My biggest amp project yet

[Jim Coash]

Greetings:  I recently acquired an old chassis that looks like an opportunity to attempt building an amp from the ground up.  It is a Masco MA-25 PA amp that lived at the K of C hall for many years.  It clearly got very hot at some point and was retired as irreparable.  The good news is that the chassis is HD, punched for everything I will need, the two transformers (Thordarson 24R05U A/C & Quality T-25 01 231 output) measure OK but it is fairly small.  Plenty deep but not very wide or long.  It had two 6L6 outputs, a 5AR4 and four pre-amp tubes.  I am having some trouble reading the transformer wiring because of the overheating/smoke problem.  I think I have 350/350v ct, 6.3v ct and 5v with multi-taps for 4,8,16 plus a couple others on the output with a center tapped input.  Here is my question.  Knowing my limited capability, what should I choose to build?  If you know me, you know I want clean, accurate, reliable power and simple tone controls.  Possibly a boost circuit, pre-out and an NFB on/off switch.  I will use the two 6L6 outputs but use a solid state rectifier.  I have space for up to four 12AX7 or similar tubes but I want only one input.  I do not need reverb but vibrato would be nice, especially if I can find a design that uses the non-optoisolator circuit.  My first thoughts are for a Fender Bandmaster, Bassman, Vibroverb, Vibrolux or similar.  It will be tight getting any of those turret/eyelet boards in there.  I can mount some things on top of the chassis.  It has a nice full metal cover and steel bottom plate.  I prefer keeping it simple in the hopes that when I'm done it will work.  AB763?  With some upgrades?
Jim

[sluckey]

Stock AB763 uses an optocoupler trem. Hoffman's modified AB763 uses the 6G16 trem circuit that modulates the bias of the output tubes. The Fender 6G12 Concert uses a two tube 'harmonic' tremolo that is the best sounding trem I've heard. A single channel version would use 4 little tubes, 2x6L6s, and ss rectifier.

[Jim Coash]

That sounds like the perfect solution.  I think I have most if not all of the resistors, caps, pots and chassis fittings to build such a unit.  Would an eyelet/turret board from Doug fit into the space I have (roughly 11"W X 71/2" D X 3" H)?  Given the tube punch outs and transformer locations the space inside the chassis is tight.  I may need to figure out a way to make the board either pivot, hinge or be open somehow to access the tube sockets.  What can I make a custom board out of?  I have 1/4" Plexi-glass, asphalt roofing that looks similar to the material used by Fender, 1/4" Masonite or I might install the parts between two long terminal strips, raised off the chassis with standoffs.  I can mount the larger caps and bias board on top, if need be.  The idea of using two long European style barrier strips seems workable.  I think they will stand the heat and the amp will be used tubes up, with the original metal, ventilated cage protecting everything.  I have a small boxer fan that would fit over the old 5AR4 opening and keep the air moving.  I won't need the 5V tap on the transformer anyway and a 6V fan should be perfect for adequate ventilation.  I will take a look at the info on that amp in Doug's store.  Shouldn't be hard to find.  Jim

[sluckey]

I would use a new chassis. Hoffman has everything you need to build a proper board. He will even make it for you if you provide a drawing.

[Jim Coash]

I appreciate your input.  Unfortunately, I need to use what I have and my budget won't permit ordering a new chassis or an eyelet/turret board.  I am considering the Plexi 50 design for several reasons.  First, it is the smallest main board that includes the features I need; one input. 6 tubes (four pre-amp and two output), suitable for the transformers I have that are already fitted to the Masco MA-25 chassis and the vibrato circuit like the Princeton I have which I also think is better than the amps that use the opto-coupler system.  I downloaded the Plexi board picture, cut it out and now have moved a few things on the chassis so it fits.  Still a little tight but I like the fact that that chassis is similar in size to my Ampeg B-15S making them interchangeable.  I may mount it to a flip-top board just like the original giving me some additional flexibility.  I am also intrigued by building an amp that has EL34 tubes for output.  It seems quite apparent that Doug sees the EL34 as the quintessential output device based on his web name.  The only amp I ever built using them was also my first; the Dyna Stereo 70 my Father and I built together in 1962.  I loved that amp dearly.  Can I assume that the Plexi 50 will be more Marshall like in sound?  Having never owned a Marshall that makes good sense too.    I have the entire back chassis built with Neutrik Speakons (4 & 8 Ohm) Neutrik 1/4" (4 & 8 Ohm) outputs in parallel, the panel mount male A/C connector and a fuse holder with non-grounded A/C outlet.
The front has a power switch with light, standby, NFB on/off switch and room for four pots.  I want volume, bass and treble but can't decide whether a midrange control is useful or perhaps a built in overdrive arrangement.  My volume control has a switch built in.  Should I use that for standby and then install Doug's boost switch?  If I go this way the only thing I don't have is a pair of EL34 tubes.  Is there a better choice than the ones on the EL34 site?  With a solid state rectifier and a little extra filter capacity I'm thinking 50 watts is no problem.  I have a choice as far as my homemade board material, MDF or Plexiglass.  I have both.  I think the clear board would look cool!  Jim 

[Jim Coash]

My new amp project, based on the EL34 Plexi 50 with the boost switch is coming along nicely.  My Dad gave me a nice matched quad of EL34 tubes, RCA NOS, so that is covered.  I fabricated a board using the EL34 design from Plexi-glass.  The Thordarson power transformer seems like a perfect fit.  I haven't yet found a good diagram of the Quality T-25 01 231 that came with the Masco MA-25 chassis.  It has  black, green, yellow and four two-color stranded wires and an enameled solid wire the I think is/was red.  Smoke from overheating has discolored most of those wires making them hard to decipher.  I think the solid wire is the CT for the input windings,  the black the common for the output, the green and yellow, the 4 OHM and 8 OHM outs but the four dk/lt wires a must be figured out yet.  I am trying to use my VOM to shed light on those.  Anyone have a schematic for that output transformer with colors?  Jim

[sluckey]

There's lots of info on the net for the Masco MA-25, including schematics. Should be helpful.

