MusicMan 2275-65

[Amp Kat]

I do agree that schemo is wicked but one thing we do know is that screen is not tied to ground but I am thinking it's tied to the 365 volt node. If it's tied to the plate without a resistor (at least 100 ohms) that's not good. But how about Triode connected 700 on the plates and 365 to 400 on G2 and the screen choke filtered for less ripple. Correct on the Cathode current sense resistor and I agree that 1 ohm is normally used. Like I said there are some with even 5 ohms but that could be repair work.

[Enzo]

10 ohms, 1 ohms, doesn't matter. it is a vanishingly small value compared to the rest of the circuit. 10 ohms math is no more confusing than 1 ohm math, move the decimal point over one.

yes, the screens go to a B+ node, there is no ground symbol there.

[Amp Kat]

10 ohms, 1 ohms, doesn't matter. it is a vanishingly small value compared to the rest of the circuit. 10 ohms math is no more confusing than 1 ohm math, move the decimal point over one.

yes, the screens go to a B+ node, there is no ground symbol there.

But which B+ H or G ? H is 700 volts G = 365. The screen resistor is only 1.3 ohms which is basically a dead short. Node F is going to the plate of the PI tube. I'm thinking G is going to the Screen connected in Triode mode.

Chandler we'll get to that other circuit with the transistors as I have some opinions and questions on it too but as of right now I'm leaning to it being a grounded grid constant source amp and not a cascode. It's hooked up like a cascode kinda but it's functions and input stage don't follow cascode characteristics.

http://www.pacair.com/mmamps/schemat...pton_HD130.pdf

Ok I just found this and the screens are 1.5k 10 watters but do check out the node which is G and 365 volts so that explains the 50mv's at 700 volts. We had this same example in a Marshall amp a few weeks back. I believe so your bias is good Chandler at.5

[H Chandler]

Bruce,

I measured the secondary side (both taps) WRT the black wire that's normally grounded. However, the OT was completely disconnected. Both taps read open WRT the black wire. With the transformer disconnected, I only should be reading the resistance of the secondary if the the OT's OK. I wouldn't expect the resistance to be infinite if there was continuity in the winding. Nor would I expect zero resistance either unless there's a dead short due to the resistivity of the wire. Am I missing something?

Also, on the primary side I measured both legs WRT the center tap. One leg of the primary is nearly open (100's of megs) and the other side looks pretty normal appx 300 of ohms. So based on all this, I assumed the the OT is toast.

I probably would have just measured the primary side at the plates like you said, except with all of the caps and fly back diodes I wanted to be sure I was reading just the transformer windings, so I disconnected the whole thing.

I'll take a closer look at the screen R to see where it's attached when i get back to the bench, but I was surprised that the resistance was so low.

Thanks,

Howard

[Amp Kat]

Note that schematic was also from an HD130 which the schemo reads at the top also for 2275 chassis so it should be the same. I'm going pick up a MM combo this afternoon so I'll do some testing and report back.

[Enzo]

The screen resistor is 1.5k, not 1.5 ohms. It conects to node G - 360 volts. Oh, you already said that...

[Bruce / Mission Amps]

Bruce,

I measured the secondary side (both taps) WRT the black wire that's normally grounded. However, the OT was completely disconnected. Both taps read open WRT the black wire. With the transformer disconnected, I only should be reading the resistance of the secondary if the the OT's OK. I wouldn't expect the resistance to be infinite if there was continuity in the winding. Nor would I expect zero resistance either unless there's a dead short due to the resistivity of the wire. Am I missing something?

Also, on the primary side I measured both legs WRT the center tap. One leg of the primary is nearly open (100's of megs) and the other side looks pretty normal appx 300 of ohms. So based on all this, I assumed the the OT is toast
Thanks,

Howard

I think you are right the OT is open.
The secondary resistance of most 2-4-8-16 ohm OTs is usually about 1 ohm down to .3 of an ohm.
If you haev anything higher then that I'd suspect the OT instantly.
Yes the primary should be hundreds of ohms from end to end and around half of that from the center tap to either end.... if you have oly a few ohms or thousands or more... I agree, the OT is toast.
By the way of the many amps I've worked on, the Fender Champ, Princeton, Princeton Reverb, Marshall 50 and 100 watters and the 50 watt Music Man amps are the ones that I have run into with bad output transformers.
Not sure why the Music Man OT have been bad but the others are usually obvious user abuse.

