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


    ST70 diode warning

    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty ST70 diode warning

    Post by jeffjmr on Sun Aug 23, 2020 4:40 pm

    Hi all,

    Old audiophile with first tube amp. Recent factory build ST70 by a VTA competitor (sorry, got it on auction at a decent price.)

    So far so good. Love the sound, and the ability to fiddle with it (and some electronic fiddling experience). I am in the middle of restoring a pair of Altec VOTT A5Xs with the intent to drive them with the ST70.

    I have read almost this entire forum, and am soaking up all the knowledge I can muster, and am thankful to Bob and all else who share. I will attempt to return the favor as opportunity arises.

    My concern is, somewhere along the line I read a post from someone who was not a Dynaco fan, who complained that he had no time for an amp with drivers that could be blown up at any moment by the failure of the single phase inverting diode!

    Yikes! That is a pretty sobering thought. Since I have yet to perform the famed yellow sheet diode mod, I can only find one diode in my amp and that is on the power supply capacitor board.

    Any idea if this is the diode he refers to, and is it that critical? If so, is there a more robust diode besides a 1N4007 called for here? Or perhaps wiring two in series as insurance?

    I have parts on order to install a CL-80 and the yellow sheet rectifier diodes mod, and a GFI that interrupts on power fails with manual reset. Any other mods suggested? By the way, I am going with the more robust DGP30-E3/54 diodes instead of 1N4007 unless there is some reason I shouldn’t.

    Thanks in advance for any advice.
    Jeff

    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sun Aug 23, 2020 5:39 pm

    Which version of Stereo 70 do you have, please?  It may help with sorting things out.

    Can you post a link to the statement about failure of a single phase inverting diode?  Out of context, I am not sure what he is referring to.  Perhaps someone else can discern it, but I’d like to see it in the context of whichever thread it was in.

    The yellow sheet diode mod reduces the peak inverse voltage demands on the 5AR4/GZ34 rectifier tube.  It does nothing to help the rectifier tube survive forward current demands.  Still worth doing, though.  Not thinking this is what your quotation was referring to.

    The diode to which you refer is likely in the bias supply.  A single 1N4007 is perfectly fine – overkill, actually.  If you are contemplating using a different diode such as the DGP30-E3/54 that you mentioned, you’d better have a look at it’s operating characteristics to make sure it isn’t some kind of special function diode.  I once picked up a Stereo 70 locally and found it had an avalanche diode in the bias supply.  It probably worked okay but really wasn’t a proper choice.  Again, a single 1N4007 is more diode than you’ll ever need for the Stereo 70 bias supply.  But, since a 1N4007 has a smaller forward voltage drop than the original selenium rectifier in the bias supply, the two bias electrolytics should be rated for 100VDC apiece.  Some folks have reported difficulty biasing the amp after replacing the bias diode, but if you can bias the amp okay, then don’t worry about it.

    CL80 in series with the power transformer primary should be fine.  Manual reset GFCI is a great idea.  A few other things… first, study-up on safe work practices around tube amps, for example, but not limited to this example: https://www.aikenamps.com/index.php/safety-tips-for-working-on-tube-amplifiers

    Second, if you don’t have one, here’s a copy of the original manual with the pictorial wiring diagrams – even though you have changes in your amp, it can still be a helpful reference: https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf

    Third, please notice in the manual on the page with the power supply schematic that there is a voltage chart and another list of reference voltages below the chart.  They are reference values that indicate the basic health of the amplifier.  If you can manage to safely check the voltages in your amp against these standard values, it should give you a good idea of how the amp is doing.  Remember that the reference voltage values are based on having the reference value of AC mains (117VAC or 120VAC depending on when your Stereo 70 was originally made) across the power transformer primary, exclusive of the voltage drop of a thermistor.  Of course, not knowing what kind of mods you have, this may or may not be practical.


    Last edited by PeterCapo on Sun Aug 23, 2020 6:02 pm; edited 1 time in total
    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty ST 70 diode. warning

    Post by jeffjmr on Sun Aug 23, 2020 6:01 pm

    Thanks Peter.

