Fender TRRI bias issue. Help!

[Steelwitch]

Hey y’all, I have a TRRI on my bench. The clients complaint was that it’s completely dead. The 10A HT fuse was blown and some
Carbon residue on all tube sockets. I replaced the fuse and checked the bias on one of the output tubes with a bias probe. I’m reading -0521 v as the plate current. This is way too high. On the bias pot from closed to open, the bias only ranges from 53 to 49 mv (-049 to -052 on my meter). I need some guidance on what to check next, or if I’m reading these voltages incorrectly. Screen resistors are within range, everything otherwise looks ok. Thanks!

[Jazz P Bass]

Sounds to me like one or another output tubes puked HV into the heaters.

If the sockets clean up nice, put in new output tubes.
The bias range is acceptable. (-52 to-49Vdc).

[nevetslab]

Welcome to the Forum!

The Fender TRRI = Twin Reverb Reissue?

10A HT fuse. I know there's now a main PCB fuse that is a recent change, though my TR65RI Service Manual doesn't show it. I don't recall what that fuse value is...thought it was in-line with the AC Mains fuse 4A Slo Blo. In either case, there aren't any 10A HT Fuses in the amp as stock. Usually, one might find a T1A HT fuse with a quad 6L6GC Output Stage.

Your bias probe reading is showing 52.1mA plate current, and yes, that would be too high. I'd run one pair of output tubes at a time so the O/T current flow is balanced.

The carbon residue on the tube sockets.....are the 470 ohm screen resistors still good, or did they open? OH, your already said those check ok. There's also the 1.5k grid stopper resistors that are between Pin 5 and Pin 1 (pin 1 is an open pin on the 6L6GC's). With the power tubes removed, see what your bias voltage adjust range is. You can read that between ground and the wiper of the bias pot. The calculated voltage range is from -59.7V to -43.6V, not accounting for current load off the wiper.

You will want to clean off that carbon residue, as that is conductive. Is there any evidence of arcing between Pin 2 & Pin 3 on the tube sockets, or on any of the tubes' bases? We need to find where the carbon residue came from.

Have you opened up the cover below the chassis which contains the power supply capacitors? You'll want to have a look in there to see if there's any damage to the caps or the dropping resistors.

I've attached the 65 Twin Reverb Service Manual to help

65_Twin_Reverb_SvcMan.pdf

After you've cleaned off the carbon deposits on the tube sockets, I'd see if any of the tubes are dead. One of the common tools used on the Forum is an AC Mains Light Bulb Limiter, which you plug into your mains, and your amp into the output from the 100W light bulb. The common lamp bulb goes in series with the Hot lead of the line. The idea being, if the bulb goes bright right away when you turn the amp on (out of Standby), you are pulling fault current from someplace. If without the power tubes installed, and the amp isn't pulling high current, I'd check two tubes at a time. I normally use positions V8 & V9, leaving V7 and V10 vacant. If you only have one bias probe, insert that into one of those sockets (V8 or V9). You can power back down to S/B, and swap the tubes so you can see what each tube is pulling. My Bias Probe system measures two tubes, and normally occupies V8 & V9.

[Randall]

That internal 10A fuse is not for HT, it is for the filament supply. It is not included on the schematic, they must have put it in later. Anyway, as has been mentioned, 10A is far too high of a current rating for HT.

[g1]

The 10A heater fuse is shown on pg.14 of the schematic set here: https://music-electronics-forum.com/...0&d=1504184323

[Steelwitch]

That internal 10A fuse is not for HT, it is for the filament supply. It is not included on the schematic, they must have put it in later. Anyway, as has been mentioned, 10A is far too high of a current rating for HT.

You’re right, it’s the heater fuse thanks for correcting me!

[Randall]

Interesting, it took them 12 years to revise that fuse in there.

[Steelwitch]

Ok so to start off with this reply, the fuse was added in Revision F, this twin was made in 2015, so very new. The 1.5k grid stopper resistors all test well. I will check what the bias range is without tubes and update here for anyone else dealing with a similar issue later on.

Luckily no arcing, I will post a pic of the tube sockets when I get home, it’s not really that bad, but should definitely be cleaned up.

I’ve already turned on the amp with a variac and these tubes are ok as far as I know (none of them are shorted) and the bias on all of them is stable, just too high for it to be safe. They need to be replaced regardless since the client hasn’t changed them since 2015..

