Cathode Follower Tube Rolling?

Discussion in 'Glowing Bottle Tube Amp Forum' started by ElliotKnapp, Jun 12, 2021.

  1. ElliotKnapp

    ElliotKnapp TDPRI Member

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    Howdy, I'm curious if anybody out there has any experience rolling 12AX7s in the cathode follower position, particularly one that's buffering a tone stack like in a 5F6-A Tweed Bassman or similar designs (for example, I've built Trinity 18-Watt Plexi and TC15 [Vox clone] amps with a similar cathode follower).

    Since the cathode follower doesn't really add gain, I'm curious if it even matters tonally what kind of 12AX7 is in there, or if different makes of 12AX7 have different tone or effects (e.g. touch sensitivity, etc.) in the position. Anybody out there have experience trying different new production or old stock tubes, and any descriptors for how specific 12AX7s sound in there?

    Request: There is a lot of anecdotal discussion out there claiming that a lot of new production tubes (especially newer Russian-made 12AX7s with spiral filaments) can't stand up in the cathode follower position. PLEASE, I'm not really interested in re-hashing this discussion and making this thread about which tubes can survive as a CF, I'm just curious if you've compared different tubes tonally as cathode followers and what you think. For what it's worth, the consensus from my reading on the "which tubes can survive cathode follower" subject is that Sovtek 12AX7WA, WB, and WC tubes, any Chinese 12AX7s, and pretty much all NOS 12AX7s can handle use as a cathode follower, though plenty of people claim to have used the "not good" tubes without problems.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Well, thanks for the inspiration to read a bit more than I have read Before. Lots of discussions…I tired early—-rehash, right—-but went
    To The Valve Wizard to see what he says. Interesting. I had always thought that the 12AX7...old U.S. And western Euro production is my only experience…was the preferred tube there due to the heater to cathode voltage difference ability.
    the Valve Wizard stays the 12AX7 is acceptable/usable in a guitar amp but the current handling ability is better in a 12AU7, which has a heater-to-cathode rating of 200v as does the 12AX7, is better suited to a cathode follower application interesting. The voltage was always my concern, but the 12AU7 has more current handling ability. Maybe I need to try some 12AU7s in a cathode follower??
    so many questions….no answers….just thanks. I learned something. Now, I may go read what Aiken has to say about such a circuit. I have never lost a cathode follower tube…everything has always been old production. I am of an age that I will have an old tube for that application if needed. I read that Mesa had gone to using the JJ spiral filament…oddly enough…after using Russian and Chinese tubes in their cathode follower amps.
    I am subscribing to this thread to see what experiences those who have had occasion to try some different tubes in such a circuit.
    What have you found with subbing in different tubes there? What amp with a cathode follower do you have?
     
  3. powerwagonjohn

    powerwagonjohn Tele-Holic

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    I also wonder about using a 12AY7 as later tweed amps used. I have rolled 12AX7 in my Allen Classic 57[tweed bandmaster] when I first bought it but I didn't really notice a difference in tone between new production, old production RCA, Sylvania or a GE so I stuck with an old production GE. I was mostly listening to the noise floor. Now I am ready to try a NOS Sylvania 12AY7. I will also try these variations in my 5F6-A clone. I guess I will have to try a 12AU7 also. When I get a chance I will try it and report back.
    thanks John
     
  4. Wally

    Wally Telefied Ad Free Member

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    in what position????
     
  5. powerwagonjohn

    powerwagonjohn Tele-Holic

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    I thought we were discussing tubes used in a CF position. Some of the Supers and others used a 12AY7 in this position.
    Thanks John
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Thanks, John. Yes, Leo used a 12AY7 in that position for several amps…the first cathode followers in the 5D Bandmaster and Twins, the 5E4A Super, 5E5A Pro, 5E8A Twin, 5E6A Bassman. It is interesting that except for the 5E5A Pro, all of the last versions of all of those amps carried a 12AX7 in that position. I have owned a 5E5A Pro, 2 5E7 Bandmasters, a 5E8A Twin, and a 5F6A Bassman. These are all the final versions of these tweed amps. Only one of them used a 12AY7 in that position. They were all great amps. My favorite of them all is the Bandmaster. It is very similar to the 5E5A Pro but for that one tube. I prefer the Bandmaster, but someone else might prefer the Pro. I never considered changing the tube in that position, but I might find it interesting to do so IF I had the Pro because I liked what the Bandmaster did.
    That said, yes, one could use a 12AY7 there. That much has been proven by Leo Fender back in those days. However, the 12AY7 has a 90 volt heater to cathode rating and that is low for the application if one takes the specs into consideration. The 5E5A Pro schematic calls for 130 volts on that cathode. One wonders why Fender was using a tube that was so far out of spec for the application? Hey…he pushed the 6V6 beyond its specs, too, though, didn’t he?
    Fwie, I provided a 12AZ7 to our brother @Bendyha for his investigation of a lower gain—-60— tube with a heater-to-cathode voltage rating that is more fitting for such an application. I do not recall what results he found or if indeed he ever tried the tube.

