MASCO PA Conversion

[bean]

I just finished my first build... a Firefly from the AX84 pages. It was a blast and I love the amp! Now moving on to the next project.

I have an old Masco MA-50 PA amp from the late 40's I believe.

It has two 5U4G rectifier tubes and four 6L6GA output tubes. The preamp tubes are two 7C7's for mic preamps and a 7F7 for a phono input. There is also a 7N7 phase inverter.

My question is... does this look promising for an amp project?

I'm planning on changing the preamps to 12AX7 tubes.

The PT is a 700V CT @ .235A. Secondaries are 4.8V @ 6A and 6.2V @ 5A. I tested all of the voltages and the PT seems to be working.

The OT specs on the schematic are as follows: Impedance- Pri. 4400ohm CT with 2, 4, 8, 16, and 500ohm secondaries. DC Res. Pri. 85ohm with .2, .25, .4, .5 and 16ohm secondaries.

Some of the caps are in bad shape... and most of the pots are frozen as well. If anything... I plan on using the transformers, tube sockets, and chasis from the amp and using new parts for everything else.

Can I change this to use two 6L6 tubes instead of four? Is there a Fender (or any other) schematic that you would recommend copying to use with the transformers I have? The PT does not have a bias tap.... so I was looking at the cathode biased 5E5 circuit.

Thanks for your input...

Brandon

[gldtp99]

Yes, you can run only two 6L6GC's into the OT's 4.4k primary without problems---and you'd only need one Rectifier tube, but could still use two if wanted-----you can run a cathode biased output stage (like a 5E5---5E5-based heads are some of the more popular ones i make for local bands)---or you can run fixed bias if wanted---- run the bias circuit from one HT leg supplying the rectifier tube (like a BF Princeton---- but make it adjustable and ignore the Trem circuit---unless you want to add a tube Trem).
Use new filter caps, pots, etc---install a grounded 3-conductor Power Cord or grounded IEC connector.
Build away---- sounds like a fun project !!!..................gldtp99

[Darkbluemurder]

That PT is quite powerful. The heater current is fine for a quad of 6L6. If you are only using 2 output tubes then you can also use EL34, 6550 and KT88 and stay safely within the heater current specs.

With dual 5U4G rectifiers you should get a B+ of about 440-450VDC. That would also be fine for a quad of JJ 6V6S. With a SS rectifier you could get a B+ of about 490VDC. But I would use two GZ34s (5AR4) which should give 450-460VDC and the advantage of a slow turn on.

The OT would be in the typical range for a 6L6 duet or a 6V6 quad.

Most blackface Fender, Dumble and Mesa preamps would work fine with this circuit. But there are really no limits. Greg Germino uses 6L6 in his Marshall derived amps so that is an option, too. Why not build a Hiwatt, Vox or Matchless preamp with a 6L6 power amp?

Have fun
Stephan

[10thTx]

I have converted a handful of Bogen PA and integrated amps successfully & they sounded great to me. I do think using a point to point wiring that one needs to be cautious about the layout when using a high gain preamp to avoid oscillation problems.

I have essentially gutted the Bogens I have done and just used the chassis, PT and OT with all new caps, pots, resistors, filter caps and preamp sockets.

I have sometimes used paralleled terminal strips for the layout when the chassis topology allowed that.

I'll offer a simple cathode biased 6L6 amp to consider. With the PAB and NFB switches, .... you can get quite a few different tones to the amp from clean to a nice overdrive.

With respect, 10thtx

[10thTx]

Here is a layout I drew up (using another layout as a foundation) to go with the schematic. You could use 6 terminal strips with 4 non-grounded terminals each. That would give you two rows of 12 paralleled terminals to wire the amp up.

I don't care if you build it or not. I have one like this with reverb and enjoy playing it. The paralleled V1 5751 and the two mini-toggles give it lots of different tones to choose from.

CHECK for errors and to see if component values match between schematic and layout. Use schematic values.

With respect, 10thtx

[bean]

Thank you all for your input so far. I'm getting some great ideas! I like the idea of a cathode biased duet of 6L6's I have a few of those laying around!

10thTx... thank you so much for posting a schematic and layout! As a complete noob... that's a huge help! One question... you show using a 600v PT... and mine is 700v... would some component values need to be changed to accommodate the higher B+?

--

brandon

[Structo]

If your B+ is too high you can do what I did on my Hammond organ amp conversion.

That is, install a stud mount Zener diode on the center tap to ground.
Just choose one with the voltage amount you want to drop.

Make sure you get one that is big enough power wise.
The other thing is to make sure it is cathode grounded (if I recall correctly) or else you have to isolate it from the chassis.
But the whole idea is to use the chassis as a heat sink for it.
And use heat sink compound when mounting it.
I used a 25 watt one. Then be sure to bypass the diode with a .01uf ceramic to kill any noise.
Works like a charm.

[10thTx]

300v X 1.3 (GZ34) = around 390v on plates. 300-0-300 PT

350v X 1.2 (5V4 or 5U4GT) = around 420v on plates. 350-0-350 PT

I'd just use a 5V4 or 5U4GT and maybe increase the dropping resistor between node C and D to 10k/3w - 15k/3w range.

The main concern for voltage to me would be the V1 & I personally like around 150-160v on the V1 plates.

The schematic is just one idea of many good ones out there. 5E5 Pro is a good amp and there are certainly others also. I like the "Blues Amp" because it's simple but still has a good variety of tones to it, IMO.

If you have limited room on the chassis for 5 pots, you can also use a James Tone Stack which has an excellent tone, IMO. I've included a schematic showing that. This schematic has a SPDT mini-toggle on the V1 cathode. The different cathode cap values: .68, 2.88, & 10.68 will each change the tone and gain levels. So, that is a VERY easy way to increase more tonal options.

Have fun with whatever build you do.

With respect, 10thtx

[Structo]

So 10thtx the first blues 6L6 schematic you showed is a kind of simplified Dumble type, correct?

Have you tried it?

Oh, and can you explain the math behind figuring out what the voltage will be for such and such type rectifier on a power transformer.

I know a full wave recto isn't a 1:1 ratio from the supplied HV.
Is this the proper formula for the DC voltage from a full wave rectifier?

[img:137:42]http://upload.wikimedia.org/math/a/2/1/ ... 267a28.png[/img]

[10thTx]

It's a quasi sorta mutuated Dumblish pre-amp. Yes, my favorite amp of 3 is this one. I play one with 6V6's & reverb in it, .....BUT I can plug in Tung Sol 5881's without changing anything and they sound fantastic. Pic below

I don't know about the formula you use. As a general starting place guesstimate, I use:

5Y3GT is 1.1 X VAC so a 300-0-300 would give 330v.
5V4, 5U4GT is 1.2 X VAC = 360v
GZ34 is 1.3 X VAC = 390v
solid state rectifier is 1.4 X VAC = 420v

Again it is a starting place guesstimate that I use when I design my amps and overall, it's been a reasonable and reliable way for me to approach my builds.

With respect, 10thtx

[Structo]

Man, that is a beautiful cab!

Thanks for the info on the amp, I may have to try one of those.

I have all the tools to build my own cabs but no room in the garage to set it all up.
Drives me crazy.

I don't dare leave our vehicles out at night so I would have to tear down and store everything every night.

But one of these days I'll get all my cutting done and routing then glue it all up.

[Baxtercat]

That is great-looking.....and thx for those diagrams too.