Guild Masteramp - I'm in the weeds here

[Randall]

Got a Guild Masteramp in today. No model no, no serial number, no ID of any kind. Tubes are two 12AX7, one 6SN7GTB, four 50L6. One Jensen P12 P, dated 49th week of 1955. All original, with insulation falling away from some of the wires. Owner thinks it is a model 99. I can find no definitive info on it, nor schematic. He wants to re-cap it and get it working. It needs an iso transformer, power cord and a bunch of oddball caps. But, there are a couple of flies in the ointment for me here. There is a cap under the board I do not relish having to replace, given the condition of some of the many components I would have to disconnect in order to get to it. As you can see in the photo, does it look to be a 4uF@450 V.N.P? Non-polarized, but if so why is it labeled + and at one end, and what would it be doing in the circuit?

Also, what is the round three terminal object at lower left? it is about 1.5 inched in diameter and the same in height. Is it a multicap? I cannot see the markings on it without removing it. I have not seen one of these before.

I must admit, I am a bit tepid on this one. I'd appreciate any advice.

[eschertron]

I'd say it's been modded/repaired in the past, from looking at the pic. Once you find a schem, ask the owner for permission to do a total restoration.

[Enzo]

The 220k and the other resistor look new, but rough looking resistor looks stock. The caps could well be original, especially the waxed one.

I had one once, I drew my own schematic.

[mozz]

Early Guild MasterAmps were likely not made by Guild (see references to early Guild amps on p.46 of Hans Moust Guild Guitar book.) There is some indication that the early amps may have bee made by Hagstrom.

The lower priced and student amps may have been build by Univox.

When Guild introduced the new line of amps in the 1956 to 57 period (66-J, 99-J, 100-J, 200-S, etc. ) the amps were likely designed and build by Guild.

From 50L6s (and No Transformers) to 6L6s Tube selection is where the Masteramp story gets weird. The catalog mentions that the 66 has a six-tube chassis—RCA tubes at that. The 99 had a seven-tube chassis, and the Double-Twin had nine. What the catalog doesn’t say is that the power tubes on all three amps are of the 50L6 variety. Mention the 50L6 to any amp old-timer and they’ll guffaw and roll their eyes. In the 1950s, the 50L6 was not used as a high-fidelity tube. Rather, it was used when the goal was to push volume through a speaker. The benefit of the 50L6, at least in 1954 and 1955, was that it had enough voltage to eliminate the need for a power transformer and allow the use of a weak (read “cheap”) output transformer. This type of construction would, at least in theory, lower the price of the amp. Using the 50L6 involved wiring the filaments of three tubes in series, creating a trio. The Masteramp 66 had a single trio of 50L6 tubes. The Double-Twin and 99 had two sets that ran push-pull. The Double-Twin also used dual selenium rectifier units that are similar to today’s diode rectifier. Preamp tubes on all three were 12AX7 and 6SN7.

[Randall]

With the 50L6 wired that way, is it advisable to add an iso transformer? Seems like it would be. Still can find no schematic, only the version with 6L6s and a rectifier tube and PT.

[The Dude]

If there is no PT and series filament configuration, yes an isolation transformer.

[Randall]

Thanks, Dude. Now, how many VA do I need? Will this 50 VA rated Triad be enough? The amp has a 3 amp fuse in it, but I currently have no way of knowing if it is the correct value or not. May be worth mentioning that it is SS rectified.

http://www.mouser.com/ProductDetail/...BiswKPjsFps%3d

[Jazz P Bass]

Quote:"The amp has a 3 amp fuse in it, but I currently have no way of knowing if it is the correct value or not"

No you not have an ammeter function on your meter?

How about getting yourself a 'Kill-A-Watt'?

[Randall]

The amp has wiring insulation that is currently falling away, and exposing the bare metal in several places. I dare not power it up in this state. I moved some filament wiring to get a look at a cap, and it was stuck to the chassis, the very act of moving it exposed the corroded copper. This one is going to be a piece of work.