S.I.R. 100W SuperLead Schematic pt. II

[Peter London]

Is this interview ever came up here in the forum?
http://theguitarzombie.blogspot.com/201 ... t.html?m=1

[Unique]

Yeah, a couple of years ago I think.

[bmwfreq]

Calling all amp guru's!

I know this is going to sound strange but, my JCM 800 #34 isn't bright enough for me. That's right, isn't bright enough.

Here's the situation. I've been running the "PPIMV" with my #34 mod for the past 9 years at low volumes. However, my amp isn't as bright as I'd like it to be. I know it's because of the "PPIMV" that the highs are missing, but is there a way to recover the lost highs at lower volumes using the "PPIMV"?? Recovering the effectiveness of the Presence control would be nice too. I wasn't sure if someone in the last 9 years hadn't found a way to recover some of the things that you lose when using the "PPIMV", so I thought I'd run it past the group here.

These are the things I've already tried in an attempt to recover the highs, but they haven't worked...

Different NFB resistor values all the way down to 10K...
Different value tone stack cap values: 220p, 100p, 47p...
Different value treble peaker cap values down to 100p...
Different Preamp Tubes.
Different speakers.
Different pickup heights.
Changed guitar volume pots to 1M.
(Amp specs: 6550 Tung Sol's, 12AX7 Tung Sol's, Greenback speaker, Biased at 36mA. Amp settings: P10-B0-M10-T10-MV8-PA1.2)

If there IS something that may work that I haven't already tried, will someone please share??

Thanks in advance.

BMWFREQ

[CoffeeTones]

Several things to try -

1. 10k to 27k input grid stopper, resistor (go too low and RF will be sometimes be heard unless other precautions are taken)
2. 100k metal oxide resistor in place of the current 100k, cathode follower, load resistor
3. smaller value or removed snubber cap across the stage 1 plate resistor
4. move stage 1 plate snubber cap to 2nd stage
5. 39k to 56k slope resistor
6. 1k stage 3 cathode resistor in place of 820r
7. different bright cap 120pF to 500pF (using 1M gain pot)
8. 500pF to 1.5nF bypassing 470k prior to gain pot instead of 2n2
9. Remove or lower the value of the PI "fizz" cap
10. Raise PI and preamp voltages 20 to 30 volts by reducing the dropping resistor prior to the PI filter cap (no lower than 3k dropper with 50uF PI filter cap)
11. 500pF or smaller value caps from the input lug to wiper of each element of the PPIMV pot (like a bright cap - a mod that I though up years ago but never really tried nor needed it)
12. 500k pre PI master pot
13. 500k gain pot with 180k to 330k to ground instead of 150k (higher value resistor to ground will bring more bass and increase gain)

One of the first eleven above will probably work for you, with least affecting the rest of the tone. They are sort of in order of how I would try them. The higher you go on the list, the more affect on gain and bass response. Possibly toy around with number 11 if you want to try and counteract the PPIMV affect and not mess with the full volume tone. No guarantees.

I would also recommend turning the preamp volume full up, the PPIMV full up and the pre PI master to .5 to 1 to begin with, then set the tone controls. That may not be what you are looking for, but it sounds great in my amps and the volume is low without the PPIMV mud.

[herbvis]

Are you reducing your treble peakers and ts treble cap? That will do the opposite of what you want. I wouldnt reduce your nfb resitor either, that will just clean up the tone. Just raise your treble peaker cap value higher than the stock 470pf value. keep raising it until your satisfied or the neighbors dog starts howling

[CoffeeTones]

IME, values higher than 500pF on TS and peaker caps will slightly lower highs and increase mids in this amp. 2n2 // 470k prior to the volume pot will add some lows, when compared to 500pF and 1nF. Same for the divider peaking cap. 330pF to 560pF seems to be a good range for the divider spot. Highs increase with 500pF and 1nF when compared to 2n2 bypassing the 470k prior to the volume, where lower values decrease bass and mids.

250pF vs 500pF - 560pF in the TS mostly affects low mids.

The slope resistor mostly affects highs and lows, plus the reaction of the tone controls.

