Will the REAL NOS 6SQ7GT please stand up?

[David Root]

This is about a tube not being what you thought it would be. Not exactly fakes, but not the real thing either.

I recently bought six 6SQ7GTs from one of the larger NOS sellers. They were all marked 6SQ7GT but somehow they didn't look right. 1) They had no manufacturer's name or any other mark on them. 2) They didn't look like an RCA or a Mullard. I finally figured out they were 6AV6 guts repackaged in octal bottles. If I wanted 6AV6s I could buy them for $2 instead of $5, which is what I paid for these. (6AV6 is a 7-pin tube "electrically equivalent" to the 6SQ7GT, but NOT sonically equivalent I think).

I sent all six back after a long discussion with a customer service person who didn't know d**k about tubes, for replacement with the proper tube. I just received the "replacements". All six are in "manufacturers boxes". Five out of six tubes are the same thing I sent back, but at least these have Sylvania or Raytheon or even Silvertone on them. The sixth tube is an honest-to-god CV1991 made by MOV in England, and is the real thing.

What is worse, I also ordered two more 6SQ7GTs from a very highly regarded NOS seller, which I paid $14.00 each for, and received two Sylvania 6AV6 cuckoo's eggs in an octal nest, the same as two of the six I got from the first seller for $5.00 each. Naturally they're all going back.

So I have gone through 14 alleged 6SQ7GTs and got one REAL one. I need three for an EH-150 style 4 build. I have one RCA coming from a third seller in Canada whoo is going to send me pics before I buy, so I'm still one short.

Am I nuts or is this only a problem with the 6SQ7GT?

[lastwinj]

no kidding. but, when those "6sq7" were made, probably wasnt much demand, but alot of demand for 6av6. almost exact electrically, so thats what they stuck in the bottle.

looked at the ratings. both have a gain of 100, with the 6av6 being slightly higher current at 1.2ma at 250v vs. .9ma at 250v for 6sq7, marginally diff plate impedance as well.

the 6av6 would probably have a ballsier sound than the 6sq7, due to increased current draw. personally i wouldnt worry about it much.

or, try to use 12sq7gt. i bet good money that all will be what you want.

damn look at that:

Code: Select all

http://cgi.ebay.com/Vacuum-Radio-Tube-12SQ7-GT-Used-tests-Good_W0QQitemZ140135623688QQihZ004QQcategoryZ7275QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

looks like what you want.

germ

[dartanion]

The 1951 Sylvania Tube manual speaks of the 6SQ7 being very similar in characteristics to the 7B6. It also mentions that the 6AV6 being very similar to the 7B4. The 7B4 doesn't have the diodes, but whatever. Do you need the diodes? If so, what for?

I attached a couple of snippets from this manual for you.

[David Root]

Thanx for responding, folks. I think the history was probably the other way around.

I suspect what happened historically is that, because the 6SQ7GT preceded the 6AV6, and was no doubt used in a lot of radios etc, there was a good demand for 6SQ7GTs, but at the end of WWII things changed quickly.
The military no longer needed the 7-pin tubes, which were designed for backpack radios and to make stuff smaller in general, and at the same time, out comes the 12AX7 with the same gain but almost half the plate resistance. Well, as we all know the 9-pin tubes took off and basically wiped out the 7-pin (except in car radios) and eventually the octal preamp tubes.
So there were factories stuck with 7-pin stuff the Army didn't want any more, and some bright spark in finance said to manufacturing, we're going to obsolete this stuff on your books and writing it down is gonna kill your bonuses this year. Then some brighter spark in manufacturing said let's use as much of it as we can and save our bonuses. So the wartime shortage of 6SQ7GTs was filled up real quick.

It could easily have been that simple, and I don't think any engineer came up with the idea either, because engineers didn't get bonuses.

As to the 7B6, that is a new angle indeed. And yes the 6AV6 and the 6SQ7GT are "electrically equivalent", but until the 6AV6 came along the design was very obviously different. I've used 6AV6s, in fact I built a Tweed Harvard some time ago entirely with 7-pin tubes and it sounds really good, but by now I have a burr up my butt the size of an orange about this wholesale substitution, plus the design I'm building dates to 1941, when the 6AV6 didn't exist, so I think I'm going to plug along until I do get three original 6SQ7GTs.

And sure, I could have used metal 6SQ7s, they're historically correct but metal tubes overheat and croak, that's why the G/GTs were born so that heat could radiate directly thru the glass to atmosphere.

Anyway, thanx and..... end of rant, I feel much better!

[dartanion]

I may have some 6SQ7s in my collection as I saw them on my inventory sheet. I just have to dig them up. I'll let you know what I find.

[David Root]

That would be great! You know what I'm looking for too!

