Impact of having 50uF on the screen supply of a JTM45 amp

[Mike_J]

I am leaning towards  building a stereo amp with both JTM45 and 800 preamps.  If I have an 800 as one of the amp choices I don't see how I can avoid having 50uF on the screen supply.  A JTM45 would normally have a 16uF screen supply.  I read the '68 Plexi also had a 16uF screen supply.

The reason I wanted to do the JTM45/800 combination is it should give a pretty good clean channel / overdrive channel difference.  I think this would be especially true with the tighter screen supply on the JTM45 side versus the way the amp was made.

The reason I started this post is to get opinions on the best way to proceed with this project.  If I went the JTM45/Plexi route then I could use a 16uF screen supply because the '68 Plexi also used a 16uF screen supply.  Have to admit one amp that could go from Clapton's woman tone to some Jimi is appealing to me.  However, I am not sure there would be enough of a difference between them to accomplish what I am trying to do by having two distinct amplifiers.

I have never owned a Marshall or played many of them.  Can anyone give me their opinion as to what they would do between the JTM45/800 or the JTM45/Plexi route given what I have mentioned above?

Thanks
Mike

[sluckey]

I would just blindly follow the schematic.

[SILVERGUN]

Stricktly my personal opinion, but I wouldn't even think twice about it...

If anything I would mark it down as an upgrade and move on to bigger issues.

But I don't care about an amp that sounds exactly like another amp...I prefer amps that sound/perform better than the ones they were making when they were worried about every penny.
More UFs = better filtering + better bass response.....simple choice

I have just revealed myself as a non-purist. 

[Mike_J]

I would just blindly follow the schematic.

Sluckey

That is my usual path.  Trying to deviate from the norm but my conformist tendencies seem to pull me pack to my paint by numbers past.

Thanks
Mike

[Mike_J]

Stricktly my personal opinion, but I wouldn't even think twice about it...

If anything I would mark it down as an upgrade and move on to bigger issues.

But I don't care about an amp that sounds exactly like another amp...I prefer amps that sound/perform better than the ones they were making when they were worried about every penny.
More UFs = better filtering + better bass response.....simple choice

I have just revealed myself as a non-purist. 

Silvergun

Thank you for the response.  Good advice.  I am trying to not be such a purist but it is hard to deviate from my conformist tendencies.

Thanks
Mike

[SILVERGUN]

Mike,
In reading some of your other posts I have gathered that you will be using a SS rectifier...this makes the option of using bigger filter caps a more viable option.

If you're using a tube rect. it is a good idea to use the smaller value filter caps because they don't stress the rectifier as much as larger caps do.

With an SS rect. you can go much larger without the risk associated with the tube rect.

[Mike_J]

Mike,
In reading some of your other posts I have gathered that you will be using a SS rectifier...this makes the option of using bigger filter caps a more viable option.

If you're using a tube rect. it is a good idea to use the smaller value filter caps because they don't stress the rectifier as much as larger caps do.

With an SS rect. you can go much larger without the risk associated with the tube rect.

Silvergun

Good point.

Thanks
Mike

[SILVERGUN]

Here's a better explanation to feed a curious mind....
About halfway down the page see :Ripple current......

http://www.valvewizard.co.uk/smoothing.html

[Ed_Chambley]

I am leaning towards  building a stereo amp with both JTM45 and 800 preamps.  If I have an 800 as one of the amp choices I don't see how I can avoid having 50uF on the screen supply.  A JTM45 would normally have a 16uF screen supply.  I read the '68 Plexi also had a 16uF screen supply.

The reason I wanted to do the JTM45/800 combination is it should give a pretty good clean channel / overdrive channel difference.  I think this would be especially true with the tighter screen supply on the JTM45 side versus the way the amp was made.

The reason I started this post is to get opinions on the best way to proceed with this project.  If I went the JTM45/Plexi route then I could use a 16uF screen supply because the '68 Plexi also used a 16uF screen supply.  Have to admit one amp that could go from Clapton's woman tone to some Jimi is appealing to me.  However, I am not sure there would be enough of a difference between them to accomplish what I am trying to do by having two distinct amplifiers.

I have never owned a Marshall or played many of them.  Can anyone give me their opinion as to what they would do between the JTM45/800 or the JTM45/Plexi route given what I have mentioned above?

Thanks
Mike

Mike,
I have a JTM45/Plexi and built another amp that is an 800.

On the JTM45/plexi I use a rectifier tube, but I changed the B+ Rail to add voltage to V1 and used a larger OT's for a 50 Watt.  It is a Heybour, but I painted it to look like a Mercury.

Seriously, the increased voltage and OT gave me a much better clean.  I would build it first them tweak.  I use the 32/32 caps and the 50/50 in the 800.

