MA25-pn mods

[anchovy]

Anyone have any good mods for a Masco MA25-pn? Bought one off ebay. Needs caps and other minor work but is in excellent condition otherwise. Would like it to be my main harp amp. Also have a MA25-P.
Thanks for the input!
Masco MA-25

[Voske]

Hey Anchovy,

Have you found any decent mods for harp yet?
I have a Masco MA-25PN on my bench right now which could use a little more "drive" and grind. At least that's what the owner requested.

I'm thinking about leaving MIC1 as is and lowering the pre amp voltages on MIC2 a bit to get a little more overdrive on this channel. The guy likes playing harp with a tube screamer..... Not my first thought, 'cause I really like the "clean" kind of harp overdrive.

Anyway, the guy also wants to play blues guitar on this amp, that why I thought to scrap the line input and convert this channel together with the MIC3 input to a Fender 5C3, with 6L6's as power stage. Ted Weber has also done this with his 5C3P kit. The paraphase PI and power stage are already into place, Now I only need to scrap on 6J7 and replace it with a 6SC7 or 6SL7 as pre amp section.

It would be nice to hear what you've done to your amp as it comes to harp mods.

The speakers which came with this amp are (home build by the owner) one cabinet with two 8" Jensen Bluetone AlNiCo's (2x 4Ohm in series) parallelled with another cabinet driving a 12" Jensen (8 Ohm) speaker.

Cya,
Marc

[anchovy]

First of all I have started on the late 40's MA25-pn. It's the brown amp. It has the triodes in it. I will be working on the newer MA25-pn( blue and silver ) next. The mods for the blue and silver amp (newer design, early 50's pentode preamp) will generally be the same.
First of all you are right to lower the plate voltage to the preamp. This should lower the gain a little and get more plate saturation. First measure the plate voltage on the preamp.Try taking out the 270K resistor and put a pot in. Turn the pot all the way down before turning on the amp. Then turn the amp on and turn up the pot to try it at different voltages. When you have the sound you like remove the pot and measure the resistance across the pot. This will tell you what resistor to put in. Also on the preamp I have removed the 15 meg resistor and .005 cap and ran the mic input directly into the tube. This should help with more hifidelity. You'll want to go to a cathode bias on the preamp. This will help filter out higher frequencies with a 220uf@ 50v cap and ran it across a 1 k resistor and ran it from cathode to ground. This can also raise the plate voltage a little. You can do this two ways. You can use one cathode bias circuit joining the cathode of the two preamp tubes. If you do this, what you can do is pull one of the preamp tubes out. This will raise the plate voltage of the other tube resulting in more gain on that tube. What I am planning on doing is, putting in a separate cathode bias on each preamp tube for mic 1 and 2. I am replacing the cathode bias cap on the 6sc7 and the 6l6g with a 220uf @50v and might lower the value of the resistor in the cathode bias circuit of the 6sc7 tube to 1000 or 820 ohms . I also am replacing the power supply caps with a 100uf and a 50uf @ 450v. The 100uf cap will be right after the rectifier tube. Another thing was to wire 2-100 ohm, 1/2 watt resistors across the heater filaments and run them to ground. This will help with reducing noise. Another thing I did on the earlier Masco ( triode amp) was the remove the phase inverter resistor and replace it with a 25k audio taper pot. What this does is it allows you to bias the phase inverter more towards cutoff. You must then reverse the speaker leads for this to work. What happens is you can crank up the volume fatten up your tone and minimize feedback. It does work. This will be a 8200 ohm resistor, number 64 on a Sams photofact. I haven't yet but will experiment with the phono line in next. Also I'm looking at 2-8A125 inch Weber speakers with the H dust caps to reduce high frequencies that cause feedback problems. Am planning on running them in parallel.
Hope this helps but is probably way late sorry about that. Been trying to study up on some of this before I posted anything more.
Anchovy

[Dave Curtis, dB AudioTech]

"The 100uf cap will be right after the rectifier tube."

You want to check the spec on the rect. tube for the maximum capacitance to use after it; I don't know of any that'll take 100uF. That will put a lot of stress on that tube to initially charge that cap.

[anchovy]

You could be right. I don't know the specs on the rect tube. All I can say is I got those mods directly from gerald Weber.

[anchovy]

I tried the mods I posted and I must say they were no where near my expectations. Should of tried them first. I apologize for that. I ended up putting in a 1/4 mic jack and connecting it to the phono input. This worked well. The plate voltage is low on this tube and it only goes through two tubes this way. The second tube is the phase inverter. The first tube is a 7n7 with a gain of 20 and the second I changed from a 7n7 to a 7f7. The 7f7 tube has a gain of 70. I changed the mic inputs back to the original contact potential bias even though there are too many stages of gain for a high impediance mic. It feeds back immediately. I won't be using them unless I put together a low impedience Shure mic with a 99c86 CM. Going into the phono input lets me get alot of volume out of it before I get any feed back. The only problem is the pot for the phono acts as a gain contol. The volume controls for the mics are before the phono pot. I am going to add a real volume pot after the phono pot so I will have a gain control and volume control. I am still experimenting on using a pot for the resistor in the phase inverter stage to lower feedback issues. Currently I can turn the volume all the way up and have the gain on 3/4. Gets a lot of volume with a good sound. Am currently running it through a 15 inch 415A biflex Altec Lansing speaker. Sounds better than my two new 10 inch Weber classic 10A125-0 speakers run in parallel. With the additional volume control I will be able to run the gain higher and turn the volume down enabling me to get better breakup at lower volumes. I'll post a schematic when I am done with it.
Anchovy