CF recovery stage - essential or not?

[PositiveNegativeMan]

Hi folks!
Couple of questions here.

Question one: I am designing a preamp section, toying with using a cathode follower to drive tonestack. I have not done one before. Considered wisdom (according to initial internet searches) is that CF goes to tonestack, and then a "recovery stage" before heading off to PS. Is the recovery stage essential?

Question two: I have heard that with CF, one should elevate heater voltage. I was given a schematic of a Marshall Superbass as an example of a CF circuit. Looking at this schematic, I cannot see elevated heater - unless I am misreading the schematic which is possible, or then again it may be incomplete diagram. Or maybe the Superbass doesn't use elevated heater?

[defaced]

It is likely anything regarding elevated heaters will be in the power supply part of the schem. Usually it's a simple voltage divider (1M/100K) coming off the screen node with a bypass cap (~10u) across the 100K resistor. The center tap, or artifical center tap of the heaters goes to the junction of the 1M/100K voltage divider.

The elevated heaters are used because the cathode of a CF is at a high DC voltage which can exceed the heater to cathode voltage spec of the tube. By elevating the DC voltage of the heaters, the voltage difference between the two is reduced.

[MWJB]

In a Tweed Fender/Marshall etc with a CF driven tone stack, the only 12AX7 tube that follows the cathode follower, is the PI tube, which is necessary to supply out of phase signals to each half of the tubes driving the OT primary. If your preamp section was being mated with a push-pull output with it's own PI tube (or if building an SE version) you would not NEED a specific recovery triode.

The cathode follower is not essential to a guitar amp preamp, but it could be considered a big part of the Tweed Fender/classic Marshall character, it gives a degree of crunch and a mid character. Whether you use it or not depends on what flavour you're after...at the end of the day there are tricks to give the CF circuit a better mids scooped tone, and a plate driven tone stack (BF/SF Fender) a more "tweedy" tone...probably best to save yourself some work though and determine which character you're after before you start soldering.

Typically Fender/Marshall/Traynor didn't bother to elevate the heaters...neither do the vasty majority of builders who build amps based on this topology. A B+ supply of anything up to 400vdc (at least) at the CF stage (with a cathode voltage of well over 200vdc) seems to be perfectly reliable.

[Gregg]

Typically Fender/Marshall/Traynor didn't bother to elevate the heaters...neither do the vasty majority of builders who build amps based on this topology. A B+ supply of anything up to 400vdc (at least) at the CF stage (with a cathode voltage of well over 200vdc) seems to be perfectly reliable.

Unfortunately some brands recent production tubes fail miserably in CF position /some of them go bad immediately/ which IMHO now makes elevating heaters mandatory especially if you're a manufacturer.
Mrashall weren't elevating heaters but their recent models are all elevated.
There's a thread on the subject here:

12AX7EH CF Failure

[Old Tele man]

...it all depends upon what the impedance needs to be going into the stage after the tonestack.

...a CF stage typically has a LOW impedance, but great current gain, so it can drive just about ANY impedance tonestack.

...however, since all "lossy" tonestacks produce LOTS of signal attenuation, its sorta a two-edged question: (1) how much GAIN do I need to recover to drive the next stage, and (2) what impedance do I need to have that signal developed across.

...life ain't simple, and neither is tube circuit design.

[mooreamps]

Originally Posted by PositiveNegativeMan

Hi folks!
Couple of questions here.

Question one: I am designing a preamp section, toying with using a cathode follower to drive tonestack. I have not done one before. Considered wisdom (according to initial internet searches) is that CF goes to tonestack, and then a "recovery stage" before heading off to PS. Is the recovery stage essential?

The answer is : it depends... It depends on where the tone stack is in the circuit. Just look at other existing schematics, and do what they do.

Originally Posted by PositiveNegativeMan

Question two: I have heard that with CF, one should elevate heater voltage. I was given a schematic of a Marshall Superbass as an example of a CF circuit. Looking at this schematic, I cannot see elevated heater - unless I am misreading the schematic which is possible, or then again it may be incomplete diagram. Or maybe the Superbass doesn't use elevated heater?

If you look in the data sheet, it will tell you , for example, the peak heater to cathode voltage for a 12ax7 is listed as +/- 200 volts. However, it never hurts to elevate the heater voltage to match the CF cathode dc voltage level.

-g

[PositiveNegativeMan]

Thanks for all the information / advice.
I have PT intended for use with bridge filter. So I take it I can drop some of the HT supply onto the heater circuit to raise the voltage?

I take it this means going to DC heaters?

Is there any advantage/disadvantage to raising the voltage in the whole heater circuit as opposed to just on the CF?

Has anybody ever included a section of circuit to switch in/out a cathode load/anode load and so making the triode switchable from a CF to a voltage gain stage? Sorry, probably a stupid question.

[R.G.]

Let me confuse the issue some more.

A power MOSFET makes a good follower to drive tube stacks without using half a preamp tube.

See MOSFET Follies at GEOFEX.

I've used IRF820s for this, as they are really tough, but Zetex and Supertex now make TO-92 plastic pack MOSFETs in 1W dissipation and 500V drain rating that do the job too.

MOSFETs make good followers and as long as you're not listening for the distortion of overdriving a follower, they sound like a cathode follower (which means they have not much sound of their own at all.)

And a MOSFET is remarkably free of the need for elevating its heater voltage.

