Vox AC30 build into a trainwreck rocket

[Curranproducer]

OK, I have built a really nice vox AC30 top boost amp. I have been hunting the site pages and see that the top boost is pretty much the same as the trainwreck rocket. I am going to do the conversions to get it there BUT I have noticed that the rocket leave one side of the 2nd valve un-used. how would I go about utilizing this half of the 2nd valve for the distortion? AND has anyone done this?

[Colossal]

You could wire it as a switchable series gain stage, parallel gain stage (with different bias point), or even another cathode follower. Yes, I believe it's been tried before, IIRC, RJ has done it with positive results. I suppose the approach depends on what you are looking for, i.e. how much distortion you want, added compression, or slight overdrive.

[Curranproducer]

I am talking stupid distortion. lots and lots, would love to push this booger into the soldano range O gain. to go from the simple nice spanky vox into that would be amazing!!!

[dave g]

You could wire up the preamp like a Soldano Atomic 16. You're going to want a MV after the tonestack or maybe a PPIMV if you go that route.

Be warned though, I don't think you're going to be able to get great clean sounds with a high-gain preamp. You could add an extra preamp tube wired strictly for clean or do some relay/optocoupler switching of a stage in and out. In general though, high gain preamps tend to be mid-heavy; bass leads to super hard clipping and nasty sounding blocking distortion and treble leads to ice-pickyness. This is why you usually see small valued coupling/bypass caps in addition to snubber capacitors in high gain preamps. Those same preamps sound weak, sterile, and muffled when not pushed into distortion.

A few ideas:
One compromise would be to use half of the first triode as your separate clean preamp (with no tonestack) a la the normal channel of an AC30 or the clean channel of the Bad Cat Hot Cat. Then you could use the remaining 3 triodes for a 3 stage high gain preamp, although you'd need to dump the cathode follower. I like cathode followers directly coupled to the last clipping stage in high gain preamps, but it's possible to build a good sounding high gain preamp without them.

A slightly more complex idea that involves some switching but lets you keep the clean Rocket preamp would be to put a switchable gain stage in between the first and second stages. I would prefer this to putting the extra stage up front because you can tune this stage to be a clipping stage, rather than using it to drive the first stage of the clean preamp into clipping.

[Colossal]

Well if you are after stoopid distortion, then by all means, insert that wasted triode in between the first and second stage. You might want to use an additional gain pot in front of it so that you can find the ideal ratio of interstage attenuation. When you are happy with the resulting level of grind, measure the pot, then replace with a voltage divider...or not. You may need to tweak some of the other values on the Voxy side of things because it's easy to get bloated, flabby distortion with all of that series gain. By this I mean, you might need to reduce the cathode bypass cap on the second (which would become your third gain stage) and play with the coupling caps and attenuation. If you were sneaky, you could use a DPDT switch and/or relay to switch in the added gain stage as well as change some values when you kick in the distortion boost. You want to keep the distortion tight.

[cbass]

http://www.youtube.com/watch?v=r2Yli8Vri1g

[gktamps]

Colossal,
I found your comment about adding a second cathode follower interesting. What would be the application for that?
Cheers,
Greg

[dave g]

I realized Colossal and I have a similar idea but it's a bit hard to explain without a schematic so I drew one up.

I have not built this amp so treat it as a "back of the napkin" doodle. It probably would sound pretty good as-is but could likely benefit from some tweaking. The output could go to a master volume or straight into a LTP PI with a PPIMV.

This drawing uses a single tonestack that's shared between the two "channels". You could conceivably have two separate tonestacks that switch in and out along with the extra gain stage. This would also allow you to only put a master volume on the high gain channel (why bother putting one on the clean channel anyway), and to tweak the two tonestacks for clean/distortion sounds. For example, you might keep the stock AC30/Rocket tonestack for the clean channel, and use a two-knob marshall tonestack (with a resistor in place of the mid pot) for the distortion channel.

Add a 12AX7 LTP PI/4xEL84 power section global cut control and you've got a mid-power amp with "separate" no-compromise clean and distortion channels and a simple, 8-knob control panel. Use a 1-button footswitch and some relays to do the channel switching.

Man, now I want to build one!

[Curranproducer]

ohhhhhhhhhh all the info and ideas!!! man I am going to have to build two or three of these for all the odds and ends of tone in here!!! ahhhhhhhhhhhhhhhhhhhhhhhhhh!!!!!

[Colossal]

Dave G, yes, that's very much what I was thinking! Thanks for drawing that up. Treble peaking and colder bias on that added stage, and reduced bypass on the third stage would do it nicely.

Greg, in addition to the usual duty of driving a tone stack, a cathode follower can add some nice compression while increasing the gain of the preceding stage. So depending on Curran's needs, the extra stage could be used as a CF just following the input stage. This would reduce the output impedance of the first stage as well.

[gktamps]

Hmm, thanks for that info, Dave (Colossal).
I built a Rocket using RJ's Rocket Booster circuit, but ended up finding it too boomy and bassy, and it had a strange interaction with the treble control, so I yanked it. I'm looking for ideas like this to utilize that unused triode, and since I built that amp as a test bed, I really appreciate the ideas you and Dave G. have contributed.
Cheers,
Greg

[renshen1957]

ohhhhhhhhhh all the info and ideas!!! man I am going to have to build two or three of these for all the odds and ends of tone in here!!! ahhhhhhhhhhhhhhhhhhhhhhhhhh!!!!!

Hi,

You might want to adapt a London Power Standard Preamp. The LPSP has the gain channel folded into the preamp channel, a Two channel switching preamp. The Clean Channel would have Vox values, the gains channel could be your Soldano values.

Traynor used the LPSP when the name was revived a few years back, these are the more recent amps, not the vintage ones.

Best regards,

Steve

[Aurora]

Just a question... how does that relay in that high gain preamp behave, vs pops and clicks..??

[renshen1957]

Just a question... how does that relay in that high gain preamp behave, vs pops and clicks..??

Hi,

No relays are used.

The switching is performed transistor shunt elements that mute either channel. The audio signals run through the transistors only to ground; so the Signal path is free of solid state components.

The Switching is silent without clicks or pops.

Best regards,

Steve

[Gaz]

I've built something very close to what Dave G. drew up, however I found you don't need all the treble peaking since the amp has no global NFB. Good call on increasing the the treble cap in the tonestack for sure.