MusicMan 2275-65

[H Chandler]

Hi,

This is my first encounter with a MusicMan amp and could use a bit of help. I've found the schematic over at Schematic Heaven, and have stared at it a while. The first thing that strikes me is that the power amp section is unlike any that I've ever encountered.

It appears that the grids of the the power tubes are held fixed at 16 volts, while the cathode voltage is controlled by transistors (Q1 & Q2). The cathode voltage is varied by varying the collector-emitter resistance , Rce which is controlled by Ibe of the transistors (Q1 & Q2). Beyond this, I've not figured it out yet. Any pointers here would be helpful including reference material.

Here's what's going on with the amp. The amp blows the mains fuse. I pull the power tubes, put in a new fuse, and it powers up OK, but when taken out of standby, it makes a mean sounding hum. So i shut it down, and disconnect the OT and find that the secondaries are open to ground, one side of the primary is nearly open (100's of megs) and the other side looks pretty normal ( 100's of ohms). So I know the OT is toast. It looks like mojomusic sells one though. Any others worth looking at?

Now, before I smoke the new transformer, I need to figure out what to check. I suppose a new set of tubes are in order, but from there's I'm figuring it out as I go.

Any help would be most appreciated.

Thanks,

Howard

[52 Bill]

When you powered it up without the tubes, where did the hum come from? If the OT secondaries were open, then it could only come from the feedback loop? Or was it mechanical?.

I would check all of the transistors in the power amp, the protection diodes on the tube sockets and the tube sockets themselves for signs of arcing, etc.

Bill

[H Chandler]

Hey look what I found!

http://www.musicmanamps.com/

[Amp Kat]

That's a good site for musicman amps which was taken over by Ernie Ball so you can get schematics there and search the forum for information if you need to. When you measured the secondary did you measure from the leads from ground or across the winding itself ? What is the ohms across the secondary and what are the ohms from center tap to each leg of the primary ? If the OT is toast a few things to do would be pop the hood and see if the Silver Mallory caps are still in it. If they are get them outta there as there 40 years old. Then chech the two +/- 16 volt zeners that run the op-amps and make sure they are ok. Those 1692 transistors don't fry very often as theres just not a lot of current there and the tube only draws about 10 to 15 ma's at idle. But it is possible I guess so do check them for diode drops. When you get it back up you can go across the Cathode resistor which the exact one is on that website and set it for 25mv and you should be good. The plate voltage is pretty mean in the Hi power mode so carefull there as it's around 700 volts.

[Enzo]

Your ohm meter does not put 700 volts on the transformer, so it can be shorted and you still can't measure it with the meter. That is the insulation may have broken down, and at voltage it arcs over or otherwise has a lot of leakage.

When you power up even without powr tubes, an arc from primary to frame or to secondary will cause large currents to flow through it. The hum you hear is the large ripple current.

[Arthur B.]

Hi,

This is my first encounter with a MusicMan amp and could use a bit of help. I've found the schematic over at Schematic Heaven, and have stared at it a while. The first thing that strikes me is that the power amp section is unlike any that I've ever encountered.

It appears that the grids of the the power tubes are held fixed at 16 volts, while the cathode voltage is controlled by transistors (Q1 & Q2). The cathode voltage is varied by varying the collector-emitter resistance , Rce which is controlled by Ibe of the transistors (Q1 & Q2). Beyond this, I've not figured it out yet. Any pointers here would be helpful including reference material.

The circuit is called a cascode. It's a transconduntance amplifier. It's used when wide bandwidth and high linearity are desired.

[H Chandler]

Thanks for the the help, I've got more questions..

Enzo, Thanks for the added insight about checking OT's. I don't think I'm gonna put 700V to check further, but it makes sense though.

Amp Kat - Hey, we're practically neighbors. Anyway, I measured across the OT secondary from black Ground wire to both of the taps and get a kajillion ohms. I'm pretty sure the OT's fried on prima fascia evidence.

Anyway, I quit staring at the schematic and took another look at the amp and noticed that I was looking at the wrong part of the pdf. DOH! There are schematics for two amps on same document, and it turns out that I have the version that includes a 12ax7.

It looks kinda like a combination of cathode and fixed bias. There's a common cathode inverting gain stage followed by a kind of split load phase inverter, but then there's also a 10 ohm cathode resistor too. Also, what's the 1.3?ohm screen resistor for?

Also, it says in the notes that to set the bias on the 6ca7's, set point Y to 1/2 volt -- at least that's what I think it says. It doesn't help that the drawing is too fuzzy to figure out where point Y is.

Anyway, any light y'all can shed here explaining how this is supposed to work would be most appreciated.

Thanks

Howard

[Amp Kat]

Is it an all tube or hybrid ? The bias is different for HD130 and 2275. The 10 ohmer is used on the all tube model and doesn't have the 6192 transistors.

