Princeton 6G2 dead tremolo

[Will V.]

The tremolo just quit working more than a year ago, the switch has no effect whatsoever. I hadn't been playing this amp much anyway, and pretty much resigned to take it to a knowledgable tech but never got around to it.

A couple of weeks ago I decided to try a new tube in V1 to see if that did anything, plugged in my Strat and lo and behold, the tremolo worked, and seemed to be working normally. Yesterday I plugged in a different guitar with hotter pickups and some clean boost, and the tremolo was dead again. The foot switch had no effect. I swapped the V1 and V2 and the tremolo was still dead but the amp worked normally otherwise, suggesting to me anyway this is not a tube issue. Tried my Strat straight into the amp to see if maybe the issue was front-end overload, and it's still dead.

I did all the requisite Googling, etc, but didn't find a thread with a situation quite like mine. Any ideas what the problem might be before I crack it open or take it to an experienced tech?

[pdf64]

How about if the footswitch is unplugged from the amp? It may have a short.

Other than that, the problem may be something in the amp chassis, eg a bad connection, failed resistor.
Ensure that you understand the hazards and their mitigation before getting stuck in.

[Will V.]

Yes, I forgot to mention that I tried it without the foot switch, it was still dead, though I wasn't sure what the behavior is supposed to be without the foot switch since I've never played the amp before without. Does the trem circuit default to on?

I'm aware of the hazards. I am completely inexperienced at diagnosing such a problem, but I have a bit of experience wiring guitars and reading layouts and schematics. I am planning to offload this amp and build a 5F11 clone kit, which would be my first amp build.

[pdf64]

Does the trem circuit default to on?

Looks like it should http://bmamps.com/Schematics/fender/pri ... _schem.pdf

Is this a real Fender 6G2? If so, is the amp properly grounded?

It looks like all the resistors / pots in the trem section can be measured in circuit, so that would be a good place to start.

Have the electrolytics been replaced? The trem oscillator cathode bypass cap needs to be functional.

[martin manning]

And the phase shift caps, the 0.02, 0.01, 0.01uF, side-by-side. See if you have any DC voltage on the CW lug of the speed pot (junction of the 0.02 and 0.01). If so, replace at least the 0.02 because it's leaking.

The new tube might have been just enough of a boost to get it working again, for a while, anyway.

[Phil_S]

If the 12AX7 is weak, it won't oscillate. You can't use other 12A?7 tubes because they have a low internal plate resistance that makes them unsuitable for tremolo. Swapping in a known good tube is worth doing before you try something else.

[Will V.]

Does the trem circuit default to on?

Looks like it should http://bmamps.com/Schematics/fender/pri ... _schem.pdf

Is this a real Fender 6G2? If so, is the amp properly grounded?

It looks like all the resistors / pots in the trem section can be measured in circuit, so that would be a good place to start.

Have the electrolytics been replaced? The trem oscillator cathode bypass cap needs to be functional.

Thanks. Yes, it's a real '64 6G2. I haven't ever cracked open the amp, so not sure about the condition inside. I bought it about 10 years ago and it was supposedly serviced in some way before I bought it, but I don't recall the details.

[Will V.]

And the phase shift caps, the 0.02, 0.01, 0.01uF, side-by-side. See if you have any DC voltage on the CW lug of the speed pot (junction of the 0.02 and 0.01). If so, replace at least the 0.02 because it's leaking.

The new tube might have been just enough of a boost to get it working again, for a while, anyway.

Yes, I've been staring at the schematic and layout for a while and wondering if the problem might be in that area, but wasn't sure how to diagnose it, so thanks for the suggestion. Also I just realized I don't have a working multi, so I need to pick one up before I even think about opening this amp.

[Will V.]

I guess the good news is, the old stock RCA USA 7025 that was previously in V1 probably isn't dead. Good thing I didn't throw it away.

[pops]

If you don't plug in your foot switch you need a shorting plug or the trem won't work. The reverb will, but not the trem. Try shorting the hot to ground on an rca plug and use that/

[sluckey]

If you don't plug in your foot switch you need a shorting plug or the trem won't work./

That's true for the later AB763 circuits, but the 6G2 will work just fine without a footswitch.

[Will V.]

I opened it up anyway today. The cap can is a JJ replacement, and you can see the somewhat poorly enlarged hole in the chassis, but other than that I think it's all original.

The tremolo issue aside (since I don't have a multimeter now anyway, one is on the way) any thoughts? I'm wondering if I should just plan on replacing all the caps and resistors, it seems to me at the 51 year mark just about everything must be out of spec or headed in that direction sooner rather than later.

Does anyone have a BOM handy for the 6G2?

[martin manning]

Looks like someone has replaced a couple of cathode bypass caps too. Overall it looks pretty good, and I would not replace any part unless it measures out of spec or is defective.

[ToneMerc]

Looks like someone has replaced a couple of cathode bypass caps too. Overall it looks pretty good, and I would not replace any part unless it measures out of spec or is defective.

Ditto, would be a shame to butcher a brown Fender amp for no reason other than it's old.

TM

[67plexi]

It may be the last of the brown Princeton's built from the last 6G2
chassis year of production 1961-1963 in 1963 Fender changed to black face
your date stamp is 7-64. I love my 1960 Brown Concert and 1961 Vibrolux
They both sound like early Marshall's when you turn the volume up past 5
The 6G2 is the last Tweed circuit made by Fender.