Question about jtm45

[Impetus]

i settled on a 100pf bright cap on my 1987 lead. the higher you go with the cap, the quicker the volume comes on (lets more signal through initially) so no cap or a lower cap will allow more sweep of the volume pot before full signal and less harshness at lower volumes

I've got 2 amps I usually leave one of them standard 1987 spec but I have come to really love the 5000pF bright cap but i use the amp at very high volume too. I used to really like the 100pF and a few other changes when i was trying for some lead/bass spec hybrid to cover more ground but have learned to embrace the 5000pF cap on the 1987's. Took a while though lol. I realize its definitely not for everyone and especially if you cant turn your amp past 8 on the dial

 

[South Park]

Yes you are correct when using a lay down transformer. When using a stand up transformer it is not the same . It is a good safety practice to watch where to place grounds

 

[hotpockitrockit]

I've got 2 amps I usually leave one of them standard 1987 spec but I have come to really love the 5000pF bright cap but i use the amp at very high volume too. I used to really like the 100pF and a few other changes when i was trying for some lead/bass spec hybrid to cover more ground but have learned to embrace the 5000pF cap on the 1987's. Took a while though lol. I realize its definitely not for everyone and especially if you cant turn your amp past 8 on the dial

i think if youre primarily using it at high volumes then it will negate the cap, or if you jumper the channels. i tend to run my amp at around 6, so for me the 100pf cap seems well balanced in terms of signal to "shrill" haha

 

[Impetus]

i think if youre primarily using it at high volumes then it will negate the cap, or if you jumper the channels. i tend to run my amp at around 6, so for me the 100pf cap seems well balanced in terms of signal to "shrill" haha

It definitely does but it depends on the value. After 3 or 4000pF it changes the frequency response I believe instead of just more and more high end so it's a different sound. I could be wrong but I believe I read this somewhere too and my ears hear it

 

[hotpockitrockit]

Yes you are correct when using a lay down transformer. When using a stand up transformer it is not the same . It is a good safety practice to watch where to place grounds

the

It definitely does but it depends on the value. After 3 or 4000pF it changes the frequency response I believe instead of just more and more high end so it's a different sound. I could be wrong but I believe I read this somewhere too and my ears hear it

that could be true, because as i understand it the cap affects the mids as well

 

[Gene Ballzz]

@Impetus
Please accept my sincerest apology for hi-jacking with my grounding rants. I'm just so adamant because I've been severely zapped and safety is no accident!
Just Apologizin'
Gene

 

[Chris-in-LA]

I’ve built 2 JTM45’s. One had the 100pf treble cap on the plans, the other had a 500pf. I like the 500, adds more sparkle, sounds more open. They both now have 500’s.

 

[Impetus]

@Impetus
Please accept my sincerest apology for hi-jacking with my grounding rants. I'm just so adamant because I've been severely zapped and safety is no accident!
Just Apologizin'
Gene

Ha! It's all good. What is the forum for if not to talk about this stuff lol

 

[Impetus]

I’ve built 2 JTM45’s. One had the 100pf treble cap on the plans, the other had a 500pf. I like the 500, adds more sparkle, sounds more open. They both now have 500’s.

Good to know! I think what im going to end up doing is having the 560pF across the mix resistors and the 100pF on volume but I'm gonna add 2 switches to disengage either of them for a little variety. I usually just find what works best and go with that instead of having switches and stuff but it sounds great in a few variations and I cant pick, at least yet. Honestly, getting the filtering right was like 90 percent of the battle to get where I wanted though.

Do you know if the 10k or 8.2k resistors by the preamp also regulate the preamp voltage like the decoupling resistors by the bias caps? I like how going from two 10k in series to a single 8.2k raised the preamp voltage with the resistors by the bias caps between PI a d screens so I'm curious what the decoupling resistors at the preamp filter cap do?

