Masco MA-35 makeover

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premiumplus
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Masco MA-35 makeover

Post by premiumplus »

Hi fellas,

I don't know where to post this so I'll put it here. I am building a custom amp for a friend who has a Masco MA-35 amp as a donor.
The amp uses a pair of 6L6 output tubes but from there back it's going to need a complete new front end. I'm going to do a Tweed Fenderish build with Vol, T,M,B, and PPIMV.
The question I've got is that the power transformer puts out 400-0-400, unloaded. Using the 5U4 that the amp was designed around, I'm thinking that I will see over 560-580 VDC. Is that going to come down significantly when loaded? As it is, I sure won't feel comfortable with anything less than 600 volt filter caps.
The original amp used a 4.5uH choke and it had a 6F6 tube driving an interstage transformer that was used for phase inversion. Interesting design from the 40's.

What do you think, should I go with the original power transformer as is, or get some Zener diodes to drop it down at the center tap?
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martin manning
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Re: Masco MA-35 makeover

Post by martin manning »

Do you have a schematic?
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premiumplus
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Re: Masco MA-35 makeover

Post by premiumplus »

Hope this works. It's a PDF of the schematic.
Note that all I'm going to use from this amp are the transformers and the choke.
masco-ma-35n-ma-35rcn-amplifier-schematic.pdf
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xtian
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Re: Masco MA-35 makeover

Post by xtian »

MOSFET B+ dropper is less expensive and more flexible way to drop HT voltage, IMO.

PPIMV sounds lame on Fender Tweed circuits, but better on JTM, JCM designs.
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Re: Masco MA-35 makeover

Post by pompeiisneaks »

I don't think you see much more than maybe 40-60 volts drop after loading with tubes from my past limited experience. You may need to use a VVR type setup to drop it or a ton of zeners.

The masco uses 6L6's so it can't be that it runs them at some massively over max voltage, so I'd wager it's just setting it up right.

Looking at this schematic: http://4tubes.com/SCHEMATICS/Music-amps ... A%2035.jpg

it seems the power transformer has a center tap and it's going straight to the rectifier, so maybe it's not as you're thinking?

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martin manning
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Re: Masco MA-35 makeover

Post by martin manning »

A 5U4 will drop a lot of voltage, maybe 40V, and you will have the PT secondary equivalent resistance. I think you will see something around 480, but the power supply caps should be rated for unloaded voltage. I'd stack a couple of 300 or 350V caps if you are not going to use some kind of voltage reducer.

In the schematic you posted, the first 6SN7 is the phase inverter. The tapped inductor is functioning as loads for the third 6SN7, which is driving the 6L6''s from its cathodes.
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premiumplus
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Re: Masco MA-35 makeover

Post by premiumplus »

I guess my main concern is that the circuit I'm putting together won't load the power supply like the original. I wish I knew what the 6L6 plate voltages were on the original amp. I'm doing away with the phase inverter, going to a more traditional design. I'll need a 12 volt filament transformer because I want to use 12AX7 tubes in the preamp and PI. It's going to be an interesting project. Maybe the best course is to go ahead and build it with conservatively rated filter caps and run it up with a the variable AC supply on my bench. Then I can measure the real world voltages and make changes as needed.
I'm pretty sure I want to do a cathode biased output section. All my Z amps use it and I love the way it plays compared to fixed bias.
The customer wants it to have just a bit of "on the edge" breakup at max gain, and my Z-Lux is outstanding at that.
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Re: Masco MA-35 makeover

Post by Stevem »

Since you will not likely have the load of all those preamp tubes I think you will end up with closer to 500 volts on the plates of the output tubes.

Now the other question will be how high will the heater voltage be with its smaller current load?

