Critique my amp please

[iknowjohnny]

me too....i've seen a lot of schematics like that which i just cannot for the life of me see correctly because of the way they are drawn. But to me mine looks pretty typical, at least compared to what i amp used to

[John_P_WI]

OK, here is my critique ,

It is nice to see someone forging ahead without explicitly copying the "norm". I'm guessing that it is a darker sounding "Marshall" without the top end fizz.

It looks like the pre-amp stages are set up for fairly high gain with a huge amount of interstage attenuation, (100k grid leaks) plus the split plate resistor halving the signal all leads up to a warm tone without the fizzy / buzzy transients on top. Nice, all of the attenuation allows no grid stops or fizz caps to be used.

No bypass caps on the cathode biased EL34s warms them up too as there is no step gain.

Post a few clips if you got 'em.

Good luck, John

[iknowjohnny]

That pretty much sums it up descriptively except that i wouldn't call it dark at all. It CAN be with the right EQ settings. But it can bite like crazy too. That was one of my problems i have with marshalls and always wanted an amp that wasn't like that. "That" being they always seems to have a certain point of setting the treble and presence that was best, and going any further aways too thin sounding and any less and it was too dark. I wanted to be able to have a darker or brighter sound that was usable instead of feeling like the only usable setting was that perfectly right inbetween setting. With this amp i am able to have either, and either one works equally well. Or to put it in simple terms, it's just very versitile instead of being a one trick pony. But the best part by far is not the tone but the feel. IMO it is feel that not only makes you play better and enjoy an amp far more, but it is almost entirely what seperates a killer amp from a good one. It also is by far the thing that is mostly responsible for the reason people do't like SS amps.

But you really nailed a lot of what i did with this amp. Good description ! i did indeed try and configure the pre so no grid stoppers were needed because i never like the way it sounds with them I also found that the huge attenuation between stages allowed me to develop a lot of gain but not slam the next stage to the point i have too much gain. that allowed me to get a lot of gain from fewer stages without blocking distortion. And the 100k resistors seemed to work worlds better then using voltage dividers, with which i could never get the tone sounding nearly as good. At this point i'm just drooling every time i turn it on. I suppose i could post clips, but it wouldn't do a lot of good because it's the feel that makes it. I even had a very good player try it today and while he was very impressed, i could not hear what he was so impressed with !!! In other words, the tone can only be truly heard by the person who's hands are on the guitar. Yes, it sounded quite good to me. But then it sounds (feels) far far better when i am playing it. Not even close. Maybe it's just me, but i have never met an amp i was extremely impressed with that i could tell from listening to someone else playing it.

[Jana]

wanna try something else?

put a .022uf cap across that 100k pot you have in the feedback network. leave everything else as is, leave the 22k, just put a .022uf cap in parallel with the pot. Maybe you like it, maybe not, but with the pot there you will have a "resonance" control.

[skeezbo]

Separate unbypassed cathode resistors on your power tubes?
You may have seen this: a new power reduction circuit has been developed (I saw it on the Hoffman site) that is based on lifting the bypass caps off of separate cathode resistors. Inspired by the Carr Vincent, it is supposed to reduce power by around 75%, which you may not want here.

I haven't experimented with the circuit, but I think they found that in a push/pull circuit you need to bypass the resistors or have the two power tubes share a common cathode resistor, which apparently doesn't need to be bypassed. If you do bypass the separate resistors, you can then switch the caps out for power reduction by lifting the ground end.
Hope this helps,
skeez

[skeezbo]

Still scratching my head.... Is it possible that this amp is only putting out around 10 watts? That could account for the diminished clean tone, but it might also explain why this one sounds better than your other JCM800 heads, which at high gain settings may be too loud to get the power tube distortion that you seem to be getting now.

With respect,
Skeezbo

[iknowjohnny]

It gets clean. I wasn't saying that. what i was saying is that i wasn't totally happy with the clean tone, not that it wasn't clean enough. I wanted a fuller clean tone, and that 22uf cap in place of the 10uf on the pre CF stage helped that out a good bit.

I can't imagine it's 10 watts, as it's way louder than my 18 watt. And at 50 it shouldn't be that much louder than 18. In any case, i measured all needed voltages and entering them in weber's calculator it shows the tubes are each dissipating 25 watts exactly. And by the way, the gain tone that is so awesome is 100% preamp generated. I can get it souding clean way way louder than where the gain tones start sounding incredible, which is about TV listening levels on up.

Jana....i had a res cap there before but took it out because it was starting to get on my nerves and i just left the pot all the way down usually. I may try it again now tho since the amp sounds so different than it did when i had the res cap there.