[Jim Coash]

I have found a decent schematic and plenty of chat out there on my unit but I can't seem to find anything that gives wire colors for the Quality  transformer.  I have the input side figured out and I am pretty sure about the common, 4 and 8 ohm taps, but that leaves the unused 2, 16 and 70v line taps to cap off.  I just can't quite figure out the enameled wire that might be yellow or red.  Could it be the NFB loop?  Jim

[sluckey]

I just can't quite figure out the enameled wire that might be yellow or red.

Does it have continuity to any other wires? If so, how much resistance?

[Jim Coash]

I measure increasing resistance between the black wire, which I believe is the common for the output side, and the yellow, green, two color and enamel wire.  The tap that I suspect is the 2 ohm (yellow) wire then the lowest  resistance (about 3.1 ohms) and ohms increase from green (8 ohm) through the various other wires.  The enameled wire with a different jacket, not cloth over rubber but just woven "shrink wrap" like material seems to be a center tap.  On the schematic I do see a wire that appears to be a CT running back to the output stage input through a 3M resistor.  That is the NFB I think.  I plan to run the NFB through a switch so it can be on or off.  Does that make sense?  Jim

[sluckey]

On the schematic I do see a wire that appears to be a CT running back to the output stage input through a 3M resistor.

We must be looking at different schematics. Can you post your schematic or a link to it?

[Jim Coash]

The best one I have found that is very similar to the amp I have is here. www.angelfire.com/vt/audio/ma25schematic.gif and this is the one I have been using for reference.  The enameled wire I think is the one connected to the 8 ohm tap and running back to the driver tube through a 270K resistor.  I assume that is the NFB loop.  My plan is to take it to a front panel switch so that it can be bypassed.  There are no colors on the schematic for the transformer leads but I am a quite sure I have isolated the inputs and the common, 4 and 8 ohm taps.  I have just capped of the remaining 15, 125, 250 and 500 taps which leaves one wire left.  I do measure continuity between it and the other taps.  The fact that it reads "0" when connected to the 8 ohm tap seems pretty conclusive.  Should I use the heated circuit in the Masco design or the one Doug recommends with a pair of resistors to ground?  I see that the CT on the 6.3V heater pair goes to ground though a 200 ohm resistor tied to the output tubes.  I am building Doug's Plexi 50 with one input and the boost switch circuit.  Three 12AX7s, two EL34s and a solid state rectifier instead of a 5V4G.  I built my own board using a Plexiglass base and #6 X 1" screws for turrets.  The chassis is tight but I used a couple stand offs to raise the board over the bottom of the chassis.  I have all the pots, switches, jacks and connectors in place and the power supply about done.  I just need to populate the board and do the point to point wiring.  Jim

[sluckey]

running back to the driver tube through a 270K resistor.

That's a big difference from the 3M you said earlier. We're probably looking at the same drawing. Does yours look hand drawn?

Should I use the heated circuit in the Masco design or the one Doug recommends with a pair of resistors to ground?  I see that the CT on the 6.3V heater pair goes to ground though a 200 ohm resistor tied to the output tubes.

I would use the hum balance pot since you have it. But the wiper will need to connect to ground since you will be building a fixed bias amp.

I built my own board using a Plexiglass base and #6 X 1" screws for turrets.

Gonna take a lot of heat to make a good solder connection on a #6 screw (even if it's brass). You're likely to either melt some plexiglass or have a bunch of poor solder connections. I highly recommend you build a proper board, or just buy one from Doug.

[Jim Coash]

Yes I was way wrong on the value of that resistor.  I did not have the drawing which is hand drawn but reasonably good albeit without the transformer colors.  So far the soldering has gone will with my custom board and it does look neat being able to see through it to the chassis wiring below.   I have been wrapping the leads around the screws and essentially soldering the parts to each other rather than worry too much about a good connection to the screws themselves.  I placed a piece of wire between those that have connections on the bottom to facilitate solid connections and I am testing them with my meter to be sure they conduct.  Another question is, the Plexi plan calls for a choke and the parts list recommends the "large" Fender one in Doug's transformer section.  So far, that's the only thing I haven't found in my parts bin.  I have a lot of similar sized transformers.  If I substitute one that has a similar DC resistance across its primary, will that work?  I'd just cap off the output leads.  I also have a bunch of ferrite coils (metal donuts wrapped with multiple turns of wire) that used to be crossover inductors.  Some are quite large.  The Fender one is 4 henrys and 500 ma with a dc resistance of about 100 ohms.  Do I have any other options?  Jim