other end with my finger tip nothing happens (except the normal hum) until I turn the volume up to say 2 or 3 then it starts to squeal too.
When I disconnect the 0.022 cap (before the PI) from the preamp and touch the plates of the PI the amp doesn't squeal so I assume the power stage is alright (I haven't inserted a different amps preamp signal here, yet).

Tried different tubes to no avail.
Voltages are all within 5% of spec.
All resistors in spec.
Have disconnected the second channel to no avail.
I'd really appreciate it if you got any ideas where to look next.

Thanks

Matt

[tboy]

When a new build squeals, it's often because the OT polarity is incorrect. Try swapping the primary leads.

[txstrat]

I've thought of that and checked the actual build against the schem. Ought to be OK.
But yes, I will try that.
Thanks tboy

[drfunkphd]

I know the Weber version states that if that happens, switch the leads from the OT to correct it. Must be a pretty common thing.

[loudthud]

Many amps designed around this circuit have a small cap of 47pF or 100pF between the PI plates. I've never read a good explanation for this. Also, the schematic shows no grid stoppers. I just read an old article in Vintage Guitar where the author thought that the lack of grid stoppers contributed to the great sound of the 5F6A.

Is the amp more prone to oscillate when the presence control is at min or max? Have you tried adjusting the lead dress in the PI region?

[txstrat]

I have the 47pF installed. I already built another amp with no grid stoppers. It plays fine - actually that is my main amp for gigs and rehearsals and this new one ought to be my backup.
The controls (none of them) don't seem to have any effect on the squeal except the volume pot turned up more than 2 or 3. Although I haven't tried to turn the presence pot up more than halfway. The lead dress around the PI is the same like I did in several other builds, which do not oscillate. The amp is dead quiet until it squeals. Actually it is so quiet, when I powered it up the first time I was wondering if I had a speaker connected.

[tubeswell]

Does it squeal if you disconnect the NFB altogether?

[txstrat]

I'll try when I'm back home. Gonna disconnect the NFB which should reveal if the primary leads have to be swapped.
Actually I could swap the primaries first, which would leave me with one solder joint less to solder in case this is the culprit.

[txstrat]

BINGO
The wires of the OT just have the colors on the opposite side. Now brown is where blue belongs and the thing plays great.
For those who are interested:
B+ 420
bias @ 30mA
25W no name OT
1x12 Jensen C12Q

BTW Ace, thanks for sharing your schem
and thanks to everybody who contributed to this thread, I really appreciate it.