[dartanion]

Hey David,

I check my inventory that I have here at home, and they are not there. The rest of my inventory is in storage as we are remodeling our house. It may take a couple of days to find that box, but I will let you know what I have. The boxes I have here had about 20+ 6SN7s and 5U4s.

Have you considered 7K7s at all? I think Jeremy has built an amp or two with these that IIRC sounded totally killer. These are a high mu triode (70) and duodiode.

[David Root]

Thanx Dartanion. I'll stay tuned.

Iwill check out the 7K7, sounds interesting, but for this build I'm going to stay with the 6SQ7 as that's what's in the '41 Gibson EH-150 I'm doing.

Actually it's not identical, in that I'm not using a field coil speaker and I'm upping the plate and screens from 270V to about 400V to get 25-30W out of it. I need to discuss a speaker for it but I think I'll start a new thread on that.

[Allynmey]

I Have 3 NOS 6SQ7's and they are metal envelope.

[PRR]

> the 9-pin tubes took off and basically wiped out the 7-pin

In large systems, maybe.

I bet that through 1960, 7-pin production exceeded 9-pin. 5-tube radios were all 7-pin.

Also the driving force for Miniature was Bell Labs looking for lower stray capacitance in VHF links. While looking at that, they found smaller size, lower cost, and better reliability. This was such a win-win-win that AT&T licensed the idea to everybody.

Anything less than 12 Watts, 8-pin Octal died a sudden death in 1950.

The mass-market for diode-triode is AM Radio. Diode detects the audio off the IF, volume control, hi-Mu triode booster, then a power bottle.

Offhand I'm thinking of one other use: a transmitter limiter. Around 1,000 of these were built, against a hundred million AM radios. There must be thousands of other odd uses for a diode-triode but they don't begin to stack up against the AM radio market.

12AX7 is NOT right for an AM radio. Don't need two triodes. Heater demand is higher. Don't want to pay the higher price of the 12AX7. Occasionally need a second diode.

> 6AV6 guts repackaged in octal bottles

Huh. Never saw that, but have seen a "2A3" which was really a 6AV5 power pentode, triode-strapped, different heater, cathode strapped to heater. Factory made it that way. 2A3 production musta declined to zero, then some important customer HAD to keep some 2A3 gear alive, so they faked it with available parts well enough to work.

Probably 6SQ7 production tailed off to zero, the metal-stampers were thrown-out or just fell to the back of storage, and out of the blue some big contract ordered more 6AQ7s than anybody had in inventory. Could even have happened a few times through the 1980s. And the AM Radio market (and most other diode-triode uses) are not about "sound", just about "works". And by objective measure, 6AV6 works a trifle better than that old octal thing we used to use.

'75 is "equivalent" in a 6-pin base and there may never have been a rush to fill orders for '75 with "anything that works". 2A6 is a 2.5V 6-pin version, and highly unlikely to have been faked; you can fiddle the heater power. 6B6 is an octal. All three are ST-12 Coke-bottle top-cap bottles. There may be much less historical trickery on these tubes. Also if older is better, these will tend to be older than even early 6SQ7. '75 (not the others) is in stock at TheTubeStore.com at $15; old-radio specialists may be a better source.

[PRR]

6F5 is the solo-triode prototype. Top-cap 5-pin from 1935 or earlier. I have never seen one: usually when you need one hi-Mu triode, you need a diode or another triode. There may not be a long bumpy production tail like 6SQ7. And TheTubeStore has 'em for $8.

6F5 comes in metal, Coke, and Straight. (theTubeStore offers no choice.)

Also shop eBay. Yeah you taking a chance, but evidently getting 6SQ7 through your current suppliers is chancey too. eBay sellers often post pictures (get them to swear it is THE tube being offered). eBay tends to turn up "grandpa's garage" tube collections which are often stuffed with AM radio tubes. Obviously they cudda been in the garage because the radio didn't work right, worked better after it was replaced. But if you are paying $5-$20, it isn't a death-defying risk.

[dartanion]

Here's a nice pictorial of an EH-150 restore. Looks like they went with a P12R in place of the Field Coil. Beautiful build though.

http://www.timeelect.com/eh-150.htm

[David Root]

In retrospect it looks like I precipitated my own problem by looking for 6SQ7GTs rather than metal 6SQ7s, which are easy to get. I'd take 6SQ7Gs too but they seem to be even thinner on the ground than GTs.

It's interesting to hear that there are other shall we say "remanufactured" tubes around that aren't built like the originals.

Judging by the boxes, the last eight "6SQ7GTs" I got were made between about 1950 (grayglass) to 1975 or so. Strangely, none had the straightforward "56-37" type date code, some had letter codes which I don't have a key to, and some had no date info at all. Four were Sylvania, two "Silvertone". None were marked RCA or Tungsol, and I know these companies made the real thing.