[KennyBB]

Originally Posted by MWJB

Whether you use it or not depends on what flavour you're after...at the end of the day there are tricks to give the CF circuit a better mids scooped tone, and a plate driven tone stack (BF/SF Fender) a more "tweedy" tone...

What are those tricks that will change the tone of the cathode follower?
Kenny

[MWJB]

"Unfortunately some brands recent production tubes fail miserably in CF position /some of them go bad immediately/ which IMHO now makes elevating heaters mandatory especially if you're a manufacturer.
Mrashall weren't elevating heaters but their recent models are all elevated.
There's a thread on the subject here:12AX7EH CF Failure " - Sure EH have built a sub standard tube in that respect (EH12AX&LPS spiral filament) my advice would be not to use that tube in a CF stage, every other tube works fine. If EH came out with a 6L6 that could only take 400v would it be the manufacturers responsibility to redesign all their amps, or would it make sense for the user to just avoid using them? You have plenty of other choices.

[Steve Conner]

Originally Posted by PositiveNegativeMan

Thanks for all the information / advice.
I have PT intended for use with bridge filter. So I take it I can drop some of the HT supply onto the heater circuit to raise the voltage?

I take it this means going to DC heaters?

Is there any advantage/disadvantage to raising the voltage in the whole heater circuit as opposed to just on the CF?

Has anybody ever included a section of circuit to switch in/out a cathode load/anode load and so making the triode switchable from a CF to a voltage gain stage? Sorry, probably a stupid question.

Yes. If you think about it, you should raise the heater circuit voltage to half of the CF's cathode voltage, not the full voltage. I leave it as an exercise to the reader to figure out why.

No, it doesn't mean going to DC heaters.

You can't easily raise the voltage on just one tube, unless you had two separate heater windings on your transformer.

Pass.

[MWJB]

"Has anybody ever included a section of circuit to switch in/out a cathode load/anode load and so making the triode switchable from a CF to a voltage gain stage? Sorry, probably a stupid question." I haven't seen this done, but it seems feasible, as a cathode follower input would use 1 1/2 12AX7s you could use an unused triode as an input triode with the plate feeding your tone stack. Switch the input of the tone stack from the cathode of one tube to the plate of another. You could have the the volume control following the treble control on a SPST so that it is switched out of the circuit when in tweed mode, or switched in as a Marshall style master volume? A 1Meg bass pot would be overkill in plate driven tone stack, so you'd have to compromise 250K to 500K?

The plate fed stack would need an additional recovery stage to bump up the gain...so your tweed channel would end up with an extra stage...you could probably overcome that with a switched output that bypasses the last triode in tweed mode.

Seems a lot of work compared to using a traditional 2 channel tweed style preamp, with a mid scoop filter/different cathode resistor & cap values, to give one channel a more BF type tone, though (which is the more practical route).

[HTH]

Originally Posted by PositiveNegativeMan

Has anybody ever included a section of circuit to switch in/out a cathode load/anode load and so making the triode switchable from a CF to a voltage gain stage? Sorry, probably a stupid question.

I've seen this done before, but can't 100% recall where - I 'think' it was in a Peavey amp, maybe one of the newer ones like the JSX or something.

[Old Tele man]

...the principle was sometimes used in "signal inversion" circuits...although as I recall, only the SELECTION was switched between Vk or Va...so, effectively, it was still a CK circuit with electronically selected output(s).

[GCA]

Originally Posted by MWJB

"Unfortunately some brands recent production tubes fail miserably in CF position /some of them go bad immediately/ which IMHO now makes elevating heaters mandatory especially if you're a manufacturer.
Mrashall weren't elevating heaters but their recent models are all elevated.
There's a thread on the subject here:12AX7EH CF Failure " - Sure EH have built a sub standard tube in that respect (EH12AX&LPS spiral filament) my advice would be not to use that tube in a CF stage, every other tube works fine. If EH came out with a 6L6 that could only take 400v would it be the manufacturers responsibility to redesign all their amps, or would it make sense for the user to just avoid using them? You have plenty of other choices.

The CF heaters should have been elevated in the first place in the amps that don't. They were saving money by not doing it and just getting away with it... now that there's a crop of tubes that die in there we have the situation where it comes back around to bite in the rear. About the 6L6's, tho... different story

As for me, I'd like people to be able to pop in any 12AX7/AU7 and go, so I elevate. It's not too hard to do.

To return to the original topic though, you only need the recovery stage if the gain suckage through the tone stack is too much in your opinion. I prefer to use the recovery personally because I like to see lots of tubes rather than just one or two

[Gregg]

Sure EH have built a sub standard tube in that respect (EH12AX&LPS spiral filament) my advice would be not to use that tube in a CF stage, every other tube works fine. If EH came out with a 6L6 that could only take 400v would it be the manufacturers responsibility to redesign all their amps, or would it make sense for the user to just avoid using them? You have plenty of other choices.

Unfortunately not only EH are dying as CF. You can't make an amp and say to the customer "The amp is OK but don't use this and that".

As for me, I'd like people to be able to pop in any 12AX7/AU7 and go, so I elevate. It's not too hard to do.

+1

The CF heaters should have been elevated in the first place in the amps that don't. They were saving money by not doing it and just getting away with it... now that there's a crop of tubes that die in there we have the situation where it comes back around to bite in the rear.

+1.

If you think about it, you should raise the heater circuit voltage to half of the CF's cathode voltage, not the full voltage. I leave it as an exercise to the reader to figure out why.

I'm not very much into theory and I don't know why but I'm elevating mine at 80-90V which in my case is approx the half CF voltage. No casualties so far