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

[niknik]

I hope i don't make disasters with wrong connections, trying the separate supply mod for screen!!
I will be super careful..

[Roe]

...
I suppose another full wave rectifier rail from low HT secondary, than straight to choke and to a filter cap (or maybe the contrary, I don't know), and than straight to screens with no res in pins 4.
Also modifying main rail supply, going from mains to PI through a 100ohm resistor..
is it quite correct?
when the amp will be ok, I'll do some clips

connect diodes to the low HT secondary but not stright to the choke. Connect it to a 550v cap first, then to the choke and another cap. Search CLC filter or Pi filter for a schematic

[niknik]

Thanx Roe,
I think I figured out how to do this mod..
What the recommended value for the filter cap before the choke is? 32uf, 50uf, 100uf, any one of these?

[Roe]

Thanx Roe,
I think I figured out how to do this mod..
What the recommended value for the filter cap before the choke is? 32uf, 50uf, 100uf, any one of these?

32uf or lower

[niknik]

Thanx Roe,
I have performed the separate supply mod for the screens in my 45/100, by using low ht taps from PT, with no disasters!
What I did is: in main rail supply I have changed the choke with a 100ohm resistor and disconnected the wire going to screens.
From low HT tap I did another full-wave rectification, same as in main rail, than I went to a 22uf filter to ground, than to RS 20h choke, after this went to a 100uf filter, and finally to the screens.
I also changed standby spst switch with a dpdt one, one side for main, and the other side for screens supply.
I reset the bias at 28ma per tube, and now the amp b+ is 548v, sagging to 490v at full power, and the screen current stays at 485v at idle, sagging to 357v at full power.
Amp tone is not so different from before, a little cleaner and focused maybe, but not huge difference here.
Main difference is that kt66 tubes show no more blue flashes, when picking the guitar strings.
sensation is that the amp is a little more "relaxed" also in full power conditions, if it makes a sense..
I hope my explanation is undertandable, it's a bit difficult express myself correctly in english language..
Next I will do some sound test with fuzz engaged and I will post my impressions..

[Tek465b]

That must have been alot of work.
Your english is very good,

Next I will do some sound test with fuzz engaged and I will post my impressions..

nice

[Roe]

Thanx Roe,
I have performed the separate supply mod for the screens in my 45/100, by using low ht taps from PT, with no disasters!
What I did is: in main rail supply I have changed the choke with a 100ohm resistor and disconnected the wire going to screens.
From low HT tap I did another full-wave rectification, same as in main rail, than I went to a 22uf filter to ground, than to RS 20h choke, after this went to a 100uf filter, and finally to the screens.
I also changed standby spst switch with a dpdt one, one side for main, and the other side for screens supply.
I reset the bias at 28ma per tube, and now the amp b+ is 548v, sagging to 490v at full power, and the screen current stays at 485v at idle, sagging to 357v at full power.
Amp tone is not so different from before, a little cleaner and focused maybe, but not huge difference here.
Main difference is that kt66 tubes show no more blue flashes, when picking the guitar strings.
sensation is that the amp is a little more "relaxed" also in full power conditions, if it makes a sense..
I hope my explanation is undertandable, it's a bit difficult express myself correctly in english language..
Next I will do some sound test with fuzz engaged and I will post my impressions..

great! does 100uf sound stiff on the screens? Originals have 16uf. Also, why not connect the phase inverter and the preamp to the screen supply with a 8k2 or 5k6 resistor instead of the plate supply

[niknik]

Roe,
I remember daveweyer mentioned 100uf screen filter, talking about half screen mod from a bridge rectifier. Also he talked about 100ohm res in place of choke, and how to go from there directly to PI, leaving a completely separate rail for the screens.
I followed these indications and also your indications, performing the low screens mod.. I think it's ok also going to PI from low screens rail, like you said..
Yes, maybe now the amp is a little more stiff, I will try to lower some filters to original specs, but this mod works!
Also I have to check the voltages in preamp tube pins, and verify all is ok.

[niknik]

I also have to change screen resistors, back to 1k

[daveweyer]

Usually you would want to use the higher value cap in the first position and lower value cap after the choke. If you run the preamp stages from the low voltage supply for the screens, you will have less hum in the circuit because you will have an 18 db rolloff from the two caps and choke. Normally the plate supply does the sagging, and the screens are kept steady with larger caps, but you can experiment with it to get the sound you like. The KT66 tubes will be much better protected with the separate supply for the screens. I tried to talk Jimi Marshall into doing this back in '69, but apparently there was too much growing and shuffling at the company to take time for this small redesign.

[niknik]

Usually you would want to use the higher value cap in the first position and lower value cap after the choke. If you run the preamp stages from the low voltage supply for the screens, you will have less hum in the circuit because you will have an 18 db rolloff from the two caps and choke. Normally the plate supply does the sagging, and the screens are kept steady with larger caps, but you can experiment with it to get the sound you like. The KT66 tubes will be much better protected with the separate supply for the screens. I tried to talk Jimi Marshall into doing this back in '69, but apparently there was too much growing and shuffling at the company to take time for this small redesign.

thanx Dave!
so you suggest to go from low HT tap to a 100uf filter, than to a choke, to another filter cap (16uf?), and than to screens? is it correct? And you suggest is better going from this point to the preamp, leaving main supply only for plate?
I have to check the voltage of the preamp tubes before, and if necessary, change some res in power rail after the mod, so to keep the same voltage as before.
I like very much when an amp sags a lot! My favourite amp (a 100w Marshall with 6550 and a Stancor A-8053 OT) sags more than 120v in the plates at full power, and litterally it sings! And this will be my next story..

[daveweyer]

Yes, use the larger cap first, 50 or 100, and then the smaller value after the choke. It will create a better supply as far as ripple goes.
A Stancor OT, I like the sound of that--they really made some nice transformers; even though the company's intention was to have a presence in the hi-fi market, the transformers sounded just great when overloaded in guitar amps.
Neil Young also loved the sagging power supplies (which really means power tube compression), and liked it when the power transformers could not keep up with the demand from the power tubes.
You get this with poorly designed transformers, like the Schumachers that Fender used.

[niknik]

Thanx Dave!
ok, I did some changes in my 45/100:
I have changed screen res, back to 1k, and I connected preamp supply to the separate screen low HT rail, leaving main supply only for plates..
After rectification of low tap, I went to a 100uf filter, than to a 20H choke, than to two in series 32uf filters, and from here to the screens and to preamp.
Plate and screen voltage remains the same as before, 554v sagging to 496v at full power for the plate, and 484v sagging to 376v (with 2k2 sag was to 357v) for the screen.
this is a very silent amp, with almost zero hum ripple.
Maybe I have to change some resistors in supply rail, 'cause voltages in v1 and in v2 are now a little too low.
215v and 230v in plates of v1, 190v and 346v in v2
But maybe is because the 20H RS choke..
Now the amp has a very cool clean and semi-dirty tone, way better than before, so I can say this mod works!

[Xplorer]

isn't lowering the screen résistors back to 1k bad for the tubes ?
what is the bias value ?

[niknik]

isn't lowering the screen résistors back to 1k bad for the tubes ?
what is the bias value ?

I think 1k screen resistors are ok, because with separate screens supply from low HT, voltage is only 484v in screens at idle, way lower then with 2k2 and main supply connection (it was 541v).
also I have to try back the 352-114 choke..
bias is -65v, with about 27ma each tube