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Thread: Looking for Supro Thunderbolt 6420 Build info

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    Quote Originally Posted by mikepukmel View Post
    Re the 0.05 cap after the two 47k resistors on the inputs, I don't even know enough to ask why its there. I did check a few build photos that i could get and there are one or two other schematics floating around, might be from the same source, but they all have it. So, there are things that affect tone, and then stuff that doesn't affect tone, but noise, rf interference etc. And, maybe something to do with input matching to certain pickups, etc. Am I wrong in guessing that the components on the inpt side of the input tube are mostly not related to tone? Grid stops (even if put in the wrong place), grid leak, and that odd cap?
    I would just duplicate the parts that they used if you want that sound. A large part of the sound of those amps is the unbalanced paraphase inverter, but even something as simple as using ceramic caps like the originals vs PIO caps in their place would change the sound of the amp. We noticed when compiling the schematic that using a 240 ohm cathode resistor instead of the stock 200 ohm made the tubes happier and would likely help them to last longer, but the sound of the amp was less rich and colder sounding with less harmonic richness. The owner of the amps I worked on insisted on the stock 200 ohm value in all three amps.

    Greg

    P.S. These amps sound better with 10 inch speakers than the stock 15 inch speaker. I'd experiment if I were you, but that is what I found.

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    Cool, thanks Greg, and thanks for the tips on speaker and stock parts. Can't wait to get done with some house repairs and get this amp running.

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  3. #73
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    Click image for larger version. 

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    I've built a few tbolt 6420 amp circuits. I like 'em with a B+ around 400 and 5881s running off a shared 270R cathode resistor that is fully bypassed. The spare triode is useful for goosing the front end up, like a switchable single-parallel input stage. (see schematic). The cathode-bias vibe is cool, but they are not the loudest of amps (you can get more oomph from the same number of tubes if you build a 1-channel AB763 circuit sans reverb and tremolo).

    The James tonestack in the attached schematic works well. The treble and bass controls have a wide range and a decent mid scoop with the controls at full and relatively flat with both controls at about mid rotation.

    Or build it how you want. YMMV

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    a shared 270R cathode resistor that is fully bypassed
    Your circuit doesn't show the bypass cap

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    Quote Originally Posted by Helmholtz View Post
    Your circuit doesn't show the bypass cap

    Fixed :-)
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Quote Originally Posted by tubeswell View Post
    Click image for larger version. 

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    I've built a few tbolt 6420 amp circuits. I like 'em with a B+ around 400 and 5881s running off a shared 270R cathode resistor that is fully bypassed. The spare triode is useful for goosing the front end up, like a switchable single-parallel input stage. (see schematic). The cathode-bias vibe is cool, but they are not the loudest of amps (you can get more oomph from the same number of tubes if you build a 1-channel AB763 circuit sans reverb and tremolo).

    The James tonestack in the attached schematic works well. The treble and bass controls have a wide range and a decent mid scoop with the controls at full and relatively flat with both controls at about mid rotation.

    Or build it how you want. YMMV
    Thanks!

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    What effect on the tone does adding the bypass cap do?

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    Quote Originally Posted by mikepukmel View Post
    What effect on the tone does adding the bypass cap do?
    It makes it a bit louder and tighter sounding when cranked

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    Quote Originally Posted by tubeswell View Post
    Click image for larger version. 

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    I've built a few tbolt 6420 amp circuits. I like 'em with a B+ around 400 and 5881s running off a shared 270R cathode resistor that is fully bypassed. The spare triode is useful for goosing the front end up, like a switchable single-parallel input stage. (see schematic). The cathode-bias vibe is cool, but they are not the loudest of amps (you can get more oomph from the same number of tubes if you build a 1-channel AB763 circuit sans reverb and tremolo).

    The James tonestack in the attached schematic works well. The treble and bass controls have a wide range and a decent mid scoop with the controls at full and relatively flat with both controls at about mid rotation.

