Bootstrapped Cathode Follower with 12DW7?

[jbefumo]

Did a search on the relevant terms and there was nothing recent, so ... I have a working amp [yay!] that uses a standard cathode follower design, but am considering redoing it with boostrapping as discussed in Merlin's book and website.  (image attached)


Was thinking about maybe using a 12DW7, with the 12AX7 half as the gain stage, and the 12AU7 side as the follower. Maybe a cathode resistor of about 18K?


Anyway, first question: Is there any benefit/detriment in doing so? (It's driving a single-ended KT-120).


Second question: If I do, should I change the value of that 47n bootstrap cap?


thanks!

[jjasilli]

You're pretty deep into the weeds here!     Merlin gives actual values at Fig. 5.22, p. 134.  With a gain chart per frequency on the next page for different C1 values. 


But I think you need to work backwards from the power tube.  I.e., KT-120 specs, single tube class A show -14V on G1.  Can this be right??? Seems low.  Anyway, usually double the bias voltage for signal voltage.  At some point the power tube will be swamped with excess voltage on G1.


The bootstrapped cathode follower should yield a mu of about 90.  So, only 1 signal volt into the 12ax7 drive may output 90V to the power tube's G1.  Is this too much?

[Paul1453]

I have no idea about your question, but I did learn something from it.

I have a few Hammond/Amperex 7247's which I now know are also called 12DW7's. 

[jbefumo]

Definitely deep into the weeds, and that's why I asked.  Thanks!  You did give me the basic direction that I needed.  Truth be told, this is the frontier of my comfort zone -- Been through his PREAMP sections pretty thoroughly, but from the phase inverter (if any) on backward, I'm guestimating.  The history of this, beyond an vague obsession to build something that I couldn't just go out and buy, is that I've had the single-ended transformer for years, looking for a project.  I finally decided to just buy one of Doug Hoffman's 5E3 boards, and built, essentially, a champ-deluxe hybrid -- size, power, and front end of the Deluxe, but with a single-ended back end like the Champ.


Had already populated the board when someone on this board pointed me to the Little Wing design, and I did a last-minute course change.  I had planned to use the 6SL7GT octal preamp tube in the 'ChaLuxe', and have used parallel preamp-first stages before and liked it, so that's where that came from.  Didn't need a phase inverter.  Wasn't really sure of putting the tone controls after the cathode follower, without a phase inverter between it and the power section, so I just moved it up after the first preamp stage.  Thought about using the second half of the dual triode as an extra gain stage (which is always a possibility), but had some ill-defined sense that being able to deliver more current to a big-ass tube was probably a good thing....   Now, AT the power amp part of the topology I'm a somewhat better footing, though not as much so as basic triode and small signal pentode stages, so yeah -- hand waving and "management by magazine" ....

You're pretty deep into the weeds here!     Merlin gives actual values at Fig. 5.22, p. 134.  With a gain chart per frequency on the next page for different C1 values. 


But I think you need to work backwards from the power tube.  I.e., KT-120 specs, single tube class A show -14V on G1.  Can this be right??? Seems low.  Anyway, usually double the bias voltage for signal voltage.  At some point the power tube will be swamped with excess voltage on G1.


The bootstrapped cathode follower should yield a mu of about 90.  So, only 1 signal volt into the 12ax7 drive may output 90V to the power tube's G1.  Is this too much?

[jbefumo]

I think it's also ECC832 or 823 .... there are two variations, one with the AX7 in the 'A' section, and the other the mirror image.  I think the 832 is the DW7, but there is no US equivalent designation of the other.  I've used them as reverb tubes, driving with the AU7 and recovering with the AX7 sides, and it works decently. 

I have no idea about your question, but I did learn something from it.

I have a few Hammond/Amperex 7247's which I now know are also called 12DW7's. 

[tubeswell]

What jjasilli said. We need to understand a bit more about the application of your 12DW7.


On top of that, optimum load resistance values for the 12AU7 triode of the tube are different to a 12AX7. I note Merlin also states (at p134 in his 1st edition pre-amps book) that if using a lower Ra triode for the CF, the positive-going output signal swing from the inverting stage will be restricted more by grid current limiting in the CF stage (and a 12AU7 triode has much lower Ra than a 12AX7).

[jbefumo]

Just got it up and stable. Just left the 12AX7.  Measuring -13.7 on G1    I measuure 115mA through the cathode.  G2 is at 215V. Plate is at 440V. It gets pretty loud, and stays clean up to about 70% & never goes beyond a healthy crunch.  Got no tone control in it at the moment -- scavenged it for another amp & waiting for parts, so that will tame it a bit, though I'm using a pull-bypass on the (BMP) tone control.

OT is an Edcor 25W SE, speaker is a 100W Celestion.  Goal was to have something pedal friendly, nice chimey clean, and enough oomph to keep up with a drummer -- As soon as I get it closed up I'll shoot some pics/vids/audio. 

You're pretty deep into the weeds here!     Merlin gives actual values at Fig. 5.22, p. 134.  With a gain chart per frequency on the next page for different C1 values. 


But I think you need to work backwards from the power tube.  I.e., KT-120 specs, single tube class A show -14V on G1.  Can this be right??? Seems low.  Anyway, usually double the bias voltage for signal voltage.  At some point the power tube will be swamped with excess voltage on G1.


The bootstrapped cathode follower should yield a mu of about 90.  So, only 1 signal volt into the 12ax7 drive may output 90V to the power tube's G1.  Is this too much?

[jjasilli]

Measuring -13.7 on G1:  I assume that verifies the bias voltage mentioned in Reply 1.

stays clean up to about 70%. . . .:  from this info it's not clear if crunch arises in the preamp, power amp or both.

[jbefumo]

Measuring -13.7 on G1:  I assume that verifies the bias voltage mentioned in Reply 1.

Yes -- actually, after some fine tuning it's now at -16 or so, but yeah, that's what the datasheet says.


stays clean up to about 70%. . . .:  from this info it's not clear if crunch arises in the preamp, power amp or both.


Will have to investigate further to be sure, my guess is the parallel first preamp stage is going to have lots of clean headroom.  Right now I have no tone control in there, so the possibility of driving the second preamp stage into distortion is not unlikely, but the preamp stage should be capable of pushing the back end in either case.