Red Knob Twin. External fuse blowing. Burnt resistors/diode on filament supply. 6L6 socket burns. Bad 6L6?

[Whatizitman]

Traded a few non-essentials for a non-working The Twin.

On first inspection, the positive lead on one of the speakers was off, and the amp was on 8ohm. It's wired in series per schematic, for 16ohm. Maybe someone was trying to make it less loud? I put it back to right before I tested further. It certainly seems to me that the tubes are all original, but can't confirm.

The external fuse (4A) was blown. I put in a 1A (all I had), plugged in through my current limiter, and it powered up. All tubes glowing similarly, no redplating. Off standby the limiter lamp lit up dimly. The amp has a loud buzz off standby, volume down, but otherwise sounds ok in the clean channel. Controls all work. Tried without current limiter, and it held until I switched channels, then blew. With current limiter I was able to use the distortion channel ok. Sounded pretty good, even with the limiter. Plugged in without limiter, died on the distortion channel.

No new smoke or burning smell.

Took chassis out to inspect, and the two 47ohm resistors on the heater supply are badly burned (R303, R303). One looks like it exploded. That even possible? Diode in parallel (anti-parallel, according to the Wiz) with the power LED looks bad, too (D307). The socket for V10 (6L6 on the far right looking at back of amp) has lots of black from something burning. The grid stopper (1.5k?) looks like it could be burnt, but haven't measured. Internal fuse is good.

Interwebz say bad tube, but it looks fine at least visually. The tubes look original to me. Output seemed ok without the limiter.

Caps look newer to me. But I will change them. Just want to get as much pre-planning before ordering parts.

What else should I be looking for? Is the filament supply to blame for blowing fuses, or is there some other culprit? If the 6L6s are ok, could anything else cause this issue with the filament supply? And what possible damage could running it with the open resistors/diode cause? If the main fuse wouldn't blow, that is. Incidentally, this is why I think it's not a bad idea to test with lower amp fuses.

The Twin schem pdf added

 

[Whatizitman]

OK, might have got some answers from the Wiz. The schem indicates the burnt resistors are an artificial CT. Merlin says CT resistors should be "flame proof". Certainly these weren't that. Hopefully replacing those will also kill the nasty buzz hum. I'm still inclined to think the 6L6s are all good.

Any issues with upping the CT resistors to 1W? Merlin says 1/2W is fine. But these were not fine.

 

[Whatizitman]

EL34world to the rescue.

Burned Artificial Center Tap

Hoffman Amplifiers Tube Amp Forum, Burned Artificial Center Tap

el34world.com

From Sluckey The Mod:

A common cause for those resistors to be fried even if they are 1/2 watt size would be a short between pins 2 and 3 on the output tube. This puts B+ onto the filament winding so basically B+ across those 100Ω resistors, so they pop. This could be a permanent short which may also kill the PT HT winding. Look for a carbon trace between pin 2 and 3 on the output tube sockets, or check for low resistance between those pins. But the short may have just been a momentary arc between pins 2 and 3. Many times the only evidence that an arc has occurred will be the fact that those resistors fried. The most likely cause for an arc (short) between pins 2 and 3 would be playing guitar loudly through an amp with no speaker connected. This may happen because the speaker just ain't plugged in or could occur if the speaker is momentarily disconnected (dirty jack, bad cable, etc) while you are playing Deep Purple.

 

[Whatizitman]

Wow. Pins 2 and 3. And like I said, the positive lead on one of the speakers was disconnected.

 

[Milspec]

I am the last guy to offer circuit advice, but the one thing that jumps out at me is the fuse....should be a 3A I believe.

 

[bebopbrain]

I recommend keeping the standby switch off so there is no high voltage until you have sorted out the 6.3VAC issue and replaced the fried components.

If you have a dead short between 2 and 3 across melted isolation at the socket, that would explain everything.

 

[Whatizitman]

I am the last guy to offer circuit advice, but the one thing that jumps out at me is the fuse....should be a 3A I believe.

Amp calls for 4A. The external fuse that was in the amp when I got it was 4A, and blown. I only had 1As to test with. But like I said, I think that's fine, since I'd rather the fuse blow sooner than later when testing. Both channels worked with a current limiter. Without the limiter, the 1A fuses blew when switched to the distortion channel. I haven't measured any voltages yet.

I am however afraid the PT might be bad. Hopefully, the fuses saved it. I'm thinking the previous owner ran it for a while with one speaker disconnected. Not sure if it was intentional or not. or how long. The impedance switch was on 8ohms, though. If the speakers are original, which I think they are, they are 8ohm, and wired in series. I reconnected the speaker and put the amp on 16ohm before attempting to power it up.

 

[Whatizitman]

Probably a good argument for wiring speakers parallel? Series means no speaker load at all if one is disconnected, which was how it was when I got it, apparently.

Costly mistake.

 

[Timbresmith1]

If that socket is carbonized, replace it.

 

[corliss1]

New tubes, new sockets, replaced the burnt stuff, you'll likely be good.

 

[Lowerleftcoast]

Most likely there was HT on the heater wiring because the resistors acted like a fuse. A short inside a tube can do that. Putting the tube in incorrectly can do that. A loose screw or whatnot rolling around inside the chassis can do that, etc.

Replace those resistors with 1/2W. When they blow next time, you don't want them any hotter than they were this time, so don't put in a higher wattage resistor. Make sure to clean off all of the carbon deposit before installing the new resistors. If you have 100 Ohm resistors on hand that is close enough. You don't have to stick with 47R.

Check the tube sockets for carbon deposits. Cleaning the socket may not be good enough and a replacement may be necessary.

I would be reluctant to trust the power tubes at this point.

 

[Whatizitman]

And then there's.... this.

