Thoughts on my Jose 2204

[FourT6and2]

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

 

[V2a]

Here's another link with lots of info, including the pros and cons of various fixes for the amp farts:
http://www.aikenamps.com/index.php/what ... distortion

 

[SpiderWars]

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

What was in Adam Jones SuperBass? Cranked.

 

[FourT6and2]

None of my amps show symptoms of mud butt lol. Here's a clip as an example. This amp is very similar to the schematic in my first post. A few voicing changes, but the meat and potatoes are the same. If I remember correctly this one has 120K bias splitters/grid leaks in the PI + 0.047uF coupling caps, 100pF fizz cap, 2K7/.68UF in V1a and V1b, 820R/.68 in V2a, 390K (or maybe 430K, I can't remember) V1a plate, 36K slope + 500pF treble, aaaaand a few other differences I can't remember. But overall, it's the same circuit pretty much.

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

What was in Adam Jones SuperBass? Cranked.

His Super Bass was modified to Lead spec. Pretty much a stock Super Lead. If I remember, the presence cap is .68uF. 0.68uF bypass caps in V1a/b and V2a. PI is stock Marshall spec. I built one to that spec years ago and it sounded pretty good.

 

[V2a]

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

You can calculate the high-pass cutoff here:
https://ampbooks.com/mobile/amplifier-c ... excursion/

With your schematic values, the cutoff is at 15.5Hz (low E on guitar ~ 70 Hz). With 22n coupling caps, the cutoff raises to 33 Hz (still way lower than the lowest note in standard tuning).

Also note that you have to have the master turned (way) up to start farting out....

 

[FourT6and2]

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

You can calculate the high-pass cutoff here:
https://ampbooks.com/mobile/amplifier-c ... excursion/

With your schematic values, the cutoff is at 15.5Hz (low E on guitar ~ 70 Hz). With 22n coupling caps, the cutoff raises to 33 Hz (still way lower than the lowest note in standard tuning).

Also note that you have to have the master turned (way) up to start farting out....

No harm in trying 0.022uF caps and stock 220K resistor value (again). I mean, I already did that before. I've got a Fortin here with stock Marshall values and it's super freakin' bright. Although it is very tight. On the opposite end of the spectrum is one of the amps I built which is much darker and stiffer (which is what I wanted). Then there's the Ceriatone Chupacabra which uses 110K PI grid leaks with 0.1uF caps and it's very brown sounding with deeeeeep bass. But it's very loose. Kind of in a good way. But doesn't track quite as fast for my taste. Still one of my favorite circuits.

Didn't Marshall lower those 220Ks to 150K at some point for US amps that shipped with 6550s?


Anyway, with all the changes people are suggesting here (and on some other forums), it's like people want me to make this amp into a stock JCM800 lol. Not really the direction I want to go in haha

 

[SpiderWars]

Anyway, with all the changes people are suggesting here (and on some other forums), it's like people want me to make this amp into a stock JCM800 lol. Not really the direction I want to go in haha

I was sort of hoping you'd make an Aldrich.

 

[FourT6and2]

Anyway, with all the changes people are suggesting here (and on some other forums), it's like people want me to make this amp into a stock JCM800 lol. Not really the direction I want to go in haha

I was sort of hoping you'd make an Aldrich.

I do have an Aldrich schematic and layout. No idea if they're accurate. But looks like an interesting amp. I've not really paid much attention to it but I can have a closer look. Any good clips of an Aldrich you like?

 

[SpiderWars]

Anyway, with all the changes people are suggesting here (and on some other forums), it's like people want me to make this amp into a stock JCM800 lol. Not really the direction I want to go in haha

I was sort of hoping you'd make an Aldrich.

I do have an Aldrich schematic and layout. No idea if they're accurate. But looks like an interesting amp. I've not really paid much attention to it but I can have a closer look. Any good clips of an Aldrich you like?

psychodave's recent clip. technomancer is building one. There's a SLOclone thread on the Aldrich and the Snorkler that has lots of info. Hey there's another, build a Snorkler.

 

[V2a]

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

You can calculate the high-pass cutoff here:
https://ampbooks.com/mobile/amplifier-c ... excursion/

With your schematic values, the cutoff is at 15.5Hz (low E on guitar ~ 70 Hz). With 22n coupling caps, the cutoff raises to 33 Hz (still way lower than the lowest note in standard tuning).

Also note that you have to have the master turned (way) up to start farting out....

No harm in trying 0.022uF caps and stock 220K resistor value (again). I mean, I already did that before. I've got a Fortin here with stock Marshall values and it's super freakin' bright. Although it is very tight. On the opposite end of the spectrum is one of the amps I built which is much darker and stiffer (which is what I wanted). Then there's the Ceriatone Chupacabra which uses 110K PI grid leaks with 0.1uF caps and it's very brown sounding with deeeeeep bass. But it's very loose. Kind of in a good way. But doesn't track quite as fast for my taste. Still one of my favorite circuits.

