voltage rating for V1 and V2 cathode bypass cap, JTM45 build

[PharmRock]

I am building a JTM45 to near-original spec, but I plan to set up the board layout to accommodate a split cathode on V1 rather than the stock shared cathode, in the event I decide to go that route later.  The split cathode will have the 0.68uF cap.  MojoTone makes a "Mojo Dijon" foil/film 0.68uf/160V cap that Doug used to sell, but he doesn't stock these anymore.  Mojo has them available but unfortunately this thing is about the size (30x18mm) of one of those vienna sausages I used to eat out of the can for an after school snack, and the 160V seems a little extreme for this position in the circuit.  I don't want to give up that much board real-estate to accommodate this cap.

Mouser has the 0.68uF/63V metallized film Cornell Dubliner Type 150 series cap that has much more board-friendly dimensions.  I also see that Doug sells a 250V Panasonic polyester cap in this value that would fit the board nicely.

Seems to me that a 25V rating would work in these positions...but obviously it couldn't hurt to go higher than that.  Anything I am missing here?  Does the material (e.g. polyester vs. foil/film wrap vs. metallized) really even make a difference in this position in the circuit? 

[sluckey]

Hoffman sells this...

     .68uf/250v 684
     250 volt Panasonic Polyester film capacitor - cathode bypass cap
     16.8mm wide x 5.39mm tall x 5.3mm thick
     Price each = $0.43

Very small. Scroll way down on the cap page.

[SILVERGUN]

I am building a JTM45 to near-original spec, but I plan to set up the board layout to accommodate a split cathode on V1 rather than the stock shared cathode, in the event I decide to go that route later.

I recently did a low B+ Plexi and really appreciate being able to blend the two volumes. I basically start with the bright half and crank that to about 80% and then fill in the bottom with the bassier 1/2, based off of which guitar I'm using.
My advice for you is to just go ahead and include both different value cathode setups from the start. I used the .68uf on one side and 25uf on the other. No need to go all the way to 320uf.


My ears aren't good enough to pickup any difference in tone between the different cap types in that or any other cathode position. Even a 25V rating is overkill, so nothing to worry about there.

[PRR]

Hoffman sells this...

He saw that:

...I also see that Doug sells a 250V Panasonic polyester cap ....

Even thin film stands hundreds of Volts. So you are not likely to find a 3V film cap. If it exists, it would cost a lot more for fabricating an ultra thin film.

0.68uFd is an odd duck. It is a very large (costly) film, or a very-very small electrolytic (which will work here).

[PharmRock]

Thanks for replies/suggestions.  Seems like the polyester cap Doug sells will do the trick, and its cheap. 

My advice for you is to just go ahead and include both different value cathode setups from the start. I used the .68uf on one side and 25uf on the other. No need to go all the way to 320uf.

My ears aren't good enough to pickup any difference in tone between the different cap types in that or any other cathode position. Even a 25V rating is overkill, so nothing to worry about there.

I ordered a 220uf as well as the 330uf cap for the shared V1 cathode, but I also have some smaller values on hand that I can try out as well.  Will definitely try that option. 

When splitting the cathode, should I increase the previously-shared 820R up to 1.5K, or keep the 820R in there?

[SILVERGUN]

When splitting the cathode, should I increase the previously-shared 820R up to 1.5K, or keep the 820R in there?

The "correct" move would be to increase it to 1.5K


If it were my build I would consider setting up those 2 halves like the Plexi, so the differences are magnified.
On mine, I set it up like sluckey because I wanted 1 input / always jumpered
I do realize that the JTM45 is a different amp.

[66Strat]

On the JMP Marshalls, the Normal channel cathode resistor was 820 ohms. The turnover frequency of a 330uF cap across an 820 ohm resistor is less than 1 hz. A 220uF cap across the same resistor is also less than 1 hz. The turnover frequencies for the following caps across the 820 ohm resistor are as follows:

• 25uF ~ 8 hz
• 10uF ~ 19 hz
• 5uF ~ 39 hz

In comparison, the turnover frequency of the .68uF cap across the 2.7K resistor is ~ 87 hz. The low E frequency of a guitar is 82 hz.

[jordan86]

I can only comment on my experience, but it matches the others. I like having two channels voiced differently and blend them in a similar manner. Bright channel 70% up, Normal to taste so it fills out the low end but not too wooly or woofy.

I went 2.7k/.68 and 1k/25uf. Also did a 5uf bypass on the third stage for a touch more gain.

Anything above 25uf seems excessive to me, but no shame in building it to exact spec just for the fun of it.

As far as cap type, you really should try your own flavors and see what you like. They do have some subtle variation. I’ve found a bigger difference can be found in changing the values of the different bright caps throughout the bright channel in a jtm/plexi circuit. I ditched the cap on the bright channel volume pot entirely but tweaked the values in other places.

[PharmRock]

but no shame in building it to exact spec just for the fun of it.

This is the direction I was heading...build a "stock" JTM but include some switching options.  Seems like splitting the V1 cathode as well as switching out the 330uf for a much smaller cap would yield some of the biggest changes in tone for the amp.  I have a single input, bright channel only 1987-ish circuit already with a cascade switch, so building the JTM with a more traditional woolly tone is ok for me. I've never owned a 4-holer before, so the suggestions for how to set up the normal and treble channels are very helpful.

Thanks everyone for all the advice.

[jordan86]

It’s be worth researching the Suhr SL68 while you’re headed down this trail. A couple threads on different forums about it. He does use the 330uf, I believe. I think he uses the obnoxiously big .68uf film caps as well. On one stage he even uses both, 330uf for the full frequency bypass, and .68uf (either film or ceramic?) for immediacy/attack. I can’t verify the validity of that scenario or if it’s a snake oil thing, but man it’s a real nice sounding plexi.

[sluckey]

It’s be worth researching the Suhr SL68...

All the old Sunns used a 250µF on the cathode of the preamp tube, Not a tone thing, rather reduce hum due to HK leakage. Probably all about hum in the 5F6A also.

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_100s_sceptre_and_sentura_ii.pdf
     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf

[PharmRock]

It’s be worth researching the Suhr SL68 while you’re headed down this trail. A couple threads on different forums about it. He does use the 330uf, I believe. I think he uses the obnoxiously big .68uf film caps as well. On one stage he even uses both, 330uf for the full frequency bypass, and .68uf (either film or ceramic?) for immediacy/attack. I can’t verify the validity of that scenario or if it’s a snake oil thing, but man it’s a real nice sounding plexi.

Thanks Jordan...after reading that, I searched the interweb for a gut shot of the SL68 and found a pic of exactly what you are referring to. There's also a pretty lengthy discussion on this on TGP forum as well as Metro forum. On V2A it appears he has a 330uf(?) electrolytic cap AND a smaller cap in parallel that looks an awful lot like the 0.68uf Panasonic polyester cap but who knows what the value actually is.  This took me down a rabbit hole that had a lot of discussion on EVH's modded plexi.  Seems to be quite a bit of speculation on those components!

[sluckey]

Conrad Sundholm (Sundhy) is a member here. He was last on the forum yesterday. You could send him a PM asking why he used such a big preamp cathode bypass cap in the Sunns and Suhrs.