[bcowen]

Good find and still a decent price for a great tube.

How many 6520s is that in your collection now? Would be interesting to see the differences between them. I stopped at 4 (or is it 3? - not at home to check) and I think there are two types so far.

This will be my first real 6520, so I'm pretty excited to hear it. I had a pair of NEC-branded 6520's I bought back when I was first learning about 6AS7's, but I sent one to a friend and broke the other one. While it always sucks to break a tube, I didn't lose any sleep as it was pretty mundane and 'meh' sounding. Turns out they were relabeled RCA 6AS7G's since RCA never made a 6520. Live and learn.

 

[CaptainFantastic]

This will be my first real 6520, so I'm pretty excited to hear it. I had a pair of NEC-branded 6520's I bought back when I was first learning about 6AS7's, but I sent one to a friend and broke the other one. While it always sucks to break a tube, I didn't lose any sleep as it was pretty mundane and 'meh' sounding. Turns out they were relabeled RCA 6AS7G's since RCA never made a 6520. Live and learn.

Here are the ones I have:

#1 - Marked "6520 Made in U.S.A." Top D-getter, high chromium, rounded micas - sounds so good it got me convinced to be on the look out for more 6520s.



#2 - Marked "6520 75-03 Tung-Sol Made in U.S.A." Bottom O-getter, low and high chromium, rounded micas - sounds great, slightly less warm than #1



#3 - Marked "6520 322512-3 Made in U.S.A." Bottom O-getter, high chromium, rectangular micas, only listened to it to ensure it's perfectly quiet, no sound profile assessment yet



#4 - Marked "6520 75-03 Tung-Sol Made in U.S.A." Bottom O-getter, high chromium, rounded micas; last 6520, haven't assessed yet other than that it works perfectly



Come to think of it, I need to get a class photo. Maybe tomorrow.

Looking forward to seeing your 6520 and the impressions @bcowen

EDIT - I might be wrong on the bottom O getters on the last two. I will take a look.

 

[gibosi]

Here are the ones I have:

#1 - Marked "6520 Made in U.S.A." Top D-getter, high chromium, rounded micas - sounds so good it got me convinced to be on the look out for more 6520s.



#2 - Marked "6520 75-03 Tung-Sol Made in U.S.A." Bottom O-getter, low and high chromium, rounded micas - sounds great, slightly less warm than #1



#3 - Marked "6520 322512-3 Made in U.S.A." Bottom O-getter, high chromium, rectangular micas, only listened to it to ensure it's perfectly quiet, no sound profile assessment yet



#4 - Marked "6520 75-03 Tung-Sol Made in U.S.A." Bottom O-getter, high chromium, rounded micas; last 6520, haven't assessed yet other than that it works perfectly



Come to think of it, I need to get a class photo. Maybe tomorrow.

Looking forward to seeing your 6520 and the impressions @bcowen

EDIT - I might be wrong on the bottom O getters on the last two. I will take a look.

Three of those are simply 5998s, deliberately relabeled by Tung-Sol as 6520. It's not clear why TS did this, but these relabeled 5998 are actually more common than "real" 6520, which to the eye are identical to a standard TS 6AS7G.

 

[CaptainFantastic]

Three of those are simply 5998s, deliberately relabeled by Tung-Sol as 6520. It's not clear why TS did this, but these relabeled 5998 are actually more common than "real" 6520, which to the eye are identical to a standard TS 6AS7G.

Thanks. Good to know. I can swear the first 6520 sounds warmer and different than my 5998s. Perhaps every tube can sound a bit different, just odd that so many 5998s sound the same and this 6520 sounds different. Must be placebo effect at its best!

 

[gibosi]

Thanks. Good to know. I can swear the first 6520 sounds warmer and different than my 5998s. Perhaps every tube can sound a bit different, just odd that so many 5998s sound the same and this 6520 sounds different. Must be placebo effect at its best!

Yes, it could be the placebo effect. But tubes manufactured at different times, in the same factory by the same company, often sound different. And your first "6520", with D getters, was manufactured earlier than the others. In my experience, 5998s manufactured in the 1950's sound better.

 

[bcowen]

Three of those are simply 5998s, deliberately relabeled by Tung-Sol as 6520. It's not clear why TS did this, but these relabeled 5998 are actually more common than "real" 6520, which to the eye are identical to a standard TS 6AS7G.

Well that makes me feel better. The one I'm getting does not have domino plates and the alarm bells went off. Thanks @gibosi !

Any odea what the code means on this: 322NF-133? I've never seen that "NF" after the EIA code.

 

[gibosi]

Well that makes me feel better. The one I'm getting does not have domino plates and the alarm bells went off. Thanks @gibosi !

Any odea what the code means on this: 322NF-133? I've never seen that "NF" after the EIA code.

Well after having one too many customers complain to the dealer, "My new television has last year's tubes! I want new tubes!", Tung-Sol, Sylvania and others switched to dating systems that couldn't be deciphered so easily. lol

Unfortunately, I have seen only a small portion of Tung-Sol's new dating system which covered the period Jan 31, 1960, "HD", through July 31, 1962, "MR". These dates indicated when the warranty expired. I would assume that "NF" would be sometime after July, 1962, but that is only a guess. It appears that they deliberately made the system such that extrapolation doesn't seem to work.