    Or build it how you want. YMMV
    So, 400v is fairly close to the other schematics folks here pointed to, e.g. one from Weatherford. It has a 0 - 325vac rms secondary and a full wave rectifier (did I use the corect term, 4 diodes in a bridge configuration). Which gives about 460 vdc at the first filter cap The transformer I found on Classictone, is 350 - 0 - 350 vac rms, so through solid state rectifiers, about 495, too high I think. Their transformer, they say is actually for the tube rectified version, so it has th 5v taps.

    Working backwards, if I wanted to build a solid state rectified version, to get 410 at the first filter cap, Id need a 0 - 290 or a 290 - 0 - 290 secondary power transformer? The second one seems a little mere common for guitar amps.

    Would this transformer work?

    *EDIT* Sory copy and paste error
    Hammond
    290UX 112 117 60 308.5V @ 250 mA 39V @ 250 mA 6.6V @ 3.75A

    its got a bias winding, but would not need that.


    As aside: I sorta get the idea how a 4 diode bridge rectifier works, with a transformer that has no center tap, vs 2 diode (like fender) with a center tap, but which is better? Not sure what better means, or why the schematics Ive seen for this amp use a 4 diode bridge.

    In the bridge, two diodes are always conducting, and the whole winding is used on every half cycle. In the other config, only two diodes are conducting, and only half (from center tap out) is used on each half cycle.

    Would one configuration heat up more, e.g. non center tap, since the while winding is used in each half cycle?

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    Last edited by mikepukmel; 06-16-2019 at 09:28 PM.
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    Quote Originally Posted by tubeswell View Post
    Fixed :-)
    Studying your schematic, all kinds of cool stuff in there, thanks! Do the elevated heaters help a lot with hum? Bass + treble is a nice add. How does the raw boost work?

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    Quote Originally Posted by mikepukmel View Post
    ... Do the elevated heaters help a lot with hum? Bass + treble is a nice add. How does the raw boost work?
    Heater elevation and humdinger is particularly useful with S:N ratio in the first stage.

    The James tone stack setup in this schematic gives a fairly flat response with the pots at mid rotation, and a decent mid-scoop with both pots at max rotation.

    The raw ('boost') switch is a TS-lift switch. Inserting a 1M resistor between the TS and the ground removes the effect of the TS from the circuit (and makes the amp even louder)

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    Quote Originally Posted by mikepukmel View Post
    ...Working backwards, if I wanted to build a solid state rectified version, to get 410 at the first filter cap, Id need a 0 - 290 or a 290 - 0 - 290 secondary power transformer?...
    Depends on what sort of rectification you use. 0-290 with FW bridge rectifier using SS diodes. 290-0-290 with 2 phase rectifier using SS diodes

    Quote Originally Posted by mikepukmel View Post
    ... Would this transformer work?
    ...
    Hammond
    290UX 112 117 60 308.5V @ 250 mA 39V @ 250 mA 6.6V @ 3.75A...
    435 B+ or so, using a FW bridge


    Quote Originally Posted by mikepukmel View Post
    ... As aside: I sorta get the idea how a 4 diode bridge rectifier works, with a transformer that has no center tap, vs 2 diode (like fender) with a center tap, but which is better? ...
    "Better" in this instance, is strictly a function of the winding configuration, which will determine which type of rectifier you need.

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    Quote Originally Posted by tubeswell View Post
    Depends on what sort of rectification you use. 0-290 with FW bridge rectifier using SS diodes. 290-0-290 with 2 phase rectifier using SS diodes



    435 B+ or so, using a FW bridge




    "Better" in this instance, is strictly a function of the winding configuration, which will determine which type of rectifier you need.
    Ah got it thanks.

    I don't have a handle on what 80 volts difference B+ would do to this amp. The 350 - 0 - 350 Classictone, they say is original design, would produce something like 490v dc at the first node. Way higher than your design, or the Weatherford, you both have around 410vdc Ive read some long discussions about B+ etc, was able to pick up some of it, even though I don't have the detailed background. Some part I got (I think) said its 1 biasing that is important, and 2 difference between the plate and screen.

    Is lower voltages better "thing" with new build tubes?

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    Quote Originally Posted by mikepukmel View Post
    ... Is lower voltages better "thing" with new build tubes?
    Depends what you mean by lower voltages I guess. I don't consider 400V particularly 'low'.

