Recent Posts

71

Other Topics / Re: Meme of the day

« Last post by shooter on September 07, 2026, 11:18:45 am »

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72

Tube Amp Building - Tweaks - Repairs / Re: Univox 202R Amplifier making noise

« Last post by astronomicum on September 07, 2026, 09:26:28 am »

Can you post a pick of the circuit?

73

Tube Amp Building - Tweaks - Repairs / Re: Univox 202R Amplifier making noise

« Last post by astronomicum on September 07, 2026, 09:25:01 am »

Attached is the U-202R schematic.

74

Tube Amp Building - Tweaks - Repairs / Re: Univox 202R Amplifier making noise

« Last post by dbishopbliss on September 07, 2026, 08:55:31 am »

The main issue seems to have been the 6AV6 tube. I started pulling preamp tubes and when I got to that one (used after the Reverb circuit) the noise went away. I installed a NOS 6AV6 and the amp is quiet(ish) now. However, the tube is VERY microphonic. I need to get another one to see if it is better. I understand these tubes are prone to microphonics but the one I have is unusable.

I saw on another forum that the schematic should be pretty close to this:

https://el34world.com/charts/Schematics/files/Univox/UniVox-U-305R.gif

The schematic appears to be similar. One big difference between this amp an the schematic is the volume and tone pots are reversed. That is, it has a 500K volume pot and a 1M tone pot. Can anyone describe what impact this would have? I'm not going to change it, but I'm curious why they would have changed it.


While the reverb "works" now it is VERY faint. I can barely hear it when the knob is turned all the way up. Not sure if that is related to the tube or something else. Any thoughts on how to approach diagnosing weak reverb?


Thanks!

75

Tube Amp Building - Tweaks - Repairs / Re: First amp build

« Last post by Mongrel714 on September 07, 2026, 04:33:29 am »

Yes that's the correct way to calculate current.  You can use either plate voltage or cathode voltage, (PI gets a bit trickier using cathode voltage).  You can also look at the power rail and see what resistors feed what parts of the circuit and take one measurement for that section.

Thank you Al, Give a man a fish, feed him for a day, teach a man to fish, feed him for a lifetime! will work more on this after work.

76

Tube Amp Building - Tweaks - Repairs / Re: First amp build

« Last post by Mongrel714 on September 07, 2026, 04:27:39 am »

Product is in Amps so:
Answer is 0.00065A
Converted to mA product x 1000= 0.65 mA

Thank you! Noted and corrected on my worksheet!

77

Other Topics / Re: Meme of the day

« Last post by shooter on September 06, 2026, 07:01:44 pm »

...

78

Tube Amp Building - Tweaks - Repairs / Re: First amp build

« Last post by astronomicum on September 06, 2026, 05:53:01 pm »

Product is in Amps so:
Answer is 0.00065A
Converted to mA product x 1000= 0.65 mA

79

Solid State / Re: DC to AC Inverter

« Last post by dpm309 on September 06, 2026, 05:39:34 pm »

It's a small fountain that's on my front porch so a solar panel would not work. Have been looking into inexpensive battery powered pumps and I think that would be the way to go.

80

Tube Amp Building - Tweaks - Repairs / Re: Pro- amp planning

« Last post by printer2 on September 06, 2026, 05:15:42 pm »

Darn it, I really should look at the date, was going to reply to the original post and drew up a schematic. Kind of embarrassing as I replied a year ago with one. Oh well, take what you want from it. The third triode which combines the channels with the tremolo is an interesting thing to boil down to something simpler, as said, may need some tweaking. Otherwise the switching from LTP to Paraphase uses a four pole toggle switch to get the plate load resistors right. The wiring would not be too bad if you keep the lines short. Didn't take into account the further discussions, drew it up on a different computer that is not connected to the world. Rather than switching the parts in the PI, switching between two PI's can be done easy enough if you have the tubes to work with. In that case a four pole switch can be used, one for the input to the two PI's, two on the output, would be best to short out the input of the unused PI in case it oscillates or picks up stay noise and amplifies it and radiates it around.