Thoughts On Cloning This Amp?

[The Ballzz]

Here is a link to a schematic for a Marshall Studio 15:

http://www.classictubeamps.com/schemati ... w_4001.pdf

I've played through one and it is pretty awesome, however given it's rarity, I would be hesitant to get one and modify it as I believe it should be. I would like to build this and reconfigure the NFB circuit to be an actual "presence" control, possibly modify it's on board attenuator to have several "switchable" levels and likely not bother with the complication of the balanced DI output.

It is kind of an unusual layout for a Marshall, given the treble and mids between the first two triodes and then having just the bass control just before the PI.This kind of eliminates the typical Marshall interaction between all the tone controls. Also kind of unusual, is the PPIMV!

One issue I see is identifying appropriate iron to be used. A member on another forum "upgraded" some or all of the iron to accommodate EL34 power tubes, but he has not yet shared how/why/where/when!



Any Thoughts/Opinions?
Gene

[martin manning]

I have one of these things and I agree it's a great little amp. Mercury makes reproduction transformers for it so you can go the full monty if you want.

[The Ballzz]

Mercury is horrible about not posting full specs for their stuff! Is their replacement/upgrade any higher current capability than the original 1.62 amps on the 6.3V tap? The reason I ask is that some folks claim that this amp was really designed for and intended to have EL34's in it, but the PT specs don't bear that out. EL34's use more than triple the heater current of the 6V6, not to mention the conversion of V1 heater power to DC chews up at least a little current. This leaves us at needing close to 4 amps on the 6.3V, if wanting to use EL34's! The 1.62 of the OEM just barely covers the 6V6's.

Just Curious Here,
Gene

[martin manning]

The original Drake transformers certainly do not look big enough for EL34's, and if the MM's fit in the same chassis I can't imagine they would be either.

[The Ballzz]

Well Sir,
I was simply asking, out of curiosity, as I will likely stick with the 6V6, if I build it. However, I've read several reports that the MM iron actually gives a considerably higher voltage than the OEM units. This also appears to be the case across most of ClassicTone's product line. What the heck is up with that?
Gene

[martin manning]

Might depend on what the local line voltage tends to be. I hear numbers from 115 to 130.

Marshall 18W transformers would work fine for this circuit if you don't mind dropping the line out. Hammond makes a lay-down (original) 18W PT, and Magnetic Components makes both the original and upright versions.

[M Fowler]

Heyboer also makes a stand up 18w Marshall PT got mine from watts audio but can order directly from Heyboer too.

[The Ballzz]

Thanks Folks,
This one was suggested by a friend who really likes them for his 6V6 builds:

http://www.classictone.net/40-18066.html

Notice on the schematic for the Studio15 that there is not a dedicated "bias" tap on the PT. Also note how the bias circuit is set up. To date, I've only built cathode biased amps and the whole concept of getting the adjustable fixed bias circuit correct is a little daunting!
Thanks Again Folks,
Gene

[martin manning]

This one was suggested by a friend who really likes them for his 6V6 builds:
http://www.classictone.net/40-18066.html

Too much voltage. My 4001 has about 385V on the plates, so you need 18W iron as I suggested above, with 290-0-290 HT.

[stephenl]

What is the function of the diodes bypassing R6 and R7 after the PI?

[martin manning]

There was discussion on this a while ago. Most thought that they are there to maintain some load on the PI at low volumes. I think it might be a noise-gating function... I'll have to check that hypothesis out.

[The Ballzz]

This one was suggested by a friend who really likes them for his 6V6 builds:
http://www.classictone.net/40-18066.html

Too much voltage. My 4001 has about 385V on the plates, so you need 18W iron as I suggested above, with 290-0-290 HT.

I was going by what I supposed were the voltages shown on the hand drawn, 1985 print. Luckily, a friend has a very lightly used one of these amps, still all original, including the tubes. He bought it new. I will check voltages, etc. before I make up a BOM.

I've also been considering the possibility of adding reverb to this. Not sure where to start on that, other than the initial thought. Maybe a tube driven Fender style or maybe even a Marshall style digital reverb, that I believe comes as a whole circuit on it's own board and may be available as a "stand alone" replacement part. The reverb and presence additions would make this quite the little Princeton Reverb killer!
Just Sayin'
Gene

[martin manning]

I was going by what I supposed were the voltages shown on the hand drawn, 1985 print. Luckily, a friend has a very lightly used one of these amps, still all original, including the tubes. He bought it new. I will check voltages, etc. before I make up a BOM.

The hand-drawn schematic shows the PT secondary at 300-0-300, and 70mA.

[roberto]

There was discussion on this a while ago. Most thought that they are there to maintain some load on the PI at low volumes. I think it might be a noise-gating function... I'll have to check that hypothesis out.

like the Peavey two diodes "coring" cicuit?

[pdf64]

I think they may be replicating the action of the power tube control grids chopping off the top of the wave, thereby maintaining a similar wave character over most of the control range of the master volume.