AC30 preamp into JTM45 power amp / Rockster gain

[VintageCharlie]

Hi guys,

so i recently ventured into AC30-land and cam to think of it as my favorite amp ever as long as i used some old EL84 japanese valves that were on the way out. They smoothed out the top end and the amp had seemingly more bottom end. At some point they stopped sounding good and are now retired. With all the new production tubes and even old original blackburn Mullards also the nasty side (entirely subjective and i'm sure many love these qualities) of EL84's came into the picture (fizziness, a bit too bright, the sand-like overtones when driven hard, etc.). I haven't found a way yet to dial that out while keeping the good aspects of the amp.

So out of interest i just injected the AC30 preamp into a JTM45 power amp. My AC30 clone has a mids control a la Marshall. The signal was a bit too low than the power amp expected as it seems, but when i cranked the mid control, it tickled the power amp enough (bumping mids bumps up the signal considerably) to get a glimpse of what this combo might be capable of. I am adding a short clip. as the mid control is cranked the mids overshadow the bass and it is a bright sound. With the stock AC30 power amp it had potential but was way too fizzy and harsh. With this, it is quite glorious - bright and detailed, but smooth and creamy. I boosted the front end a bit as it was a bit cleaner than this on its own.

I know about the Rockster amp, which has been circling on these forums and i suppose there is little use of inventing the wheel again - that is pretty much the same thing i guess (probably similar concept to Matchless Phoenix, Bad Cat Wild Cat/Cub, Top Hat Super Deluxe and probably many others with their variation on the theme. What i am wondering though, is how much gain the Rockster gets compared to the AC30 with el84s? As i saw now in my little expewriment, the AC30 preamp itself has similar level of gain as the plexi circuit - quite cleanish. I assume that the el84s are driven hard in the AC30 and contribute a lot to the much higher gain levels. AS the available amount of gain is one of the selling points for an ac30 in my view, i am wondering, if the Rockster type amp is not far behind in this respect?

Thanks a bunch in advance for any input!

[Roe]

sounds like a small ac50 with LTP phase inverter almost

[gtomax]

@Roe for sure

The AC10 and AC50 get overlooked for the AC30 but are both epic amplifiers

[gtomax]

@OP check out the Russian 6P14P-K or -EV I've found these to have excellent overdrive characteristics and be a touch darker than a typical EL84 which matches nicely with Vox circuits

[VintageCharlie]

@OP check out the Russian 6P14P-K or -EV I've found these to have excellent overdrive characteristics and be a touch darker than a typical EL84 which matches nicely with Vox circuits

Thanks for the hint. This recommendation is surfacing a lot, i guess time to take it serious. Will check them out.

[gtomax]

The good news is they are cheap so it's an easy experiment!

[VintageCharlie]

sounds like a small ac50 with LTP phase inverter almost

I didn't know the AC50 does NOT have an LTP PI, i assumed it does. What type of PI DOES it have?

But yeah, thereabouts would be a starting point. Then i just want to see how much fixed vs cathode bias effects the tone and what disconnecting NFB does. I did my tests with the presence knob at 0, which i assume lessens the effect of the NFB circuit.

[Roe]

sounds like a small ac50 with LTP phase inverter almost

I didn't know the AC50 does NOT have an LTP PI, i assumed it does. What type of PI DOES it have?

But yeah, thereabouts would be a starting point. Then i just want to see how much fixed vs cathode bias effects the tone and what disconnecting NFB does. I did my tests with the presence knob at 0, which i assume lessens the effect of the NFB circuit.

the very first ac50s had LTP PI, but most of the others had a paraphase inverter. all the ac100s had paraphase but some of the later 120w and 125w amps went back to the LTP. It does not make a great difference until the phase inverter breaks up

[pdf64]

Of the 4 versions, it was only the AC50 MkIII that had the (floating) paraphase. But it’s that version which seems by some way the most common.
The paraphase will have perhaps 6dB higher gain, and greater potential output signal, than the LTP.
Clipping characteristics would seem to be largely determined by the output valves and their operating point; somewhat higher signal levels are required to overdrive the input of the phase splitter than the output valves.
https://www.voxac50.org.uk/ac50_schematics.htm