    As to new build tubes, manufacturing tolerances tend to be a bit variable (not so many items going into the reject pile). Screen over-dissipation is a big factor in tube failure. One way to reduce that is keep screen voltages lower. But there are also other ways, like cooler biasing, running an optimum load resistance (and not allowing the PI to drive the output stage 'too unbalanced' - which can be a problem with Long-Tail-Pairs).

    Bear in mind that the output tubes in a stock t-bolt are biased to maximum plate dissipation at idle, and while these are auto-(cathode)-biased, you are still running the tubes hard, even at 410V.

    Also, the cathode-biasing of the 6L6s tends to bring the B+ voltage down quite a bit from what you'd expect if you otherwise ran them with fixed bias. If you got a B+ of 470 with fixed bias, it could well end up being anything from 400 to 420 or so with cathode biased 6L6s, depending on the load regulation of the power transformer.

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    Quote Originally Posted by tubeswell View Post
    Depends what you mean by lower voltages I guess. I don't consider 400V particularly 'low'.

    As to new build tubes, manufacturing tolerances tend to be a bit variable (not so many items going into the reject pile). Screen over-dissipation is a big factor in tube failure. One way to reduce that is keep screen voltages lower. But there are also other ways, like cooler biasing, running an optimum load resistance (and not allowing the PI to drive the output stage 'too unbalanced' - which can be a problem with Long-Tail-Pairs).

    Bear in mind that the output tubes in a stock t-bolt are biased to maximum plate dissipation at idle, and while these are auto-(cathode)-biased, you are still running the tubes hard, even at 410V.

    Also, the cathode-biasing of the 6L6s tends to bring the B+ voltage down quite a bit from what you'd expect if you otherwise ran them with fixed bias. If you got a B+ of 470 with fixed bias, it could well end up being anything from 400 to 420 or so with cathode biased 6L6s, depending on the load regulation of the power transformer.

    Very interesting, more stuff I don't understand yet! Re
    "Also, the cathode-biasing of the 6L6s tends to bring the B+ voltage down quite a bit from what you'd expect if you otherwise ran them with fixed bias. If you got a B+ of 470 with fixed bias, it could well end up being anything from 400 to 420 or so with cathode biased 6L6s, depending on the load regulation of the power transformer."

    OK so this is why they could have run that 700vac transformer with 495 (roughly) at the first power node, since cathode biased it would end up much lower actual V at the plate? Could you explain ' depending on the load regulation of the power transformer" does this mean if it doesn't sag?

    *** sorry, so you mean the actual voltage between cathode and anode?

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    Quote Originally Posted by mikepukmel View Post
    Could you explain ' depending on the load regulation of the power transformer" does this mean if it doesn't sag?
    About PT regulation https://www.ecmweb.com/basics/basics...age-regulation A PT’s load regulation is a ‘measure’ of the PT’s ability to cope with a load.

    Besides, When you run the tubes in a stock t-bolt, they’re idling at Pmax, so the B+ load is more than what it is if you’re idling them at 0.7Pmax using fixed bias. (More tube current in the stock t-bolt = more load to pull the B+ down further)

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    Another thing to keep in mind is that if you have a 350V power transformer like you suggested above, with a solid state bridge rectifier that will be 490V with no load. When you put a load on it then that voltage will drop quite a bit. Take a look at Fender transformers and do the simple math to see what a no load voltage is vs what those amps actually had for B+ voltage. For the multiplier, SS is 1.4, GZ34 is 1.3, 5U4 is 1.2, and 5Y3 is 0.9. So with a 350V power transformer as you noted above, SS = 490V no load, GZ34 = 455V no load, 5U4 = 420V no load, and 5Y3 = 315V no load. Then if you look at the voltages in a Fender and compare to what they would be no load, you can get an idea of how much it drops with different kinds of amps. Then you can get an idea of what voltage the power transformer should put out to get to the B+ voltage you are looking for. I say to use Fender because they are the most documented vintage amps.

    Greg

    Greg

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    Thanks!