    Well it may take a lot of re-reading to find that comment, but I will try.

    I have the factory built Triode USA ST-70. It is stock as far as I can tell, with a relatively new complete set of all JJ tubes, EL34s as drivers.

    The DGP30-E3/54 diodes are recommended in the “yellow sheet mod” thread. They are electrically virtually identical to 1N4007 with a bit bigger body and supposedly better heat dissipation. I’m all about overkill if the price difference can be reduced to single digits!

    I have the original manual and schematic, and am hoping to acquire the Triode USA differences schematic. But there is only one discrete diode in this amp, and it is on the power supply cap board.

    I’m off in search of that comment.

    Jeff
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sun Aug 23, 2020 6:44 pm

    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty St70

    Post by jeffjmr on Sun Aug 23, 2020 8:05 pm

    I have the Triode USA build manual. It shows only one diode. Unfortunately it does not show a schematic of their modded cap and driver boards.

    I didn’t remember the cryptic quote perfectly. I can’t copy and paste but it went like this;

    “I enjoyed the big iron of the design, but I didn’t like that if the negative bias diode blew it would take out the finals in nothing flat”.

    Does that make any sense?

    I can bias fine. Triode recommends 1.00 volts. I’m at .990, with just one adjuster per side of course. Any significant advantage to modding for four individual bias adjustments?

    Really only had one problem so far, probably self-induced. Until the Altecs are ready I am running a pair of B&W DM605S2, nominally 8 ohms. I first listened on the 4 ohm taps. When I tried the 8 ohm taps the fuse blew. Could be a transient from turning on the ST-70 before I turned on the B&W bass amps, or that they could be as low as 3 ohms with no signal. Or it could have been unrelated. Either way I’m back on the 4 ohm taps and all is well. I am still awaiting instructions from Triode about how to hookup the 16 ohm taps as that is what the Altecs are. It would seem that swapping out the 16 ohm tap for the 4 or 8 would be appropriate but the 16 ohm taps are soldered to the driver board and without a schematic I can’t tell if they are just parked there and/or if the 4 or 8 would need to take their place.

    Thanks again for your help.
    Jeff
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sun Aug 23, 2020 9:09 pm

    Sounds like it is saying that if the bias diode goes, the bias supply will fail and allow the power tubes to go into a runaway current condition where they'd be ruined.  Some have offered power supply boards with built-in protection against this scenario.  I recall that Classic Valve Design had one at one time, but I think he no longer sells them.  I recently read of another but cannot remember where I saw it.  In any case, I don't think it's likely to be a problem.  The original selenium bias rectifier lasted for decades, and a new, appropriately rated silicon diode would last even longer.  

    However, there are other reasons why a bias supply can fail. It's not uncommon to hear about "red-plating" events.

    Not sure what's up with blowing the fuse on the 8-ohm tap.  Wouldn't think that should happen.
    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 diode warning Empty Re: ST70 diode warning

    Post by peterh on Mon Aug 24, 2020 9:25 am

    jeffjmr wrote:Hi all,

    Old audiophile with first tube amp. Recent factory build ST70 by a VTA competitor (sorry, got it on auction at a decent price.)

    So far so good. Love the sound, and the ability to fiddle with it (and some electronic fiddling experience). I am in the middle of restoring a pair of Altec VOTT A5Xs with the intent to drive them with the ST70.

    I have read almost this entire forum, and am soaking up all the knowledge I can muster, and am thankful to Bob and all else who share. I will attempt to return the favor as opportunity arises.

    My concern is, somewhere along the line I read a post from someone who was not a Dynaco fan, who complained that he had no time for an amp with drivers that could be blown up at any moment by the failure of the single phase inverting diode!

    Yikes! That is a pretty sobering thought. Since I have yet to perform the famed yellow sheet diode mod, I can only find one diode in my amp and that is on the power supply capacitor board.

    Any idea if this is the diode he refers to, and is it that critical? If so, is there a more robust diode besides a 1N4007  called for here? Or perhaps wiring two in series as insurance?

