5C2 princeton clone build

[ThisLifeILead]

howdy folks, I got an old hammond 12" extension speaker that houses the amplifier. It has hammond trannies from the early 60's. choke and OT are potted. (Ot is a 30W P-P OT but has been wired for SE use). Seems to work fine as its quite large.
tubes are 1x6sc7 1x5y3 and 1x6l6
single vol and single tone.
I have a NOS 6SC7 tube (metal) that tests really really strong but i know they are inheritantly microphonic. (no others to try)
I followed the schematic for the princeton 5C2 except im using a 6L6 for the output and used a 470 ohms screen resistor. with 350v on the 6l6 plate im drawing 55ma of current. something around 15W dissipation. 270ohm cathode resistor.
All tone caps i used are random parts laying around the shop. The filter caps are new but im suspecting one might be bad. 1st stage is 33uf, 2nd and 3rd are 22uf.
The problem i am experiencing is with the volume turned up, im getting a vibrato like sound on held notes. i cant tell if its a bad 6sc7 tube or my filter stages are decoupling. or possibly a leaky coupling cap between the two gain stages?
Any suggestions?

[lowell]

I guess describe this vibrato a bit more. Fast? Slow? Is it consistent in frequency? Happening all the time or just sometimes?

[ThisLifeILead]

its a slow but steady vibrato and it cuts in and out.some notes it doesnt start until the note is held. it seems to be consistent in freq too.
there is also some scratchy noises coming through on vol pot which im starting to suspect some leaking coupling caps. the 0.022uf coupling caps are from the 60's and were in the amp to begin with.
i havent gotten very far troubleshooting it as i just finished wiring it yesterday and got the 6l6 to bias up nice.
i have never heard this sound from an amp before and between gain stages on the oscilloscope you can see the baseline moving up and down with the vibrato pulses. slight signal distortion is detected on the output signal with the vol turned low and guitar muted.
i will get a sound clip tonight.

[lowell]

Ok. Try to narrow it down to what stage it's in. I'd input some sound, then clip an audio probe around and see where it's at. Start at first stage, if it's there, it'll probably be everywhere else.

[ThisLifeILead]

http://www.davidsonamp.com/sf/images/princeton5c2.gif

i did poke around alittle yesterday and if i remember correctly it was not visible on the scope after the first gain stage coupling cap but i will do a full voltage/signal check and report my findings!
Thnx Lowell.

[ThisLifeILead]

okay, input grid on v1a measures 0v, input grid on v1b measures 1.07v. when i place the scope probe on this grid it makes alot of noises. this is also measured at idle. no input signal.
cathode voltage was unstable. 6sc7 has a shared cathode. it wavered from 2.40 - 2.43vdc.
plate voltages were fluctuating aswell. V1a plate voltage measures 175-178vdc v1b measures 96-106vdc.
even when amp is set for clean it sounds distorted.
heaters are 6.5vac and stable.
input grid doesnt show modulation on scope. v1a plate does however....

[lowell]

Good voltage check. Looks like V1b grid leak resistor is either open or has a cold solder joint.

[lowell]

Oh and OR the coupling cap is leaky from V1a plate. I was misreading the schematic. Could still be my first guess too though. More likely that cap.

[ThisLifeILead]

Oh and OR the coupling cap is leaky from V1a plate. I was misreading the schematic. Could still be my first guess too though. More likely that cap.

thats my thinking too! i will resolder the voltage divider off v1a and replace the coupling cap off v1a plate tonight.
cheers!

[ThisLifeILead]

It alive!! i replaced all the old coupling caps with new Xicons. Installed a 5.6K grid stopper on the 6l6 and installed a huge 470uf cathode bypass cap. tone control works good.
i discovered the speaker is a smooth coned Marsland. it breaks up extremely early and has some voice coil rubbing. (speaker in pic)
So i swapped it out for a ribbed cone alnico hoepner from a hammond organ. the speaker has no high end and when the volume control is near maxed out it oscillates still. Sounds like motorboating. I havent dug any further but my guess is that the preamp is oscillating. i also installed a speaker jack so i could try different speakers and use the dummy load on the scope. i tried it with a pair of 1979 G12-65's (sounds amazing wound up) and a 1963 Jensen C15N that has some VC rub at high voumes. The jensen is the winner! it sounds so good, plenty of high end pick attack, really compliments the amp so i am now looking for a C12N style speaker.

[Leo_Gnardo]

when the volume control is near maxed out it oscillates still. Sounds like motorboating.

In post 1 you mentioned one hi voltage filter cap was suspect, have you replaced it? Motorboating/low frequency instability is an indicator of insufficient filtering. Replacing bad filter caps can clear up other problems besides. Amp stages can modulate each other thru a dodgy power supply & cause audio havoc.

That way oversize 470 uF cathode bypass, is that on the output stage? If yes, might also be a problem. There's a reason smaller values are used. Up to a point you can extend clean low frequency response, and it may be all to the good. 5 to 25 uF commonly found in this application. Try different values & pick one that satisfies your ear with whatever speaker you finally settle on. Beyond that you're trying to boost sub-sonic "garbage" & that can send your amp into a tizzy. It's a small guitar amp after all, no point in trying to develop power at organ pedal-tone frequencies and below.

[ThisLifeILead]

i have not replaced the one suspect filter cap. i will look for leakage when im in there tonight or tomorrow. the reason i say its oscillating is because i was farting around in there with a sub box and was clipping in snubber caps in various values from 100pf to 2200pf and i could completely eliminate the motorboating/pulsing with max volume. so i think my layout is faulty

[Leo_Gnardo]

i have not replaced the one suspect filter cap. i will look for leakage when im in there tonight or tomorrow. the reason i say its oscillating is because i was farting around in there with a sub box and was clipping in snubber caps in various values from 100pf to 2200pf and i could completely eliminate the motorboating/pulsing with max volume. so i think my layout is faulty

Freshen up that filter then before you rearrange the room, OK? Early Princetons are more or less Champs, there's not all that much gain. Also have a re read at my last post, I added another potentially useful paragraph.

Speaker, you might consider something a bit lighter weight than the current Jensen c12N although it is a darn good one. For smaller amps along this line I've had good success with their C12Q, or somewhat mellower MOD series 35W Jensen.

[ThisLifeILead]

aha! i see what you mean. i will remove the massive cathode bypass cap ( all i had was 25uf or 470uf) and parallel in a new cap on the "recycled" input filter cap. (note: the input filter cap value is 33uf for a 5y3...another item to investigate).

[ThisLifeILead]

okie dokie, Removed the large 6L6 cathode bypass cap. Replaced with a 1uf. needs alittle bypass as it sounds thin at low volumes without it. I paralleled in some new caps and it made no difference. But i did notice when i move the 6sc7 tube around while playing it can trigger and arrest the motorboat/swirly sounds. the tube sockets are all old as hell. time for a good socket cleaning or replacement.
Also Time to find a good speaker to pair it up with :jam: humbuckers are alittle muddy/crunchy but a tele through the amp sounds amazing!
Thnx fellas! :highfive: