Hall VVR in an AC4TV

[clengman]

I'm thinking about buying the hall amplification vvr kit to replace the attenuation circuit in my ac4tv. I haven't done any amp building or modding before, and I'd like to try and make sure I can figure out how to wire it before I place the order. I know that there are the connections that need to be made; to B+ in, B+ out and ground. I can handle the ground connection, but I'm wondering, if I post a schematic of the amp, can anyone help me locate the other two circuit points? I think if I know where they are on the schematic, I'll be able to find them on the board. Thanks!

[Nigel Goodwin]

Should be no problem, it's absolutely trivial to spot - post the scematic and we'll show you where.

[clengman]

Thanks! Here's a link to the schematic: http://website.lineone.net/~m0dzo/AC4TVschem.pdf

And here's a link to the thread that got a few people working on putting it together: Vintage Amps Bulletin Board • View topic - AC4TV schematics anyone?

I was thinking about this more as I was falling asleep last night and I think I have a little better understanding of how the whole thing is supposed to work. And it leads me to a few questions/statements I'd like to submit just to get your thoughts.

1) I believe that the kit can be installed to scale the B+ voltage for both the preamp and the power amp sections of the amplifier OR it can be applied just to the power amp section. I think installation is easier when it's applied across the whole amp. What would be pros/cons tone-wise or control-wise to one method over the other?

2) The installation that I saw online used the lugs on the standby switch for a B+ in and B+ out. It's convenient because the B+ power rail is already interrupted by the switch. This would scale both preamp and power amp sections?

3) My amp does not have a standby switch. If I want to scale the whole amp is there another means to get the VVR into the B+ in series without cutting traces and drilling holes on the circuit board? (I'm not opposed to trying this, just wondering)

4) If I want to scale just the power amp it seems to me that I have to interpose the VVR somewhere on the node common to R16, R19 and pin 9 of the EL84. Is that correct? Is that likely to require cutting a trace and drilling holes? How big a deal is that?

Thanks so much!

[Nigel Goodwin]

That diagram is particularly badly drawn to explain it

But the junction of D2, D3 and the 100uF 350V to their right is the B+ in.

The junction of R16 and OT is B+ out.

Essentially make a diagonal cut top left to bottom right through the junction above the 100uF 350V.

[clengman]

Thanks. I can follow that on the diagram. I guess I was pretty far off.

I'll have to have another look at the amp tonight to see if I can find everything for real.

[clengman]

Okay, so I located the relevant part of the circuit board and I think I have an idea of where to solder in the VVR. If you look at the of the front of the board, the lug labeled "Tag 6" is the spot where the wire from the OT primary attaches. ("Tag 4" is the other end of the primary that runs to the anode of the EL84.) There's a jumper from the T6 lug to the node shared by R16, Q2, Q3 and the 100 uf cap. In the photo of the back of the board I have the lug side of that jumper labeled "T6 (B+ out)" and the other end of the jumper labeled "T6 (B+ in?)" I also labeled the locations of the two diodes and the big cap for reference.

Can I take the jumper out, and wire in the VVR in its place?

[clengman]

Nevermind... I see that that would put R16 on the wrong side of the VVR. So...on the photo of the back of the board, the solder connection in the upper right corner is one side of R16.

What if I drill a hole just to the left of that R16 solder joint. Then I remove the jumper that's there (between the points I have labeled as B+ in and B+ out) and instead jump from the T6 lug to the new hole next to R16, then solder the B+ out right to the T6 lug? Then I cut the trace to the left of the hole I drilled and use the point labeled as"T6 (B+ in?)" for B+ in?

Or is it okay/better to disconnect the end of R16 from the board and solder a jumper right to the free end of R16 and to the lug at T6 and solder the B+ out to the same lug?

[Nigel Goodwin]

You could make two cuts in the track, and run a wire from R16 to the OT track.

White lines the cuts, red line the wire.

[clengman]

Great! Got it.

One last thing. Do I need to worry about the kind of wire I use to make that connection?

I can scavenge pieces that were used to hook up the attenuator network after that's removed. Is there any reason that those wouldn't be suitable? Proper voltage rating would be the concern for that part of the circuit right?

[Nigel Goodwin]

Any sort of plastic insulated wire should be fine.