I luckily picked up one of these at the local Ham Radio swap meet. I'm going to turn it into a guitar amp / experimentation platform, to try out some ideas I've had. It will not be a "get a fender schematic and go for it" type build. An additional bit of luck, the amp came up on the Variac with no red plating of the 7868 output tubes and connecting a speaker, I've confirmed a signal makes it through.
Here's what the amp looks like with the cover off;
I gutted the whole thing, leaving only the output stage intact. Bought two new sockets, salvaged one from the mic transformer position. I was hoping to use at least some of the original circuits, but after wrestling several crimped wire leads off socket pins, I decided on a clean sweep.
Even testing my filament connections - smoke! How could this be? Turns out one wire at the upper left corner of the black socket possibly was touching a ground tab provided by that socket. Dont know why that would short circuit the heater winding...looking at the terminals where the PT heater winding attaches, neither are grounded. I ended up replacing the TWP from the power tube sockets and now all 5 tubes light. The original TWP was in bad shape; the short may have been there in a breached jacket in the first place. Nice way to beat up my PT, right out of the chute. Hopefully, this incident isnt foretelling of how things are going to go with my project.
I'm going to start completely **s backward, by making the phase splitter be the very first tube. Dual pots for all controls isnt economical, but this isnt a production design. From what I'm reading, using a 12AX7 as this first tube, LTP with a current sink, I need a small negative voltage somehow at a small current. I realize I cant steal it from the existing -20V output tube bias supply, because, there's current and that would make that voltage not be -20 anymore.
Hopefully, my next post will show a solution and the first stage built from input jack to two, 180 deg out of phase signals, coming from a dual pot which I'd label "Pre". Thanks for watching, be sure to click "subscribe" ;')
Here's what the amp looks like with the cover off;
I gutted the whole thing, leaving only the output stage intact. Bought two new sockets, salvaged one from the mic transformer position. I was hoping to use at least some of the original circuits, but after wrestling several crimped wire leads off socket pins, I decided on a clean sweep.
Even testing my filament connections - smoke! How could this be? Turns out one wire at the upper left corner of the black socket possibly was touching a ground tab provided by that socket. Dont know why that would short circuit the heater winding...looking at the terminals where the PT heater winding attaches, neither are grounded. I ended up replacing the TWP from the power tube sockets and now all 5 tubes light. The original TWP was in bad shape; the short may have been there in a breached jacket in the first place. Nice way to beat up my PT, right out of the chute. Hopefully, this incident isnt foretelling of how things are going to go with my project.
I'm going to start completely **s backward, by making the phase splitter be the very first tube. Dual pots for all controls isnt economical, but this isnt a production design. From what I'm reading, using a 12AX7 as this first tube, LTP with a current sink, I need a small negative voltage somehow at a small current. I realize I cant steal it from the existing -20V output tube bias supply, because, there's current and that would make that voltage not be -20 anymore.
Hopefully, my next post will show a solution and the first stage built from input jack to two, 180 deg out of phase signals, coming from a dual pot which I'd label "Pre". Thanks for watching, be sure to click "subscribe" ;')
Here's a schematic of what's left after the gut. Mozz is right - voltage doubler B+. I have 3 empty 9 pin sockets besides, with the potential for mounting up to 3 more - probably more tubes than the heater winding can handle.
There's why the heater winding didnt like one lead coming into contact with ground...
I'm taking a stab at the small negative supply today, we'll see if it works out. I've bought some octal sockets, in case one day I want to give up on the 7868s and their wire pins. The final amp will be a mix of purchased parts and junk I happen to have lying around.
That free (if I disconnect from ground) 70V winding on the OPT looks interesting; I'll see if I can make use of it somehow in a clever way.
I got rid of the 15 Ohm resistors going from OPT primary to the tube plates, if only because I've never seen that before and they were in my way. Hope the output stage survives without them. I can always replace them again - still have the parts.
There's why the heater winding didnt like one lead coming into contact with ground...
I'm taking a stab at the small negative supply today, we'll see if it works out. I've bought some octal sockets, in case one day I want to give up on the 7868s and their wire pins. The final amp will be a mix of purchased parts and junk I happen to have lying around.
That free (if I disconnect from ground) 70V winding on the OPT looks interesting; I'll see if I can make use of it somehow in a clever way.
I got rid of the 15 Ohm resistors going from OPT primary to the tube plates, if only because I've never seen that before and they were in my way. Hope the output stage survives without them. I can always replace them again - still have the parts.
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Last bogen i converted, had 8417 outputs and ran 650v B+. I ditched the doubler and ran some Russian 6l6gt at about 330v. Kind of unknown specs on those tubes. They are not a 6L6GC. The output trans was shorted and smoked. I put in a 6v6 plexi board from Hoffmans with fixed bias. I see yours has the cap off the high voltage to get bias, i don't like those, rather have a resistor but no center tap. Those input transformers may have some use as a mic preamp, i put them on the side for another rainy day project.
Yep, I have those 3 set aside - even got one more with it, in the form of a small metal box with an RCA plug and a 3 position screw terminal strip. Sometimes people buy such things on ebay, for a moving coil phono cartridge step-up. I guess only certain ones work; sold a pair one time - I think DuKane - that had really low Z taps in addition to the ordinary mic windings.
