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
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. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes I would use between 220R to 270R for the shared 6V6 cathode resistor.

    If copying the 5C1 6SJ7 input stage, you could leave the Valco input jack and resistors as they are.

    Did you check the plate resistor wiring for the pre-amp tubes? The plates voltages you report are identical to the HT voltage for the pre-amp stages, so you must have mis-wired the plate resistor connections.

    The order of connection is: plate pin & plate coupling-cap* & plate resistor on one junction (e.g. at the plate pin),

    With the other side of the plate resistor connected to the junction of the pre-amp filter cap (positive pole) and the supply resistor. (The power-supply-side of the plate resistor should be sitting at a higher voltage than the plate-side of the plate resistor)

    (* The other side of the plate coupling-cap goes to the following signal circuit)
     
    Last edited: Oct 24, 2017
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  2. D'tar

    D'tar Tele-Holic

    949
    Jan 11, 2013
    WNY
    I believe the tubes were removed so this makes sense?

    Don't mean to interrupt, just following. I'll be quiet now.
     
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  3. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes he says that in his post, but he also reports voltage across the cathode resistors in the pre-amp, which wouldn't happen if the tubes were pulled out.
     
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  4. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    I've modified the amp. There's a 4.7K resistor in place of the choke now, and an 18K resistor in place of the 47 K that split the two smaller filter caps. The 470 ohm cathode bypass resistor is now a 225.

    I didn't have a chance to map the voltages, but I did play it quickly afterwards and the volume has increased, but nowhere near as much as expected. It's almost as loud as a Champ, say. The tone is a little lackluster and of course there's still hum, but it's different in that it is no longer affected by the volume knob. Not in the least.


    I don't remember measuring the voltage across the cathode resistors, so I must have either said something that revealed that or I didn't post my findings correctly.

    Hopefully I'll get a look at the voltages this evening and report back with some accurate data.

    Many thanks to everyone for sticking with me.
     

  5. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    in this case = voltage at the cathodes. When you measure the cathode voltage with one gator clip on the cathode and one on the ground return/Chassis, the voltage you are measuring is effectively the voltage dropped across the cathode resistor.
     
    Last edited: Oct 25, 2017
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  6. D'tar

    D'tar Tele-Holic

    949
    Jan 11, 2013
    WNY
    10-4

    +1
     
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  7. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Measured the voltages. It's obvious even to me that there are some odd things going on here, so I will have to go through it from one end to the other and find out where I've made the mistake(s)

    This is all tubes installed, speaker connected.

    For what it's worth, here's what I got:

    At the first filter cap. 341.8
    at the second 342.5, and that after a 4.7k resistor, so something is wrong there, methinks.
    at the third 188.7

    V1, the 6SJ7

    1. shield 3.9mv
    2 heater 5.8
    3Grid ( suppressor) 5.5
    4 Grid 4.8
    5 Cathode 5.3
    6. Screen 118.8
    7 heater 5.8
    8 11.29

    I checked the plate several times and that's what it read, so problem there.

    V2 6SN7

    1.Grid 2 1V
    2. plate 2 102.7
    3 Cathode 2 4.5
    4. Grid 1 0
    5 Plate 1 100
    6 Cathode1 4.8
    7 htr 5.8
    8 htr 5.8

    V3 6V6

    1 nil
    2 htr 5.8
    3 plate 332
    4 screen 333
    5 grid 4.3 mv
    6 nil 4.7 mv
    7 htr 5.8
    8 cathode 32


    V4 6V6

    1 nil
    2 htr 5.8
    3 plate 309
    4 screen 1
    5 grid 1.5 mv
    6 nil
    7 htr 5.8
    8 cathode 19.1


    Obviously an issue on both screens. I'll have to put my magnifying hood on and root through it, but anything jumping out?
     
    Last edited: Oct 26, 2017

  8. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    And had a good rummage through it. The voltage divide between the first two filter caps was simple. I had a 4.7 ohm in there, not 4.7k. All I had to hand that was man enough for the job was a 3.3k, so I put that in and found a reasonable split - at least enough to be getting on with. So with no tubes in place I'm getting about 380 from the first filter cap and 355 from the second. I also found a poor solder joint on one of the 6V6 sockets so I repaired that and had a good look around, fully expecting to find a major error, but didn't. I made certain that nothing was even close to touching its neighbour and fired it up with tubes installed. It's louder, alright, but not by much. The tone is okay - nothing to get excited about. The hum was much lower and once again related to volume. IOW, turned down - no hum at all. Because of the remote panel it's still a bit of a mess when it's on the bench, so I tinkered with a few leads, thinking that the earth was less than perfect. I discovered that the unused secondaries were a bit too close to the input jack, and once removed brought the hum down significantly to nearly acceptable levels. So that's that problem mostly solved. I was surprised that they had that effect, since they're not making any sort of circuit, so another lesson learned.

    I'll have another run through the voltages, but at this stage I'm thinking of altering the pre-amp as suggested, or just discarding that 6SJ7 altogether in favour of an altogether different installation.
     
    Last edited: Oct 26, 2017

  9. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    For those heater readings - did you have your V-meter set to VAC (and measure between the pairs of heater pins)?

    For the 6SJ7, can you check the resistance of the plate resistor with your R-meter?

