Ampeg VT40 problems

[falcom]

Have a 1971 VT40, the Bright channel has lost gain, harsh early breakup & the mid
control (which usually can add massive amounts of high end w/ the sensitivity switch)
seems not to do much of anything at all... swapped tubes in the preamp channels, no change. Any experience with this amp?

[Amp Kat]

Treble loss is usually a hent of power tubes. Are you still using the 7027's and when is the last time you changed them ? How about the 6K11,6CG7 & the 12DW7 ? That amp is getting old and the filter caps may need changing if you hadn't already done it.

[falcom]

NOs 7027 less than a year ago, I tried some spares w/ no change as well as the other oddballs you mentioned. Channel 2 still works well with the exception of the mid rocker
switch not doing anything. A guess a cap job is in order. Another note....I tried a new set of the Winged C 6l6sand noticed no difference from the 7027! I guess when I get
the thing running back at 100% I'll have a more accurate test.

[Enzo]

Look at the schematic. If channel 2 works, that limits where channel 1 can be bad. Channel 2 is V2, and the plate at pin 6 shares a load resistor with pin 6 of V4. That point is where the two channel join for the rest of the trip through the amp. So we know from that point forward things are OK. Both channels run off the same B+ node, so I doubt filter caps are your issue.

So that leaves us with V1 and that half of V4. If you swapped tubes, then the tubes are likely OK. SO check the DC voltages on those three triodes. AC volts are spec'd on the drawing, so apply a test signal and trace it through. Alternatively, use signal injection starting at V4 pin 7 and work back towards the input.

And make sure that kakamamie distortion circuit isn't hanging up or half of it maybe.

[falcom]

That midrange sensitivity switch that works for both channels quit working at the same time this ch 1 problem started, which makes me believe the problem is related..... no
distortion circuit on this model.

[Enzo]

Then we are looking at different schematics. Post yours or link it? I was using http://www.schematicheaven.com/ampeg..._vt22_vt24.pdf


I'll have to check the circuit you have, but if the same control serves both channels, as it does on the drawing I had, then it would not specifically be involved, otherwise it would affect both channels.

OK, here is another VT40 preamp with no distortion circuit. It's one of those damned Joe Piazza re-draws, but the circuit is similar to the other. The two channels combine after the early stages, and the midrange is a few stages later and common to both channels. http://www.schematicheaven.com/ampegamps/vt40preamp.pdf

[Albert Kreuzer]

Does this one help?

[falcom]

Ok, that's the right schematic....I'll look into V4- did a swap with an old used 12u7, it's possible that one was rummy.....I assume a known 12ax7 would work for test purposes?

[Enzo]

That looks like the one Piazza redrew, but I hate his drawings. In this version, the two channels come together at the plate of each channels second triode, so no V4 in this one. The second triodes of V1 and V2 share a load resistor and are thus mixed together at that point. If channel 2 is OK and channel 1 is not, the trouble sure looks like it needs to be in the V1 circuit.

[g1]

Are the ultra high and sensitivity switches good on the bright channel? Is the mid problem with the control or with the switch? Seems unlikely your 2 problems are related as Enzo stated, the mid is common to both channels. I'm thinking maybe dirty/bad mid switch and maybe bad solder/resistor/cap in bright channel. Those amps have board mounted tube sockets so bad solder at the sockets is fairly common.