In regards to having a heater voltage above 6.9 you can certainly leave some tubes in place with just there filiments powered just so they have there load on the heater string
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Re: Masco MA-35 makeover

Post by martin manning »

premiumplus wrote: Tue Jul 18, 2017 10:39 amI guess my main concern is that the circuit I'm putting together won't load the power supply like the original.
Most of the current load comes from the power stage, so you won't be far off.
premiumplus wrote: Tue Jul 18, 2017 10:39 amI wish I knew what the 6L6 plate voltages were on the original amp.
I think 500 or less, and it looks like you have a 120V primary.
premiumplus wrote: Tue Jul 18, 2017 10:39 amI'll need a 12 volt filament transformer because I want to use 12AX7 tubes in the preamp and PI.
No need for that. 12AX7's are most often run at 6.3V by paralleling the filaments: Join the free ends (pins 4 and 5) and apply filament voltage there and at the common pin (9).

Early Tweed Fender amps used octal preamp tubes so you could do that too. See 5C8 Twin Amp. Member rp built a very nice point-to-point version in a Hi-Fi style chassis. http://ampwares.com/schematics/twin_5c8.pdf
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premiumplus
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Re: Masco MA-35 makeover

Post by premiumplus »

Thanks for all the input, fellas. You guys are great. Martin, I had completely forgot about paralleling the two filaments in a 12AX7. I am a retired EE but I'm subject to brain cramps...

So for filtering, should I be safe setting up the power supply like a Super Reverb or a Twin, with a couple of 70uf/350V capacitors in series, then through the choke and the string of series dropping resistors and filter caps to ground between them?

I was surprised at how high the unloaded AC out of the transformer is. Most amps I've built have used closer to 300-0-300, not the 400-0-400 volts like I see here. Being that the Masco is such an old amp I expected to see a much more conservative rating. But this thing is so old that it doesn't even have a line fuse on it, which I found surprising.

One other thing I need to do before going much further is ensure that the power transformer isn't leaky. I did check with my Fluke 83 for inter-winding leakage and it looks good. I wish I had a megger. I've got just about every other piece of test equipment but that.
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martin manning
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Re: Masco MA-35 makeover

Post by martin manning »

That all sounds good. I think you'll want to find out what the OT primary resistance is and set the screen voltage accordingly with a dropping resistor and screen resistors on the sockets. I agree the voltage is pretty high... I thought it might have been a choke input power supply until i saw the schematic. Some old amps used a big resistor in the CT (back bias) to get negative bias voltage, but not here. I don't believe I've ever seen a vacuum rectified (6X5) bias supply like that before.
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premiumplus
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Re: Masco MA-35 makeover

Post by premiumplus »

I ordered some filter caps for the power supply, and I'm excited to get underway with this project. I'm cleaning the chassis up and gutting it while I wait for parts delivery and I'll post results as it progresses.

A thought just occurred to my caffeine starved brain that I want to toss out here...since I am dealing with a pretty hot power transformer at 400-0-400, what if I used a variable voltage regulating (VVR) mosfet to drop the voltage by about 100 volts? Or is this just going to kill my rectifier tube and create heat?
Or would a chassis mounted Zener diode in the transformer CT be a better way?

I realize that the best solution is to get a new power transformer but that's contrary to the whole idea of this project, which was to get a cheap donor chassis and build a useable amp for my buddy. He told me about this amp at church one morning a couple of weeks ago and I told him to buy it and I'd build him an amp. Stupid me assumed that it would at least have a usable power supply. Expectations always kick my butt.
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xtian
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Re: Masco MA-35 makeover

Post by xtian »

As I mentioned above, MOSFET is the way to go. A simple B+ dropper on the HT CT is easy to implement, and adjustable with a single 1w Zener (I use a socket so they're easy to swap).

And Lou recently developed an excellent VVR PCB for fixed bias that I beta tested. Also great. Cathode bias VVR is even easier.

These options are so much better than expensive, non-adjustable power Zeners, IMO.
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Re: Masco MA-35 makeover

Post by dorrisant »

Yes!!
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premiumplus
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Re: Masco MA-35 makeover

Post by premiumplus »

Very cool thanks a ton for posting that layout diagram, it looks like it can't be simpler. Is Mouser a good source for the MOSFET?
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