I am quite sure if you are building Sluckeys design, it will be nice right off the bat and there may only be a couple of things to do.

I have a PPMIV in both and a hot switch and a Preamp MV as well.  I put the NFB on a pot and this allows me to loosen up the bottom to achieve the woman tone.  Great mod.

[Mike_J]

Here's a better explanation to feed a curious mind....
About halfway down the page see :Ripple current......

http://www.valvewizard.co.uk/smoothing.html

Silvergun

Thanks for the information.  I did the math and according to the 3H choke I will be using I couldn't use a 16uF cap I don't think.

Thanks
Mike

[Mike_J]

I am leaning towards  building a stereo amp with both JTM45 and 800 preamps.  If I have an 800 as one of the amp choices I don't see how I can avoid having 50uF on the screen supply.  A JTM45 would normally have a 16uF screen supply.  I read the '68 Plexi also had a 16uF screen supply.

The reason I wanted to do the JTM45/800 combination is it should give a pretty good clean channel / overdrive channel difference.  I think this would be especially true with the tighter screen supply on the JTM45 side versus the way the amp was made.

The reason I started this post is to get opinions on the best way to proceed with this project.  If I went the JTM45/Plexi route then I could use a 16uF screen supply because the '68 Plexi also used a 16uF screen supply.  Have to admit one amp that could go from Clapton's woman tone to some Jimi is appealing to me.  However, I am not sure there would be enough of a difference between them to accomplish what I am trying to do by having two distinct amplifiers.

I have never owned a Marshall or played many of them.  Can anyone give me their opinion as to what they would do between the JTM45/800 or the JTM45/Plexi route given what I have mentioned above?

Thanks
Mike

Mike,
I have a JTM45/Plexi and built another amp that is an 800.

On the JTM45/plexi I use a rectifier tube, but I changed the B+ Rail to add voltage to V1 and used a larger OT's for a 50 Watt.  It is a Heybour, but I painted it to look like a Mercury.

Seriously, the increased voltage and OT gave me a much better clean.  I would build it first them tweak.  I use the 32/32 caps and the 50/50 in the 800.

I am quite sure if you are building Sluckeys design, it will be nice right off the bat and there may only be a couple of things to do.

I have a PPMIV in both and a hot switch and a Preamp MV as well.  I put the NFB on a pot and this allows me to loosen up the bottom to achieve the woman tone.  Great mod.

Ed

That is funny.  If you have any of that paint left over I would like to upgrade my amp too.

I will be using a 100 watt PT that I had Heyboer custom wind for me for my first Sluckey inspired Plexi/800 build.  I kept adding so many tubes to do more and more things that eventually I exceeded the PTs heater supply current rating.  So I had to have Heyboer build me another PT with a larger heater supply rating.  Therefore, I have the first PT sitting around that must have an amplifier to go with it.

I got enough grief for making a 100 watt amp on the Plexi/800 build that I decided there is probably something to what so many people were saying.  Therefore I decided the best way to solve that problem is to have two separate amps using individual 50 watt OTs.  I would use KT66s and EL34s because they can both be used pretty well at the voltage the PT produces.  Putting a PPIMV on each should hopefully get the amp to play at a not too unreasonable level.

Thanks again for the laugh
Mike

[SILVERGUN]

Silvergun

Thanks for the information.  I did the math and according to the 3H choke I will be using I couldn't use a 16uF cap I don't think.

This math?:
(quote from valve wizard site below)
A rule of thumb is that the sum of the two capacitors (in micro-Farads), multiplied by the inductance of the choke (in Henrys) should equal 200 or more for excellent performance. For example, using a 22uF reservoir capacitor with a 47uF filter capacitor and a 5H choke:
 (22 + 47) * 5 = 345

That's a 'rule of thumb' for "excellent performance"....some can settle for less than excellent, and some original amp designs didn't qualify

Reading back through this now, I realize that the screen supply cap that you initially asked about is not as much of a culprit as the main filter cap (first node - plate supply), also referred to as the reservoir cap.
....so our discussion becomes even less relevant.

[Mike_J]

Silvergun

Thanks for the information.  I did the math and according to the 3H choke I will be using I couldn't use a 16uF cap I don't think.

This math?:
(quote from valve wizard site below)
A rule of thumb is that the sum of the two capacitors (in micro-Farads), multiplied by the inductance of the choke (in Henrys) should equal 200 or more for excellent performance. For example, using a 22uF reservoir capacitor with a 47uF filter capacitor and a 5H choke:
 (22 + 47) * 5 = 345

That's a 'rule of thumb' for "excellent performance"....some can settle for less than excellent, and some original amp designs didn't qualify

Reading back through this now, I realize that the screen supply cap that you initially asked about is not as much of a culprit as the main filter cap (first node - plate supply), also referred to as the reservoir cap.
....so our discussion becomes even less relevant.