    Thanks again, powerwagonjohn, for your kind response.I was just beginning my cup of morning tea and should have reviewed the few posts in this thread before asking. However, your response has caused me to review what Leo did back in the day….and to come to realize why I prefer the 5E7 Bandmaster to the 5E5A Pro. I preferred my two Bandmasters to the 5E8A Twin, and I owned those three amps at the same time. The 5F6A Bassman is a different animal compared to those in that it has a LTP phase inverter, so I am not comparing it.

    IF one were to try a 12AY7 in one of those amps, I personally would think one would want to use a NOS tube. Those are pricey tubes, ime. I have five of them…four 6072s and a Tung Sol 12AY7. Imho, they are applicable and valuable in the V1 positions of such amps.
    JJ sells a 12AY7/6072, and they spec them at 90 volts for that heater to cathode voltage.
     
  7. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    ...for use in my Lead 75W, where yes, the 12AT7 (rated for 90V) was being used with ca.200V on the cathode, and although it was working, I feel a lot better with your 12AZ7 in there (seemingly made for the job by being rated for 200V Cath. to Fil.) and although I don't use the amp often, it does sound fine.......thanks again Wally for that tube.:)
     
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  8. Wally

    Wally Telefied Ad Free Member

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    Tangential offshoot…informative and temporary hijack. That is high voltage for that phase inverter. The schematic shows 110vdc like other AB763 based Fenders, in which I have come to understand many people have used the 12AZ7. Why is that voltage so high in your amp on that phase inverter’s cathodes?
     
  9. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I can't remember the measured voltages for the amp, I will note them the next time I have it open.

    Indeed, the schem shows 110V on the PI Cath, which might be reason enough to us an AZ in-place of the AT.
    But in this case it is the second half off the reverb driver AT, the stage that is used as the "Lead" gain, 3B, that has the higher voltage Cath........BUT......this depends on which schem you look at. :confused:

    upload_2021-6-15_8-40-38.png upload_2021-6-15_8-41-45.png .....can't recall which is correct but there the tube sits, and works a treat, and helps sooth my nerves.

    200V on one Cath. of the tube, and 5V on the other, means that heater elevation to the middle would then put both halves out of the safe limit. With a LTP PI, both Caths. are tied, at the same voltage, not too far over the limit, and heater elevation would be no problem to implement.
     
    Last edited: Jun 15, 2021
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  10. Wally

    Wally Telefied Ad Free Member

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    Thanks, Bendyha. Neither of the schematics show what we see in the schematic to the right there. I will keep this info in mind when I next look at a 75.
     
  11. powerwagonjohn

    powerwagonjohn Tele-Holic

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    Elliot and all, I did some tube rolling the other day in my 5F6A clone, actually a Chicago Blues Box BG-60. Now I am not really a tube snob but I do like to run vintage tubes in my vintage amps so I do have a small stash of good old tubes. When swapping tubes in the CF position I did notice a difference in 12AX7 tubes. The new production EX and JJ tubes were bright but a bit harsh on breakup while some top seam Canadian tubes I have, I don't remember who made these but they test as new, sound very good but slightly less bright. The best to me were some Sylvania 12AX7A tubes that I bought several years ago in a new sleeve and these test as new also. The difference in these tubes were minor to my old, 64 year old ears but discernable.
    The biggest difference was installing a NOS Sylvania 12AY7 or a like new ANOS Mullard 12AU7. The AY and the AU has progressively lower gain. Since the 5F6A has plenty of volume and I don't go into high gain territory often I left the 12AU7 in place and I will experiment a bit with this set up.
    So for now I have a ANOS Sylvania 12AY7 in VI and a ANOS Mullard 12AU7 in V2 and a NOS Sylvania 12AX7 in V3. I may have a couple hours tonight to revisit this set up and see if I hear things the same as last week.
    I am not sure if this is sound engineering but this is what I experienced.
    Thanks John
     
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