Carlsbro 100 top weird powertube resistors

[Alf]

Hi,

I have this Carlsbro 100 top on my bench and it differs from your regular 100 poweramp in that it has these 3.9 - half watt- resistors between pin 8 and 1 where they are usually just tied together.
What I would like to know is , what are they for . Can I remove them because I would like to add 1 ohm resistors between cathode and ground to measure the bias current.

Can someone shine a little light on this ?

[IMG][/IMG]

It has great Partridge Xformers by the way.

Regards , Alf

[Mark Black]

My guess is somebody was trying some fanciness utilizing the fact the EL34 suppressor grid is not internally tied. Interestingly they look like flameproof or fusible resistors.

The schematic (at least the one I could find) for the 100 Top doesn't show them, and shows pin-1 tied to pin-8 tied to ground as is common.

[Alf]

Mark,

That's what I found too but then someone reminded me of a Sound City 50 plus schematic where there are also reasistors used tied from grid 3 to cathode but here two different values.

Very strange , I take it that in the Carlsbro amp maybe they were use to measure bias current but normally you do this with 1 ohm resisitors and tied from cathode to ground, so that's what I will do .

[IMG][/IMG]

Alf

[Enzo]

Looks like 4.7 ohm from here.

I see it different. Pin 8 is not connected to anything but the resistor to pin 1, and the pins 1 are connected together and are served by the green/yellow wire. I cannot imagine connecting supressor grids together and running cathode resistors from there.

I think it would be inappropriate to run EL34 that way, so I bet the amp liked 6L6 or other power tube with pin 1 free.

Where does the green wire end up? I bet it goes to ground, or maybe to a standby switch and THEN to ground. If that is the case, replace the 4.7 with a 1 ohm and there you go. or forget the resistor and strap 1 and 8 together. Then you can add a pair of 1 ohm resistors to the hreen wire, or just use a common single 1 ohm resustor at the other end ot the wire.

As to what they were doing, 4.7 ohms seems like an odd value to calculate current across, so I'd guess maybe the small value was a way to let slight mismatches between tubes even themselves out a little.

[Alf]

Enzo,

Thanks for the reply , they are 4.7 must have measured with a less accurate meter. With my Fluke they are spot on 4.7.
The yellow-green wire goes to ground so I'll take your advice and replace the resistors with 1 ohm .
The amp was originally designed for El34 tubes ,it looks a bit like a Hiwatt , it has the big Partridge xformers and the metal shielding between preamp tubes and powertubes.
The bias resistors are 100 k , would it be better to use the regular 220 k type ?
One pair of tubes draws much more current than the other pair.
As they are old Philips el34's I want to leave them in there. Their quality is very good still. So I'm thinking of putting in seperate biaspots to bias each tube seperately.

Alf