Masco 25N - Just making some noise

[LooseChange]

I have this old PA on my bench. Just messing with it for now.
Curious about the first gain stage (Mic 1, 2 and 3). They are all identical. I think they are Grid leak biased. Is there an advantage to changing them to cathode biased for use with guitar?

[DummyLoad]

a cool sounding amp - i tried selling one a while back in the classifieds here - no bites.

i'm posting a schematic that richard and i drew up - it seemed as though mine had been untouched. PS resistors were waaay out of whack and needed a cap job.

we hacked it into what we thought was a best compromise in sound and minimally invasive to the original ckt.

[LooseChange]

Thanks ISO.
You didn't change much. I was hoping you would have modded one of the inputs to be cathode biased.
The filter caps and most of the coupling caps are bad. I'm working through that first but thinking ahead.

Did you do anything with the output jacks? Mine has two 6 or 8 pin sockets and the selector is a 8 pin socket with a little pin you use to select the impedance.

[DummyLoad]

LC, i didn't do much - cap job, most resistors were way out of spec. and were replaced. mine was in pretty poor shape cosmetically so i didn't want to throw a lot of time and money at it. the original ckt sounded really good so i changed as little as possible. i bumped the gain and mids up in summing stage w/ a 680nF, changed the low-freq roll off w/ smaller CC and that's about it. i added a 1/4" phone jack for the speaker and strapped an octal plug for 8ohm, a couple of light bulbs, grounded power cord and called it all good. i have had on consignment at a music store, but it's so damn ugly no one wants it, 

[LooseChange]

Sorry. I should have said you didn't change much.  I did everything you mentioned except the mods.  Got the thing working like it used to. I sounds good.
That Phono input is useless.
I think these things are pretty cool looking with the slanted and lit control panel.

[DummyLoad]

try those mods... they really wake it up. mine is a rust bucket. it is cool looking amp, i just didn't want to spend so much time refinishing it, however, in retrospect, perhaps i should have... 

[PRR]

> That Phono input is useless.

It would work fine if you had a 1940 magnetic cartridge and played 78s.

It could also be your iPod input.

If you do not have a knob on your guitar, the gridleak inputs will "soft" overload on hard strum. If you do have a gitar knob, set the channel gain for "slight hiss", control overall volume/gain with the guitar.

If you fiddle a lot (more than ISO did), I have a suspicion that the mixer tube driving tonestack is lame. In this case no real problem, because the pentode-triodes into 6L6 gives so much gain that levels out and into the tonestack are small (1.4V into tonestack).

[LooseChange]

I messed with one channel and made set it up to be cathode bias.
I used a the 270k plate, a 2.2M screen and a 2K2 cathode resistor. Kept the 100n to ground.
How do I maximize the gain with this type of tube? I'm thinking of adding a reverb stage and want to push a pan using a reverb transformer off of one of the 6J7's and mixing it back in.

Thanks!

[LooseChange]

One more question:
Why the 10K dropping resistor at the screen node? It drops the voltage from 400v to 300v. Do we need it for a guitar amp?

[DummyLoad]

Why the 10K dropping resistor at the screen node? It drops the voltage from 400v to 300v.

design is in RCA datasheets.

400Vp/300Vg2 -25Vg1 into 6600Ω - 50Vpp drive - 34 watts.


edit: oops! it's self bias - 32watts... 57Vpp drive.

 

[DummyLoad]

How do I maximize the gain with this type of tube?

bump up B+ then fiddle with Rsg and Rk.

 

[DummyLoad]

here's a thought...

rework V1 and V2 for self bias; disconnect V3 and phono from V4-4, swing 270K summing R of V2 level control from V4-3 to V4-4; strap V1 & V2 inputs together. you can use a switched jack on V1 and strap V2 in || with V1 when plugged into that jack - use the level controls as a blend. bump up B+ to node "B" w/ smaller (<100K) dropping resistor.

other options: strap V5 as pentode and fiddle w/ NFB.

[LooseChange]

Thanks!
Increase gain at V5 and too much noise.  I think it's just going to be as it is.. It's loud and it's fat.
Here is a picture.

[RicharD]

>I think it's just going to be as it is.. It's loud and it's fat.

deja vu

I remember when Iso got his up -n- goin'.  It sounds great like it is.  Why mess with it?

[LooseChange]

And.... It looks pretty cool too.

[DummyLoad]

yours is great shape - cool looking amp. octal pentodes rock.  :glasses9:

[PRR]

> How do I maximize the gain...

MASCO already did that. The most gain for the least cost, therefore some profit.

You could cut-out the mixing and the tone network.

It is VERY likely that any great increase of gain from input to output just brings up the hiss and makes "clean" impossible. Mike-amps are usually goosed to the point that hiss is a problem when turned-up. Normal operation is turned-down a bit, but the gain (and hiss) are there when you need them (and to put large numbers on the spec-sheet).

If you drop the 6L6 G2 resistor for more "bite" and less blocking, you must raise the cathode resistor to keep the plate current in the same area (probably 101% of 19W, the rating of a 6L6G). I'm guessing 280-300 ohms.