Tube substitution for a reverb tank.

Discussion in 'Amp Tech Center' started by jguitarman, Apr 3, 2020.

  1. jguitarman

    jguitarman Tele-Afflicted Silver Supporter

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    I have a reverb tank which was made in the early to mid '90's by a local builder who has sadly passed. I'm pretty sure he just copied a Fender tank schematic. I used to gig with it with no issues and after sitting for a few years I decided to fire it up and now I'm having a problem with it.
    I'm trying to isolate the problem which is that within a minute of playing time I just lose power; I have no sound. I want to try a different power tube to see if that's the problem. The tube he installed in the unit is a Ruby 6V6 GTA R.
    I'm almost surprised that he put a tube like that in because the preamp tubes are a Sylvania and a Bremer.
    I have some spare tubes and wonder whether or not I can use a JJ 6V6 S or an Electro Harmonix 6V6 EH?
    Am I OK to do this or do I need a 6V6 GTA R?
     
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  2. scelestus

    scelestus Tele-Meister

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    If it's a direct copy it should work OK with a 6v6, though I think the originals used a 6k6 (which is even lower wattage).
     
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  3. dogmeat

    dogmeat Tele-Afflicted

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    I heard the 6v6's are all the same. only possible worry is the ones in metal cans. the #1 pin goes to the metal case and is not connected on glass tubes. some amps are wired using that pin as a solder connection for the control grid resistor
     
  4. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Twenty five year old capacitors could be your problem.
     
  5. Wally

    Wally Telefied Ad Free Member

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    Tech time.....and a bad time to need a tech. Letting an amp or a reverb unit such as this sit unused for years puts the electrolytic capacitors at risk for early failure. One should run some current through one of these devices once a week or once a month in order to help the caps maintain viability.....once a day would be better, but that is hard to do when one has a number of devices.
    Fwiw, jguitarman, the term ‘reverb tank’ refers to the spring reverb tank that is part of that ‘reverb unit’ that was built for you.
     
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  6. muchxs

    muchxs Doctor of Teleocity

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    6V6s are not all the same.

    You have your 12 watt 6V6s.

    You have your 14 watt 6V6s.

    Who knows what a JJ really is but I'd guess around 18 watts max dissipation.

    6K6s tubes dissipate 8.5 watts max.

    It's generally safe to sub a 6V6 for a 6K6 but not vice- versa. A 6K6 will redplate PDQ when subbed for a 6V6 in a circuit designed for a 6V6.

    When you say, "reverb tank" I assume it's a Fender derived circuit designed for a 6K6 driver.

    If you see a 1k resistor on Pin 8 it's designed for a 6K6.
     
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  7. dogmeat

    dogmeat Tele-Afflicted

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    so... not the same. safe to interchange? I got that from one of the antique tube dealers
     
  8. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Just food for thought: the 6V6 or 6K6 in this device only drives the reverb pan. The tube responsible for passing the dry signal is the 7025/12AX7/ECC83. It also provides the reverb recovery gain. Even with the reverb driver tube removed, you should be able to turn up the Mixer control and whack the reverb pan and get clashing sounds, or turn down the Mixer control and get some of your dry signal, if everything but the reverb driver tube is functioning.

    If this thing is gonna be remotely trouble-shooted, I think the phrase 'I just lose power" needs some explanation. Lose power as in the wet signal disappears? Or all the signal (including the dry) disappears? Or the pilot light goes out and the tube filaments cool off and the whole unit is electrically powered off?

    Good luck!
     
    Last edited: Apr 4, 2020
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  9. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    If there is a 1k resistor on Pin 8 it's designed for a 6K6. It would be safe to use a 6v6. The 6v6 would be biased cold.
    If the resistor on pin 8 was smaller than 1k, A 6k6 would bias hot and possibly red plate.

    It all depends on the value of the bias resistor on pin 8. If it has been optimized for a 14watt tube a 12watt would be biased hot and the 6k6 would be out of the question.
     
