The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


+3
Sfguitarworks
peterh
j beede
7 posters

    Blown fuse

    Sfguitarworks
    Sfguitarworks


    Posts : 18
    Join date : 2018-10-31

    Blown fuse Empty Blown fuse

    Post by Sfguitarworks 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
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Wed Oct 31, 2018 8:14 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:47 pm; edited 1 time in total
    j beede
    j beede


    Posts : 473
    Join date : 2011-02-07
    Location : California

    Blown fuse Empty Re: Blown fuse

    Post by j beede 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.

    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Wed Oct 31, 2018 9:19 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 6:28 pm; edited 1 time in total
    Sfguitarworks
    Sfguitarworks


    Posts : 18
    Join date : 2018-10-31

    Blown fuse Empty Re: Blown fuse

    Post by Sfguitarworks 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

    Sent from Topic'it App
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Blown fuse Empty Re: Blown fuse

    Post by peterh 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.
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Thu Nov 01, 2018 11:33 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 6:28 pm; edited 1 time in total
    Peter W.
    Peter W.


    Posts : 1351
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Blown fuse Empty Re: Blown fuse

    Post by Peter W. 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!

    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Thu Nov 01, 2018 3:59 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 6:29 pm; edited 2 times in total
    Peter W.
    Peter W.


    Posts : 1351
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Blown fuse Empty Re: Blown fuse

    Post by Peter W. 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.
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Thu Nov 01, 2018 4:31 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 6:29 pm; edited 1 time in total
    Sfguitarworks
    Sfguitarworks


    Posts : 18
    Join date : 2018-10-31

    Blown fuse Empty Re: Blown fuse

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

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


    Blown fuse Dc8b0710
    Blown fuse Ed735710
    Blown fuse B31a3f10
    Sfguitarworks
    Sfguitarworks


    Posts : 18
    Join date : 2018-10-31

    Blown fuse Empty Re: Blown fuse

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

    I modified it for 6gh8a tubes, FYI.
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Fri Nov 02, 2018 3:34 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:46 pm; edited 1 time in total
    Sfguitarworks
    Sfguitarworks


    Posts : 18
    Join date : 2018-10-31

    Blown fuse Empty Re: Blown fuse

    Post by Sfguitarworks 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

    Sent from Topic'it App
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Sat Nov 03, 2018 3:57 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:46 pm; edited 1 time in total
    deepee99
    deepee99


    Posts : 2244
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Blown fuse Empty Re: Blown fuse

    Post by deepee99 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
    ttownscott
    ttownscott


    Posts : 40
    Join date : 2018-07-31
    Age : 59
    Location : Tacoma, WA

    Blown fuse Empty Re: Blown fuse

    Post by ttownscott 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.
    Tubes4ever
    Tubes4ever


    Posts : 167
    Join date : 2015-07-14
    Location : Star, Idaho

    Blown fuse Empty Re: Blown fuse

    Post by Tubes4ever 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.
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Mon Nov 05, 2018 11:16 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:46 pm; edited 1 time in total
    Peter W.
    Peter W.


    Posts : 1351
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Blown fuse Empty Re: Blown fuse

    Post by Peter W. 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.
    avatar
    Guest
    Guest


    Blown fuse Empty Re: Blown fuse

    Post by Guest Mon Nov 05, 2018 11:38 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:45 pm; edited 1 time in total
    Peter W.
    Peter W.


    Posts : 1351
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Blown fuse Empty Re: Blown fuse

    Post by Peter W. 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.

    Sponsored content


    Blown fuse Empty Re: Blown fuse

    Post by Sponsored content


      Current date/time is Thu Mar 28, 2024 4:20 pm