Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on September 01, 2026, 01:14:24 pm
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A friend asked me to look at this amp. Very little info out there. I'm basing the model number off of an amp with similar controls and tubes but the cabinet tolex and badge are completely different.
It has somewhat recently been serviced because it has a 3 prong power cord and all of the electrolytics, some film and some resistors have been replaced.
Anyway... it's making crackling, hiss, popping noises. Here's video with no instrument plugged in and the volume controls turned down all the way. eature=share
Is this just tubes?
It has a number of tubes I don't have (6an8, 6av6, 6973). What would be the best way to determine which tube is bad without a tube tester?
Thanks,
David
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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
Here's a link to the thread.
https://offsetguitars.com/forums/index.php?threads/broken-old-univox-202r-can-i-make-something-out-of-this.61455/
Sounds like it could be a tube. Do the volume controls have an effect? Reverb control? You could try pulling one tube at a time while following the schematic to see where the noise originates. You could also trace the signal chain with an oscilloscope or listening amp to find the source. I'd try giving everything a good clean first, including the jacks and sockets. Also, I'd check voltages to make sure the power tubes are biased correctly at least, before doing much more.
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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 (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!
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Attached is the U-202R schematic.
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Can you post a pick of the circuit?
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Where did you find that schematic? I searched a lot of sites and could not find one.
Here's a picture of the amp internals. The curve is just lens distortion.
(https://i.ibb.co/WNBz4Wq9/F67-E1133-4-D22-4879-AEF7-1-D5320-B9901-C-1-201-a.avif) (https://ibb.co/zWP2Thjc)
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I had it for years. Can’t remember where I found this one. Have not gotten around to having it included in the library.
Trying to open the image, it is taking me to imgbb and it won’t open.
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What a weird phase inverter. I may have to make this one.
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Trying to open the image, it is taking me to imgbb and it won’t open.
I was having the same issue yesterday but it is working for me today.
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What a weird phase inverter. I may have to make this one.
Same circuit we discussing recently in the Univox U-152.
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I was having the same issue yesterday but it is working for me today.
It is working for me today also. Thanks.
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The schematic has 1M volume and 500K tone pots so my amp was assembled incorrectly.
If you notice, there is a pot between the 6AV6 and 12AX7 tube. I’m trying to locate it on the schematic. Or… that’s the pot near the tube in the schematic, but then where is the reverb level pot located on the front panel?
Another odd thing… my amp seems to have verb in both channels but I don’t see how that would work in the schematic. But it is very faint. Maybe it really isn’t the reverb… maybe it is the microphonic tube??? There is a hum when turned up and it goes away l when all the way down.
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If you notice, there is a pot between the 6AV6 and 12AX7 tube. I’m trying to locate it on the schematic. Or… that’s the pot near the tube in the schematic, but then where is the reverb level pot located on the front panel?
Univox used a pot for Reverb recovery. You should find that the pot is located directly after the Reverb tank return. The 220K showing on the schematic is probably not there, replaced by the 500KB.
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my amp seems to have verb in both channels but I don’t see how that would work in the schematic.
There is a hum when turned up and it goes away l when all the way down.
There can be some cross talk between the two channels due to the close proximity of the volume and tone wires. A little crosstalk is normal. See if the wires feeding the volume pots and returns from the tone pots run parallel and if so, separate them.
A few things to check for hum:
The early 202Rs used an open frame reverb tank with a paperboard cover mounted in the bottom of the cabinet. The later used Belton tanks contained in a pouch on the side of cabinet. Univox had a habit of grounding both tank feed and return jacks. If there is continuity between the grounds on the tank and both jacks are grounded, a ground loop is formed. If there is continuity between the grounds on the tank and both jacks are grounded, remove the ground from the tank feed jack to break the loop.
If the tank is mounted on the side of the cabinet in a pouch, make sure the tank feed is at the top of the pouch. You want to keep the tank return as far from the PT as possible.
Make sure that the tank feed wire to the jack, and the tank return wire from the jack to the pot, and from the pot to the grid of the 6AV6 do not run close to the heater wires where they can pick up hum.