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  10. dogmeat

    dogmeat Tele-Afflicted

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    ahhhhh I was only talking about 6v6 tubes... one of the big tube dealers had charts etc, and stated that any 6v6 variant would be OK in a circuit built for 6v6. I had that site bookmarked in a computer that crashed and now can't find it. now that seems to be in question. I'd like to know
     
  11. jguitarman

    jguitarman Tele-Afflicted Silver Supporter

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    IMG_1748 (1).jpg IMG_1744.jpg

    Here are some shots of the inside.
     

    Attached Files:

  12. jguitarman

    jguitarman Tele-Afflicted Silver Supporter

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    The pilot light stays lit and I have no signal; dry or wet. I tried a different power tube and the unit is still silent so its not the tube.
     
  13. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Assuming yours is a copy or work-alike to the Fender 6G15 Reverb Unit, then the dry signal is handled by the 12AX7.

    Replacing that and seeing if you get any change in behavior would be a reasonable thing for an end-user to do, but Wally's advice about waiting for assistance from a pro is good. There's only so much you can learn about how an amp is working with tube swapping and looking and listening.
     
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  14. Wally

    Wally Telefied Ad Free Member

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    If that 12AX7 sub does not give you a working unit, then one would need to be able to draw the schematic...mentally or on paper....in order to understand with what one was working. If one had some understanding of the circuit, then voltages could be taken.......hey, maybe taking voltages would help in understanding what was going on. if one has no experience, it is an excellent time to start reading and studying in order to prepare one’s self for working on this circuit.
    do you understand a schematic?
    https://schematicheaven.net/fenderamps/reverb_6g15_schem.pdf
    Do you understand tube elements and pinouts?
    History of vacuum tubes...Wikipedia has a quick study with some good basic instruction on what a vacuum tube is and what the elements are/do.
    Basic safety rules for working on these circuits is a necessary concern. There is a lot written, and Uncle Doug has some good tutorials about basics and beyond. I like Aiken Amps..Tech Info, white pages. Hey, the glossary of terms is worth a read, imho. .
    The unit hasn’t been used for some years, and it might not be the ideal time for interaction with a tech. It is a good time to study in preparation for learning by doing. Or....maybe a new 12AX7 gets it going. If not and if one understands the safety precautions, then taking voltages starting at the AC source though the rectifier and filter caps to the socket pins with all tubes pulled would be of interest as a starting point. Is the fuse good...measured with a meter if it looks good? If the voltages are there with the tubes pulled, install the tubes? What happens? Do the heater filaments light up?

    Interesting project.
     
    Last edited: Apr 4, 2020
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  15. jguitarman

    jguitarman Tele-Afflicted Silver Supporter

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    I substituted the preamp tubes and it worked. To be sure, I put the originals back in and it still works as it should. I don't know if one of the tubes was not making good contact or what but everything seems fine. I cleaned the sockets and pots.
    Wally, I should dedicate some time and effort to learn how to do my own servicing. I have had a hard time justifying time required to learn because I've been busy working full time and playing in two bands. I have instructional books and videos and have done some basic stuff over the years like biasing a Deluxe Reverb and building a 5E3 kit. I will dig in deeper and see what I can do. as always, I really appreciate your help.
     
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  16. SnidelyWhiplash

    SnidelyWhiplash Friend of Leo's

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    Glad you got it working. Lucky it was a tube problem. Wish I possessed 1/10th of the knowledge & patience to work on my own amps. Glad that there are helpful resources that one can refer to like TDPRI.
     
  17. Wally

    Wally Telefied Ad Free Member

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    Kudos, jguitarman. A slight bit of oxidation or dirt in a tube socket can stop the whole process. Or...there could be a connection that has an intermittent problem and jiggling the tube socket while pulling and installing tubes corrected it. This is one reason for subbing tubes one at a time in a diagnostic manner rather than all at once.
     
    Last edited: Apr 6, 2020
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