25W Kodak projector to Sunn guitar amp

[Jerry garrcia]

I’ve got this untouched 25W Kodak Pageant projector amp. 12ax7, 6an8 and 2x6L6’s. Made to function in Europe so a 220->110V pound step down transformer already mounted in the cabinet.
So should be quite easy to rebuild to a Sunn guitar amp. A tight chassis and a PCB board so thinking of a rebuild PTP and change all electrolytics and add an adjustable bias circuit.
So to my questions.
1) what’s with the second half of the 12ax7? Local NFB? No Rk? Keep it that way or just do it the regular way and add a gain pot between them with Rp 100K and Rk 1.5K?
2) keep the tone stack as it is or change to something else (if so, to what)? The face plate and chassis has a single tone knob and two volume pots.
3) for adjustable fixed bias, the JCM900 type?
4) since a tight chassis it would be great if the can cap lugs could be used as “tag strips/connection points. Is that possible? Could the can caps be made non functional so this could work?
Any suggestions appreciated

[ShoemanGB]

Start simple.  Cobble a fender guitar input to pin 2 of the 12ax7.  See how you like it.  The rest of the circuit is pretty much already a guitar amp, or close enough.  If after that experiment you need more, the sky's the limit.  Many of us here have done similar conversions to various things.  Best advice from the pro's was don't reinvent it right off the block.

[sluckey]

If this was my project I would simply rewire the 12AX7 to a Sunn preamp using the attached schematic. Leave the power supply and power amp alone.

[Jerry garrcia]

If this was my project I would simply rewire the 12AX7 to a Sunn preamp using the attached schematic. Leave the power supply and power amp alone.

Thank you. Was hoping for your reply since you are supposed to be the Sunn god 🙂. That was my initial idea but just needed a kick in the butt.
Already ripped out the PCB, might be able to put it in, otherwise I’ll just go with a “regular” power supply and add the Sunn preamp and the rest of the circuit. Unfortunately there is only one tone control in the chassis/face plate but I think the one you suggested is probably far better. Looks or sounds 🤔. There is kind of a strange filament winding in the original schematic so will have to think if I should use a known one and an adjustable bias also. A lot to consider

[sluckey]

Look closely at that odd power supply. It appears to be a voltage doubler, one that I've never seen before. And the filament circuit is a bit odd also. There is an adjustable bias circuit, although a range of -11v to -37v seems very odd for 6L6s. Gonna need a regular PT to go along with a regular power supply. And if you want to capture more of the Sunn vibe you'll need a U/L OT.

Gonna be interesting to watch you stuff all that back in that tiny chassis. I'll pull up a chair. 

[Jerry garrcia]

Look closely at that odd power supply. It appears to be a voltage doubler, one that I've never seen before. And the filament circuit is a bit odd also. There is an adjustable bias circuit, although a range of -11v to -37v seems very odd for 6L6s. Gonna need a regular PT to go along with a regular power supply. And if you want to capture more of the Sunn vibe you'll need a U/L OT.

Gonna be interesting to watch you stuff all that back in that tiny chassis. I'll pull up a chair. 

Yep. That power supply makes my brain hurt. Have a few PT’s that should be sufficient current handling but unclear yet on the secondary voltage.
Would be nice to use the one that is stock. Probably have to map it.
If anyone can explain how it functions and how to wire it for a HV secondary and a regular 6.3V filament winding I’ll truly be happy.

I think everything will fit if it’s possible to utilize the can cap lugs as connection points and do it PTP.
After modding a Filmosound 621 to guitar everything is spacious 🙂

[astronomicum]

Look closely at that odd power supply. It appears to be a voltage doubler, one that I've never seen before.

Very interesting. 50% more parts than the Delon circuit so the questions is, why use it  Going to breadboard it and see how the scope trace for each compare. I would speculate with more parts, smoother ripple maybe

[PRR]

Look closely at that odd power supply. It appears to be a voltage doubler, one that I've never seen before.

Very interesting. 50% more parts than the Delon circuit

It is a (Cockcroft-Walton) tripler. Probably limited to 600V diodes that year.

