Masco MA17N

[madkatb]

I've been checking several forums I belong to about harp amps and this is definitely the place to be.
I have several old tube PA's that I have "collected" over the last few years and it's now time to start some mods/upgrades on some of them. I've previously modded a couple of Epi Vjr's (basically Lonewolf mods) with good results.
This is the next amp on the project table:
Some photos are here:masco ma17n pictures by madkatb - Photobucket
(6SL7 plate loads are 100k, cap on screen grid of 6SJ7 is a larger Mallory OD, probably .01? have to remove to read)
So I'm looking for ideas. As a start, I'm thinking three inputs: grid leak, cathode biased and straight into the 6SJ7 (the phono input). But before that I have to try to reform the caps or replace them, replace some of the coupling caps (they aren't suppose to "ooze" are they?), rewire the heaters, finish cleaning the chassis, sort out a better grounding scheme,etc.
As I said I have several of these old PA's so this is just the first step in a long journey...

[Bruce / Mission Amps]

The pentode preamp tube cap, with the ?, can be anything around .1uF to .22uF @400v to 630v and should decouple the screen fine.
I've worked on lots of these old 6L6 Masco amps and they sound great for harp and will push about 18-20 watts when tuned up right.

Wash it all off and dry it. Keep the good 6SL7s and 6SJ7 in the circuit... use new power and rectifier tubes if needed.
Strip the crap out, use new discrete modern 450v-500 main filter caps, 50v cathode bypass caps, 1 watt carbon film resistors, a 10 watt 200 ohm cathode biasing resistor and leave the first two stages grid leak biased.
Use either inexpensive Mallory 150s or Russian K40Y paper in oil caps for a nice sounding amp.
Dump the hum balance rheostat on the chassis and use a pair of 100 ohm resistors across the filament string with their common point connected to the cathodes of the power tubes.

[madkatb]

What I was suggesting for the first stage (the 6sc7) was mic1 as grid leak and mic2 as cathode biased, either through switching on the input jacks (for the 6sc7, common cathode) or replacing the 6sc7 with a 6sl7 or 6sn7 (two cathodes). That, along with the "phono" input direct into the 6sj7, would give three different possible preamp choices. But please don't get me wrong, those were just my suggestions and I'm here to find out what you guys think, and why. Is grid leak a preferred input for harp? If so, why? As in, what does it do differently than cathode bias? Wouldn't cathode bias give more voicing choices or am I thinking too much along guitar lines?
That should (begin to) take care of the first stage...LOL

[MWJB]

"Is grid leak a preferred input for harp? If so, why? As in, what does it do differently than cathode bias? Wouldn't cathode bias give more voicing choices or am I thinking too much along guitar lines?"

Grid leak presents the mic with a huge impedance load, the resistor at the tube grid is only there to bias the tube. Grid leak sounds good with crystal & dynamic mics (rounder,fuller, wetter) but I don't much care for the sound with CM/CR/Hi-Z dynamics. Grid leak inputs can choke up easier when hit with a big signal too.

Yes, I would suggest that you convert one of the inputs to cathode bias, say a 2.2K cathode resistor, 5.6Meg grid load. You would need to either switch to a tube with individual cathodes for each triode, or employ a switch to ground out the 6SC7 cathode resistor in grid leak mode (half the suggested cathode resistor value if staying with the common cathode).

[madkatb]

Thanks for your reply.
I was thinking along the lines of 3 inputs (grid leak, cathode bias and phono) in keeping with the present circuitry of the amp. I can switch cathode circuitry on the 6sc7 with grounding switches on the input jacks. I wasn't sure about the grid leak vs. cathode bias or which was better for what type of mic. Just trying to make the amp a little more versatile.
I don't really want to add a bunch of "unnecessary" mods but would prefer to keep the original circuit pretty much as is and only tweak things that need it to improve the tone or usability. I have new electrolytics, Mallory 150's and metal resistors on their way from AES.
So I've been reading through all the previous posts looking for ideas and info. I'd like to keep posting updated photos as this project progresses. But I still have lots of questions regarding refurbishing the amp. Like "What's the best way to remove surface rust and patch the paint? Or do I just remove the rust and then lacquer the bare spots to keep the chassis more original?". And other questions along the same lines...

[Bruce / Mission Amps]

I'm with MWJB.
I find grid leak/contact bias bias inputs can be limiting and the first stage can be over driven very easily with a real hot mic... I think that sounds nasty thick and muddy when that happens. On second thought, that might be OK for novice players with very little articulation.
My friend Rick Davis has a couple mics that put out over 750mVac with not very much effort. That's crazy!
Most grid leak biasing is under .5vdc so you can see what happens at the very first stage when hitting it hard.