cathode follower fed tone stack mod in DRRI

Discussion in 'Amps and Cabs' started by SmoothBluesman, Mar 24, 2015.

  1. SmoothBluesman

    SmoothBluesman Member

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    [​IMG]Hello fellows, I have an idea I want to run by some tech minded folk. I never ever use the normal channel in my DRRI. I keep V1 pulled for gain increase for the tremolo/reverb channel. Since I never use the normal channel I was thinking about completely disconnecting the V1 socket and using the V1 socket as a cathode follower for the tremolo/reverb channel's tone stack.

    I want to go one up and make it switchable from stock plate fed tone stack to cathode follower fed tone stack using a high voltage DPDT switch so I can A/B it. Once I decide on what I like best I may make it permanent. I'm afraid the switch may be flipped by accident when powered on in the future.

    I'm very low vision, legally blind since my early childhood and losing vision everyday due to RP, so I had my wife, who helps me with solder work and mods, draw a schematic of what I am describing that I have attached. I hope she drew what I described correctly.

    I will use a 12AU7 as the cathode follower tube. The pins of the unused half would be grounded (6, 7 and 8).
    I would power the amp down and "REMOVE the 12AU7 from V1 before flipping the switch to plate mode and I would power the amp down and flip the switch to cathode follower mode before installing the 12AU7. I could be back up and going from plate fed to cathode follower fed tone stack in just a couple minutes to get an A/B comparison.

    What do you tech minded think. I'm not the best technical minded fellow and amp schematics are fairly fresh with me (imagine my wife trying to explain what goes to what lol). I'm still learning. I believe the trace between C7 and R16 is where I would make the cut to install the diagram I provided. I could keep wire runs short and neat. I think I can install the DPDT switch between the V1 and V2 sockets on the bottom of the chassis.

    Here is a link to a list of schematics on Fender's website. You have to click downloads below the Amplifier schematics drop box. 65 Deluxe Reverb is in the list.

    http://www2.fender.com/support/articles/?category=amplifier-schematics
     
    Last edited: Mar 25, 2015
  2. SmoothBluesman

    SmoothBluesman Member

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    I can't figure out how to upload the picture of the diagram. Is photobucket required? I can upload from my PC on other forums.
     
  3. dharmafool

    dharmafool Member

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    Yes, photobucket or another image-hosting service is required.
     
  4. SmoothBluesman

    SmoothBluesman Member

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    Diagram loaded. What a headache fooling with photobucket.
     
  5. sfryette

    sfryette Member

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    There's a lot easier way to do that which would involve utilizing both halves of the tube and switching between plate drive and cathode drive. A 12AU7 is probably not an appropriate choice because it won't sound great driving a stock tonestack in CF mode or in PD mode. That's going to take some trial and error but the upside is you could switch it easily and not have to do a tube swap and power down/up procedure to switch it.
     
  6. DaveKS

    DaveKS Member

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    Sorry but this priceless, your talking about wiring up your amp so you have to shutdown, flip a switch, pull a tube, then power back up to change modes on your amp but uploading a jpg to a image hosting site is to much of a PIA.

    :rotflmao :rotflmao :rotflmao
     
  7. SmoothBluesman

    SmoothBluesman Member

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    sfryette, thanks for making me think. I'll have a reply with another diagram for you to look at later. I think I have figured out a way to make it switchable without having to remove the tube. All I would have to do is switch to standby to flip the mode switch.

    DaveKS, you wasted your time and my time to post your sarcastic reply. Photobucket is slow, plus I had to bother my wife to help me upload the picture because I can't do it alone. I can't see to drag and drop the picture. My screen reader is no help for stuff that requires sight. On other forums I can upload a picture from my PC directly to the site. Using photobucket is an option on other sites, not a requirement.

    Plus this site times out if the message takes a while to type and signs me out. Even if I check the keep me signed in box when I log in.

    I like the knowledge here on TGP though. That is why I'm here. I'm not here for sarcasm. It is no help to me nor any other reader.
     
  8. sfryette

    sfryette Member

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    I agree - the photobucket business is a PITA, but I suppose it helps keep the forum database managable, so...

    Try this:

    [​IMG]

    Naturally, a CF preceding the stock tone stack will be problematic, but your goal is to experiment and learn something and I applaud your effort, especially given your circumstances. You're fortunate to have a partner who cares enough to support your curiosity. By the way, no need to power down or remove a tube when switching this. Even a garden variety SPDT mini-toggle will suffice. That'll help with your A/B comparison too.

    Some people are too quick to judge and not patient enough to read a post before responding (subtle hint DaveKS, prehaps an apology is in order?).
     
  9. dbeeman

    dbeeman Gold Supporting Member

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    Mr Fryette, thanks for the diagram and circuit commentary
     
  10. DrainBamage

    DrainBamage Member

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    So instead of the second half of the valve being the tonestack recovery stage it will now give more gain and the amp will be dirtier right? I think if I wanted more dirt Id just change one resistor in the feedback first. Unless the CF compression in half of the sine wave is the tone you were hoping for.

