Modified V76 is Kicking My Butt

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WOW, that would be nice!

[quote author="Winston O'Boogie"]After I've built a couple of prototypicals and if there was enough interest from "the group 'n' ourselves" to buy more than say 30 of the buggers then I think they'd end up being about $40 apiece[/quote]I'd be into enough for 4 preamps. Aren't there about 3 different choke values in a V76?
Kelley

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Winston,
I understand completely. I'm not getting my hopes up, I wouldn't be able to start a V76 anytime soon anyway, just wanted you to know that if everything did work out and you needed some purchasing muscle, I'd thrown in for a few in a group order.
Kelley

same about me!

I updated my schematic with current values of components and voltages. As listed in my schematic, I substituted the EF40 for the RF804 tube. The EF40 is the precursor for the EF804 and EF86 tubes and specs similarly. I got nice Miniwatts for $4 and, being the cheap bastard I am, I went with them. The major drawback is the Rimlok socket. They are nearly impossible to find. I just hpoe they sound as good as they spec out. We shall see.

I did some frequency measurements under the following conditions:
Input from my oscillator at 1V Peak sine wave. Osc measures 6.3k static resistance.

No output termination. I'll do one with termination later.

Measurement taken at output of the LL7903 and LL1680 at minimun, mid, and maximum gain settings.

Freq Min Mid Max
in out in out in out
10 .1 .2 1 15(distorted) 1.8 80(dist)
25 .1 .32 1.8 20 1.8 100
40 .1 .48 2.4 120 2.4 120
80 .1 .9 3.2 150 3 150
250 .1 1.2 3.8 140 3.2 150
500 .1 1.2 3 125 3.4 135
1k .1 1.4 1.6 75 1.6 140
2.5k .1 1.4 .75 40 .75 120
5k .1 1.2 .38 20 .38 130
10k .07 1.1 .2 19 .2 120
25k .03 .6 .12 6 .14 75
50k .02 .28 .1 5 .1 50
100k .02 .12 .15 4 .15 15

Max gain will clip the signal at most settings. My oscillator only goes to 1v min so I will have to set up a devider to get smaller signals.

I graphed the frequency measurements.

I put in a PI filter between stage 2 and 3 similar to the one used in the original. With this in, I was able to modify the feedback resistor down to 80K on the output pair. I graphed the output for this:


This gives a little boost th the high end just before dropping like a rock.

I didn't check all the gain settings, but the middle setting I chose to look at seems to drop the high end quite a bit. I will check the rest of the settings to see it thes loss is consistent.

I still have a roll off above about 3kHz starting at the 5th switch position, when the input pad is no longer switchrd in. If I disconnect the pad I get no roll off.

I found this was caused by the bypass cap for the phantom power. It was not switched out when phantom was off but remained across the input to bleed off phantom slowly. It did that but it also boosted the bass response out of proportion to the rest of the signal.

I ditched the pi filter, thats the pf caps and little inductor.

Click to expand...

CJ, is the pi filter you removed in your v76 the three components in the below image, before the third tube? The two caps go out and the the inductor is replaced by a wire - correct?



are there any other modifications that could be interesting for a stock V76? I have a very early V76 (SN 144) that will need a recap and while I'm at it, I might as well do some other things... By the way, the unit came with a schematic from Telefunken that is a bit different from all the schematics from Kubi's site, when I find someone with a scanner I can post it. It has some dB values marked at different points in the circuit and is drawn different from all the schematics I've seen on the web.

cheers and thanks, Marten

Hello, time travelers...  Marten, CJ, you want to follow up?