Hiwatt Hybrid Preamp

[mhuss]

Getting there. You probably should add a 10k resistor and filter cap in the V2 power line, that's too many tubes on one power supply node.

[ryanthomas1323]

Should it go here, or in between v2 and v3?

Hybrid Preamp (5 tubes).jpg (173.51 KiB) Viewed 3723 times

[ryanthomas1323]

Clarification, here or between v2 and v3a?

[mhuss]

Try both ways. Ideally both, but then the component count goes up. (According to Jones every stage should be decoupled , but that's a bit of overkill in most situations). However, higher gain amps are always more likely to have oscillation problems...

[ryanthomas1323]

Thanks Mark ...so then let this be the final schematic (maybe)

Hybrid Preamp (5 tubes).jpg (361.3 KiB) Viewed 3701 times

....but I just have to ask because its really erking my brain, would the circuit work like this:

(thus putting the channels back in-phase and giving you the option of linking them)

Hybrid Preamp (5 tubes no v3a).jpg (351.05 KiB) Viewed 3701 times

[mhuss]

Looks good, the only issue with the second scheme is they'll be mixing out of phase, so you'd likely get a really thin sound unless one or the other was much louder.

[ryanthomas1323]

Hey Mark, by removing one gain stage from the normal channel wouldn’t it be putting these channels in phase? ie:

Brilliant 3 gain stages -> mixer
Normal 1 gain stage -> mixer

[mhuss]

You're right. The way you drew it is a little confusing (to me).

[ryanthomas1323]

The way I drew it confused me!

Thank you Mark I have two preamps I'm going to try, I'm thinking it would be interesting to compare also the difference in the above schematic (the one with an empty v3 triode), one like its drawn where the v3 mixer feeds the tone stack via the cathode and one where the mixer feeds the tone stack via the anode like a DR.

I think having the 70's and 80's circuit built into one amp could be really interesting! Hi-Tone should do something like this! ...forever putting an end to the DR enthusiast dilemma of 70's or 80's DR

[Emiel]

I'm late to the game, but are you familiar with the Trinity Triwatt? It features the early '70s pre amp but with a switchable '80s OL mode and a separate overdrive control. Seems like a nice alternative.

https://www.trinityamps.com/product/trinity-triwatt/

https://www.trinityamps.com/ForumGaller ... e_v.94.pdf

[ryanthomas1323]

This is interesting, the one thing I would want to know is how they are able to turn off or on the extra gain stage, and still keep the channels in phase.
They obviously must have accomplished this as they have a linked input. And the fact that they did it while still using 4 preamp tubes makes me think that some consolations might have been made to the circuit, but this is just speculation.

Also I'm not a huge fan of lowering the power output via the transformers (KT66's at 33 watts). I would have preferred a lower output tube running in class AB, like a 6aq5 or something.

[Emiel]

This seems like the schematic:

https://www.yumpu.com/en/document/view/ ... inity-amps

[mhuss]

He just mixed the two input stages together, then switched an extra gain stage in and out after that. So, both channels low gain or both high, which is probably not what you wanted. Otherwise, it seems fairly true to the original.

[ryanthomas1323]

I just checked out this schematic, yeah not exactly what I wanted. Having the choice of high gain mixed with low gain opens up a lot of cool options with an A-B-Y pedal.

[ryanthomas1323]

Hello Again,

I thought I would give a little update on this thread, because of the quarantine situation going on I've had loads of time to experiment (and research and read and learn) with this circuit! First of all I have discovered that it is somewhat difficult to keep a preamp with 4 gain stages before the tone stack from OVER over-driving. Many attempts to make this circuit work failed because of being unable to control the amount of gain. I should also say that I decided in the process of my experimenting that I would only be happy if I kept the normal channel true to Reeves early 70's design. It makes it easier to accept that even if the high gain channel isn't the greatest high gain channel ever created, that I could always fall back on the normal channel as being that sound that I love (I never used the brilliant channel much anyways). So that being said it meant that no matter what I did v3 had to reach the tonestack via the plate, which in contrast to the cathode follower would add a lot of gain, which compounded my problems.

I should say that I was able to get Mark's cathode follower (v3) circuit to work quite nicely, like the schematic below it required that I dump a bunch of gain off in between the v2 stages. An idea that I stole from looking at the Marshall 2204 schematic, but it did make the normal channel kind of have a different flavor, it actually sounded pretty similar to the low gain input on my 2204, and I really love the sound of that channel once you open it up. But anyways by succeeding with the cathode follower design, I took what I learned and tried to get it work where v3 was like the original. But even with the voltage divider it was still completely unstable, but just on a whim I thought what would happen if I swapped out v2 from 12ax7 to something with less gain? I decided to start with the most drastic gain cutting tube I had, the 12au7 dropping the gain factor from 100 to 10. And because I had a 12DW7 as well (which is half 12ax7, half 12au7) I was able to put that into the V1 spot so that all 3 gain stages of the high gain channel were 12au7. To my surprise this actually worked. Now even still this channel is EXTREMELY high gain, I played through it for a couple hours and I felt like it was riding a razor's edge and I kept waiting for it start oscillating, farting or spitting but it never did.

Now for the downside, for some reason I feel like it is really dragging the normal channel down, just in terms of headroom, and I don't know why? I attached the high gain channel to the v3 mixer with an alligator clip so that I could easily disconnect it, and even with it disconnected the normal channel sounded the same. Now keep in mind my guinea pig amp for all this is a DR201 clone (which I will be converting from a 6 x EL34 to a 4 x KT88 after this experimentation phase), and I was able to get the normal channel volume all the way up and the master volume to 50% and still stand right in front of the speaker cab, and not die ...or spontaneously combust, which unless I'm dreaming was not possible before. For the life of me I can't figure out why this would be, even with the high gain channel not connected to v3 is it still having an effect on B+? I did beef up the filter caps just in case there could be an issue with power sag, but I didn't notice a difference. Am I imagining this?

Here is the schematic as I have it now.

Hybrid Preamp 5.jpg (350.21 KiB) Viewed 3564 times