MusicMan 130, questions.

[Matthias]

Hey guys
It's been a while.....life.
Anyway I'm helping out a friend with this head.
it's a 2100-130 chassis, which originally came with 6ca7's
He had it re-tubed about 6 years ago and now it's on the fritz, he call's me up and I say cool, let me look it up.....just get some good el34's and well go thru the thing and check values and retension the sockets and bias it up and all that.
well, he comes over with it....minus one tube...problem #1
and a new set of el34's.....ruby's/unscreened.
bust it open and.....it's got 6l6gc's!...problem#2.....o.k. no big deal, check the circuit and it appears as if they just stuck 'em in and biased 'em up.....so I'm thinking....hell pretty good for 6 years.
so I figure I'll look it over for a bit more before throwing in the el34's and biasing...
today I jump in and reheat a couple joints...solder. double check some values and go for it.......I got the dmm clipped in across the 10ohm and throw the switch.....starts climbing and climbing...hit's 800mV before I shut it down...
o.k. 6l6 to el34....bring it down...or up, however you want to look at it.
so I dial up the resistance and try again the current is a little low so I dial down the resistance and get it to sit around 490mV...ok for now... let 'em sit and cook. I hit 'em with the laser thermometer and over 300 deg. F! I cringe...mostly because I've never used the thermometer on a tube, so I really have no point of reference....it's a new toy from the infamous harbor freight. I feel the chassis and it seems warmer than I would expect.
so I had shut it down and look at the data sheet ....
sez max bulb temp can be 250 degC, or 480F. ok I say.....
I turn it back on and go to hit it with 1000Hz and look for the notch on the scope. I throw the standby and I hear the tone for a split second then I blow the fuse....ok. old fuse, standby spike, etc.
well I pull the old and it's a 4A when it calls for a 5A.
o.k., maybe a precaution for the 6l6's?
either or, I have neither in stock so I'm off to the shack.
Basically,
It's been a while since I've messed with this stuff and Seeing as this isn't your straight forward retube/bias
I figure I'd drop in and throw it out there.
see what the pro's think.
-Matthias
wow, I didn't realize that this log in was actually for the old sunn forum. I don't remember my ampage info.

[Matthias]

o.k.
New 4A fuse works fine....havn't got back to checking the notch.
There seems to be some slight pops and crackles....going to have to go back and check those resistors and coupling caps....tomorrow.

[Enzo]

I'd think the IR thermometer would have a hard time measuring the glass temperature without looking right through teh glass and seeing the metal inside the tube, which indeed will be a lot hotter.

I would prefer to monitor the tube dissipation electrically.

The thing to do would be start with the tubes removed, and verify all the operating voltages around the output tube sockets. Chances are if the bias is half what it ought to be, the tubes will draw tons of current.

ANd another personal preference, settting the bias by tube dissipation. Guitar amps are not hifi amps.

[Matthias]

Hello Enzo,
true, that's what the IR is measuring, the surface that deflects it. The 350F was the highest reading, which is on the Plate. And of course, four power tubes will radiate a good amount of heat. In this situation, I'm just not sure what to expect.
Tubeless we get:
#4@380.5VDC
#6@-35.2VDC
#5@-35.1VDC...of course
#3@760VDC
heaters @7.2VAC
a touch over spec, but we're tubeless here.
As far as bias methods go, yes I'm with you....I just like to look at it all...you know...besides the fact that this thing has so much head room...it kinda is a hi-fi amp.
-MS

[Matthias]

well, I got it going and it's off to fight another fight.
There are a few thing that make me nervous on this amp.
all of which actually seem to be ok.....I'm just the nervous type.
Thanks for the insight, Enzo. it did get me thinking.
Only thing I'm really not to sure about is the 730VDC hitting those el34's that have only been tested/matched to 600VDC....Time will tell.
Sounds awesome though. real tight, fat bass tones, break-up only happens with a hard strum....he loves it again.

[Matthias]

well, it lost the fight in the second round.....blew 4A fuse....I missed something.
caps seem good...rect. good....tubes new....
Ideas...help...slander?

[jbltwin1]

music man

Well, first off, 700v on a mm is about right. Mine routinely run between 700-725 when cooking right. I assume that since you are checking the bias and things that you have one with the 12ax7a PI tube? That means that you have an actual bias voltage conrol on the main circuit board. The little trimmer at the far right hand side, next to the edge. I'd trash the power supply caps and start there. Make sure that the el34's that you have are good, then check and make sure that you have plenty of control grid voltage at pin 5 with the power tubes out. Like I said, you SHOULD have 700 volts. If you got it REAL low, chances are that you have the tubes conducting WAY too much current. I'd prolly try either hooking up the variac to monitor current or do the lightbulb in the hot side thing to keep from blowing fuses until you find it. Also, check the bias cap-it's on the little board over by the trans.

[52 Bill]

I've had pretty good luck using JJ E34L's in these amps. They are supposed to be rated a little higher.

[Matthias]

Hello jbltwin1, And Bill
yeah, it's the one with the 12ax7 driver/inverter.
I've gone over the caps and they seem good, but you know....
right now with the tubes in we see:
#3@722vdc
#4@360vdc
#6@-34.5vdc
and the fluke bench meter says there is .5VDC across the 10 ohm.
It's has to be a cap or leaky diode? they all test good, but then again @700vdc....it's gonna be another story.
I'm gonna go put a clamp meter on the mains.....
.72A on standby
1.12A off standby
And As much as 3.8A on a hard strum!
so throw in a big muff and.....pop!
it does ask for a 5A fuse though, so...
Doesn't mean the caps are in the clear though.

[Twist]

Does this amp have two or four output tubes? 0.5 volts across the 10 ohm resistor is 50ma. I would think that is too high for a single EL34 @ 700 volts on each side. It would be ok for two tubes per side. With 700 volts and two tubes I would adjust it for about 250 mv across each resistor for each side.

[Matthias]

Hello Twist,
Yeah, It's four el34's, so that's 500mV....I believe.
I went ahead and put in the 5A sloblo that it asks for....so far so good.

[Rick Erickson]

Yeah, It's four el34's, so that's 500mV....I believe.

Look closely at the schematic and you will see that a four tube MM HD-130W has TWO 10 Ohm resistors from the power tube cathodes to ground. I think these are on the outside two sockets when you look inside the chassis. Take another look at the schematic and you will see that these resistors are connected in parallel, so no matter which one you read across you are actually reading across both of them. This is why the bias setting of 500mV across the 10 Ohm resistor is the same for a HD-65 or a HD-130. With the HD-130 you are actually setting 500mV across 5 Ohms. Now do the math and tell me how many mA per tube this is. Extra credit if you can also tell me how many watts dissapation per tube this works out to. Extra extra credit if you can tell me why this method of figuring power per tube is very inaccurate even if the place you bought your tubes from said they were "matched".

RE

P.S.- Measure the 10 Ohm resistors to make sure they are within spec. Keep in mind that with an HD-130 you will read 5 Ohms across them unless you disconnect one from the socket.

[Matthias]

Hello Rick,
The 10 ohm are exactly where you remember them to be. I noticed them being parallel while I was measuring/Testing components, the ten showed as five so one leg was lifted and they both read @ 10 ohms.
500mV/5 ohms=100mA
so that's 25mA per tube.
and if all this is correct,
we should have about 18W sd per tube?
B+@722VDC*.025A=18.05
so,everything would have to be matched for the math to more closely reflect reality. and still thats not gonna happen, because Besides the tubes, which are assembled by hand, there is the Rk, and the Rg, and we got some Ig in there and?
what else?
Man, commercial coffee equipment is so much easier.