Identification help - please

[ricach]

For reference sake, its a:
Merrell Electronics Model MA 12 High-Fidelity Tube Amp.
ITT tubes - 12AT,12AX7, and 2-6V6GT. 5Y3GT Rectifier

Power Transformer numbers:
12580-3
138926
I'm guessing the 138 tells me its a Stancor-ChicagoStandard but I can't find anything on it. And no other manufacturer cross references it.

Output Transformer:
No numbers anywhere. Multi-tap 4/8/16 ohm.
Primary -
Grn/Yellow(Screen V3), Blue(Plate V3), Red (Rectifier), Red/Yellow (Screen V4), Brown (Plate V4)

Secondary -
Red(16ohm), Yellow(8), Green (4), Black (Gnd)

Any help in finding specs on these transformers would be greatly appreciated. Also, a schematic would be nice as well.

Thanks

[Enzo]

The numbers would be the amp manufacturer's own part numbers. OEMs as a rule do not use off the shelf parts. SO you won't find it in a catalog.

The OT? I'd just look at it as one suited to use with a pair of 6V6s. They are unlikely to have used a 2 watt transformer there just as they would not have used a 40 watt transformer there. It will be what most any 2x 6V6 OT is. What do you want to know about it? It is an ultraliniear OT, those are nice. You can use the UL taps or not, as you chose.

The PT? You can add the tube heaters up and have an idea what current the heater winding can supply, and the HV winding will be similar to other small amps that have a similar tube complement. SInce it is an MA-12, I am going to guess it might put out about 12 watts.

[ricach]

Thanks Enzo. I'm not familiar with ultralinear so I'll need to research that. I'm relatively new at this but do notice this is not a normal circuit design, and my research into the hi-fi audio circuits confirm this as well. I'd like to learn more about the output transformer. In general, most circuits I've studied tie the power tube plates to the xformer, and the screens to B+. In this case both plates and screens go to the xformer. How does this work? B+ does feed the xformer as well, is this just a 'feature' and as such just a round about way to feed the screens with B+?

The other unusual aspect to this design is that the plate on 12AT7a is a direct connect to the grid of 12AT7b. And the plate and cathode of this 12AT7b each feed to the grid of seperate 6V6's. What's happening here? Is this a phase inverter?

I'm a ways from finishing a schematic, but I can sketch out these last stages if it'll provide a better picture.

Thanks again for any help. BTW, the amp works fine, and I understand its not designed for guitar amplification, but I'm still surprised how really 'blah' it sounds with a guitar plugged into it. I'm looking forward to changing that. :)

[ricach]

Reading up on ultralinear OT's I agree they are very interesting - but can't say I truly understand what's going on. I also am now familiar with long and short tailed phase inverters - coming from being only somewhat familiar with single ended designs, I had no clue what I was looking at in this circuit initially.

Ok, so now I figured I'm up to speed on this design - until I started to draw out the tone and preamp section. That is by far the wildest, and most confusing part of this design. I've redrawn it 3 times now trying to arrange it so it flows better when reading it. But considering that the existing BT doesn't sound that great, I'm not sure its worth the effort. So I'm looking for an existing tonestack design that works well using a 12AX7 (feeding a 12AT7 short tailed phase inverter if that matters). I know there's a bunch out there, but I realize they all have their inherit weaknesses. I, obvioulsy, want crystal clear highs, and booming bass as needed. Don't we all? lol. :)

Actually a better question is - what old fender amps have a nice responsive bass/treble control? I might think of adding a presense control if necessary.