How to reduce the number of tube sockets needed?

[proaudioguy]

Is it possible to drive the reverb on an AB763 with 1/2 of a 12ax7/ 12at7 (or some alternative that fits)?


If yes, what can the other half be used for?  Can it be used for the reverb return or the mixer?  If I used Sluckey’s tremonator, which reduces the trem to 1/2 a tube, can that oscillator be on the same tube as the mixer, or the reverb return or the reverb send assuming it can be reduced to half a tube?


Trying to cram a Deluxe Reverb 1 channel into a smaller chassis with only enough room for 4 of the little tubes.  I don’t see a way to eliminate the mixer without affecting the tone too much.  Not sure about the reverb though.  I understand the drive uses the entire tube for impedance reasons?  Is there a work around to that?

[sluckey]

Use the one tube reverb circuit but reverb effect will not be strong. Then you have a whole 'nother tube for tremolo.

Better, build a princeton reverb but use deluxe reverb iron.

[proaudioguy]

Better, build a princeton reverb but use deluxe reverb iron.

I’m trying to preserve the long tail.  I think it has an affect on the sound.  I’ve never built 2 otherwise identical amps to compare though.


So I need 1/2 for the input, 1/2 for the post tone,  1 for the reverb driver, 1/2 for the return, 1/2 for the mixer, 1/2 for the sluckey trem, 1 for the LTPI.  If I could drive the reverb with 1/2 then that would fit the 4 holes in the princeton.


I meant to ask, does your tremonator cut the gain down like the bug does?  One thing I like about the bias vari is the amp sounds a lot more raw.  I tried the bug and it killed the tone.  Sounded like every other ab763.  Not bad, but not nearly as good.


Is the vactrol thing the same thing in my Superchamp, DRIl, and Concert?  Where do you buy those?

[mresistor]

I fail to see how any ldr is going to decrease gain in an amp, and you don't specify where you think gain would be decreased..   


For sale here   and there are many other places  easy to find..   https://www.tubesandmore.com/products/optocoupler-vtl5c1-vactrol-channel-switching


I definitely also think the long tail pair PI sounds different than the PR cathodyne PI ..  I have built a Hoffman PR with a DR OT and an upgraded PT and it doesn't sound like any DR I own or have serviced.


It might sound like a tweed deluxe though.

[sluckey]

I meant to ask, does your tremonator cut the gain down like the bug does?

Short answer, yes. But, it's not the bug that cuts the gain. And it's not the Vactrol in my circuit that cuts the gain.  It's the 50K INTENSITY pot that cuts the gain.

If you want to keep the DR sound (and reverb is a big part of that sound), I think your best path would be A single channel DR with real Fender reverb circuit but no tremolo. This will keep all that DR vibe. Buy a tremolo pedal. There are plenty that sound way better and are more versatile than any tube tremolo.

[Bieworm]

Easy.. make it a bias vary trem with a mosfet irf840. Tone king imperial uses that for trem and it's kickass good

[sluckey]

Easy.. make it a bias vary trem with a mosfet irf840. Tone king imperial uses that for trem and it's kickass good

Perfect solution!   

Here's Hoffman's single channel AB763 with the Tone King tremolo circuit...

[Leevi]

Easy.. make it a bias vary trem with a mosfet irf840. Tone king imperial uses that for trem and it's kickass good

Mosfet LND150 works perfect as well and is much cheaper.

/Leevi

[Joel]

Here's Hoffman's single channel AB763 with the Tone King tremolo circuit...

Looking at this, are all those caps necessary at the oscillator output?  Could you drop one of the caps I've circled in the attached pic?

Mosfet LND150 works perfect as well and is much cheaper.

/Leevi

Dropping the LND150 into this circuit?  Or a different circuit?

[Leevi]

I think I built my Mosfet oscillator according to the circuit you (Joel) shared in the "More Tremolo" thread.
/Leevi

[sluckey]

Looking at this, are all those caps necessary at the oscillator output?

No. You can eliminate C30. I should have done that when I spliced the two schematics together.

[d95err]

Easy.. make it a bias vary trem with a mosfet irf840. Tone king imperial uses that for trem and it's kickass good

Mosfet LND150 works perfect as well and is much cheaper.

/Leevi

You could also use an LND150 for the reverb return stage, to save yet another half tube.

It would probably also reduce noise and hum for the reverb.

