Military projector amp - last edited 9/26/22 - layout help needed

[yello]

Whole schematic:



Preamp (oscillator deleted):



Output:



Pic of whole amp circuit:



Pic of turret board:

[yello]

PT:



Filter caps, replacing the original failed can type, built on tube bases per original spec:



220 ohm resistor, connected to CT via pin 2 of power tubes:



1000 uf 50v cap to replace the original 100uf 25v cap, per recommendation:



This is one filter cap section, common ground on pin 1, each 20uf 450v cap on pins 3, 5, 7. The unused pins used to have parts related to the projector circuitry, they were clipped off. The orange wire on pin 7 cap goes to r240, the 5w 360ohm resistor (3w 350 ohm original spec). The red wire on pin 5 goes to R228, the other 5w 360 ohm. The red wire going left from pin 3 goes to other end of r228. The red wire going right from pin 3 goes to rectifier tube. None of the wires are grounding out on the screws or chassis.

[yello]

v3 and v4 sockets:



v3 the problem tube:



v3 up close, the solder on pin 2 with the three orange wires looks fresh, though I didn't touch that. The entire circuit is lacquered to withstand humidity and corrosion in military zones, so the joints typically look brown not silver :



v4:



Output section of turret board, replaced c212 and c213 with .022uf caps in place of the stock .1uf. Replaced r220 and r221 while doing the caps. Removed R222 and C223 which was negative feedback. The v3 short problem was there before these mods, and old parts were all tested and within spec, no continuity, and new parts all test well, no continuity. v3 problems remains, and I think I have replaced all the parts that go directly to wires going to v3 and v4.

[JMPGuitars]

I thought you were referring to a preamp tube, it looks like you're talking about one of your power tubes?

If that's the case, go backwards for a minute. Put in only the rectifier and power tubes. If no short, then add the PI. If no short, then add the last preamp tube.

Thanks,
Josh

[yello]

I thought you were referring to a preamp tube, it looks like you're talking about one of your power tubes?

If that's the case, go backwards for a minute. Put in only the rectifier and power tubes. If no short, then add the PI. If no short, then add the last preamp tube.

Thanks,
Josh

Yes, it is the power tube, that is what I desired earlier in explaining going tube by tube to test for shorts with my light bulb limiter.

Preamp tubes: v1 and v2 are 12ax7's.
Power tubes: v3 and v4 are 6aq5's.
Rectifier tubes: v5 and v6 are 6x4's.

I had been following the directions from Rob Robinette, which was test with tubes, then rectifier tubes first. Then add tubes one by one starting with first preamp tube, one at a time, then remove that tube and go to next to test. There was no short in rectifiers, no short in first preamp tube, no short in second preamp tube. I put in one power tube in v3, and the light bulb limiter lit up bright. I tested 4 different tubes in v3 to make sure it wasn't a bad tube, bright every time.

The direction I followed are here:

https://robrobinette.com/Tube_Guitar_Am ... lows_Fuses

Mentioning it, as the order of testing is backwards from your suggestion, which was rectifiers in, and BOTH power tubes, then try v2, then v1. I'd love to learn the difference in suggested methods between what you are saying and what RobRob said.

[JMPGuitars]

I thought you were referring to a preamp tube, it looks like you're talking about one of your power tubes?

If that's the case, go backwards for a minute. Put in only the rectifier and power tubes. If no short, then add the PI. If no short, then add the last preamp tube.

Thanks,
Josh

Yes, it is the power tube, that is what I desired earlier in explaining going tube by tube to test for shorts with my light bulb limiter.

Preamp tubes: v1 and v2 are 12ax7's.
Power tubes: v3 and v4 are 6aq5's.
Rectifier tubes: v5 and v6 are 6x4's.

I had been following the directions from Rob Robinette, which was test with tubes, then rectifier tubes first. Then add tubes one by one starting with first preamp tube, one at a time, then remove that tube and go to next to test. There was no short in rectifiers, no short in first preamp tube, no short in second preamp tube. I put in one power tube in v3, and the light bulb limiter lit up bright. I tested 4 different tubes in v3 to make sure it wasn't a bad tube, bright every time.

The direction I followed are here:

https://robrobinette.com/Tube_Guitar_Am ... lows_Fuses

Mentioning it, as the order of testing is backwards from your suggestion, which was rectifiers in, and BOTH power tubes, then try v2, then v1. I'd love to learn the difference in suggested methods between what you are saying and what RobRob said.

It's just two different methods. Try it out and see what happens. Maybe won't make a difference, but worth a shot.

The confusion earlier was that I didn't know you removed your oscillator tube, which logically would have been V2 or V3, regardless power tubes would have been V4 and V5.

Thanks,
Josh

[yello]

It's just two different methods. Try it out and see what happens. Maybe won't make a difference, but worth a shot.

