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    Blown fuse

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    Sfguitarworks

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    Blown fuse

    Post by Sfguitarworks on Wed Oct 31, 2018 7:04 pm

    Hello,

    Been lurking a bit and have learned a ton. I was gifted a dumpster find ST-70, kind of beat up, but all there. I did a full recap, including quad can, installed grounded power cord and diode rectifier. I initially installed tge diode backwards, so when i powered it without tubes, one of the bias caps got really hot. I shut it off quickly and reversed the diode. It powered on and was stable. I installed the tubes, set bias, and it works fine. Today, i listened to it for a few songs at moderate volume, turned it off, and when i turned it back on after abouf 15sec, the rectifier tube flashed and the fuse blew. I put in a new fuse and it seems fine. Any ideas? Also, it has a low level buzz, so i’m replacing the bias caps, as they got hot, and all the resistors. Thoughts? Thanks so much for your time!

    Geoff
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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Wed Oct 31, 2018 8:14 pm

    A few thoughts for starters...

    Never power cycle the amp quickly.  Give it several minutes after turning the power off before turning it back on again.  Quickly power cycling the amp may or may not be the reason why the fuse blew, but it should not be done in any case.  Verify your fuse is slo-blo and not more than 3A.

    The buzzing might just be the grounded power cord.  Try lifting the power cord ground with a three prong to two prong adapter at the wall socket.  But, there could be other causes, as well.

    If you do not already have one, here's a complete copy of the original Stereo 70 manual including the pictorial wiring diagrams: https://dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf   There are lists of reference voltages on the same page as the schematic for the power supply.  I suggest taking readings at the indicated nodes in your amp and see how close they come to the reference values.  This is a good and fairly easy way to establish the basic health of your amp.  If there is an issue, this could be a good way to start narrowing it down.

    Observe safe work practices around high voltages.
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    j beede

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    Re: Blown fuse

    Post by j beede on Wed Oct 31, 2018 9:04 pm

    Re: rectifier flash: You can increase the effective peak inverse voltage of your 5AR4 by inserting a pair of forward high voltage diodes (1N4007 like you used in the bias circuit will work fine) ahead of pins 4 and 6. Do a search for "yellow sheet" mod.

    I have never achieved buzz-free operation with my Dyna amps when using a grounded power cord. If safety is a priority, running an amp with high voltage present on the accessible side of the input PCB (like some Dynaco tube amps) may be a bad idea to begin with.

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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Wed Oct 31, 2018 9:19 pm

    A GFCI can provide a measure of safety, especially if the power cord's safety ground wire is "lifted" in some way.  The GFCI should be the kind that requires a manual reset after it trips, which will also prevent rapid power cycling of the amplifier in the event of rapid fluctuations in the AC mains.

    Which 5AR4/GZ34 are you using?  I'd suggest the current production Gold Lion from McShane Design.  Some other current production 5AR4/GZ34 may not be robust enough for the Stereo 70.  Whichever rectifier tube you use, it will be helped with handling the inverse voltage stress by using the Yellow Sheet diode mod that j beede mentioned, but of course this is a mini project requiring a little rewiring.

    But, there could also be other issues, so I suggest proceeding with studying the manual and checking the voltages as I mentioned in Post n°2.  Again, there are harmful or fatal voltages present on and in the Stereo 70, so please be careful.
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    Sfguitarworks

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    Re: Blown fuse

    Post by Sfguitarworks on Thu Nov 01, 2018 2:36 am

    Hello,

    Thanks so much for all the info! It’s a tung sol tube. I removed the ground wire and the hum
    is almost completely eliminated. The amp itself has a slight hum, but almost nothing through the speakers. The amp tech at my repair shop won’t work on an amp without a three prong, so that’s why I went that way. I’ll do the diode mod and I think I’ll be in good shape. The amp sounds really nice, so I’m pretty happy. On a side note, what sound improvement could I expect from the vta driver board? Have any of you used the octal in a stock amp? I don’t need to replace the power transformer now, but wouldn’t rule it out if the octal board is way better. It just sounds so good now, I am curious as to benefit.