 

[bcowen]

Well after having one too many customers complain to the dealer, "My new television has last year's tubes! I want new tubes!", Tung-Sol, Sylvania and others switched to dating systems that couldn't be deciphered so easily. lol

Unfortunately, I have seen only a small portion of Tung-Sol's new dating system which covered the period Jan 31, 1960, "HD", through July 31, 1962, "MR". These dates indicated when the warranty expired. I would assume that "NF" would be sometime after July, 1962, but that is only a guess. It appears that they deliberately made the system such that extrapolation doesn't seem to work.

Ah, thanks. I guess I probably shouldn't have thrown away that secret decoder ring.

 

[Ripper2860]

I still have mine, let me see what I can find. OK After deciphering it seems to say...

...'Be sure to drink your Ovaltine'.



For those not wise in the ways of 'A Christmas Story'...

 

[hmscott]

Three of those are simply 5998s, deliberately relabeled by Tung-Sol as 6520. It's not clear why TS did this, but these relabeled 5998 are actually more common than "real" 6520, which to the eye are identical to a standard TS 6AS7G.

Yes, it could be the placebo effect. But tubes manufactured at different times, in the same factory by the same company, often sound different. And your first "6520", with D getters, was manufactured earlier than the others. In my experience, 5998s manufactured in the 1950's sound better.

Well that makes me feel better. The one I'm getting does not have domino plates and the alarm bells went off. Thanks @gibosi !

Any odea what the code means on this: 322NF-133? I've never seen that "NF" after the EIA code.

@gibosi - thanks for that warning, I would have easily bought the "6520" ala 5998 before learning about the Domino plates.

You (someone) did mention to not get the non-Domino plated 5998's earlier, if I am recalling it correctly?

So how do we identify a real 6520? Will it look like a "bad" 5998 - without Domino plates? Sorry, getting confused

I still have mine, let me see what I can find. OK After deciphering it seems to say...
...'Be sure to drink your Ovaltine'.
For those not wise in the ways of Christmas Story...

Yup, can confirm I did Drink My Ovaltine! Bosco too!

 

[hmscott]

@bcowen , I'm on the hunt for date codes, and found this one that says NF is June 1963:

It's not Tung-sol, probably not even the right "power" tube realm, but at least there is such a thing as the "NF" date code out there somewhere...
I wonder if the companies do standardize on these 2 letter date codes?
http://pax-comm.com/rcadates.pdf

I'll keep looking

Update: Several confirmations that Tung-Sol changed hands many times and no-one thought to preserve the code sheet(s) for date codes....also customers during the 50's/60's didn't want "old" tubes, and customers learned to read the simple year/week codes, so that is why Tung-sol originally obfuscated the manufacturing date, so old tubes would sell.

 

[abvolt]

I've never been able to find a 6520 just 5998's which is ok I really enjoy them.

 

[gibosi]

You (someone) did mention to not get the non-Domino plated 5998's earlier, if I am recalling it correctly?

So how do we identify a real 6520? Will it look like a "bad" 5998 - without Domino plates? Sorry, getting confused

1) Yes, if the tube doesn't have domino plates it is not a 5998 or a WE421.

2) Regarding the 6520, it looks exactly like a TS 6AS7G. There is no way to identify it by looks. The differences can't be seen. And I doubt that anyone would deliberately make a fake. So if it is labeled Tung-Sol 6520 and it looks identical to a TS 6AS7G, I think it is safe to call it a 6520.

Essentially it is a 6AS7G that has been tweaked in ways you cannot see. You can read about the tweaks on the datasheet.

https://www.radiomuseum.org/forumdata/upload/6520~~1.pdf

 

[gibosi]

I wonder if the companies do standardize on these 2 letter date codes?

No they all used their own scheme.

 

[hmscott]

No they all used their own scheme.

What I am finding is a much longer manufacturer code+numeric postamble that became the date code. No mention of 2 letter date codes for Tung-sol tubes, here's what I found:

The first describes the 6 digit mfgr (322 for Tung-sol)/date code scheme, the 2nd has links to the articles I'll include below, that further expand the lists of mfgr codes, IDK about date codes...
https://community.klipsch.com/index...ung-sol-tubes/&do=findComment&comment=2230674


Do these 3 articles combined have enough info to decode the 6 digit Tung-Sol Code?
https://community.klipsch.com/index...ung-sol-tubes/&do=findComment&comment=2231049

http://www.triodeel.com/eiacode.htm

https://www.tubemuseum.org/SearchResults.asp?Cat=30

http://www.tubemaze.info/deciphering-european-tube-date-codes/

So I wonder if @bcowen has a Tung-Sol labeled 6520 made by RCA with RCA date code NF, for June 1963, from the chart I posted back a few posts from here...

Is this an example of RCA made 6520 tubes labeled as Tung-Sol?

Update: My postulation isn't correct, @gibosi set me straight, RCA didn't make 6520/5998 tubes, only Tung-sol made them. So the NF date code on @bcowen 's 6520 is still an undecoded mystery...