    I have parts on order to install a CL-80 and the yellow sheet rectifier diodes mod, and a GFI that interrupts on power fails with manual reset. Any other mods suggested? By the way, I am going with the more robust DGP30-E3/54 diodes instead of 1N4007 unless there is some reason I shouldn’t.

    Thanks in advance for any advice.
    Jeff


    You should ignore unconfirmed Internet sayings that has no documentation supporting it.
    It's just rumors or even worse trolls in action.

    1n4007 is a wellknown diode that fulfills the spec in all positions in a st70.
    UF4007 ( https://www.vishay.com/doc?88755 ) is an alternative that makes less noice, not
    needed here.
    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty Re: ST70 diode warning

    Post by jeffjmr on Mon Aug 24, 2020 2:09 pm

    Couldn't agree more, Peter. That's why I came here to the experts for their take on the idea.

    The DGP30 idea came from this forum, in the yellow sheet thread, and it sounded like a logical, if not entirely necessary, upgrade.

    I have 120-122vac house current here, and running speakers that don't perfectly match up impedance wise so am trying to take all the precautions within financial reason as possible.

    Thanks for your feedback!
    Jeff
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Wed Aug 26, 2020 6:27 pm

    I must have missed it, because I looked in the yellow sheet thread but did not find reference to a DGP30 - can you post the link or quote the Post n° in that thread?  I'd like to read it in context.

    For the impedance matching thing, I'm not understanding why your fuse blew.  Putting a speaker on taps that don't match it's nominal impedance usually means either cleaner power or power with increased distortion.  Never heard about it blowing a fuse - perhaps something else is afoot?
    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty Alternate diode for rectifier tube mod

    Post by jeffjmr on Fri Aug 28, 2020 11:49 am

    PeterCapo wrote:I must have missed it, because I looked in the yellow sheet thread but did not find reference to a DGP30 - can you post the link or quote the Post n° in that thread?  I'd like to read it in context.

    For the impedance matching thing, I'm not understanding why your fuse blew.  Putting a speaker on taps that don't match it's nominal impedance usually means either cleaner power or power with increased distortion.  Never heard about it blowing a fuse - perhaps something else is afoot?

    Post in question below. The post has a link and I am not allowed to post links yet, so I removed it but it is post No. 88.

    My understanding as still a novice in all things tube, if you connect output taps of a significantly higher rating than the actual load you could exceed design current flow. Driving a 16ohm speaker with 4 or 8 ohm outputs will decrease power and possibly fidelity, but using 16 ohm outputs to a 4 ohm load is a no-no. Similar to the admonition not to power up your amp with no load at all. In my case, the B&Ws are nominally 8 ohms, but could be as low as 3, or even less with the bass amp unpowered (which in retrospect may have been what happened). This is what I have read. Still learning.

    Jeff

    Post n°88
    tube rectifier "yellow sheet" diode mod - Page 4 Empty Alternate diode
    Post by KenGaler on Mon Oct 21, 2019 12:39 pm
    At the risk of flogging this dead horse.. This is a good choice for a diode also. Yes, it's overkill but that's the mode we're in anyway. Same forward voltage drop as the 1N4007 so it will dissipate the same power but because the case is larger the temp rise will be slightly lower. I plan on using them in a M-125.

    (Link to the diode was here)

    Regards,
    Ken
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Fri Aug 28, 2020 12:27 pm

    Oh, okay, I see. I searched the pages for "DGP30" but he didn't actually type it into the post, he linked to it instead.

    So, the DGP30 has a PIV rating of 1500V where the 1N4007 has a PIV of 1000V. The spec for the Stereo 70 is in the vicinity of 60V. Hence, the 1N4007 is already gross overkill, but a DGP30 turns overkill into an art form.

    The forward current rating for the 1N4007 is 1A and the DGP30 is 3A. Someone correct me if I am wrong, but the bias circuit doesn't have much in the way of current flow into the grids of the EL34s. There's the voltage divider network made up of the fixed bias resistors and the two bias pots, but, again, I'm not thinking it's anything close to 1A of current.
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Fri Aug 28, 2020 12:39 pm

    jeffjmr wrote: ... if you connect output taps of a significantly higher rating than the actual load you could exceed design current flow. Driving a 16ohm speaker with 4 or 8 ohm outputs will decrease power and possibly fidelity, but using 16 ohm outputs to a 4 ohm load is a no-no. Similar to the admonition not to power up your amp with no load at all. In my case, the B&Ws are nominally 8 ohms, but could be as low as 3, or even less with the bass amp unpowered (which in retrospect may have been what happened). . ...

