Masco tweak

[Diverted]

Hi all,
Long time no see. I've been doing lots and lots of old radio work so have not been concentrating on amps. However I just got a Masco MA-35 that I'll be recapping and hopefully tweaking a bit.
It has three channels: two mike, one phono. They are all grid leak input stages summed together with 270K mixing resistors feeding the second stage.
I'll probably leave the phono and one of the mic channels alone, and play around a touch with the other mike.
What I'm looking for is one channel that would perform decently as a bass channel, and the other with a bit more breakup so it could be pushed with guitar. I think the amp as is, when recapped, would probably give me nice clean tones all around, so I'd just 'dirty up' one channel. I'm not averse to changing the input stage bias setup.
I'll be reworking the power supply just a bit - grid stoppers, screen grid resistors, etc., upping first filter cap to 20uf@1kv (two 500v 40s in series with equalizing 220K resistors), and probably upping all the other filter caps from 8-10 to 20.
Any thoughts on giving me a variety of tones from one mic input vs the other?

[dunner84]

Have you considered adding a switch to Cascade the two mic inputs?

[HotBluePlates]

... What I'm looking for is one channel that would perform decently as a bass channel, and the other with a bit more breakup so it could be pushed with guitar. I think the amp as is, when recapped, would probably ...

If you need to recap to get the Masco to be healthy, do that.

Then play the amp.  Play through it as-is with guitar, play through it as-is with bass.

What's right/wrong about the amp as-is for each application?  Knowing that will guide how to modify the preamp.  The mistake folks make is conceiving modifications whose goodness will be judged on subjective preference without knowing what is the starting-point.

[Diverted]

This is good advice. It's what I suggested when I told him I'd recap it. He really wants to have different tones out of each channel and since he lives about an hour away, wanted me to do some stuff in a "one and done" kind of arrangement.

I'm intrigued about the cascading channels. I've read about it a bit since the first post but am not sure how I would implement it in this amp. I'd like to at least try, and undo if not effective or good. Can anyone help?

Thanks.

[dunner84]

Ok a few caveats here. I am not sure how the grid leak bias affects something like a cascaded stage. You may have to cathode bias the second tube to make it work, and I also have very little experience working with the pentodes, but I attached a mod to your schematic for where I would start. Hopefully some more experienced eyes can chime in.

[Diverted]

Hi all,

So I'm leaving the amp stock as is for the moment, just trying to stabilize it. I'm running into a couple of issues I think caused by these crappy loctal sockets. They seem very worn and loose. Consequently the amp chassis is super microphonic and touching the tubes causes all sorts of interesting problems from no volume at all to full out and everything in between. Just not stable at the moment.

I'm thinking about switching out the 7C7s for octal 6SJ7s as they seem pretty much equivalent electronically. I have one question:

This amp runs the three preamp stage filaments with DC fed from the 6L6 cathodes. According to specs the 7C7 is a 6.3v filament tube. But the amp is running them around 24VDC when I checked ... this jibes with the voltages chart which specifies 26VDC on the heaters (pins 1 and 8).

Spec says 7C7 powered by 6.3v, ac or dc. I didn't know it was safe to run a 6.3v filament off so high a DC voltage.
If I switch to octal 6SJ7s, will this be an issue?

[sluckey]

This amp runs the three preamp stage filaments with DC fed from the 6L6 cathodes. According to specs the 7C7 is a 6.3v filament tube. But the amp is running them around 24VDC when I checked ... this jibes with the voltages chart which specifies 26VDC on the heaters (pins 1 and 8).

The dc voltage from the 6L6 cathodes is simply being used to elevate the heaters. The 7C7 heaters are being powered by 6.3VAC. Measure the AC voltage ***BETWEEN*** pins 1 and 8. You can change to octal tubes with no worries.

[PRR]

> This amp runs the three preamp stage filaments with DC fed from the 6L6 cathodes.

No. It IS confusing. There is 6.3VAC across the heater, and 26V DC from heater to ground.

