Masco ME-27 Amplifier Part 1 Let's Take a Look 

If you think this Masco amp has tubes that are unfamiliar to you, they made other models that used Loctal tubes, including 7C5 outputs which are basically a 6V6 but with short skinny pins and a locking base (and a 6.3V filament/heater, despite the tube nomenclature beginning with a 7)! I don't know if the 5V4 tube is original or was installed to replace a stock 5Y3 or 5U4 by the previous servicer (presumably to increase the B+ voltage), but anyway, the 5V4 is kind of like a baby version of a GZ34/5AR4, with an independent cathode, and therefore it usually has less voltage drop than a 5Y3 or 5U4, a slower rise/warmup time, and it can also handle a little bit more first-stage filter capacitance without arcing. If you want more voltage in order to increase the dissipation of the 6L6GC tubes, then you might actually want to use a GZ34, or even a solid state rectifier.

You've got a juicy piece of equipment there my friend. This is the same amp and model that Rhett Shull bought and demoed on his channel. Very cool Martyn!

Really enjoying the channel. The harmonica player I work with collects these, and has a couple of the square dance caller models with built in turntables. One of the more rare Canadian amplifier manufacturers Verlage used to take these power amps, add their own preamp circuit and Altec 15" speakers. They roar.

Wow Martyn, I didn’t expect to see this one on your bench. You obviously have the bonez to make a wonderful minimalist type of design. I say keep it cathode biased on the outputs, and voice it with your ears and a guitar in hand, and it will sound great coming from your bench. I look forward to the conclusion and final design. Cheers my friend..!

Sounds so good, that clean is lush

Interesting amp that one,loveley tones.Been trying to get my head around the DIY Layout app,its a handy tool for working out schematics.

There are 2 slightly different versions of the Masco ME27 (and a ME27P-3 with a phonograph player mounted on top of the tube cage); I have the Sam's for both of these. Comparing the 2 schematics, the resistor and capacitor values are virtually identical but they do use different power transformers. From looking at the pictures in the service info it looks like you have the later version of this amplifier, with a transformer that is top mounted to the chassis with its bracket-feet while the earlier version has a rectangular cutout and the transformer lays down sideways into the chassis cutout. Nominal power input is given as 117VAC. DC working voltages are pretty close to each other, within 10 to 20 volts in most respects, except that the later version ---- presumably the one you have ---- runs the plate voltage to the 6SC7 phase inverter at 190 V, and the earlier unit at 160 V. The later version has 04, 8, 15, 70, 350 and 500 ohm outputs; the earlier version has 2, 4, 8, 15 and 500 ohm outputs. Neither unit has global negative feedback. Sams manual for the earlier (1952) unit is from Set 155, Folder 11; the later unit (1955) is Set 270, Folder 9. Hope this helps!

Grid leak bias can rock! It’s a whole different sound! Before you decide to rewire that first stage, have a look at the supro 1690T Coronado schematic traced from multiple examples of the amps by Eric Kettenburg. That’s the Jimmy page/Zeppelin 1 amp that Mitch Colby cloned for the Sundragon. Grid leak bias first stage into cathode bias 6L6GB output section. Try larger value plate (360k+) and grid leak (6.8M) resistors and much smaller value couplers. If you need a copy of the schematic, just let me know where to send it.

Can't wait to see how it comes out Martyn. It's in the best hands for sure!. Thanks.

I recently found your channel and have been enjoying it very much. Amps like these are among my favorites and I think you did a great job with it. Subscribed, and looking forward to the next one!

Early in the video when you said "6L6GC", my first thought was, they are the wrong tubes. I would probably try to find a pair of 6L6GBs to avoid stressing the output transformer too much if in the future someone tried to bias the amp too hot.

Masco was short for Mark A Simpson Company, of Long Island New York. Many of their small PA amplifiers are famous for use as harmonica amps in pretty much stock form. Sam's schematics posted service information for many Masco models and so that's where I would look if you have trouble finding a reasonably accurate schematic for it. Unfortunately somebody still owns the publishing rights to Sam's and so you might not necessarily find one for free online. PS, im old fashioned, and no longer have a full-size computer anyway, so I draw out schematics by hand, with pen and paper, and Whiteout! I've only watched the first minute of the video so far so we will see how you do! ( And by the way, British finger picker Martin Simpson used to play an acoustic slide guitar instrumental titled Masco Blues but I have no idea what the Masco in the title of that song refers to or where the song came from; it appears to be a traditional that he learned from somewhere else. You can find a video on RU-vid of him playing It).

Hmm... the 6SC7 got me thinking and so i checked the early tweed deluxes, 5A3, 5B3 and 5C3 and sure enough they have a 6SC7 as V1 and it is wired with grid leak bias via a 5M resistor. Is the phase inverter a cathodyne? (I think you said it was wired as a triode) - which would be very 5E3... Interesting stuff. Obviously what to do to it depends on what the owner wants. I'd be tempted to go for a 5A3 deluxe which are great sounding amps. Not sure about the grid leak bias though, might want to change that to cathode biased and I'd consider using different values of bypass caps to get different tones for each triode. Obviously it probably needs smaller coupling caps too. Interested to see what you do with it though!

Good job tracing out circuit . Looks like someone has one Ive got a selmer truvoice same sort of era 6l6 valves havent a clue whats going on in it but the bass moves the floorboards..cant find any info ..great toons 😊

Watching how you go at aroiund 16:00, the 6SC7 has a common cathode for both triodes, they're tied together *internally*.