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    MkIII blown fuse.

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    Ernstmach

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    MkIII blown fuse.

    Post by Ernstmach on Tue Sep 12, 2017 9:02 pm

    Hi,
    blew a fuse in one of my MKIII's tonight. Noticed a while back that there was a bit of a buzz coming from the speaker hooked to it but didn't think much of it. The buzz did not get louder with increased volume from the preamp. The other amp is dead quiet.

    Been using these since I assembled them about 18 months ago. Checked bias every couple of weeks. Have been trouble free since initial power up. Have about 1500 hours on them.
    Using the original GZ34s rectifiers as supplied in the kit. I did a search here for blown fuses and found the usual comments, "reflow solder joints, ect". I'll be doing that tomorrow.
    Nothing unusual noticed as the fuse blew. No bright tube displays. Just the amp going dark. Pulled the bottom off and the fuse was smoked but no evidence of other issues.

    Anything else I should check out?
    I'm going to check the GZ34s with my tester but it is an emissions tester so it may not be helpful.

    Thanks for any suggestions!
    Bob.
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    j beede

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    Location : California

    Re: MkIII blown fuse.

    Post by j beede on Tue Sep 12, 2017 9:50 pm

    Do you have a clamp-on ammeter? if so replace the fuse and pull all the tubes from both amps and measure and compare ICC. Replace all tubes except for the 5AR4 and measure ICC again. If the amps will power up with the 5AR4 installed, power them up and measure ICC again. This will help locate the current path that is blowing the fuse.

    If the ICC is not symmetrical left/right, try swapping 5AR4 and see if the current follows the rectifier tube.

    Are you in the habit of turning your amps on after they have been off for just a short while? I would not do that. Have you added forward 1N4007 silicon diodes ahead of rectifier pins #4 and #6? I would do that.

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    Ernstmach

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    Re: MkIII blown fuse.

    Post by Ernstmach on Tue Sep 12, 2017 11:13 pm

    I don't have a clamp on ammeter. I will have Thursday as I just ordered one. Where should I use this? Have never used one before..
    I'll do as you suggest.
    I have never turned the amps on a short time after shutdown. These are powered up, used for about 3 hours then turned off till the following session 24 hours later.
    As for the  1N4007 silicon diodes  they are as built. If they were supplied in the kit they were installed.
    Since these are identical amps can I expect the other amp to experience the same issue?
    Thanks!
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    peterh

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    Re: MkIII blown fuse.

    Post by peterh on Wed Sep 13, 2017 12:27 am

    Ernstmach wrote:Hi,
    blew a fuse in one of my MKIII's tonight. Noticed a while back that there was a bit of a buzz coming from the speaker hooked to it but didn't think much of it. The buzz did not get louder with increased volume from the preamp. The other amp is dead quiet.

    Been using these since I assembled them about 18 months ago. Checked bias every couple of weeks. Have been trouble free since initial power up. Have about 1500 hours on them.
    Using the original GZ34s rectifiers as supplied in the kit. I did a search here for blown fuses and found the usual comments, "reflow solder joints, ect". I'll be doing that tomorrow.
    Nothing unusual noticed as the fuse blew. No bright tube displays. Just the amp going dark. Pulled the bottom off and the fuse was smoked but no evidence of other issues.

    Anything else I should check out?
    I'm going to check the GZ34s with my tester but it is an emissions tester so it may not be helpful.

    Thanks for any suggestions!
    Bob.

    Hum that is independent of volume and existing even if input is shorted usually emanates
    from one of two places : broken B+ can cap or unbalanced 6550 tubes.

    If this is an original MkIII the cathode resistor used to set bias is common to both tubes,
    thus you cannot see if the tubes are unbalanced. Du you have another pair to test with ?

    The can cam could be broken this would manifest itself if fuse blows ( or gz34 flashes)
    after 5-10s from power up. Test this with power tubes removed.
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    j beede

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    Re: MkIII blown fuse.

    Post by j beede on Wed Sep 13, 2017 1:25 am

    Ernstmach wrote:I don't have a clamp on ammeter. I will have Thursday as I just ordered one. Where should I use this? Have never used one before..
    I'll do as you suggest.
    I have never turned the amps on a short time after shutdown. These are powered up, used for about 3 hours then turned off till the following session 24 hours later.
    As for the  1N4007 silicon diodes  they are as built. If they were supplied in the kit they were installed.
    Since these are identical amps can I expect the other amp to experience the same issue?
    Thanks!

    The clamp on ammeter is easy to use if you plug your amp in via an extension cord that has had its hot and neutral conductors "split" so you can clamp onto one or the other--not both conductors. This permits contactless current measurement.