[Amp Kat]

The circuit is called a cascode. It's a transconduntance amplifier. It's used when wide bandwidth and high linearity are desired.

No I really don't think it is a Cascode. It is however Cathode driven as the signal from the PI is coming in on the Cathode driven by the transistors. What do you mean by transconductance amplifier ? A Cascode has it's Plate tied either directly to or coupled to the Cathode. This circuit does not. Now there are two different types of splitter circuits used in the Musicman and three diffent preamp topologies. One is a 12AX7 splitter and the other is a op-amp splitter There is also an all tube circuit and an op-amp circuit. The tube circuit with the 12AX7 splitter has a 10 ohm current sense resistor that is used to set the bias to .5 volts or 50mv's and I've seen 5 ohmers in them also. The other circuit uses an op-amp splitter and those signal go into the Cathode injected thru the transistors into the grid. The bias is set reading across the 3.9 ohm emitter resistors for 25 mv.

[H Chandler]

Amp Kat,

Don't know what you mean by "all tube" but the last gain stage and phase inverter are all tube. I've attached the power section drawing.

The fixed bias supply and trimmer is on the main preamp board.

MM-PowerSection.pdf

[Amp Kat]

The HD130 is an all tube. Yours is a hybrid. If you measure across the 10 ohm resistor from the Cathode to gound it should be around 50mv's. Schematic says -46 so that's about right. I think it's RV2 so your amp doesn't have the transistor drive circuit. Let me know where you stand on this Chandler and feel free to drop in our shop any time.

[H Chandler]

It's the first drawing on this pdf file.

http://www.ernieball.com/mmonline/te..._&_2275-65.pdf

Sorry about the confusion. At first I assumed the amp was like the second drawing, and someday I'd like to look into the nitty-gritty of this design as well. I would not have thought to call it a cascode amp, but my only exposure to cascode amps was from opamp design courses in school 20 years ago.

Anyway, it's the first drawing with the all tube driver. It's hard to read, but if what they call point Y is the cathode voltage, then if it is set to 500 mV like the instructions at the bottom of the page, that would give a cathode current of .5/10 = 50 mA or 25mA per tube. At appx 700 V on the plates (yikes!) that would give about 17.5 watts dissipation which is about 70% of 25 watts, so about right for an el-34. That is assuming I got my math right.

But, what is it called? Is it cathode biased since it has a cathode resistor, or is it fixed biased since it puts a dc offset on the grids of the el-34's. Or what? And why was it designed this way?

I guess I'll find out when it's up and running, but does it sound more like one or the other?

Next time I'm in your neck of the woods I'll try and stop by fer sure.


Thanks

Howard

[Enzo]

Designed what way?

It is plain old vanilla fixed bias. The 10 ohm cathode resistor is only there to read across for bias setting. WOuld it be less confusing to you if it were a 1 ohm resistor instead of 10? THat 500mv is half a volt, hardly enough to be bias for anything. It is not cathode biased.

I agree it might look like a cathode bias circuit, but once you read the component values it should be evident that it is not.

And that is a poor copy they scanned alright. The cramped layout and the missing tube outlines don't help.

[H Chandler]

Hmm, well dang I guess I was fooled. How come they didn't use a 1 ohm resistor?

While I'm at school, what's the deal with the diodes and caps on the plates of the el-34's.

And what about the resistor from the screens to ground? Aren't these usually tied individually to the power supply?

Wait, maybe it is. I'll have to check when i have the amp in front of me. It's hard to tell what's going on in the drawing.


Thanks

Howard

[Bruce / Mission Amps]

I think I read you are measuring your OT from ground to the outside legs?
Don't measure the OT with respect to chassis ground.
With your DVM set to ohms, measure from the OT's center tap to the plates, lugs 3, of each power tube. I can't read what letter that center tap is on the poor schematic scan but maybe you can tell... you should read something on each leg of a few hundred ohms or less... probably less the 100 ohms from the center tap to each leg.

The diodes are called flyback diodes and are to help protect the OT from winding up with a large voltage bump, like if you have no speaker connected accidentaly... at their breakdown voltage, they basicly short the OT to ground for an instant, probably blowing the fuse in the process so the voltage doesn't get so high it will blow the OT.
The .001uF caps look like they are there to snub high frequencies and maybe to give the power tubes a more constant impedance to work against.
The caps are in parallel with the output of the power tubes and sort of in parallel with the inductance of the OT.
I think it forms a low pass filter of sorts.
That screen resistor doesn't look grounded to me at either, it looks like it goes the the main B+ rail too... the resistor is to limit current a bit.
The schem is pretty bad but I'm sure you'll find that screen node resistor going to the B+ rail.