Victor model 40BX to Ampeg J-12B conversion project

[filmdiary]

Hi,

Working on a conversion project and was wondering if anyone has made a hybrid of a M12 and a J12B? I've got a 1940's Victor projector amp with a 5U4 rectifier and 6L6G push pull power section. It came with nice coke bottles which seem almost new so I'd love to change up the V1 6J7 pre and V2 6n7 PI for a couple of 6SL7's. The J12B uses one triode of V1 as the input and the other as the trem oscillator so I think this will be a good solution as there are only 5 octal sockets on top of the chassis.

I'm looking at Steves redrawn J12B schematic and I've made a couple of changes that include the 6L6's and 5U4. Does anyone have advice about the biasing change (a 250 ohm 10 watt) and using 32uF 475V at the first node?

I'm pretty deep into it... power section is wired... volume and tone pots are wired... and I just drew up a layout to squeeze the modified trem circuit into the lower portion of the chassis by running shielded cable from pins 1,2 and 3 of V1 down to where there is (slightly) more room! Just waiting on some resistors before wiring up the rest.

Any thoughts about using the 6L6G's would be welcome!!! The PT is pretty monsterous so I think 3A of 5V wont be an issue despite the tube socket having a '5V4' stamp on it.

Thanks in advance!

John.

[sluckey]

Nice looking project. I'll be following with interest.

I'm looking at Steves redrawn J12B schematic and I've made a couple of changes that include the 6L6's and 5U4. Does anyone have advice about the biasing change (a 250 ohm 10 watt) and using 32uF 475V at the first node?

250Ω is the typical cathode resistor for 6L6s or 6V6s and will work just fine in your conversion. I chose to use 500Ω because I only have a 6" speaker in this radio conversion and don't really need much power. Also my PT is small and originally powered a pair of 6K6s with a 500Ω cathode resistor so I was trying to keep the current down.

32µF should be fine as long as the voltage rating is high enough to handle your B+.

You don't really need shielded wire for the tremolo since there is no guitar signal involved. But it's OK to use it. Just always keep in mind that the plate lead will have B+ on it.

Keep us posted... Steve

[filmdiary]

Thanks a million Steve! Very kind of you to post a reply :)...

I'm waiting on an order of resistor values that I did not have in stock but should have them by the weekend.

I figured I should just go with the specs for the M15 regarding the bias... the amp had a 200 ohm bias resistor so I dont think cooling it by 50 ohms will make too much difference. I'll check the PV on the 6L6's and if it's way high I'll drop it. The schematic for the original projector amp was full of inductors...

The OT in the amp sent a 100V signal through a polarized speaker cable to the cab. There was another transformer mounted on the speaker basket with taps for 3 impedances... assuming 4/8/16 ohms...

I took the speaker tranny and mounted it inside the chassis and installed a 1/4" jack... connected everything as it was. Not sure how that will go as the 2 OT's are side by each. Worst case (loads of hummmmm) I'll source one from the fine folks at Hammond. Curious to see what this configuration sounds like...

[filmdiary]

Okay...

Had to wait for some resistors but got them today and went at it... completed the wiring for the preamp and tremolo... started it up on a variac and had no issues... the old 5U4 pushes 380VDC of B+ and the 6L6G's have 370VDC on their plates... voltage drop across the 250 ohm brown devil is 29.63 which equates to 19 watts of dissipation... 100%! That's a first for me and I'm a bit shocked - a 1940's 16mm projector amp still rocking!!!

With the amp volume at 2 a Les Paul sounds warm and clean... at 5 it starts getting gnarly... at 10 with the tone control dimed it's Neil Young :)... super happy with this one!!

I have some filmosound knobs installed for the trem speed and intensity controls but will swap them out for something period correct :)... otherwise the switch, fuse, input volume and tone knobs, the transformers, the 5U4G, 6L6G's preamp shield are original to the amp... looks like a battleship :)...

I do have 2 issues though... the first is a low 120 cycle hum at idle that becomes unnoticeable with the volume at 2. With the volume turned all the way down it's there. The second is that with the trem switch off the LED is oscillating. But it shuts off when the switch is on... will double check the circuit wiring shortly. Not too concerned about it as it's a joy to play even through my little bench speaker! Cant wait to hook it up the the original Victor speaker cab!

[filmdiary]

I just tried elevating the 6.3V center tap to the cathode of the output but it made no difference so I soldered it back in place. My hunch is that the 2 output transformers in that configuration are causing something... I'll unscrew them tomorrow and see if moving them around / out of the chassis effects the level.

I've got 143 VDC on the plate of the 6SL7 triode that is being used as the trem oscillator.... 1.64V on the cathode... and 0 everywhere else. Just noticed the green text on the J12B schematic and will check for AC voltage there tomorrow...

Love how raw the attack sounds on these old 6L6G amps!

[sluckey]

The second is that with the trem switch off the LED is oscillating. But it shuts off when the switch is on...

I think you misunderstand how the circuit works. When the switch contacts are OPEN the trem is enabled and the LED will flash. When the switch is CLOSED the trem is disabled and the LED will not flash.

[filmdiary]

Yep! I made two big errors… one was using a 2.2K rather than a 2.2Meg resistor across the speed pot to ground… the other was more basic than that but I figured it out.

The trem in the fast position is working but so far it’s not working in the slow position… looks like the led is flashing around 3Hz with the speed backed off all the way. When I flip the switch the led goes dim and does not oscillate… the intensity and speed pots have no effect… but they seem to work well from 3Hz to very fast :)… I'll see if there's something up with the switch.

I’ll post the modifications I made to the schematic (because I did not have some of the resistor values on hand) with voltages…

I also recorded some tone samples which I’ll post tomorrow… the brown tone from a Les Paul sounds really nice…

I’ve also recorded the hum at idle relative to the signal and I’m tempted to loosen the output transformers to see if their positioning is causing the hum…

[filmdiary]

Solved the trem issue...

Must have been tired when I was wiring it because it was a stupid error... slow and fast controls working perfectly. With the intensity cranked the speaker pumps into distortion but it's fine backed off a little. Added an on/off toggle to the chassis but might wire up a foot switch. I'm experimenting with variable negative feedback. Decided that I'll put it on a switch in case I want to take it out completely... starting with a 10k resistor into a 50k pot. Cleans things up (but I like the hair when it's off :))...

Posting changes to Sluckey's AMPEG J-12B JET schematic that I used as a guide. THANK YOU STEVE FOR POSTING THESE AMAZING RESOURCES!!!

Also posting the ugly... jammed the tremolo into the bottom of the chassis and it's definitely not the prettiest looking thing... fact is, it works and sounds great for an old 1940's 16mm projector conversion! It's a lot louder than the RCA 201 that I put together so I'll probably open that one up and make some changes! Quite a versatile amp and I'm happy that the Victor speaker cab sounds great with it... pretty boxy and reedy but it's the perfect vibe for the amp!

The PI plate voltage is way higher than Sluckeys and I'm not sure what effect this will have on the performance of the 6SL7. I have a stack of 6SN7's that I might sub in to see if there's much of a tonal difference. Curious what you all think.