Didn't Marshall lower those 220Ks to 150K at some point for US amps that shipped with 6550s?


Anyway, with all the changes people are suggesting here (and on some other forums), it's like people want me to make this amp into a stock JCM800 lol. Not really the direction I want to go in haha

I'm not trying to sway you from your previous decisions; I was just wondering about the reasons behind the decisions. There are plenty of amps out there with 47n or 100n coupling caps, and most sound great. But I think that it's always good to know a little math and electronics behind the design and component-value choices.

Your other amps sound killer, btw.

 

[FourT6and2]

I can also always use a plate bypass cap instead. That's another can of worms. Because I can go with:

330K resistor bypassed
100K resistor bypassed with 220K in series
220K bypassed with 100K in series
220K in series with 100K that's bypassed
100K in series with 220K that's bypassed

Does it make a difference what order it's in or if it's just a single bypassed resistor? I've seen it done in each of these ways.

The order of the series Rs does not matter unless you take the output from the intersection of the Rs rather than from the anode.

Ok, thanks! So that said... why would someone build a NEW amp (not a mod of an existing amp) with a 100K bypassed with a 330pF cap and then a 220K resister in series, to the socket? Why not just use a single 320K or 330K resister?

In short, too much bias excursion and recovery time leads to bad distortion. The amp might get a case of the farts.

Ahhhh ok. Well that shouldn't happen here. I've already tried all three values in the PI in 4 other similar builds (2204-based), with no issues. A big clue to what I'm doing is spelled out on Robinette's site: "Reducing the size of the [PI} coupling caps will reduce the bass content of the signal." I think the stock 0.022uF value is just too bright for the sound I want.

You can calculate the high-pass cutoff here:
https://ampbooks.com/mobile/amplifier-c ... excursion/

With your schematic values, the cutoff is at 15.5Hz (low E on guitar ~ 70 Hz). With 22n coupling caps, the cutoff raises to 33 Hz (still way lower than the lowest note in standard tuning).

Also note that you have to have the master turned (way) up to start farting out....

No harm in trying 0.022uF caps and stock 220K resistor value (again). I mean, I already did that before. I've got a Fortin here with stock Marshall values and it's super freakin' bright. Although it is very tight. On the opposite end of the spectrum is one of the amps I built which is much darker and stiffer (which is what I wanted). Then there's the Ceriatone Chupacabra which uses 110K PI grid leaks with 0.1uF caps and it's very brown sounding with deeeeeep bass. But it's very loose. Kind of in a good way. But doesn't track quite as fast for my taste. Still one of my favorite circuits.

Didn't Marshall lower those 220Ks to 150K at some point for US amps that shipped with 6550s?


Anyway, with all the changes people are suggesting here (and on some other forums), it's like people want me to make this amp into a stock JCM800 lol. Not really the direction I want to go in haha

I'm not trying to sway you from your previous decisions; I was just wondering about the reasons behind the decisions. There are plenty of amps out there with 47n or 100n coupling caps, and most sound great. But I think that it's always good to know a little math and electronics behind the design and component-value choices.

Your other amps sound killer, btw.

Absolutely The engineering and math behind all of this stuff is still very much a mystery to me. I've played around with all the calculators and graphs and whatnot. But at the end of the day, looking at a graph with frequency cutoffs is still meaningless to me because I don't really know how to interpret it for the most part.

For example, grid bias excursion calculator. I get that you don't want a flubby farty amp. That's a given lol. So, stock Marshall values are 220K grid leak, 0.022uF couplers, 5K6 grid stoppers on the power tubes. At one point Marshall changed the grid leaks to 150K when they used 6550 tubes instead of EL34. But I've got a modern-day amp here that uses EL34 and it has 150K grid leaks + 10K (instead of 5K6) grid stoppers. With 0.022/150K/5K6 = cutoff of 33 Hz instead of 25 Hz, excursion is basically the same, but recovery sped up by 2 milliseconds. Is any of this even audible at the end of the day?

Now, change the grid stoppers to 10K with the 150K grid leaks, and cutoff hasn't changed (still 33). Recovery hasn't changed. But excursion increased by 0.1 milliseconds. Again... does that even make a difference? And if not, then why would someone design a new/modern amp with 150K grid leaks and 10K stoppers (0.022uF caps)?

 

[V2a]

Ok, thanks! So that said... why would someone build a NEW amp (not a mod of an existing amp) with a 100K bypassed with a 330pF cap and then a 220K resister in series, to the socket? Why not just use a single 320K or 330K resister?

