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    bad output tube?

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    sendwaves

    Posts : 14
    Join date : 2015-08-12

    bad output tube?

    Post by sendwaves on Wed Oct 21, 2015 11:33 pm

    I replaced the rectifier tube in my VTA ST-70, readjusted the bias, and have been happily listening to music for the past few days, but today I heard pops from the left channel. After hearing the pops, I did a quick replay, heard the pops again, and immediately powered down the amp. I confirmed that the pops were not coming from the source or the program material.

    I think the problem is a bad output tube, but would appreciate some feedback on whether my diagnosis is going in the right direction.

    My diagnosis steps:

    1. Check bias with the inputs shorted

    • V3 (left front) was ~0.435V
    • other three tubes were holding 0.400V +/- 0.002


    2. Readjust bias on V3 to 0.400V.

    • bias started to climb continuously and increased above 0.450V
    • heard pops in left channel


    3. Measure mains voltage

    • 123.3 VAC +/- 0.1V


    4. Remove tubes from right channel and install tube from V6 in V3 socket; readjust V3 bias to 0.400V.

    • V3 bias steady at 0.400V +/- 0.002V
    • no pops heard


    5. Reinstall tube originally in V7; install suspect tube in V6.

    • as the amp powered up, the bias for V6 climbed above 0.400V and kept increasing
    • bias for V7 was constant at 0.400V +/- 0.002V
    • heard pops in right channel
    • increase in brightness from heater of V6 and sizzle from right channel


    After the flare-up and sizzle I powered down the amp as quickly as I could reach the switch and have not turned it back on.

    The tubes are Shuguang EL-34s that were new with the kit. My guess is that I have ~100 hours of operation on the amp and the tubes. The attached photo is the suspect tube. Note the white loops at the top of the heater. I see similar protrusions on the other left side tube (V2), but they are more pronounced on this tube. The tubes that I've been running on the right side do not exhibit this feature. My impression the past few days is that the tubes on the left side have been a little bit brighter than those on the right.



    I put the sheet of paper behind the tube so that the brown cast on the tube was more visible. All of the tubes have a similar cast in roughly the same orientation.

    My questions:

    1. Is the output tube that had the high bias and the increase in brightness likely bad and the cause of the pops/sizzle?
    2. Is it just an unhappy coincidence that I had a problem with the rectifier and now one of the output tubes?
    3. Is there anything else I ought to check on the amp?
    4. Is it normal to see the protrusions from the heater element?
    5. Is the brown cast normal?


    Thanks again for your help.

    GP49

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    Re: bad output tube?

    Post by GP49 on Thu Oct 22, 2015 12:34 am

    Yes, from what you tell us of your diagnostic steps, that is a bad (runaway) tube.  Most likely a grid winding is loose, sagging when the tube starts getting warm. The tube then begins to draw too much current, heats up some more, making the situation worse.

    The runaway tube may have overstressed the rectifier; if it was weak to begin with, that may have sealed its fate.

    I don't think the filament protrusion is of any concern.

    We have to be aware that today's tubes are not of the quality and consistency of those from the 1950s and 1960s.

    Tubes4ever

    Posts : 59
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    Location : Star, Idaho

    Re: bad output tube?

    Post by Tubes4ever on Thu Oct 22, 2015 1:01 am

    Sendwaves,

    Sounds like a coincidence between the rectifier and the EL34 previously both failing. I have a quad of the EL34 Mullard reissues. They all have the heater protruding from the top and bottom of the cathode. This is in no way an indication of the tube quality. Many NOS tubes have protruding heaters. Seems like you are just having a run of bad luck with the tubes. Buy a replacement for just that one EL34 and rebias and press on.

    peterh

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    Re: bad output tube?

    Post by peterh on Thu Oct 22, 2015 1:43 am

    The visible filament is an independent and benign thing. The unstable bias ( raising current) is a
    malign symptom of a bad tube.  
    replace with a matched pair!


    Last edited by peterh on Thu Oct 22, 2015 1:44 am; edited 1 time in total (Reason for editing : spelling)

    mazeeff

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    Location : Grand Rapids, Michigan, USA

    Re: bad output tube?

    Post by mazeeff on Thu Oct 22, 2015 7:35 am

    I use the Shuguang El34's in two of my ST-70 amps. I have experienced the exact same problem on two of the Shugaung's over the past year. At $38 a quad, the Shugaung's are a great buy, and sound very good, but the mortality rate seems to be much higher than average. Just buy a quad, and replace the tubes if/when they fail.

    Mike

    Tubes4ever

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    Re: bad output tube?

    Post by Tubes4ever on Thu Oct 22, 2015 1:18 pm

    peterh wrote:The visible filament is an independent and benign thing. The unstable bias ( raising current) is a
    malign symptom of a bad tube.  
    replace with a matched pair!

