Ckt for feedback

[rutledj]

My other thread about the 372 Hammond xfrmer sparked some good debate. Basically, from that thread I gather that the xformer should be adequate for this design.

This is basically a deluxe front in with an extra switchable gain stage taken from 1/2 the reverb driver. The pi and 6l6 output xfrmer is from the DR. The tremolo ckt is from the earliest vibroverb.

Results: With extra gain stage switched out it sounds very nice. just like you would expect a DR to sound. Starts crunching aroun 10:00 on the vol.
Bias is set at 33mv measured across the 1 ohm resistor. With the vol set around 1-2:00 and playing hard for about 10 min. the PT starts getting quite warm. For 20 min, it is hot. Voltage at bias point can be seen jumping to 150mv.

Problems:

Reverb has a bad hum when level is turned up any at all. Also, it sounds distorted (in a bad way) with reverb on. Not sure is signal is too high going in or what.

Vibrato doesn't work at all. I can see the oscillator working with the scope but changing the speed pot doesn't seem to change the sign wave much at all. Intensity has no effect. This ckt does vary in that I have the additional bias pot in the ckt. Not really sure how to troubleshoot this section. On the intensity pot wiper the scope shows more of a sawtooth wave then a sign wave.

Extra distortion stage kicked in give a rather buzzy distortion. Not nice sounding at all.

Any suggestions would be greatly appreciated.

Thanks,
Rut

[Andy Le Blanc]

the scheme isnt clear ...... is the center tap grounded on PT?...
with a total of 33ma. draw for two tubes at 446V thats a static dissipation of
14.7 W total or 7.3 W per tube.....even with th 70% rule of thumb and 19W
conservative guess as to tube type ....13W per tube.....you should be shooting
for 26 W tot. or .0585 A (58.5 ma.) with a B+ at 446 at the very least......you might have biased the
amp really cold ...... Id temporarily dissconnect the trem. from the bias ckt.
and get the power side of the amp nailed down.....totally go around the intensity pot.......
double check your dress around the power componants
power trannies do get warm but not overly hot...... every time you touch the
bias pot. you must recheck the B+ at the tube plate and do the math...

[rutledj]

Thanks. I'll try this. And yes, the secondary ct is grounded. Just forgot to draw it. Otherwise, do the preamp supplies and connections look ok? Any suggestions on why the reverb is so crappy sounding?

rut

[Andy Le Blanc]

in general things "look" ok..... whats the reverb transformer?......
whats the verb tank I.D.?...... the fender verb tranny is ment to match a 12at7
wired parralel, to the input of a verb tank, a big mismatch could make it sound like pooo.....
its easy to overdrive a tank too..... the 500p cap that
supplies the verb is good.... going to large there promotes a LF occilation
you could go larger after the recovery.....but it looks fine
double and triple check the dress looking for mistakes, bad joins.... etc
but id focus on resolveing any issues on the power side first.....
youll want to post voltages so others can chime in.......

[rutledj]

Well, I changed this back to use the standard 12at7 ckt to drive the reverb and removed the extra gain stage. Updated voltages on prints. Sounds ok but reverb still doesn't work. Well, it works but as soon as you turn the level knob up any you get a bad hum. Otherwise the amp is quiet.

I've tried moving the grounds around. Isolating the tank rca connectors from the chassis, moving wires around, changing tubes, added shielded wire throughout the reverb ckt. No help. On a scope I see a very ugly spiked wave on the output of reverb recovery ckt. Ground it out and of course it quietens.

Still no luck with the tremolo ckt. Disconnected it from the ckt and set the bias to around 54mv. I can see a sine wave on the output of the ckt but the level is very low on the output side of the .1 cap.

Rut

[jjman]

I'm thinking the bias adjuster tap should be on the diode/filter/outer trem lug junction instead of the wiper. Having it on the wiper allows the trem signal an easier path to ground thru the bias control (bad.)

I agree on disconneting the trem for now. Just remove the cap-to-pot connection on the trem side of the trem pot.

[rutledj]

I'm thinking the bias adjuster tap should be on the diode/filter/outer trem lug junction instead of the wiper.

You are correct. I actually have it that way but didn't draw it correctly. I have updated the poweramp section.

Any ideas on the reverb problem?

[rutledj]

OK. I found the problem with the reverb ckt. Missing a ground on the 2nd B+ cap.

I don't know how to go about troubleshooting the oscillator ckt. Not sure what signals I should see where.

Rut

[Andy Le Blanc]

is that the filter section that supplies the screen grids of the power tubes? ....

[rutledj]

Yes. I guess it affects the reverb also since the xformer connects there also.

I have double-checked, even triple checked the wiring for the tremolo ckt and can't see any errors. Voltages seem in line with the schematic. Changed tubes several times. With the intensity pot disconnected from the ckt I would expect to see some type of oscillation going on but not sure what exactly. Should this be a nice even (proportional) sine wave?


Rut

[rutledj]

dpost. ignore

[Andy Le Blanc]

you should be able to get an analog meter .... one with a needle.... to tick
at the circuit output..... you can inject its out put in to another amp if you
dont have a scope...... theyer really gain dependant..... and if theres a issue
with the caps or connection in the phase shift network it wont go.....
double check the chathod resistor and bypass cap of the occilator tube for faults....

[Andy Le Blanc]

is that .02 cap going from the plate to the chathod in the actual circuit...
or a typo in your schematic?....

[Andy Le Blanc]

found an example in a vibroverb...... its taken to ground in a vibrolux
but omitted in most other fender scheme....

[rutledj]

s that .02 cap going from the plate to the chathod in the actual circuit...

Yes. It is in the 6GlC vibroverb schematic. I do have a scope. With I could post the wave I'm seeing in the ckt. It is not a pure sinewave. It's like the amplitude of every other wave is twice as high as the other. Varying Speed know doesn't seem to do anything.

Rut