ST70 diode warning

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

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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

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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

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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.

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

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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

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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

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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.

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

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.

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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.

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

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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.

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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.

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