    Most speakers do not have a flat impedance curve.  A nominal rating is an approximation.  Whatever the nominal value of the speaker's impedance, it will typically have widely varying impedance by frequency.

    Are you using a powered speaker?  Or a powered subwoofer?  If so, what's your system architecture from the Stereo 70 output?  Is your Stereo 70 "looking into" a powered speaker or subwoofer?  If so, this might be the problem.
    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty Alternate diode

    Post by jeffjmr on Fri Aug 28, 2020 9:57 pm

    Overkill as an art form. LOL! I like that. And you are right. I am not yet knowledgeable enough on the ST-70 circuitry to recognize when I’m way out of bounds.

    I do have powered speakers for now, until my Altec A5Xs are ready. They are B&W DM605S2 with 200 watt powered LF. Sounds great. But I have learned to turn the speaker amp on first before the ST-70 and that has worked well so far.

    But I would appreciate your comment on the high impedance out to a lower impedance load concept.

    Jeff
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sat Aug 29, 2020 6:14 am

    jeffjmr wrote:Overkill as an art form.  LOL! I like that. And you are right. I am not yet knowledgeable enough on the ST-70 circuitry to recognize when I’m way out of bounds.

    I do have powered speakers for now, until my Altec A5Xs are ready. They are B&W DM605S2 with 200 watt powered LF. Sounds great. But I have learned to turn the speaker amp on first before the ST-70 and that has worked well so far.

    But I would appreciate your comment on the high impedance out to a lower impedance load concept.

    Jeff

    The only thing I might add to my comments in Post n°12 is that I would not expect even a 16Ω speaker connected to the 4Ω tap to blow the fuse.  Perhaps someone else could chime in to either confirm or deny.

    If I correctly understand that you blew the fuse when attempting to drive a self-powered speaker with your Stereo 70, then that might be the problem, though I cannot say for sure.  I can say that I once tried running a powered speaker with my Dynaco Stereo 35 and the sound was screwed up.  Didn't blow a fuse, though.  Perhaps there's something about the load a powered speaker presents that isn't palatable.  Pretty sure amplifiers of the Stereo 70's time didn't expect to drive a self-powered speaker.

    I'd suggest cracking open the manual, finding the voltage chart (on the page with the power supply schematic), SAFELY take readings at the indicated nodes and compare your readings with the standard values given in the manual on that page.  Note that the standard nodal voltage values are referenced to 117VAC across the power transformer primary (the two black wires).  If you have a a later production Stereo 70, you might have a 120VAC PT primary.

    Manual can be downloaded here: https://www.dynakitparts.com/wp-content/uploads/Dyna-ST70.pdf and I'd crosscheck with the Triode USA manual, looking for standard voltage values in there for sure.
    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 diode warning Empty Re: ST70 diode warning

    Post by peterh on Sat Aug 29, 2020 8:49 am

    Connecting a MkIII to a self-powered speaker would be similar to an open output. This is dangerous
    and might cause overvoltage and flashover, which in turn could make the fuse blow.

    ALWAYS have a load connected, minimum 100 ohm when a tube amp is powered on.

