How do you increase headroom in a 50w JMP?

[ELS]

Note that GEC info directly correlates valve life to anode dissipation, see under ‘life performance’ p3 of https://tubedata.altanatubes.com.br/sheets/084/k/KT66_GEC.pdf

Note the higher, stiffer HT supply. That’s where the uprated power output comes from. I think that the 12.5% g2 taps (my measurement) off the OT primary are coincidental to that. I think it’s incorrect to describe 12.5% taps as UL, I don’t think Fender described them as such.

Note the 33k grid leaks of the output valves. They put a heavy load on the LTP phase splitter outputs. CF buffers are probably necessary to drive sufficient signal l

Note that GEC info directly correlates valve life to anode dissipation, see under ‘life performance’ p3 of https://tubedata.altanatubes.com.br/sheets/084/k/KT66_GEC.pdf


Note the higher, stiffer HT supply. That’s where the uprated power output comes from. I think that the 12.5% g2 taps (my measurement) off the OT primary are coincidental to that. I think it’s incorrect to describe 12.5% taps as UL, I don’t think Fender described them as such.

Note the 33k grid leaks of the output valves. They put a heavy load on the LTP phase splitter outputs. CF buffers are probably necessary to drive sufficient signal level across them.

well then the book is wrong. its common knowlage that tubes like EL500s or 6LQ5s last forever, even tho the plate dissipation isnt tha high on the EL500.
it doesnt say that... it says that reducing the plate dissapation increases tube life.
what Im saying is that twice the max plate dissipation and running it at the same dissipation doesnt mean double then life.

12.5% isnt wrong to describe as UL, it still does the same thing. its just not the most ideal value that usually is around 40%.

I said 47-100k and also did say that it will load the PI and reduce gain. with a CF you dont need a grid leak, since before the tube warms up the cathode load resistor will act as such.

 

[william vogel]

Okay so there are bias test points on the back of my amp which are connected to a 1ohm resistor to measure bias, Ive set it to where it reads 26mV at those test points using my DMM. With what you said it mind is the actual bias current only 21mV?

Because you haven’t measured the voltage drops across the screen grid resistors, it’s impossible to know for sure. Measure the resistance of the screen grid resistors, I’m guessing they are 1k ohms. Power on the amp and measure voltage across the resistors. It’s probably about 4-5 volts. 4 volts divided by 1000 = .004 or 4mA. Whatever this measurement, subtract it from the reading from the bias port. In your case, your measurement of 26mA will be reduced just like you’re saying. If it’s 21mA x 450 volts = 9.45 watts or 38% dissipation. That’s way too cold of a bias setting and I promise it’s got crossover notch distortion before reaching full clean rms output. Tomorrow morning I’ll open up my amp and show you some waveforms of various bias dissipation settings. Question is, can we hear crossover distortion? That’s subjective. If you like the amps sound with a cold bias, good and run it. I think they sound better up at 60-70% and prefer to bias as hot as possible to keep the clean rms output waveform as clear of crossover distortion as possible without exceeding 75% dissipation. The higher the plate voltage, the hotter the bias setting is required to remove the crossover distortion. The crossover distortion is the point of operation when the class AB amp transitions from class A to class B. The crossover notch is the indicator of reaching true cutoff of each output tube. I’m simplifying the explanation with this observation because the amp is not true class A up to the point of crossover being observed in the waveform but once crossover is observed it is true class B. I hope this isn’t confusing.

 

[sdn25]

Because you haven’t measured the voltage drops across the screen grid resistors, it’s impossible to know for sure. Measure the resistance of the screen grid resistors, I’m guessing they are 1k ohms. Power on the amp and measure voltage across the resistors. It’s probably about 4-5 volts. 4 volts divided by 1000 = .004 or 4mA. Whatever this measurement, subtract it from the reading from the bias port. In your case, your measurement of 26mA will be reduced just like you’re saying. If it’s 21mA x 450 volts = 9.45 watts or 38% dissipation. That’s way too cold of a bias setting and I promise it’s got crossover notch distortion before reaching full clean rms output. Tomorrow morning I’ll open up my amp and show you some waveforms of various bias dissipation settings. Question is, can we hear crossover distortion? That’s subjective. If you like the amps sound with a cold bias, good and run it. I think they sound better up at 60-70% and prefer to bias as hot as possible to keep the clean rms output waveform as clear of crossover distortion as possible without exceeding 75% dissipation. The higher the plate voltage, the hotter the bias setting is required to remove the crossover distortion. The crossover distortion is the point of operation when the class AB amp transitions from class A to class B. The crossover notch is the indicator of reaching true cutoff of each output tube. I’m simplifying the explanation with this observation because the amp is not true class A up to the point of crossover being observed in the waveform but once crossover is observed it is true class B. I hope this isn’t confusing.

