Francesca vs Schemo

[gearhead]

I've been having a long look at the Francesca pics (whoever did them, THANKS!) and can't seem to match up the Kelly and/or A-series schematics around the power tubes.

For instance, aren't there supposed to be a 1.5k and .02uf between the power tubes pin 5 and preamp pin 1 or 6, but in pics 1109 and 1112 it looks like a straight wire run with no components? What are the two (on each power tube) 100ohm resistors to ground?

Does anybody have a chasis/tube socket/pot component connectivity drawing? There are (thanks again!) the two perf board layouts, but can't seem to find the former.

Thanks,

Dave the confused

[doctord02]

That would be part of the phase inverter wiring on the end of the circuit board. Take a look at the image DSC01125. The two orange capacitors at the top of the board are the .02 caps, and they connect via yellow and blue wires to 1.5k resistors wired directly on the socket visable in DSC01124 (- the yellow wire). The resistor is pointing straight up and is behind the green enamel wirewound resistor that is horizontal over the socket.

It's also visible in DSC01156...

another Dave

[gearhead]

Got it on the first one. Thanks!

How about the two 100 ohmers to ground? On one power tube socket (but tied to the same points on the other pt), especially visible in DSC01165 and DSC01157?

[UR12]

Got it on the first one. Thanks!

How about the two 100 ohmers to ground? On one power tube socket (but tied to the same points on the other pt), especially visible in DSC01165 and DSC01157?

Those are to create a pseudo center tap for the heater circuit. If the transformer you use has a center tap on the heater winding, you shouldn't need the 100 ohm resistors to ground.

[gearhead]

Ah, this is one of those things you would know if you are "in the know".

The pin 7 green/yellow wire goes under the perf board to where?

[UR12]

pins 2 and 7 are the heaters so the G/Y wire on 7 goes under the board to the Power Tranny

[gearhead]

Ah, couldn't find the g/y after that, but makes sense.

Last one (for a while); what's the white wire that, in the middle of DSC01122, goes from the junction of the 15k, 47k, and 10uf (not on any schematics) that then runs under the 1k 25w and (as far as I can tell) runs past the big cap block and under the other perf board?

[UR12]

That would be your bias voltage wire that comes off the PS board, runs down along the chassis in front of the filter caps turns 90 degrees and runs along side and between the filter caps and the preamp board and then connects to the preamp board on the row of turrets (4 of them on the end of the board). It connects to the 1st turrett towards the back of the chassis.

[gearhead]

Now, that really has me puzzled. All the schemos depict that bias coming in after that 15k, 47k, 10uf junction (past the 47k). Francesca has it coming in at that junction?

[UR12]

Now, that really has me puzzled. All the schemos depict that bias coming in after that 15k, 47k, 10uf junction (past the 47k). Francesca has it coming in at that junction?

I am not sure which schematics you are looking at. Most have the bias voltage coming from the wiper of the pot. This isn't the way Franchesca was laid out. If you use the wiper to take your bias voltage from and for some reason the pot goes bad and the wiper opens up you will loose bias and cook your tubes. The way Fran is designed if you loose the wiper, the voltage goes to max and you will be way over biased and won't cook your tubes. So the schematics you have been looking at are wrong.

I'm not much of an artist but here is the way the circuit is layed out.

[gearhead]

I'm no artist either

I'm just looking at the Kelly 90 schemo and have attached my interpretation of where Fran bias really connects (using the Kelly 90 gif).

[UR12]

Dave

The schematic Kelly 90 or any other for that mater is drawn wrong. Franchesca has the bias coming from the point you have your arrow going to and not the pot. When some of these schematics were drawn up (years ago) there was a little guessing going on as some people had not seen anything but pictures of the amps. It appears that some looked at it and recognized it as a copy of a Marshall Bias circuit and got their values from that others just did the best the could I guess. Now thanks to a couple of wild and crazy guys ( Allyn and Greg) we have all of the Franchesca pics to go by.

Check out the schematics at this link and look how many bias circuit varaitions there are.

http://maximoaudio.com/download/schemat ... eckxpr.pdf