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    So with a 350V power transformer as you noted above, SS = 490V no load, GZ34 = 455V no load, 5U4 = 420V no load, and 5Y3 = 315V no load.
    There won't be much difference in no-load DC voltages between the different rectifiers. It requires load current to produce the different voltage drops.

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    so the final question: 350-0-350v secondary power transformer from Classictone, which will result in higher B voltages, or a different brand that has about 300 -0 - 300v?

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    Quote Originally Posted by mikepukmel View Post
    so the final question: 350-0-350v secondary power transformer from Classictone, which will result in higher B voltages, or a different brand that has about 300 -0 - 300v?
    If you’re using SS diodes in a 2- phase rectifier, I vote for 300-0-300. Others may have different views.

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    Also, make sure you have enough currrent rating on the HT (and heater) winding(s).

    For a ballpark guesstimate for a 300-0-300 HT, you want about 200mA to be sure the PT will cope well with the draw from a pair of 30W 6L6GCs under full signal. With a B+ = 424, figure about 390V plate-to-cathode with ~33V across the shared 200R bias resistor. Each plate idling at 77mA, plus around 6mA for each screen plus 3-4mA tops for the pre-amp tubes = 170mA. Under full signal, this could peak to a factor of about 1.2x*, - so that's close enough to 200mA.

    * allowing for auto biasing and inefficiencies, and we want it to sag a bit with power chords.

    And about 3A (minimum) for the heater winding (if you're just using 2 x 6L6 and 2 x 12AX7 and maybe a 6V lamp)

    YMMV

    Others may have different views.

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    Quote Originally Posted by tubeswell View Post
    Also, make sure you have enough currrent rating on the HT (and heater) winding(s).

    For a ballpark guesstimate for a 300-0-300 HT, you want about 200mA to be sure the PT will cope well with the draw from a pair of 30W 6L6GCs under full signal. With a B+ = 424, figure about 390V plate-to-cathode with ~33V across the shared 200R bias resistor. Each plate idling at 77mA, plus around 6mA for each screen plus 3-4mA tops for the pre-amp tubes = 170mA. Under full signal, this could peak to a factor of about 1.2x*, - so that's close enough to 200mA.

    * allowing for auto biasing and inefficiencies, and we want it to sag a bit with power chords.

    And about 3A (minimum) for the heater winding (if you're just using 2 x 6L6 and 2 x 12AX7 and maybe a 6V lamp)

    YMMV

    Others may have different views.
    Thanks Tubeswell. Having a tough time finding that transformer. Found quite a few with higher B voltage secondary, but most with lower B have a lot lower current rating, unless maybe Im looking at the wrong spec. Hammond 290DX is 325vac quite a bit higher than 300. (Has a bias tap and 5v winding, that won't be used as well) 459v dc approx, vs 424vdc after rectifiers. Will send them an email, maybe they have a suggestion. The only transformer makers I know are Hammond, Classictone, both have spec sheets on their site, Heyboer, have to contact them, and Mercury, too pricey for most stuff.

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    Quote Originally Posted by mikepukmel View Post
    THeyboer, have to contact them, ...
    Heyboer also sells stuff via the WebetVST site. (They are pricier equivalents of the Weber spec transformers)

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    Thanks Tubeswell, I 'll check them out.

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    Layout question

    I found a few layouts, this is one. critique is that some components are laid out so that a long wire has to go all the way across the board, to get to the tube pin. And the connection to pin8 on th PI was routed way to the far side of the board.

    Click image for larger version. 

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    Tough for a weakly tech person to 'get there'. I keep comparing to classic Fender and Marshall layouts, where the wire from the tube to the board is really short, and components for a given tube are right near the tube on the board.

    ***
    Working on a version of above.
    What software do you guys used to draw out layouts? Something easy to use and cheap?

