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    id like to improve but keep true, my 66 ST-70

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    dmagazz

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    id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Fri Jun 22, 2018 1:00 pm


    this is where i am at.
    i was going to save up and get a VTA-st70
    AND THEN!! this 66 great condition dynaco st-70 fell in my lap for $440.
    i thought i was taking a chance because it was powered on,but untested,
    so i was considering using it to finance the VTA-st70
    HOWEVER Smile
    the unit is quiet, no hiss or hum.
    my transformers are good, old,cloth wire and are in good shape.
    power cord is in great shape.
    the housing is great,cover is in good shape, all the connectors seem to be ok, no burn spots, or traces lifting off the boards, all solder connections look good.
    the choke has heated a bit and melted the wax out onto the bottom cover a bit, but its yellow not brown.
    ALL the original DYNACO branded tubes are in it and work good.
    i ran my HK-730 preamp outputs into it and it sounds SPECTACULAR!!

    BUT*the bios jumps around* so im not using it anymore till i "rectify" the situation :p
    the quad cap and rectifier have to go, just because they are old sketchy and hazardous.i get that!

    NOW! maybe this is asking alot, or too much at once. BUT! there are so many upgrades/kits/mods.
    id like to know which ones may deface it, or cause its value to degrade.
    which serve a safety purpose and are absolutely imperative.(besides the bios diode/res's/caps).
    which will change it for the better( in keeping with the idea of its original intent- (wigiwig)a flat "what you give is what you get"output response.

    i KNOW the bios kit is a must, as is the quad cap, but do i keep the quad cap similar with a slightly higher voltage?
    or should i get one with a higher uf and much higher voltage. like 600v 80/40/20/20.

    RESISTORS,ALL? with metal film? or just a few(besides the ones covered in the bios kit).

    CAPS,ALL? with film,electrolytic,poly? or just a few.

    wire?
    power cord / grounding?
    the boards?
    pots?
    sockets?
    switches?
    connectors?
    retain the oem driver tube type. or not?(adapters obviously don't degrade value)

    triode mod?
    auto-bios addition?

    also ive seen addressing's toward fluttery low bass response, limited high response, and references to the solid state recto,

    there is SOOOO much to consider.
    Im trying to wrap my head around improving it, while retaining its value, keeping it true to its nature, or even fixing what they didn't have the technology or ability to pull off back when it was designed.

    so many of you have been down this road already, i hope to gain from you. and thanks if you allow me to do that.
    thank you all in advance for your knowledge in getting me to my end result.


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    PeterCapo

    Posts : 561
    Join date : 2008-12-05

    Re: id like to improve but keep true, my 66 ST-70

    Post by PeterCapo on Fri Jun 22, 2018 3:27 pm

    this is where i am at.
    i was going to save up and get a VTA-st70
    AND THEN!! this 66 great condition dynaco st-70 fell in my lap for $440.
    i thought i was taking a chance because it was powered on,but untested,
    so i was considering using it to finance the VTA-st70
    HOWEVER  
    the unit is quiet, no hiss or hum.
    my transformers are good, old,cloth wire and are in good shape.
    power cord is in great shape.
    the housing is great,cover is in good shape, all the connectors seem to be ok, no burn spots, or traces lifting off the boards, all solder connections look good.
    the choke has heated a bit and melted the wax out onto the bottom cover a bit, but its yellow not brown.
    ALL the original DYNACO branded tubes are in it and work good.
    i ran my HK-730 preamp outputs into it and it sounds SPECTACULAR!!


    Before you do anything, I’d recommend you become familiar with the following two indispensable documents, in case you don’t already have them:

    https://dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf
    AND
    http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

    The original Dynaco Stereo 70 circuit comprises a fine amplifier that, when functioning properly, still does what it always did: it gets you most of the way to the sonic performance level of hifalutin amplifiers costing a lot more.  Beyond this thought, different amplifiers have different sonic signatures, and which one is "better" usually boils down to a matter of personal taste.

    Much is sometimes made of the burn spots on the old Dynaco PC boards.  But, as long as the board substrate and etches are intact, then I don’t think it is a concern.  At the same time, the old phenolic substrate is not terribly strong, so it might be better to support it from the bottom when inserting 7199s.  As far as the etches lifting, this is usually only an issue when the board is reworked (desoldered/resoldered).

    All that said, an old, original Stereo 70 likely will need some 1:1 parts refreshing to bring out more of its sonic potential.

    *************************************
    BUT*the bios jumps around* so im not using it anymore till i "rectify" the situation :p
    the quad cap and rectifier have to go, just because they are old sketchy and hazardous.i get that!


