Biasing & Gain questions

[Howlin' Mad Mac]

Hi y'all, back after a somewhat lengthy hiatus & trying to help out a friend in need of cheap quality amplification. He's never built anything before but wants to with my help so of course my initial suggestion was a decent little SE champ kit but he wants a little more power/headroom & more of a British flavor. Oh, and did I mention he's about broke? So anyway I dug through some of my stuff & have the Weber & Classic-Tone transformers & all the tubes on hand (and willing to donate to the cause) and think I've drawn up about as simple a P-P amp as possible that hopefully will still provide decent tone. So... 1st I could use some more eyes on my schematic to catch any blatant errors (yes, I know I didn't draw the pilot light/filament connections but this is only for me to follow while providing guidance), 2nd I'm a decent enough tech but I'm no engineer so it would be nice to also know 2 things from the more knowledgeable gurus here since this is basically a stripped down mash up of a Marshall 18watt & a BF Fender Princeton (I suppose) only sans Tremolo circuit, so is there enough/too much gain and are any adjustments needed to properly bias the 6CA4s for (about) 12 watts of output? Any assistance is appreciated, explanations/formulas/calculations even more so. Oh, 1 more thing, voltages approximated using Duncan's with a (guessed at 50k load.) Thanks.

Simple 12 Watt.pdf

[Howlin' Mad Mac]

Oops folks, don't know why I marked down 6CA4 for power tubes... I know they are 6BQ5s. Doh! Making corrections (grumble, grumble, grumble)

[J M Fahey]

Dear Howlin.
May I respectfully suggest you build a true and tried schematic?
Either the Marshall 18W if you want, or some similar Fender or any other you fancy.
It will be easier that way, specially if it happens to need some troubleshooting .
Plus often you will have some suggested layout, quite helpful.
This schematic has a couple gross errors which make me feel uneasy about using it .

[printer2]

So it looks like you want to build a 18W, skip the VVR and use the Marshall tone circuit.

http://www.rh-tech.org/public/18-Wat..._PA_PI_141.gif

[jazbo8]

Ditto about the schematic, there are a few parts missing, anyway, they can be easily corrected... But I think there isn't enough gain to get a good British crunch if that's what the friend is looking for. Another alternative might be the TW Liverpool.

Jaz

[Tom Phillips]

+1 to the previous three posts. They provided words of wisdom.
Cheers,
Tom

[Howlin' Mad Mac]

Thanks for your input Juan. I do agree it would be easier to build an 18w however I'm limited by my friends lack of funds & therfore need to use what I have, not what he can't afford to buy. I've been looking at a print out of the schematic I'd drawn up & the only thing really bothering me is the PI. I'm realizing that as drawn there's no signal feeding the one triode & I need to fix that before I repost. I also suggested like a JTM 45 pre with a more 18w power amp but he likes the sound when he plugs into the non-tremolo channel of my own personal 18w clone I built awhile back which is why I drew it up this way. My big questions regarding gain is I know even when you're not using the trem channel it still adds something to the sound & when I've built other amps leaving out the trem circuit they work ok but sound lacking compared to counterparts with & just wanted to make sure there was enough to be able to overdrive w/o a pedal. I was also concerned that using the smaller Princeton OT (12 watts I think) that I might need to change the value of the 6BQ5s bias resistor or is output wattage determined by the transformer alone. As is I know how to take readings, replace burnt out parts on a broken amp (or Hammond Organ for that matter) & build tried & true clones from parts but I still have issues when I try to change things up a bit. It would be easier if I had more time to devote but alas that's not going to happen anytime soon so I'll mess around with this a bit more & try to get it right before I tell my friend he's just going to have to make due with his crappy little SS Epiphone practice amp that sounds like ass a while longer.

[Howlin' Mad Mac]

Ok folks here is revised schematic complete with omitted capicitor in the PI. I'm also not quite sure that my voltages will be as high as Duncan's says. Other than swapping a 5v 5Y3 for the 6.3v EZ81 rectifier (which as I understand have very similar output) this IS the same supply my 18w uses & its voltages are (about) 325/320/270. I just don't know how much gain I'm losing (if any) w/o the tremolo. I'd also like to make sure I'm not trying to push 18w through a OT designed to handle 12-15w. Does that help?

Simple 12 Watt.pdf

[trem]

Excuse me for asking...
...but what program/software was used to make your schem.?
It is very nice looking.
Thank You

[Tom Phillips]

... I do agree it would be easier to build an 18w however...