[skyboltone]

It gets clean. I wasn't saying that. what i was saying is that i wasn't totally happy with the clean tone, not that it wasn't clean enough. I wanted a fuller clean tone, and that 22uf cap in place of the 10uf on the pre CF stage helped that out a good bit.

I can't imagine it's 10 watts, as it's way louder than my 18 watt. And at 50 it shouldn't be that much louder than 18. In any case, i measured all needed voltages and entering them in weber's calculator it shows the tubes are each dissipating 25 watts exactly.

But that really only tells you what the static "bias" or "idle" condition is. What might be instructive is to know what sort of voltage it's putting across an 8 ohm load before onset of clipping. Just before the crossover stage. Have you got a scope?

Also, my 50 watt builds are rather extraordinarily louder than the 18 watters. Depends on the speaker lineup of coarse. I don't imagine Jimi could have been happy with a JTM 1974 because it was almost as loud as a 1959. Or a pair of them. Or six. There's no substitue for horsepower. Just depends on the size of the track.

And I think the drawing is fine, for what it's worth. Clear as a bell. I just don't get the power tube cathode bias without bypass either.

[iknowjohnny]

The lack of cathode bypass on the PA is simple....it sounds/feels better w/o them. I find the tone gets harder with any value i have tried. Spongier w/o. Why i feel that way and others use them i have no idea, tho maybe there are some who just think they are supposed to be there and never try it w/o them. Granted, i haven't tried but 2 or 3 values, but i decided i liked it unbypassed even on my 18 where i tried all the way from 10uf to 2000uf. Plus it's louder than i'd even need as is and doesn't that increase volume?

[RB]

Hey iknowjohnny I just finished moding an amp to match your preamp chain changes. The amp started out life as a plexie with 4 6V6s in the PA. The plexie circuit was good but I wanted more. So I changed it to a JCM 800 preamp chain. That was Ok but never got the tone I wanted and could never resolve some of the noise issues I had.

The PA is fixed bias so we would be talking apples and oranges trying to compare the amps on that point.

Having read through the posts here I want to say that you are correct that it is not a dark amp. It has some serious top end bite depending on speakers.

I don’t quite understand your edit regarding the bypass cap on the gain side of the CF. Are you using a larger value than in the schematic?

All in all I am very happy with the overall balance and performance of your design. It is definitely an improvement over the JCM800 preamp chain I had in my build.

Nice work!

RB

[iknowjohnny]

Wow, thats cool ! i never had anyone copy my work. Then again, this is the first design i feel i accomplished something truly special with. As to the CF bypass cap, i did increase it to 22uf, but thats not really important because for some reason the cap size there makes far less difference than on the preamp stages. What it DOES do however is give the tone a more elastic or spongy feel. No matter how many times (and there have been MANY) i have tried to eliminate that cap, i always go back to it because it just plain feels/sounds better with it.

I must say tho that the second biggest secret to the tone i am getting is the cathode biased PA. That schematic is outdated. I currently have 22uf caps across the resistors on a switch. without them the amp has less PA gain and the tone gets real squishy and rich. With them you sacrifice that a little but you also get a louder amp and a cleaner clean tone when you turn the guitar down.Either way it blew away the tone i was getting with fixed. It was the final key.

Also, the PSU node resistors are another key. That huge 27k at the first stage node makes for a much less harsh tone. I tried a lot of different resistors on the 3 nodes till i came to realize the first stages needed low voltages but the others didn't. getting the high gain tone and feel was a neverending quest.

So you have some of what mine has, but unless you got the voltages similar and cathode biased it with seperate resistors for each tube, it isn't going to be real close. Because mine is very different than it was in fixed bias and with more "normal" PSU resistor values/voltages. It has been cathode biased before, but the preamp wasn't good at that point so i didn't realize how much better it was than fixed. I think sometimes it's GOOD to be an idiot ! If i were a great tech i may not have tried a lot of the things that turned out to give me killer tone and feel. In fact, after doing some of them i later found info on how others had used the same unorthodox methods to great success. they key is to be persistant and don't give up, use common sense, and trust your instincts. Doing that i found even a non tech like myself could pull it off. heck, i've owned a ton of amps including some expensive boutique numbers, and i built an amp i feel is the best i've ever owned. Not bad for an idiot like myself ! Now i'm starting the VVR quest so i can get the same great tone at low enough levels to practice at home. By the way, if anyone knows of a schematic or is willing to explain to me how to do it to the output section ONLY, i'd appriciate it. I have a schematic for scaling the whole amp. But i cannot envision how i could do just the PA without adding a second cap can and choke.