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    Blown fuse ... what now?

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    hup_d_dup

    Posts : 14
    Join date : 2009-04-06

    Blown fuse ... what now?

    Post by hup_d_dup on Tue Jun 16, 2009 8:24 am

    Last night one of my MkIIIs blew a fuse. I heard it happen ... loud pop. (the speaker is fine) Pulled the tubes and they look good. Tube sockets are clean. I can't see any physical damage to anything on the unit. I don't have a tube tester but I have some spare tubes. FWIW, the caps on this amp were replaced by a previous owner. The amp sounded good until this event.

    What is the best way for me to proceed?

    Bob Latino
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    Re: Blown fuse ... what now?

    Post by Bob Latino on Tue Jun 16, 2009 9:37 am

    Even though a tube LOOKS GOOD it can still be bad. Also even though a tube lights up OK it can still be bad. The filament may be OK but the plate, cathode or screens can have a problem.

    1. Try replacing the rectifier tube and the fuse. When you turn the amp on keep your hand on the power switch and your eyes on the rectifier tube. The key time is after 10 or 15 seconds - if you see any arcing (sparks) in the rectifier or the output tubes shut the amp off immediately.

    If the fuse blows ...

    2. Try replacing the output tubes and do the same procedure as above ....

    3. You can try replace the 6AN8 (if your Mark III is stock) but this tube is least likely to cause a problem.

    General info > If a fuse blows IMMEDIATELY the power transformer may have a problem. If the fuse blows after 15 seconds or so you usually have a high voltage problem somewhere in the amp ...

    Bob

    hup_d_dup

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    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Tue Jun 16, 2009 10:04 am

    Thanks, Bob, this is exactly what I will do tonight.

    One other question, is it necessary or desirable to have a load across the output terminals? Would a high wattage 8 ohm resistor be satisfactory?

    Bob Latino
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    Re: Blown fuse ... what now?

    Post by Bob Latino on Tue Jun 16, 2009 10:17 am

    Yes - you shouldn't ever run a Dynaco tube amp without a load on the speaker terminals. Probably nothing will happen without a load on there when the amp is at idle but if you accidentally have your preamp connected and run a signal through there you can blow an output transformer. If you just have the amp at idle with an 8 ohm resistor across there (a 5 or 10 watt 8 ohm resistor) you will probably be OK. Radio Shack sells 5 and 10 watt 8 ohm resistors which will be fine as long as the amp is at idle. Parts Express has 20 and 50 watt versions made specifically to work on the amp without connecting your speakers. Personally, from what you describe, I don't think it would be harmful to just connect your speakers ...

    Bob

    hup_d_dup

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    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Sun Jun 21, 2009 1:55 pm

    OK, so I replaced the fuse, with a resistor hooked up to the output terminals. Everything OK. Then put the amp back into the system. Have been listening for a couple of days. No problem so far.

    GP49

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    Re: Blown fuse ... what now?

    Post by GP49 on Mon Jun 22, 2009 12:54 am

    I know you say the capacitors were replaced by a prior owner, but take another look at the amplifier and make sure that the power supply capacitors (the four-section can capacitor) were replaced, and that it was done with capacitors of sufficient voltage rating.

    Also, a tube...either an output tube or the rectifier...could arc over intermittently and blow the fuse.

    hup_d_dup

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    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Mon Jun 22, 2009 8:29 am

    The replacement cap is a 30/20/20/20 525v, which is what the schematic calls for.

    I seen recommendations that the new cap should be rated at a higher voltage. Also one vendor is selling a cap that has higher capacitance, I think 80/40/30/30 ... something like that.

    Is this a worthwhile improvement?

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: Blown fuse ... what now?

    Post by Sal on Mon Jun 22, 2009 8:58 am

    If you were going to replace quad capacitor then all by means order the 80/40/30/20, wire the capacitor with the 40uf connected to the 5AR4 and the choke between the 40uf and 80uf section with the audio transformers red wires (center tap) connected after the choke to the 80uf section.

    The improvement you might/will hear is tighter bass if you are playing the amp loud that is bass heavy.

    Sal

    GP49

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    Re: Blown fuse ... what now?

    Post by GP49 on Mon Jun 22, 2009 10:35 am

    hup_d_dup wrote:The replacement cap is a 30/20/20/20 525v, which is what the schematic calls for.

    I seen recommendations that the new cap should be rated at a higher voltage. Also one vendor is selling a cap that has higher capacitance, I think 80/40/30/30 ... something like that.

    Is this a worthwhile improvement?


    525 volts is marginal at best. Quite often the AC line can be at 130VAC which puts a 525V capacitor in the danger zone, especially during turn-on. The delayed-on characteristic of the 5AR4 helps, though. I have gone to series-connected 350V electrolytics on my Mark II, making a 700V rating, safe with its 5U4 rectifier which has no such delayed-on characteristic.

    The previous poster said that if you were to use the 80mfd section of that 80/40/30/30 cap, the feed to the output transformer should be placed in the second position, after the choke...and I quite agree.