Stuffing a Sunn-gain amplifier in that small chassis is begging for oscillation. Obviously Kodak did it (the opto-sound starts weak) but they had a lot of experience on staff and on rent.

[sluckey]

If anyone can explain how it functions and how to wire it for a HV secondary and a regular 6.3V filament winding I’ll truly be happy.

First tell us how much voltage is between terminals J and K. Filaments are easy. Just connect the 12AX7 across terminals M and N. The other tubes stay connected as shown in the schematic.

...if it’s possible to utilize the can cap lugs as connection points

I would not do that!

[Jerry garrcia]

If anyone can explain how it functions and how to wire it for a HV secondary and a regular 6.3V filament winding I’ll truly be happy.

First tell us how much voltage is between terminals J and K. Filaments are easy. Just connect the 12AX7 across terminals M and N. The other tubes stay connected as shown in the schematic.
Will do and report back.

...if it’s possible to utilize the can cap lugs as connection points

I would not do that!

Neither would I but it would have been nice if it would work. Should save a lot of space. Gutted it a few hours ago (paper and wax board with some parts already removed by previous owner had pressed the soldering iron through the board so hard to trace it. The traces on the board is just solder between each connection points) and if I just get the pt to function as intended I’ll think it will all fit. Might cover the cap holes in the chassis with some sheet metal and place all large electrolytics on top like a doghouse. I’ll just have to locate J and K… the PT is removed by all wires cut long but no markings. I’ll bring out the variac tomorrow and hopefully I’ll be able to map the PT. Usually it’s quite easy once you locate the 6.3V winding.
Then read up on how a voltage doubler functions. I’m not familiar with that setup for a voltage doubler. All connected on one of the taps🤔.

[Jerry garrcia]

Look closely at that odd power supply. It appears to be a voltage doubler, one that I've never seen before.

Very interesting. 50% more parts than the Delon circuit

It is a (Cockcroft-Walton) tripler. Probably limited to 600V diodes that year.

Stuffing a Sunn-gain amplifier in that small chassis is begging for oscillation. Obviously Kodak did it (the opto-sound starts weak) but they had a lot of experience on staff and on rent.

Pressed wrong button so gave an unlike. Hope you wasn’t offended.
Thanks for the circuit. They were actually awarded the Nobel prize. Very interesting. Witt map the PT and try out this circuit 🙏🏽
One more question. With this kind of voltage tripler, how important is it that the capacitance is the same in all three involved caps, do they all have to be the same value and if so is 80 uF important or could I go with 47 or 100uF?

[sluckey]

It is a (Cockcroft-Walton) tripler.

The Kodak circuit is not the same as your tripler circuits.

[astronomicum]

It is a (Cockcroft-Walton) tripler.

The Kodak circuit is not the same as your tripler circuits.

Yes. Looks the same up to C3 where the Cockcroft-Walton circuit has the negative side of C3 attached the the positive side of C1, and the Kodak circuit, the negative side of C3 is attached to ground. Still working on how the operations would differ but I am thinking that the output of the Cockcroft-Walton version would be influenced by the charge on C1 where that would not be the case for the Kodak version.

[Jerry garrcia]

It is a (Cockcroft-Walton) tripler.

The Kodak circuit is not the same as your tripler circuits.

Yes. Looks the same up to C3 where the Cockcroft-Walton circuit has the negative side of C3 attached the the positive side of C1, and the Kodak circuit, the negative side of C3 is attached to ground. Still working on how the operations would differ but I am thinking that the output of the Cockcroft-Walton version would be influenced by the charge on C1 where that would not be the case for the Kodak version.

Does the capacitors need to have the same capacitance though out the triplet circuit? If so must they be 80uF or would 100uF work as well? Hard to find 80uF caps. And have a load of 100uF, 450V in a drawer somewhere

[sluckey]

That info is on your schematic. I would use a 100µF rather than an 80µF.

[Jerry garrcia]

If anyone can explain how it functions and how to wire it for a HV secondary and a regular 6.3V filament winding I’ll truly be happy.

First tell us how much voltage is between terminals J and K. Filaments are easy. Just connect the 12AX7 across terminals M and N. The other tubes stay connected as shown in the schematic.