    Take care,

    Geoff

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    peterh

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    Re: Blown fuse

    Post by peterh on Thu Nov 01, 2018 4:14 am

    Sfguitarworks wrote:Hello,

    Been lurking a bit and have learned a ton. I was gifted a dumpster find ST-70, kind of beat up, but all there. I did a full recap, including quad can, installed grounded power cord and diode rectifier. I initially installed tge diode backwards, so when i powered it without tubes, one of the bias caps got really hot. I shut it off quickly and reversed the diode. It powered on and was stable. I installed the tubes, set bias, and it works fine. Today, i listened to it for a few songs at moderate volume, turned it off, and when i turned it back on after abouf 15sec, the rectifier tube flashed and the fuse blew. I put in a new fuse and it seems fine. Any ideas? Also, it has a low level buzz, so i’m replacing the bias caps, as they got hot, and all the resistors. Thoughts? Thanks so much for your time!

    Geoff
    Change the bias cap, once connected backwards you cannot trust it. And if it, even for a moment,
    shorts your tubes are in danger.
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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Thu Nov 01, 2018 11:33 am

    Glad to hear you achieved a reduction in hum.  I'd definitely get it on a GFCI, though.

    The physical buzzing is not uncommon with power transformers.  You could try a set of isolation mounts: https://www.dynakitparts.com/shop/power-xfmr-isolation-kits/

    DC offset on the AC mains might also be contributing to the physical buzzing https://emotiva.com/collections/accessories/products/cmx-2
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    Peter W.

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    Re: Blown fuse

    Post by Peter W. on Thu Nov 01, 2018 3:49 pm

    I have sat on my fingers until now, giving it both some thought, and hoping for some points to be covered. Most here have seen most of this before:

    There are two types of delayed-action fuses. The first is the typical SLO-BLO fuse, which consists of a wire element wound around a bit of ceramic. They look like this:

    https://assets.alliedelec.com/c_scale,w_400,f_auto,d_no_image.png/70184163.jpg

    The second is a TIME DELAY fuse, AKA a "Dual element" fuse. They look like this:

    https://assets.alliedelec.com/c_scale,w_400,f_auto,d_no_image.png/70149818.jpg

    The former will tolerate up to a 100% overload for tens of seconds, a 50% overload for tens of minutes and a 20% overload for far too long. These fuses should not be let anywhere near tube equipment unless its ONLY purpose is to protect real-estate. These fuses will tolerate short-cycling quite well. So, if this fuse type was in place, then short-cycling is likely not the issue.

    The latter will accept a substantial, but very brief overload at turn-on (when the filaments are cold) but act as a conventional fuse thereafter. These fuses do not like short-cycling, and should be allowed to cool for several minutes, at least before a re-start. Such as these have some ability to protect equipment as well as real-estate.

    Next: Fuses are wearing parts. They get old, and they fail. It is a function of start/stop cycles, how close to full rating they operate, and actual operating voltage as to whether that failure is 1 wee, 1 year, or 30 years down the road, but fail they will. Those fuses sized closest to actual operating parameters will fail soonest, but will also protect the equipment best.

    And: Pretty much every regular operator of tube equipment should have the wherewithal to measure within very small increments. By such means, one may tell whether or not there is an impending failure mode - a dozen watts too many, and something is up.

    And: Whereas I agree that a red-plated tube may be toast, at least with post-blight devices, I have any number of pre-blight tubes that have been through all sorts of trauma in equipment that has crossed my bench over the years - and as I have a decent tube-tester, I am able to separate the wheat from the chaff. IF (and only if) you have the means to make a reliable test, do so. Otherwise, yes, toss the tube.

    Also: Even the best polarized cap does not like reverse voltage. They are cheap enough not to take the risk. NP caps don't care. I will use screened NP caps when practical and optimal, mostly on solid-state circuits, however.

    Also: On a dead-cold amp, try tightening the transformer bolts (holding the laminations together) and if that does not work, obtain some super-thin super-glue and drip that over the top of the windings. A little goes a long way. This should reduce mechanical hum.

    Lastly: Lose the 3-way line cord. A polarized cord with the hot to the switch is OK, however. Even suggested.

    And, Enjoy!

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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Thu Nov 01, 2018 3:59 pm

    Dynaco originally called for a slo-blo fuse.
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    Peter W.

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    Re: Blown fuse

    Post by Peter W. on Thu Nov 01, 2018 4:10 pm

    PeterCapo wrote:Dynaco originally called for a slo-blo fuse.

    They did, in words. Absolutely true. However, 100% of vintage dynaco equipment up through Blackwood, that was fused was supplied with dual-element fuses. The nomenclature has "matured" for lack of a better word, and the fuse suppliers are at pains to differentiate now. Given that a Dual-Element fuse may cost 20x as the same value slo-blo in certain values, this has some validity.

