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Adcom GFA 565 Diodes
Adcom GFA 565 Diodes
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Old 30th July 2011, 04:16 AM   #1
vphifi is offline vphifi
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Default Adcom GFA 565 Diodes

Hi,

I am looking for the KB262 diode to replace the one on my Adcom 565 mono amp. Can some ones help me to buy one or two?

Thanks
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Old 31st July 2011, 08:29 PM   #2
embrown057 is offline embrown057  United States
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Default KB262 Diode

You can order then from here:
Search results for kb262

They are out of stock but shows there on order, lead time <30 days. You can also use RVDKB262D as a sub, that's a Panasonic part. I only have 4 in stock at this time, I also need to order another 12 to keep on hand. Other than that you have also:
https://www.encompassparts.com/webwi...tnumber=521885

You may be waiting forever if you order from them, they sometimes take months to get the part and ship it out. They have a habbit of selling parts they do not have and then scramble to fill the order only after you call over and over again. Even then you might recive another sub like NTE or ECG that you do not want. I would stick to MCM for that part unless some else has a sugestion.
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Old 11th March 2015, 01:46 PM   #3
dikt123 is offline dikt123  Canada
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Adcom GFA 565 Diodes
I'm also looking for those KB262 and KB362. Is there any substitude out there like from Digikey etc.

Thanks to everyone for Help

D.K.
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Old 23rd November 2016, 08:29 AM   #4
Phloodpants is offline Phloodpants
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Resurrecting this thread, as I think I've found the answer! Please give feedback!

Scant information is available on these KB362 and KB262 diodes. The service manual lists them as "Varactor diodes", but I think this is a mistake. I believe they are actually "Stabistor diodes". They are avalanche diodes designed for highly stable forward voltage versus current. They come in packages containing 1-4 P-N junctions in series, and so the parts numbers tend to reflect this. KB262 has two diodes, and KB362 has three.

I found an scan of an old parts catalogue on the internet somewhere with the KB262 and KB263 listed. I could not find an actual datasheet, but various sources suggested crossing with Phillips BZV86, and this is what led me to believe they are stabistors.

KB262 - Si-St Vf 1.4V (@10ma?)
KB362 - Si-St Vf 2.1V

This makes sense. If the 262 has two diodes with a Vf of 0.7V, then the 362 should have three diodes, and be 2.1V, which it is.

I did some measurements of actual diodes from a GFA-565, and the Vf versus If jibes well with what I've found on the datasheets.

So, I found a few parts that should work, only one of which is still being made. Their Vf vs. If seems pretty close.

1N4156 and 1n4157
Digitron MPD200 and MPD300
Phillips BZV86-1V4 and BZV86-2V0

All the above are hard or impossible to find.

BUT! Central Semiconductor makes surface mount stabistors!

CMXSTB200 = KB262
CMXSTB300 = KB362

Mouser has the 200 (two diodes) and 400 packages in stock, but not the 300. However, one can simply buy the four-diode package and not use one of them. Here's the pinout...

Click the image to open in full size.

And here's where they go in the circuit.

Click the image to open in full size.

Anyone see any issues with this?
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Last edited by Phloodpants; 23rd November 2016 at 08:40 AM.
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Old 30th November 2016, 06:39 AM   #5
Phloodpants is offline Phloodpants
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OK, I am more and more confident this is the answer. I received the CMXSTB200 and 300 diodes, and the measurements match the original diodes closely.

This latest pair of 565's I'm working on don't have the usual KB262 and KB362 in the small epoxy-blob package. Instead, there are black and pink diodes in cylindrical cases, which I assume are 1N4156 and 1n4157 or BZV86.

Mystery Stabistor #1 (KB262 position, two-junction diode)
0.1ma = 1.05v
1ma = 1.18
3ma = 1.25
5ma = 1.29
10ma = 1.35

Mystery Stabistor #2 (KB362 position, three-junction diode)
0.1ma = 1.55v
1ma = 1.74
3ma = 1.84
5ma = 1.9
10ma = 2.0

CMXSTB200
0.1ma = 1.0v
1ma = 1.2
3ma = 1.32
5ma = 1.37
10ma = 1.44

CMXSTB300 (Actually STB400 with one diode unused)
0.1ma = 1.5v
1ma = 1.8
3ma = 1.98
5ma = 2.05
10ma = 2.17

Here's how I converted the surface-mount diodes to through-hole...

I soldered thin leads to them
Click the image to open in full size.

Then a blob of JB Weld putty epoxy.
Click the image to open in full size.

I'll be trying these diodes in an amp soon, and I'll update this thread, as well as the other thread where this is being discussed...
http://www.diyaudio.com/forums/solid...thread-32.html
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Old 30th November 2016, 02:37 PM   #6
cogeniac is offline cogeniac  United States
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Looking at the diode stack in the schematic, I see that the total voltage drop is about 80 volts (85 volt rail, and about 4-5 volts in diode drops). The resistance in that stack is about 39.25K ohms, so the total current is about 2 mA, not 10.

That puts the drops for your Stabistors at about 1.2 v and 1.8 v, but your parts seem to match over the range, so this looks like a good solution.

