cathode follower billm

Billm AudioSmall amps, big sound.

The Cathode Follower Modor, what does this mod actually do?Almost everyone who has looked into the heart of a Blues Junior has noticed that extra half of a12AX7 thats just sitting there, doing nothing. There are lots of things it could be doing a secondinput, a line input, another gain stage. Or take a page from the venerable Fender Bassman, anduse it as a cathode follower. The cathode follower is a non-gain stage that separates thepreamplifier from the tone stack.Why would you want to do that? The tone stack, with multiple paths to ground through capacitorsand tone control potentiometers, places a rather heavy load on the previous preamp stage. Thepreamp stage is high impedance and does not drive a load all that well. The cathode follower hasvery low input capacitance and very high input impedance, so it is a very light load for the preampand a better match impedance-to-impedance than preamp-into-tone stack. On the other side, ithas very low output impedance and is a powerful current source, so it can drive the tone stackeffortlessly.So why wouldnt you want to do this?The biggest reason is that it doesnt sound all that different. The theoretical bark is worse than theactual bite, and despite all the impedance mismatches, lots of signal gets through to the tonestack. So you have to question whether its worth the effort. On the other hand, nobody everaccused a Bassman or a Marshall (which copied the circuit) of sounding bad.Mark Huss did this mod on the green board Blues Junior years ago. He has instructions and somesound clips on his site:http://mhuss.com/BluesJr/index.htmlAt a customers request, I recently re-engineered and adapted the mod to the cream board. Itsnot all that difficult. Heres how, below. Ive also re-engineered Husss mod for the rev. C and Dcream board. Photos are at the end of this page.

Pagina1 di 22Billm Audio Cathode follower mod for Fender Blues Junior

12/04/2014http://billmaudio.com/wp/?page_id=141

This is the circuit were trying to create. I cut-and-pasted another tube section into the Blues Junior schematic.Its laid out just like the schematic for the Bassman, and its technically a DC-coupled cathode follower. The half-tube connected by the colored lines is the unused half of V2, designated V2A. The only additional component,other than the wires, is the cathode resistor. Although its unmarked on the schematic, its 100K, just like theplate resistor of V1A. This may seem strange if youre used to cathode resistors in the 1K-2K range, but the high-potential balance between the plate and cathode is essential to the cathode followers operation.The voltage drop across the cathode resistor raises the cathode to a high voltage that gets modulated bywhatever signal appears on the grid. Unlike amplification stages, the cathode follower does not change thephase of the signal. What goes in is what comes out.V2B, by the way, is the tone stack recovery amplifier.(Note: If youve changed the plate resistor to some other value, as with the high-voltage preamp, the newcathode resistor needs to be the same value.)



This is the stock circuit board. The first step is to clip both ends of this jumper, close to the board.This jumper is very convenient for our purposes; its the path from the output of V1A to the tone stack. Well feedthe cathode follower from one end and feed the output of the cathode follower to the tone stack with the other.Its essential to clip components when desoldering them on printed circuits because the leads are typically bentonto the trace on the back of the board. Trying to pry or wiggle the entire part loose while keeping the soldermolten is an excellent way to tear the solder pad from the circuit board.



Ive removed one of the stubs from the jumper and am about to remove the other. A solder sucker is the bestway to do this because you dont have to linger on the joint and it cleans out the stub and the excess solder.



This trace brings filtered high voltage from the X point of the power supply to the plates of V2 and V1, but onlyhalf of V2 is suppliedno sense in sending power to an unused portion of the tube. Ive marked the best place totap into the supply.



I used a #60 drill in a Dremel tool fitted with an inexpensive accessory chuck. The chucks are readily available bymail order and in home centers and hardware stores.If you drill at low speed (I use a variable-speed foot controller), you have excellent control over the drill. If youstart at high speed, it could skitter off over the surface of the circuit board.



We also need a place for the new 100K cathode resistor. I created a pair of holes, one that connects to the newwire that will go to the V2A cathode, one that connects to the diamond-patterned ground area on the circuitboard.Scrape away the green coating so you can make a good solder connection. A 1/2 watt resistor is a perfect fitbetween these two points.



