Nothing Sounds Good

[davent]

If i recall...

Dropping the screen resistor brought on the crunch.
Dropping the screen cap brought on the sparkle.
Upping the cathode cap added some body lost with the smaller screen cap.

Played with each one individually so there was only one variable different at each audition.

[thelonious]

I'm interested in Dave's and Ron's thoughts on this, but my first inclination is to say that for V1, for guitar purposes, 212V at the anode is pretty high. I'm not super familiar with the 6SJ7, but many people like the sound of preamp tube distortion at lower plate voltages (see Dave's @ 90V). You could try a larger dropping resistor just to see if you liked it.

BTW, Dave, I have a 6AQ5 I've been wondering what to do with... think I might give your schem a try!

[PRR]

> Does the palatability of a booster pedal depend on the gain of the first preamp tube?

Perhaps better to look at Input Overload.

12AX7 with 350V supply has gain near 50, output overload near 70V, so input overload about 1.4V.

Chip with +/-12V supply has output overload like 10V, so to get a similar 1.4V input overload it can only run a gain of 7.

What may be more important: what happens after overload? Some stages just clip benignly. Others have their biasing thrown way off-center, can even be pushed to "cutoff" and may not recover quickly or smoothly. ("5M grid leak bias")

In some sense the distortion should be integrated in-side the amplifier. However that concept has been around for 45 years and this forum is still here.

[PRR]

> I have a 6AQ5 I've been wondering what to do with...

It is a baby 6V6. It can be used in any 6V6 way that does not push the tube too hard.

The "12W" 6V6 is often run in Champs at 14W or 16W. Does OK. The 6AQ5 would not. Drop the supply voltage to stay inside the 6AQ5's ratings. 250V is often ample.
_________________

> This seems like a very high screen resistance

Pentodes are complicated.

As a general thing, the ratio of Rs/Rp can be taken as a fairly fixed thing, and you get this ratio from the tube-maker's tables. Many pentodes do OK with 4:1 or 5:1. 5.5:1 is not out of the ballpark.

But also: the input overload of a pentode goes as some fraction of Vs. You put Vs up at 100V-150V for big current and large output power; OTOH you put Vs down below 40V for low current and high voltage gain.

As a rough guide: input overload is similar to Vs/Mu(g2). So a 'J7 with 29Vs has about 1.5V input overload. I believe Vs could be even lower for raw guitar work. However it also interacts with Vk. And if pushing overload, all your R-C time-constants.

Aside from not using a 10Meg grid-leak, it may also be {edit} generally useful to have a 34K (47K) series grid resistor, as seen here, to limit grid blocking.

[seedlings]

Aside from not using a 10Meg grid-leak, it may also be useful to have a 34K (47K) series grid resistor to limit grid blocking.

There is already a 47k series resistor to the input grid.  Maybe need more resistance, or... just make sure that 47k is there.

CHAD

[davent]

It's been a number of years since

I'm interested in Dave's and Ron's thoughts on this, but my first inclination is to say that for V1, for guitar purposes, 212V at the anode is pretty high. I'm not super familiar with the 6SJ7, but many people like the sound of preamp tube distortion at lower plate voltages (see Dave's @ 90V). You could try a larger dropping resistor just to see if you liked it.

BTW, Dave, I have a 6AQ5 I've been wondering what to do with... think I might give your schem a try!

Pretty sure the 230v i used was what i thought the component values from the Octal One schematic would yield, plugging them into to Duncans power supply software, then using info from Merlins website and preamp book. Can't offer any insight into what might/should work, it's pretty much cut and paste, trial and error to get it to where i thought it sounded good.

The 6aq5 power section started with values from the Savge Croaker schematic, (6u8->6aq5, worked up by the guys at Wattkins.com) then a few tweaks tweaks for my higher voltages. Readings shown are the range available with power scaling.

[thelonious]

^ Thanks Dave! Definitely going to give this a try...

[ulysses]

a lot of pedals sound shit. thats the truth of the matter.

i have found that the ROG 18 is an awesome pedal on amps that dont like to be driven.

some amps hate being pushed. fact of life. if yours hates being pushed, try turning the amp up to 10 and then try pushing it with a pedal. that sometimes works.

if you want a driven amp i would suggest building one that can be driven simply by turning it up.

cheers