Clean boost with unused triode

[loverocker]

Happy as I am with my Lite II build, I can't leave the unused triode sitting doing nothing. I could parallel the first stage - like the Lite II, but I want the kind of gain that you get with a clean boost stompbox, but I don't like pedals.

So I sketched out the simplest of clean boost circuits: http://www.firstmail.com/18w/cleanboost.gif - the red circuit is the new stuff, and the black part shows how it fits on to the first bit of the amp. (Note: I didn't forget that I need another power supply stage for the new triode, I just forgot to put it in!)

Could you chaps give me a reality check on it? I've just adapted the Marshall 2203 method of switching by using alternate input jack. If I've got it right, plugging into the top jack in the diagram gives the standard 18W circuit, but plugging into the lower jack gives an adjustable clean boost... Hopefully

Let me know if you spot any mistakes or bad ideas. I plan on using the same values as the first stage to start with and then adjusting values if the EQ needs tweaking one way or the other.

[markh]

I think "clean boost" is a misnomer here. The stock circuit already provides plenty of voltage to slam the PI and output tubes, so you'll just end up overdriving the preamp more. You may need to do a lot of tuning, as many have reported here over the last few years, too much gain in the 18watt tends to sound, um, not that great. OTOH, if you do hit on the right values to make it sound good, I thing many people would be interested.

--mark

[loverocker]

I know what you mean I'm just talking about hitting the first stage harder, like what a lot folks these days like to do with Tube Screamers/etc (Level up, Distortion down). It may make it fizzy, etc, etc. - I'll see...

I also spotted a mistake in my diagram - I also need to add a 1M resistor from the grid of the second stage to ground. D'oh!

By the way, this weekend I plugged it into a 4x12 (mid-'70s Marshall G12Hs) - sounds far better clean than the 1x12 combo, and sounds very big when cranked.

[SteveinTempe]

you will need massive attenuation,
I tried this and gave up, you may try some marshall tricks
like the 470k in series with the grid with a bypass cap for that
screaming high end crunch,
and voltage divider instead of 1 meg grid go lower...
you may even try the 1 meg in series and 100k grid
possibly bypassing the 1 meg with like 180 pico
??
curious what others think here I have extra triodes in mine cus I got no trem
good luck with it.



another marshall trick would be a 10k cathode resistor
also lose the bypass cap..
I have fought this gain problem a lot..it sounds horrible
when there is too much.

perhaps someone on the list could draw a true paralell
connection with like a summing node..
or maybe you can just add a channel with different values for
frequncy response and paralell the channels with the input jacks
That is what I ended up doing

T.O.T. there are many examples
of course my book is too new to make me an expert..
I do like Kevin O'Conner's Book.

It has been a big help

[SteveinTempe]

[SteveinTempe]

I tried for an hour or two to get two triodes in series in front of the PI
I cant say my amp is a real 18watt but I do know that I keep
coming back to only 1 12ax7 stage is needed!! holy cow does it get unstable
even with all kinds of attenuation I think high gain and high octane
can stay over at the ax84 project
this thing is a delicate , sensitive design..
that sounds great wiht one half of a 12ax7!
my two cents

[loverocker]

Hi Steve - thanks for the info. I wondered who'd been down this route already. I thought I might have to add some attenuation, and there may be some squeal, too. Yeah - I've seen that [470pf || 470K] + [470K] divider in Marshalls, etc, so it'll be my first stop if I need some attenuation.

Thing is, the reason I used the 2203 input jack switching is that I don't want to mess with the basic 18W tone. This circuit will give a 100% standard Lite II circuit when I plug into the upper jack in that diagram. So I'll be able to go back and compare tone with and without that extra stage. We'll see...

[loverocker]

OK - I wired the boost up and it does produce a bit more preamp-style gain. Stating the obvious First thing - I have a post-PI Master Volume in my build, and this extra stage makes the amp more flexible at low volume levels.

When the Master is dimed, this extra stage pushes the overdrive tone quite a bit more than the 18W alone, but the distortion is still way short of the ultra-compressed stuff you get from a maxed multi-stage Mesa. The amp remains perfectly usable (IMHO!) and the 18W tone is there if you want it (I'll also experiment by ear and add a front-panel mark at the 'unity gain' position - just for reference).

I do get some oscillation at about 8-10 on the dial, so I'll add a voltage divider after the pot to attenuate that out. I don't want to mess with any part of the second stage itself, because then I'd lose the ability to plug into the standard 18W circuit.

Of course, next there's the hard work - making up a faceplate...

[JIG]

I am interested in these mods. I am gathering materials to buils the 18 watt lite. I plan on doing it with 6v6 in the power section and a master volume mod. Could you not just directly parralell the inputs? Please post a schem or a layou of your work. I would like to see and hear the results. An unused triode is a terible thing to waste>

JIG

[loverocker]

Yup - I was going to do the parallel input socket that's shown on the Lite II schematic (which is what the rest of my amp is pretty much). But once the amp was made I just wondered what it would sound like with a hotter signal from the guitar. But I don't like using pedals, so... ...let's use that triode instead.

The mod diagram is still at http://www.firstmail.com/18w/cleanboost.gif - there's got to be a 1M from the grid of the second stage to ground.

I didn't put in the voltage divider that Steve mentioned. Yet. Playing with valves in V1 yesterday, I noticed that with some there was no squeal at all - lower gain I presume. I'll do some more experimenting before deciding. May also lower the cathode bypass cap and/or coupling cap, too.

[JIG]

What kind of valves were you playing with? All 12AX7's? Havew you tried a 12AT7? not quite as much gain (60 vs 100) but in my Electar the 12AT7 really sound quite good

JIG

[zaphod_phil]

Playing with valves in V1 yesterday, I noticed that with some there was no squeal at all - lower gain I presume. I'll do some more experimenting before deciding. May also lower the cathode bypass cap and/or coupling cap, too.

The parallel triode thing would roughly double your voltage gain - I've discussed this through with Paul Ruby on the AX84 board, and we know this now to be correct, given certain provisos. Yes, you certainly should reduce the value of the cathode cap (down to around 1uF or 0.68uF) or you could get some very mushy bass happening. The cathode resistor value looks OK for a first stage. I also recommend against trying out the 10k cathode resistor in the 2nd triode (which will bias it very cold and asymetric), unless you really want the amp to sound like a JCM800!

[loverocker]

JIG - I experimented with AX7s (Mullard, Brimar, JJ Teslas). Will try out some AT7s and AU7s soon, too <thinks: really must get myself an oven mit for the gear room> But I really want to keep the top input to be pure 18Wer though, and putting an AT7/AU7 in there would undo that... Might sound interesting though...

Phil - thanks for the info - I may try that 1uF on the first cathode, but there's no way I'm going to 10K on the second stage (already got a 2204, and that now has 3K-ish on the second stage).

And another thing - it's great to be back (thanks gabster-dude!)

[zaphod_phil]

Have you tried a 12AT7? not quite as much gain (60 vs 100) but in my Electar the 12AT7 really sound quite good

Last night I found out why your Electar wasn't sounding right with 12AX7s, whereas mine is great. I discovered that R10 in mine is 47K, not the 22K I thought I had used! As this is now O.T. , I'll tell you about it offline....