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Pioneer Stv 4h Bias Diode Substitutions?

MrMonster

MrMonster

Totally Lost In The 70's
Hello all,
After a lot of searching, this diode is now ubobtania. Has anyone been able to replicate a STV 4H bias diode using a 3H and a 1H ganged together? if so how well does it work, and how is it done?
I have 2 Spec 2 amps, and one has a 4H bias diode on the verge of collapse, and I know I will need to replace them all someday.
Thanks,
Donald Bowman
 
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dbow19 said:
Hello all,
After a lot of searching, this diode is now ubobtania. Has anyone been able to replicate a STV 4H bias diode using a 3H and a 1H ganged together? if so how well does it work, and how is it done?
I have 2 Spec 2 amps, and one has a 4H bias diode on the verge of collapse, and I know I will need to replace them all someday.
Thanks,
Donald Bowman
Click to expand...

Verge of collapse? mechanically? electrically? Who says that?

Unless I am remembering incorrectly the spec-2's bias circuit includes a potentially crappy relay (bias instability).


This thread discusses bias instability in the spec-2 amp and the cure of a relay replacement.

The spec-2 bias circuit uses an stv-3h and a 470 ohm pot, with an stv-4h in what looks to be a SAFETY function, so that if the stv-3 or the pot ever opens up, the stv-4h will clamp the maximum bias voltage to a safer (but higher) level , keeping the amp from being instantly converted to a smoking ruin.

Thus the stv-3h really controls the level of bias current, BUT the relay can really mess it up. The stv-4h as a CLAMP may be able to be substituted for with a diode string.

With the stv-4h being used in that safety function, I would match a diode string USING THE SAME CURRENT as flows through the stv-3/pot circuit to match the voltage drop across the stv-4h 's and make a multi-diode replacement. The forward voltage drop is usually specified at 7 milliamps at a particular temperature on the stv-xh's and while there are graphs of forward drop versus current associated with the replacement diodes, nothing beats measuring the actual conditions.

ALL the above is null and void if the stv-4h is also mouted on the heatsink.

I also defer to EchoWars 's recommendations as well.
 
You might be able to replace the number of junctions with diodes. I have my doubts at any real temperature stability, which is critical with these amps. Hopefully, EW will chime in.

On another note, STV4 is not quite unobtainium yet: http://www.partstore.com/PartSearch.aspx?PartNumber=stv4

Those folks will combe back as not in stock, but just may come up with a few if you are willing to wait.

STV3 is also listed: http://www.partstore.com/PartSearch.aspx?PartNumber=stv3

Note that many times this source shows availability, until they are unable to obtain, then show as unavailable, rather than showing available only if "in stock" is known to be true. I think that, rather than this place being a parts warehouse, it is a loose association of places that can be tapped. Whenever I get parts, they don't come from them, but from some other business or individual.

One an additional note, these 2 parts are used in many lower end designs, which you can buy for peanuts, as frankensteins. Not as good as buying new parts, but better than kludges.

Rich P
 
markthefixer said:
The spec-2 bias circuit uses an stv-3h and a 470 ohm pot, with an stv-4h in what looks to be a SAFETY function, so that if the stv-3 or the pot ever opens up, the stv-4h will clamp the maximum bias voltage to a safer (but higher) level , keeping the amp from being instantly converted to a smoking ruin.

Thus the stv-3h really controls the level of bias current, BUT the relay can really mess it up. The stv-4h as a CLAMP may be able to be substituted for with a diode string.

With the stv-4h being used in that safety function, I would match a diode string USING THE SAME CURRENT as flows through the stv-3/pot circuit to match the voltage drop across the stv-4h 's and make a multi-diode replacement. The forward voltage drop is usually specified at 7 milliamps at a particular temperature on the stv-xh's and while there are graphs of forward drop versus current associated with the replacement diodes, nothing beats measuring the actual conditions.

ALL the above is null and void if the stv-4h is also mouted on the heatsink.

I also defer to EchoWars 's recommendations as well.
Click to expand...
The STV4 is indeed mounted on the heatsink, but you are correct in surmising that the STV4 is pretty much there as a failsafe, in case the pot opens up or something. Still, a small trickle of current flows through the diode, and this current increases as the bias is raised. I simply would not trust a Spec 2 where the diode had been removed or replaced with something else, and I sure can't think of a way to do it neatly (neatness does count, in my book).
pustelniakr said:
You might be able to replace the number of junctions with diodes. I have my doubts at any real temperature stability, which is critical with these amps. Hopefully, EW will chime in.
Click to expand...
I'd rather use the original part, and not have to wonder...so far, I've always been able to find a STV-4H diode when I had to have one. Lucky, I suppose.
On another note, STV4 is not quite unobtainium yet: http://www.partstore.com/PartSearch.aspx?PartNumber=stv4
Click to expand...
Damn! I swear I've searched that site many times and never found those. Only the 'H' version is a direct replacement for the original radial lead 'horseshoe' device, the others are axial lead (and I already have a few of those). I just ordered 15 of the Toshiba parts...we'll see if they come through. If not, I'll try a few of the $5 Kenwood devices.
One an additional note, these 2 parts are used in many lower end designs, which you can buy for peanuts, as frankensteins. Not as good as buying new parts, but better than kludges.
Click to expand...
The STV-3H, yes. The STV-4H, not so much.
 