    I am something of a fuse nut, if you have not figured that out by now. I admit to not being entirely rational about it, either.
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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Thu Nov 01, 2018 4:31 pm

    Oh, that's perfectly fine, of course.  I would say it's good for readers to be informed of the various, relevant parameters of the matter being discussed.

    I'd like to see some photos inside and out of Sfguitarworks Stereo 70.  There may be additional things that can be done to bring out more of its potential.
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    Sfguitarworks

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    Re: Blown fuse

    Post by Sfguitarworks on Fri Nov 02, 2018 3:22 pm

    Thanks for the info. Here are a few pics. Interested to get your thoughts.




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    Sfguitarworks

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    Re: Blown fuse

    Post by Sfguitarworks on Fri Nov 02, 2018 3:23 pm

    I modified it for 6gh8a tubes, FYI.
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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Fri Nov 02, 2018 3:34 pm

    Images of the top side could be helpful, too.

    The original bias capacitors were 50µF @75VDC.  If the selenium rectifier is changed for a silicon diode, as you have done, and if the AC mains runs a little higher, then you kind of start pushing up against the original 75VDC rating. The bias capacitors in your photos are 47µF @500V.  Not necessary to be rated for this much voltage, but it won't hurt anything.  100µF at 100 volts or so should be safe under normal operating conditions and reduce the ripple a bit more.

    Wires from the power cord are kind of all over the place.  Wonder if some of the hum might be coming from the way they are routed.  Might be better to shorten them and stuff them behind the power transformer.

    Might get a further reduction in hum by shorting the two 10 ohm resistors on the PC board.

    Looks like you retained the original carbon comp resistors underneath.  Did you check their values out-of-circuit, with a meter?

    Is that a 1N4007 diode for the bias supply?  A UF4007 or other fast or zero recovery diode could be an audible upgrade.

    I've considered, for an old original like this one, whether there might be a sonic benefit from resoldering all of the connections throughout the amp.

    Do you have the original carbon comp resistors and original coupling capacitors on the PC board?  Again, the resistor values should be checked out-of-circuit.  Replacing the original coupling capacitors on the PC board with polypropylene should be a sonic improvement.

    Looks like a three lug power switch?  What are the current and voltage ratings on that switch?
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    Sfguitarworks

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    Re: Blown fuse

    Post by Sfguitarworks on Sat Nov 03, 2018 2:11 am

    Hello peter,

    Thanks for the info. I’ll send pics of the top next time I open it. I’ve replaced all the caps up top with orange drops and micas. I’ll rerun the power leads.

    Last night I made plates for, and installed, gold banana plugs on the 4ohm circuit and wider spaced gold rca’s on the input. I also removed the stereo/mono circuit. Plugged it in tonight and it is dead quiet at the speakers. Sounds great too.

    I have not checked the resistors yet, but I have all new ones on the way, with 100uf bias caps. I’m kind of scared to mess with it at this point, I’ll be installing the 100uf bias caps for sure.

    The switch was ordered as a stereo mono switch, but I needed it for my power switch. It’s rated the same as the switch dynakitparts sells for power. 6A. Any reason that won’t work?

    I have two questions. Have you seen 8pin socket plugs? I’d like to cover the sockets on the front of the amp for safety reasons. Also, do you have thoughts on upgraded driver boards? What type of sonic difference could I expect with a vta board? Classic vs octal?

    Thanks again!

    Geoff

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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Sat Nov 03, 2018 3:57 pm

    Orange drops are good capacitors.  From my experience, plus comments from others, I think they are a little on the warmer side.  If you ever want it a bit brighter, you might try Multicap RTX or certain other polypropylenes.  Again, if you think the sound is too warm or rolled off on top for you, try brighter tubes like the EH EL34 or brighter yet KT77.

    The 390pf mica on top of the board should be rated for 1000VDC - the original was 1000VDC: https://www.audioasylum.com/forums/dynaco/messages/12/6416.html

    6A rating is fine for the switch.  Again, make sure fuse is not more than 3A.

    For the front panel sockets, sure, you could tape them over or something, if you think someone might decide to stick a fork in them or otherwise manage to touch a hot contact.  Not sure how likely that would be, but it’s your call.

    One other thing I should point out about all of the sockets.  They should be cleaned and retensioned, which could prevent or cure a variety of gremlins.

    As for "upgraded" driver boards, there have been and continue to be a variety available.  I have never heard any of them, so I cannot tell you first hand what I think they sound like.  I have read a lot of comments from others, and the different boards each have their own following.

