Tremoverb Design Flaw?

[LockStock]

I was speaking with one of my favorite amp builders and I mentioned my recent purchase of a 1998 Tremoverb. This is what he said:

1) Screen grid resistor is underspec'd and will end up burning up and charring - needs to be upgraded to a 5 watt.
2) There is a resistor in the low voltage switching supply - also underspec'd. There's a cap next to it that will start turning brown. You have to buy a power resistor and move it off the board and run it back- you have to screw it into the chassis, which is a laborious task

Does this make sense? Anyone here hear of this before?

 

[Vigo1999]

this subject also popped once on the HC forum, but as long as i remember, no one ever told what was the problem, they just said it has a flaw in the project :?

also curious about that . . .

 

[AdmiralB]

I was 1) Screen grid resistor is underspec'd and will end up burning up and charring - needs to be upgraded to a 5 watt.

Only if you use EL34s (or other pentode - EL37, etc.), and really crank the amp. If you stick with beam tubes (6L6, KT-series, 6550) the stock ones are fine. FWIW this is true of most Mesa amps...the screen resistors are undersized for EL34s.

2) There is a resistor in the low voltage switching supply - also underspec'd. There's a cap next to it that will start turning brown. You have to buy a power resistor and move it off the board and run it back- you have to screw it into the chassis, which is a laborious task

This is true of the early ones, and the DRs too. It's not that the resistor (it's the 5W sandbox mounted on-end) is underspeced, it's that it's placed in a location that'll toast the PCB. It's a 5W resistor, and it only dissipates about 2W, but it's located such that the heat bakes the board and (maybe) the components next to it.

Moving it isn't a big deal, though, I usually stick a terminal strip under one of the power transformer nuts and mount a 10W resistor there.

The later amps - the ones with the 561140 power transformer - don't have this problem.

 

[LockStock]

By 1998 were they using that 561140 power transformer?

 

[AdmiralB]

Mine is an early '98 and has it. I suspect it came about earlier than that however.

If you look at the schematic for the TOV that's on the net, there are two pages each on the switching section - one set is the earlier version with the switching power taken from the bias supply, and the other has the later design where there is a dedicated 12V winding on the PT.

561140 seems to be the 'standard' PT for 100W amps. The TOV and DR use it, the Heartbreaker used it, the Mark IVB uses it. I suspect the Roadster does as well.

 

[gooberizer]

I wonder if that would explain the Trem-o-Verb combo I had in the mid 90s that spontaneously caught fire. I was jamming at rehearsal and suddenly no sound, I turn around and there's smoke pouring out of it. By the time we got an extinguisher and put it out the whole thing was charcoal. Imagine the looks on the guys' faces when I brought it into the store I bought it from and asked for a diagnosis! :lol:

They did check it out but never found the problem because there was just nothing left that was recognizable. They did give me a full refund, though.

I currently have a T-Verb head that works fine, but considering this thread I'm wondering if Mesa would be willing to "upgrade" it if the one I have is an older model. I'm not sure of the year, I usually just play the things and don't get too into the specs and all.

 

[TP Parter]

I was 1) Screen grid resistor is underspec'd and will end up burning up and charring - needs to be upgraded to a 5 watt.

Only if you use EL34s (or other pentode - EL37, etc.), and really crank the amp. If you stick with beam tubes (6L6, KT-series, 6550) the stock ones are fine. FWIW this is true of most Mesa amps...the screen resistors are undersized for EL34s.

2) There is a resistor in the low voltage switching supply - also underspec'd. There's a cap next to it that will start turning brown. You have to buy a power resistor and move it off the board and run it back- you have to screw it into the chassis, which is a laborious task

This is true of the early ones, and the DRs too. It's not that the resistor (it's the 5W sandbox mounted on-end) is underspeced, it's that it's placed in a location that'll toast the PCB. It's a 5W resistor, and it only dissipates about 2W, but it's located such that the heat bakes the board and (maybe) the components next to it.

Moving it isn't a big deal, though, I usually stick a terminal strip under one of the power transformer nuts and mount a 10W resistor there.

The later amps - the ones with the 561140 power transformer - don't have this problem.

Wow, this is the first I've ever heard of this. I've got a '96 combo, and run a pair or a quad of SED EL34 along with one or two 5AR4 rectifiers and crank the amp when I play it (master at 2:00 + often, gain around 12:30-1:00). Do you think a problem might be looming in my future?

 

[AdmiralB]

Can't say. The spec for EL34 screen resistors, even going back to the Mullard data sheets, is 1K/5W. In the TOV Mesa used 1K/2W.

You might pull your chassis and look at them. If they're looking cooked, you might want to upgrade. If they look good, well, maybe you're fine.

 

[LockStock]

I took my amp to a Mesa-certified tech, and he said that he'll take a look, but he preliminary warned me that Mesa uses 5W resistors in what looks like a 2W package in these late-model amps. So, he said, many people are confusing it because on sight it looks like a 2W, but is in fact a 5W. But he'll take a look. My Tremoverb is a 1998, and I'll report back what he finds.

 

[AdmiralB]

Well, the schematic clearly states "1K/2W'. And it's true that some types of resistor are smaller for a given rating than others...but my '98 clearly had 3W at best.

 

[LockStock]

Just as a follow-up, this is what my Mesa-certified tech found in my 1998 Tremoverb that I had bought used:

"In your Tremoverb we found that the screen resistors and the high current resistors associated with switching, have been replaced. Two of these screens resistors have failed. I'll plan on replacing all four with 5 watt packages."

 

[MesaENGR412]

Only if you use EL34s (or other pentode - EL37, etc.), and really crank the amp. If you stick with beam tubes (6L6, KT-series, 6550) the stock ones are fine. FWIW this is true of most Mesa amps...the screen resistors are undersized for EL34s.

When you say most amps, does this include my 3 channel Triple?
thanks

-AJH

 

[guitarmechanic]

I just checked the schematics I have for a Dual Rec 3 channel and the grid resistors are spec'd at 1k 2watt. I can't say what they did in the Triple Rec.
They did however get it right in the Stiletto starting with Series 1 they are spec'd at 5 watts since this is a dedicated EL34 amp.