New AC4 build trem and 1 tube reverb not working

[nateflanigan]

Hello,
I just finished up stuffing an AC4 into one of the solid state vox cambridge carcasses.  The amp works and sounds quite nice but the trem doesn't work.  Additionally, I added the one tube reverb circuit, that is not working either.

First things first...
I'm using the schematic and layouts from www.thevoxac30guide.com (which is a nice book BTW)
but here's the original...
http://www.el34world.com/charts/Schematics/files/Vox/ac4.pdf

The thing that's throwing me off right now is that on the layouts I have, on V2 pin 6 is connected to pin 2 which then goes to C6.  Perhaps this picture will help.


Where as on the schematic C6 is connected directly to pin 6 and R9 and it appears that pin 2 is unused.  What do you make of that?

Secondly, the verb.  I inserted the circuit between the volume pot and the el84.  It's on a dpdt switch so I can take it out of the circuit.  The dwell pot does nothing, with the reverb pot all the way up the signal gets muffled, that's about all that's observable.  I've got about 230v on the plates and 1.5v on the cathodes.  The B+ is pretty low coming off the rectifier, so I don't know what kind of options I have.  Can the one tube reverb work in  this amp?

Let me know if there's anything I can clarify.

[sluckey]

The thing that's throwing me off right now is that on the layouts I have, on V2 pin 6 is connected to pin 2 which then goes to C6.  Perhaps this picture will help...
Where as on the schematic C6 is connected directly to pin 6 and R9 and it appears that pin 2 is unused.  What do you make of that?

It's the same thing. Pins 6,7,8 are the trem oscillator. The signal from the plate (pin 6) is directly coupled to the grid (pin 2) of a cathode follower. Pins 1,2,3 are the cathode follower. That schematic shows a connection between pin 6 and 2 but the vertical part of the line did not scan well. It's there though.

I'm not sure how well the one tube reverb will work in that amp. Can you tell us exactly how you have it inserted into the stock circuit? A schematic or entire layout would be very useful.


EDIT... added pic.

[tubenit]

Yeah, I am not sure a one tube reverb would work either?

I'd probably opt for a passive effects loop and insert a reverb effects pedal there.

with respect, Tubenit

[nateflanigan]

Hmmmm, thanks for explaining that.  I was hoping I would get off easy on the trem not working.  I'll draw up something that shows the reverb implementation this weekend.  Right now I'm thinking it'd be cool to pull the bells and whistles out and make a second schedule 40 channel.  I can save the reverb parts for that stand alone build. 

[sluckey]

The tremolo should be fairly easy to troubleshoot. I'd start by double checking the wiring for V2 and also be sure that 100K resistor is connected between V2-3 and V1-3. Measure all voltages on V2 and compare to schematic. Post them here too. V2-7 is not listed on the schematic but it should be zero. The footswitch contacts needs to be open for the trem to work. Take two sets of V2 voltages, one with the F/S open, and another with it closed. With the switch closed the trem will be defeated and all V2 voltages will be steady. With the switch open the trem will be enabled and the voltages on V2 pins 6, 2, and 3, should be fluctuating at the trem speed. The fluctuations will just appear to be erratic readings on a DVM, but you can actually see the smooth changes on an analog meter. Fluctuating voltages means the trem oscillator is working.

Do some checking, post vour voltage readings,  and let us know what you find.
 

[nateflanigan]

be sure that 100K resistor is connected between V2-3 and V1-3

The layout (and my amp) has the 100k (R8) going from V2-3 to V1-8 which is then connected to V1-3



V2 voltages, the voltages are the same regardless of the switch being open or closed.

1 232
2 148.3
3 148.3
6 146.7
7 0.8 mv
8 1.13

I checked over my wiring again, couldn't see any errors.  I'll post some pictures tonight, I gotta go to work today.

[sluckey]

The layout (and my amp) has the 100k (R8) going from V2-3 to V1-8 which is then connected to V1-3

That's fine.

V2 voltages, the voltages are the same regardless of the switch being open or closed.

That means the oscillator is not working. Concentrate on the wiring for V2-6,7, and 8 and associated components.

Any chance you could scan the entire layout rather than just bits and pieces?

[nateflanigan]

Ya know, I was thinking about that and I feel like it's a little unethical. The schematic is public domain I guess but the guy that made that book spent time and effort redrawing everything and cleaning it up. What do you think?

[sluckey]

I think it would help get your tremolo fixed.

[sluckey]

Have any luck?

[nateflanigan]

No, thanks for asking though.

Here's some pics...









And the layout...



Let me know if you can think of anything else to check or if anything jumps out at you as a problem. 

In other news I ditched the reverb and added a schedule 40 channel, right now the channel switching is sort of unelegant.  I was thinking I'd set up some mixing resistors like on the AB763.  I really like this amp a lot, especially with the two channels.

[nateflanigan]

Man I just can't find it.  I've been over the layout and schematic 100 times.  The only thing I can find that I did differently is the B+ goes from the first filter cap to the power tube, then V2 then over to V1 as opposed to splitting after the power tube and going to the preamp tubes.  I guess I would describe it as series rather than parallel.  I really don't know if that would keep V2 from oscillating or not.

[sluckey]

I still think it's a wiring error although I can't see it in the pics.

[kagliostro]

Have you completely swapped tubes with other that you are sure are OK ?

Did you checked the layout with the old schematic ?

have you enough current for the added tube ?

have you checked resistors and caps to be OK ?

Kagliostro

[nateflanigan]

yeah, it's gotta be something.  I went over it again against the original schematic, nothing new.  I've tried a few 12ax7s, I pulled the schedule 40 tube, parts are checking out fine with my meter and identification codes.  The only thing I can think is to try a more literal take on the schematic.