MusicMan Phasor

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

MusicMan Phasor

Postby seniorLoco » 10 Dec 2007, 04:47

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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Postby soulsonic » 10 Dec 2007, 05:15

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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Postby seniorLoco » 10 Dec 2007, 08:02

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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Postby soulsonic » 10 Dec 2007, 09:49

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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Postby seniorLoco » 10 Dec 2007, 09:54

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