Here's what's happening to the Spec 2

The on the verge of collapse term,

The amp went down this Summer in an odd way, it was on and playing sweet, when all of a sudden it started booming with an incredible bass thump. A fast shut down, then it was dead. Off to a local tech for replacement of a few lesser diodes and one of the STV bias diodes. He only had one available, so that was that.
Now, the amp will play sweetly with no distortion until all of a sudden, with no extra load on it, the amp will heat up to a blistering temperature, make a few screeching noises and then shut itself down. After a few hours of cooling, it comes back to life as if nothing happened at all.
so I am beginning to think that this lies in that bias diode going down and letting the current build untill the protection circuit says enough.

Thankfully Echowars has added me to his schedule for an overhaul of this tragedy. I have been thinking of buying another Spec 2 from a source for a good price, but if the life of the Spec 2 depends on an extinct diode, I may just bag the whole thing and switch to the Spec 4 amp.
Thanks one and all
Donald Bowman
 
The Spec 4 uses them too. Matter of fact, probably 85% or more of the larger (100W+) vintage amps from about '73 to '83 use this same diode.

They rarely go bad...I've never seen a blown one, only broken ones from rough handling.
 
dbow19 said:
The on the verge of collapse term,

The amp went down this Summer in an odd way, it was on and playing sweet, when all of a sudden it started booming with an incredible bass thump. A fast shut down, then it was dead. Off to a local tech for replacement of a few lesser diodes and one of the STV bias diodes. He only had one available, so that was that.
Now, the amp will play sweetly with no distortion until all of a sudden, with no extra load on it, the amp will heat up to a blistering temperature, make a few screeching noises and then shut itself down. After a few hours of cooling, it comes back to life as if nothing happened at all.
so I am beginning to think that this lies in that bias diode going down and letting the current build untill the protection circuit says enough.

Thankfully Echowars has added me to his schedule for an overhaul of this tragedy. I have been thinking of buying another Spec 2 from a source for a good price, but if the life of the Spec 2 depends on an extinct diode, I may just bag the whole thing and switch to the Spec 4 amp.
Thanks one and all
Donald Bowman
Click to expand...

Your description says to me, a tech, that it is doing e.x.a.c.t.l.y what the bias instability and relay thread describes....

The resistance of the bad contacts is SO much that it is changing the operation of the carefully controlled constant current circuit.

The BEST thing you can do for the spec-2 is what you have done, let EchoWars work his magic upon it....
 
I am amazed

Hell again,
well it just goes to show the power of the net.
I have been looking for this diode for 6 months, I post one time on AK, and in 48 hours I have 8 of them.
the parts store had them in stock, so I took the initiative to buy eight in the wild event I need them for my other spec amps. $18.00 is cheap insurance, as I intend to keep these great old amps, and have them playing 70's Rock at my 90th birthday, and subsequent funeral. I do look forward to having Echo wars finally bring the Spec 2 to the front of the class. I have 2 spec 4's that just go everyday for 12 hours a day, with nary a complaint, and I hope to see my spec 2 to finally be able to hang with the cool kids.
Thanks one and all for the help. now I need to overhaul a F1250 tape deck and a 630 turntable. I am pretty confident of doing the 630 myself, but is there someone her that can restore the Pioneer F1250?

Again, thanks,
Donald
 
dbow19 said:
Hell again,
well it just goes to show the power of the net.
I have been looking for this diode for 6 months, I post one time on AK, and in 48 hours I have 8 of them.
the parts store had them in stock, so I took the initiative to buy eight in the wild event I need them for my other spec amps. $18.00 is cheap insurance
Click to expand...

Don't count your diodes untill they are in your hands.... more than once they have cancelled the order after a few months....
Plus those are toshiba parts, hopefully they are the same form factor.
 
Not too suprisingly...
Dear Mr. McDonald,

Thank you for your order with Partstore.com.

We regret to inform you, but part # STV-4H is no longer available (NLA) from the manufacturer. This means the manufacturer discontinued the production of this particular part. We attempted to locate a substitute part, but currently there are no valid substitutes available.

Your order for this part has therefore been cancelled and you will not be charged for it. Any authorized hold(s) will be automatically lifted.

We apologize for any inconvenience. If you have any questions, please feel free to contact us directly, by replying to this e-mail.

Thank you for your patronage and please keep us in mind for your replacement part needs.

Jennifer Liebel
Customer Care Specialist
Partstore.com
Click to expand...
 
If you can't find the correct ones you can probably get by with NTE600.
I know everyone hates Nte but in this case they are readily available and cheap.
It's a temperature compensating diode. You may have to series a couple to get the correct voltage drop. Or Use one and series with a small sig diode.
Hardest part is making sure the diode body will stay in touch with heatsink to detect the heat.
I've used heatsink compound to create a bond at times in the past.
 
Well, just a few days later, I got the same Bon Mot, that EchoWars received from Parts store,
Sorry, blah, blah, blah!

With all the people looking for this little piece of electronic fluff, that some maker would produce a few hundred.

So, back to the hunt.
Donald Bowman
 
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