The brown wire will go to V2As cathode; the yellow wire (green on the schematic above) goes to the grid.



But first I had to remove the ground connections between pins 1, 2, and 3 of V2. If a tube section is unused,good design practice is to short all of the elements to ground so there is no chance of parasitic oscillation orother electronic weirdness.Always test with an ohmmeter to make sure there is no hidden thread of copper connecting the pins.Ive desoldered pin 1 here. Theres a nice pocket right next to the socket pin. When I desolder the other two, Illbe able to poke the ends of the wires down along the pins and solder them securely.



All finished! The new 100K cathode resistor is under my thumb; sorry for not having a clearer view. The red wirecarries the plate voltage, the yellow wire connects the plate signal from V1A to V2A, and the brown wire carriesthe signal from the cathode to the tone stack.You can see how the cathode follower essentially replaced the jumper wire, with the addition of plate power andthe 100K bias resistor.Rev C and D green board cathode follower:



On the rev C and D green board, the layout isnt nearly as convenient as that of the cream board. The first step isto tap into the trace from the plate of V1A to the grid of V2A, our cathode follower. Huss did this on the tubeboard; so did I. The yellow wire runs from pin 1 of V1A to pin 2 of V2A. Note the cut traces between pins 1, 2,and 3 of V2A. including the ground.



The trace where the red wire goes into the circuit board is power to the preamp. We tap into it and send it to pin1, the plate of V2A. The trace just above it is ground. One end of the cathode resistor goes into the ground trace.The other end is in an open area of the board. holes for the brown wire and the black wire shown in the nextpicture are drilled next to the lower end of the cathode resistor and all three are connected together.



The black wire sends the signal developed on the cathode into the Blues Junior tone stack. Note that the tracebetween the black wire and the plate side of R10 is cut. Thus V1A receives power from R10, but the signal fromV1A never reaches the tone stack. Instead, the yellow wire (above) carries it to the grid of V2A to drive thecathode follower.One final thought:The 12AX7 in V2 may not be the best choice for a cathode follower, since the 12AX7 is designed to be a high-gain voltage amplifier. Its not designed to supply significant amounts of current. It also has high internalimpedance, which is part of the problem were trying to solve in the preamp/tone stack interface.The cathode follower, however, is a current amplifier, not a voltage amplifier. Other tubes in the 12A*7 family arebetter suited to deliver current, most notably the 12AT7 and the current-driving champ, the 12AU7. They havelower internal impedance and are designed to deliver significant amounts of power. Putting a 12AU7 or even a12AT7 into V2, however, would reduce the gain of the tone stack recovery stage, exactly like turning your mastervolume down halfway or more. Thats a bad idea.Theres one tube, however, that is ideally suited for V2 in the Blues Junior: the 12DW7, also called an ECC832.Its a hybrid tube; one half is that of a 12AX7 and the other is a 12AU7. Its specifically designed for tasks thatrequire an amplification stage and a driver stage. Fortunately, the triode on pins 6, 7 and 8 is the high-gain12AX7 side, a perfect match for V2B. The section connecting to pins 1, 2 and 3 is the 12AU7 side, perfect for acathode follower.

Whats the Difference?So does the cathode follower actually sound any different? My customer was looking for more touch-responsiveness and feels that cathode follower tone stacks deliver this. I couldnt say for sure, but I felt that thefollower gives the amp an additional measure of clarity, especially in the bass. I havent done an analysis of thetone stacks impedance, but it stands to reason that the modded tone stack, with the increased bass and mids,loads the preamp more on the low end. As a result, you get unequal amounts of distortion on the low strings vs.



the high strings. Remove the load on the preamp, and everything evens out. This is not an easy mod to switchon and off, so I couldnt do A/B comparisons. But Ive heard enough Blues Juniors that the tones are pretty muchengraved into my brain. This mod slightly increases clean headroom; the amp takes pedals better.Swapping the 12AX7 for a 12DW7 (actually a JJ ECC83 and a JJ ECC832) did make a small additionalimprovement, but it may just have been the small, but audible difference between the two 12AX7 stages in V2B,the tone stack recovery stage. Then again, the internal capacitance of the 12AU7 stage and the 12AX7 stage isdifferent enough that it could also affect tone.My conclusion: If you make the effort to do this mod, you should try a 12DW7 or ECC832 to see if you getadditional benefit.