    Edit- will the dc coupled CF affect the tone of the first stage? As the Dc is now shared for the plate and the grid of the CF. Curious for my own interest.
     
  11. SmoothBluesman

    SmoothBluesman Member

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    [​IMG]Sorry my wife can't tell heads or tails of the diagram. She really doesn't know how any of this works. lol She drew the last one with me showing her what to do with my fingers. If she can't relate to it I can't learn it. Based on what has been asked by DrainBamage I don't want to change the tone stack recovery amplification (V2B). I don't want to alter the tone of the amp much. Just hoping to improve signal through the tone stack. The cathode follower is the only change I want to the circuit (plus the normal channel disabled and other mods I have).

    I had my wife draw a new, better, diagram with my new idea. I thought long and hard on your suggestion and I think I came up with a solution using a DPDT switch as shown in the diagram. This way the cathode follower tube can be completely bypassed in plate mode and, I think, left in. The bias resistor is left to ground and the second pole is used to switch the cathode wire using the inner lug to one outer lug to do the disconnect of the cathode. I realized there was no need to switch the resistor in and out. I'm still learning this stuff and hoping I'm correct. lol

    Just to clear it up. I'll be using V1 for the cathode follower. Only one half is needed. The other half will be grounded. The normal channel will be completely disconnected (pins 1, 2, 3, 6, 7 and8) to empty the socket for this purpose.
    I'll leave V2A as it is now (plate fed tone stack mode). I plan to just cut the trace between C7 and R16 and install the circuit in my diagram there. The cathode follower would just grab the signal after the plate resistor (R16), be routed to the newly wired V1A, then fed by the cathode follower to C7 to enter the tone stack. Then plate mode would just reconnect R16 to C7 as it is stock when the switch is flipped out of cathode follower mode.

    Do you see any problems with this? This is very much the same thing, without switching, that is done to the Fender Blues Junior by Bill Macron.

    I assume you are saying a 12AU7 won't sound good for a cathode follower because most amps, probably all, that have a cathode follower feeding the tone stack have a voltage amplification stage in the other triode. A 12AU7 sure wouldn't sound good for that. This is where a 12DW7-ECC832 or a JJ ECC823 (reversed 12DW7) can be used.

    About the tube choice for the cathode follower half.
    A 12AX7 has a higher internal impedance than the 12AU7 and it is a voltage amplifier tube instead of a current driver. It isn't a very good option to drive current.
    The 12AU7 has a lower internal impedance and can drive a lot of current.

    Since the purpose of the cathode follower isn't to increase gain(not a voltage amplifier), rather drive signal from the previous stage at slightly less than unity gain (buffer amplifier), I believe the 12AU7 triode would be the best option.

    Keep in mind the other half of the tube will be grounded because it is unused. I could very well use a 12DW7 and make a switchable gain stage too, but I'm not really interested in that. At least not for now. HAHA

    Another interesting thing I may add to make the tube last longer is described here. Speaking of the diode and resistor between the grid and cathode in diagram A.
    http://valvewizard.co.uk/dccf.html

    Link to the cathode follower mod to the Blues Junior on BillMaudio.com.
    http://billmaudio.com/wp/?page_id=141
     
  12. DrainBamage

    DrainBamage Member

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    May I ask why you think that you need to help the signal through the tonestack? The 1 tube tremolo channel preamp is simply enough to feed the tone stack then the recovery stage is there to reamp the lost tone stack signal to drive the powerstage. Im not really sure there would be that much of a difference if everything else was equal. I think the BillM article says that more or less.

    To be honest the risk reward for the cathode follower probably isn't worth the time or the possibility of devaluing / messing up the amp including more possibility noise. Not to mention it sounds like a fairly involved project. Do you and you wife have any experience working on high voltage amps?
     
  13. SmoothBluesman

    SmoothBluesman Member

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    I replied last night. I guess it didn't go through. I thought it did.

    Yes sir, we have experience working on tube amps. Thank you for watching out for our safety.

    Here is a mod we did.
    http://www.strat-talk.com/forum/amp...6-tremolo-disconnect-switching-your-drri.html

    As you can see I don't care about devaluing the amp. I want it to sound the best it can while looking stock up front. The rear, underside and inside, who cares how it looks.

    We've also installed most of the 64 Vibroverb Custom values inside. I have a larger OT installed and instead of putting two 220uF 285V main filter caps in series I did away with the balancing resistors and put two 50uF 600V caps in parallel. It made more sense to do that to me and removing the two balancing resistors gave more room to fit the larger caps.

    I just need to have confirmation from people more knoledgable than me with amp circuitry to guide me before we do this cathode follower switching mod on the DRRI. We have done the cathode follower, without switching, on a Blues Junior before. There is instruction on Bill's site for the Bj though.

    The difference in tone isn't huge, but you know how we guitarists are when it comes to tone. I don't want a huge difference anyway. I'm hoping for a suttle improvement of the same sound.

