Univox U45 (not U45b) schematic - is the NFB connection missing ??

[kagliostro]

I was sending to Doug a schematic of the Univox U45 (not the U45b) I've found

but looking to it seems that the NFB connection was deleted

look to the little dot on the upper side of the 8ohm winding and at the piece of "wire" connected between the 100R and 33k on the cathode of the 6av6 tube

what do you think about ?

K

[tubeswell]

Yes - no NFB there

[kagliostro]

So, for you, the NFB isn't present in the original amp

but what to think about the "wire" under the 6v6 ??

K

[sluckey]

but looking to it seems that the NFB connection was deleted

I agree. Looks like a draftsman was told to remove that part of the circuit but he didn't get it all removed. I have no idea if the actual NFB circuit is actually in that amp nor what value was used for the missing series resistor.

[tubeswell]

but what to think about the "wire" under the 6v6 ??

What Steve said. The wire going to the 100R beneath the 6AV6 looks like it was part of the NFB loop as initially drawn.

[kagliostro]

Knowing that, if someone build it it, will be easy to find the right resistor value using initially a trimmer connected as rheostat

I'll send to Doug the schematic with the red line

K

[jeff]

Looks possible it had NFB.

What's with the 1.5M going from the first tube's plate to ground?

[kagliostro]

What's with the 1.5M going from the first tube's plate to ground?

Yes, interesting question

a voltage divider to tamer B+ or something as to permit the Tremolo in V1 ????

K

[tubeswell]

What's with the 1.5M going from the first tube's plate to ground?

Yes, interesting question

a voltage divider to tamer B+ or something as to permit the Tremolo in V1 ????

K

A crude method of fixing the plate idle voltage

[PRR]

> What's with the 1.5M

The tremolo has to over-power the preamp.

They both start from the same supply, which would be a fair fight.

So the preamp is crippled with a lower effective supply, so its 150V 220K can cut-off the preamp's 90V 470K||1.5Meg.

[PRR]

The NFB resistor can be 3K for "small NFB". Higher values have little effect, smaller values will make the amp too "tame".

I suspect it was stolen/researched from the 2K7 in the late Champ. However the slightly smaller output bottle, generally lower B+, and lamed preamp, made it tamer than a Champ. And resistors cost money!!! If it worked fine (less tame) without a 2K7/3K3, then right away the factory would leave that resistor out. The 100 ohms can go too, but then you have to bring the cathode RC down to ground, which means a layout change, or a jumper, and maybe they didn't bother.

For yourself: put in the 100 ohm, and be ready to try 10K 3K 1.5K 1K from the speaker. Smoke-test, it should play fine and brash. Then try 10K then 3K NFB. If it howls, reverse OT primary. The "right" value depends a lot on speaker and style. For acoustic with hot active pickup, and large closed speaker, 1K NFB will be very clean up to clipping. For trash-amp with small open cab, no NFB may rock-out better.

[kagliostro]

Many Thanks Sluckey - Tubeswell - PRR

your answers are always very instructive


K