61 Comments1. Scott Pope says:

January 20, 2009 at 11:06 pm12DW7?! All right! Old Ampeg tubes are NOT dead after all! Im actually going to have to getone now. Now I know why my old SVT head that had been modded from 12DW7s to all12AX7s in the front end, ostensibly to make it easier to get tubes in a pre-internet world inthose years of wandering through the desert tube-wise, after most of the USA & UK tubefactories shut down, but before overseas factories got their act together, sounded a littlemore harsh than others. I have known for years about the gain stage differences in 12A*7tubes, but didnt know about the current characteristics. Did I say Im now going to have toget one?

2. Scott Pope says:January 28, 2009 at 11:13 pmUpdate: the 12DW7 came in. Along with the tube, I got a 47pf capacitor, and modded C2 to apair of jumpers to a DPDT mini switch installed next to the input jack so I can have aswitchable bright and brighter switch with a choice of the stock 100 pf or a total 147 pf.With this configuration, and the preamp tubes going 5751, 12DW7, 12AT7, and this brightswitch in, I am getting a rich, silky, clean sound with just the right sparkle that is almostidentical to a 1978 Fender Silverfaced Vibrolux I bought brand new and had to sell a fewyears ago to make a house payment when I was between jobs.

bill says:January 29, 2009 at 11:58 amNote: Scott has a green board and the capacitor in question is the bright cap on thevolume control that keeps trebles alive when you play at low volume. The effect of thispot diminishes as you turn up the volume. The equivalent pot on the cream board is C3.

3. Ron says:February 4, 2009 at 6:01 amWithout making this mod. Would you still suggest the 12DWY or go with the 12AT7 in the V2position? I have a non-mod Blues Junior and was looking to change V1 to 12AY7, V2 & V3 to12AT7. Your thoughts and suggestions please. Thank you.

bill says:February 4, 2009 at 8:11 amThe What About Tubes page has more info on general tube substitution. The 12AT7 is adriver tube, not a preamp. Use a 12AY7 instead to preserve frequency response andtone. If you want darker tone, turn down the treble. The 12AT7 can be used in V3 toreduce drive to the output tubes, but a 12AY7 would work as well, without dulling thehighs. There is absolutely no point in using a 12DW7 in V2 without doing the cathodefollower mod.



4. mike@frog_corner says:August 10, 2009 at 7:57 amBill, another possible use for the unused section of V2 would be for valve reverb. In thatsituation, the 12AX7 would be fine. This is a mod that is incorporated into Torres BBQ Bluemods. Ive just got my hands on the hand wired version of the kit so Ill post back how valvereverb sounds. Im not a big fan of the stock reverb sound anything past 3 and I feel Ishould be wearing shorts and a Hawaiian shirt!

bill says:August 10, 2009 at 10:21 amYou realize, of course, that the Torres BBQ Blue isnt a modded Blues Junior in anysense of the word. He guts the amp and installs a new circuit board. So you have adifferent amp in a Blues Junior box.Ive seen others use the unused half for reverb recovery, but youre still left with solidstate drive. If you think the stock reverb comes on too strong, you can install the audio-taper pot, which extends the low end greatly. Also, the basic mods deepen the tone ofthe amp, which has the byproduct of improving reverb tone, too. Finally, the Rubyreverb tank has longer delay and deeper, slushier reverb. It works well in combinationwith the audio-taper control.

