Critique my amp please

[iknowjohnny]

Ok, after being asked several times i finally updated my schematic. And after fixing a few things i have been asking about here it sounds great and i think i'm pretty much done with it, but i'd like to hear any thoughts on the final circuit. note that there are several things you may find odd but i kept it like that because those things just simply work for me. But what i would like to hear is whether anyone sees anything that appears like it is not good practice and could be changed to improve things even more. If it's one of those things that i kept because it just always works best for me i'll let ya know. But anything that you think could be detrimental to the tone i'd like to hear about before i zip this puppy up. Thanks for all the help so far....i appreciate it !

[img603]http://i24.photobucket.com/albums/c39/dazco/amp.jpg[/img]

[jjman]

Only strange thing to me is the "missing" resistors on the input. If you have no radio stations coming thru I guess you’re ok. But it you bring the amp elsewhere maybe a problem would occur?

Is this an area you experimented with ?

[CaseyJones]

I see you're using the rare "cathodeless" version of the EL34.

That's good 'cuz there's no apparent way to bias 'em.

Sorry, you asked...



Stick at least a 33k grid blocker on the input, it will save you all sorts of trouble later.

[iknowjohnny]

The input is a typo on the schematic. Theres a grid stopper and a ground ref R. I'll fix that on the drawing, thanks. As for the cathode-less PA....what do you mean? I may be spacing, but i keep looking at it and don't see the issue. Both cathodes are tied to ground via dual 470R's. What am i missing?

[stoo]

oops. beat me to it.

[Andy Le Blanc]

your just a hair away from a JCM 800....... you might not need the bypass
cap around 820r just prior to the follower ....

[iknowjohnny]

That bypass is one of those things i mentioned. No matter what configuration i've had the amp, it always sounds better and has a certain spongy feel with that cap there.

Still wanna know what it is i am missing about the PA biasing. I mean that seriously... for the life of me i cannot see what is wrong. Amp sure sounds awesome tho, so i cannot figure out how it isn't being biased unless there is another schematic typo.

[RB]

You have drawn the PA as if were cathode biased. So you are missing the EL34 cathode resistor(s) and bypass cap(s) and their ground connection. If you intended it to be fixed bias you need to remove the ground from the EL34 220k grid resistors and apply the bias voltage there and then you can draw in the EL34 cathodes and ground them, and last but not least the bias supply circuitry.

Regards
Randy

[CaseyJones]

I may be spacing, but i keep looking at it and don't see the issue. Both cathodes are tied to ground via dual 470R's. What am i missing?

Oooohh.

It's me who is spacing. Pin 8 is indicated on the schematic but there's no cathodes shown on the tubes. So I'm lookin' around for a fixed bias circuit or cathode bias in between the tubes, the network is actually down there by the ground plane. Just like you say it is....

I'd try several things. Try a single 250 ohm 10 watt resistor, bypass it with a 50 microfarad cap at 50 volts or greater. I still think cathode bias is working against what you have. In general high gain preamps are teamed up with fixed bias, it keeps the amp from bein' too saggy and mushy.

Splitting it into seperate resistors for each tube like you have it is a good idea, that way you can get it done without cooking one big resistor.

Carry on!

[Andy Le Blanc]

I see 470r resistors at the cathodes of the power tubes.......
you have a few choices there..... a shared chathode resistor or one for each
tube.... and what to bypass with...... I like the tone when each has its own
but it seemed to get fuzzy when cranked.... I would stick a 1 ohm in there
between the 470r 's and ground to measure static dissapation with to make sure you know where your operating point is.....

[CaseyJones]

You have drawn the PA as if were cathode biased. So you are missing the EL34 cathode resistor(s) and bypass cap(s) and their ground connection. If you intended it to be fixed bias you need to remove the ground from the EL34 220k grid resistors and apply the bias voltage there and then you can draw in the EL34 cathodes and ground them, and last but not least the bias supply circuitry.

Regards
Randy

Yaayyy! Now I don't feel quite so silly.

[iknowjohnny]

man, you guys have me so confused at this point i'
m no longer even sure who is spacing ! the schematic clearly shows TWO 480 ohm resistors tied from pins 1 and 8 to ground. the amp is SUPPOSED to be cathode biased. i did just notice another typo where the presence control is....the ground line is missing for tiny bit where i changed something and forgot to connect it. But even that shouldn't matter because even if you accept that missing part, both sides are still shown to be connected to ground at the input and output jacks.

So will someone please clearly tell me where i amp wrong here so i know whats going on? I don't doubt you, but then i also no longer know if any of you are saying theres a problem or that you now see what i'm saying !

[CaseyJones]

There's nothing wrong with your amp. The only thing that's "wrong" is that when I look at a schematic that looks like a JCM800 my eyeballs play tricks on me, I expect to see a JCM800. You've got a few clever changes from a JCM800 which threw me a little. RB didn't see it, either.

I'll bet it's a kick-ass blues amp.

[RB]

My apologies for my previous post. My mind totally block those PA cathode resistors. That’s the problem some times with the human mind. We get used to certain patterns and project over reality what we have repeatedly seen and want to see and then you end up with egg on your face. I haven’t seen a guitar power amp schematic organized in that way that I can recall. I am not saying it’s incorrect, just different and it flat jammed my pea brain up.

Regards
Randy

[iknowjohnny]

thanks. And i can actually point out quite a few differences between this and a 800. You'd really realize that if you heard it. I've owned three 800's and none sounded this good. It's of course a similar sound, but very much improved, which is exactly what i was shooting for. Has more drive too. I will say i think the 800's cleaner tones when turning down the guitar volume are a little better, but this amp's drive tone is much richer. I tried to get the cleaner tones as good, but i could not seem to get that w/o sacrificing the driven tone, which seems odd to me because i always felt the better the clean tone is, the better the driven tone will be. But no biggie, as it sounds very close clean anyways, just not as perfect as i'd hoped.

EDIT:well, after saying that i decided to try a larger cap on the pre CF stage because the main thing about the clean i wasn't totally happy with was that there wasn't enough body. That seems to have rectified that issue ! (not to mention even improving the high gain tones) Right now i'm so friggin happy with this tone that i have surpassed what i had hoped for initially, and my hopes were pretty high. It's mainly due to a few changes i tried due to what some of you told me in other threads i started. Thank you !

[img618]http://i24.photobucket.com/albums/c39/dazco/el34.jpg[/img]