Can Someone Explain The Carmen Ghia Power String & PI?

[rp]

The schematic was posted a few days back and then removed, here is a detail hopefully without infringing on anything. I have no idea what the power string is doing at the non-typical points.

Nodes D &K: Why are the PI grids and cathodes connected to power supply electrolytics here?

Nodes E & F: Why the desire for separate power caps for V1a & V1b? Seems like overkill. Is it really that important to the tone to have 22mf on one plate and 8mf on the other?

A - OT CT
B - G2
C - PI Vp
D - PI Grid
E - V1b Vp
F - V1a Vp
G -
K - PI Cathode

[diagrammatiks]

the e and f are to have different voltages for the respective stages.

I can't tell what the pi is doing but it's not an ac coupled cathode biased long tail pair.

it looks like it might be biased or voltage referenced via grid leak

[es345]

Ok, looking to the schematic: it is a LT PI but with an interesting generation of the operating point for the 2 triodes.

Point D and K set the virtual ground (AC) from Cathode and grids at different DC levels. The difference of 6,4 V defines the operating point for both triodes via the common cathode resistor R11 (33K).

The grid of the lower triode is basically AC grounded compared to the grid of the grid of the upper triode.
This essentially leads to a differential amplifier operation like that what we have in a long tail PI.

[DR.Z]

ES 345 has it right.
It is a differential FIXED DC BIASED amplifier.
The use of a fixed bias as you all know allows a more linear responce.
And when coupled to the Conjuctive Filter forms a nice Limiter/Compressor.
This gives a very balanced output from top E to bottom E string.

The two filter caps are at differant voltage nodes.


Editors Note:
I can not take credit for this circuit, it is a Hammond Organ design that I liked and thought it was a bit unusual.

DR.Z

[martin manning]

Thanks for posting the explanation DrZ, I like your style. Others here should do all they can to encourage (and not discourage) professionals in this line of work to participate in the forum as they see fit. That will maintain the forum's status a cut above the other gear sites. Cheers!

[tubeswell]

Nodes E & F: Why the desire for separate power caps for V1a & V1b? Seems like overkill. Is it really that important to the tone to have 22mf on one plate and 8mf on the other?

Re: the lower voltage on V1a - the 100k drops it by about 50V, and when you use a largish resistor (100k is large for a power supply) you only need a smallish cap for the same RC freq roll-off (compared to if you use a smaller resistor and a larger cap). Keeping the caps small where possible probably also reduces inrush (charging) current demand on the power supply, helping keep the other voltages on the other nodes up a bit more. That's how I figure it anyhow.

[rp]

A doctor who still makes house calls. Thanks!