Hiwatt Hybrid Preamp

[ryanthomas1323]

Hey gents,

I am curious if it would be possible to create a Hiwatt circuit that uses the OL's extra gain stage on the Brilliant channel, but uses the original early 70's circuit on the Normal channel (ie: bypassing the extra gain stage). I have modified one of Mark's schematics to better explain this idea and I have attached it. Note that everything from the tone stack onwards is consistent with the early 70's circuit and not the OL circuit.

>Would this work?
>You could not link these inputs together because they would be out of phase with each other correct?
>What should v2b plate resistor be 100k (as shown) or 220k (early 70's).

Hybrid Preamp.jpg (297.67 KiB) Viewed 3928 times

Thanks for any thoughts

[mhuss]

I don't think that'd work very well, you basically have the two anodes connected together, it would really drag down the Normal channel. You'd really need an extra triode to do it right.

Another option would be to replace the 47k with a gain control (as in the LEAD amp). This way you could adjust it from normal-ish gain to OL levels.

[ryanthomas1323]

Would it work if I moved the 470k mixing resistor over so that it was after the first v2 stage instead of before? something like this:

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[ryanthomas1323]

Ignore the other changes, I am tinkering with how to voice the brilliant channel which is why v1 has 470nf (a little less low end roll off) and 4.7k (a little bit of cold clipping) on the cathode and 3n3 for the plate coupling cap. Also I flipped v2a and v2b because it was necessary to wire a few components directly to the socket to get everything to fit on a standard 31 spot Hiwatt style turret.

[mhuss]

You could try it; might be noisy.

[ryanthomas1323]

Just to clarify what you thought would be the best option, which is to add a 5th valve so v2 didn't have to share an input. I'm confused on whether the 470k mixing resistors are still necessary, and also on the best way to connect the normal channel to the tone stack and B+?

Hybrid Preamp (5 tubes).jpg (341.18 KiB) Viewed 3902 times

[ryanthomas1323]

Or does it make sense to put the mixing resistors from pin6 on V2 and V3 and before the tonestack?

[mhuss]

No; the driving impedance is an important factor in the frequency response of the tone stack controls. Perhaps use the extra triode as a buffer between the 470k mixer and the tone stack.

[ryanthomas1323]

Mark,

Can you explain what you mean on the schematic?

[mhuss]

Not easily.
1) Add a coupling cap to both stages you want to join, put 470k Rs after the caps, connect them together, and feed that into
2) a self-biased cathode follower stage (anode to +, cathode output to tone stack plus 100k to ground, 1 M from grid to cathode.)

[ryanthomas1323]

Like this?

IMG_1599 (1).jpg (473.79 KiB) Viewed 3847 times

[ryanthomas1323]

For the couple people paying attention to this thread I thought I would just share with you the results of my testing this weekend. The first thing I tried (because it was an easy 15 minute mod) was to wire it just like the 2nd schematic I attached, where both the normal and the brilliant channel share v2a, and there is a 470k resistor between v2b and v2a. The brilliant channel sounded great, granted it's been 15 years since I played an "OL" but it sounded like the fire breathing gain monster that I remember.

The biggest question was how would it affect the normal channel, because as Mark said now the normal channel kind of feeds back into the anode of v2b (though the hope was that the 470k resistor would fix most of the bleed-thru). To my ears the normal channel now sounded a little quieter and a little less bright, but I fully accepted the fact that I could be imaging this. The big reason being because I was switching back and forth between a very brightly voiced hi-gain channel to a less brightly voiced clean channel so of course it might appear quieter with less treble! (I should also add that there was no added noise after I completed this mod). But I had to know for sure what this mod was doing to the normal channel so I came up with this rather crude idea of putting a switch between the anode of v2a and the 470k resistor. This way I could kill any signal traveling back up to v2b when playing thru the normal channel, and turn the switch "on" when playing thru the hi-gain channel. See Below

Hybrid Preamp.jpg (324.64 KiB) Viewed 3806 times

So I disassembled the lower gain input on the brilliant channel, pulled that input out and used that hole for my switch (using shielded wire (grounded right underneath the inputs) to travel back of forth between v2). I was expecting to hear a very noticeable difference when playing thru the normal channel whether the switch was on or off, but to be honest it was very very slight. With the switch in the off position (not allowing any signal back up to v2b) it seemed to have 1% more gain, which my ears kind of heard as 1% more "crunch". But like I said it was very very slight, and if forced to do a blind test it is very possible I would fail in being able to tell whether the switch was in the on or off position.

[ryanthomas1323]

Also see attached for the completed schematic of Mark's idea, which is the more efficient and elegant way of combining these channels.

If my translation of his self-biased cathode follower stage is correct, then I will try to finish this mod over the weekend and let you know how it comes out.

I am still holding out hope that I will find a way to make these channels in-phase with each other, as not being able to link these channels remains the biggest drawback.

Hybrid Preamp (5 tubes).jpg (346.37 KiB) Viewed 3805 times

[mhuss]

You forgot the plate resistors on V2 and the half of V3.

[ryanthomas1323]

Yeah those might be important!

Here is the corrected schematic.

Hybrid Preamp (5 tubes).jpg (353.88 KiB) Viewed 3784 times