MusicMan Phasor

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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seniorLoco
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Post by seniorLoco »

musinman hd150 Phasor i like :D

Just wanted to throw this here to see if anyone else wants to build just the PHASOR ... as it's perrty good ...liquid rush :lol: 8)
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soulsonic
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Post by soulsonic »

Yes, that Phasor looks nice, and something that could become a stompbox without too much difficulty.

But, something that's been bothering me for awhile.... can anyone here please explain to me how the output amplifier works. I see a driver transistor connected to the Cathode of the 6L6... a Common Grid setup? Why? Why? This really bothers me.
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seniorLoco
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Post by seniorLoco »

Initial MM had pre tubes but due to high voltage on the power tubes they had too many failing ..... something like that :scratch:
here is what i got from the MM forum....

"The earliest series of Music Man amps were made between 1974 & 1977. These are identifiable by the "script" "One-Thirty" as opposed to the "HD130" of later models.

Originally, this design had a 12AX7 phase splitter (the circuit that derives the 180 degree phase shift and drives the output tube pairs in push-pull).

These amps had a very warm overdrive sound when pushed to the max (very loud, though), partially due to this tube. Sometime in 1976 or so, it was decided to replace this tube circuit with a solid-state design that was more reliable.

Apparently, if the 12AX7 tube itself shorted (due to mechanical abuse primarily) it could take out the ouput tubes AND output transformer if not immediately shut down. The solid-state circuit minimizes this particular problem.

The solid-state phase-splitter circuit doesn't sound as warm at or near output tube overdrive (again, VERY loud with an HD130 model), but it is a much more reliable circuit that can take the physical punishment typically applied to guitar amplifiers.

At anything less than max output, I am hard pressed to notice any difference (your mileage may vary). Apparently, many amps that were returned to the factory for repairs were upgraded to the solid-state circuit and many of the later "old-style" amps were modified on the assembly line while retaining all the external cosmetic attributes usually used to identify those with the 12AX7 circuit.

The modified "old-style" amps and the first units of the newer series all had the 12AX7 tube socket hole plugged with a plate and a couple of screws (until the supply of pre-punched chassis ran out). The solid state circuit was a retrofit circuit board inside the chassis. "

"However, the later models were more reliable as it was found that under certain circumstances, you could wipe-out a set of output tubes and the output transformer if the 12AX7 shorted and the amp would quickly "run-away" thermally and lead to a "melt-down" of the output stage (Think "Chernobyl" or "3-Mile Island")."

There you go :wink:
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Post by soulsonic »

I'm sorry, but that did not explain to me the reasoning behind using a Common Grid arrangement as opposed to the more traditional Common Cathode - it only says why they chose to switch from a tube driver to a transistor, and it suggests that with either circuit it's still a Common Grid, so could someone please tell me why they would want to use a Common Grid... any advantages?

Trying to understand the logic behind this circuit is like having an itch that I cannot scratch.
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Post by seniorLoco »

I not sure ssonic ...high voltage (700vdc) calling for a design as such ?!? :hmmm:
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Post by dbrdly »

In response to the cathode drive used in the Musicman output stage. It allows class AB2 operation where the grid voltage with respect to Cathode voltage can go positive. The standard capacitor grid drive will not allow AB2 operation due to the fact that the grid appears as a forward biased diode. Any attempt at driving more voltage at the capacitor input simply pushes the average grid bias voltage more negative. Allowing the grid to cathode voltage to go positive results in much more plate current. Holding the grids at +23 volts and allowing the cathode to approach 0V gives a plate current double of a standard class AB1 design. It is a very clever design allowing 150W from 4 6L6GC tubes. I don’t think I ever read a post that explained this circuit correctly. One other point. You can’t simply rewire your amp with a driver like this using the old output transformer as the output will clip due to the double current through the primary. You will need a transformer with half the turns ratio. You could try using a 4 Ohm load on the old 8 Ohm output to get the proper impedance match, but the transformer would probably blow due to the smaller wire gauge used for the original output power. Peavey copied the design in the Classic 8 VTX series. Hope that helped.
-73-

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Post by teemuk »

Peavey introduced the VTX series around the same time so it's difficult to say who copied who. Both likely took the design from some electronics journal of the days.
As a side note, Peavey also added a solid-state tube amp emulation circuit to their version of the circuit so that this tube amp would behave and sound more like people expect a tube amp should do.

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Post by dbrdly »

Thanks for the info. I did read the two patents Peavey were granted. One on the input tube modeling and one on the solid state cathode drive making it limit more like an all tube output stage. Interestingly they used the Ultralinear configuration on the OT and ran the plates at a modest 500V rather than the MM 700V. I can only assume they were trying to differentiate themselves from Musicman. Looking at the 212-HD150 schematic with the Phasor looks like the same engineer designed them both, moving from MM to Peavey.
I own the 212-HD150 with the phasor. Still has the original 6L6’s and starts to clip at 160W into a 4 Ohm load. I replaced the original JBL E120’s with D120’s. These were re coned. The same cone is used on the E120, so I should be safe on power according to the recone shop, about 20 pounds lighter overall.

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Post by dbrdly »

I just went through some of my notes on the Ultrlinear configuration. There is more power available due to the screens helping the current draw on the OT primary. Not much though, maybe 5W max. Of course you get better distortion characteristics, but the main reason is, you don’t need a clean supply for the screens. This saves the cost of a choke. I worked on a 135W Twin Reverb that had the UL hookup. If you didn’t balance the bias, you would get hum. It was very picky on balance. It would drift out after a week and start humming again. Maybe the cascade configuration eliminated this problem.

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Post by dbrdly »

Cascode ! Spell check got me..

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