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    Your drawings look like they might have originated here: https://el34world.com/Forum/index.php?topic=4089.0

    The file named Supro Thunderbolt layout.sch was created using ExpressSCH which is part of the ExpressPCB package. It can be downloaded for free: https://www.expresspcb.com/expresssch/

    ExpressSCH is meant for drawing schematics, but as shown, it can be used for layouts. Some of the super-nice looking layouts are made with Visio, but that's an expensive program.If you happen to alread own it, you can download a library of components from Steve Luckey's web site (scroll straight to the bottom or else you'll get stuck for weeks checking ot his builds): http://www.sluckeyamps.com/

    If you want to take the time to learn, DIYLC is an awesome tool for creating layouts (totally free): http://diy-fever.com/software/diylc/

    One thing that makes DIYLC nice (besides how nice the results look) is that you can use components that are actual size so you know everything will fit right. After installing DIYLC, you can download and install a library of actual size components from Doug Hoffman's site: https://el34world.com/Forum/index.php?board=26.0

    When you're done, you can send the DIYLC file to Doug and he'll make a nice turret board for you for just a few bucks.

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  28. #98
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    Awesome, thanks Tony, will try them out when I get off work tonight.

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    Well I got my by-hand drawing fairly close (I think, maybe). will try out the PCB software tonight.

    Question for you all: I have a fiberglass eyelet board from one of the versions of Fender Deluxe AB763 that I tinkered with last year. I managed to get all of the first preamp tube components right across from the tube. Also managed to get the PI components fit where the channel 2 components went on the Fender.

    There is so much room on the board, I fit the ss rectifiers, all of the electrolytic filter caps and dropping resistors at the other end of the board, where the tremolo and phase inverter went in the fender ckt. This leaves two potential problems:

    1) The two grid (input) wires leading out of the phase inverter to the output tubes are pretty long to make it all the way over to the output tubes, pin 1 (parking pin for the grid stop).
    2) The power supply caps, and dropping resistors are on the board right across from the output tubes.

    All other wiring looks ok to my untrained eye. Leads are short (exc for grid wiring for output tubes).

    So, questions:
    - will longer output tube grid wiring cause a problem, is there something I can do to alleviate, maybe route them a certain way?
    - Will having the HV power supply components on the board near the output tubes cause problems?
    (e.g. oscillation, noise/hum).

    Thanks!
    As an aside, besides making the amp smaller (width wise), is there any good reason to have the power supply caps on the other side of the chassis, under a metal box, like Fender did?

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  30. #100
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    Its hand drawn, will get into PCB software asap. Don't laugh. OK go ahead, laugh, (I did too).

    Click image for larger version. 

Name:	suprothunderbolt_hand_drawn_layout.jpg 
Views:	23 
Size:	154.0 KB 
ID:	54274

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  31. #101
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    Grid wires to output tubes look OK to me. When I do a layout for an existing board, I get a big sheet of paper and use the board to mark where all the eyelets are

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

  32. #102
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    Quote Originally Posted by loudthud View Post
    Grid wires to output tubes look OK to me. When I do a layout for an existing board, I get a big sheet of paper and use the board to mark where all the eyelets are
    Thanks Loud! (Doesn't look so good, but that is what I did. Took a couple of pieces of printer paper, held over the board, and marked the eyelets out. Make copies of that, then drew components in by hand. Added about 11 new eyelets since this ckt components don't line up well with existing eyelets)

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    Last edited by mikepukmel; 07-12-2019 at 12:22 PM.
    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    Does the negative side of the power supply rectifiers need to be tied to the chassis, or can it just be hooked up to the - side of the filter caps and the power tube cathode?

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

  34. #104
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    will longer output tube grid wiring cause a problem, is there something I can do to alleviate, maybe route them a certain way?
    I would at least twist the grid wires. As the grid signals are opposite phase/polarity twisting will provide some shielding and produce a little capacitance (maybe 10 to 20pF) between the wires which may help to avoid oscillation.

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    Quote Originally Posted by mikepukmel View Post
    Does the negative side of the power supply rectifiers need to be tied to the chassis, or can it just be hooked up to the - side of the filter caps and the power tube cathode?
    Will hum less if it’s connected directly to the ground lead of the reservoir cap. I suggest following the grounding layout in Merlin Blencowe’s grounding article. (Especially Fig 15.12 and Fig 15.14) http://www.valvewizard.co.uk/Grounding.pdf

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    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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