I think im getting a hang of it, I've gotta ask though, when the amp is cranked and is just a big square wave generator wouldn't the crossover distortion just blend in and disappear?

 

[william vogel]

well then the book is wrong. its common knowlage that tubes like EL500s or 6LQ5s last forever, even tho the plate dissipation isnt tha high on the EL500.
it doesnt say that... it says that reducing the plate dissapation increases tube life.
what Im saying is that twice the max plate dissipation and running it at the same dissipation doesnt mean double then life.

12.5% isnt wrong to describe as UL, it still does the same thing. its just not the most ideal value that usually is around 40%.

I said 47-100k and also did say that it will load the PI and reduce gain. with a CF you dont need a grid leak, since before the tube warms up the cathode load resistor will act as such.

With an AC coupled cathode follower you have to have a grid leak. The example you use is AC coupled. With DCCF no grid leak is used but few amplifiers use this as a driver stage for the output tubes.
Explain the 100% THD. Unless the speaker load is in thousands of ohms region, it’s impossible to omit the output transformer.

 

[william vogel]

I think im getting a hang of it, I've gotta ask though, when the amp is cranked and its just a big square wave generator wouldn't the crossover distortion just blend in and disappear?

Yep, it’s still observed as the little squiggle in the middle of the waveform. Like I said, if you enjoy the sound of the cold bias, run it. I don’t spend all my play time digging hard on the strings so I like the amp to sound full of life even at light touch. You have to experiment and find what sounds best to you.

 

[Pete Farrington]

I didnt know that power output of an amplifier was meassured at 100% THD. in that case just connect the speaker to mains and call that your power output.

How is 100%THD from an amp possible? The 1st harmonic would need to be removed.

what Im saying is that twice the max plate dissipation and running it at the same dissipation doesnt mean double then life.

Straw man alert!
Why do you assume that the relationship must be linear?

Yes I know the internet is a free for all, but I’m doubtful you’re anywhere near being helpful here. Please read what @sdn25 ’s goals are. How about starting a thread where you put forward and properly explain (including technically credible citations) your somewhat unusual hypothesis, rather than derailing this one?
Every post you make introduces yet more diversions, Ii’s not just feasible to debate them all here

 

[ELS]

With an AC coupled cathode follower you have to have a grid leak. The example you use is AC coupled. With DCCF no grid leak is used but few amplifiers use this as a driver stage for the output tubes.
Explain the 100% THD. Unless the speaker load is in thousands of ohms region, it’s impossible to omit the output transformer.

when did I say AC coupled? AC coupling the CF would ruin the benefit of having one, low frequency response would be terrible at higher volume levels. you need to DC couple them.

you were saying pentode mode has higher power output, and that ultralinear modes have lower output but higher distortion.
then how are you measuring power output? peak watts no matter what THD?
if you compare any ultra linear amp with an equivalent pentode mode amp you'll see that at a constant THD level, 1% is pretty standard to meassure clean power output, the ultra linear amp outputs more power than the pentode one.

if you want a high powered pentode amp you would build a class AB2 ultralinear amp.

How is 100%THD from an amp possible? The 1st harmonic would need to be removed.

Straw man alert!
Why do you assume that the relationship must be linear?

Yes I know the internet is a free for all, but I’m doubtful you’re anywhere near being helpful here. Please read what @sdn25 ’s goals are. How about starting a thread where you put forward and properly explain (including technically credible citations) your somewhat unusual hypothesis, rather than derailing this one?
Every post you make introduces yet more diversions, Ii’s not just feasible to debate them all here

I was overexaggerating, was implying that he just cranks the input voltage to like 50V ptp on the outupt tubes and messures power output then, ignoring THD.

when did I assume the relationship would be linear? I literally said that it wouldn't.

OP asked how to increase clean headroom in a 50 watt marshall, I replied with answers to that. how naive was I to think that people on the internet would like what I'd say instead of arguing over it and when I explain my answers they say that I'm derailing the topic... if it were so... if it were so...