I always wondered about the "Varistor" aspect. A varistor has a dual I-V curve, so it acts like two parallel diodes with opposite polarity, that is it has a negative I-V part and a positive one. In this circuit all of the current is forward, so a varistor would be useless. In addition, the match between the diodes in the stack (top matched with bottom) is important for maintaining the DC balance bias, which translates to DC offset.

Nice work! For a more truly representational equivalent, I think you should paint the epoxy covers properly though (half black and half orange for one, and half black half cream for the other!) :-D

Scott
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Old 30th November 2016, 10:25 PM   #7
PRR is offline PRR  United States
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> "Varactor diodes", but I think this is a mistake.

Agree. (Though I think "varactor" would be an alternate name for variable capacitance diode for radio tuning.)

> I believe they are actually "Stabistor diodes". They are avalanche diodes designed for highly stable forward voltage versus current.

"Avalanche" would be several volts or more, reverse-biased. "Zeners" above a few volts are really avalanche diodes, and as-stated sold as stable several-volt references.

Here they are used in *forward* conduction giving about 0.6V per junction.

> packages containing 1-4 P-N junctions in series, and so the parts numbers tend to reflect this. KB262 has two diodes, and KB362 has three.

Yes. Many semi shops had a large range of "bias diodes". They could use-up the "reject" junctions that had mild leakage or didn't have good PIV performance. They could stack several in one package. For a few bucks they would tight-select voltages and print your part number.

Looking at the circuit-snip, here they are not even precision references used, say, to set idle current. They are just blocking-up one transistor above the next, with just enough voltage to let the transistor be "active".

Myself I could see wads of 1N914/4148 or even 1N4007 diodes stacked to the correct number of junctions. I don't even think JB thermal bonding matters. The CMXSTB part is a sweet find.

I do think it may matter that, say, D101 is similar to D107. So it may be wise to replace all with "the same thing".

Wonder why they are blowing? There's no stress (<1mW/diode) normally. Can there be big reverse voltage kicks? Can't tell from the snip, and don't really want to see the full monty-- that plan is too complicated for my brain. Or OTOH were these "bias diode packs" really and truly assembled from "reject junctions", which failed normal tests because the dopant or sealing was poor and they fail over time?

Last edited by PRR; 30th November 2016 at 10:28 PM.
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Old 30th November 2016, 11:56 PM   #8
cogeniac is offline cogeniac  United States
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Quote:
Originally Posted by PRR View Post
> "Varactor diodes", but I think this is a mistake.

Agree. (Though I think "varactor" would be an alternate name for variable capacitance diode for radio tuning.)
My bad.. My RF background is showing through. They are listed as VARISTORS..But I think they are really sable forward voltage references.
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Old 1st December 2016, 12:06 AM   #9
cogeniac is offline cogeniac  United States
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Quote:
Originally Posted by PRR View Post
> Myself I could see wads of 1N914/4148 or even 1N4007 diodes stacked to the correct number of junctions. I don't even think JB thermal bonding matters. The CMXSTB part is a sweet find.

I do think it may matter that, say, D101 is similar to D107. So it may be wise to replace all with "the same thing".

Wonder why they are blowing? There's no stress (<1mW/diode) normally. Can there be big reverse voltage kicks? Can't tell from the snip, and don't really want to see the full monty-- that plan is too complicated for my brain. Or OTOH were these "bias diode packs" really and truly assembled from "reject junctions", which failed normal tests because the dopant or sealing was poor and they fail over time?
This was where this started. I had one amp that had several 1N914 diodes in series...The originals in many amps are unmarked little blobs, Black on one end and white or orange on the other.

A few of mine have been blown, but not often. I think the issue probably arises when/if one of the transistors fails shorted, or if a tech accidentally shorts one to ground. This might also happen with an electrolyte pickled board (from the leaking capacitors), allowing some stray current path. Any of this could put a big jolt of current through them, and they are very small, so presumably not made to tolerate that. Under normal operation they are barely pushing 2 mA.

It is also possible that, being custom, they were not well potted, and ended up with some internal oxidation.

I'd say less than about 1 in 10 I have seen have been bad...BUT, when they ARE bad, then they are hard to replace.

S
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Old 1st December 2016, 07:40 AM   #10
Phloodpants is offline Phloodpants
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Gah, yes! Service manual says varistors, not varactors. Anyways, they're stabistors.

I only did the jb-weld potting so the leads don't get stressed. One could probably do without.

I haven't seen any of these diodes blown out before, but this set of 565's came to me with the stabistor diodes installed in the wrong spots! And they weren't the usual KB262 and 362, so I wanted to find out what's really supposed to be there.

PRR, thanks for the correction. Stabistors are indeed tunnel diodes. I went down the rabbit hole and discovered this 1964 GE Transistor manual, chock-full of nerdy stuff about stabistors, snap diodes and the like. I thought this was an interesting passage...

Quote:
When the multi-pellet stabistor is used as a voltage regulator, the temperature coefficient of the stabistor will be larger than a breakdown diode of comparable voltage. However, this is offset by the stabistor's tighter initial tolerance, lower dynamic impedance, and absence of noise at low currents.
Would a stabistor be a better performing reference for a transistor current source that feeds a long-tail-pair, versus the usual string of two 4148's?
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