Question about Changing from Shared Cathode to Split Cathode... Sorta...

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ESR
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Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by ESR » Fri May 29, 2020 1:42 pm

I've got a Marshall circuit build that started out as a JTM45 with 50w transformers...
I ran KT66's for a while and then switched to EL34's...
Then I switched it from tube rectified to solid state rectified...
I changed the bright channel coupling cap from a .022 to a .0022...
And I also wired in a mini switch to switch between original 27k NFB resistor and a 47k NFB resistor...

Somewhere along the way, I realized that I wanted a Super Lead (after all my mods started to reveal the truth), so I built myself a Super Lead. Love it.

Now, I want to adjust my 50w hodge-podge amp to be a *little* closer to a 1987 circuit, so I figure the V1 cathode follower is the next best thing to adjust.

It currently still has the stock 250uf/820ohm shared cathode follower like JTM45's have... I thought about splitting pins 3 and 8 on my V1, and ADDING in the .68uf/2.7k cathode follower for the bright channel... BUT, then I thought about it and realized I NEVER use the dark/"normal" channel on the amp at all. And I have no plans to do so.

SO, rather than *adding* in the second cathode follower option (.68uf/2.7k) to make it a true split cathode setup, what if I simply REPLACE my existing 250uf/820ohm pairing with the .68uf/2.7k pairing...?

If I'm after the sound of the lead spec bright channel cathode follower (.68uf/2.7k), could I attain that for this hodge-podge amp by using those values and keeping it "shared"...? Or would the "shared" aspect still hinder what I'm going for somehow?

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by danman » Fri May 29, 2020 6:18 pm

The cathode follower is part of the circuit associated with the v2 tube. It is directly before the tonestack and helps to drive the tonestack. What you are after is going from a shared cathode arrangement (bass spec) to a split cathode arrangement (lead spec) on the v1 tube. You can split the cathodes on v1 and use any pair of resistor/cap combo that you prefer. Whatever you choose to use on each half will rebias the tube slightly but as long as you keep them in the normal range you will be fine. There is a pair of 470k mixing resistors directly after the v1 tube. There is usually a 470pf cap over the 470k resistor associated with the bright channel. This allows extra highs to pass through and helps to make the 2.7k/.68uf channel slightly brighter than the normal channel. Make sure that this cap is still across the correct resistor that will be associated with 2.7k/.68uf that you plane to install. If it accidentally ends up on the 820r/250uf side of v1, that will end up being brighter and the other channel won't sound like it should.

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by ESR » Fri May 29, 2020 7:00 pm

danman wrote:
Fri May 29, 2020 6:18 pm
The cathode follower is part of the circuit associated with the v2 tube. It is directly before the tonestack and helps to drive the tonestack. What you are after is going from a shared cathode arrangement (bass spec) to a split cathode arrangement (lead spec) on the v1 tube. You can split the cathodes on v1 and use any pair of resistor/cap combo that you prefer. Whatever you choose to use on each half will rebias the tube slightly but as long as you keep them in the normal range you will be fine. There is a pair of 470k mixing resistors directly after the v1 tube. There is usually a 470pf cap over the 470k resistor associated with the bright channel. This allows extra highs to pass through and helps to make the 2.7k/.68uf channel slightly brighter than the normal channel. Make sure that this cap is still across the correct resistor that will be associated with 2.7k/.68uf that you plane to install. If it accidentally ends up on the 820r/250uf side of v1, that will end up being brighter and the other channel won't sound like it should.
Thanks for your reply, danman. You're right--my main question here is about changing from a shared cathode arrangement to a split cathode arrangement on the V1 tube--SORT OF.

Here's why I say "SORT OF":

Right now, my V1 has the shared cathode arrangement, with pin 3 and pin 8 bridged, leading to the 250uf/820ohm combo. I know that I want to have the .68uf/2.7k combo in play for the bright channel of this amp (just like a lead spec)... But since I never ever use the normal (dark) channel of the amp, I'm wondering if I can simply *REPLACE* the 250uf/820ohm combo with the .68uf/2.7k combo while LEAVING it in a shared arrangement. I *WAS* going to make it split cathode, but I would be happy leaving it in a shared arrangement with the "lead spec" cap/resistor values if that will work.

So, to reiterate--I wanna know if I can LEAVE it in the "shared" arrangement, and change the values of the the cap/resistor combo coming off pins 3 & 8 to be .68uf/2.7k (lead spec). Will that make it sound more like "lead spec" even though it's still "shared"?

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by danman » Sat May 30, 2020 8:00 pm

If you want to leave it as a shared cathode arrangement but you want the sound of the lead spec 2.7k/.68uf from each channel, you will need to double the value of the 2.7k to correctly bias the tube to lead specs sound. You need to install a 5.4k (or closest value) because when in a shared cathode arrangement, each triode sees half the value of the shared resistor. The channel that has the 470pf peaker across it's mix resistor will sound like a proper lead spec and the channel without it will be slightly duller.

http://forum.metropoulos.net/viewtopic.php?t=22748

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by ESR » Sat May 30, 2020 10:07 pm

danman wrote:
Sat May 30, 2020 8:00 pm
If you want to leave it as a shared cathode arrangement but you want the sound of the lead spec 2.7k/.68uf from each channel, you will need to double the value of the 2.7k to correctly bias the tube to lead specs sound. You need to install a 5.4k (or closest value) because when in a shared cathode arrangement, each triode sees half the value of the shared resistor. The channel that has the 470pf peaker across it's mix resistor will sound like a proper lead spec and the channel without it will be slightly duller.

http://forum.metropoulos.net/viewtopic.php?t=22748
Awesome! Thanks a bunch danman! Much appreciated!

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by danman » Sun May 31, 2020 6:46 pm

Your welcome! Let us know how it turns out. It may not sound identical to a regular split cathode/lead spec amp but it should get you very close. You can tweak the value of the .68uf bypass cap to really fine tune the sound to your liking.

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by ESR » Mon Jun 15, 2020 3:38 pm

danman wrote:
Sun May 31, 2020 6:46 pm
Your welcome! Let us know how it turns out. It may not sound identical to a regular split cathode/lead spec amp but it should get you very close. You can tweak the value of the .68uf bypass cap to really fine tune the sound to your liking.
Got it wired up and cranking again. I decided to just go with traditional split cathode arrangement and values. I'm digging it a lot. Kind of has a cool blend of lead spec and bass spec character going on. Has the available crunch of a lead spec circuit, while also maintaining the muscle and oomph of a bass spec circuit.

I also ended up removing the bright cap from the bright channel. I like it this way, I've discovered.

Thanks again for your help, danman!

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Re: Question about Changing from Shared Cathode to Split Cathode... Sorta...

Post by danman » Mon Jun 15, 2020 6:44 pm

Glad to hear that it worked out well! There are lot's of options available for you to try between those two setups. I'm constantly tweaking mine trying to get the perfect sound.

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