 

[lespaul339]

 

[Chris-in-LA]

Good to know! I think what im going to end up doing is having the 560pF across the mix resistors and the 100pF on volume but I'm gonna add 2 switches to disengage either of them for a little variety. I usually just find what works best and go with that instead of having switches and stuff but it sounds great in a few variations and I cant pick, at least yet. Honestly, getting the filtering right was like 90 percent of the battle to get where I wanted though.

Do you know if the 10k or 8.2k resistors by the preamp also regulate the preamp voltage like the decoupling resistors by the bias caps? I like how going from two 10k in series to a single 8.2k raised the preamp voltage with the resistors by the bias caps between PI a d screens so I'm curious what the decoupling resistors at the preamp filter cap do?

The 8k2 and 10k resistors regulate how much high voltage gets to the plate resistors in the preamp but I don’t know what happens if you change the values. And I’m confused about what you mean by coupling resistors. Also, you might try checking out this Uncle Doug clip on phase inverters, it might answer some of your questions.

 

[Impetus]

The 8k2 and 10k resistors regulate how much high voltage gets to the plate resistors in the preamp but I don’t know what happens if you change the values. And I’m confused about what you mean by coupling resistors. Also, you might try checking out this Uncle Doug clip on phase inverters, it might answer some of your questions.

Sorry, I may have that wrong. I've just always heard them called decoupling resistors. Or maybe that's just for the 1 or two resistors by the bias caps that help regulate preamp voltage. But my thinking was that this other 10k resistor that's connected to v2 tube right next to the preamp filter cap was the same thing? So the resistors by the bias regulate preamp voltage and the resistor by the preamp filter cap and connected to v2 tube also regulate the preamp voltage but it's for the preamps plate resistors instead?

Gonna check out the video, thank you

 

[Metroman]

I know very little about building amps. These are the guts of my 2013 LTD ED Metropoulos JTM45 replica head, if thats any help. Whatever the values he used I wouldnt change anything about the amp. I run it through 4 x 12 Pulsonic G12Ms in a Replica JTM45/100 TVB cabinet with EC Collins Pinstripe cloth.
It uses NOS 1956 Marconi Osram KT66s, and all Mullard NOS Preamp, and Rectifier tubes.

 

[Pete Farrington]

The value of the HT dropper resistors was probably determined by the type of output valves that the amp was intended to use.
KT66 need a fair bit more signal to drive them than EL34. So the V3 LTP circuit needs a higher HT voltage at its supply node. So an 8k2 or 10k dropper to it is used. Whereas for EL34 amps, a series pair of 10k droppers is used, ie 20k total, in order to reduce the HT voltage supplied to the LTP, and so limit the max signal voltage it can hit the output valves with. That mitigates blocking distortion, avoids the amp farting out.

 

[Dblgun]

Yes, that 10k next to the pre-amp filter cap is the dropping resistor for the first pre-amp tube and cathode follower(v2). Marshall's pre-amp voltages through the years have been all over the map. As Pete said voltages to the PI are higher in some amps and generally dictated by HT voltage provided by the PT and tube intended to be used. Keep in mind, any changes to the HT dropping resistors will effect those provided in the remainder of the circuit, eg: pre-amp.

Find a resource for a voltage chart for your given model and compare to readings you take to see where you are. You can then see what if anything is out of the original specs.

 

[Impetus]

I know very little about building amps. These are the guts of my 2013 LTD ED Metropoulos JTM45 replica head, if thats any help. Whatever the values he used I wouldnt change anything about the amp. I run it through 4 x 12 Pulsonic G12Ms in a Replica JTM45/100 TVB cabinet with EC Collins Pinstripe cloth.
It uses NOS 1956 Marconi Osram KT66s, and all Mullard NOS Preamp, and Rectifier tubes.

Dang, that sure is pretty. I wonder why he used those ceramic caps all face down instead of showing the value lol. They look like some NOS CRL or mallory ceramics I have. That cab sounds like a dream!

