Recent Posts

51

Tube Amp Building - Tweaks - Repairs / Re: First amp build (5F1) would anyone mind double checking my turret board layout?

« Last post by HexEnduction on September 08, 2026, 11:50:42 am »

I would prefer to see a flameproof resistor for the 22k power rail dropping resistor.
Also a little more space between the 500 ohm/5 watt cathode resistor and other components to prevent possible overheating.

The 22k power rail dropping resistor is 1/2 watt but searching around for ones I can only find 1w 22k metal oxide resistors so far. Just to double check, using a 1 watt resistor here would be no problem right?

And I'll add some more space between those components.

Thanks a bunch for the suggestions.

52

Tube Amp Building - Tweaks - Repairs / Re: Tweed BluesMeister needs a tweak

« Last post by Helmholtz on September 08, 2026, 10:50:07 am »

Those notches are actually single cycles of HF oscillation. Might be triggered by the class A to class B transition.
This could mean the amp is conditionally unstable.
A square wave signal would show better.

Is the cathode cap good?

53

Tube Amp Building - Tweaks - Repairs / Re: Tweed BluesMeister needs a tweak

« Last post by acheld on September 08, 2026, 10:08:35 am »

This amp came about when I was perusing work done by Da Geezer and by Tubenit on this forum. And I can't recall the exact links, but there were quite a few.  You can start by reading https://el34world.com/Forum/index.php?topic=32419.msg359347#msg359347   chatting with Tubenit and pdf64.   Going back to that post,  Tubenit is saying that the OD in this style of amp sounds harsh, and there is no question that holds true for this amp, probably the main reason I put it back on the bench.

I had never seen notches like this on any amp I've built, so I panicked.  I should have traced back, and then posted. 

I'm thinking an unbalanced PI or bias issues on the 5881's, or both.  We will see!  (I'm usually wrong with my predictions . . .).   Definitely oscillation is in the differential diagnosis.   Good idea about removing the NFB.

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Tube Amp Building - Tweaks - Repairs / Re: Tweed BluesMeister needs a tweak

« Last post by Helmholtz on September 08, 2026, 09:38:56 am »

To me it looks like the amp is at the edge of HF oscillation.
What happens if you disconnect the NFB from the output?

55

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

« Last post by astronomicum on September 08, 2026, 08:58:13 am »

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

« Last post by dbishopbliss on September 08, 2026, 08:40:13 am »

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

« Last post by astronomicum on September 08, 2026, 08:27:38 am »

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

« Last post by astronomicum on September 08, 2026, 08:26:46 am »

What a weird phase inverter. I may have to make this one.

Same circuit we discussing recently in the Univox U-152.

59

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

« Last post by dbishopbliss on September 08, 2026, 08:16:14 am »

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.

60

Tube Amp Building - Tweaks - Repairs / Re: First amp build (5F1) would anyone mind double checking my turret board layout?

« Last post by ac427v on September 08, 2026, 07:28:19 am »

I would prefer to see a flameproof resistor for the 22k power rail dropping resistor.
Also a little more space between the 500 ohm/5 watt cathode resistor and other components to prevent possible overheating.