[H Chandler]

Enzo and all

Thanks for helping me sort this all out. I see now that I could have avoided a lot of the confusion if I'd done a bit more examination of the circuit with the drawing in hand.

You are absolutely right, the screen R is 1.5K-10W. The "cathode" resistor was connected to pin 7 heater pin of the last el-34, which gets grounded at the next tube in. It was actually 2-5watt 22 ohm carbon comps in parallel. Whatever caused the OT to go must have been related to high current as these two resistors got so hot that they started sweating a resinous goo and kind of melted together from excessive heat. The heat generated even caused the insulation from a nearby wire to melt.

By the way, this is the second amp that I've worked on recently that had excessive heat problems and included instructions on the drawing to set some voltage to a specified level, without taking into consideration the actual plate dissipation. Could it be that the OT was taken out by a well meaning tech dutifully following instructions, and what followed was a case of thermal runaway due to the increases in wall voltage over the years, and variation of power tube characteristics.

Thanks for putting up with this sloppy diagnostician's questions.

Howard

[Mars Amp Repair]

I recently had questions about this amp on this forum. Try a search onbandmaster.
I learned that this ckt idles at nearly NO idle bias. The bias tracks the drive signal leverl. As the signal is increased, so is the bias current. All this accomplished by driving the transistors that are in the Cathode to ground path with the audio. It makes it extremely efficient as as well as lengthening the life of the tubes tremendously.

I don't see where you resolved the fuse blowing issue...Have you disconnected the centertap to the OT to see if it still draws current. if the amp still draws major current with the power tubes out & centertap disconnected, it would make sense to look at the power supply first....rect diodes, filters, etc. ....glen

[H Chandler]

Yea, that other circuit (whatever it's called) with the cathodes driven by the transistors is a pretty cool design.

As for the cause of the fuse blowing, I think Enzo may have hit upon it in an early post when he observed that what appears as an open to an ohmmeter can behave as a short when the 700+ volts cause internal arcing.

I've checked out the rectifiers and they seem OK. I will probably just replace the filter caps just because they're old.

Whew! it looks like the parts list is adding up -- OT, Power Tubes, Filter caps....


Thanks,

Howard

[Amp Kat]

I'm sure all of those things you mentioned Chandler contribute to the OT blowing but the least I would say being the change in wall. The new amps seem to adjust pretty well to it and the transformer manufacturors haven't changed the ratio to compensate for it so it must be ok. I see heaters running at about 7 to 71/2 volts and rectifiers at 6 on the heaters with no ill effects. I also agree that at 700 volts especially if cranked could cause some serious damage from a shorted tube or coupling cap or bias failure. Another possibility that OT's hate is an open load which can be the result of a bad speaker,bad cord,jack or loose connection and at high power levels could take it out quite easily. The tubes back then were 6CA7's and not sure if you've seen them but the big bottle ones which work the best in this amp were very stout and 6L6'ish sounding IMO so with that amp was super clean. When you get it right we'll have to meet at the sandbar on the east side of Lake Maurepaw for a Sunday jam. I did pick up that MM it's a 2275 HD130 100watter from my buddie so they should both be ready hopefully around the same time. Music Man on the bayou !

[H Chandler]

I still think it's better/safer to set the bias based on dissipation.

So it looks like I will need to get the Mojo transformer. Should I buy direct from them, or would I do better buying it from someone who does enough volume to get the discount?

AmpKat -- Sounds like a nice thing to do on a Sunday afternoon. Check your private messages.

[Bruce / Mission Amps]

I still think it's better/safer to set the bias based on dissipation.

So it looks like I will need to get the Mojo transformer. Should I buy direct from them, or would I do better buying it from someone who does enough volume to get the discount?

AmpKat -- Sounds like a nice thing to do on a Sunday afternoon. Check your private messages.

This version of the amp idles it's power tubes at around 18 watts each.
Driving a good OT, with a PT having enough currently capability and at +700vdc, a pair of these 6CA7s can make way way over 50 watts!!
The driver is standard cathodyne driver and I'm pretty sure yours is a normal class Ab1 amp.
Most agree, the transistor cathode driven amp (grounded grid) is a class B amp and those power tubes idle in very low current, like around 7ma each.