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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Valco clone

Discussion in 'Shock Brother's DIY Amps' started by Prophetsnake, Sep 17, 2017.

  1. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    I'm thinking of building a Valco clone and I'd like to float my ideas on it here.

    Most of the simpler push-pull, early amps seem to follow the general arrangement of the schematic I've attached. It's the one I found easiest to follow, but if anyone knows of a better schematic?
    I have a large PT I'd like to use. It's got two 275V secondaries and two 6.3V secondaries. One winding of each is more than enough power as far as I can tell. 275 is .185 and the 6.3 is 3 amp. Obviously this requires a bridge rectifier, but I'm hoping to leave as much of the original circuit intact as I can after that.
    To take up the slack left by the lack of a field coil I reckon I will need a choke. I've read up on their use, but I'm really at a loss as to size and position - before or after the first filter cap?

    The only other mods I would be thinking of at this stage would be to make the two input jacks different from each other, and possible add as simple a tone control as possible.

    Any thoughts?

    Many thanks!



    Valco_valco_1212.jpg .
     

  2. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Okay, still moving onwards with this project and hoping to stimulate a little interest! I have a chassis, a 13.5 x 5 x 2 inch Hammond, and I've been tinkering with the layout. That's an obviously very hefty PT on the right, a choke and output. The two sockets more or less centre are the 6V6 outputs and to their left is the 6SN7 and behind (on top of in the pic) that, the 6SJ7. Any glaring errors here? 2017-10-01 22.12.48.jpg
     
    fenderchamp likes this.

  3. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Are you planning on wiring "point-to-point", via "grommeted" fiberboard, or using eyelet'ed terminal strips?

    And, am I seeing a toroidal OT?
     
    Last edited: Oct 1, 2017

  4. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Yep, toroidol. Think it's beefy enough? ;) Bridge rectifier, but after that, as close to the Valco digram as is practical.
    As much as possible, point to point, using terminal strips to keep things tidy. This will be my first build like this.
     

  5. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Visually, a true "point-to-point" eventually ends up looking like an electronic spider web of "high-wired" components.

    Grommeted fiberboard can produce a very tidy layout leading to "easy" troubleshooting.

    Terminal strips tend to be somewhere "between" the two configurations.

    Also, the torioidal OT should be as far from the PT as possible, because it produces a spherical magnetic field rather than the two-lobed field that common E and I core OTs do, so mounting it perpendicular (90-degrees) to the PT won't help.
     
    Last edited: Oct 1, 2017

  6. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Yeah, I think I understand. All of my builds to date came with turret boards or something like them, and clear layouts. I have a rough idea of where I want everything to go, and I've been playing with the components to get an idea of where I want to nail down the strips, but I want to avoid wire as far as I can without getting stupid about it.
    How about the arrangement of the large components, Tele-man? I was a little concerned about having the OT as close to the pre-amp tube as it is.
     

  7. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Well, vacuum tubes ARE definitely affected by strong magnetic fields -- think CRTS and o'scopes, etc. The good news, however, is that the fields diminish as the inverse-of-distance-squared, so distance is important; however, when spacing is NOT possible, there's mu-metal walls and shields to consider.

    Preamp tubes are MOST affected while power tubes are lesser affected...but, preamp tubes (ala' Fender amps) commonly CAN use slide-on metal "shields" which DO work; but, power tubes CANNOT because of their much greater heat dissipations.
     

  8. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Thank you for that.

    I do have some metal sleeves that should fit the octal pre-amp tube. And I have read that the 6SJ7 is not the quietest tube to begin with, so maybe that would be better.
    .
    Would I be better off moving the OT to the right and the output tubes to the left? Or is there a downside to having the tubes clumped together, outside of the obvious heat issue?
     

  9. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Preamp tubes are your 1st worry, the power tubes will be where you can place them, because their HEAT will ultimately be more of a concern than magnetic field
     

  10. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Sure. I think you're right. I'll try shuffling things around. The tubes clustered together might help with wiring as well. Might being the operative word.

    A couple of other things that have me scratching my head regarding this schematic.
    The control grid on the input tube is pretty much straight in from the input. Just that 100k resistor there. I presume the stage relies on the suppressor grid and cathode to keep everything hunky dory here?

    And this amp has no cathode bypass cap on the power stage. I notice a lot of amps from this period are set up like that. Is this a good or bad thing, or simply a matter of taste?
     

  11. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Regarding the 6J7 sharp-cutoff pentode preamp tube: having the suppressor screen tied back to the cathode effectively holds off secondary emissions but isn't really enough voltage to overcome the plates influence. If the input is TOO sensitive (picks up radio stations, etc.) you can simply hang a 1MΩ grid-leak resistor from grid-to-ground and then tweek its value downward as necessary.

    I'd try it first without the by-pass capacitor across the single common-cathode biasing resistor; and, add a cap only if the total volume/gain isn't what you expect or want. Adding a by-pass capacitor eliminates "negative AC-voltage feedback" between the grids and the cathodes of the power tubes; by-passing that common cathode resistor WILL increase the gain of the output tubes.
     
    Last edited: Oct 1, 2017

  12. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Thanks again. I think I actually understand some of that!
    When you say to hang a
    resistor from grid to ground, you're talking about the control grid, right?

    As for the bypass cap, that answers my question perfectly. I was only worried that maybe that was a typo or maybe something that Valco got wrong back in the forties. I'd like to try the amp as it was originally manufactured, otherwise what's the point? having said that, if it doesn't work, I'm relishing the opportunity to tweak it and hopefully learn something along the way.
     

  13. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    Oops...sorry. Yes, all that first paragraph pertained to the 6J7 sharp-cutoff pentode preamp tube: http://rtellason.com/tubedata/6J7_6J7-G_6J7-GT.PDF

    The second paragraph pertained to the 6V6 output tubes.
     