The confusion earlier was that I didn't know you removed your oscillator tube, which logically would have been V2 or V3, regardless power tubes would have been V4 and V5.

I see, I forgot about oscillator. It is marked v5 but I deleted it from the circuit. Rectifiers are technically v6 and v7 therefore on the schematic.

Regardless, I tried both methods (yours and RobRob's) and same result, I still have a short in the power tubes, I think the first one.

Not sure what to try next to figure it out. I cannot find any shorted caps, no wires out of place, no continuity where there shouldn't be, etc. I'm wondering if I need to replace the power tube sockets, or the filter socket that feeds the power tubes? On the pictures above I see some black residue on the solder tabs on the filter cap tube socket, which is a brown octal socket, on the tabs that go to the power tube circuit.

This fuse blowing problem started after I sprayed deoxit into all the tube sockets to try to clean them. The amp was working before that, albeit with a crackling sound thus my attempt to clean the tube sockets directly, beyond just dipping the tube pins in cleaner. In hindsight overkill and may have caused a shorting issue?

[JMPGuitars]

Go ahead and try replacing the power tube sockets. If the only change was you cleaning the sockets, then that could be the problem.

[zaphod_phil]

I'm starting to think that your deoxidizing spray has been shorting out some power tube socket pins, causing the fuse blows.
As for the "crackling and static" you described, your sound clip appears to be more a case of what I would call a "fizzy" distortion, possibly due to grid blocking. To counter this, I suggest you insert 100k grid-stopper resistors in series with pins 2 and 7 of your 12AX7, V202. Also, possibly a 47k in series with the grids of your power tubes (V203 & V204 pins 1/7), as well as reducing the 0.1uF coupling caps, C212 and C213, to 0.022uF.
for reference, see https://guitar.com/features/opinion-ana ... ar-amp/and

Hope that helps!

[yello]

Go ahead and try replacing the power tube sockets. If the only change was you cleaning the sockets, then that could be the problem.

Makes sense, seems obvious to me now but I had read beforehand the cleaner wasn't conductive. I wish I would have realized that sooner as I've been chasing it for a week and a half, but at least I am learning the circuit and techniques better, and have had a chance to do some mods I was intending anyway while in there.

I'm starting to think that your deoxidizing spray is shorting out some power tube socket pins at the high voltages you have there, causing the fuse blows.
As for the "crackling and static" you described, your sound clip appears to be more a case of what I would call a "fizzy" distortion, possibly due to grid blocking. To counter this, I suggest you insert 100k grid stopper resistors in series with pins 2 and 7 of the 12AX7, V202. Also, possibly a 47k in series with the grids of your power tubes (V203 & V204 pins 1/7), as well as reducing the 0.1uF coupling caps, C212 and C213, to 0.022uF.
for reference, see https://guitar.com/features/opinion-ana ... uitar-amp/

Thanks for the tips! I have already changed C212 and C213 to .022uf per your recommendation a few weeks ago, as well as the cathode bypass cap from 100uf to 1000uf.

I'll get to work on the grid stoppers while I sort through the tube socket issue. Are the 100k and 47k values spec'd per the stock 5.6k/560k preamp load and cathode resistors, or per your recommendation earlier to switch them to 1k/100k? Or fine either way?

[zaphod_phil]

This is a generic issue, unrelated to my earlier recommendation to switch the preamp cathode/anode resistors to 1k/100k, hence fine either way.

It appears that apart from the 5E3 Tweed Deluxe, some Hiwatt amps also suffered from this same issue - https://hiwatt.org/2_mod.html

[yello]

This is a generic issue, unrelated to my earlier recommendation to switch the cathode/anode resistors to 1k/100k, hence fine either way.

It appears that apart from the 5E3 Tweed Deluxe,some Hiwatt amps also suffered from this same issue - https://hiwatt.org/2_mod.html

I think I have lots of 1w 100k, but might be out of 1w 47k. Would a close value like 1w 56k metal film work? Or I have 2w 47k metal oxide.

[Bieworm]

This is a generic issue, unrelated to my earlier recommendation to switch the cathode/anode resistors to 1k/100k, hence fine either way.

It appears that apart from the 5E3 Tweed Deluxe,some Hiwatt amps also suffered from this same issue - https://hiwatt.org/2_mod.html

I think I have lots of 1w 100k, but might be out of 1w 47k. Would a close value like 1w 56k metal film work? Or I have 2w 47k metal oxide.

You can put 2 x 100k in parallel. That gives 50k theoretically.. close enough.

[zaphod_phil]

56k will work fine there. We just want to limit some current, so the exact Value isn't critical.While on this topic,

I've also noticed that most of the other experts recommend 470k on the 12AX7 grids. The 100k I had suggested, was just a random value, off the top of my head, so I think I might defer to the others for that one, but like I said it's not critical.