[Leevi]

LND150 as a first gain stage.


http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm


/Leevi

[2deaf]

The Tone King tremolo is interesting in that it adds bias as the tremolo is turned up, thereby giving the tremolo more room to operate without flattening out the positive side of the signal.  I designed one with a similar idea, but never tested it in an actual amplifier.

 

[2deaf]

Here is an LND150 bias vary tremolo that works in an actual amplifier.

 

[Joel]

I tried this out today after reading this;  http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm and studying the various circuits that have been shared so far.

At first I tried a single LND150, but the Trem effect is a bit stronger with 2 as shown in the schematic.  The LND150's and 12V zeners are soldered directly onto the tube socket.  Fortunately the Source, Gate, and Drain line up perfectly with pins 1, 2, 3, and 6, 7, 8!

All in all pretty similar to the tube trem, but a bit stronger and comes on as the amp is powering up.
 

[Joel]

New development.  I turned the amp on this morning and there was a "galloping" kind of thumping in time with the trem (that wasn't there yesterday...).  A double kind of tick/thump sound.  Not affected by volume at all.  So I go probing around with the scope.  I can't find anything that would give that type of sound.  Then, as I go back over the trem circuit again, it disappears.  No trem - one or both of the LND150's blew. 

I did have a similar thump with the original valve trem that was removed by creating a new power node of much lower voltage.  I think I'll try powering the trem from the D node when I replace the LND150s so its running off 280V instead of 450V. 

Anyone have 2c to add?

[Leevi]

I think one reason for the blow could be the high voltage on the Gate on the second Mosfet. You have the zeners there but if they have broken then the Mosfet is gone. Would it work if you add a cap between Mosfets? Leak resistor is needed then as well.
/Leevi

[proaudioguy]

I fail to see how any ldr is going to decrease gain in an amp, and you don't specify where you think gain would be decreased..   


For sale here   and there are many other places  easy to find..   https://www.tubesandmore.com/products/optocoupler-vtl5c1-vactrol-channel-switching


I definitely also think the long tail pair PI sounds different than the PR cathodyne PI ..  I have built a Hoffman PR with a DR OT and an upgraded PT and it doesn't sound like any DR I own or have serviced.


It might sound like a tweed deluxe though.

I’m sorry for not being specific about the bug.  I don’t mean the bug itself is the cause but the circuit in general.


So what sounded better, the DR, the PR, or the PR with the DR transformer?

[Bieworm]

In my 'sorta' tone king imperial clone the trem doesn't thump. Not even a little bit. It runs like a charm!!

[Joel]

My Trem circuit once again has it's own power node - 315V. 

I took a picture of the waveforms I discovered within the circuit.  Would you expect to see spikes such as these in an oscillator circuit?  The red trace is at the junction of C27 and C28.  The yellow trace at the junction of C28 and the Gate of the LND150 at V4A. 

[Leevi]

Would you expect to see spikes such as these in an oscillator circuit?

Hard to say, might be realistic.
I propose another point for measurement. Add first a temporary cap to C31 in order to block DC and measure tremolo pulse there.


/Leevi

[sluckey]

I've never looked at those particular points. But the FET drain should have a healthy sinusoidal wave. Some of my tube oscillators have a near perfect sine wave. Others are a bit bent. But no spikes. Here's a pic of my Magnatone M2 oscillator plate...

[PRR]

> expect to see spikes

I think you have WAY too much gain. Disconnect one leg of the source cap C29, is that less over-excited?

[Joel]

Nope, wasn't a gain thing.  I removed the cap.  No difference.  I removed the second LND150 device.  Still thumping.  So I gutted the trem section and started from scratch and built an Imperial-ish circuit (attached).

The FDPF12N50NZ is very similar to the IRF840, and I had them in my parts stash.  I kept the 3M RA pot in the circuit which meant I had to reduce the 0.047uF to compensate and speed up the Trem a bit.  As built it's a bit over 2Hz to 7.5Hz.  Sounds different to the original tube one and the LND150 one.  It's nice.

[Bieworm]

Nope, wasn't a gain thing.  I removed the cap.  No difference.  I removed the second LND150 device.  Still thumping.  So I gutted the trem section and started from scratch and built an Imperial-ish circuit (attached).

The FDPF12N50NZ is very similar to the IRF840, and I had them in my parts stash.  I kept the 3M RA pot in the circuit which meant I had to reduce the 0.047uF to compensate and speed up the Trem a bit.  As built it's a bit over 2Hz to 7.5Hz.  Sounds different to the original tube one and the LND150 one.  It's nice.

Thumping gone?

[Joel]

No thumps.