Masco MA-8

[hearforever]

Hi again... I posted about this a couple of weeks ago but then went away on vacation and wasn't able to follow up. I recently got a Masco MA-8 amplifier from the late 40's that I'm trying to set up for guitar. I recently modified a Bogen PA amplifier in this way, but I'm pretty inexperienced and have a relatively limited understanding of electronics.

I'm looking for a practice/recording amp for my apartment with kind of a bright, Fender-like tone but there's qualities about the tone that the amp produces that I like, too, so I'm not looking to directly copy a Champ circuit or anything like that. I was hoping I could post the schematic for the amplifier and gather some suggestions for sensible modifications I could make.

I've been playing through the Mic input rather than the Phono because it has much more gain and a brighter tone that's better for guitar. I'm surprised, though, that the volume is so low when I play my guitar through the Phono input. Why might that be?

There's 3 knobs on this amp, Mic Volume, Phono Volume, and Tone. Ideally, I guess, I would have the knobs set up Volume after the 6SF5, Volume after the 6SJ7, 1 tone knob, or a volume knob and Bass and treble controls. Any thoughts on this?

Why is the tone control connected to a capacitor at C8 on this circuit?

Also, the volume pots are 250K...in the Bogen and most circuits I've seen volume pots are 1M. What should I know about these 250K pots, what difference will it make to the circuit?

Many thanks for your time!

http://www.hereforever.org/ma8n.jpg

[Enzo]

Looking at the schematic, the phono input bypasses the first stage that the mic input uses. SO naturally there is less gain for it. COnsidering there is no RIAA EQ on the phono input, it is apparent it was meant to have a crystal cartridge instead of dynamic. Those can put out something like a volt of signal, so they don't need a lot of gain.

C8 is the interstage coupling cap leaving the first tube, that is not a tone control, that is the MIC volume control. R3 and C11 are the tone control.

250k or 1 meg volume pots - won't make any difference, sound the same.

If you don't want the phono input, then remove R7 and use the control itself as you please. You already have a control after the 6SF5.

Unless I am out thinking myself, I hope the drawing is wrong on the bias supply cap. C6 is it? Off R12. I would think that was a negative voltage there and the cap should be drawn facing the other way.

The simplest way to add a post 6SJ7 - let's start calling them V1, V2, etc - a post V2 volume control would be to wire the control in parallel to R3, and connect its wiper to pin 5 of V3 instead of where pin 5 goes now. Then break the connection at the top of R3 as it would no longer be needed.

[woodyc]

You might want to remove R14. Its cutting a little gain.

[hearforever]

Enzo,

thanks for your thoughts. I'm still confused why the bottom of the Tone pot connects down to C8/R12. doesn't the bottom of a pot like this usually connect to ground? I'm probably misunderstanding something pretty basic here...

[Bruce / Mission Amps]

Enzo,

thanks for your thoughts. I'm still confused why the bottom of the Tone pot connects down to C8/R12. doesn't the bottom of a pot like this usually connect to ground? I'm probably misunderstanding something pretty basic here...

Yes, they usually are grounded and this one is too.
As far as AC or in this case audio goes, the 50uF cap is ground.
The impedance of a 50uF cap at audio frequencies is much much lower (close to nothing) ...compared to the 270K resistor, which is grounded through the inductor.

[hearforever]

Oops...I was confusing C6 and C8 from the bad jpeg compression.

I have a couple more questions.
Is capacitor C7 before V1 there for a Mic signal and therefore sucking away some of the guitar tone? Also, what affect would lowering the value of R4 have... why is it so high compared to something like the Champ 5F1 or even 5C1 circuits?

[J Martin]

C7 keeps DC off the grid so as not to upset the bias. You will notice there is no cathode resistor. I wouldn't lower R4 to less then 5 meg. Or, eliminate C7 and cathode bias the tube with a 1.5K to 3.3K resistor. Then R4 can be one meg or less. You can increase C7 a little in the existing circuit, but if its too high you will get some bad distortion.

I just re worked a slightly later version of this amp a few months ago. It had a 6AV6 and a 6AU6 in place of the 6SF5 and 6SJ7. It was a little too much gain to manage without a lot of changes so I changed the second tube to another 6AV6. It had a rotary switch tone control which worked through the feed back loop. Great souding amp.

[Enzo]

That input stage is called a grid leak stage. The grounded cathode is the clue. The extra large grid resistor allows a buildup of negative charge on the grid to bias the tube. You could rewire the input stage more conventionally, but that grid leak stage may well be part of the tonal charm you like about the amp.

[hearforever]

I've made some changes to the circuit
The editted schematic is at http://www.hereforever.org/ma8nv2.jpg

I removed R7 from the original schematic, although I should probably put it back if I'm going to use both channels, otherwise there will be interference between the channels, correct?

I changed the tone stack to the 'Moonlight' circuit I found at http://amps.zugster.net/articles/tone-stacks and added a volume control here. It acts rather strange though...There is almost no volume unless the volume is at 10 and the tone pot doesn't seem very responsive either. I'm thinking I have used the wrong pot values here. Will reversing them do the trick?

I'm tempted to just make it a single channel amp. If so, should I remove R6?

Joe Jasniewski suggests increasing the volume of the decoupling capacitor to .01 with a 250K Pre volume pot (http://www.netads.com/~meo/Guitar/Am...tar/index.html) Should I consider changing C7 and C8 to those values?

It was suggested to remove R14 but when I did so the gain was out of control, very distorted and a "pumping" effect with the volume.

Any other suggestions? I appreciate all the replies and patience with my ineptitude. I'm happy with the changes thus far.

[Enzo]

Interference between channels? Are you planning to plug instruments into BOTH and play them at the same time? Besides, you don't have two channels, you have one channel that has inputs at two points along it.


You have the labels misplaced, the tone pot is labelled master volume and the MV pot is labeled tone.

I would change values of C7 or C8 or any other C based upon some shortcoming of the tone as it stands, not because someone says they are cooler values. How does it sound with the values in there now? LArger caps will add to the bottom end, and part of why it is designed the way it is I'd bet was to keep too much bottom from running away with the amp and leaving it out of breath... so to speak.

[hearforever]

Oops, yes Tone and Volume are mislabeled. The Volume pot does ground to to where the Tone pot appears to on the mislabeld diagram. The Volume pot is actually 250K and Tone is 500K

Any idea why they are acting funny? The volume cuts out very drastically unless they are both at about 10.

[Enzo]

well for one thing, you new tone circuit adds a lot of voltage division to the signal path.

[hearforever]

Well I had the tone and volume pots wired wrong and figured out that problem. Dumb mistake.

I feel like this amp could be making better tone. I'm wondering if I can use what is in there an try adapting to something like the 5C1 champ. only because it seems simple and within my means and similar to the Masco circuit in some ways. Here's what I have so far... I'm a bit confused by C5/R8 and C9 compared to the Champ circuit.

http://www.hereforever.org/ma8nv3.jpg

http://www.ampwares.com/ffg/

[MWJB]

C5 is a cathode bypass cap for a cathode biased preamp tube, the 5C1 uses a preamp tube in grid leak bias. You're not comparing like with like. 5C1 is not going to make as much power as later champs. I would stick with the cathode biased preamp.

R8 is a voltage dropper, it's doing the same job in both amps (2Meg in the champ) just in a slightly different place & value. Try both ways and see what you lke best.

However, if you're looking for better power & fidelity, perhaps you might want to look at 5F1/5F2A as the basis of your circuit, you'll need to drop the 6SJ in favour of a twin triode octal tube (6SL7?), or buy an adaptor for a noval socket (12AX7 type).