[yello]

Tube sockets arrived, so I changed v3, the first power tube socket. My light bulb limiter was lighting up brightly when a tube was in the socket (did not go bright with just rectifiers, or with either preamp tube).

Does that mean the short is in the v4 socket, the second power tube socket? That is the only socket left. (other than the two sockets that take all the filter caps, though I would have thought if the short was there that it would be noticed in testing the rectifier and preamp tubes, unless the short is in the socket pin just going to one of the power tubes?).

I suppose I need to change the second power tube socket and go from there. If there is still a short after that, then maybe its the filter cap sockets?

Go ahead and try replacing the power tube sockets. If the only change was you cleaning the sockets, then that could be the problem.

[Bieworm]

Tube sockets arrived, so I changed v3, the first power tube socket. My light bulb limiter was lighting up brightly when a tube was in the socket (did not go bright with just rectifiers, or with either preamp tube).

Does that mean the short is in the v4 socket, the second power tube socket? That is the only socket left. (other than the two sockets that take all the filter caps, though I would have thought if the short was there that it would be noticed in testing the rectifier and preamp tubes, unless the short is in the socket pin just going to one of the power tubes?).

I suppose I need to change the second power tube socket and go from there. If there is still a short after that, then maybe its the filter cap sockets?

Go ahead and try replacing the power tube sockets. If the only change was you cleaning the sockets, then that could be the problem.

For the cost of a socket it's well worth trying. On my sockets I always bend the gris lugs a little outward. Nogal sockets tend to be very small, and I have seen arcing between lugs before. Are you sure the tube pins fit really snugg into the socket, all pins? Bad connections may spark, sparks can arc...arc = short circuit.

[yello]

For the cost of a socket it's well worth trying. On my sockets I always bend the gris lugs a little outward. Nogal sockets tend to be very small, and I have seen arcing between lugs before. Are you sure the tube pins fit really snugg into the socket, all pins? Bad connections may spark, sparks can arc...arc = short circuit.

Last night I replaced the second power tube socket. Those little pins on the 7-pin sockets are tiny and the amp is compact, so it was quite difficult to solder all the wires in but I got it done. I did bend the little tabs apart as you mention to make sure there is as much room as possible between them. I had done the same on the originals too before replacing to make sure that wasn't the problem.

Upon removing both power tube sockets, I did not see any signing of arcing or carbon.

I didn't get a chance to test last night with the new socket, but will this afternoon. If a short remains, the only thing I can think is that the short is somehow in the octal tube socket where the power filter cap plugs in, and is on the pin that feeds the power tubes. Does that makes sense as the next logical step to trace/replace?

[Bieworm]

For the record.. without tubes there is no short? Amp stays on?

[yello]

For the record.. without tubes there is no short? Amp stays on?

Correct, no tubes in and no short. That is with light bulb limiter.

I add rectifier tubes (there are two), still no short.

Rectifiers stay in, add preamp 1, no short.

Rectifiers in, pull preamp 1, try preamp 2, no short.

Rectifiers in, no preamps, add power tube 1, SHORT. Light bulb glows bright. Same with just power tube 2, or with both power tubes, SHORT whenever a power tube is in.

I have replaced both power tube sockets. I have replaced every component (I think) on the turret board that is connected to the power tube section.

The only thing left that I can think of is the power filter cap area. The caps are brand new, but in this particular amp they plug in to an octal tube socket - the caps are mounted on a tube base and plug in. The caps all test correctly, leaving the socket. There are wires going from the pin on this filter cap octal socket (C216) to pins on the power tube sockets and components. I'm wondering if the wiring of C214 A/B/C and C216 A/B/C are correct.

Its hard to see but inside the amp it looks to me like C216C goes to pin 6 of power tube sockets and to R240 (350 ohm 3w) and R226. C 216B goes too R228. C216A goes to other end of R228 and also to pin 7 of rectifiers.

I remain at a loss for where the short is, other than the light bulb limiter seems to be telling me output section (I think).

[Bieworm]

It appears that something is drawing excess current when the power tubes are plugged in, hence a short in that part of the circuit. This should be detectable
Another hypothesis is the power transformer secondary windings. It's possible there is a flaw in one of those windings. It might stay stabil until a certain amount of current is being drawn. This could be the winding itself or some bad connection perhaps. This is a long shot though.. but we need to consider all possibilities...how do we test this? Disconnecting the secondary windings and running them through a variable resistor to ground reference? Watch out! This is advanced tech stuff... You say you have other similar amps.. you could try another power transformer from one of those working amps. You don't have to remove them, just disconnect secondary wiring and jumper from one amp to the other. don't forget to connect both chassis grounds...
Firts rule out the heater winding by disconnecting the HT wiring from the power transformer and test if the amp stays on with all tubes in. If there's a bias tap you'll have to disconnect that too. That will narrow it down to the HT

Did you try posting this at the Ampgarage? There are some smart oldtimers over there that will feast on problems like these