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    VTA ST-70 rectifier tube & quad cap diagnosis

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    sendwaves

    Posts : 14
    Join date : 2015-08-12

    VTA ST-70 rectifier tube & quad cap diagnosis

    Post by sendwaves on Thu Oct 15, 2015 9:08 am

    Hello all, I've been greatly enjoying my VTA ST-70 for the last two months, but it went quiet yesterday. Sad Blown fuse.

    Could only find a 2A and 4A slow blow in local store yesterday evening, so I opted for a 2A and replaced it.

    Perhaps 10 seconds or more after turning on the power, I hear a "zzzt" and saw a brief a blue-white flash in the vicinity of the rectifier tube, then all of the tubes went dark again. Another blown use. I think I saw arcing inside the rectifier tube.

    Based on the troubleshooting thread, it sounds like either the rectifier tube or quad cap is the problem.

    What is the best way to isolate the problem?

    Should I test the quad cap before I replace the fuse and try to power up again? Is the procedure Bob describes for testing the quad cap in his reply Re: Blowing rectifier tubes appropriate for my situation?

    I'm assuming I should remove all of the tubes and discharge the quad cap before doing anything else. Any other precautions to follow in addition to those described in the "Safety" sticky?

    Recommendations for a replacement/spare rectifier tube? Original with the kit is a Ruby 5AR4C.



    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by deepee99 on Thu Oct 15, 2015 7:07 pm

    Not trying to one-up The Master here, but if the tube fix doesn't do the trick check your soldering on the quad cap. Those are some pretty hefty lugs, IIRC.

    Tube Nube

    Posts : 603
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by Tube Nube on Thu Oct 15, 2015 9:32 pm

    Buy one rectifier at the guitar store, and order a couple of spares on line, in order to save a bundle. Last guitar store 5AR4 I bought was $45!

    I get a kick outta the name of it: Mesa Boogie. But most certainly it is a rebranded inexpensive Russian or Chinese tube underneath the logo.

    And, by the way, welcome to the forum and the sweet soul sounds of modern ST 70. They really stir the soul! (to paraphrase the inimitable Cheech & Chong).

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by deepee99 on Thu Oct 15, 2015 10:56 pm

    Tube Nube wrote:Buy one rectifier at the guitar store, and order a couple of spares on line, in order to save a bundle. Last guitar store 5AR4 I bought was $45!

    I get a kick outta the name of it: Mesa Boogie. But most certainly it is a rebranded inexpensive Russian or Chinese tube underneath the logo.

    And, by the way, welcome to the forum and the sweet soul sounds of modern ST 70. They really stir the soul! (to paraphrase the inimitable Cheech & Chong).

    I'm about to retire my nearly ANOS British Mullards just because I may need trading stock some day, and have ordered from Jim McShane a quad of his Gold Lion (Genalex) GZ-34s, which are the electrical equivalent of a 5AR4, for $39.95 a pop. A bit spendy but I've had good luck with upper-grade Gold Lion and Tung-Sol Russian tubes. I had an experience similar to yours with a fresh out-of-the-box-new Sovtek except I didn't even get a 10-second warning before it all went pear-shaped. Rather pretty to watch, but more than a bit stressful, to watch a rectifier self-destruct so spectacularly.

    Tubes4ever

    Posts : 59
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by Tubes4ever on Thu Oct 15, 2015 11:04 pm

    If you can wait a week or so you can buy a 5AR4 online for at least half what you will pay at a guitar store. TubeDepot.com and Tubesandmore.com are good places to check.

    Keep a 5U4GB as a backup tube. They are more resistant to internal arcing and you can get them fairly cheap from Ebay.

    sendwaves

    Posts : 14
    Join date : 2015-08-12

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by sendwaves on Fri Oct 16, 2015 9:58 pm

    Thanks for the suggestions.

    It turns out that I live only a mile or two from a very reputable guitar amp repair shop. They told me that in recent years they've been seeing a lot of rectifier tube failures. Of the current production 5AR4s, the Sovtek has been working the best for them. Picked one up for about the same price I'd pay with shipping from an online store. I'll probably order a spare or two after doing a bit more research.

    Tube Nube wrote:I get a kick outta the name of it: Mesa Boogie. But most certainly it is a rebranded inexpensive Russian or Chinese tube underneath the logo.

    I noticed a Mesa Boogie amp head on the shelf at the shop, with it's complement of at least six tubes. Amusing history to the name, which is even footnoted in Wikipedia.

    deepee99 wrote:if the tube fix doesn't do the trick check your soldering on the quad cap. Those are some pretty hefty lugs, IIRC

    Yep, I was planning on inspecting the wiring on the rectifier socket and the quad cap. The solder joints appear to be good and the connections match the instructions and pictorial.

    I also checked the capacitance with my DMM. The measurements on two sections of the quad are different than in Bob's reply to the "Blowing rectifier tubes" post I linked above.

    40 section (square) to ground

    • expected: 70–100µF
    • measured: 156µF



    30 section (triangle) to ground

    • expected: 40–50µF
    • measured: 5.5µF intermittently


    By "intermittently", what I mean is that the DMM displays an indication that it's charging the capacitor, then displays the 5.5µF reading for only a second or so, then repeats the charging/measurement cycle for as long as I observe.

    For all of the other measurements the meter was stable and did not cycle like it does on the 30 section. I also checked a few of the individual capacitors on the driver board just to confirm that the meter measurements are consistent with the marked value +/- tolerance.

    Are the measurements of the 40 and 30 sections something to be concerned about? Are they indicative of anything?

    Tubes4ever

    Posts : 59
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by Tubes4ever on Sat Oct 17, 2015 12:03 pm

    Sendwaves,

    As Bob Latino said, it is highly unlikely that there is anything wrong with your quad cap. Measuring a cap in circuit will almost always give differing readings from when the cap is out of the circuit.

    It is highly likely that this is simply a 5AR4 failure. The tube has very close spacing between the cathode and plate. If the manufacturer doesn't pay close attention to their construction tolerances, then the cathode and plate can be too close and arc. Especially after several thermal cycles as you experienced.

    Put in the new tube and enjoy the music.

    sendwaves

    Posts : 14
    Join date : 2015-08-12

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by sendwaves on Sat Oct 17, 2015 9:50 pm

    Tubes4ever wrote:
    It is highly likely that this is simply a 5AR4 failure. The tube has very close spacing between the cathode and plate. If the manufacturer doesn't pay close attention to their construction tolerances, then the cathode and plate can be too close and arc.  Especially after several thermal cycles as you experienced.

    Thanks for the explanation. I have much to learn about tube amps, so I appreciate you taking the time to explain details like this.

    Put in the new tube and enjoy the music.

    The tubes are glowing happily once again, and the music is brining a smile to my face.

    Tubes4ever

    Posts : 59
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: VTA ST-70 rectifier tube & quad cap diagnosis

    Post by Tubes4ever on Sat Oct 17, 2015 11:14 pm

    sendwaves wrote:
    Thanks for the explanation. I have much to learn about tube amps, so I appreciate you taking the time to explain details like this.

    My pleasure! Please feel free to inquire about how amps and tubes work.  I'm sure there is someone here who will be happy to answer your questions. Me included!

    The tubes are glowing happily once again, and the music is bringing a smile to my face.

    Tube amps just seem to to have that effect Smile

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