Looking for cathode bias amp with only 3 preamp tubes & both reverb & tremolo?

[Redwood]

I'm restoring an old Japanese made Kingston amp that's been modified to add reverb, but in so doing it appears to have disabled the tremolo with which it was originally configured.


The amp has a pair of cathode biased 6AQ5 power tubes (sort of like 7-pin EL84s 6V6 in a smaller package) and three 12AX7's in the preamp. The original tone stack had Bass and Treble controls and the tremolo had Depth and Speed controls.  A prior owner or tech added a tiny reverb transformer and reverb pan and dedicated the Treble pot to its circuit so the Bass pot became in effect a single Tone control.  I have been unable to find an original schematic. I've created very poorly drawn hand drawn block schematics of various parts of the amp but haven't tied it all together yet.

The original Tremolo didn't work when I got the amp so I've been trying to figure out how to get it going but the problem is I'm having is I'm almost certain the original design used two triodes, one of which was robbed to power the Reverb driver. 

I've studied the single half tube tremolo design but it's based on modulating fixed bias of power tubes and this amp is cathode biased. The only designs for cathode biased amps I'm finding require two triodes.  I've tried several different routes of injecting the tremolo tube's plate output into either the grid or cathode of different preamp stages as well as the power tubes but nothing seems to get the oscillation going.  Actually in a couple configurations I have been able to get it going such that I can see it on the scope with 1kHz injected but it's so weak I can barely hear it with a clean sine wave and not at all with a guitar signal.

I suppose it's possible, or probable even, that the tremolo's plate voltage is insufficient (only 75-125Vdc after it passes through the Intensity pot) or various resistance levels are incorrect but other than replacing leaky coupling capacitors I haven't played around with values yet.

I'd just like to know if what I'm trying to accomplish is even feasible before wasting any more time.

[passaloutre]

Princeton reverb does reverb and trem in just about the most minimalist way I can think of and it takes 4 bottles to do it. You could use a “one tube reverb” design and free up a triode, but I’m guessing you already have that.

There’s a couple of high voltage MOSFETs you can use (LND150, IRF820) in various places when you run out of available triodes. People have had good luck using the IRF in cathode follower roles.

Or try an optical trem. Lots of options.

[SEL49]

Here's a single triode tremolo that will work very well with your cathode biased 6AQ5s...

     https://sluckeyamps.com/RCA/Ampeg_J12B.pdf

PS... The 6AQ5 is like a 6V6 in a smaller bottle.

[Redwood]

Thanks, that gives me at least another direction from which to approach the problem, though there are significant differences in my platform. I guess the other thing I should have mentioned is my phase inverter only uses a single triode with one power tube's grid fed from the plate and the other fed from the cathode of V3b.  I'm also working with much lower available B+ but there's only a single diode in my rectifier so I might be able to goose that a bit by installing a proper 4 diode bridge. 
All that having been said, when I tried to inject the oscillator's output into the power tube grids previously it didn't occur to me to put it in the middle of a voltage divider like that so I'll give it a try this weekend.
Also, point taken on the similarity to 6V6.  I'd read that of course but they still remind me more of EL84's not just because they look like them but this amp has a very British EL84 tone.

Here's a single triode tremolo that will work very well with your cathode biased 6AQ5s...

     https://sluckeyamps.com/RCA/Ampeg_J12B.pdf

PS... The 6AQ5 is like a 6V6 in a smaller bottle.

[SEL49]

All that having been said, when I tried to inject the oscillator's output into the power tube grids previously it didn't occur to me to put it in the middle of a voltage divider like that so I'll give it a try this weekend.

Those two 270Ks are not a voltage divider. They are the grid leak resistors and would usually be connected to ground. But when you disconnect the two resistors from ground and connect the resistors to the trem circuit, the trem voltage will modulate the grid bias and cause the trem effect.

Your amp will have two grid leak resistors most likely connected to ground. Your resistors may be some other high value but they function the same. Just disconnect the resistors from ground and reconnect to the oscillator just as seen in the Ampeg J12 schematic.

If you need more specific answers, then post an as-is schematic and maybe even some hi-rez pics.

[Redwood]

Thanks for clearing up my confusion on that point about the grid leak resistors and yes, you were right there were a pair of 500K in there that I lifted after I installed the 270Ks per the schematic provided above.


I did finally get the tremolo working but not until I substantially increased B+ to that stage.  It was still doing the same thing as before where it seemed like it was trying to get the oscillation going but couldn't maintain it, and the LED gave me more confirmation of that, so I was pretty stoked when I got enough DC to it and that LED started blinking rapidly.  I wish I'd tried that before completely modding the circuit to the Ampeg design because I now wonder if I had it mostly right and starving the plate of voltage was my problem all along but I'm not going to undo all that at this point.

I'm still not stoked with the overall sound of this amp but that's a whole different topic.  I suspect robbing the treble pot for the reverb left the bass pot ill configured to act as the sole tone control so I'll start looking at that next.

[mountainhick]

So sounds like cathodyne PI?

Fender princeton 6G2 has cathodyne PI, and same concept as the Sluckey..

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_6g2.pdf

[roarshock]

The Swart AST does what you're talking about with three preamp tubes. https://www.tdpri.com/threads/swart-ast.551853/ It does use a single tone control and a cap coupled reverb, but the idea is there.