    By "rectifier" I assume you are referring to the selenium rectifier in the bias circuit - sure, replace that.  If you are referring to the 5AR4 rectifier tube, the one you now have may still be good, but, if not, replacement 5AR4s are readily available at reasonable cost.  You don’t need to replace the 5AR4 rectifier tube with a solid-state device, by the way, unless you really want to, and, if you want to, I recommend making another post to talk about the ramifications of doing so.  Can you say more about how the bias "jumps" around?  It is okay if the hundredths or thousandths decimal place run around.

    *************************************
    NOW! maybe this is asking alot, or too much at once. BUT! there are so many upgrades/kits/mods.
    id like to know which ones may deface it, or cause its value to degrade.
    which serve a safety purpose and are absolutely imperative.(besides the bios diode/res's/caps).
    which will change it for the better( in keeping with the idea of its original intent- (wigiwig)a flat "what you give is what you get"output response.

    i KNOW the bios kit is a must, as is the quad cap, but do i keep the quad cap similar with a slightly higher voltage?
    or should i get one with a higher uf and much higher voltage. like 600v 80/40/20/20.


    So, again, "better" here in terms of sonic results is quite subjective.  If you desire to retain its original identity and original sonic signature, then a 1:1 parts refresh is the way to go, meaning, you replace original parts one at a time with a new part of the same or better value and/or rating.  It is not necessary to swap out the original circuit for a different circuit, and, were you to do so, the amp would no longer be a Dynaco, IMO.

    As far as value is concerned, it’s kind of hard to say.  It has good value now as a functioning original.  But, I have seen plenty of originals that have had 1:1 parts refreshing that sell for a decent price on eBay, for instance.  I guess what helps resale is if it looks clean and orderly and if you can say it is working well and then post good-looking photos with good ad copy.

    For safety, I suggest independent study of safe work practices around high voltage electronics like tube amps.  And, as far as safe operation of the amplifier, the original Dynaco manual mentions some things to look out for.  But, AFAIK, this type of amplifier, and quite possibly also its various offshoots in current production, might not be designed to present day electrical safety standards.  You might want to look into this more, on your own.  Never leave it on unattended.

    Which parts to start replacing 1:1 is something that sounds like you have already gotten some input on.  The usual suspects are: the quad cap, the selenium rectifier in the bias supply, and the two bias supply electrolytics.  The quad cap with the higher voltage ratings should not be necessary.  You can get 525VDC 40/80/30/20 quad cap and install it in that order.

    *************************************
    RESISTORS,ALL? with metal film? or just a few(besides the ones covered in the bios kit).

    CAPS,ALL? with film,electrolytic,poly? or just a few.

    wire?
    power cord / grounding?
    the boards?
    pots?
    sockets?
    switches?
    connectors?
    retain the oem driver tube type. or not?(adapters obviously don't degrade value)

    triode mod?
    auto-bios addition?

    also ive seen addressing's toward fluttery low bass response, limited high response, and references to the solid state recto,

    there is SOOOO much to consider.
    Im trying to wrap my head around improving it, while retaining its value, keeping it true to its nature, or even fixing what they didn't have the technology or ability to pull off back when it was designed.

    so many of you have been down this road already, i hope to gain from you. and thanks if you allow me to do that.
    thank you all in advance for your knowledge in getting me to my end result.


    One thing worth doing as soon as possible would be to clean the tube socket contacts and retension contact number 5 for the four EL34s.  This can be tedious.  I recommend NOT spraying them.  I prefer scrubbing them with 91% or purer isopropyl and "interdental brushes," blown out immediately with compressed air before the alcohol evaporates.  To retension the contacts...  the contacts in many of the original octal tube sockets Dynaco used look like the letter "C" when viewed from above with the tube removed.  Take a small flat blade screwdriver or an awl and carefully press against the top or bottom of the "C" but not opposite the split in the "C."  Don’t close the contact too much, or you won’t be able to get the tube back in.  Cleaning and retensioning the contacts is important, as tubes are sometimes ruined by bad contact, again, especially contacts number 5 that provide the bias voltage for the EL34s.

    Don't try to do everything you are asking about at once.  My suggestion is to study as much as you can and take it in incremental steps.  For instance, after cleaning and retensioning the contacts, changing the quad cap, the bias rectifier and the bias electrolytics, fire it up and take readings to compare with the reference voltage chart found in the original Dynaco manual.  If the voltage readings are right and if it sounds good, then you have a working amplifier that doesn’t really need anything else.  If you then want to bring out more of the sonic potential of the original Dynaco circuit in the amp, then additional parts refreshing can certainly be considered.