    As to your original fuse-blow, it's possible that a short occurred across a section of the quad electrolytic, which was then blown out by the current before the fuse blew. This happens quite a bit on old, marginal electrolytics, leaving no evidence that anything ever happened.

    You didn't say...did you replace the tubes? When one of my Mark II blew a fuse lately, the cause was one of the Mullard EL34 output tubes that arced over intermittently. No evidence of runaway or of any other problems...just a bright arc, a POP and the blown fuse.

    hup_d_dup

    Posts : 14
    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Mon Jun 22, 2009 12:20 pm

    Wow, this is great info. Thanks for such quick replies!

    Bob Latino
    Admin

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    Re: Blown fuse ... what now?

    Post by Bob Latino on Mon Jun 22, 2009 5:53 pm

    Hup,

    The best quad cap you can find for your Dynaco Mark II, Mark III, Mark IV or ST-70 are made in Germany and are distributed by Dynakitparts. This is an 80, 40, 30, 20 cap and is 550 volt rated (continuous) and 600 volt surge rated. I have used a number of these in my ST-120 tube amps which have a working plate voltage of about 495 VDC. None of these caps has ever failed. They are expensive at $49 but worth it. They are also a low ESR cap ...

    Dynakitparts quad cap

    Bob

    Sal

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    Re: Blown fuse ... what now?

    Post by Sal on Sun Jun 28, 2009 10:44 pm

    What was the outcome with your amp, did you find out what was causing the fuse to blow?

    Sal

    hup_d_dup

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    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Mon Jun 29, 2009 6:05 am

    No, the amp is running. However, I have discovered that my bias is high even at minimum setting. I don't know if this was a previous condition or if it's related to the fuse problem. I'm going to get a bias repair kit.

    Thanks for your concern.

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Blown fuse ... what now?

    Post by Bob Latino on Mon Jun 29, 2009 8:17 am

    Hup,

    Most people start on the bias system by replacing the stock selenium rectifier with a diode. Because a diode has more forward voltage flow than a selenium rectifier, the replacement usually throws off range at which the bias system operates.

    The usual procedure is to add resistors in parallel to the two bias resistors on the 7 lug terminal strip. This will drop the total resistance and allow an expansion to the range in which the bias system operates. Try paralleling another 10K resistor with the one that is already there on lugs 1 and 2. This will give you 5 K. Try paralleling a 15K to 27K resistor to the 10 K resistor on lugs 3 and 4 to give an effective resistance of about 6K to 7K or so. A 1/2 watt resistor should be OK here. While your at it you should replace the two 50 Uf capacitors between lugs 1 and 3 and 1 and 4 with new 100 Mfd at 100 volts or higher. Make sure that you orient the capacitors properly with the positive (+) side of both capacitors connected to lug #1. Some people, when they do this, trace the wire from lug # 1 and find that it goes to ground? and say "This can't be right ? Positive to GROUND" but it is. The bias system runs a negative DC voltage to control the flow of current through the output tubes and everything is in REVERSE - positive to chassis ground.

    Bob

    Sal

    Posts : 221
    Join date : 2009-02-05
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    Re: Blown fuse ... what now?

    Post by Sal on Mon Jun 29, 2009 10:55 am

    Bob Latino wrote:Hup,

    Some people, when they do this, trace the wire from lug # 1 and find that it goes to ground? and say "This can't be right ? Positive to GROUND" but it is. The bias system runs a negative DC voltage to control the flow of current through the output tubes and everything is in REVERSE - positive to chassis ground.

    Bob

    Is that why my bias capacitors keep blowing up... Rolling Eyes

    Only kidding.... Very Happy

    Sal

    hup_d_dup

    Posts : 14
    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Wed Jul 01, 2009 2:15 pm

    I discovered something interesting today. I have two schematics of the MK III; one has a 3 amp slow blow fuse and the other schematic shows a regular 3 amp fuse.

    I'm using regular 3 amp fuses. Maybe that's why my fuse blew. (it's been OK since I replaced it)

    Jonathan

    hup_d_dup

    Posts : 14
    Join date : 2009-04-06

    Re: Blown fuse ... what now?

    Post by hup_d_dup on Thu Jul 02, 2009 8:55 am

    I was looking at my schematics of the Mk III and saw something I didn't notice before; one schematic shows 3A slow blow fuse and the other one shows a regular 3A fuse.

    What's up with that?

    Jonathan

    Bob Latino
    Admin

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    Join date : 2008-11-26
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    Re: Blown fuse ... what now?

    Post by Bob Latino on Thu Jul 02, 2009 9:28 am

    Jon,

    The pictorial that shows "3 amp slo-blo" is probably the pictorial that I enhanced with color. All the original Dynaco pictorials had no color. In the Stickys above under "Dynaco Tube Pictorials" is a large version of the enhanced Mark III pictorial. Below is a smaller version of the same pictorial.

    Bob



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