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6G2 plate voltage confusion

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, Jan 18, 2021.

  1. Mongo Park

    Mongo Park Tele-Afflicted

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    I have looked up a number of threads on this topic and get more opinions and more confusion.
    So I thought somebody here could help me. I have built the 6G2 a while back, sounded good, off it went to a friend. It came back for a to me just recently and I thought to check this thing with the plate voltages being unique in these amps, higher wall voltage, different power transformers, and my lack of knowledge when I built the amp. Not that my knowledge has gone up since, but effort to increase it a bit here.
    So I had purchased a Hammond 290CAX and used the 550CT, followed the schematic, but have a 12ax7 as the first tube instead of a 7025, but that shouldn't affect anything. I fired up the amp, all tubes in and working, speaker connected. Wall voltage 120V, 280V DC going to the 100K resistor on the plate of V1. All good so far, since this is what is on the schematic. After the 100k resistor I get 190V DC to the plate, not the 135V DC printed on the schematic.
    So is this a misprint on the schematic, or should I try and achieve the 135V on the plate, I haven't checked the voltages on the other tubes, just trying to deal with the V1 first.
    Does the amp sound good, yes but does it sound as the amp was intended, I don't know, I have never played a real one in the wild.
    Any thoughts, tweak it with some advice from here, close it up, nothing to see here and move on.
    cheers Ron.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    A 7025 is just a low noise 12AX7. Samo-samo

    Did you 0 all the controls on the amp?

    If the amp sounds OK you're probably good. If the tremolo works especially.
     
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  3. Mongo Park

    Mongo Park Tele-Afflicted

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    Last edited: Jan 18, 2021
  4. Wally

    Wally Telefied Ad Free Member

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    Both plates on V1 read 180vdc? I would suggest a full voltage chart just out of curiosity...and as a baseline if you have never done that. .
     
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  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    .. and a schematic to begin with.

    Your bias resistor might be a bit off. But since I don't know what it is and should be, that is a wild guess.
     
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  6. symbiotic

    symbiotic TDPRI Member

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    Doesn't Ohm's law have something to say about this? If you know there's 280V on one side, and 190V on the other side of a 100K resistor, there is 0.9mA flowing through there.

    If there is 280V on one side and 135V on the other side of the same 100K resistor, there is 1.45mA flowing.

    Could it be a difference in test conidtions? Are your measurements at idle while the specs are with a signal flowing through the amp? Or, have you tried changing out the tube?
     
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  7. FenderLover

    FenderLover Poster Extraordinaire

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    I have original '62 6G2 and 6G3 amplifiers, and yes the schematics are not accurate. I have no idea why people think all Fender schematics are the yardstick by which all measurement should be compared. It would make sense, it would be nice, but isn't a reality.

    If it sounds good, you're happy with your B+, you're not red-plating, you're using schematic component values, then I don't know what there is to worry about. Cloning is a hole that can get deep and wide for no good reason.
     
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  8. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Yes, Ohm's law has something to say. Maybe the 100k is not what is should be or, after looking at the tube curves, the bias resistor is different...
     
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  9. Mongo Park

    Mongo Park Tele-Afflicted

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    OK, I am good with what Fender Lover said but I will check the whole thing and post the numbers, might as well do it ad then I can save it for later reference.
    Thanks everybody for their help.
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Heh, @FenderLover helped me with a very similar answer to a similar question a few years back: like he said, on a good day it’s just that the preamp plate and cathode voltages, in particular, don’t exactly match Fender vintage values, for several reasons. On a bad day there are other causes. So a complete voltage table is a good thing.

    And the 6G2 is a good example of "Were the schematic voltages even right?" You can find debate among owners of originals about B+; some say it's nowhere near as low as 315V even on vintage-correct wall volts. Others say it is. Most folks agree though that to get closer to 315 makes the amp 'more brownface' -- tubey-er, bluesier, earlier drive, etc.

    BTW, you note you used the 275-0-275 taps (good idea if you're chasing vintage B+). Did you make any changes to schematic values? Sovtek 5Y3? Different resistors? Etc? And as noted above your bias setup and setting is relevant.

    princeton_6g2_schem copy.jpg
     
  11. Mongo Park

    Mongo Park Tele-Afflicted

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    I will put up the voltages later today. Yes it is all stock for this sort of thing, a few things in there like a NFB switch and a diode on the trem. I have a bias pot in for the negative voltage bias. As far as it goes I have it set to all stock built to the schematic, resistors are 1 watts where 1/2 watts is called for. Shielded wires for the signal path.
    I buy old radios and such for the transformers and they frequently have 5Y3’s in them so I use them. Here is the one in the amp , no idea of its age or anything, other than it works. so I will try another one and see if it changes any voltages. 50C4110A-64D0-4A2A-A404-46A7F6790D3F.jpeg
     
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  12. schmee

    schmee Doctor of Teleocity

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    Many actual pre tube voltages are way off it seems.
     
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  13. D'tar

    D'tar Friend of Leo's

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    If it works out proper....

    It is proper

    upload_2021-1-18_12-49-36.png
     
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  14. 2L man

    2L man Tele-Meister

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    Back when those schematic voltages were measured 10% tolerance resistors were usually more than 5% off the value because more accurate were painted more accurate tolerances and I recall readind Fender did use 10% resistors.

    You need to bias tubes changing cathode resistors if you want to see other voltages. Optimum operation is not necessarily good on guitar amps when 2nd harminic distortion sound good and it increase when bias is not perfect.

    Or change different tubes.
     
  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Good idea -- get some voltages before you start doing too much work to change anything. We don't have any reason (so far) to think it isn't right. Here's the 'King Fan corollary' of Murphy's Law: If something *isn't* wrong, trying to fix it will change that.... :)
     
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  16. Wally

    Wally Telefied Ad Free Member

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    Before fretting about another rectifier, what is your B+? I am thinking that if you have that 280vdc at the last power rail mode, your B+ must be close to the schematic, correct? If you get really involved with wanting to see if you can make those V1 voltages look more like the schematic, you could change the cathode resistance since according to your post the voltage there is a bit low. A lower voltage there will cool the bias and raise the plate voltage a bit. Decreasing the value of the cathode resistor will increase current and decrease plate voltage. One could also change the plate resistor.
    This thread makes me wish I had a voltage chart from one or all of the 6G3s that I have owned.
     
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  17. Mongo Park

    Mongo Park Tele-Afflicted

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    E5E82789-AD97-4F6B-9E9A-C1224DDC314A.jpeg 653252AD-E7F6-4D62-8507-4B5D8C2B8C32.jpeg Here are my voltages. I don’t really see anything that is that far out except:
    On the plates of the preamp tube Oddly they are 191 and 182 volts.
    I did notice the power tubes screen grid and plate voltage are the same at 304V , I would like the screen grid to be a few volts below the plate, but that is easy enough to fix. I put in a different 5Y3 and the voltage went from 307 to 309 so I put the original back in, 2 volts lower.
    So I guess I could put in another dropping resistor to lower the B+ for the first tube.
     
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  18. Mongo Park

    Mongo Park Tele-Afflicted

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    I think Wally has the fix here.
    Thanks to all for the excellent input.
     
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