    PS - I wouldn't try cleaning the sockets on the PC board, BTW, unless there is some compelling reason to do so.


    Last edited by PeterCapo on Fri Jun 22, 2018 11:13 pm; edited 1 time in total (Reason for editing : Spelling.)
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    dmagazz

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    Re: id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Fri Jun 22, 2018 5:14 pm

    thanks peter capo.
    very good advice.

    BIOS
    the bios when i got it, was 1.78 and 1.65
    i turned the pots up and down rapidly a 1/4 turn or so centering the original position,then i zero'd them in to 1.56 - i noticed 1 effects the other by 2 or 3 hundreths, (.0x)x being the effected)
    i was putting music through it, giving it a light push, id seen the bios's both were fluctuating between 1.44 and 2.11
    i then set the bios's to 1.56 with the signal running.
    i then turned off the input signal(music) and noticed the bios's went down to 1.44 and 1.48
    they were coming up ever so slowly- a 100th of a volt every 2-5 seconds,
    so i set the bios's yet again to 1.56, and left it alone but id noticed the bios's were still moving and differentiating from one another.

    i definitely want to keep it sounding true to its design

    i ordered a dynaco selenium recto swap kit. it has a diode, couple resistors and a couple ceramic(i think) caps.
    i was lookin at the 525v 30/20/20/20 quad cap. so ill go for that next.
    today ill gingerly clean and re-tension the tube sockets.
    i will gingerly clean the driver board with a old toothbrush,and some tuner cleaner.
    while i have the cleaner out ill get the switches and pots.
    i will check the voltages as you say

    after all that i think ill opt for some tube adaptors for the 7199 sockets, so i can use alternatives.
    and ill be keeping the rectifier tube.

    my concerns for safety was for the amps power supply,and tubes as i do push it.
    id like to be assured its going to be reliable with no sketchy overheating issues.
    considering voltage difference's between when it was designed and now.





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    PeterCapo

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    Re: id like to improve but keep true, my 66 ST-70

    Post by PeterCapo on Fri Jun 22, 2018 6:13 pm

    I wouldn't clean the PC board with any kind of cleaner.  The PC board's phenolic substrate is vulnerable to chemicals and could be destabilized by them.  Also, there is a wax coating on the foil side that may result in a mess if chemicals contact it.  A safer approach is a dry cleaning of the top side of the PC board only, using a toothbrush or something like that.  I also recommend that you not spray the pots or the switches for basically the same reason.  My rationale for this can be found here: http://audiokarma.org/forums/index.php?threads/dynaco-pas-3x-and-the-tone-control-modification.772264/#post-10575372

    the bios when i got it, was 1.78 and 1.65
    i turned the pots up and down rapidly a 1/4 turn or so centering the original position,then i zero'd them in to 1.56 - i noticed 1 effects the other by 2 or 3 hundreths, (.0x)x being the effected)
    i was putting music through it, giving it a light push, id seen the bios's both were fluctuating between 1.44 and 2.11
    i then set the bios's to 1.56 with the signal running.
    i then turned off the input signal(music) and noticed the bios's went down to 1.44 and 1.48
    they were coming up ever so slowly- a 100th of a volt every 2-5 seconds,
    so i set the bios's yet again to 1.56, and left it alone but id noticed the bios's were still moving and differentiating from one another.


    After a cold start, the bias should be set after about ten minutes.  It should be set with no signal on the inputs and will vary by hundredths or thousandths of volts – this is normal enough.  The bias reading will vary with fluctuations or drifting in your AC mains.  If the amp runs for a while, the bias may slowly drift downward, again, I think this is normal enough.  You might try taking a series of bias measurements beginning right at a cold start and then, say, every ten minutes and record the readings.  At the same time, record corresponding readings of your AC mains.  Maybe do this over the course of an hour.  Again, no signal at the amplifier’s inputs.

    i definitely want to keep it sounding true to its design

    i ordered a dynaco selenium recto swap kit. it has a diode, couple resistors and a couple ceramic(i think) caps.

    Should be electrolytic.  What kit is it?  Can you post a link to it?

    i was lookin at the 525v 30/20/20/20 quad cap. so ill go for that next.

