1962 6G3 re-cap advice/affirmation please...

Discussion in 'Amp Tech Center' started by capohk, Jul 25, 2020.

  1. capohk

    capohk TDPRI Member

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    Inspired by Uncle Doug's recent video, I pulled out the chassis to my 1962 Brown deluxe. I have had this amp for ages but I rarely play it because it is noisy and too loud for the house. It stopped working a while ago so I have finally got around to trying to see what the problem is. One of the diode legs on the bias board has snapped, and now that I know what I'm looking at, I can see the death cap on the ground switch and the two core power cable both need 'modernising'. I've also done the NFB switch that Doug suggests in his vid.

    The filter caps in the dog house are some older blue Cornell Dubilier 40uF @ 450VDC. I pulled a couple to get a reading on my meter and they read 50uF and 52uF so are obviously changing in value. The datasheet on the GZ34 rectifier tube gives max C as 60uF so there isn't much more room to move.

    The schematic asks for 16/16/16/8uF, but from what I've read, these were on the low side because of the high cost of electrolytic caps back in the day.

    What would be good replacement values? suggestions of brands also would be welcome.

    The cathode bypass caps have also been replaced at some point with 33uF 35V caps. The schematic calls for 25uF 25v, so this is not correct but I wonder how much difference it will make? I was thinking to go up to 50v if I replace them. (Might as well do it all at the same time)

    The pencil notes on the top of the dog house say that the caps were last replaced in '01. Given the sparse use the amp has had, I think it best to replace all of the electrolytics?

    Thanks in advance, your knowledge and advice here always appreciated!
     
    Last edited: Jul 25, 2020
  2. capohk

    capohk TDPRI Member

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    Also, as I go through the amp, it appears that the V1 load resistors have been changed from the schematics 220K to 100K. No idea why that would be, other than the tech looked at the wrong schematic by mistake?
     
    Last edited: Jul 25, 2020
  3. Jon Snell

    Jon Snell Tele-Meister

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    Electrolytics lose their value not gain.
    As the tolerance for older electrolytics was +50 -20% they read fine.
    The values 16/16/26/8uF is the calculated vale, there were plenty of larger value capacitors, the issue was voltage, in the day these were made and the values are chosen for minimal load on the GZ34 and transformer plus the CR time making them perfect values for decoupling. Don't mess with the values!
    Most of the cathode voltages, apart from the output stage, are in the region of 5 to 10volts. In this case the maximum voltage is a mere 2volts!
    The capacitance value governs the low pass gain. Too much low pass will stress the output stage and transformer.
     
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  4. Ricky D.

    Ricky D. Doctor of Teleocity

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    If it was mine, I'd go with the original values.
     
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  5. Wally

    Wally Telefied Ad Free Member

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    I would go back stock everywhere in the circuit. After getting to know the amp as it was originally, I might make a change in the first stage of filtering. That would happen only if I wanted the low end to be punchier and hang together better with humbuckers. There is no reason to go to larger capacitance in those cathode bypass positions, imho. The change to the power resistors in that first stage would be that there would be less gain with the 100k ohm versus the stock 220ks. I like what a stock 6G3 does. It looks as if someone was trying to ‘clean’ the signal up and make it more articulate when pushed??? Unnecessary for my purposes...ymmv.
    Enjoy the amp.
     
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  6. FenderLover

    FenderLover Poster Extraordinaire

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    Usually the best advice. Starting from the beginning is the best way to figure out where to go. Trying to guess where you'll be in the end could get you lost somewhere in the middle.
     
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  7. BobbyZ

    BobbyZ Doctor of Teleocity

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    For replacements I use F&T caps for the filter and bias, normally use a 100uf-100v for the bias. For the cathode bypass caps Sprague normally those are 25uf-25volt.
    I'd also go with stock values, with the exception of the bias cap.
     
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  8. Ricky D.

    Ricky D. Doctor of Teleocity

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    Just have to add, those old brown Deluxes are known as great amps. They got that reputation in their original configuration. Keep it that way and find out why they are loved. You might just decide not to change anything. If you do ultimately decide to mod the circuit, at least you'll know where you are starting from and have a specific goal in mind.
     
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  9. capohk

    capohk TDPRI Member

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    Seems to be the consensus, so that's what I'll go for. Except for the bias filter at 100uf 100V.