...if it’s possible to utilize the can cap lugs as connection points

I would not do that!

Between J and K is 110VAC when injecting 110V.
Between H and N is 18.4V
Between L and M is 6.45V
Between N and M is 6.5V

My cap question was regarding the voltage tripler. Been trying to read up on the circuit addressed by @PRR but haven’t found any info on how the cap value affects the voltage tripler circuit. The three caps are 80uF in the schematic but what I was wondering about was if I could utilize 100uF or 47uF in the voltage triple circuit instead? I can understand if my knowledge in understanding schematics is questionable 🙂 but usually I’ve got a pretty good understanding of a regular power supply and filter caps. In this case most are 30uF but this voltage doubling thing is hard for me. That’s why the question. Also not used to DC filament conversion.

A good news is that I will only use one input so will have the possibility to include a Sunn tone stack.

[shooter]

the capacitance, 47, 80, 100 is way less important than the rated VDC of the cap.  That should be your guiding number.
do you know what the original "output" of the Tripler is/was? 400vdc, 600vdc....
you want your caps rated voltage greater than the "unloaded" (no tube) value the PS will put out.


one of my amps had a B+ of 440vdc, but for 30 seconds before the tubes warmed up, the caps "saw" 520vdc

[sluckey]

The three caps are 80uF in the schematic but what I was wondering about was if I could utilize 100uF or 47uF in the voltage triple circuit instead?

Did you not read what I said?

[PRR]

It is a (Cockcroft-Walton) tripler.

The Kodak circuit is not the same as your tripler circuits.

It is equivalent. The classic form was made to stack to infinity. At 3X it makes sense to bring caps back to zero.

[Jerry garrcia]

The three caps are 80uF in the schematic but what I was wondering about was if I could utilize 100uF or 47uF in the voltage triple circuit instead?

Did you not read what I said?

Now I did. Probably a language barrier. Sorry if I offended you. This whole alternative PS made me a bit out of focus. But did finally map my transformer. Unfortunately no ul tap in this one. Have a pair of hifi OT’s that has an ul tap but was planning on using them for a HIFI build. The OT has an impedance ratio of about 1000:1 so a 4 ohm speaker should suit it perfect. I think I have one 10” somewhere

[Jerry garrcia]

It is a (Cockcroft-Walton) tripler.

The Kodak circuit is not the same as your tripler circuits.

It is equivalent. The classic form was made to stack to infinity. At 3X it makes sense to bring caps back to zero.

Last question? With this voltage tripler, will it still be able to deliver the needed current? Read somewhere that with the increase in voltage through such a circuit the current delivery capacity decreases a lot. Think the HV winding is 710mA so it has some capacity. Would a choke at the input before the first filter cap make it handle the current demand better?

[PRR]

710mA divided by 3 is 236mA, plenty for a couple 6L6. (It is very unlikey to suck over 180mA because plate dissipation; drivers and preamps are a dozen mA at most. The Kodak a bit more because sound-lamp.

It is not that simple, but also: it powered two 6L6 for Kodak. If you haven't damaged anything when you "Gutted it", it should work as good for you as for Kodak.

[Jerry garrcia]

It is not that simple, but also: it powered two 6L6 for Kodak. If you haven't damaged anything when you "Gutted it", it should work as good for you as for Kodak.

Thank you. I was gentle with the amp and projector during the gutting procedure. Almost as gentle as Macchiarini was when doing the procedures in my previous work place. Now I’m having a different employer.
But over all the components were removed in a “sensitive” fashion. I will “bread board it in in a Princeton reverb build with the HV tap in the build utilizing the same spec as the Kodak. And with 4 12a*7 tubes and 2x6v6’s and a 80-100 ohms resistor and 450 V caps I’m hoping for success. That pt has a 110V (710mA) secondary with a 230V primary, including 4A 6.3V tap. A good experiment circuit since it’s a “give away” amp. 450v caps at 22+47+47+100uF. For the PSU 100+47+22+22uF at 459V. Might have to include a varistor if the sag resistor doesn’t bring down the start up B+ to <450V.  A lot of complaints about using a voltage tripler. But what could one do with 110V secondary