    That will retain absolute originality, however using the 40/80/30/20 is still in accord with the original design and could offer a bit of sonic improvement.

    today ill gingerly clean and re-tension the tube sockets.
    i will gingerly clean the driver board with a old toothbrush,and some tuner cleaner.
    while i have the cleaner out ill get the switches and pots.

    i will check the voltages as you say


    Remember that the values in the manual's voltage chart are referenced to the AC mains that the power transformer primary winding was intended for.  For most of the original Stereo 70s, this was 117VAC mains.  Some later Stereo 70s might have had a 120VAC PT primary.

    after all that i think ill opt for some tube adaptors for the 7199 sockets, so i can use alternatives.
    and ill be keeping the rectifier tube.

    my concerns for safety was for the amps power supply,and tubes as i do push it.
    id like to be assured its going to be reliable with no sketchy overheating issues.
    considering voltage difference's between when it was designed and now.


    If the amplifier is in proper working order, and if it has proper ventilation, then it should be okay being driven hard.  In its original configuration, the Dynaco manual specified that the Stereo 70 is okay with AC mains up to 125VAC.  I suggest taking readings and recording your level of AC mains a few times a day for a couple of weeks to see how it varies.  If your AC mains gets to 125VAC or higher, then I'd suggest putting an appropriately rated NTC thermistor in series with the PT primary.

    Finally, be sure that you have a 3A fuse in the fuseholder.
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    dmagazz

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    Re: id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Fri Jun 22, 2018 8:04 pm

    thanks pete
    bios.
    glad that was acting all fairly normal
    as to the kit. it was $8 and supposedly new old stock
    i cant post links yet, just joined, i tried that initially.
    but this is the description.
    (1) Diode Kit for the Dynaco Stereo 70 Tube Amplifiers; this kit replaces the original outdated selenium rectifier with a modern diode.  We consider these NOS as they came from the SV / SCC inventory which dates back to the 1980's.  These are not Dynaco brand "kits", rather they are aftermarket kits that were offered by SCC back in the 1980's.  These are a simple and inexpensive way to replace your old crusty selenium rectifier; included are a very basic instruction sheet, diode, terminal strip, and hardware.  The parts are sealed in a small paper envelope with the instructions folded around it; some have dust from warehouse storage but perfectly usable.  (D544042R)

    and holy crap. that number?!D5540942R...might be the rectifier itself! i just found it online at
    double u double u double u. dot.thermionic.de/tdyna5.html as part 5540942

    and the cap 525v 80/40/30/20 - duly noted.
    tranny
    how would i know if mine was a 120v primary tranny?i do know its probably year 66-67
    i can tell you now, my mains run from 118-125, ive seen that in the past on the display of a battery backup i had.
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    PeterCapo

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    Re: id like to improve but keep true, my 66 ST-70

    Post by PeterCapo on Fri Jun 22, 2018 8:55 pm

    Is this the one you found: https://www.ebay.com/itm/DYNACO-DYNAKIT-ST70-Tube-Amplifier-Diode-Kit-replaces-Selenium-Rectifier/372340605905?hash=item56b13ac3d1:g:TJoAAOSwxH1T8Nv5  It does not seem to be NOS Dynaco.  It is NOS by a company that was known as "Stereo Cost Cutters" a.k.a "Sound Values."  It looks like it replaces the diode, but it doesn't say what kind of diode comes with the kit.  The two bias supply electrolytics should also be replaced, but it doesn't look like this kit includes them, or did I miss it?  The other website in Europe may very well have original selenium rectifiers, but I have read that they degrade over time even if they have not been used.  Don't know how true this might be, but, if it is going to be replaced, might as well get a replacement that is known to be reliable.

    You might do better with the following kit, which includes everything for not much more money: https://www.dynakitparts.com/shop/st-70-bias-circuit-kit/  Not sure which diode this kit comes with, though it may be a 1N400x.  A possible upgrade to the diode would be a UF4007 available from places like mouser.com or digikey.com.

    If your Stereo 70 is from 1966, it is probably a 117VAC PT.  If your AC mains runs as high as 125VAC, then I would put two CL80 thermistors in series with the PT primary.  Going forward, I'd still continue to monitor your AC mains with your mutlimeter and record the readings.  If it goes high enough, you could have problems even with CL80 thermistors in use.  Other thermistors might be more appropriate, depending on what your AC mains does. Some end up using a variac to adjust the AC the amp sees.
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    dmagazz

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    Re: id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Fri Jun 22, 2018 10:32 pm

    yes! thats it.
    i saw the dynaco kit, that was first on my list, i saw they were out till july, so i looked for it on ebay to get it faster. maybe that was a bad idea, i took it from the post that it was a dynaco part.. my bad thinking.
    incidentally, you think there is any use to keeping the parts i am replacing?
    also today my service mains are L1 122 AND L2 119 the lug running my amp and other stuff like my guitar amps are the lower voltage lug, the lower lug seems to have changed from the high position to the lower position since the tornadoes(in new fairfield, ct
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    PeterCapo

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    Re: id like to improve but keep true, my 66 ST-70

    Post by PeterCapo on Fri Jun 22, 2018 11:02 pm

    dmagazz wrote:incidentally, you think there is any use to keeping the parts i am replacing?