I can think of two reasons (one good, one not so good
1. Copy the popular Friedman setup
2. Because the partial bypass results in less attenuation in the stop band. In the case of a partial anode bypass, you'll attenuate but not completely lose those high frequencies.


So what was the reason MF used the Friedman setup in that expensive new amp you just purchased? It has to be either 1 or 2.

Absolutely The engineering and math behind all of this stuff is still very much a mystery to me. I've played around with all the calculators and graphs and whatnot. But at the end of the day, looking at a graph with frequency cutoffs is still meaningless to me because I don't really know how to interpret it for the most part.

For example, grid bias excursion calculator. I get that you don't want a flubby farty amp. That's a given lol. So, stock Marshall values are 220K grid leak, 0.022uF couplers, 5K6 grid stoppers on the power tubes. At one point Marshall changed the grid leaks to 150K when they used 6550 tubes instead of EL34. But I've got a modern-day amp here that uses EL34 and it has 150K grid leaks + 10K (instead of 5K6) grid stoppers. With 0.022/150K/5K6 = cutoff of 33 Hz instead of 25 Hz, excursion is basically the same, but recovery sped up by 2 milliseconds. Is any of this even audible at the end of the day?

Now, change the grid stoppers to 10K with the 150K grid leaks, and cutoff hasn't changed (still 33). Recovery hasn't changed. But excursion increased by 0.1 milliseconds. Again... does that even make a difference? And if not, then why would someone design a new/modern amp with 150K grid leaks and 10K stoppers (0.022uF caps)?

In my opinion (based on limited experimentation), it makes a difference once the excursion plus recovery starts to last more than 10 ms. A change of a few milliseconds isn't going to be noticeable (I don't think).

Why use 150k grid leaks and 10k stoppers? There are many reasons, including not-so-good reasons (1. people are experimenting without having a decent understanding; 2. people are holding on to vintage values or other legacy values).

Good reasons include the following factors:
- all tubes have limits to the resistance in the grid circuit. The limits depend on whether the tube has fixed bias or cathode/self bias. The 6V6 has a 100k ohm limit in fixed bias, so that the total resistance between the PI coupling cap and each output tube grid should be less than 100k! That includes the grid leaks (normally 220k), the grid stopper (1-10k), and any other series resistance you put in place to limit the signal (see the Aiken link that I posted earlier).
- someone might lower the grid leak to be within limits
- someone might increase the grid stopper to reduce oscillations, etc. The stopper can be much higher than typical before the audio frequencies are affected.

 

[FourT6and2]

Ok, thanks! So that said... why would someone build a NEW amp (not a mod of an existing amp) with a 100K bypassed with a 330pF cap and then a 220K resister in series, to the socket? Why not just use a single 320K or 330K resister?

I can think of two reasons (one good, one not so good
1. Copy the popular Friedman setup
2. Because the partial bypass results in less attenuation in the stop band. In the case of a partial anode bypass, you'll attenuate but not completely lose those high frequencies.


So what was the reason MF used the Friedman setup in that expensive new amp you just purchased? It has to be either 1 or 2.

Interesting. The amp you're referring to is very bright. So personally, I want more attenuation of some of the highs. My thoughts are to try one or more of the following if I ever build something similar.

1. Up the PI fizz cap from 47pF to 100pF
2. Go with a 330K resistor bypassed for V1a plate
3. 0.001uF cap off V1b grid to ground
4. 0.001uF cap in series from V1a coupling cap to ground

On a related note, yes the expensive new amp is a bit... surprising... since it's a 2204 with some diodes and it seems I've already built a bunch just like it
It's not a bad sounding amp. It's just too bright for my taste. There are a small number of useful tidbits I might implement into my next build. I just prefer a darker voicing since I don't play 8-strings tuned down 5 steps.

In my opinion (based on limited experimentation), it makes a difference once the excursion plus recovery starts to last more than 10 ms. A change of a few milliseconds isn't going to be noticeable (I don't think).

Why use 150k grid leaks and 10k stoppers? There are many reasons, including not-so-good reasons (1. people are experimenting without having a decent understanding; 2. people are holding on to vintage values or other legacy values).

Good reasons include the following factors:
- all tubes have limits to the resistance in the grid circuit. The limits depend on whether the tube has fixed bias or cathode/self bias. The 6V6 has a 100k ohm limit in fixed bias, so that the total resistance between the PI coupling cap and each output tube grid should be less than 100k! That includes the grid leaks (normally 220k), the grid stopper (1-10k), and any other series resistance you put in place to limit the signal (see the Aiken link that I posted earlier).
- someone might lower the grid leak to be within limits
- someone might increase the grid stopper to reduce oscillations, etc. The stopper can be much higher than typical before the audio frequencies are affected.