Except my choke is 3H I think and I was going to use a 47uF and 16uF caps which equals 189 which is less than the 200.  The JTM45 and Plexi would not meet the "excellent performance" test because I read somewhere that even their 5H chokes measured 3H. 

What I do know from experience is if you don't have enough filtering you can have noise problems.  I also know if you put too much filtering in the amp can become pretty clean.  Okay if that is what you want but not so good if you don't want clean.  In the case of this amp the 800 side will not be overly filtered so the overdrive should be there.

Thanks
Mike

[SILVERGUN]

Except my choke is 3H I think and I was going to use a 47uF and 16uF caps which equals 189 which is less than the 200.  The JTM45 and Plexi would not meet the "excellent performance" test because I read somewhere that even their 5H chokes measured 3H. 

"Excellence" is over-rated 

But I agree,,, and that's why I'd throw a 50/50 in there and never look back

[Mike_J]

Except my choke is 3H I think and I was going to use a 47uF and 16uF caps which equals 189 which is less than the 200.  The JTM45 and Plexi would not meet the "excellent performance" test because I read somewhere that even their 5H chokes measured 3H. 

"Excellence" is over-rated 

But I agree,,, and that's why I'd throw a 50/50 in there and never look back

I agree.  Putting a 50/50 in there and never looking back.

Thanks
Mike

[HotBluePlates]

Nothing beats firsthand experience. If I were you, I'd just build the thing with one value, and if you still have a question about the effect, swap in the other value.

Some people could tell you wrong. Some will tell you what's technically-right, but leave you an exaggerated expectation for the difference. So if the change is a minor part-swap and not a major re-design or re-build, it's much faster to just try it out. And you'll always get the best answer.

[Mike_J]

Nothing beats firsthand experience. If I were you, I'd just build the thing with one value, and if you still have a question about the effect, swap in the other value.

Some people could tell you wrong. Some will tell you what's technically-right, but leave you an exaggerated expectation for the difference. So if the change is a minor part-swap and not a major re-design or re-build, it's much faster to just try it out. And you'll always get the best answer.

HBP

Good advice.  Thank you.

Thanks
Mike

[terminalgs]

Valco had two sets of 6L6' & OTs on all the 70W amps from 66-69 (Supros, Nationals, Airlines, etc).  They designed the power supply in such a way that you could have one value filter cap on set of screens and one value on the other.   See:


http://el34world.com/charts/Schematics/files/valco/Valco_Supro-Statesman_S6699.pdf


Although Valco made budget amps, and budget decisions, the higher wattage amps from this era used premium components and were well designed.

[Mike_J]

Valco had two sets of 6L6' & OTs on all the 70W amps from 66-69 (Supros, Nationals, Airlines, etc).  They designed the power supply in such a way that you could have one value filter cap on set of screens and one value on the other.   See:


http://el34world.com/charts/Schematics/files/valco/Valco_Supro-Statesman_S6699.pdf


Although Valco made budget amps, and budget decisions, the higher wattage amps from this era used premium components and were well designed.

Terminalgs

Not only do they use different value caps on the screens but they also wire both of their OT center taps directly to the HT B+ point which is another question I had.  Good find.

Thanks
Mike

[darryl]

Nothing beats firsthand experience. If I were you, I'd just build the thing with one value, and if you still have a question about the effect, swap in the other value.

Or install the smaller value capacitor, and temporarily parallel it with the larger value. Use a switch to quickly add or remove the larger capacitor from the circuit. This will allow you to instantly A-B the two configurations.

If you find yourself straining to detect any significant difference in performance between the two settings, there is no reason to install the smallest usable capacitor. Modern electrolytic capacitors are much cheaper, and physically smaller than capacitors were in the days of yore.

[Mike_J]

Nothing beats firsthand experience. If I were you, I'd just build the thing with one value, and if you still have a question about the effect, swap in the other value.

Or install the smaller value capacitor, and temporarily parallel it with the larger value. Use a switch to quickly add or remove the larger capacitor from the circuit. This will allow you to instantly A-B the two configurations.

If you find yourself straining to detect any significant difference in performance between the two settings, there is no reason to install the smallest usable capacitor. Modern electrolytic capacitors are much cheaper, and physically smaller than capacitors were in the days of yore.

Darryl

Thank you for the advice.  I am leaning towards keeping the 50uF screen supply.  I doubt that I would play the amp as loud or with as much gain as someone else might.  Therefore, I would be less likely to experience ghost noting or other oscillation problems if I used a lower value.  Better safe than sorry.  Marshall tended to keep their filtering pretty close to the edge so after giving it some thought I don't think it would be a good idea to be less conservative than they were.

Thanks
Mike