Hot PT in Mesa Boogie

[Platefire]

Friday night after I got through playing and turned my amp off I heard a "clinking" noise like something makes when it's real hot and starts cooling off. I had been playing steady for several hours! I reached into the back of the amp and the PT was hot enough where I couldn't leave my hand on it over a few seconds. I turned it back on so I could observe the power tubes. I didn't see any red plateing. Amps sounds fine with no problems with sound and operation. Guess I'll keep a closer eye on it--any suggestions? Platefire 

[tubenit]

Install a fan to cool it.

With respect, Tubenit

[Platefire]

Duh! That should have been a no-brainer  Thanks, Platefire

[PRR]

"Clinking" sounds more like a tube.

I was once abusing many 6550s. Some tinked when temperature went up and down, others didn't.

[jojokeo]

Plate, another suggestion you could do is lessen the bias a bit which will reduce the Pdiss of the power tubes. This will lessen some current demand of the PT.

[Platefire]

Yeah PRR, I think it was the tubes makeing the noise and that drew my attention to the PT. I don't recall having an amp get that hot before---warm but not hot. That's why I am a little concerned. Enjoying the amp to much to let it go bad. The power tubes are old and maybe should be replaced. Platefire

[stratele52]

Hot transformer ; Normal , no problem there.  Be sure you have the correct fuse size and forget that.

[SILVERGUN]

Had a Rectoverb(2001) combo and PT got so hot that it heated up the board and lifted the trace for the 6.3 filaments right in the middle of a show...board was mounted right above PT and had a push connector that was blocking my view of where the spot had burnt the trace...
indicator light went out and I was F'd ....(right before a big solo too)

I should've seen it coming when the indicator light had gone out at our last rehearsal...I changed the bulb and there was just enough of the trace left that it actually worked until halfway through the 1st set..

Make sure the board isn't mounted above the PT and check the connector/board for any signs of heat......REPLACE the PT when you get a chance, heat is a sign of resistance, and too hot is too hot.....
If reliability matters, do your job and get that thing out of there before it shorts and takes out your board
PT's can fail because of insulation breakdown caused by HEAT....110-120 deg. f is ok.....there was a spot on mine that was 185!!

If you want to check it,,,get a non-contact thermometer and take readings from every angle

I felt like an idiot,,,being a tech,,,with a damaged amp that wouldn't work....what a DB......kinda like an auto mechanic broken down on the side of a road in a Corvette

Here's the article I found:
http://www.mercurymagnetics.com/pages/mainframe.htm

[SILVERGUN]

So after repairing the board trace and adding a hardwire, I turned the amp on and let it idle for about 15 minutes, went back and the PT was HOT with no load except the filaments...no good!

TRY THAT, and if it heats up @ idle with no (output) load, you might be onto something

This was the amp that made me hate pc boards,,,I don't care if they sound "as good as hand wired"

In YOUR amp, ALWAYS use overrated PT....just spend the extra $30, make room, and never think about it again

[Platefire]

It may have been running this hot all along. It's just the tinkeling cool off sound of the tubes drew my attention--I don't recall hearing that before---so I started looking around. it's got my attention now--I will be watching now! It's the amp I use at Church and I checked it last night and it was pretty warm but not as hot as Friday night. It may be time to bring it home and give it a little attention. Plate

[Platefire]

Those EL84's in this amp have shaded glass around the plates. You couldn't tell if they were red plating of not---why do they do that?

[jojokeo]

Those EL84's in this amp have shaded glass around the plates. You couldn't tell if they were red plating of not---why do they do that?

That's really unusual for EL84s. I've got almost every kind of EL84 new & old and I can't even think any of them have shaded glass. That's something you only see w/ most older 6V6s.

[Dave]

Hmm......

I have seen EL84's assume a "shaded glass" appearance around the plates are they have red plated, or gotten particularly hot.

Dave

[Platefire]

OOPs! Bad sign--and it's sounding so good. As much as I hate to, I guess I need to drag that heavy thing home and have a closer look. Platefire

[Dave]

Update?

Dave

[Platefire]

Well--------I haven't brought it home yet. I got sidetracked by an Ampeg SVT Classic(another post).
Things happen     Hopefully soon!

[Platefire]

OK, I brought my amp home tonight for a closer check out. First I was wrong about the 4 EL84's having shaded glass. All tube glass is clear, Mesa type tubes. I have run the amp off standby about 30 minuets. No sign of red plating. Using my InFraRed Thermometer I took several readings:

PT=100/116/106/128
EL84#1=192/189/182/195(all pointed right at center of plate area)
EL84#2=225/225/223/220
EL84#3=233/226/224/232
EL84#4=186/195/180/216

Back of chassis right behind power tubes is at about 165.

[SILVERGUN]

It's never a bad idea to run the thermometer slowly across every component on the board, while running the amp.....don't forget your dummy load

[Platefire]

I did a google search regarding operating temperature on EL84's and didn't fine much. One said 180 to 250 degrees is a normal range. As an alternate prosepective, I checked the temperature on the PT and tubes on my DIY AB763 Deluxe reverb. PT only got barley warm and power tubes about 125.