 

[Impetus]

The value of the HT dropper resistors was probably determined by the type of output valves that the amp was intended to use.
KT66 need a fair bit more signal to drive them than EL34. So the V3 LTP circuit needs a higher HT voltage at its supply node. So an 8k2 or 10k dropper to it is used. Whereas for EL34 amps, a series pair of 10k droppers is used, ie 20k total, in order to reduce the HT voltage supplied to the LTP, and so limit the max signal voltage it can hit the output valves with. That mitigates blocking distortion, avoids the amp farting out.

When you talk about the two 10k in series on the el34 amps are you talking about the resistors by the bias caps? I only ask because I want to be sure we are talking about the same thing because I dont think I've ever seen 2 10k resistors in series on any Marshall's by the preamp filter cap. I've always seen either 1 10k or an extra 10k between a dual filter cap? But I DO see them in series by the bias caps. Now, I could just be misunderstanding but when I went from two 10k in series for the decoupling resistors at the bias caps to a single 8.2 it gave me much more headroom and also more gain. I know that that particular resistor(s) are regulating preamp voltage. At least I think.

I'm specifically curious about the 10k next to the preamp filter cap and if it works the same way as well as if it is also regulating voltage somewhere. For example an 8.2k here would theoretically give more headroom and gain than a 10k even if the difference is small? I see the person below mentioned that this resistor is for the first preamp tube and cathode follower. But what exactly is it doing to the first preamp tube and CF? Regulating voltage as well and the same principal applies that the lower the value the more headroom/gain?

I'm really trying to learn though lol. Sorry for these kind of questions

 

[Impetus]

Yes, that 10k next to the pre-amp filter cap is the dropping resistor for the first pre-amp tube and cathode follower(v2). Marshall's pre-amp voltages through the years have been all over the map. As Pete said voltages to the PI are higher in some amps and generally dictated by HT voltage provided by the PT and tube intended to be used. Keep in mind, any changes to the HT dropping resistors will effect those provided in the remainder of the circuit, eg: pre-amp.

Find a resource for a voltage chart for your given model and compare to readings you take to see where you are. You can then see what if anything is out of the original specs.

This is where I'm a little lost. If that particular 10k resistor is for the v1 and v2 preamp tubes then what does the PI have to do with it? Is it that v1 and v2 ultimately send the voltage on to the PI tube? But then that doesnt make sense i guess if the PI voltage is dictated by the PT?

So this resistor drops the voltage for v1 and v2 preamp tubes? Which in turn effects other things down stream? (Higher the value the lower the voltage, right)

Sorry. I'm really trying. Is the same principal of the lower the value resistor the more headroom/gain the same with this resistor as it is for the resistors by the bias caps? I mean, the only values ever used here is a single 8.2k or a single 10k, right? I guess I'm not gonna hurt anything by trying both values?

 

[neikeel]

The resistors next to the bias caps are between the screens feed and PI. Anything down stream of these is regulated by the voltage in from the rectifier and the value of these resistors (study the schematic). Early amps (some very early JTM45s and PA 100w used 4k7 here). Usually you will see a single 8k2 and on the board for v1 and v2 another 10k and a 16uF filter cap.
Later the extra 10k for v2 was introduced and an dual 32uF for an extra preamp node. Yes you can call the resistor a decoupler or dropper, both will be interpreted the same way.
Later on as lower headroom/more preamp distortion was desired the 8k2 became 8k2+10k or 8k2+8k2 or 10k+10k between PI and screens. The preamp resistors were 10k and 10k during this era.
You are talking about a JTM45 so 8k2 between screens/PI and a 10k in preamp are all that was ever used.
If you want to go to different values, added nodes that is your custom/mod choice to adjust your preamp voltages to suit your sounds and your preamp tubes etc.

 

[Pete Farrington]

This is where I'm a little lost. If that particular 10k resistor is for the v1 and v2 preamp tubes then what does the PI have to do with it?

To help with your learning process, have a good read through Merlin’s site. The section under the ‘smoothing filters’ heading here being particularly relevant to your queries in this thread.