    I've found the 56k resistor at the speaker changed to 100k. That will get changed back to stock too.

    The only thing that I'm not sure about now is that there are added resistors on the power tube sockets - 470 ohm from pins 1 - 4 (as screen resistors?) and 1.8k across 5 and 6

    [​IMG]
     
  10. Dacious

    Dacious Poster Extraordinaire

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    It sounds like someone tried to turn the preamp closer to BF/SF tone with the 100k V1 resistors That's opposite to the way most people would go.

    The 35uf bias cap would be no drama.
     
  11. Wally

    Wally Telefied Ad Free Member

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    That negative feedback circuit top resistor change from 56k to 100k would cause the output to be richer and livelier. I like a potentiometer there to go from stock to richer/livelier/hotter. A 250k pot might be a good value for that circuit....but knowing what it sounds like in stock format is a good starting point.
    That 470ohm resistor is an added screen grid stopper...not a bad idea. The 1.8k is a control grid stopper...again not a bad idea. Note: Pins 1 and 6 of a 6V6/6L6 tube are blank, so those pins are being used as contact points for those resistors and the voltage supply to them. Fender used the #1 pin as a contact for the control grid stopper, which is pin 5, and pin #6 as a contact point for the screen grid stopper, which is pin 4. I would suggest leaving those.
     
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  12. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I'd go with stock 16 or 20 uf on the filters.
     
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  13. capohk

    capohk TDPRI Member

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    Thanks, Wally - so what I've done is taken the (now unused) ground switch and used it to switch the negative feedback in and out. Are you suggesting that rather than a switch, I install a pot in its place to have adjustable NFB? Or would it be better to replace the 56k resistor with a trimmer pot? Or both...
     
  14. Wally

    Wally Telefied Ad Free Member

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    For adjustability/variety, a pot needs to be placed in series with that resistor. If you use the 56K resistor, then you can always get back to the stock NFB level. Increasing that resistance with the pot will decrease the amount of negative feedback signal. I like to use the ext spkr jack for that pot...unless one expects to use that extension jack.
     
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  15. capohk

    capohk TDPRI Member

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    Thanks again Wally. That makes a lot of sense.
     
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  16. ocduff

    ocduff Tele-Meister

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    Sweet amp. Not my amp but I’d go 100% stock values. Including not adding screen or grid resistors.

    Start there and I suspect you’ll be floored.
     
  17. capohk

    capohk TDPRI Member

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    I've got a 10K fender style bias pot to do my bias adjustment circuit. I can put it on a little bracket inside the chassis, or I could drill a hole to make it like the AB763. Obviously this would involve drilling the chassis, but I can't imagine it would damage the value of the amp significantly, and it's a much easier/safer solution in my view. The other option would be to put it in the hole for the now unused ground switch, but that is currently an NFB switch. Thoughts?
     
    Last edited: Jul 31, 2020
  18. Wally

    Wally Telefied Ad Free Member

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    Capohk, if this 6G3 is basically all-original with regard to transformers, speakers, pots, tolex, and grille cloth and in good cosmetic condition; I would advise you not to drill any holes. I am going to think that there is a way to mount a trim pot for bias adjustment without drilling any holes. There are Small trim pots that will do the job. These can be mounted on a terminal strip which can be attached to existing bolts....PT mount bolts usually....or soldered to the chassis with a large soldering iron.
    Well done, a hole to mount the vintage type of bias adjustment pot might not be frowned upon by some. If the amp is really clean, there are those who would take exception to that and would detract some value due to it.

    on the other hand, if either tranny, the tolex, the grille cloth, or the speaker is non-orig8nal; then a properly executed hole in the bottom of the chassis will not further detract from the value, ime.
     
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  19. BigDaddy23

    BigDaddy23 Tele-Holic

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    What about positioning it Uncle Doug style with a Z shaped bracket of a PT bolt? This way, you don’t have to touch the chassis and the pot is well secured. You need to take the chassis outta the cab anyway to re-bias so it isn’t necessary to have access to it with the chassis mounted.

     
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  20. tubegeek

    tubegeek Friend of Leo's

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    Bracket. Gerald Weber would advise you to drill TWO holes in case you decide you want to change it. But he was a hack.
     
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