6G12A Concert Amp vibrato stopped working

[jrmintz]

Hi All,

The vibrato in my 6G12A Concert stopped working. I swapped out the three preamp tubes but it made no difference. Otherwise the amp sounds great. Can anyone point me in a direction?

Thanks,

Seth

The schematic is here: http://www.ampwares.com/amp.asp?id=49

[HotBluePlates]

Do you have a meter? Check V3 pin 6 for voltage. Set for a.c. you should see some signal; set for d.c. you should see a confusing bouncing meter reading (assuming a digital meter).

Assuming you get signal there, check V4 pins 6 and 8; you should have a.c. signal on both (if you had an o'scope, they'd be push-pull signals; a scope is not needed to troubleshoot this, though).

Assuming good, check V5 pins 2 and 7 for a.c. signal. If present, then there should be signal at pins 1 and 6. That's the output of the trem circuit, so if there is signal present, the only thing that could kill the trem would be open resistors to where the trem and normal channels are mixed.

[jrmintz]

Thank you, I will check those.

[zapped]

Is the 6G12A a bias vary type tremolo?

[Geezer]

http://www.schematicheaven.com/fenderamps/concert_6g12a_schem.pdf

[HotBluePlates]

Is the 6G12A a bias vary type tremolo?

No, it's a much more complicated circuit.

Nutshell-talk rather than blow-by-blow:
There is an oscillator that produces a slow sine-wave. The signal to be "trem'd" is split into a low frequency band and a high frequency band. The oscillator sine is applied to a phase splitter to generate a push-pull signal, which is then passed on to a push-pull amplifier stage and mixed with the 2 signal bands; highs are on one push-pull side and lows are on the other side. The overall signal is then re-combined into 1 single signal at the push-pull amp output, which then sounds as though it swirls from trebly to bassy along with a volume fluctuation at the same rate.

[jrmintz]

Thanks for your help, HotBluePlates. I finally got a chance to open up the amp, and I saw that the original Astron dual 25uf electrolytic caps are still in it. I changed the one in the vibrato circuit and it works fine.