Jose mod: .68uF/2.7K shared cathode = .68uF/2.7K split?

[scriptphase]

If I'm converting a 50-watt bass spec (1986) to a 3-pre Jose style, then is it necessary to eliminate the jumper from pins 3 to 8 and run a separate .68uF/2.7k to V1b instead of simply changing the V1a cathode from 339uF/820ohm (250uF on my amp) to .68uF/2.7k and keeping the V1 cathodes shared with the cascade?

There's no turret to the V1b cathode on a bass spec, so this seemed to make more sense to me considering both halves are the same combo in the Jose, but I'm not sure that I understand this correctly. Any input would really be appreciated.

 

[Anonymous]

Steve

 

[scriptphase]

Thanks, Steve.

Why do the two cathodes need their own cap/resistor pair to ground when they're the same value? When channels are jumped on a bass spec, both sides are blended using the same combo. Is this due to both halves of V1 being cascaded in a Jose-type vs. blended in a jumped bass spec?

Sorry for the dumb questions.

 

[MississippiMetal]

I'm not entirely sure what you are asking, but the stages need to be biased separately in a cascaded arrangement, even if they're biased the same. Otherwise you're just ending up with parallel stages, which can be done, but Arredondo cascaded the parallel arrangement on those old Plexis for a reason, and that reason was the amount of clipping that could be generated from having An extra gain stage. That coupled with the Zener clamping, Jose was going for all the distortion he could muster from these old amps without adding tubes.

 

[lespaul6]

cool thread!

 

[scriptphase]

I'm not entirely sure what you are asking, but the stages need to be biased separately in a cascaded arrangement, even if they're biased the same.

Thanks!

Here's a pic of my amp:

I had been thinking I could just change V1a cathode to .68uF/2.7k, then add the cascade using a DPDT mounted to a 1Meg pot, and not have to run a separate .68uF/2.7k because of the bridge from pins 3 to 8.

This would cause oscillation due to phasing issues?

 

[mightymike]

Not sure if I understand your question, but you do need both .68uf caps on a Jose front end because it is not split or shared cathode. It is cascaded. The tube has 2 sides. The signal is going out one into the other versus a plexi circuit that uses one side for channel 1 and the other for channel 2. If I remember right, the 2 sides use the same ck/rk values except that v1a plate is increased from the standard 100k to either a 220k or 330k with a plate bypass cap from anywhere between 250pf to 500pf. Typing this from my phone. So I'm just going from memory.