5. mike@frog_corner says:August 11, 2009 at 4:42 amYes the Torres kit wont be a genuine BJR for sure! A comparison of his schematic with thefactory schematic shows a much simplified preamp circuit, though the reverb appears to bevalve driven as well as valve recovered- output from V2A into a reverb transformer throughthe pan then on to V2B. The audio taper pot is a definite upgrade! Ill check the Ruby Tubetank also.Im doing the build myself and planning to incorporate several of the mods you havedeveloped. My plan is to substitute 6V6s in the output stage and hopefully achieve moreclean headroom. I was planning a 6V6 kit build when I read your A Very Special BluesJunior and was inspired (or deluded). By building a hand wired board (and with carefuldisassembly) I can always restore the BJR to its present state if the results are truly awful!Thanks for the many useful tips you have generously given us over the years. I will email youseparately about mod kits for the Torrestein!

6. Roki says:January 21, 2010 at 5:42 pmAnd if you cant find 12DW7 or ECC832 anywhere, go to Hammond organ repair shop and askfor 7247. That quite rare tube seems to have many names. Found that out only few daysago. (Could have helped a Ampeg owner on tour earlier with that knowledge, since we had7247s for Hammonds but no information about 12DW7)

bill says:January 21, 2010 at 7:24 pm12DW7 and ECC832 are now readily available from online tube vendors. Thanks for the7247 tip!

7. JesseO says:January 22, 2010 at 12:23 amI just did this mod but havent tryed it yet because i ordered some of your other mods and imnot gonna close it all up just to open it again, but does the 12DW7 make a large difference, ijust bought new preamp tubes but if it does ill get the 12dw7

bill says:



January 22, 2010 at 8:39 amThe benefit of the 12DW7 is theoretical. I dont really hear a difference.

JesseO says:January 22, 2010 at 12:51 pmalso should i twist the wires to reduce hum?

bill says:January 22, 2010 at 1:41 pmNo need to twist the cathode follower leads. Just position them so that theydont parallel one another for any appreciable distance.

8. Sho says:February 27, 2010 at 1:24 amfirst question, did the fender 57 Twin (5E8A) have a cathode follower? i think it does butnot quite sure. can you clarify please?second question, the main part of the cathode follower is that the resistor between pin 1 and2 should be equal to the resistor between pin 3 and ground?finally, your wires you used look pretty thick. can I use a 22~24 AWG stranded wire for thismodification?

bill says:March 1, 2010 at 9:05 amYes, the 57 Twin used a cathode follower to drive the tweed tone stack. Yes, theresistors should be equal in size for a full, symmetrical waveform. The wires I use are 22gauge unistrand with 600V insulation. Theres vibration on any freestanding wire in acombo amp, so the heavier insulation helps to damp the vibration. I also reinforce theconnection on the main circuit board with a dab of hot melt glue.

Sho says:March 1, 2010 at 6:25 pmHey Bill, thanks for the response.I installed the cathode follower this morning, and heres my outcome:with the tone stack mod (.1uF bass cap), the amp sounds very very sharp. Itsounds somewhat metallic. I guess its because the increased bass cap, but thebass knob governs bass AND low mid frequencies. From this, the mid knob governsthe higher mids and some treble. Treble knob controls the chime. What could I doto get a bit less treble? What do you think of replacing the bass pot with a 1M, andthe mids pot with a 50k?Thanks Bill

bill says:March 2, 2010 at 8:40 amDid you do the TwinStack mod? That gives you more separation andindependent operation of the tone controls. A 50K mids pot with the TwinStackis sort of a super mids control. I do that on harp amps along withthe .015uF / .1uF tone stack mods. I havent found it as useful on guitar, but ifyou like more mids, its worth trying. I think a 1M pot in the bass will throwthe stack out of whack. You can model the changes with the Duncan ToneStack Calculator.I cant think of any reason that the amp would sound more metallic with thecathode follower.



9. Jason says:February 28, 2010 at 11:36 pmWhy not turn the unused half of the 12AX7 into an extra gain stage? Would it make adifference? Could you use the farty fat foot switch to trigger it instead of the farty fat =pThanks!

bill says:March 1, 2010 at 9:07 amWith the other mods, the Fat switch is not as farty. Turning the extra triode into a gainstage is a lot of work because you probably dont want it there all the time. Making itswitchable is a hassle. Youre better off getting a Hot Rod Deluxe and saving the effort.

