Vox Ac Preamp with two power-amps

[THEZED]

Hello Guys, I have this crazy idea and I'm wondering if it would be possible to realize it.
Beeing a huge Queen fan, I love the sound of the Vox Ac30, expecially the normal channel driven by a treble booster, BUT I would love to have a stereo sound with two effect loops in order to play with delays etc.
Another thing I would like to have is less power so I'm thinking of a Vox AC preamp into a stereo Vox poweramp with el84, maybe twox8watts or twox10watts with two separate effect loops (1 for each stereo channel).
Any Idea on how to build something like that?
Thanks

[mth5044]

You could build them in modules. A power amp module and two power amps. Or a dual power amp.

But 10W is still going to be quite loud!

[Ben N]

To go full Brian May with this idea, you'd need three power amps (dry + 2 delay wets), in order to approximate his 3 separate (pairs of) AC30s. I you decided to go that way, I'd recommend a single stock AC15 clone with a post-PI (assuming your amp mixes channels at the PI) line out, and, separately, a stereo power amp for your delays based on the AC15 design, using one big (AC30) power transformer and maybe a single 12a_7 ahead of each PI configured as a V941 effects loop buffer to adjust your levels and match impedances. That way, you have the flexibility of a nice compact, not-exceedingly-heavy combo amp when you need it, which also acts as the "brain" of your amp system when you go wet-dry-wet.

[THEZED]

yeah, I was thinking about the triamp thing, but probably a stereo set-up with send-return would be flexible enough.
I'm thinking about building in modules, kind of rack thing, this way I could use different cab/speakers.
The big question is: how to start? where I can find resources on how to build these pieces of gear, and aproximately how much would it cost?

[Ben N]

A lot of folks have built and documented AC15 builds, and there are commercial materials around for DIYing one, so search and ye shall find. As for a Vox power amp, there you are in virgin territory, pretty much, although once you have schematics and layouts for the AC15, it should be reasonably doable to cut off the power amp part and execute it separately, or even double it in a single chassis. Putting two power amps and a preamp in a single chassis--now that truly must be virgin territory. I'm certainly no expert, but I could see things like transformer placement presenting challenges. You want to keep them at the fat end from the preamp, but do you then hang a double-sized power transformer, two output transformers and a choke all at the same end of the chassis? That may affect your choice of chassis material, and may create noise issues.  You might want to look into hi-fi stereo tube amps and see how they handle layout--although from what little I know of that world, high end stuff tends to involve, if not entirely separate "monoblock" amplifiers for each channel, then at least preamps entirely separate from the elegantly symmetrical power amps.

[thermionix]

I've worked on some "stereo" Magnatone amps.  "Stereo" in that there were two power amps, for a stereo vibrato effect, but mono preamp.  They were combos, had the power supply in a separate chassis at the bottom, output transformers were in the main chassis up top.

[blackcorvo]

Check out the Vox AC4C1 preamp, you could use that and a PAM8610 power-amp board (2x 10w @ 12v, 8R).
For an extra-compact build, you could use subminiature tubes for the preamp (6N21B, 6N17B, or similar), powered off an SMPS board for the B+.

Damn, now I'M tempted to do this build!

[THEZED]

Another way could be buying a second-hand Ac30c2x, then modding the power amp in order to split it in two with 1 single el84 and 1 send-return per channell. What do you guys think?

[thermionix]

Why only one EL84 per channel?  Also, PCB amps are not exactly mod-friendly.  You'd probably end up gutting the chassis entirely to accomplish your goals.

[printer2]

You want less power than an AC30. Actually less loudness. Since loudness is a function of the amp int of watts and the sensitivity of the speaker you need to find out the sensitivity of the AC30 speakers and for the ones you want to use. Then it is easy to determine how many watts you need, 5 - 10 -15W. If 5W then a stereo single ended amp out back would do. Would be a good project with iron from a 60's console stereo. If 10 or 15W is needed then a push pull output transformer is needed. A little more complicated than the single ended design needing a phase inverter and a second tube. Also a buffer stage before both phase inverters coming from your effects.

The preamp is straight forward. Copy the Vox preamp and then send a buffered output to the effects. The less complicated way of doing things would be to get a kit and break the circuit before the phase inverter for the effects buffering. Then you get a pre-punched board for turrets (or other suitable tube era board) and reproduce the phase and power section as in the kit. Will the power transformer be enough to feed the two amps? Ask the kit supplier. If you are going your own way without a kit then you look at the current draw of a single section and double it.