Hiwatt Hybrid Preamp

[RyanThomas13]

Would it be possible to build a hybrid Hiwatt preamp where the Normal channel follows the early 70's circuit design and the Brilliant channel follows the early 80's "OL" circuit that has the extra gain stage? I've modified the attached schematic (from Mark Huss' original "early 70's" schematic) roughly showing what I mean.

Hybrid Preamp.jpg

[pompeiisneaks]

I don't know the exact topology of the two preamp's you're mentioning, but it seems to me that you'd want the two channels to both input to the second stage of V2A's grid with those 470k mixing resistors, not have the normal channel go into the next gain stage? They'd be out of phase at that point and cause issues possibly if you jumped the channels etc. basically, you've got the output of V2A's brilliant channel mixing into the less amplified (one less gain stage)'s normal channel.

Something like this makes more sense to me.

mixedstages.png

~Phil

[RyanThomas13]

Thanks Phil,

What you have drawn is basically the 80's Hiwatt circuit or the OL version, I've attached it.
I've also attached the 4 input early 70's schematic.

The phasing issue I knew would be one downside, being that one channel would have an extra gain stage. The goal of combining one channel from each circuit into one amp is to achieve the crunch of the 80's version, but also the great clean tones from the early 70's version.

DROL_2InputPre.gif

DR_Pre4Input_v1a.gif

[pompeiisneaks]

Oh I see the difference between the two. Then in your case, doing what you've posted is close to what you're talking about, except that I think the lack of a mixing resistor on the output of the normal and the output of the brilliant second tube stage, means there's potential for some signal bleed issues. I.e. if you plugged into the normal channel, its signal could go to pin 2 of V2B, but then back into V2A via both anode and/or cathode... not sure how that might impact the signal, it may end up with that 47k to earth being that you'll lose a lot of signal.

You would likely, at a minimum, want another 470k resistor between the output of V2A and V2B's grid so that they have the similar balance of the either the 70's or 80's version, You may not need the 470k mixer resistor after the brilliant volume, since it's not mixing two channels there. BUT it does work as a grid stopper as well, so I'm not 100% sure if it is or isn't needed. Removing it, though, would possibly change the response of that brilliant channel compared to a stock Hiwatt, so maybe leave it just to remain consistent with the way the original circuit worked.

~Phil

[RyanThomas13]

Phil,

I see your point about the normal channel feeding v2b but also the anode and cathode of v2a - creating a strange kind of loop. Hmmm I'm wondering the best way around this? I could always install of 5th preamp tube so that I didn't have to share the input of v2b?

-Ryan

[pompeiisneaks]

Phil,

I see your point about the normal channel feeding v2b but also the anode and cathode of v2a - creating a strange kind of loop. Hmmm I'm wondering the best way around this? I could always install of 5th preamp tube so that I didn't have to share the input of v2b?

-Ryan

Just adding another 470k mixer resistor between V2A and V2B's connection should help block that I'd think.. as I mentioned earlier. (I'm sure I went on too long about other things so maybe that wasn't clear).

Like this, see the 'optional' 470k I was referring to... maybe it should stay to keep the gain on the brilliant channel exactly as it is. But since the 'mixing' is being done post V2A, it's not mandatory anymore.

hybrid.png

~Phil

[RyanThomas13]

Thanks Phil,

This makes sense, I did miss this detail the first time. I suppose the only way to find out is to build it and go from there. Thank you again for your help!
-Ryan

[RyanThomas13]

I'm resurrecting this thread because while trying to find the best solution to the Hiwatt tonestack bypass switch it occurred to me that maybe my attempts last year to design a Hiwatt Hybrid preamp (the original goal was to have the 80's high gain Hiwatt sound and the early 70's Hiwatt clean sound in one amp) that I may have overlooked a simpler option.

So using Mark's early 70's schematic as a template this is what occurred to me, a simple DPDT switch like recommended by sluckey in the tone bypass switch.

hybrid preamp.png

Would this not be the easiest way to switch in or out the extra gain stage of the 80's version?

[mhuss]

I'd add a 1M to ground on the grid of the 'left' triode so it has a consistent ground reference.

[jazzbass]

I'd add a 1M to ground on the grid of the 'left' triode so it has a consistent ground reference.

Re: Hiwatt hybrid preamp
Hi Mhuss,

Excuse me for my unfamiliarity with electronics but could you explain me better, perhaps with a diagram, how exactly you would put the 1M resistor.

Thank you. Franco

[jazzbass]

Another question Mhuss,

First of all, my compliments for your research on the Hiwatt world, thanks to your magnificent work I fell in love with Hiwatt amplifiers and would like to build one.
I have read that the choice of transformers is critical for the success and I would like your suggestion. I live in Europe and due to the cost of transporting these heavy components I would like to purchase them from a European distributor/manufacturer.
I normally buy from Tubetown or Tube Amp Doctor.
In your experience, can I find suitable transformers from these or others in Europe?

Thank you. Frank

[mhuss]

Sorry, I missed this thread Q - any good quality transformers with appropriate specs should work fine. The circuit and the stiff power supply are the elements most responsible for the Hiwatt sound, imo.

[jazzbass]

Sorry, I missed this thread Q - any good quality transformers with appropriate specs should work fine. The circuit and the stiff power supply are the elements most responsible for the Hiwatt sound, imo.

Thanks Mhuss for the reply but what are the "appropriate specs" for these transformers or where can I find this specs?
I think I build a clone of the DR 504.

Thank you.
A hug, Franco

[pompeiisneaks]

Mike's site details a LOT of this, but it does seem to be lacking the detail on the 504 only the 103 and 201 here:
https://hiwatt.org/tech.html

More detail here:

https://hiwatt.org/tech2.html

BUT again not specific to 504. OTOH the listed transformers int he schematics there do show the exact partridge part numbers and you can likely get detail on those.

~Phil

[jazzbass]

Hi Phil,

I'm looking for the name of who imports Heyboer transformers for Hiwatt in Europe or Italy. I would like to build a 504 and will be using the power transformer from a Triwatt Trinity that I built over a decade ago. Unfortunately its output transformer has an impedance of 8K only suitable for 6V6 while I would like to build an amp with 2x EL34.

Thank you
Franco