68 shared cathode audio clip

rockstah · Post by **rockstah** » Thu Jul 20, 2006 10:53 pm

so lets see, shared cathode,
we have one wire connected to both cathodes of the tube..
both sides of the tubes cathode gets input from one side of the tube...so isnt this setup simply using one side of the tube?? the other cathode has no effect cause its no getting input? so the atribute to the sound we are hearing here is merly 250u/820/single side of the tube

where is Larry when ya need him?

rgalpin · Post by **rgalpin** » Thu Jul 20, 2006 11:41 pm

that's the way i see it too - but i wonder if sharing the juice provided by the 820/250uf between both sides has some effect? the juice is "serving" both sides of the tube so it would seem that there would half as much of something. maybe that's too logical to apply to anything electronic.

BrownSound1 · Post by **BrownSound1** » Fri Jul 21, 2006 1:03 am

The resistor on a shared cathode circuit will be seen as double what the actual value is. So if you have an 820 ohm resistor there it is "seen" as a 1640 ohm on each channel separately. The higher the RK resistance, the lower the gain. At least that's the way I think it is.

SDM · Post by **SDM** » Fri Jul 21, 2006 1:09 am

Brownsound's got it. This is the same as any shared cathode amp. Using a jumper on the socket and one wire, or running two wires from the same cathode resistor/cap makes no difference. So, just as any shared cathode Marshall, V1A and V1B get inputs from normal or bright inputs. The shared cathode effects both channels. The shared 820 ohm resistor roughy acts like having a 1.6K (1640 ohms) cathode resistor on V1a, and V1b seperately.

rockstah · Post by **rockstah** » Fri Jul 21, 2006 2:07 am

so it is a shared cathode! i learn something new every day!

rgalpin · Post by **rgalpin** » Fri Jul 21, 2006 1:48 pm

BrownSound1 wrote:The resistor on a shared cathode circuit will be seen as double what the actual value is.

shared means double. perfectly illogical!

BrownSound1 · Post by **BrownSound1** » Fri Jul 21, 2006 8:19 pm

It can be confusing for a lot of people, but that's the way it is.

The single resistor basically becomes two 1640 ohm resistors in parallel, because you have current from both channels flowing through the one resistor.

I'm sure George or Larry could explain it much better than I ever could.

awangotango · Post by **awangotango** » Fri Jul 21, 2006 9:00 pm

don't most people who switch their split superleads to a shared setup, also change out the resistor at the same time? something like 1k . In other words, isn't the stock 2.6k acting like a 2.6k in a split, but acting like a 5.2k in shared? Which is very high.......or is it the other way around? regardless, sharing cathodes and leaving the stock resistor is not acting like a stock shared amp. ( a stock pre-68 superlead or superbass)

BrownSound1 · Post by **BrownSound1** » Fri Jul 21, 2006 9:31 pm

I don't see how it is different. He doesn't have the 2.7k ohm resistor hooked up to anything, and is just using the 820 ohm. That's the exact same thing a JTM45 has.

awangotango · Post by **awangotango** » Fri Jul 21, 2006 10:48 pm

now i see.......my bad.

BrownSound1 · Post by **BrownSound1** » Sat Jul 22, 2006 4:02 am

That's ok...I missed it at first too.

Bainzy · Post by **Bainzy** » Sun Jul 30, 2006 9:17 am

What value is that Channel 1 coupling cap - .022uF or .0022uF?

Edit: nevermind, seen the 22n marking in the second pic..

MARCO · Post by **MARCO** » Fri Oct 20, 2006 12:19 pm

I am interested in knowing what kind of EQ you are using for those clips. Sounds cool. My amp sounds very much like that old tone but sometimes needs a little "push" on the front end. My REPLEX gives it a nice push but....

rgalpin · Post by **rgalpin** » Fri Oct 20, 2006 1:23 pm

it's an mxr 6-band - 70's era i think -

like this one:

although those are not my settings...

MARCO · Post by **MARCO** » Fri Oct 20, 2006 1:37 pm

thanks, i know davbe freidman said to get one with an overall volume control on it.