Jimi Hendrix' Gear and Mods at West Coast Organ and Amp

[bill bokey]

Which do you think is closest to what you hear in the room ?

[niknik]

Which do you think is closest to what you hear in the room ?

Unfortunately, my listening is not optimal, because my pc speakers are not of good quality..
My impressions are that modded sm57 is somehow flat or little scooped, and standard sm is more mid pronunced..
I would like to listen to your impressions.

[niknik]

Ok,
my 45/100 surely performes better than before, with separate screens supply, unfortunately it still presents ghost problems at high volume, expecially with fuzz engaged and full guitar vol..
I next will try what Roe suggests in another section of this forum, that is to balance with trims some resistors in PI section, in the traynor or dumble style.
I will report the result.
now I would like to perform another mod in another amp of mine (super lead west coast 6550 with stancor OT)
In page 5 of this topic, Dave suggested to utilize a 5687 tube in the driver section, using an extra triode from a 12ax7..
I'm trying to figure this correctly.
I have to wire these tubes in a concertina arrangiament, like in marshall major 200w? running from tone stack to extra 12ax7 and utilize a 5687 in place of 12au7? Same res values? And where to connect the negative feedback, to the cathode of the extra 12ax7?

[daveweyer]

Use the 5687 in the PI slot. Change the plate resistors to 27K and 30K respectively, 2 watt. Use an 18K to ground on the cathode, 2 watt. Set it up for about 255 plate volts on the 30K side, and 260 volts on the 27K side, about 340 to 380 volts supply.
The filaments are 4/5 and 8, so you have to change that at the socket.
Place the 12AX7 in front of the 5687, tone circuit goes to 12AX7; output of 12AX7 goes to grid of 5687 directly, there should be around 130 volts on the grid of the 5687.
Supply for the 12AX7 should be 230 volts, with 100 volts across the plate resistor. Tie both sections of 12AX7 in parallel, use a 50K plate resistor, a 3.3K cathode resistor in series with a 220 ohm to ground. Bypass the 3.3K with an electrolytic; take it to the junction of the 220 and 3.3K--that is also where your feedback will be injected. Then switch the output transformer primary leads so that your negative feedback will be in the proper phase; you have added one stage of inversion.
Use 47K grid resistors on the output tubes to the negative supply.
Your feedback resistor will probably be in the 5.6K range. You will have WAY more gain in the power amp section, so take care with your feedback or you'll have an oscillator.
The 12AX7 will draw about 2ma. The 5687 will draw about 4ma per side.
This circuit will give you much more power amp overdrive, more power too if your power supply can handle it.
Use the 30K plate resistor on the side of the 5687 tube whose grid is AC grounded with the .47 film cap.

[niknik]

Thanx Dave!
I'll perform this as soon as! I let you know.

[daveweyer]

Please tell us exactly what you mean by "ghosting".

[niknik]

Please tell us exactly what you mean by "ghosting".

It's like an out of tune note effect, at the same time you play a note.
a sort of pitch shitfing, a fixed third below. This issue is more evident when you are playing single notes, and the amp is on full power, specially when a fuzz is engaged.
I know that 100w marshall with low filtering are prone to this issue, and some of my amps present this, but in less audible way than my 45/100, in which volume of ghost note is almost the same of actual note, so it is an awful thing.
Never considered before to further unbalace the 82k and the 100k plate resistors in PI, maybe with a 10k trim, with lateral legs connected one to each res plate, and the wiper connected to B+, like Roe mentioned in the marshall section of this forum..
I think I'll try this arrangiament, and see what happens.. 
this make sense in your opinion, Dave?

[niknik]

Use the 30K plate resistor on the side of the 5687 tube whose grid is AC grounded with the .47 film cap.

I'm not sure if I understood this correcly, with the rest of explanation I think I'm ok

[daveweyer]

That kind of ghosting is power supply ripple. When the output tubes draw a lot of current, the DC rail gets a lot more ripple on it; it can easily find its way into the preamp and driver stages, anywhere there is a triode stage, or a pentode stage acting like a triode. Triodes have low plate resistance, which means they can be modulated with external AC on the plate.
Just try a 100uf cap on the first filter stage and notice what happens.

There is another kind of ghosting when two notes play, the sum and difference frequencies appear along with the two notes, especially if the amp is in intermodulation distortion.

On the driver grids, one has a 1 meg resistor and a cap to ground, the 1 meg resistor biases the grid, and the cap shorts out all the AC on the grid; there will be DC bias only on that grid. Because the stage is driven at the cathode, it has less gain than the input stage, hence the larger 30K resistor for that side.

[niknik]

There is another kind of ghosting when two notes play, the sum and difference frequencies appear along with the two notes, especially if the amp is in intermodulation distortion.

Do you mean a sort of ring modulator effect like?
This seems what happens to my amp..
Is it an issue in the PI section?
Maybe I have to try a better plate resistors balance in v3..
thanx Dave, you are very special

[crazyplexi]

That kind of ghosting is power supply ripple. When the output tubes draw a lot of current, the DC rail gets a lot more ripple on it; it can easily find its way into the preamp and driver stages, anywhere there is a triode stage, or a pentode stage acting like a triode. Triodes have low plate resistance, which means they can be modulated with external AC on the plate.
Just try a 100uf cap on the first filter stage and notice what happens.

There is another kind of ghosting when two notes play, the sum and difference frequencies appear along with the two notes, especially if the amp is in intermodulation distortion.

On the driver grids, one has a 1 meg resistor and a cap to ground, the 1 meg resistor biases the grid, and the cap shorts out all the AC on the grid; there will be DC bias only on that grid. Because the stage is driven at the cathode, it has less gain than the input stage, hence the larger 30K resistor for that side.

Hello david an honor and respect towards you. I'm from Argentina and a guitar player, hendrix is my god. Please could make a separate post for the mod fuzz face, because I just put one together and I can not understand well that 15k resistance to input, sorry my English is not good. Thanks for making history in this forum. peace

[niknik]

Hello david an honor and respect towards you. I'm from Argentina and a guitar player, hendrix is my god. Please could make a separate post for the mod fuzz face, because I just put one together and I can not understand well that 15k resistance to input, sorry my English is not good. Thanks for making history in this forum. peace

15k resistor is to connect between input wire after the on off switch, and the first capacitor (2.2uf).
Also you could try a 50k pot, in the place of this resistor, connecting one external leg of pot to the input, and the wiper of the pot to the 2.2uf cap, so you can adjust the value of input res at your taste, or when you feel your wha starts to breathe.

[Tek465b]

50k is a bit too much. a 20k pot is enough.

[niknik]

50k is a bit too much. a 20k pot is enough.

May it depend on the wah? My vox completely breathes at about 30k.

[Tek465b]

I think Dave use 3k in the wah. Just check the video and read the color code(brown/black/red) .
Also i think he said somewhere that over 15k has little effect. i personally use 10k( but i use different value for different fuzz/circuit) and find that anything over 30k is mostly gain reduction.(i used the 50k pot before and it was just too big a value to me).