10. wm says:April 27, 2010 at 8:50 pmCould the other half of the tube power a tremolo circuit?

bill says:April 29, 2010 at 8:45 amSurea tube is a tube. Youd have to figure out a way to do bias modulation or optomodulation, though. The bigger problem is where to mount two additional controls.

11. Shot says:June 10, 2010 at 2:15 amwouldnt using the 12AU7 side of the 12DW7 for the cathode follower draw a lot morecurrent, dropping the HT supply? i think leaving V2 a 12AX7, then lowering the cathode loadresistor on the cathode follower, along with the plate resistor of the driving stage to maybe68k/68k would give nice results? i put in a 5751 in V1 on my blues jr. also, it might be nice toadjust the cathode resistor of the driving stage. ive been experimenting with this part of myblues jr. putting a cathode load resistor of 47k instead of 100k gave me a little bit smoothersound.

bill says:June 14, 2010 at 11:59 amThe 12AU7 can deliver more current, but the tone stack doesnt load it all that heavily,so it doesnt draw down the HT supply.47K will give you a different harmonic structure than 100K, but a smaller voltage swingand thus more compression. You also get some loss in volume, so its a tradeoff.

12. evi says:June 25, 2010 at 10:32 amHi Bill,Modded Jr user here! A question regarding 12DW7 and cathode follower circuits: You say thatwith this mod 12DW7 tubes can become a good option for V2. I was wondering if 12DW7scould be also a good option for V2 in the Bassman 59 I was looking for a good 12AX7 forthis position, and your article made me wonder. Thanks

bill says:June 27, 2010 at 8:12 pmI read a learned dissertation by Merlin Blencowe on how the values chosen in theBassman and the 12AX7 get along perfectly. But its just a tube, no harm in trying it.

13. Craig says:July 10, 2010 at 2:43 pm



Where do i purchase the capacitors and wire for this mod?Thanks

bill says:July 14, 2010 at 6:56 pmI can send what you need if youre buying other kits.

14. Cliff says:July 12, 2010 at 7:14 pmLeaving a tube on with no plate voltage/cathode current whatsoever will wear down the tubealot faster than in normal use. (See: Cathode Poisoning)Youd think that they would start making 6C4s again if they needed one triode (1/2 of a12AU7). (or, could just use a 6/12AV6 and ground the diode plates)Excellent mod! I myself hate having unused tube sections. (Even in AM tube radios whereone diode plate in a 12AV6 is left unused)

bill says:July 12, 2010 at 9:06 pmOn the stock amp, the unused cathode, plate, and grid elements are grounded together.Theres no voltage present. So the cathode may be hot, and may even poison itself, butif that half of the tube is unused, I guess it doesnt matter.

15. John says:July 16, 2010 at 10:08 amJust did this mod. I like it. To my ears, theres a definite difference. The overall tone balanceis definitely brighter, and the bass is much cleaner.A 12DW7 in v2 leaves the overall sound a little cleaner. A 12AT7 in v3 makes the amp a littlequieter. A 12AY7 in v1 makes it quieter still, and also pushes the breakup to a higher volumesetting.For jazz and clean fingerstyle strat, the alternate tubes are great. The clean boost modcompensates nicely for the drop in volume. Crunchy strat needs a 12AX7 in v1. The breakupis much more uniform across the frequencies with this mod. Before the bass would tend tobreakup before the treble.

16. John says:July 18, 2010 at 4:30 pmAdditional observation I usually run the amp with no fat. This makes the output impedanceof the second preamp stage 68k. The output impedance of the 12AU7 half of a 12DW7 in thecathode follower is 425R. According to Duncans tone stack calculator, under my typicalparamters (TMB all on 12) going from 68k to 425R gives a 6db boost in treble, 4db in midsand 1db in bass. Your results may differ. Turning treble down to about 9 fixes it.