OK. This is the way you learn but consider that you can still build the 18Watter circuit using the major components that you have.
If you want to proceed with your design then things I see right off that you need to correct are:

1. When your tone control pot is all the way “up” as drawn on your latest schematic all the signal is shorted to ground.
2. V4’s cathode is connected directly to ground.

Tom

[jazbo8]

I think the 125r for the PA cathode is fine, although the 340v on the screen is a bit high unless you have older or NOS EL84s... By dropping one gain stage from the 18W design, you can just get a clean amp but don't expect much if any OD. The LTPI and the parallelled V1 are both fine if you have tubes to spare, but in this case, I think you might be better off turning V1 into 2 cascaded gain stages and split V2 into either another gain stage or CF to drive the tonestack and use the other half as a cathodyne PI to drive the PA. e.g., V1a -> TS -> V1b -> V2a -> V2b (cathodyne) -> PA, or V1a -> V1b -> V2a (CF) -> TS -> V2b (cathodyne) -> PA. Either configuration should be able to provide enough OD.

Jaz

[J M Fahey]

It makes me somewhat nervous that you didn't notice on your own the gross errors pointed by Tom Philips, and also that others didn't comment on them.
I'd also add that C5 must be placed in parallel with R11 or, much better, from V2a grid to ground.
As-is, it heavily compromises the function of R9: being a high impedance current source feeding the PI cathodes.
These problems must be addressed before suggesting improvements.
You can only improve what's already working ... which is not the case.
A schematic is not "a drawing" but shows a logic series of connections.
To "look almost like" some other one, is not enough.
You don't need to know much, if any, of electronics theory ... if you build an amp "painting by the numbers"
There are thousands of successful amp builders which did just that, but as soon as you want to modify anything, you must know why.
That's why I don't discourage you on building an amp, far from it, but *strongly* suggest you follow a proven design, to the letter.
Wish you the best, specially avoiding frustrations on your build.
And I respect your idea of keeping costs down, why don't you post the 18W schematic you built?
Maybe we can save time going straight to the point insted of going back and forth many times.
And yes, the tremolo channel can be chopped off but let's all work on the same original schematic first.

[Enzo]

I know it isn't what you want to do, but if parts cost is an issue, then why have two inputs? Change the PI to a split load, and you can eliminate an entire 12AX7.

[Tage]

You only want to run two pre-amp tubes and want a brit"ish" sound then an old school Orange/Matamp preamp would be great, you can simplify it a lot, but remember, little parts, caps, resistors, pots, aren't the place to save on cost and they help add versatility. So you could keep that whole preamp, or at least the james tone stack and volume.

here is an example of that pre-amp. Just sign it up for weight watchers. No faq switch or drive control

http://www.orangefieldguide.com/OFG_SCHEM/ODschem_A.gif

[printer2]

It makes me somewhat nervous that you didn't notice on your own the gross errors pointed by Tom Philips, and also that others didn't comment on them.
I'd also add that C5 must be placed in parallel with R11 or, much better, from V2a grid to ground.
As-is, it heavily compromises the function of R9: being a high impedance current source feeding the PI cathodes.
These problems must be addressed before suggesting improvements.
You can only improve what's already working ... which is not the case.
A schematic is not "a drawing" but shows a logic series of connections.
To "look almost like" some other one, is not enough.
You don't need to know much, if any, of electronics theory ... if you build an amp "painting by the numbers"
There are thousands of successful amp builders which did just that, but as soon as you want to modify anything, you must know why.
That's why I don't discourage you on building an amp, far from it, but *strongly* suggest you follow a proven design, to the letter.
Wish you the best, specially avoiding frustrations on your build.
And I respect your idea of keeping costs down, why don't you post the 18W schematic you built?
Maybe we can save time going straight to the point insted of going back and forth many times.
And yes, the tremolo channel can be chopped off but let's all work on the same original schematic first.

I was hoping that he would look at the schematic I listed and see that it used basically the same circuit and values but with the parts connected properly. The first thing that I noticed is that the cathodes on the input triodes share the same resistor but one cathode is connected properly and the other is tied directly to ground, even in the revised circuit.

Howlin', take your schematic and the 18 Watt Lite II side by side and look at the two together. I think you might find it is closer to what you were planing in the first place.