6S100 Driver Circuit

[fusionbear]

Hello Gents. I am contemplating my next build. I really liked the 6S100's I previously built and sold and now am considering a 4-6V6 version. I want to keep the EF86>12AU7 driver part of the circuit. Do you see any issues doing this into a quad of 6V6's? I don't but many of you are much more knowledgeable so I would humbly ask for your advice.

[SilverFox]

Hey FusionBear, My build of just the 6s100 power section sounds really great with an amp emulator. It reproduces that changy grind of the high gain amps. I made a couple changes to Webbers design by using one side of a 12AX7 instead of (2) AU7's in parallel, since I was driving 6L6 power tubes instead of, were they KT66 in the original? and didn't need the higher current drive of 2 AU7 valves.

I also used a .1 uf on screen(2) of the EF86 since I didn't have a 1 uf in a high voltage. I wanted to find out what voltage rating that cap has if you happen to know? C11, screen(2) to cathode...

silverfox.

[sluckey]

I also used a .1 uf on screen(2) of the EF86 since I didn't have a 1 uf in a high voltage. I wanted to find out what voltage rating that cap has if you happen to know? C11, screen(2) to cathode...

My Sceptre uses a .1µF @ 600V BlackCat for that cap between screen grid and cathode. I know it's a different tube but basically the same schematic.

That cap needs to have a voltage rating that is greater than the B+ node C that feeds the screen.

[fusionbear]

Hey FusionBear, My build of just the 6s100 power section sounds really great with an amp emulator. It reproduces that changy grind of the high gain amps. I made a couple changes to Webbers design by using one side of a 12AX7 instead of (2) AU7's in parallel, since I was driving 6L6 power tubes instead of, were they KT66 in the original? and didn't need the higher current drive of 2 AU7 valves.

I also used a .1 uf on screen(2) of the EF86 since I didn't have a 1 uf in a high voltage. I wanted to find out what voltage rating that cap has if you happen to know? C11, screen(2) to cathode...

silverfox.

Solen makes a 1 uf 600v that works really well. Instead of the 12AU7, I will experiment with either a 12AT7 or a 12AX7 We'll see. Inn the planning stages. This build will be next years project hopefully...

[fusionbear]

Anyone care to share their insights? Please?

[SilverFox]

It may be the case that 4 6V6 tubes will place too great a load on the 12A _7 if you get into the output tubes drawing grid current and in a sense, shorting the driver tube. It may be the case the parallel AU7 is needed after all.

With these search terms, "sunn s100 cathodyne 6v6 output", found something here:

http://www.ax84.com/bbs/dm.php?thread=264091

http://archive.ampage.org/threads/6/gad ... ack-1.html

Something that keeps showing up is the need for a strong signal at the phase inverter to generate sufficient gain to drive the output tubes to full power. The 6s100 preamp output is probably what you need to have from your preamp design.

Why the 6V6 output?

btw- I ordered some of the Solan 1uf 600V caps. That cap on g(2), (not sure if it's g(2) or (3) since the schematic is buried in my rearrangement process here), that 1uf cap influences the bass frequency response but I don't know how much yet.

silverfox.

[fusionbear]

It may be the case that 4 6V6 tubes will place too great a load on the 12A _7 if you get into the output tubes drawing grid current and in a sense, shorting the driver tube. It may be the case the parallel AU7 is needed after all.

Something that keeps showing up is the need for a strong signal at the phase inverter to generate sufficient gain to drive the output tubes to full power. The 6s100 preamp output is probably what you need to have from your preamp design.

Why the 6V6 output?



silverfox.

I really like the way 6V6's affect the tone. Even on an amp used primarily for clean tones. I really appreciate the links. Thank you very much!

I like to build amps that are a little different than usual. I built a JTM45/100 with 4 6V6's and it came out fantastic. I would love to do a 6S44 version....

I was even thinking that V1 could be a 6SL7, V2 the EF86 driver, 12AU7 PI, 4driving 4- 6V6's... we'll see brainstorming right now...

[brewdude]

Can you please point me to a schematic?

[fusionbear]

Can you please point me to a schematic?

This is the foundation, but not my final design. I am still in the brainstorming stage:

http://www.tedweber.com/media/kits/6s100_schem.jpg

[gingertube]

The parallel 12AU7 triodes of the cathodyne are running at just under 2mA total.
you could easily use 12AX7 parallel triodes instead.

I believe that the 12AU7 was used to try to lead the behaviour when you overdrive the output tubes, may also have less voltage across the tube so leave more signal swing at anode and cathode.

When the output tubes grids approach zero volts the grid to cathode looks like a forward biased diode and the input impedance dips dramatically.
This means that the cathodynes anode and cathode loads are dipping dramatically on opposite half cycles.

Output Impedance at the anode and cathode of the cathodyne are equal and equal to 1/gm while the anode and cathode loads are equal.

If the cathode load drops dramatically then the anode output impedance increases toward a max value of RL (from 1/gm towards RL, RL = 47K in this instance).

If the anode load drops dramatically then the output impedance at the cathode increases (from 1/gm)by a max amount of RL/ mu

If you do all the algebra there are some advantages in running low value RL (the anode and cathode loads) which means a beefier tube to run larger current swing for the same signal voltage developed, and also to run a low mu triode. Hence 12AU7 in lieu of 12AX7.

Here they compromised the 12AU7 design so that a 12AX7 could be plugged in. Otherwise they would have taken the load resistors down to 39K or 33K and upped the running current, the AU7 would benefit from that but the 12AX7 would then be over stressed. So current circuit values chosen for either to work.
Example: I have a 4 x EL34 HiFi amp which has a 12AU7 cathodyne splitter with 22K RL in anode and cathode.


That treats the symptom but not the disease.

The way to fix this is to run much larger than normal grid stops on the output tubes. Run 33K or even 39K. That minimises the cathodynes load changes as we overdrive the output tubes. That minimises the cathodynes "bad" behaviour and also limits the result of that bad behaviour.
A win win win.
6V6 need big grid stops anyway.

I'm running 33K on a Fender 5E3 circuit with 12AX7 cathodyne driving a pair of vintage 6V6G.

So by all means run the AU7 splitter, there is some engineering behind its choice. But also increase those output tube grid stops. Then you will happily be able to crank 4 x 6V6.

Cheers,
Ian

PS.
You will probably never reach these limits but if you start to overdrive the cathodyne itself then big gridstops for its triode(s) too are the answer, 100K or even bigger.

[fusionbear]

@ gingertube: Thank you very much for your advice. I will draw up a schematic this week and post for a review and critique. Your points help quite a bit...

[fusionbear]

One more question: Can you use the same driver circuit in the above posted schematic to drive a pair of KT66's?

Obviously, I would consider changing the resistors on the 12AU7 to 33k load resistors. Would I still need the higher value grid stop resistors?

This idea would presuppose I use a 8K primary OT as well...

[SilverFox]

KT88, KT66, and 6550's should all work with the same OT but someone else should chime in with a more experienced opinion.

See also the Sunn S200 it still uses one valve to drive the grids and the same OT is used with the 6550's as the KT88. From what I've read the main difference in the various KT series relates to the frequency response of the tube, well power handling capacity also.

For what it's worth I've got the 6s100 power amp sounding fantastic now with an amp emulator. I'm working on an EQ section to place before the PA that has removed perhaps the last vestiges of SS or Digital sterility.

silverfox.