I haven’t opened the filter cap encasement yet, I have a suspicion that it could be the dropping resistors, I will post the results from that as well!

[Steelwitch]

Ok, so there's some cool stuff going on in the filter cap encasement.

1. one of the 22uf filter caps is reading 11uf and the positive end is covered in discharge, and likely where the carbon residue on the tube sockets came from-- needs to be replaced.

2. R65 and R66 220k resistors are open-- they need to be replaced

3. Both of the 220uf filter caps are reading as 260uf-- a little surprising to me, but I'm going to go ahead and replace those as well.

I might go ahead and do a full recap on this one. If anyone has any additional thoughts, please chime in. I think this is just about as clear of an explanation of what happened as I can get, but if folks have suggestions for other things that might have been affected or caused it, I would appreciate additional input.

[nevetslab]

Ok, so there's some cool stuff going on in the filter cap encasement.

1. one of the 22uf filter caps is reading 11uf and the positive end is covered in discharge, and likely where the carbon residue on the tube sockets came from-- needs to be replaced.

2. R65 and R66 220k resistors are open-- they need to be replaced

3. Both of the 220uf filter caps are reading as 260uf-- a little surprising to me, but I'm going to go ahead and replace those as well.

I might go ahead and do a full recap on this one. If anyone has any additional thoughts, please chime in. I think this is just about as clear of an explanation of what happened as I can get, but if folks have suggestions for other things that might have been affected or caused it, I would appreciate additional input.

Good find! Though the caps don't spew out carbon soot, and whatever does smoke and fume, tends to stay in the 'doghouse'.

Don't buy Illinois Electrolytic Caps. Short life span. F & T make nice 500V rated 22uF caps, and suitable voltage-rated 220uF caps for the first series-stacked stage.

FTAxial Electrolytic Cap_A.pdf

[Steelwitch]

I’m way ahead of you! Just ordered my client some beautiful F&T caps, I think he will he really happy with the repairs to his amp!

Can I throw out a question to someone who might know? Why would these 220uf caps be reading over capacitance? It doesn’t make sense to me unless it was a manufacturing error. I tested them in circuit which could explain why they’re testing so high, but typically I get a ballpark reading if it’s within 10% of its rating.

[Chuck H]

So... Why was the 10A filament circuit fuse blown? Something caused excessive current through the filament circuit. Preamp tube? Maybe the arching on the socket was caused by a wire filament that has vaporized? And certainly something has happened to the 22uf cap and that resistor. I don't like Illinois caps either, but at three years old I doubt the cap failed of it's own crappiness and took a resistor with it. At face value this isn't obviously related to blowing the 10A filament fuse, and coincidences happen, but I always suspect a relationship first. So I'm still wondering if you may have a shorted preamp tube. I wouldn't fully trust the power tubes yet either. Not before the amp is making full power without failure.

[Steelwitch]

It’s possible. My objective #1 is getting the bias down to a reasonable voltage before checking out preamp tube world. I already have a matched quad of JJ’s to put in once I do that. I do what I can without a tube tester, and will not put them in until I know they won’t get totally cooked!

[Leo_Gnardo]

So... Why was the 10A filament circuit fuse blown?

Two things I find happen on filament fuses. 1) Fatigue. The turn on pulse is way more than 10 amps for a fraction of a second. Repeat hundreds of times and the fuse wire breaks. I see a similar thing in modern (last 25 years) SVT and Sunn/Fender Bassman 300 where the quik-connect tabs that bring filament wires from the PT to the circuit board overheat during the stressful second, to the point where insulation over them turns brown & black, and sometimes partially melts away.

and B) Corrosion on the surface of the fuse holder clip, and the metal electrode on the fuse itself. With a lot of current passing through these junctions, if the surfaces are not squeaky-clean, that little bit of resistance builds enough heat to eventually corrode the metal. On it goes in a vicious cycle, until there's enough heat generated to melt the fuse element. Or so much corrosion that current can't pass. Even worse if the amp is used in a corrosive environment say damp salty oceanside locations.

[Chuck H]

I'd still keep my eyes peeled (Who ever came up with that term ) for something that may have connected the filament circuit and that failed cap and resistor in the power supply. But I did acknowledge that coincidences happen.