    I like to think of the purpose of the cathode follower as being similar to putting a good buffer in front of a pedal board with all true bypass pedals. It does make a difference in the bypassed tone in a good way. A low impedance output can drive a lot more signal through a heavy load. And true bypass pedals really aren't even a heavy load, but I like the difference. I know it isn't the exact same thing, but the reasons are similar, IMO.

    Back to post 11.
     
  14. SirGilmour

    SirGilmour Supporting Member

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    And you just wasted my time by responding and making me read this!
     
  15. SmoothBluesman

    SmoothBluesman Member

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    Yes, I agree, but you just wasted others time by replying it with a quote and replying to it. And oops, I just did the same. lol See folks how a sarcastic comment can derail a thread?...

    Back to post 11. lol
     
  16. SirGilmour

    SirGilmour Supporting Member

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    Ha! Good stuff here. Ok back to the main article. Seriously.
     
  17. DrainBamage

    DrainBamage Member

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    Sorry I cant help with you CF idea. Try the ampgarage as they are builders who would be of more help. Fwiw ive never seen an amp with 1gain stage then a CF for the tone stack the way you want. Dosnt mean its not out there but theres probably a good reason why I haven't seen it
     
  18. SmoothBluesman

    SmoothBluesman Member

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    [​IMG]
    I've been studying.

    I would think the fact there is only one gain stage before the tonestack would be all the more usefulness of the cathode follower. The only downside could be noise, maybe, since the signal isn't amplified as much as in a design with two gain stages before the cathode follower. I think it's worth a try. I do want it switchable just in case it isn't worth while. I need to be able to A/B it to get a good and fair comparison. Plus I want the stock plate fed tones even if the CF tones are great.

    From what I've learned this should work. It is a DC coupled cathode follower. The plate of the first gain stage (V2A) will feed the grid of the cathode follower tube (going through the DPDT). The cathode of the cathode follower tube will then drive the tonestack (through the DPDT) where C7, the treble cap, will block DC voltage and only allow the AC voltage through. The same goes for C8 and C9 as DC will pass through the slope resistor, but not the mid and bass caps. DC makes it that far in plate fed mode too, so there is no difference.

    I think I have a place to get filtered B+ to the cathode follower tube really easily and neatly. I have to do more studying, but I think I can disconnect the trace to the normal channels tone stack (X marks the place where I would cut the trace between R4 and C2/R6), remove and jumper R4 and that should put filtered B+ on the plate of V1A. I'll have to study and check to see, but I think that does it. This will be very convenient because the trace from R4 goes directly to the plate of V1A already.

    Also, with my better understanding, I believe I can feed the plate of V2A to the DPDT from pin 1 on the V2 socket so the wire run will be short. I was originally gonna feed it from R16, but R16 goes to the plate anyway. This eliminates a wire over the PCB that I was gonna run from R16 to the DPDT. I can get the signal directly from the plate of V2A to the DPDT to feed the grid of the cathode follower. The trace from R16 to the wire that goes to V2A pin 1 will be left in place to power the tube.

    The connection between R16 and C7/R18 will be broken (X marks the break) and the DPDT, as in the diagram), will be wired to the trace that goes to C7/R18 (a single wire over the PCB). This is the wire that will feed the tone stack from the DPDT.

    I then need the cathode follower's bias resistor to ground. I can remove a component for the normal channel for this (the stock cathode bypass cap for V1A, C1). I will install a 100K 1/2W resistor in place of C1 and I have to remove R5 as well so it won't parallel the new 100K cathode bias resistor. The lead spacing of R5 is too narrow for a 1/2W resistor, so C1 is a better location for the new 100K resistor. This location is convenient because it goes directly to pin 3 of V1A already.

    I'll remove the wires from the plate, grid and cathode of V1B (pins 6-8), jumper them and run them to ground where R5 was. Since that half of the tube will be unused the elements need to be grounded.

    V1A's grid, pin 2, has to be disconnected from the input of the normal channel. I'll just remove that wire and use it somewhere when wiring the switch. We certainly do not want high voltage on the normal channel input jacks.

    Then wire the DPDT, like in the diagram to V1A and it is done. We'll have to unsolder pin 3 on V1A to put the shared wires for the cathode bias resistor and the cathode follower wire to the DPDT in it. We'll also have to unsolder pin 1 on V2A to put the wire that feeds signal to the DPDT in with the wire from R16.

    That does it utilizing supplied traces and wiring as much as possible so there shouldn't be any problems with unwanted noise. Wire runs will be short, several using the stock wiring, and only a single wire going over the board to feed the tone stack of the tremolo/reverb channel. Pretty clean, me thinks.

    With a flip of a switch I should have CF mode and another flip should give me stock sounding plate mode. I was gonna put it on the bottom of the chassis, but now I'm considering putting it on the rear since the tube can be left in.
     
    Last edited: Mar 30, 2015
  19. SmoothBluesman

    SmoothBluesman Member

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    I made an edit to the image to correct a mistake. So if anyone had looked at it and was confused I think it should be clear now. We had put V1A where V2A belonged in the diagram.

    Oops, made the same typo in the text too. I think it's all fixed now.
     
    Last edited: Mar 30, 2015

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