    Peterh,

    He doesn't need to buy a matched pair.  That's the great thing about having the VTA board with individual bias pots.

    peterh

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    Re: bad output tube?

    Post by peterh on Thu Oct 22, 2015 2:04 pm

    Tubes4ever wrote:
    peterh wrote:The visible filament is an independent and benign thing. The unstable bias ( raising current) is a
    malign symptom of a bad tube.  
    replace with a matched pair!

    Peterh,

    He doesn't need to buy a matched pair.  That's the great thing about having the VTA board with individual bias pots.
    To keep distorsion down he needs matching Gm ( and bias, even if the Vta makes small
    adjustments possible). But unmatched Gm means raw distorsion that the feedback needs
    to correct.

    Tubes4ever

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

    Re: bad output tube?

    Post by Tubes4ever on Thu Oct 22, 2015 2:29 pm

    peterh wrote:
    Tubes4ever wrote:
    peterh wrote:The visible filament is an independent and benign thing. The unstable bias ( raising current) is a
    malign symptom of a bad tube.  
    replace with a matched pair!

    Peterh,

    He doesn't need to buy a matched pair.  That's the great thing about having the VTA board with individual bias pots.
    To keep distorsion down he needs matching Gm ( and bias, even if the Vta makes small
    adjustments possible). But unmatched Gm means raw distorsion that the feedback needs
    to correct.

    Bob says this on his website referring to the VTA board,  " Matched pairs or quads of output tubes are not necessary with this design."

    Matched tubes are matched for bias current, not Gm.  Some sources will match both but not most.
    Small differences in Gm cannot be heard anyway. Same as biasing the EL34s at 40mA instead of 50mA. 40mA causes higher distortion. As Bob says, you can't hear the difference.

    If the OP had lots of hours on his EL34s, I would agree that he needs to swap them in pairs, but his system is fairly new.


    Last edited by Tubes4ever on Thu Oct 22, 2015 2:46 pm; edited 2 times in total (Reason for editing : More info)

    Bob Latino
    Admin

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    Re: bad output tube?

    Post by Bob Latino on Thu Oct 22, 2015 4:56 pm

    sendwaves wrote:

    3. Measure mains voltage

    • 123.3 VAC +/- 0.1V



    123.3 VAC is too high a voltage to run either a stock Dynaco ST-70 amp or the VTA ST-70 amp. On the back of the parts list for all VTA amp kits it mentions this ..

    "NOTE - You should also check your home AC LINE VOLTAGE. Higher line voltages (122 VAC or higher) will raise the amp's internal AC and DC voltages. This can cause the amp's tubes to have a shorter life. (especially the rectifier tube). If your AC line voltage is in the 123+ range, you should run the amp on a variac (any 6 amp or higher model will work fine) set to 117 - 118 volts AC."

    Everyone should check their line voltage. In some parts of the USA line voltages are much above the 120 VAC "standard" for USA line voltage. In my experience with these amps, (for USA and Canada amp users) 122 VAC is the upper limit that you should operate your stock Dynaco or VTA amp. Above 122 VAC, tube life will be noticeably shorter. It is cheap insurance to use a variac set to about 117 - 118 VAC to operate your tube amp. This problem is even worse for vintage gear that is expecting 115 - 117 VAC. Filament voltage can rise above 7 VAC and the B+ on a stock Dynaco ST-70 which is supposed to be about 435 VDC can rise up to 475 VDC. EL34 tubes do not like 475 VDC B+ and will not last as long as they should. 475 VDC B+ is in KT88 territory ..

    If your incoming line voltage is 123.3, you should run the amp on a variac set no higher than 118 VAC ...

    Bob

    sendwaves

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    Join date : 2015-08-12

    Re: bad output tube?

    Post by sendwaves on Thu Oct 22, 2015 6:57 pm

    Thanks for pointing out the mains voltage threshold. I remember reading about that when I was building the amp, but couldn't find it in the manual when I looked yesterday. I see now it is on the "Do's and Dont's" page.

    The mains voltage must fluctuate quite a bit in my neighborhood. I measured it several times at the end of August and it was always < 120 VAC, generally < 119 VAC. Now it's higher, but yesterday was the first time I measured it above ~122 VAC.

    Given this, it sounds like a variac is a good investment.

    Does a variac typically need to be adjusted as the mains voltage fluctuates, or does it hold a max output, dipping lower only if the mains voltage falls below the output set point?

    GP49

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    Re: bad output tube?

    Post by GP49 on Thu Oct 22, 2015 7:51 pm

    A VariAC® or generic equivalent, once the big knob is set, will output a set percentage of what voltage is coming into it; so its output will vary directly as the mains voltage. You can only vary that set percentage by moving the knob.

    sendwaves

    Posts : 14
    Join date : 2015-08-12

    Re: bad output tube?

    Post by sendwaves on Thu Oct 22, 2015 11:15 pm

    It's perhaps no surprise that the USA-made Staco Variac costs a couple of hundred more than the examples made in China.

