5F6: reduce gain at cathode follower with split resistors

[Baguette]

Hello tinkerers,

I want to reduce the gain before the PI in a 5F6 type build.
Here are two ways I'm thinking of.



Are those 2 ways (split plate Vs. split cathode) doable?
Differences in tone?
Is one way prefered over the other?

Thanks a lot for the help!

[jjasilli]

Yes, but also a 3rd way:  you could also put a voltage divider between the driver stage and the follower stage.  Strongly suggest that you read "Designing Preamps for Guitar & Bass", by Merlin Blencowe.  There's an entire chapter on cathode followers.

[12AX7]

I've done that and it softens the high end. Not sure if you will find it does that in a way you like, but thats why i undid that tweak. Didn't care for the change, so i tried other ways to reduce gain after the CF and nothing was really satisfactory.

[tubeswell]

@ Baguette. So you tried a 12AY7 in V1?

[Baguette]

Well, the beast ain't really a 5F6. It's got a 6SL7 as first preamp tube (gain stages before and after the volume control). No NFB.
Not VERY gainy, but the player wants the most clean headroom.
I'm wondering whether I should reduce gain after the first stage or after the second stage (just before the PI) or both. Will have to experiment I guess.
As for voltage dividers, I thought about it but like the split resistors approach better.

[HotBluePlates]

"Reducing gain" at the cathode follower doesn't "read right" to me, because the cathode follower already has a smaller output signal than its input signal; so, there's "no gain" there already.

As a result, if I had to choose from your options (though each works), I might consider the split plate-load more sensible. But this is very similar to what JJasili suggests: an interstage voltage divider.

Building up a signal, then knocking it down with a voltage divider (whether a volume control, fixed interstage divider or a tapped plate/cathode load) makes a lot of sense in a high-distortion preamp. I suggest it makes less sense in a clean preamp. Instead, if your cathode bypass caps are bigger than ~5uF, you could simply remove a bypass cap to cut that stage's gain by about half.

Well, the beast ain't really a 5F6. It's got a 6SL7 as first preamp tube (gain stages before and after the volume control). No NFB.
Not VERY gainy, but the player wants the most clean headroom.

This is a soapbox item for me. You really have to get inside the player's head to know if they say, "more headroom," but really mean more volume capability before output tube distortion (which will require a bigger/different output stage or something akin to Super Scaling), or if they mean "turn the volume higher before distortion kicks in.

You can fake "more headroom in the latter case by removing a bypass cap, or you can add a resistor between the prior stage's coupling cap and the "input" of the volume pot. The latter makes it so there is an upper ceiling on how high the volume pot can be turned up, which will be less than you'd get with a stock volume control. The end result is a smaller output signal and the impression the amp stays cleaner until a higher volume pot setting.

You can experiment with the limited volume control by wiring a pot as a variable resistor in series with the original volume pot, or the split plate load by replacing the plate load with a typical pot. Vary each with the player testing the amp, and settle in on the value that works for him.

I have a sneaking suspicion that the player will think the amp works fine for his needs until he tries one of these approaches, then discover that what he really wants is "louder" not "more headroom" (whatever that means to him). The are pluses and minuses to the several approaches to trying to increase the real output power of his setup.

[jjasilli]

As for voltage dividers, I thought about it but like the split resistors approach better.  Split resistors are a voltage divider!

BTW:  I was focusing specifically on the cathode follower part of your question.  Regarding overall gain before the PI, the other posters make good points.  Swapping in lower mu preamp tubes is a good alternative.

[darryl]

Using split resistors in the plate circuit won't work correctly, because the two stages are DC coupled. The plate voltage of the driver stage sets the grid bias of the cathode follower.

[HotBluePlates]

Good call Darryl, I overlooked that part.

But the principle holds in other stages, outside of the direct-coupled stage.

[Baguette]

Thanks for the answers folks!

1/ I can't just swap in a low mu preamp tube. There's an octal 6SL7 here.
2/ So the split plate resistors is not an option in a DC coupled cathode follower config?
>Will try the split cathode then. I remember a german amp (Klemt BS40) which was basically a JTM45 with a Master control added. The master was actually the cathode follower's cathode resistor, and the wiper was tapping signal betwen the ground and the cathode. Cool approach me thinks.

[jeff]

"I want to reduce the gain before the PI in a 5F6 type build."

How about simply adding a voltage divider after the tone stack?

"The master was actually the cathode follower's cathode resistor, and the wiper was tapping signal betwen the ground and the cathode."
Most cathode resistors are 1/2 watt. Most pots are 1/4 watt.

[Boots Deville]

You might try increasing the CF cathode resistor to 120K.

Refer to the note in the preamp section of this schematic:
http://ax84.com/static/october/October_Club_101010.pdf

[HotBluePlates]

So the split plate resistors is not an option in a DC coupled cathode follower config?

As Darryl said, that changes the d.c. bias of the following stage; so, no.

Why can't you just lose some gain earlier in the amp? Removing a cathode bypass cap cuts gain, introduces local negative feedback, and causes that stage to be able to handle a larger input signal for the same amount of distortion. If the guy wants "headroom" that will do it, unless the output power is the issue.

[Baguette]

Well, gain stage 2 has the classic 5F6 config with 820 ohm cathode res, and no bypass caps.
I could remove the bypass cap at the input gain stage and see what happens. Very few amps are made like this IME (some Ampegs like the portaflex feature no bypass cap at the input stage, but no other examples spark to my mind).

[sluckey]

1/ I can't just swap in a low mu preamp tube. There's an octal 6SL7 here.

Sure you can. Just plug in a 6SN7.
High-mu 6SL7 amplification factor is 70.
Medium-mu 6SN7 amplification factor is 20.
Same pin-out.

[Baguette]

Thanks Mr Luckey, For some reason I thought the 6SN7 had a different pinout.

Sounds like a great option.

[tubeswell]

BTW, splitting the CF cathode resistor will reduce gain but this will be at the expense of increasing the stage's output impedance somewhat.

[nateflanigan]

Seems like you have plenty of cooks in the kitchen, but have you experimented with NFB?  I mean it's purpose is to make an amp cleaner. 

[HotBluePlates]

Just curious, does any have experience making adapters? so you can try the 12a_7 series in an octal plug? 

You could, but why bother? 6SL7 is close-enough to 5751, and 6SN7 is essentially the 12AU7's big brother.

Or, you could get an octal plug and 9-pin socket from Hoffman, solder the interconnecting wires to the 9-pin socket, pull the ends through the pin-holes on the octal plug, fill with whatever potting compound suits you to hold it all together.