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Adding Master Volume To Vintage Univox

Started by surkis_magurkis, July 29, 2023, 05:04:23 PM

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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


No no no, :o
         Those 2 transistors are DC coupled they work as one, as the amp seems to be working after removing the mod then consider yourself very lucky. :-X

Research a small attentuator on the speaker output or throw a heavy blanket over the speaker but never interfere with the power section unless you know exactly what you are doing.


Is that a 50k-B volume pot (B50k)?
If it is change it to a logarithmic pot for better low volume control.
A50k is what you want for a volume pot, it should make it better.
There are no stupid questions.
There are only stupid mistakes.


Those 2 transistors are coupled and voltage dependant upon each other. I didn't realize that. Oops! I do consider myself very lucky, indeed.
I think if I decouple them, isolate the volume pot with capacitors, and supply the base of the 2N405GN with the correct voltage with a divider, it should work without frying the pot or anything more important in the circuit. Please correct me if I'm wrong.
If that doesn't work, I'm not going to mess with the power section. I don't have enough knowledge to do so confidently


Most of the magic you hear is most likely created in the power stage ,,,so highly likely if you mess up that you will regret.

you have negative Feedback connecting the output back to those 2 transistors so there is a lot to consider before you mess with that section,,, as I said you need to fully understand before you mess it up. If you blow that OTx you will struggle to find a replacement Rendering it land fill.

I'd advise you to check out simple Non invasive options.
Google L pads, or add a fixed resistive divider at Spk output.
Or try to find a less efficient speaker.


Thanks alot for you advice, Phil. I'll check into those options you listed. Anything to tame the volume to bedroom-friendly levels is what I'm looking for.


Tassieviking, thanks to you as well for your idea. I had thought of that, too. It doesn't help with getting distorted tones at lower volumes, but it should help with the lousy sweep at the bottom of the pot.


If the crunch is developed in the Power Amp then you are out of luck, unless you have lots of thick blankets.
I don't know enough to go into that but if you can check what the waveform looks like at the 1uF capacitor after the Tone pot but before the 2SC536F you will find where its coming from.

If the voltage is right at that 1uF cab you could add an effects loop there I think or even another volume pot, but I can't see it doing anything that the current volume pot doesn't do.
I would do what Phil said

This might be of some interest: https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/
There are no stupid questions.
There are only stupid mistakes.


Now that I'm considering attenuation rather than a master volume, I'm wondering if this option is any safer. Is there any chance of harming the amp (output transistors or speaker)? In researching attenuators with SS amps, I can't really find a definitive answer.

Since the U65RN uses output transistors rather than an OT, does that make it different to how an attenuator works with a tube amp?


Quote from: Tassieviking on August 02, 2023, 12:16:23 PMIf the crunch is developed in the Power Amp then you are out of luck, unless you have lots of thick blankets.

An L-pad would take care of this as it takes on some of the load at the speaker level.
Life is what you make it.
Still rockin' the Dean Markley K-20X