Adding Master Volume To SS Amp

[surkis_magurkis]

I have a Univox U65RN that breaks up real nice when cranked (especially for SS), but it's pretty loud for playing around the house. I'd like to add a master volume to the amp. As far as I can tell, there are 2 places in the circuit where this may be possible.

I installed a basic volume pot to ground (from the collector of the 2nd 2SC536F to the base of the 2SB405GN). It worked like it should for a couple weeks, then the amp developed a loud buzz with no guitar signal coming through. When I removed the volume pot, the amp was back to normal. I expect the potentiometer failed and was dumping all the signal to ground. There wasn't much voltage going to the pot, and it was rated at 1/2watt (I believe). but I guess it couldn't handle the current. Does this make sense?

I think the transformer that is after Q3 (the 2SB405GN in the main part of the circuit) splits the phase before the output transistors. Could something similar to the PPIMV mod that is popular with many tube amps be used here?

Here's the schematic http://vintageunivox.com/pics/schematics/u65rn.jpg

[shooter]

Could something similar to the PPIMV mod that is popular with many tube amps be used here

I'm in the NO boat
You have a "gain" knob at the guitar, you have a Volume knob at the amp, be happy, there are windows to clean, toilet bowls to scrub, don't make extra work, just so you can troubleshoot a working amp that flamed out from curiosity.

[PRR]

That type of amplifier is funky already. Don't make ignorant mods. You were lucky this time, but you sure can blow-up something you can't replace.

Put 4r 20W to 100r 2W between OUT and speaker. (Couple-dozen 100r 2W in parallel. Snip out several or many to get less power.)

This *may* significantly change the "toan" of this funky amp.

[surkis_magurkis]

That type of amplifier is funky already. Don't make ignorant mods. You were lucky this time, but you sure can blow-up something you can't replace.

Put 4r 20W to 100r 2W between OUT and speaker. (Couple-dozen 100r 2W in parallel. Snip out several or many to get less power.)

This *may* significantly change the "toan" of this funky amp.

I know that it was lucky nothing failed in the amp. I'm very fortunate, indeed.

As far as looking into non-invasive options, I know of attenuators, but I'm not sure I understand what you're describing here.
Is there a name for this kind of thing, so I can look into the details?

[shooter]

it's a "build your own" attenuator PRR is describing.


try this simple idea;
put a gain/OD pedal between the guitar and amp, use the amps Volume to "attenuate" and the guitar volume for "OD"  If that doesn't work for you
 use a search engine to surf the web for "solid state amplifier attenuators"

[sluckey]

I'm not sure I understand what you're describing here.
Is there a name for this kind of thing, so I can look into the details?

It's called putting a resistor in series with your 4Ω speaker. 24 100Ω resistors in parallel is the same as one 4.2Ω resistor so only half the amp's power would reach the speaker. The reason for using 24 resistors in parallel rather than one 4Ω speaker is you can clip one resistor out of the parallel kludge at a time, causing the total resistance to increase a bit everytime you clip one out, and this will cause the power delivered to the speaker to decrease a bit. So, clip resistors out until you are happy with the results. When you are happy with the results measure the resistance of the remaining parallel resistors and replace with a single high wattage resistor.

Probably not what you wanted but at least you will not destroy the amp.

[surkis_magurkis]

Now that it seems like attenuation is the best option, my next question is "Is this safe?" Attenuators are most often used with tube amps for obvious reasons, but I can't seem to find much about using them with SS. Some people say it will work, others say it won't.

I'm guessing it depends alot on the type of output in an amp. The U65RN doesn't use an OT like most amps do. Does this affect the ability to use an attenuator at the speaker?