As you probably already know this Boogie has no bias pot but the negative bias voltage is set with fixed resistors. Part of their advertisement in the manual is that "It never needs a Bias Adjustment"--just change the tubes and go! I haven't pulled the chassis yet but normal plate voltage according to the schematic is 380. I'm not totally sure but I'm thinking that the high operating temperature is wrapped up how Boogie has the amp biased. Of course this could be effected by the fixed resistors drifting. Feedback from other Boogie owners so far indicate this is pretty normal. I would think "no red plating" is a good sign. Platefire

BTW--Here is a pretty interesting thread from another formum on the Mesa Boogie Biasing subject on older amps like mine. Tubenit was probably right on about adding a fan
http://www.thegearpage.net/board/archive/index.php/t-879672.html

[phsyconoodler]

Lots of amps' PT's run hot to the touch.How hot is the question.
 The 'never needs biasing' mesa stuff is only if you use their tubes.If you pop in a different brand of tube you takes your chances.
 If you check Fender Champs and Princetons you will find the PT's cooking away.Hot to the touch cause they were underspec right from the get go.I suspect the Boogie was the same too.

[PRR]

IR thermometer in degrees C or F ??

120 deg _F_ is not hot for a transformer. But 120 C is mighty toasty.

What strikes me about the tube readings is the 30+ differences between tubes. If running normally-hot, radiation is significant. A small rise in temp is a large rise in radiation, so the tubes should fall in a narrow range of temps. These numbers *may* mean some tubes are throwing twice the dissipation of others.

I'd get individual current readings on the tubes, find the loafers and toasters. Swap 'em, is it tube or socket?

[Platefire]

Well PRR you got me wondering if I had it set right so I went back and measure again and made sure it was set on F. I fired up the amp and measured it about after 10 min several times during the next hour. The progression from lowest to highest one hour later:

PT=92 to 137
#1 EL84=187 to 204
#2 EL84=214 to 224
#3 EL84=190 to 209
#4 EL84=151 to 162

Looks like I might need to try #4 in the #2 position and vice versa. Platefire

EDIT: I did swap #2 & #4. Results: PT=120, #1=188, #2=190, #3=191, #4=186---seems to of have a balancing effect.   

[PRR]

> seems to of have a balancing effect.   

That says you have TWO issues: a hot tube and a cold socket.

Since you have not identified the model/version, I won't try to guess why you might have a cold socket.

Assuming deg F--

Everything is cold.

137 is quite warm to hold forever, but a few seconds should be easy.

HARD-worked tubes go 400+ deg _F_. 190 is loafing.

> I got through playing

I bet the amp is fix-bias and idles cool, but hard work makes it hot. Can you beat it for 20 minutes, annoy the neighbors, and see what happens?

[Platefire]

I'm sorry I haven't properly introduced the Boogie. It's a Rocket 440 about the same size cab as a Super Reverb with 4-10 Speakers. The schematic posted is a pdf file scan of a hard copy of a Rocket 44, same circuit as the 440 but smaller cab. The previous owner marked the schematic pages up all over with a red marker. So here is a few pixs of the amp and schematic of power amp section and power supply.
   In Spite of it being a PCB board amp, I really like it and want to keep it healthy as possible for as long as possible. I recently moded it to remove the little midi chassis mounted reverb tank and installed a full size accutronics tank in the bottom of cab. The older pictures posted don't show that mod.
   On cranking it for 20 minutes I don't think my neighbors would be a problem but with my wife it might be? Timing would haf to be right on that one!   Platefire

[PRR]

It's a 40-Watt Ampeg VT40. Four lightweight Tens in open-back. Only with quad-EL84 in place of a pair of 7027/6550, at 400V instead of 600V.

> a PCB board amp

Use good healthy tubes. I can see why you don't want to dig into it. But I bet the tubes work HARD when played LOUD. EL84 are pretty robust, M-B knows that, and probably flogs them to-the-max. If a tube gets sick, it could do a lot of damage. (That's what happened in 'my' VT40.) I am not real happy to see the different temperatures; I wonder if a tube is getting sick.

[Platefire]

I can't seem to find a match Quad in the Mesa tubes from MB, only a match pair. Platefire

http://mesa.stores.yahoo.net/6bq5elrus.html

[phsyconoodler]

So get two matched pairs of the same color.They rate the distortion characteristics by color.Yellow,red,green.

 

[Platefire]

So my existing tubes have white lettering identical to the Mesa tube of attached link. They say nothing about color coding there. I just want the same carteristics in a new tube as I'm getting with the old or in other words I don't want to change the sound or response. Platefire

http://mesa.stores.yahoo.net/6bq5elrus.html

[smackoj]

Most guys I know that use 84s use JJ tubes when they put a new set in. They may not be the exact tube you are replacing but should be as good or better. AES has matched quads of EH, Sovtek and JJ EL84s. The Mesa tubes I have are 6L6 and they look like they are made by Sovtek to me. However, I don't know if Mesa uses Sovtek for all their power tubes?  Here's a link where I have bought tubes with good results:

http://www.tubesandmore.com/products/T-EL84-JJ

[Platefire]

I made an effort to read MB's take on their tubes. What I got out of that read in general is the tubes they buy and put their name on are not so much better than you can buy somewhere else but their testing and weeding out process requirements are more stringent so supposibly theirs is the best of the lot with closer tolerances. I can't prove it, that was just the essence of their speal.

I did e-mail MB about my amp in reference to the tube link previouly posted and they did verify that tube to being the right match for the existing tubes I now have with the exception the lettering it now black instead of white because the black text/numbers holds up better than the white without wearing off. They also said I need to get two matched sets, one set would go on the two outside sockets and the other set on the inside sockets. Still not sure what I will do yet, just getting information. Platefire