Knob - Vintage, set screw, brass insert | Amplified Parts

Generic white or brown vintage, priced per single knob. Set Screw. Height: ½", Top Diameter: ¾", Base Diameter: 13/16" Replacement for Fender: 099-0933-000 Multiple color options available.

www.amplifiedparts.com

Boom. No more red knobs.

 

[Milspec]

Amp calls for 4A. The external fuse that was in the amp when I got it was 4A, and blown. I only had 1As to test with. But like I said, I think that's fine, since I'd rather the fuse blow sooner than later when testing. Both channels worked with a current limiter. Without the limiter, the 1A fuses blew when switched to the distortion channel. I haven't measured any voltages yet.

I am however afraid the PT might be bad. Hopefully, the fuses saved it. I'm thinking the previous owner ran it for a while with one speaker disconnected. Not sure if it was intentional or not. or how long. The impedance switch was on 8ohms, though. If the speakers are original, which I think they are, they are 8ohm, and wired in series. I reconnected the speaker and put the amp on 16ohm before attempting to power it up.

I was wrong, had to walk over to look at my SF Twins to see the 4 amp fuse....really thought they were just 3 amp until the "The Twin" came out. I guess I haven't replaced a fuse in quite awhile.

I would guess that the previous owner pulled a couple of power tubes and disconnected a speaker for lower volume, then put the tubes back in later on without re-connecting the speaker. Starting pushing the amp until smoked. I have known of 2 players who did that very thing about 15 years ago in my area.

 

[Whatizitman]

I was wrong, had to walk over to look at my SF Twins to see the 4 amp fuse....really thought they were just 3 amp until the "The Twin" came out. I guess I haven't replaced a fuse in quite awhile.

I would guess that the previous owner pulled a couple of power tubes and disconnected a speaker for lower volume, then put the tubes back in later on without re-connecting the speaker. Starting pushing the amp until smoked. I have known of 2 players who did that very thing about 15 years ago in my area.

Thanks for bringing this up, cuz it's my next question. I'd rather not buy an entire quartet tubes if I get by with two. Preferably if at least two of my remaining tubes are ok. Plus not having to change out the burnt socket right away might be nice.

After some digging, apparently the manual for the RK Twin says it's ok to pull the inner tubes, and of course half the impedance. In my case, I would be either pulling the outer, or 2 and 4.

I would be keeping both speakers connected, since they're in series. If the previous owner tried to run it with one speaker disconnected there would been so sound. The selector was on 8ohm, though. So who knows. If it were run properly with two tubes and one speaker, the proper impedance setting would be 4ohm.

So, my question. Is is safe to run it with the inner two tubes pulled (or 2 and 4), when the manual only says to pull the outer (1 and 4)?

 

[Milspec]

Thanks for bringing this up, cuz it's my next question. I'd rather not buy an entire quartet tubes if I get by with two. Preferably if at least two of my remaining tubes are ok. Plus not having to change out the burnt socket right away might be nice.

After some digging, apparently the manual for the RK Twin says it's ok to pull the inner tubes, and of course half the impedance. In my case, I would be either pulling the outer, or 2 and 4.

I would be keeping both speakers connected, since they're in series. If the previous owner tried to run it with one speaker disconnected there would been so sound. The selector was on 8ohm, though. So who knows. If it were run properly with two tubes and one speaker, the proper impedance setting would be 4ohm.

So, my question. Is is safe to run it with the inner two tubes pulled (or 2 and 4), when the manual only says to pull the outer (1 and 4)?

If it is like all other Twins....

 

[Milspec]

Wait, I had to correct myself. It is either pull 2/3 or 1/4. I don't think 2/4 would work

 

[Whatizitman]

Socket is shot. Low resistance between pins 2 and 3. No continuity on the tube 2 and 3 pins, though. Tube looks ok, but I have no other way to test it without rolling it other sockets. I’m gonna just mark it and put aside, and continue testing with it and 1 pulled after I replace the burned CT resistors.

 

[Lowerleftcoast]

Socket is shot. Low resistance between pins 2 and 3. No continuity on the tube 2 and 3 pins, though.

There is a connection between pins 2 and 3 under normal circumstances. Both have a reference to ground through the CT connections to ground.

The socket looks like it has a layer of burnt carbon. If it is cracked or cannot be cleaned well, it should be replaced. If there is a carbon trace between pins 2 and 3, it will need to be addressed or disconnected when you run two tubes (or four).

 

[Whatizitman]

Hmmmm. I replaced the resistors. I think they’re 1/4watt, but it’ll have to for me. Ran it with two tubes, and it was fine with the current limiter. Tried without, and it died. Tried two newer good tubes, and it worked on wall power on clean channel, but died when switched to dirty channel. The resistors did their job, though, since the buzzy hum was gone. No smoke. Just another blown mains fuse.

Is it possible my fuses are blowing at this point since they’re underrated (1A)? I thought any big current surge can cause damage. But the amp’s rated for 4A fuses for a reason, one would guess.

Where do I go next to hunt down the short?

 

[Whatizitman]

There is a connection between pins 2 and 3 under normal circumstances. Both have a reference to ground through the CT connections to ground.

The socket looks like it has a layer of burnt carbon. If it is cracked or cannot be cleaned well, it should be replaced. If there is a carbon trace between pins 2 and 3, it will need to be addressed or disconnected when you run two tubes (or four).

Ok just read this. No more goofing with it until I change the bad socket.

I was thinking keeping it in circuit without a tube wouldn’t be an issue. But if it’s leaking B+ into the heater line, then that might be the culprit?

By low resistance between 2 and 3, I mean significantly lower than the other sockets. They measured around 130k, iirc. The burned socket was ~10k.