1946 Masco MA-35 PA Conversion To Guitar Amp

[keithb7]

Hi folks, I recently purchased a used 1946 Masco PA head. 35W. Price was good, it needed work. I found one bad 7N7 phase inverter tube, replaced it and fired it up. It works.
I had bad crackling on the pots so I pulled the original coupling caps in the tone stack and replaced. The crackling cleared up great. There are a few more coupling caps I will replace and have ordered them.
All the B+ caps have already been replaced by previous owner to new F&T caps. These look new and will remain. 3 prong cord has been installed. It appears to have a 270K negative feedback resistor.

I have questions about what I should consider doing to an old PA such as this to convert it to a guitar amp? Should I put 68K resistors off the input jacks? Or maybe 33K as the jacks are not wired to be a shared input?
Any tips on what I should do are appreciated.

I traced the signal circuit through the schematic here. Green is tone stack. Orange arrow is cathode resistor. Blue is through phase inverter.



Here are a few pics of the amp. Any tips appreciated. Thanks, Keith

[trobbins]

Suggestions:
Lower output stage coupling cap value and add grid stoppers.
Add cathode current sense for each 6L6.
Add grid stopper to V5a.
Add protection 1N4007 diodes to 5U4 anodes.
Change one input stage to cathode bias, and remove it's input coupling cap and add grid stoppers and 1Meg leak. Seto other stage grid leak for ok anode idle voltage.
Play and fault-find all creaks and groans.
Check hum level is not objectionable.
Check how much feedback is applied - may need to reduce feedback to ensure no instability for all load types.
Check which stages are overloading first, and maybe attenuate or increase stopper to move overload more to output stage.

[mozz]

First, i would change any and all wax caps, that looks to be about all of them. Chances are also that most every high value resistor is going to be way off.

[Justin Thomas]

Call it "Pentode-A-Saurus." Love all those pentodes!

Justin

[Broosh61]

I just bought an MA-35 yesterday, and I discovered this thread as part of my research. Any chance you could repost your pictures and schematic? A good schematic is proving very hard to find, so any pointers would really be appreciated. I dont want to start this project with the one schematic I have as there are no voltages shown and it is not the easiest to read. thanks

[tboy]

This thread is an excellent illustration of why we always recommend uploading pictures and other relevant files here instead of hosting them elsewhere. Time goes by, people move on, and their files disappear never to be seen again.

What would you like to do with your amp? Maybe you could start by uploading a photo to get the ball rolling.

[sgelectric]

http://www.angelfire.com/vt/audio/ma35schem.jpg

[Broosh61]

I just bought an MA-35 as well, and I'm starting the rebuild process. Unfortunately I cant find any schematics that match mine. I've paid for the content on one site that provides voltages and a schematic, but again it does not match my MA-35. I've got two large chokes in my circuit, and three of the large paper wrapped 16uF caps mounted on the chasis.

Everything inside the amp looks in great shape, but I really dont want to mess around with 75 year old caps, so I guess I'll start with replacing them.

If anyone has better / additional schematic information please post it if possible. thanks

[sgelectric]

I just bought an MA-35 as well, and I'm starting the rebuild process. Unfortunately I cant find any schematics that match mine. I've paid for the content on one site that provides voltages and a schematic, but again it does not match my MA-35. I've got two large chokes in my circuit, and three of the large paper wrapped 16uF caps mounted on the chasis.

Everything inside the amp looks in great shape, but I really dont want to mess around with 75 year old caps, so I guess I'll start with replacing them.

If anyone has better / additional schematic information please post it if possible. thanks

There are several MA-35 variants.
Many will have a suffix after the MA-35.
Exactly which one do you have?
As T-Boy noted, pictures would also be useful

https://www.google.com/search?q=masc...w=1679&bih=884

[Broosh61]

The model tag simply says MA-35. However, this model is clearly different. It has 3 mic inputs, and one phono input. It also has 3 large paper wrapped caps (yellow in the picture) mounted to the chasis. Here's a few images to help. I'm in process of tracing out the circuit, but a real schematic sure would be nice.

Do any of you have any advice on what should be changed to turn this into a good guitar amp? I realize the inputs will have to be changed, and I intend on swapping out most of the caps (at least the electrolytics). I have heard that the coupling cap values may not be ideal for guitar usage, and perhaps I should add some cathode bypass caps in the preamp stages - not sure

[trobbins]

It's a good looker.
Perhaps re-read the thread for advice.
A good guitar amp starts with a good amp that operates well and that has a schematic to appreciate why it may sound as it does, and then allows you to think about what tone or control changes that you would prefer. I'd suggest not trying to cut/paste controls/circuitry until you have had time to get the amp working well as it is, and how it presently sounds with your guitars and speakers.

[Broosh61]

It's a good looker.
Perhaps re-read the thread for advice.
A good guitar amp starts with a good amp that operates well and that has a schematic to appreciate why it may sound as it does, and then allows you to think about what tone or control changes that you would prefer. I'd suggest not trying to cut/paste controls/circuitry until you have had time to get the amp working well as it is, and how it presently sounds with your guitars and speakers.

Agreed. My first steps will be to add a three prong plug & fuse, replace the filter caps, and wire and input & output jack. That should be enough for me to figure out what I'm working with and possibly adjust from there. No schematics, so I am hand tracing it. While a bit painful it is forcing me to learn the layout

[sgelectric]

FYI: This amp uses a transformer as the PI. It is driven by the 6V6.
There is also a PT, an OT and the choke under the chassis.
All the tubes that begin with "7" are Loctals as opposed to Octals.
The amp design is pre-1946

SG

[sgelectric]

This thread may also be helpful.

https://www.tdpri.com/threads/noad-masco-ma-35.653035/

[Enzo]

Should not be hard to trace out the schematic.

All those waxed caps will be bad. By teh way, if the wax is too cloudy to read the print, hit the cap with a heat gun and the wax will melt away. And the wax that sy=tays will have cleared enough to see through.