    I can tell you that my dynakitparts Stereo 70, having the original driver circuit, and with the kind of refinements I have been suggesting for yours, holds its own very nicely against my other amplifiers that include a Cary SLI-80 Signature, Parasound Halo A21 and McIntosh MC275 MkIV.  I am not suggesting that the original Dynaco driver board is a "world beater," but it surely does have the high end vibe.  Whether or not someone likes one of the drop-in replacement circuits better than the original may just boil down to personal taste.  YMMV.
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    deepee99

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    Re: Blown fuse

    Post by deepee99 on Sun Nov 04, 2018 10:48 am

    Peter W. wrote:
    PeterCapo wrote:Dynaco originally called for a slo-blo fuse.

    They did, in words. Absolutely true.  However, 100% of vintage dynaco equipment up through Blackwood, that was fused was supplied with dual-element fuses. The nomenclature has "matured" for lack of a better word, and the fuse suppliers are at pains to differentiate now. Given that a Dual-Element fuse may cost  20x as the same value slo-blo in certain values, this has some validity.

    I am something of a fuse nut, if you have not figured that out  by now. I admit to not being entirely rational about it, either.
    Peter, we are *all* fuse nuts. Why else would we spend so much money on all that expensive equipment just to protect one of the little buggers? Very Happy
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    ttownscott

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    Re: Blown fuse

    Post by ttownscott on Sun Nov 04, 2018 9:06 pm

    [quote="PeterCapo"]

    DC offset on the AC mains might also be contributing to the physical buzzing https://emotiva.com/collections/accessories/products/cmx-2[/quote


    This device made a big difference in my house. I bought it from Amazon just in case it did not so that returns would be easy.
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    Tubes4ever

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    Re: Blown fuse

    Post by Tubes4ever on Mon Nov 05, 2018 1:50 am

    ttownscott wrote:
    PeterCapo wrote:

    DC offset on the AC mains might also be contributing to the physical buzzing https://emotiva.com/collections/accessories/products/cmx-2[/quote


    This device made a big difference in my house. I bought it from Amazon just in case it did not so that returns would be easy.

    Tried the link and it didn't work.
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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Mon Nov 05, 2018 11:16 am

    https://emotiva.com/collections/accessories/products/cmx-2

    Frank Van Alstine also made a similar product. Not sure if he is still making it or not.
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    Peter W.

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    Re: Blown fuse

    Post by Peter W. on Mon Nov 05, 2018 11:31 am

    On DC-offset on the AC Mains.

    Whatever goes through a transformer will not have any DC offset. If DC offset is defined as the sine-wave not crossing over at 0, that will be stopped dead by a transformer (Or choke, or auto-transformer). And why it is back in the day - and in a few cases to this day - some solid-state equipment manufacturers used interstage transformers. These include, but are not limited to:
    McIntosh, AR, Scott and others....

    For the purposes of a ST70, OEM or otherwise, this will not be an issue. The power-transformer will eliminate it from the mains, and the output transformers will remove any DC from the speakers - and why OPT-equipped devices clip so softly. They will not deliver DC into the speakers.
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    PeterCapo

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    Re: Blown fuse

    Post by PeterCapo on Mon Nov 05, 2018 11:38 am

    Okay but isn't the point of this thing to eliminate the DC offset ahead of the power transformer, in order to achieve a reduction in physical, or mechanical, buzzing from the power transformer itself, not to keep it out of the circuit after the power transformer?
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    Peter W.

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    Re: Blown fuse

    Post by Peter W. on Mon Nov 05, 2018 11:55 am

    PeterCapo wrote:Okay but isn't the point of this thing to eliminate the DC offset ahead of the power transformer, in order to achieve a reduction in physical, or mechanical, buzzing from the power transformer itself, not to keep it out of the circuit downstream?

    Why smells the goat on yonder hill? It eats naught but chlorophyll!

    If that reference is obscure to you, there was a time when adding Chlorophyll to various things was touted as a means to make people smell better, have better breath, and find better looking partners. The Chicago Tribune did an expose on the practice using that phrase as the headline. In other words, it was a solution in search of a problem. Similarly, DC-Offset.

    A transformer will respond to Chopped DC exactly as it does to AC. Those of us who dabble in vintage radios and have come across vibrators for car radios. The vibrator would make chopped DC, from which the transformer could then make B+ at sufficient voltage to drive a tube oscillator. The transformer does not care what it sees, offset or otherwise. Mechanical buzzing may be fixed by mechanical means, and is typically caused by a mechanical failure. Put another way, if what is coming off your mains is bad enough to cause a transformer to act out, you will have far worse problems than a bit of buzzing in that transformer.

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