    The difference between the plate and screen supply nodes could be because one of the 6V6s doesn't appear to be conducting (only 1V on the screen - could be a bad tube, or the screen pin on that side isn't wired up). (And have you got separate cathode resistors on each 6V6? Either that or your meter isn't reliable/giving inconsistent readings)
     
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  10. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Thanks Tubeswell

    These are with the tubes in.

    You spotted my heater error. I measured again using the AC scale and they are all 6.29

    The 6SJ7 resistor is 330K and measures 305K. The supplied voltage is 161, but it's 9.9 at the pin. I can't see any errors there so I'm thinking the tube itself may be the problem. I have a 6J7 I can try. I think the only difference between them is the shield is grounded on top, and I have a clip for one of those. I might see if I have another tube that I can try as well.
    I found a bad solder joint on one of the 6V6 pins and that may have accounted for the bad screen on the 6V6. Or maybe I didn't get good contact with the pin. I'm getting 309 on both 6V6 now. I've had the thing warmed up in the dark and the two 6V6 seem to glow normally, the 6SN7 has a barely visible glow.
     

  11. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    If could well be a shorted 6SJ7. If you pull just the 6SJ7 out, where does the HT pre-amp supply voltage (at the pre-amp filter cap supply node) land?
     
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  12. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Yeah, I'm getting 170 on the powered side of the plate resistor and 160 at the pin. I checked and rechecked the voltage with the tube in and I'm certain it was 9.9, so I'm guessing the tube itself must be the problem.
    One thing about this tube is that the centre key pin broke off the first time I removed it. I have been very careful to put it back in the correct orientation each time and had no reason to believe this might be an issue, but could it?
     

  13. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Yes, if you got the keys in the wrong way. Look at the 6SJ7 schematic (on the tube datasheet) and work out the number of combinations of mis-connections e.g. just getting it wrong by one 'notch' (say tube Pin 7 to socket pin 8) could connect the heater filament between the suppressor (which is connected to the cathode resistor) and the plate resistor, thereby pulling the plate down to cathode voltage, as well as connecting the screen to the AC heater supply, and the cathode to the screen supply (causing the maximum H-k insulation voltage to be exceeded), and the signal grid to the cathode resistor, and the suppressor grid to the signal grid. You can figure out other combinations of bad connections.
     
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  14. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Sure. I was pretty careful to make sure it was correct each time, but it's by no means impossible that I still got it wrong. I'll try again, but I don't see how the amp would even play if it wasn't in correctly. In any case, Ill order another one or two. They're cheap enough.
     

  15. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Tried it again. I'm 100% that I got it in correctly, but perhaps I put it in wrong once with all the coming and going and ruined it. I'll have to wait until I can get another to try again. the 6J7 I have won't do without rewiring the socket and I ain't doin' that! So it'll take a week or two to get another. I will report back when I do.

    Meanwhile thank you so much for the help here. Aside from making a good amp, my intent was to learn something and I think I have picked up a good bit from everyone who has contributed, and especially Tubeswell. You're all very generous!
     

  16. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Right. I got the new tube and installed it. Same problem. The voltage at pin 8 (plate) is 120 when the tube is out, which seems reasonable, and 9 volts when the tube is installed. I've checked the wiring against the schematic and Im not seeing any issue. There doesn't seem to be any sort of leak between the pins with the tube in or out. The components are the correct values. The way I have the #8 pin wired is this: both the 330k resistor and the coupling cap are wired directly to the pin and each goes its merry way. The 330k to the power supply and the cap to the volume pot. I've checked the pot and it seems fine.

    Unless both tubes have an identical fault, I can't see how a plate can do this. Anyone have any ideas?
     

    Attached Files:


  17. D'tar

    D'tar Tele-Holic

    949
    Jan 11, 2013
    WNY
    With 9vdc on the plate what is vdc on the b+ side of the 330k, trace back to the filter cap and dropping resistor. Are these values, vdc, as expected with a loaded circuit.
     

  18. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    Thanks D'tar. Tried it and I'm getting 157 volts on the power side of the 330k resistor, and the same at the supply node.

    With the tube out, the supply is 170 and the pin is 160 volts, not 120. I made an error posting earlier.

    By the way, the reading at the pin with the tube in drifts a good bit and never really settles down. It's generally about 7.8 to 9 volts or thereabouts.
     
    Last edited: Nov 10, 2017

  19. D'tar

    D'tar Tele-Holic

    949
    Jan 11, 2013
    WNY
    Kinda draws attention back to that screen supply network.

    Edit: or not just thinking out loud I suppose.

    I have to re read some earlier postings......

    How about ohms from each pin to ground without tubes?
     

  20. Prophetsnake

    Prophetsnake Tele-Meister

    134
    Aug 2, 2016
    ireland
    I'm not really sure what you mean by 'the screen supply network'.

    As for the resistance to ground - Here we go:

    1 Shell - 0
    2.heater 276 ohm
    3.grid 3 ( suppressor) 5K
    4.Grid 1 -50K
    5 Cathode 125K
    6 Grid 2 276 ohm
    7 heater 276 ohm
    8 plate 346K

    By the way, the data sheet I used is for a GE 6SJ7 and the tube is also GE and definitely a 6SJ7. I presume that aside from small variations in performance ( and mojo) it doesn't really matter from manufacturer to manufacturer anyway. a 6SJ7 is a 6SJ7
     

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