    Might be worth keeping for future reference if any questions come up.

    If you can, it might be helpful if you post a series of photos of the amp, topside and inside.

    Don't know if you have spent much time studying the manual. The manual I previously posted the link to includes the pictorial wiring diagrams. Before you begin working on the amp, it would be helpful to go through the entire assembly procedure, step by step, to see if the amp is built according to the manual or if there any differences. While you describe what sounds like an original Stereo 70, there could have been some undocumented changes made to it over the years. I suggest it only makes sense to establish the configuration of the amplifier before diving in and making further changes.
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    dmagazz

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    Re: id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Fri Jun 22, 2018 11:13 pm

    hey hey pete Smile
    here. i posted pics
    http://dynacotubeaudio.forumotion.com/t3755-just-got-this-puppy
    i will be studying into that manual, ive downloaded it (both) to my computer.
    thanks for putting that up.
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    PeterCapo

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    Re: id like to improve but keep true, my 66 ST-70

    Post by PeterCapo on Fri Jun 22, 2018 11:24 pm

    Looks like a really nice original sample.  If those are the original EL34s, like it has the original 5AR4, I'd suggest getting new, current production 5AR4 and quad of EL34 to run in the amp until after you've established the amp is stable.  Don't want to damage those precious original tubes.

    Have you verified it has a 3A fuse?
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    dmagazz

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    Re: id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Sat Jun 23, 2018 12:11 am

    they are pete Smile
    ALL dynaco branded tubes
    yea, 3a fuse it is, its a littel USA(old rounded type set text) fuse, wouldnt be surprised to find it was oem also,looks like a quick blow style spring type,with copper flat wire on one side for the actual fuse link.and solid thin wire on the other connecting the spring to the end
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    PeterCapo

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    How original to risk, and also more thoughts on replacing the selenium rectifier

    Post by PeterCapo on Sat Jun 23, 2018 9:52 am

    You know... when running across such a nice original sample, and in reading a comment from Peter W. at the link in Post n°9 of this thread... I am reminded that there is the option of not changing out the original parts.  I am tempted to say:

    1) clean and retension the five octal socket contacts
    2) get a new, current production tube set to use until some more checking is done to increase confidence that the amp is stable
    3) install two CL-80 thermistors in series with the PT primary
    4) get a variac and run the amp up slowly to full AC mains level, over several hours
    5) adjust the variac so you have 117VAC across the power transformer primary and then check all of the voltage test points indicated in the manual's chart

    If the voltage readings line up with the chart, then do nothing else to the amp.  You would then have a totally original Stereo 70, working well.

    The problem with this, of course, is that the selenium rectifier could fail, which is said to be a nasty experience.  Also, if the aged electrolytics fail, there is the potential for collateral damage, though I am not sure how much of a problem this really is.  Also, you can buy a new production quad cap that could fail, and there have been reports online of early failures in them, though you never know what else might have occurred to cause this (pilot error, for instance).

    By the way, replacing the original selenium rectifier is not necessarily a straightforward situation.  The original selenium rectifier, even when functioning as new, had a significantly greater forward voltage drop than present day replacement diodes do.  This, plus a potential 125+VAC mains, results in the original bias supply electrolytics being pushed hard against their voltage rating.  The two original bias supply electrolytics were rated for 75VDC each.  You'd want that bumped up to 100VDC each if replaced, which does appear in the image of the electrolytics supplied by dynakitparts.com.

    However, there is another potential issue, as well.  With the lower forward voltage drop of the replacement diode, some have reported an inability to bias the power tubes, which then necessitated replacing the resistors in the bias supply with different values.  For future reference, should you not be able to bias the amp after replacing the selenium rectifier, here is an article that might help: http://www.curcioaudio.com/ST7%20Bias%20Expansion%20Mod.pdf
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    dmagazz

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    Re: id like to improve but keep true, my 66 ST-70

    Post by dmagazz on Sat Jun 23, 2018 10:45 am

    well there it is! Smile believe me, id like to be able to do minimal changes if possible, but i want to be able to sleep at night.lol knowing its running well.. thing is, i will push it.

    incidentally, i have a variac, but its only protected as 2A.

    on another note, look at the extreme lengths this young man went through to mimic the selenium diode.
    youtube.com/watch?v=r5fquGccuSk
    YYYEA. no....not doin that!! lol

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