Music man Bias

[Valvehead]

I have a Music Man 112 RP 100 , it has a SS preamp with 2 , 6L6 power tubes...getting odd readings at the 6l6 sockets....measuring at pin 3 i have absolutely nothing.... Right where the OT connects.... Amp works fine .

I just want to check the bias..and im getting a super high reading at pin 5.

I havent seen a scem yet on this...but is the bias circuit oddball on these ? Or the output tube circuit?

[Valvehead]

to add to this.......i found a scem and it shows 6CA7 tubes .............??


Music Man 112-RP-65 Ch= 2165-RP Ampl/Mixer Music Man Inc.,

[Enzo]

Music Man amps are cathode driven. They have a positive grid voltage, the cathode is lifted quite high. Bias spec is on schematic. MM amps used both the 6L6 and the EL34. For example:
http://bmamps.com/Schematics/Musicma..._&_2100-rd.pdf

If the amp works, then you must have B+ on pin 3.

[Leo_Gnardo]

If the amp works, then you must have B+ on pin 3.

A MM bass amp showed up here one day, no hi voltage on pin 3 of the output tubes. The output transformer was missing! And of course its owner said it was working before, and nobody had been messing around inside it.

"Amp works fine" well there must be hi voltage on output tubes' pins 3.

If I'm not mistook there's a pair of low value resistors, 6.8 ohms (?), in the transistor circuit that drives the cathodes. Measure voltage across these resistors, also check their actual value which may have drifted some, you can calculate the cathode current in idle that way for your bias check. Bias current tends to be very low in MusicMan amps. Best to accept "if it sounds good, it IS good" in MM's. I've spent many hours trying to fiddle with them, to little avail.

[g1]

There should be a chassis number somewhere, with that number you can find the appropriate schematic here:
MM Amp Schematics

You may need to dig in harder with your meter probe to get that reading on pin3. Be careful, these things run on very high voltages.

[Justin Thomas]

My Fluke wouldn't read the plate voltage in the last MM I saw in "High Power" - my meter tops out @600V, so, as g1 said, be careful. For your meter's sake, too. Maybe you're getting no reading for a reason...

Justin

[J M Fahey]

My Fluke wouldn't read the plate voltage in the last MM I saw in "High Power" - my meter tops out @600V, so, as g1 said, be careful. For your meter's sake, too. Maybe you're getting no reading for a reason...

Justin

That´s the point.
I always grind myn teeth when (too often) I see here comments such as "no voltage/resistance/current/whatever" or like now: "i have absolutely nothing" or "no reading"

Only way to get no reading is just not mesuring at all, otherwise display shows *something*.

It may be "0" or "infinite" and should be stated as such, instead of "no resistance" , "0" or "overload/out of range" instead of "no resistance" or in this case, "no reading" probably shows a blank screen or, more common, a lonely "1" and no other digits, which means "impossible to display value" , anything above "1999" in a common 3 1/2 digit multimeter.

Sorry for insisting so much on this, but I think that blank display actually means DEADLY >600V there, the multimeter´s way of saying "take me out of here" .
Problem is you may brush against that very high voltage (probably 650 or 700V DC) and the amplifier becomes :

[Valvehead]

Thanks for all the info guys----------bias is set........and yes, my meter didnt see that 739 Volts !!! wow !! i had no idea...good lord ...

[J M Fahey]

Glad you are safe, I was worried, not because of your personal skills which I trust, but because of the false security given by that meter.
Hey, the meter says there´s nothing there !!!!

Not dissing the meter either, in modern days who works with more than 500 or 600 VDC?

Most (I mean more than 90%) modern amps are roughly 300V something powered or most around 400/450V
Even in the old days, hairy Marshalls or ultralinear Fender or the odd Traynor reached 530/560V ... but not beyond.

But when I was a noob, late 60`s, many (needle) multimeters reached way higher, a few on their own, many with High Voltage probes, because of Tube TV sets.

This was my very first multimeter, a Central 200H , bought in 1969 thanks to my first guitar pedal sold, a Treble Booster straight from the pages of Popular Mechanics
Notice the separate 2500VDC socket.


Mind you, this was a mid priced and very popular general purpose meter !!!!
Yet it would have properly measured those 790V

Also notice the separate "Output" probe jack, it had its own internal 400 or 600V capacitor in series leading straight to the AC voltage section, so you could read Audio voltage directly at tube plates, without worrying about the high DC voltage present there.
Clearly a "Tube Era" meter

We all envied this one, the Hansen FN, "what the Pros used" (think TV repairmen).

The meter scale went "only" to 1000VDC, then you had to use the red 17000V (or was it 25000V?) Red special probe, with those wide ridges in the middle, to avoid user´s fingers slipping or even getting too near to very high voltage.
FWIW TV CRT screens regularly used 14kV to 17kV voltage.
Some TV sets used an auxiliary or boosted high voltage rail, some 1200V or so.

Just thinking without speaking aloud: "I hope they don´t notice I had to go to Radio Museum to illustrate my early test gear"

[Valvehead]

what brand of 6L6 / 5881 is out there that will handle 739 volts on the plate ?

[Justin Thomas]

Probably your best bet is the Sovtek 5881WXT. If any 6L6 will survive, that'd be it... yeah yeah yeah, I know, 250V Max plate voltage affording to data sheet... bunk. I'll see of I can link a pic...

This guy:
https://tubedepot.com/products/sovte...er-vacuum-tube

I think that's the one. The one with the oblong holes. It's been around forever and isn't a 5881 at all. Don't let the name fool you. Of course, no guanrantees, but they're cheap at least...

Edit: had a VT-40 w. a pair of these. 590 on the plates, had been in it a year, and I rocked that amp while I had it. Yes, on 10. No failures. Or even a problem.

Justin

[The Dude]

what brand of 6L6 / 5881 is out there that will handle 739 volts on the plate ?

Remember that the tube doesn't know if it's cathode is grounded or not. The voltage rating on the tube is plate to cathode. So, because your B+ measured to ground is 739V, doesn't mean that's what the tube "sees" if the cathode is above ground potential. Be sure to subtract the cathode voltage from the B+ measurement if you want to know what the tubes are actually "seeing".

[Valvehead]

Remember that the tube doesn't know if it's cathode is grounded or not. The voltage rating on the tube is plate to cathode. So, because your B+ measured to ground is 739V, doesn't mean that's what the tube "sees" if the cathode is above ground potential. Be sure to subtract the cathode voltage from the B+ measurement if you want to know what the tubes are actually "seeing".

ok thanks--just making sure.
the largest rating i could find on a new one was the TAD at 550v by the way

[J M Fahey]

Yes, I wrote some of that in the other thread about suitable transistors: if tubes are biased a b5 or 6 mA which is near cutoff, they must be getting some -60V bias ... which means cathode is 60V *above* grid.
IF grid sits at +30V (not -50V as in conventional amps), cathode then sits at 60+30=90V above ground.
Substract that from 739V and we have 649V plate-to cathod.

And that supply voltage is way too high, and probably intended for EL34 which can handle it better; as I remember 6L6 equipped MM tended to have 650V supply, meaning actual 560V Plate to cathode.
Definitively high voltage, but a value we have seen in other amps, with conventional voltage drive.

[Valvehead]

Would you recommend trying to knock down that voltage? If so , what would be an easy and safe way to do so?

or do you think some 550v rated tubes will live for a while

sorry if thats a stupid question but ive never dealt with more than 500v plate voltage before !