Thank you

 

[V2a]

To clarify your proposed fizz destroying cap plan:

Cap 2. Since you are increasing the R that is bypassed, you might want to decrease the C (to keep cutoff at same frequency as the BE.

Are the two 1000p both between stage 1 and 2? And both to ground?

You really need only the anode bypass or the cap to ground since both attenuate high frequencies between the first and second gain stages

 

[FourT6and2]

To clarify your proposed fizz destroying cap plan:

Cap 2. Since you are increasing the R that is bypassed, you might want to decrease the C (to keep cutoff at same frequency as the BE.

Are the two 1000p both between stage 1 and 2? And both to ground?

You really need only the anode bypass or the cap to ground since both attenuate high frequencies between the first and second gain stages

Yeah I wouldn't use both of those caps. One or the other. I'll play around with it to see how each one sounds.

By "cap 2" you're referring to the plate resistor and plate bypass cap, correct? The BE schematic I have (not verified) shows 100K bypassed with 500pF then 220K to plate. The MF amp uses a 330pF plate bypass. If it's a low pass, then 330K would need 100pF cap to get same cut-off (4.8 KHz) as 100K/330pF?

I could also just ditch this cap and go with a 1nF cap in one of the other common locations like I initially planned. Easy enough to try all of 'em. My early Chupacabra has this cap in position 1. Later versions have it in position 3. People suggest it should be in position 2 instead. I have a Cali schematic that shows it in position 4.

 

[fusedbrain]

I could also just ditch this cap and go with a 1nF cap in one of the other common locations like I initially planned. Easy enough to try all of 'em. My early Chupacabra has this cap in position 1. Later versions have it in position 3. People suggest it should be in position 2 instead. I have a Cali schematic that shows it in position 4.

And I have an "alleged" Cali schematic that shows a cap in position 1 AND 4 LOL.
That's the beauty of all this amp tinkering. You just have to experiment and find what works for you.
But it sure is rewarding when you hit on something that just kills after a few setbacks.
I think there is enough info available in places like this and SLOclone etc. that anyone who puts in the time can come up with something that's pretty cool.
Judging by your other build clips, I'm sure this next amp will sound great too!
Good luck with the build and keep us posted

 

[FourT6and2]

I could also just ditch this cap and go with a 1nF cap in one of the other common locations like I initially planned. Easy enough to try all of 'em. My early Chupacabra has this cap in position 1. Later versions have it in position 3. People suggest it should be in position 2 instead. I have a Cali schematic that shows it in position 4.

And I have an "alleged" Cali schematic that shows a cap in position 1 AND 4 LOL.
That's the beauty of all this amp tinkering. You just have to experiment and find what works for you.
But it sure is rewarding when you hit on something that just kills after a few setbacks.
I think there is enough info available in places like this and SLOclone etc. that anyone who puts in the time can come up with something that's pretty cool.
Judging by your other build clips, I'm sure this next amp will sound great too!
Good luck with the build and keep us posted

Thanks!

Yeah I have the same schematic. Shows both caps

 

[ChurchHill]

Just wanted to say thanks to everyone in this thread. I'm definitely learning here... great stuff!!

 

[V2a]

Yeah I wouldn't use both of those caps. One or the other. I'll play around with it to see how each one sounds.

By "cap 2" you're referring to the plate resistor and plate bypass cap, correct? The BE schematic I have (not verified) shows 100K bypassed with 500pF then 220K to plate. The MF amp uses a 330pF plate bypass. If it's a low pass, then 330K would need 100pF cap to get same cut-off (4.8 KHz) as 100K/330pF?

I could also just ditch this cap and go with a 1nF cap in one of the other common locations like I initially planned. Easy enough to try all of 'em. My early Chupacabra has this cap in position 1. Later versions have it in position 3. People suggest it should be in position 2 instead. I have a Cali schematic that shows it in position 4.

Yeah, I intended on using your number points, but I was typing on my phone at an airport, so I just said 'cap 2' etc.

390p would be the nearest standard silver mica to get you near the Friedman cutoff; but you could also use a much larger value to get closer to the 'Chupa' sound.

Notice that the 1000p shunt cap in "position 1" (right after the first coupling cap) would be a bit different than in other 'standard' Jose-style amps that have a 2n2 coupling cap in that position. The coupling cap and the shunt cap form a capacitive voltage divider with Vout = C1 / (C1+C2), where C1 is the coupling cap and C2 is the shunt cap. So the 'standard' arrangement (with C1 = 2n2) is more lossy than the one in your schematic. And loss at this stage is pretty important.

 

[FourT6and2]

Too many options lol. I'll try them all. I've got a capacitor decade box here. So I can just clip it in and spin the knob.