Ac15/trainwreck inspired amp...

[riscado]

Ok folks, any corrections/sugestions regarding this schematic would be highly appreciated... A friend of mine recently got a hold of a vox cab with alnico greys, and he was looking into getting a nice ac15 with a top boost. Turns out, we got to do some thinking and decided maybe we could incorporate a "wreck sounding" channel where the vibrato would otherwise go...

this is a rough version, and the amp will be built into a head rather than a combo which is a perfect situation for trying a wreck channel. So basically we're looking into a little input from you guys on what could be done to improve this design... therefore a schematic is included!

thanks

[LeftyStrat]

Look at the Rocket schematic in the files section. It is basically the same as you have for your Vox channel, with just a few minor tweaks. Might give you some ideas for tweaking.

Sounds like a great project.

[riscado]

I'm quite familiar with the rocket... it's the lack of negative feedback loop, choice of rectification, and most important, the power rail arrangement in this circuit that I'm concerned with.

thanks, I'm hoping it'll turn into a nice amp

[jjman]

If you might ever want to use both channels for the same signal they would be out of phase from entering different sided of the PI. If one is delayed (wet only) than you’d have no problem with phase cancellation. Otherwise, you might want to bring both channels into the same side of the PI with mixing resistors.

[riscado]

actually I avoided using passive mixing (mixing resistors) on purpose, I'm not intending to "jump" the channels, so the fact that they're are out of phase is a non-issue... I also didn't really want to use negative feedback loop, so that is also out of the equation!

thanks for the sugestion though

[klingo]

Hi,

first with your PI plate resistors (82k&100k) VOX channel is OK but TW channel is unbalanced. both 100k will be better and you are able to bridge both channel (VOX has 2 inverting stage & TW has 3, so they are 180° one each input of PI...that's OK).

As TW are using a negative feedback loop you may want to use it for the TW channel only. it's doable with a 10/47k pot across the OT secondary winding (8 ohms...) then from the wiper you go to a 100K resistor and then to the 10k TW third stage cathode resistor. this pot set the amount of negative feedback and behave like a gain pot. With this pot turned down you paralleling the 100k to the 10k so have a 9K cathode resistor...so select a 10k+10%, ~11k with your DMM. if it squeal like a pig you have positive feedback so reverse el84 grid or plate leads.

it's not my design: http://www.nintendork.org/images/akai/akai.gif
but it is in my RT77( & will stay in)

now, the feedback may interact also with the VOX channel (not sure in a good way) but you may turn the pot to 0

Hope this help

[riscado]

thanks klingo, yes you're right about the phase of the channels, I was a little unsure due to the cathode follower, however it's not an issue as I don't intend to bridge them.

As for the balance of the PI, I'll try the 100K, actually I had it like that to begin with and changed, because I went back and checked the values used on "francesca", which atually uses the 82k on the upper side.

The negative feedback idea is something that I'd like to get input on, glad you mentioned it, it's worth a try for sure.

Thanks for the ideas

Ps - I'd really appreciate if someone could check the power rail/filtering section, because 3 triodes were added, and I'd like to be sure, I didn't screw that up. Thanks.

[klingo]

with quick calcs (and from liverpool schem voltage)
i'll go around these values 15k-4k7-3k3-8k2 instead of your 18k-10k-1k-10k
Caution: your values may sound better

I would prefer, if room isn't an issue, to go with // supplies after the 15k PI filter resistor node:
-10k+10k for the TW side like in a liverpool
-10k 2nd stage//22k 1st stage for the VOX side (you may decrease these values because they are connected to the PI node instead of screen supply node).

the thing i don't like too much is having the third stage (TW) & the CF (VOX) sharing the same supply node. the CF (when unsued) will stiffen the supply of your TW 3rd stage, no sag, no bias dancing,no sing.
Caution: i have no proofs for this assertion

it is not a simple project!
are going with the combo or the head route?

[riscado]

would something along these lines work better? (schematic posted)

I realize this will not be simple, it's mostly the layout on the wreck side that I'm concerned... however it will be a head not a combo, which is a plus!

I'm also working on a visio layout already, so I'll post the final layout once I get this power railing issue sorted out, I think I'm going to leave the negative loop out for this one, even though it was a nice idea too.

[klingo]

would something along these lines work better?

Not sure...but i will try these values first or do it this way...schem attached

you may need 10/33k grid stopper on the TW first tube and a 100pf cap across PI plates.
as a vox may prefer different speakers than a TW i will build both separatly...but you may find the perfect combination

as the negative feedback loop needs only a pot, a resistor and three wires...it's an easy try & decide mod.

good luck with the layout & be happy with your amp!

Ps- have you an adress for some tube custom components to put in my expressSCH?

[riscado]

Hey klingo, thank you I'll try that...

The grid stopper I'll add after first assembly if needed... but I'll add the cap across the pi, to the schematic.

As for the loop you're right, it's simple to try out, I was actually leaving it out, because I really wanted to avoid it's interaction with the vox channel.

As for the components, I got the ones I'm using at the hoffmanamps board, and some others I had to make myself while drawing, anyway here's a link to the custom components made available by Doug Hoffman.

components

[klingo]

thanks for the link!