    You refer to your MkIII as a kit. I suppose this could mean a NOS Dynaco MkIII or a modern reproduction? A photo would help. Does it use a single 6AN8 input tube? The different MkIII implementations have circuit differences that need to be taken into account.

    If you can acquire four 1N4007 diodes you can insert these to pre-rectify your B+ before passing it through the 5AR4. This is a reliability enhancement for your amps.

    If you can acquire some 47uF, 450V electrolytic capacitors they may come in handy during debug as well. These will enable the bypassing of individual segments of your quad caps.

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    Ernstmach

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    Re: MkIII blown fuse.

    Post by Ernstmach on Wed Sep 13, 2017 5:50 am

    "You refer to your MkIII as a kit. I suppose this could mean a NOS Dynaco MkIII or a modern reproduction?"

    The amps were kits I purchased from Roy. Since assembled I have run KT88's and 6SN7's and GZ34S.

    They have worked very well since finishing assembly.

    I will pick up new fuses asap. I have a new pair of 5AR4's and power tubes so I'll get started with trying the suggestions.

    Please realize I am no electronics expert.
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    LeGrace

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    Re: MkIII blown fuse.

    Post by LeGrace on Wed Sep 13, 2017 7:43 am

    One fuse after 18 months you're doing well. I've gone through two 5 packs in my first year with my M125's! A weakened 5AR4 is one possibility as it is continually operating near its design limit. Stressed 5AR4's were responsible for probably 3/4 of my fuse blowing incidents. To the point I've since moved away from 5AR4 completely and now running GZ37. That has helped a lot. But it also could be mismatched power tubes from one declining at a faster rate then the other. Have also had that pop a fuse. I had about 12 months on a pair of 88's that I then had to swap out. Same MO as you're describing.
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    pichacker

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    Re: MkIII blown fuse.

    Post by pichacker on Wed Sep 13, 2017 8:09 am

    Did the fuse blow whilst operating or at switch on?

    If whilst operating then look for a fault, if at switch on then it may (if you are lucky) have just been fatigue due to switch on surges..
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    Ernstmach

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    Re: MkIII blown fuse.

    Post by Ernstmach on Wed Sep 13, 2017 9:24 am

    LeGrace, your comments on the 5AR4 is interesting... I did install a new pair of 5AR4's in the amps a few days ago. I had purchased them awhile back and wanted to try them out. I pulled them after that session and put the GZ34S back in. Thanks for your comments!



    pichacker, the fuse went a couple of minutes after power up.


    I suspect it is more than fatigue because I was hearing hum at the speaker. The hum started a couple of weeks ago. Just appeared.


    Never became louder over time. I'm careful to power up my gear in order and always keep an eye on them while they are on.


    Thanks for your comments as well!




    Well I replaced the fuse after looking everything over very carefully. Connected all cables, powered up my dac, preamp then the amp. Seems to be operating normally. and the hum is no longer present. Nice and quiet.


    Is it possible that the fuse was degrading (if that's possible) over time causing the hum?

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    Peter W.

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    Re: MkIII blown fuse.

    Post by Peter W. on Wed Sep 13, 2017 2:16 pm

    OK - please forgive me if I seem a bit strident, but fuses are a pet peeve of mine, for what they can do, for what they cannot do, and for what they tell one when doing (or not) it.

    The fuses as-supplied for the typical VTA amplifier (Sorry, Bob) is designed primarily to protect real-estate and people, not necessarily the equipment. In general, they are too heavy and too slow to do anything else. So, if a fused blows within a few minutes of turn-on, then there is a serious condition in the making. And *you* need to determine what that is. As already noted, the first thing to do is determine the amount of current drawn by the unit. And if you have a clamp-on ammeter, that is a start. You may need to wind a couple of turns if wire in the measuring loop  to get the meter to read finer increments, however. Or, you can use the AMPs function on your VOM by putting the probes in series with the hot side of the primary winding on the transformer. Or, you may use a metered Variac that will read very fine increments as well as show you how B+ ramps up - which is my preferred method.

    Fuses are wearing parts, certainly. But they are also go/no-go devices. They do not degrade in any meaningful way. Nor can they cause or contribute to hum.

    Generally, I look towards Filter Caps as the primary source of constant-level hum in any system. And a partially shorted filter *will* cause over-current, eventually full shorting will cause the fuse to blow instantly at turn-on - with other conditions in between. As these caps begin to fail, the hum will grow to a point - whereupon the cap is essentially toast. The fuse blowing (or not) will depend on whether it is failing open or failing short.