I got the "-5V" supply in place. Didnt go as I was hoping - to generate it from a small 600 - 600 ohm mic tranny connected to the 6.3 heater winding to ensure that circuit floats, but just couldnt get enough peak for the L05, 5V regulator that I cut out of a saved board.
So I installed the small 200 mA AC power tranny from the saved device onto the chassis and, with 12 VAC going into the regulator board, I get 5V out - should be good for 100 mA worth of constant current sinks that need a little voltage compliance below ground to work.
Maybe better than a wall bug plugged into the accessory outlet on the back of the chassis...
Next!
So I installed the small 200 mA AC power tranny from the saved device onto the chassis and, with 12 VAC going into the regulator board, I get 5V out - should be good for 100 mA worth of constant current sinks that need a little voltage compliance below ground to work.
Maybe better than a wall bug plugged into the accessory outlet on the back of the chassis...
Next!
I figure I'll use the 7868s until they burn - accidents happen. I did buy some octal sockets, anticipating an eventual replacement with something "else". I only have a 6550 pair and some Russian 6V6s on hand at the moment. Running two 6550s doubles the heater current for the output stage - no idea if the PT can take that.
Most of what I want to do is in the preamp part of the amp. I've got the 1st 12AX7 LTP stage built and connected to the dual gang "Pre" control; been moving signal generator and oscilloscope onto the bench to see what the through signal looks like. Maybe tonight.
The other two nine pin sockets dont have anything connected besides the heater, so the B+ string lacks their loading and is higher than what it would be with the other tubes pulling current. So I'm not even in the ballpark of changing anything from my initial guess for component values. Maybe I'll find the two outputs from the first stage arent very symmetrical voltage wise...
Most of what I want to do is in the preamp part of the amp. I've got the 1st 12AX7 LTP stage built and connected to the dual gang "Pre" control; been moving signal generator and oscilloscope onto the bench to see what the through signal looks like. Maybe tonight.
The other two nine pin sockets dont have anything connected besides the heater, so the B+ string lacks their loading and is higher than what it would be with the other tubes pulling current. So I'm not even in the ballpark of changing anything from my initial guess for component values. Maybe I'll find the two outputs from the first stage arent very symmetrical voltage wise...
Voltage wise, with 1V RMS at 1kHz going into the TR input, I'm getting 19.75 VRMS and 20.5 VRMS out of the 2 pot wipers, cranked all the way up. With my Fluke 123 scope doing the measurement, still showing a sine wave. Next I'll see what voltage overdrives that first stage...should have thought of that last night!!
Glad you're enjoying it - I will further explain and the possibilities I believe it opens up. Hopefully with scope'd signal screen shots, harmonic profiles and what's happening stage by stage.
The NOS yet leaky RIFA X caps really threw me for an hour or so, waveforms not being at all what I was expecting. With that fixed at least temporarily until I get the better condition NOS components in hand, I dare say it works and is doing what I'd hoped for by this architecture.
Hopefully the old beast hangs in there for me hardware wise, while I get it tuned up. All the parts are just tacked at this point; no need to cinch component wires to the socket tabs when you just may be changing that plate resistor "tomorrow". Or trying a current source instead of a common cathode resistor.
I also need a real guitar speaker / cab, an instance of which I blissfully sold last year figuring I'll not be needing it any more; my electric days are done. Wrong and wishing I still had it to play this amp through today.
Something I figured out is this Bogen's power supply crashes 70V (from 460 to 390) going from idle to full power on a resistor load. I dont mind the compression that might give on the output stage, but the droop of the B+ follows to my signal tubes causes the diff pairs to go out of balance.
That's right, at just over 300V B+ both tube sections amplify pretty much the same, Crash that to 230, now one side of a stage amplifies more than the other.
Part of the design is the assumption that both identically built inverting and non inverting signal chains operate identically. Until I tell them not to via some control or signal, versus a coincidence with something else.
The next issue I've found is that the dual pots I purchased dont track very well, causing discrepancy in amplitude in the middle of their rotation, while at the "end" or wide open, the + and - amplitudes of the gain stages are pretty close.
The old 40 uf 350V caps in the voltage double circuit both measure better than the value (47) unless I'm getting tricked by doing that in circuit. If I replace them with fresh and larger value caps, intuitively that would lower the output impedance of the voltage doubler. Is this true?
That kind of droop is not uncommon. It may be better with fresh caps, but that voltage doubler does produce high peak secondary currents so some droop has to be expected. Whatever happens, the circuit has to be designed to work with it. Might take a regulated 300V for the front end. For ahi-fi application, I will always do that. That phase splitter stays in balance.
Any pointer to such a regulator? I've built the LM317 / pass fet version 30 years ago, but I'm sure there's better ways now to skin that cat.
I picked out a pair of also 30 year old 100 / 450s last night, but they're the wrong form factor. I'll try them tacked across the existing caps, realizing that'll put even more peak current on the PT secondary. Just to see if any pos effect, if so then I can order axial psrts.
But I think the regulator is the way to go, to isolate the preamp part from the power amp's current draw effects.