    Any advice on whether the quality or performance of the Staco is significantly better?

    Would you buy a used Staco, General Radio, or Powerstat instead of a new unit made in China?


    Kentley

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    Re: bad output tube?

    Post by Kentley on Fri Oct 23, 2015 12:13 am

    You may wish to refer to this recent thread: http://dynacotubeaudio.forumotion.com/t2817-line-ac-reduction

    Tubes4ever

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    Re: bad output tube?

    Post by Tubes4ever on Fri Oct 23, 2015 12:43 am

    sendwaves wrote:It's perhaps no surprise that the USA-made Staco Variac costs a couple of hundred more than the examples made in China.

    Any advice on whether the quality or performance of the Staco is significantly better?

    Would you buy a used Staco, General Radio, or Powerstat instead of a new unit made in China?

    The China examples will work fine. All it is is a toroidal autotransformer with a wiper attached to a knob on top.
    I have one that is rated at 3 amps which is the same as the fuse size in the ST70.

    There are good examples on Ebay. New and used for much less than $100. Just look up variac.

    Bob Latino
    Admin

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    Re: bad output tube?

    Post by Bob Latino on Fri Oct 23, 2015 8:35 am

    Some customers of the VTA ST-70 and VTA ST-120 have used this 5 amp model sold by Circuit Specialists in Tempe, AZ. $64.95 with free shipping. Sure, it is made in China but they work ... If you use one, I would set it for an output of 118 VAC. I would not use the meter on the variac because it is probably no where near as accurate as a digital multimeter. Plug your multimeter into the output on the variac while you turn the knob until you read 118 VAC on your multimeter.

    5 amp variac from Circuit Specialists

    Bob


    j4570

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    Re: bad output tube?

    Post by j4570 on Fri Oct 23, 2015 10:45 am

    If you are patient you can usually get a 6-10 amp variac used model off fleabay for $50-60 shipped.

    If you only want a 2 prong, they can be had a little cheaper. I ended up with one of each (as I really wanted 3 prong, but had a 2 prong for about a year before), and sold it (here or at AK) for what I had in it.

    Jason

    wgallupe

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    Re: bad output tube?

    Post by wgallupe on Fri Oct 23, 2015 10:56 am

    My house voltage is a pretty constant 122VAC so I bought the one in the link below. It has worked perfectly for the past year. I simply set it to 117VAC and forget it. Well, I do check it every once in a while to make sure it holds at 117 (and it does). My ST120 and SP13/PH13 both run off of it without incident.

    http://www.ebay.com/itm/20-AMP-110V-VARIAC-AUTOTRANSFORMER-VOLTAGE-REGULATOR-POWERSTAT-0-130V-OUTPUT-NEW-/381222191973?hash=item58c29d0365:g:rPwAAOSwrklU9OTQ

    GP49

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    Re: bad output tube?

    Post by GP49 on Fri Oct 23, 2015 2:23 pm

    At least use the multimeter to verify the meter on the device. You can then use the meter to TRACK the voltages going to your equipment through their ups and downs, once you know how far off it is and how far you have to offset its reading to compensate..

    wgallupe

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    Re: bad output tube?

    Post by wgallupe on Sat Oct 24, 2015 7:00 am

    In my case the variac is located where its not easily accessible so i don't bother with the rather crude, low resolution meter on it. All setting and checking is only done with a multimeter. But I agree, you could use it to monitor variations.

    sKiZo

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    Re: bad output tube?

    Post by sKiZo on Sat Oct 24, 2015 1:07 pm

    Hmmmmmm ... looks familiar!

    You can see one peeking out from behind my latest restoration project ... had mine for years, and it's still kicking. I only use it on the bench, as I've got my handy lil DIY bucker for the amp when I need it. Seems to be well built with steady output and minimal drift.



    My new preciousssssss ...

    PS ... if anyone want's to keep their sanity, don't attempt a full recap and restoration on a Sansui QRX-9001 .... blubba blubba blubba ahahahahAAAAHHHHHHAAAA!

    (It's not for the faint of heart - but certainly worth the effort!)

    sendwaves

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    Join date : 2015-08-12

    Re: bad output tube?

    Post by sendwaves on Sun Nov 08, 2015 11:45 pm

    I ended up buying the little baby blue 5A variac from Circuit Specialists, like the one on sKiZo's bench.
    Thanks for all of your suggestions.

    After a week without music, it's been nice to have the amp back online again. I tried listening to my desktop Audio Engine speakers, which sound ok for their diminutive size, but it wasn't as enjoyable. I think I'm beginning to understand why fans of tubes often have a stock of spares and more than one amp: always good to have a backup!

    I installed a replacement pair of Shuguang EL34s and have been enjoying that. But I also bought a quad of the Genalex KT66s that I'm eager to try.

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