Shot says:August 6, 2010 at 2:24 ami think changing the tone stacks slope resistor to 68k might make it a bit better. ivenoticed that after a cathode follower, standard fender tonestack tend to sound thin, andif you look at a 5F6 tone stack, the slope resistor is 56k. we can experiment with thattoo.

bill says:August 9, 2010 at 8:49 amYour tone stack sounds thin with the .1 F bass cap and .015 mids cap? Have youchanged the coupling caps to .02 F?



John says:August 11, 2010 at 10:34 am

Actually it turned out I had a bad solder joint in the tone stack. Gettingessentially no bass or mids! After fixing that, it sounds just great.

PS Bill is correct in his advice not to use a cheap soldering iron

17. Golney says:August 10, 2010 at 6:23 am

Hi bill

I think I have the necessary resistor for this mod, its definitely 100k but is there anyway oftelling if it is a 1/2 watt resistor? I received it in a byoc kit for an overdrive, it is blue andabout 6mm in length. this may be a stupid question!Ive just ordered the to-20 OT and an input jack, along with coupling caps. any chance youcould include one of these 1/2 watt resistors if this isnt correct? ill happily pay for it, wouldsave me having to pay extra postage!

18. TP says:January 20, 2011 at 8:04 am

Do you experience more noise (hiss) with the cathode follower since there is one tube stagemore? Im thinking about this mod, but Im afraid to get some more noise.thanks

bill says:January 24, 2011 at 9:23 am

I have not heard any additional noise of any kind with the cathode follower. No one whohas installed it has mentioned it, either.

19. blakes says:January 31, 2011 at 3:33 pm

Series II owners may want to make sure to add the treble taming cap before doing this mod.

I added the cathode follower mod to a Series III, and it almost made the amp too bright. Ihad also done the twinstack mod, and changed the tone caps to .1 bass and .015 mid, butthe treble was just ice pick at anything over 9 oclock on the treble knob. I did, howeverreally like the touch response of the amp and wide open springy bass after doing the cathodefollower mod. My solution was to change the slope resitor to 82k, and add a 300pf cap to C9,the treble knob is much more useable as a result.

bill says:February 2, 2011 at 11:34 pm

For every person who thinks the Blues Junior is too bright, theres another who thinksits too dark.

20. Eric says:February 18, 2011 at 9:14 am

Ive got all the mods in place, and I love the sound of my amp. One question did occur to meafter I completed this mod.

For the cathode resistor, you recommend 100k (or 82k if youve done the high voltagepreamp mod). Is this still appropriate when using the 12DW7/ECC832 instead of the 12AX7?

bill says:February 18, 2011 at 5:51 pm

Yes, the same resistor applies to either tube type.



- Bill

21. leslie says:March 18, 2011 at 9:25 pm

I did the cathode follower mod and it does take pedals better! Im glad I did it!!! ,,,Thanksfor all you info on yor site Im glad I found it..THe only thing this amp dosent have is thereverb pot, the volume pot and the switchcraft input jack and of course the 6v6s mod butit dosent really need itSOUNDs AWESOME!!!!!!

22. tronjohn says:May 4, 2011 at 4:23 pm

I am doing the cathode follower mod and the high-voltage mod. Since the cathode resistor ofthe cathode follower (V2A) has to be changed from 100K to 82K, should the plate resistor(R16) of the tone stack recovery amplifier (V2B) also be changed from 100K to 82K?

bill says:May 4, 2011 at 5:11 pm

No, there is no need to change the plate resistor of V2B. It does not interact with thecathode follower.

23. Wil says:September 27, 2011 at 4:57 am

If I understood everything, the 100k resistor goes from the brown wire to a new groundhole ?

bill says:September 27, 2011 at 8:13 am

It goes from the diamond-pattern ground plane to the brown wire and the trace whereIm pointing in the picture.

24. Wil says:October 5, 2011 at 12:45 pm

Second time my Bj kills a 12ax7 in V2 with the cathode follower mod Silly little boy !

bill says:October 5, 2011 at 4:10 pm

What brand of tube was it? Dont use a Tung-Sol tube in V2 with the cathode follower. Ithink the most reliable are the standard Sovtek and JJ 12AX7s. The small, boxy platestructure is probably better than the long, thin plates.