    Good diagnostics avoid haring down dead-ends and chasing false leads. "It could be....." suggestions are all good, and good to know as avenues to consider. But, again, good diagnostics followed by systemic repairs will avoid wasted time, energy and much frustration.

    If you are going to take on tube gear more seriously, and get into basic diagnostics and basic repairs, you might consider investing in a metered Variac, preferably one that is isolated as well (rant, rant, harp, harp).  

    http://www.ebay.com/itm/Heathkit-IP-5220-Variable-Isolated-AC-Power-Supply-Tested-Working-SP-5220-/232472717038?epid=1701509401&hash=item36207426ee:g:Xv0AAOSwXq5ZqVTN   I keep this unit - and have found it adequate for anything discussed here. The large meters do show B+ ramp-up very nicely. However, there are other options by other manufacturers out there.

    Best of luck with it - but unless you know *exactly* what you are doing, do nothing yourself. Voltages on and under that chassis are potentially lethal.
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    Bob Latino
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    Re: MkIII blown fuse.

    Post by Bob Latino on Wed Sep 13, 2017 3:52 pm

    Peter W. wrote:OK - please forgive me if I seem a bit strident, but fuses are a pet peeve of mine, for what they can do, for what they cannot do, and for what they tell one when doing (or not) it.

    The fuses as-supplied for the typical VTA amplifier (Sorry, Bob) is designed primarily to protect real-estate and people, not necessarily the equipment.

    Peter .. The "general rule" for tube gear is to use a slo-blo fuse that is about 2 X the average continuous current draw. Consider the current draws below. The first 4 were worked out by Dynaco engineers many years ago who I tend to think know quite a bit about the amps they designed.

    Dynaco ST-35 draws 100 watts or about 1 amp continuously and uses a 2 amp slo-blo fuse
    Dynaco SCA-35 draws 110 watts or about 1 amp continuously and uses a 2 amp slo-blo fuse
    Dynaco Mark IV draws 115 watts or about 1 amp continuously and uses a 2 amp slo-blo fuse
    Dynaco ST-70 draws 190 watts or about 1.5 amps continuously and uses a 3 amp slo-blo fuse (VTA ST-70 is about the same)  
    VTA ST-120 draws about 320 watts or about 2.6 amps continuously and uses a 5 amp slo-blo fuse
    VTA M-125 also draws about 320 watts or about 2.6 amps continuously and uses a 5 amp slo-blo fuse

    You may have a different idea about this but your thoughts about this (that the fuse selection used is too large) may be based upon your experience with solid state amps, car stereo gear or house wiring where the parameters for fuse selection is quite different.

    Bob
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    Peter W.

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    Re: MkIII blown fuse.

    Post by Peter W. on Wed Sep 13, 2017 4:38 pm

    http://www.mouser.com/ds/2/240/Littelfuse_Fuse_218_Datasheet.pdf-310015.pdf

    Bob:

    Generally, I would agree with such rules-of-thumb, but for the unique factor in tube equipment - which is heat. You will note from the data-sheet attached that a slow-blow fuse such as you describe will accept 150% of the rated load for a minimum of sixty (60) minutes. Emphasis on the minimum.

    Looking at the VTA M-125 with its 5-A fuse, it will accept a 7.5 A load for sixty minutes.

    At 120 V, that comes to 900 watts, mostly expressed as heat, of which, being generous, 500 watts is excess heat. All of which, as it happens, is going through the power-transformer primary, and depending on where and what the fault might be, through one-or-another secondary.

    The fuse will accept a 600-watt load forever. 200 watts of excess heat.

    http://www.mouser.com/ds/2/87/Bus_Elx_DS_2044_MDQ_Series-335854.pdf

    A 2.8 A MDQ fuse will accept full operating load forever, accept a starting surge as large as the 5-A slow-blow, and serve to protect the amp a bit more effectively. It will blow before the 5 A slow-blow even reaches its rated load.

    Yes, these fuses are expensive. And they were expensive back in the day as well. However, even an individual as, for lack of a better word, frugal as David Hafler used them as OEM on all of his tube equipment, all of it. He was looking to protect his transformers from melt-down.

    Referring back to SS equipment, fusing such as a much more close-run thing as internal destruction can be very nearly instantaneous. In such cases, I will run the item at a moderate level and observe the load. Then I will fuse to that load - and no more - with an MDL/Q-type fuse.

    No fuse is perfect, and every fuse choice should be driven by the application, not the expense. But some fuses are definitely less perfect than others.

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