    PeterCapo and 10-E-C like this post

    avatar
    Dale Stevens

    Posts : 150
    Join date : 2014-07-06
    Age : 71
    Location : Loris, SC

    ST70 diode warning Empty Re: ST70 diode warning

    Post by Dale Stevens on Sat Aug 29, 2020 9:49 am

    Peterh, Why a "min 100 ohm" ? why not closer to spkr impediance . say, like 10 ohm ? Dale
    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 diode warning Empty Re: ST70 diode warning

    Post by peterh on Sat Aug 29, 2020 10:08 am

    Dale Stevens wrote:Peterh, Why a "min 100 ohm" ?  why not closer to spkr impediance . say, like 10 ohm ?  Dale
    100 ohm could be permanently soldered inside the amp without any detrimental effects and
    is enough to prevent arcing.
    10 ohm would also do, but then one will have to remove it before connecting speakers.
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sat Aug 29, 2020 10:11 am

    peterh wrote:
    Dale Stevens wrote:Peterh, Why a "min 100 ohm" ?  why not closer to spkr impediance . say, like 10 ohm ?  Dale
    100 ohm could be permanently soldered inside the amp without any detrimental effects and
    is enough to prevent arcing.
    10 ohm would also do, but then one will have to remove it before connecting speakers.

    Will adding the 100Ω affect the amplifier inside the speaker? I suppose not.
    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 diode warning Empty Re: ST70 diode warning

    Post by peterh on Sat Aug 29, 2020 12:04 pm

    PeterCapo wrote:
    peterh wrote:
    Dale Stevens wrote:Peterh, Why a "min 100 ohm" ?  why not closer to spkr impediance . say, like 10 ohm ?  Dale
    100 ohm could be permanently soldered inside the amp without any detrimental effects and
    is enough to prevent arcing.
    10 ohm would also do, but then one will have to remove it before connecting speakers.

    Will adding the 100Ω affect the amplifier inside the speaker?  I suppose not.
    It won't affect the amp. If soldered on the 8 ohm tap it will at most absorb 5watt but it will
    protect the amp from loose connectors.
    And it certainly affects the external speaker, in fact it will have a cleaner signal then if
    the amp is unloaded.

    BTW, an active speaker will work best if driven direct from the preamp.
    jeffjmr
    jeffjmr

    Posts : 15
    Join date : 2020-08-22

    ST70 diode warning Empty Alternate diode

    Post by jeffjmr on Sat Aug 29, 2020 3:30 pm

    Thanks for all this great feedback. I assume too much though, that everyone is familiar with the B&W 605s.

    Only the LF is self powered. Effectively a built in 200 watt subwoofer. Specs say 8 ohms nominal, 3 ohm minimum, but does not say if that is with or without the sub-amp powered on. It is switchable, though there is NO bass without it so on is normal.

    Why I surmised the impedance mismatch might be the problem is the universal warning that one should not power up a tube amp without a load. I figured too low a load would be nearly as bad. I like the 100 ohm resistor idea as another inexpensive insurance policy mod. But am not following where it would be inserted in the circuit. And I assume it would be wise to remove once I’ve hooked up the A5Xs? And I’m definitely not following why a 10 ohm would have to be removed but the 100 ohm wouldn’t?!?!

    Jeff
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sat Aug 29, 2020 4:43 pm

    I found a manual for B&W 605 S2.  It containes the following diagram of the connections:

    ST70 diode warning Bw_60510

    Is that what yours looks like?  

    It has full-range speaker-level inputs that permit bi-wiring or single wiring.  In the above diagram, the jumpers are in place and it appears as a single-wired connection from an external amplifier.

    There is also a pre-amp level RCA input to the powered woofer only.  It appears to be intended to make the 605 function only as a woofer if this connection is chosen.  I say this because another diagram in the manual, as well as the written instructions, indicate no connection to the speaker-level inputs if the RCA woofer input is used.  In this configuration, the 605 is functioning as an LFE, or Low Frequency Effects typified by home theater systems

    So, for full range operation of these B&Ws, it looks like an external amplifier, like your Stereo 70, is driving the high frequency terminals directly.  The external amplifier also directly drives the low frequency speaker-level terminals that go to the speaker's internal power amplifier - kind of like how a booster amplifier for a car stereo takes a speaker level output from an amplifier and then amplifies it some more.

    The Nominal impedance of the speaker is specified at 8Ω with a minimum of 3Ω.  If this is what the Stereo 70 "sees," it should work - all things being equal.  However, I will go back to what I originally said about the possibility of there being some other kind of problem with a vintage tube amp driving a setup like this.  Or, your Stereo 70 isn't operating quite right to begin with.  I'd suggest e-mailing B&W and ask them if they know of any problems associated with driving the 605 full-range with a vintage tube amp like the Dynaco Stereo 70.