Bright caps largely depend on the setting of the volume pot until you use large value bright caps. When using large value bright caps, the volume control setting gets somewhat negated as far as volume level adjustment is concerned, but more volume / gain and mids are introduced, often causing the tone controls the need adjustment.

I believe its better to use a small to moderate value bright cap that works best in the volume range you use most, but turn the volume up to get the volume / gain increase that a large value bright cap like 5nF provides at a low volume control settings. The smaller value bright cap will still affect the attack and woody tone, even with the volume full up. It won't be identical to a 1nF or 5nF bright cap, but it will be more usable throughout the volume sweep and the volume control won't be like an on / off switch. The 1nF bright cap isn't terrible but still affects the volume control in a way I do not care for.

[Unique]

Did you try raising the screws on your pickups? This will increase the treble response of each string individually. Tilting the pickup, by raising the treble side closer to the strings while lowering the bass side will greatly increase the highs as well. Simple things to try before opening the amp up.

[bmwfreq]

Thanks a lot, coffee!

Good to hear from ya! It's been a long time. Hope things are well with you and yours.

I really appreciate the in-depth detailed response. I'll give your suggestions a try and see what happens. I'm sure one of em has got to accomplish my objective.

I did recently replace my 8 year old Tung Sol 6550s (that had thousands of hours on em), and this did recover some of the highs I was missing, but didn't quite get me where I'd like to be.

Anyway, thanks again for the lengthy reply.

I'll post my findings after I've had a chance to try a few of your suggestions.

Talk to ya soon.

[bmwfreq]

Are you reducing your treble peakers and ts treble cap? That will do the opposite of what you want. I wouldnt reduce your nfb resitor either, that will just clean up the tone. Just raise your treble peaker cap value higher than the stock 470pf value. keep raising it until your satisfied or the neighbors dog starts howling

FUNNY!!!

Hi herb!!

Been a long time.

So, you're saying that I should try and raise the value of the TS cap from 470p? I believe I tried this by paralleling some different value caps using my cap sub box, and this didn't help any. Since I'm running the #34 mod in my amp, my TP cap is a 1000p. I tried raising this value as well, but didn't realize any benefit.

Thanks for the suggestions!

[bmwfreq]

IME, values higher than 500pF on TS and peaker caps will slightly lower highs and increase mids in this amp. 2n2 // 470k prior to the volume pot will add some lows I know...I tried this and it didn't work just as you stated, when compared to 500pF and 1nF. Same for the divider peaking cap. 330pF to 560pF seems to be a good range for the divider spot. Highs increase with 500pF and 1nF when compared to 2n2 bypassing the 470k prior to the volume, where lower values decrease bass and mids. I could do this, but it'll just take me away from the #34 mod, and I don't want to change anything with respect to the #34 mod components

250pF vs 500pF - 560pF in the TS mostly affects low mids. Yep. Found this out

The slope resistor mostly affects highs and lows, plus the reaction of the tone controls. I tried messing with this also, but wasn't happy with any of the results

Bright caps largely depend on the setting of the volume pot until you use large value bright caps. When using large value bright caps, the volume control setting gets somewhat negated as far as volume level adjustment is concerned, but more volume / gain and mids are introduced, often causing the tone controls the need adjustment.

I believe its better to use a small to moderate value bright cap that works best in the volume range you use most, but turn the volume up to get the volume / gain increase that a large value bright cap like 5nF provides at a low volume control settings. The smaller value bright cap will still affect the attack and woody tone, even with the volume full up. It won't be identical to a 1nF or 5nF bright cap, but it will be more usable throughout the volume sweep and the volume control won't be like an on / off switch. The 1nF bright cap isn't terrible but still affects the volume control in a way I do not care for.

As stated before, I hate to change any of the #34 component values for fear of losing some of the #34 tone

[CoffeeTones]

Since I'm running the #34 mod in my amp, my TP cap is a 1000p. I tried raising this value as well, but didn't realize any benefit.

As stated before, I hate to change any of the #34 component values for fear of losing some of the #34 tone

But you have changed the TP cap to 1nF instead of 2n2 prior to the volume pot? The only 1000pF in #34 is the bright cap.