{POST} ah, Steve didn't hesitate.....

[Diverted]

Thanks. It threw me. I’ve never seen amps with tubes that didn’t have 6.3vac windings to all of them. On this one the earliest ac winding is on the phase inverter.

[Diverted]

So, I'm having more issues with this Masco-35. I have a few questions and I'm sorry, this post will be all over the place.

First, I'm trying to figure out the grounding on this. As shown in pic, it looks like the last filter on the B rail is grounded. Mistake in the schematic? The original inputs were not isolated from ground (I replaced them with switchcraft switching jacks). Plus the node it connects to is definitely at ground, because the pots are wired straight into ground and that is shown below. What is going on here with that filter cap?

Also, the amp works, sometimes, on the second mike channel, but the microphonics are out of control. Just the loudest noisiest microphonics ever. Mic 1 does not work at all, just Mic 2, and I have not tested the phono channel because the pot is completely frozen.
Thinking the microphonics might be a noisy tube, I rotated them all around, no change. I also tried taking out the preamp tube for the channel I'm not using. But because these amps are made with a series string DC-referenced heater supply, removing any of the preamp tubes kills the signal dead.

Also: I replaced the 7C7 #3 (second gain stage, top, second one from left) with an octal 6SJ7 because a pin tab was bent and I thought that might be the issue. No change, and since then I bought a good new loctal socket and put it back in. So no more octals in the set apart from the rectifier and 6L6s.

I replaced two pots which read dead open. No change.

I reflowed all solder joints, no change.

I'm sorry I'm all over the place with this post.
But I would like to get the amp running smoothly and with stability.

I'm considering pulling out the non-functional channel 1 preamp and replacing it with a 6SF5, and setting up that input in a traditional triode manner (IE a fender input). If I ran that input stage to the mic 1 pot, would I have any issues? Thanks! Again sorry for the OCD post. This amp has a million problems and I'm not sure how to isolate/address them.

[PRR]

> it looks like the last filter on the B rail is grounded. Mistake in the schematic?

Yes, or....

> I also tried taking out the preamp tube for the channel I'm not using. But because these amps are made with a series string DC-referenced heater supply, removing any of the preamp tubes kills the signal dead.

That is not what the schematic shows. Is it the wrong plan?

[Diverted]

I'm sorry, that's my mistake. I cropped out the part of the schematic that shows that. Here's the full one.
On my amp, the 6.3V filaments run into the two 6L6s, the 7N7 phase inverter and the pilot light. They do not go directly to the first, second and third gain stage 7C7s. The only thing attached to those sockets are jumpers that start at the 6L6 cathode, hit all the 7C7 heaters in series, with a 10uf to ground off that node.

I just took the mic 1 completely out of circuit, the problem persists.

What do you all think of loctals in general? They appear to be super noisy though I've cleaned all the sockets and pins.

Would I be able to just take them out completely, and rewire the front end completely with triode 6J5s and a fender-style input scheme?
Or would I be better off keeping the grid leak inputs in, and rewiring the 7C7s for octal 6SJ7 pentodes?

I'm kind of at my wit's end. I've worked on this so much and it seems like I'm not any closer to a stable amp than I was when I started. Very frustrating.

[sluckey]

The only thing attached to those sockets are jumpers that start at the 6L6 cathode, hit all the 7C7 heaters in series, with a 10uf to ground off that node.

I'd like to see a hi-rez pic that clearly shows that. The attached revised schematic fixes the schematic error.

[Diverted]

Thanks, yeah that was a pretty grievous error!
I'm at work so can't take photos, but here's a drawing of the four 7C7s I drew a while ago. The jumpers are the only thing on filament pins 1 and 8. The 6.3 winding runs to the 6L6s, to the 7N7 phase inverter and over to the pilot light, where it stops. A very odd arrangement, at least to me. I've never seen it.

[sluckey]

Ahh, I'm finally on the same page with you. I've seen that in a few other amps. It would be nice if that was included on the schematic.