    At another place in the specifications, it states the amplifier Input impedance is 100kΩ.  I have to believe that this is referring only to the woofer input via the RCA jack, which is something the external amplifier will never "see."  No hi-fi amplifier can drive a 100,000Ω load.


    Last edited by PeterCapo on Sat Aug 29, 2020 6:15 pm; edited 6 times in total
    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 diode warning Empty Re: ST70 diode warning

    Post by peterh on Sat Aug 29, 2020 4:44 pm

    jeffjmr wrote:Thanks for all this great feedback. I assume too much though, that everyone is familiar with the B&W 605s.

    Only the LF is self powered. Effectively a built in 200 watt subwoofer. Specs say 8 ohms nominal, 3 ohm minimum, but does not say if that is with or without the sub-amp powered on. It is switchable, though there is NO bass without it so on is normal.

    Why I surmised the impedance mismatch might be the problem is the universal warning that one should not power up a tube amp without a load. I figured too low a load would be nearly as bad.  I like the 100 ohm resistor idea as another inexpensive insurance policy mod.  But am not following where it would be inserted in the circuit. And I assume it would be wise to remove once I’ve hooked up the A5Xs? And I’m definitely not following why a 10 ohm would have to be removed but the 100 ohm wouldn’t?!?!

    Jeff
    I suggest that the 100 ohm is soldered inside the amp between zero/ground and 8 ohm taps. As
    the load is minuscule there is no need to detach it . A 10 ohm at the same location would
    load the amp too much if a "normal" speaker is also attatched.
    Note that too low load wont hurt a tubeamp, but it will reduce power and increase dist.

    A permanent 100 ohm will "vaccinate" the amp from all future loss of load at a low price of power.
    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sat Aug 29, 2020 5:01 pm

    And here is some recommended reading on the matter of operating a tube amplifier without a load: http://tronola.com/html/unloaded_vt_amps.html
    peterh
    peterh

    Posts : 1403
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST70 diode warning Empty Re: ST70 diode warning

    Post by peterh on Sat Aug 29, 2020 7:13 pm

    PeterCapo wrote:And here is some recommended reading on the matter of operating a tube amplifier without a load: http://tronola.com/html/unloaded_vt_amps.html
    I think this article is simplified and not fully describing the hazards. I have personally seen
    violent sparks across tube sockets. Phillips amps ( yes they made a large series in holland)
    even has a spark-gap to protect in case of overvoltage.
    To be able to state categorically as in the article a full coverage of all amps under all
    conditions has to be made. Which is not the case. The article also claims that an amp using
    feedback is immune, in my opinion the opposite is true, feedback will in the event of saturating
    make the saturation even worse.

    Even if one amp seems unharmed provided it's not saturated one can never promise that
    an external event creates a spike that will saturate the amp and cause a flashover thereby
    destroying a transformer.

    It is however always safe to "play safe" and always have some load.

    Shorting the output however is usually no problem, while on transistor amps a shorted
    output may give an immediate destruction of output transistors. ( some transistor amps
    has protection circuits that is claimed to cope with this, but i would not take the risk with
    any transistor amp unless someone else will pay for the repair). A transistor amp with no load
    is in general no problem as there is no large iron core that builds up energy.


    PeterCapo
    PeterCapo

    Posts : 892
    Join date : 2008-12-05

    ST70 diode warning Empty Re: ST70 diode warning

    Post by PeterCapo on Sat Aug 29, 2020 9:07 pm

    Okay, better to take a safer route just in case.

    But his B&W speakers are specified as 8Ω nominal with a minimum of 3Ω, "looking into" the speaker terminals.  Why did he blow a fuse? His Stereo 70 is not looking into a high impedance load. The high impedance load is via the preamp-level RCA jack on the back of the speaker.

    Sponsored content

    ST70 diode warning Empty Re: ST70 diode warning

    Post by Sponsored content


      Current date/time is Thu Sep 24, 2020 1:20 pm