If you use a 500pF TP cap prior to the volume pot and highs don't increase, something is amiss I believe Herbvis was talking about the peaker cap at the divider. Raising that above 470pF - 560pF will not increase highs, but increase mids and change the #34 tone.

One thing to note - many things you change to increase highs, requires another change in the circuit to get the mids, lows and gain back where they were. For example, raising the slope resistor removes some bass, mids and gain, but increases highs. You probably heard this already. That list above attempts to get you some highs without changing much else as well as possible, until you get further down the list as I previously stated.

You could try the little circuit below to see if you like it. It's something one of the well known modders uses. I tried it, it works okay. Still NFB related though.
http://www.mediafire.com/view/vemzgdbyu ... ontrol.png.

[bmwfreq]

Since I'm running the #34 mod in my amp, my TP cap is a 1000p. I tried raising this value as well, but didn't realize any benefit.

As stated before, I hate to change any of the #34 component values for fear of losing some of the #34 tone

But you have changed the TP cap to 1nF instead of 2n2 prior to the volume pot? The only 1000pF in #34 is the bright cap.Yes. I did try this, but it changed my #34 tone, so I stayed with the 2N2 cap value to retain my #34 tone.

If you use a 500pF TP cap prior to the volume pot and highs don't increase, something is amiss I believe Herbvis was talking about the peaker cap at the divider. Raising that above 470pF - 560pF will not increase highs, but increase mids and change the #34 tone.This was my experience with this cap, and that's why I left it alone.

One thing to note - many things you change to increase highs, requires another change in the circuit to get the mids, lows and gain back where they were. For example, raising the slope resistor removes some bass, mids and gain, but increases highs. You probably heard this already. That list above attempts to get you some highs without changing much else as well as possible, until you get further down the list as I previously stated. Good to know; thank you.

You could try the little circuit below to see if you like it. It's something one of the well known modders uses. I tried it, it works okay. Still NFB related though. I'm intrigued by this little circuit you shared. What exactly does it do to the tone of your amp? Does it add some sort of added presence control or something?
http://www.mediafire.com/view/vemzgdbyu ... ontrol.png.

[CoffeeTones]

It gives a smooth, glassy, high increase from what I remember. I don't recall it being real extreme though. It has been a while since I tried it. You might also try removing the 47pF "fizz cap" and / or add a cut control to take it's place.That way you can dial back to the original tone with the PPIMV full up. That 47pF usually cuts a good bit of highs and attack, plus you are always getting a small cut from the PPIMV wiring and PI output wires, especially if they are twisted. Cut control could be a 1M pot and 500pF to .0022uF cap for maximum brightness and can be wired to have the high cut at counter clock or clockwise control position. Vox uses a cut control. It can go to the input or output side of the PI, but output side is safer and more convenient for front panel mounting.
http://www.chasingtone.com/yourguitaram ... 4/Vcut.jpg or https://robrobinette.com/images/Guitar/ ... Layout.png

[bmwfreq]

Thanks a lot, Coffee!!!

I'm gonna give this a try and see what I think about it. Its got me really intrigued.

I'll let ya know what the outcome is, and what I think of it after I give it a whirl.

Thank you for all of your input, links and suggestions.

Much appreciated.

[bmwfreq]

It gives a smooth, glassy, high increase from what I remember. I don't recall it being real extreme though. It has been a while since I tried it. You might also try removing the 47pF "fizz cap" and / or add a cut control to take it's place.That way you can dial back to the original tone with the PPIMV full up. That 47pF usually cuts a good bit of highs and attack, plus you are always getting a small cut from the PPIMV wiring and PI output wires, especially if they are twisted. Cut control could be a 1M pot and 500pF to .0022uF cap for maximum brightness and can be wired to have the high cut at counter clock or clockwise control position. Vox uses a cut control. It can go to the input or output side of the PI, but output side is safer and more convenient for front panel mounting.
http://www.chasingtone.com/yourguitaram ... 4/Vcut.jpg or https://robrobinette.com/images/Guitar/ ... Layout.png

Coffee;

On the "Cut Control" circuit, since I am running the PPIMV, I'll have to come off of the resistors that are now on the PPIMV pot, right?? I'm guessing so, since the "Bias Feed" resistors are no longer on the board.