    Last edited: Oct 1, 2017

  14. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Yes, thanks. It wasn't clear at first, but I got that.

    As you say, if it works, it works, and if it doesn't then it can always be modded after.

    A question about cathode bypass, though. I've read about them, seen videos, etc. The issue that they address; would I be correct if I put it like this? The buildup of excess electrons between the cathode and the grid are presumably bogging down what is a natural rythmic release of electrons when the grid is open. Similar to the way a crowd of people crushing at a row of turnstiles would actually slow things down?

    Just trying to get my head around some of this stuff.
     

  15. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    The 6J7 is a true 5-element pentode: cathode(K), control-grid(G1), screen-grid(G2), suppressor-grid(G3), and plate(P or A).

    The cathode(K) "sources" the electrons being pulled toward the high-voltage elements (G2 and P).
    The control-grid(G1) "modulates" & controls(*) that electron stream; (*) set by the DC-bias voltage.
    The screen-grid(G2) is the ACTUAL accelerating element; its voltage provides the MOST electron acceleration (far more than P).
    The suppressor-grid(G3) is the "splatter collecting" element and is usually connected to ground (functionally identical to beam-forming plates in beam tubes).
    The plate(P or A) is the "catcher" in that it receives/collects the 'modulated' electrons sent to it by the screen-grid.

    The by-pass capacitor on a power tube functions exactly the same as it would for a preamp tube; it's all about suppressing the "negative AC-voltage feedback" that occurs within the tube between the cathode and its control-grid. The samething occurs in preamp or power tube; the only difference being the current magnitudes: a few millamps in a preamp and 100's of mA's in power tubes.
     
    Last edited: Oct 1, 2017

  16. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    OK, I pretty much understand all of that. I was just having a little trouble understanding how the cathode bypass actually relieves the congestion. I'm guessing that it doesn't actually bleed off the electrons, it only quiets them down by dampening their activity around the cathode when they have nowhere else to go.
     
    Last edited: Oct 1, 2017

  17. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Here's plan B. From left to right, the two output tubes, the phase inverter/second stage and the pre-amp bottom right. IMG_6302.JPG
     

  18. Old Tele man

    Old Tele man Tele-Afflicted

    May 10, 2017
    Tucson, AZ
    No, it's about "feedback" that occurs within the tube: It's sorta like a teeter-totter, where the control-grid is one seat and the cathode is the other seat.

    For example, assume the 6V6 control grids are biased at -32Vdc and the cathode is at ground level (what happens with fixed-bias). Now, as the current through the 6V6 cathode resistor generates its voltage (Vk = Ik*Rk), it raises the cathode voltage ABOVE ground (assume +32Vdc). Since the control-grid(G1) and the cathode(K) are "out-of-phase" their apparent voltage levels are INVERSE, so +32Vdc at the cathode is identical to -32Vdc on the control-grid.

    However, add an AC-signal into that originally constant DC-cathode voltage and you have an AC-signal "riding" above & below the original +32Vdc cathode-bias voltage. But, now, that same AC-voltage is adding-to & subtracting-from the bias...which is INVERSELY / CONCURRENTLY being "seen" by the control-grid as an "addition-to" & "subtraction-from" the AC-signal present on the control-grid that is creating the changing current flow through the cathode resistor, which is INVERSE or NEGATIVE FEEDBACK.

    Adding a capacitor across the cathode resistor maintains the DC-bias voltage while simultaneously shunting the AC-signal to ground (AC passes thru capacitors, but NOT DC), thus "eliminating" the inverse voltage feedback between the cathode and the control-grid. Changing the value of the by-pass capacitor affects both gain (how much feedback is eliminated) and low-frequency roll-off (low-end response).


    Regarding pix: are you going to edge-mount the tubes or vertical-mount them as shown? Is the chassis "thickness/depth" enough for edge-mounting?
     
    Last edited: Oct 1, 2017

  19. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    OK. Thank you for that. I went back and read some more, and I'm getting there, but it's still not sunk in 100%.
    However, I am beginning to comprehend the relationship between feedback and the bypass circuit, at least, even if I don't get it entirely. It will come by and by, I'm sure.
     

  20. Nickfl

    Nickfl Tele-Holic

    681
    May 24, 2016
    Florida
    Is the toroidal the OT, or is it the PT? That classic tone looks like an OT to me, but I can't see the full model number and they have both PT and OTs that would fit the 40-180x2 numbers that I can see so I can't be sure.

    I would think you would want to rearrange things so that the preamp tube is as far to the left as possible. get it away from the transformers and let the power tubes sit by the output transformer if something has to. Standard practice would be to arrange things so from the input side of the amp it would be: Preamp, PI, Power tubes, OT, Choke, PT.

    That PT should put you in a reasonable ballpark for a 2x6v6 amp, although I have the impression that these older field coil amps tended to run at lower voltages than the later Fenders and such that we are all more familliar with. That field coil would probably have pulled down the voltage a lot more than a choke will, though the original PT was probably higher than 275v anyway. Also the original amp may have had an OT primary impedance to match the lower plate voltage if it indeed did run at a low B+ and you may be better off with conventional voltage with your modern OT.

    Still, you might consider using an ez81 rectifier, they run on 6.3v and can use the same heater winding as everything else so you can use one with pretty much any PT. If your PT doesn't have center taps you can still use the EZ81 if you use a couple of SS diodes to create a hybrid bridge rectifier. Actually, since you have two 6.3v secondaries you could probably rig something up to drop the voltage of the other secondary to 5v and use any tube you want for a rectifier, which again may give you more of the sound of the old valco amp with the voltage drop and sag of tube rectification.
     

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