 

[Pete Farrington]

when did I say AC coupled? AC coupling the CF would ruin the benefit of having one, low frequency response would be terrible at higher volume levels. you need to DC couple them

You linked to the Super Twin and mentioned the CFs. That amp is AC coupled.

if you compare any ultra linear amp with an equivalent pentode mode amp

How exactly do you mean equivalent? If the screen grids are moved from regular 40% taps to the CT, power output will almost certainly increase.

when did I assume the relationship would be linear?

In the GEC info.

OP asked how to increase clean headroom in a 50 watt marshall

No he didn’t.
You entered without apparently reading even all @sdn25 ’s posts to it, and used the thread as a platform for your hypothesis.

 

[ELS]

In the GEC info.

oh I didn't know I wrote that GEC datasheet, must've been a GEC employee so long ago that I don't remember.

 

[sdn25]

@Pete Farrington Since the change to a single 8k2 dropper, when measuring bias current through the 1 ohm bias test points, during playing it spikes to upwards of 270mV. Before the absolute Max was about 220mV. Is this right? Does it degrade the tube faster?

 

[Pete Farrington]

It may just be a signal symmetry thing, has it increased for both output valves?
Bear in mind that the particular mains voltage at the time of the test will affect things, so if not already doing so, keep note of that too.

 

[sdn25]

It may just be a signal symmetry thing, has it increased for both output valves?
Bear in mind that the particular mains voltage at the time of the test will affect things, so if not already doing so, keep note of that too.

It has increased negligibly on v5 compared to v4.

Its actually weirder than I thought, when I strum a chord and let it decay, v4 goes from 180-190 to about 260 where on v5 when I let it decay it goes from 165 to about 110mV.

 

[sdn25]

@Pete Farrington is this to be expected ?

 

[Pete Farrington]

It’s probably just the overdrive characteristics of the amp, which will change a bit as HT rises. Volume and tone settings will affect it, as will the guitar. If scoped, the peak levels should be the same, it’s just the average levels that differ.
As long as it sounds like it’s supposed to, it’s good.

 

[william vogel]

oh I didn't know I wrote that GEC datasheet, must've been a GEC employee so long ago that I don't remember.

If you’ll re read my post, I stated that ultra linear produces less distortion than pentode mode. Also you referenced a Fender amp with a cathode follower driver and it’s AC coupled. I can’t read your mind about because there’s AC and DC coupled common sense would relate your post to the amp in your post.

 

[ELS]

If you’ll re read my post, I stated that ultra linear produces less distortion than pentode mode. Also you referenced a Fender amp with a cathode follower driver and it’s AC coupled. I can’t read your mind about because there’s AC and DC coupled common sense would relate your post to the amp in your post.

so me explaining how to wire the cathode follower doesnt matter at all because I used an amp that contradicts that as an example for an ultra linear amplifier. yes?

if I use your logic then, you were implying that ultra linear amps have less clean power because the topic of this thread is how to increase clean headroom.
or if I dont, then you stated something that nobody asked

 

[sdn25]

Hey guys I think im almost there in terms of what I set out to achieve. Im positive I don't need to spend money on this now. I just installed a 250k volume pot in my guitar as opposed to a 500k and think the resonant peak of the guitar signal has changed or something because the low mids are much more thumpy and aggressive! it also helps tame some high end fizz off the amp. With 450V on the plates and the higher PI voltage setup im really happy with how its sounding. Im pairing it with a 8k wound PAF and it sounds killer. I think beyond this point Im gonna have to buy new tubes and a new PT which I don't think I need anymore. So yeah, thank you guys for the plethora of suggestions!

 

[william vogel]

so me explaining how to wire the cathode follower doesnt matter at all because I used an amp that contradicts that as an example for an ultra linear amplifier. yes?

if I use your logic then, you were implying that ultra linear amps have less clean power because the topic of this thread is how to increase clean headroom.
or if I dont, then you stated something that nobody asked

This thread is about a 50 watt Marshall that the owner would like it to have response similar to a 100 watt Marshall. He cranks the amp into an attenuator to reduce volume. I understand the title is maybe a little misleading but in the beginnings of the thread it’s been explained. Somehow the thread is being turned into ultra linear and CF drive circuits. Why don’t we get back to the point of the thread and help the guy get his 50 watt to react more like a 100 watt, all being attenuated.