25. lor says:November 5, 2011 at 11:37 am

bill, In order to reduce the number of wires crossing from board to board would it be okay torun the followers grid wire (green in your little schematic above) direct from pin 1 of V1-Aalso on the tube board rather than from the main board?

bill says:November 5, 2011 at 6:41 pm

Yes, you can connect right on the tube board. My approach is more schematic anduses the two jumper holes, but you can go either way. Since the signal and voltagelevels are high feeding the cathode follower, there is no downside to the longer wireit isnot sensitive to interference.

26. lor says:



November 5, 2011 at 8:59 pmThank you. In the same context, I checked the full schematic and it appears also possible toconnect the 100k cathode resistor from V2s pin 3 on the tube board (shared with the wireback to the tone stack) to the V2 ribbons ground lead which seems otherwise abandoned onthe tube board. If there wouldnt be any significant signal or noise problems with that, itwould avoid drilling one of the holes in the main board.To take this to extremes, one could avoid drilling the other hole by snagging the high-voltageplate source off the positive lead of C28 (TP29), but that wire would then be pretty longpassing over a lot of circuitry; maybe causing problems?I agree your schematic approach is cleaner, more elegant, in that it puts all the relatedmodifications in a physical proximity that is lost when the pieces are spread around.Thanks again.

27. TP says:January 13, 2012 at 10:17 amI found a way to do the cathode follower mod without drilling any hole. A description ingerman and english with pictures here:http://www.palmenreich.at/documents/CathodeFollowerTiPiMods.pdfI took the plate voltage directly from R8 and I soldered the additional 100k on the back of theboard to a near ground point. The rest is like in Billms very good description.hope you like it

bill says:January 16, 2012 at 9:38 amI dont like soldering components to the back of the board. Drilling a couple of holes iseasy and gives a secure mounting and professional appearance.

28. Slestak71 says:June 17, 2013 at 4:55 pmI noticed that you said you have been using the JJ 12dw7 in your follower circuits. I haveread not so good things about the tone of the JJ 12ax7s. Have you noticed any ill effects onthe tone from the ax7 half of the JJ tube?

bill says:June 17, 2013 at 9:24 pmV2 is not that important for tone. It just re-amplifies the output from the tone stack. V1has a much greater effect on tone. So the JJ 12DW7/ECC832 works fine.

29. Slestak71 says:July 11, 2013 at 9:53 amI did get and install the JJ 12dw7 and it made a nice difference. That plus changing from aMullard 12ax7 to an EH12at7 in the PI has made a nice change to the amp. It added a gooddeal more punch to the tone, more solid sounding. Cant wait to get the TO20 installed andeventually go octal. Tons of thanks Bill for sharing info as you do. You ROCK.

Slestak71 says:July 11, 2013 at 9:55 amAs an additional benefit I was able to replace V1 with the Mullard I removed from the PI.Also a nice change from the Chinese I had in there. I like the OD of the Chinese but theMullard smokes it in overall tone quality.

bill says:July 11, 2013 at 10:46 am



Ive had a number of early failures with the Mullard reissue. If it starts to rattle oroscillate, youll surely hear it in the V1 slot.

30. Phil Connolly says:January 30, 2014 at 12:29 amIm looking at your photos for the green board, in particular the one showing the black wirecarrying signal from V2A into the tone stack (the trace on the plate side of R10). Your captionalso says to cut the trace so that the signal from V1A no longer gets into the tone stack. Butthere it looks like C25 is still connected to V1A. Based on the circuit diagram at the top of thepage, that didnt look right.Sorry if I am missing something, but I am about to do this one on a green board and wasntsure what do with C25, which is the 250pF cap going into the treble.

bill says:February 3, 2014 at 4:49 pmSorry, Im going to be away from my tech docs for a while. And I hate doing the cathodefollower on the green board.



cathode follower billm

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