Marshall el34 PI values to el84

[RockinRocket]

Im taking a Jca 20h and rebuilding the circuit board. Im trying to figure out what the proper PI values would be to drive el84s in the same way it drives marshalls el34s. The JCA has typical marshall PI vales 470/10k long tail. But that is driving the el84s alot harder than it would if it were el34s. Do all I need to do is change the tail resistor to a common 47k? Thanks for the help!

[sluckey]

The guys at ax84.com used some different values for their "November" amp. Here's the schematic. Sounds pretty good to me. Maybe give you some ideas to try.

http://home.comcast.net/~seluckey/amps/ ... vember.pdf

[RockinRocket]

Interesting they use the 47k tail resistor I had in mind but the 15k plate resistors?

[roberto]

You can use 100k on both plates, as the 47k tail will balance the two triodes.

[martin manning]

Interesting they use the 47k tail resistor I had in mind but the 15k plate resistors?

The idea is to reduce the output signal to something more compatible with EL84's, which don't require as much voltage swing, and just increasing the tail resistance won't have much effect on the output. A similar effect could be achieved with split-load plates, or by replacing the grid resistors on the power tubes with voltage dividers (like a PPIMV).

[Phil_S]

The Marshall 18W schematic has the LTPI with 100K plate load, 470K grid leak, 820 Rk, and 47K tail. I found that I didn't like how hard it was driving the EL84s. Though it is heresy in the 18W camp, this is typically tamed by reducing the 470K grid leaks to 220K. That provided what I thought was a satisfactory result. Of course, there is no accounting for personal taste

The 18W also uses 8.2K grid stoppers on the EL84's and 470K grid leaks.

Good luck with your amp.

[matt h]

(deleted)

[shane]

Interestingly if you look at the November pdf, though the schematic shows 15K plates on the PI, the layout shows 150K...hmmm

Cheers
Shane

[sluckey]

Interestingly if you look at the November pdf, though the schematic shows 15K plates on the PI, the layout shows 150K...hmmm

If you closely look at the color bands and compare yellows to oranges you'll see that those plate resistors on the layout are indeed 15K (brown/green/orange). The 47K tail resistor would be a good one to look at. Not all monitors are created equal.

[RockinRocket]

Thanks for the feedback everyone.

You don't need to post a schematic of the original amp, but can you at least provide a little bit of information about the circuit? The following information is really necessary to say anything useful:

1) what is the bias method for the power tubes?
2) what is the bias voltage for the power tubes?
3) what are the voltages (plate, screen for power tubes, and b+ supply for the PI node).
4) Is there a PPIMV already in place?
5) is there an NFB loop ("presence" circuit)

Well im striping the board so the possibilitys are basically limited to 3 gain stages ,cathode follower, Long tail PI, fixed output el84 with NFB presence. Not sure of the voltages.

[Phil_S]

What is the unloaded secondary high voltage, unrectified AC?

[RockinRocket]

The idea is to reduce the output signal to something more compatible with EL84's, which don't require as much voltage swing, and just increasing the tail resistance won't have much effect on the output. A similar effect could be achieved with split-load plates, or by replacing the grid resistors on the power tubes with voltage dividers (like a PPIMV).

So it seems like keeping the PI the same values(470/10k marshall values) and adding a voltage divider for the grid be the closest to el84s being driving like marshalls el34s?

Because this way your not messing with how the PI sounds. Would you agree to that?

[martin manning]

Yes, but if you are using NFB, you might want to experiment with increasing it in the same proportion as the PI output has been reduced.

[matt h]

(deleted)

[RockinRocket]

Try maybe a 33k/68k resistor split for each 100k plate resistor value to start with and adjust up/down as necessary.

My concern with adjusting the plate resistors is now the PI sounds different than what would be driving el34s. No?