The 6550 Experience

[Roe]

the GE 6550As are hard sounding tubes. They are nice for bass, but hardly for guitar (unless you're playing metal or something)

[daveweyer]

Bill, if you'd like to try a grand experiment we could probably arrange for a shipment of old 6550s so you could try them out in your amp mod and report the differences between actual 6550s from the late '60s and the Russian models.

I still have two tubes from Jimi Hendrix' amp which would make the experiment especially interesting, like, if the tone was particularly BOG or something, it would verify some of the speculation about the sound from that period.
It would probably be the most authentic thing we could do to isolate the sound of those actual tubes in a way that everybody could share.

So if you'd be interested in borrowing some of Jimi's tubes to try, just let me know, maybe send me a PM.

[Xplorer]

wwwo !!!! ha ha ha !!! Bill, say yes !!!!!!
i'm dreaming awake ...
if you don't , i'm willing to go for it

[bill bokey]

Oh yes I'd love that !!!! Thanks Dave !


I'll also try and find better sounding speakers and some kind of Univibe and a real fuzz as well.

[Tazin]

Dave,
What's the dimension/diameter of the top on the old Tung-Sol 6550 valve? This would be the glass part above the shoulder.

[daveweyer]

Tazin, it looks to be pretty close to 1.5", maybe a little bit smaller. I'm measuring one of the tubes from Jimi's amp, the measurement taken from the most vertical part of the envelope above the shoulder.

[daveweyer]

I wrote Bob Hovland, the engineer at Vox during the late '60s regarding our use of the 6550 and why we moved away from the EL34 in our own amp builds. He responded with a great letter for understanding the basis of the mods done for Jimi Hendrix. There is also some really good information for using alternative brands available today.

"Dave,

Wow, quite a flashback. I think that my first experience with EL34s was with the early ones with the rather narrow metal base rings I think I remember using those for awhile, but when I thought about getting more of them, I could only find the ones with the brown micalex? bases, and so looked around at TV Supply and found some Sylvania branded 6550s, little did I know that they were made by Tung-Sol. When I tried them in the stereo amp I built with the A-431 output transformers, I liked the sound they made. I thought that they did as good a job as the EL34s, with better sounding bass. From then on, I used them.

The Marshall story probably still needs some research, I would think that they started with EL34s because the larger British Vox amps used them. But I don't know the time frame, like when Marshall began making their EL34 amps. Amperex Holland made a lot of the Mullard EL34s, as the two companies had big financial ties with each other during this time. The Amperex EL34s of the day were some of the finest, both from a quality and a sonic standpoint. Today, New Old Stock Amperex Holland EL34s with the XF3 or XF4 date codes sell regularly on eBay for $150 apiece. Telefunken also made EL34s but they never were as popular as the Mullard/Amperex ones.

I think that we also should try to find a time line for when musicians began using the big tube guitar amps driven into clipping most of the time during a song. The EL34s would do OK in this type of service as long as the output transformers had screen taps and used them on the output tube screens, or if the screens were operated at lower than about 300 volts fixed at all times; but if the amp designers started using the pentode-connected output tube configuration with the screens at full or near-full B+, the EL34s would not take being run at clipping for long, the screens couldn't take this, and overheated.

The aligned-grid power tubes were the only thing that could stand the over-driven condition in pentode connection, and as you unfortunately discovered, the 8417 was not really capable of taking it either. Even 6550s would not take running the plates and screens at much over 400-450 max in pentode connection. I still wonder if maybe the 8417s were not aligned-grid tubes; the pitch of the grids in the 8417 may have been so fine that it was literally impossible to align them; also, I think that the screens in the 8417 were not located very close to the control grids, possibly obviating the alignment advantage altogether.

The GE 6550s were awful sounding; I didn't know the guitarists didn't like them either. Many of the different brands of output tubes that VTV and others rated in guitar amps didn't sound all that good to me in hifi amps, I thought that the disagreement was due mainly to the fact that the tubes were used in generating the music rather than reproducing it; maybe this is not a good conclusion to come to. The fat bottle GE EL34s were awful too.

The Sylvania/Philips fat bottle EL34s were another story, they were very powerful sounding and musical, and had great energy from bottom to top. The only EL34 that had better upper register was the RFT narrow bottle ones made in E. Germany, I think, but those had weak bass impression in listening comparisons with other EL34s. The RFTs had a high register that sounded as though you added a super tweeter to the system, always amazed me.

1974 was about the end of good British and American output tubes, the QC just went to hell after that. Nobody cared about making good tubes at that point, everybody's attention was on solid state.

Today, the best EL34s and 6550/KT88 tubes I have worked with and listened to are those with the Winged C brand. Their build quality is usually quite good, and they have a very similar sound quality to the original Amperex EL34s and the Tung Sol 6550s and Genalex KT88s. I believe that the Winged C brand offers both the 6550 and the KT88, if I remember correctly the two models are in a slightly different glass envelope shape, but basically the two tubes are the same internally. I have heard slight differences in the two, but was never sure if it was just production variations----and for a very low price, the Electro Harmonix 6550s are quite close in sound quality, probably plenty close for guitar work. They may not be as high quality of build as the Winged C but appear to be pretty well made.

The new issue Tung Sol KT120s are amazingly powerful sounding, and their larger plates and screen assemblies make them pretty rugged; the only thing about them is that the build quality varies, especially the hand-welds from the tube elements to the base pin connections inside the tubes. I had one of them short out, and found that the short was where these connections were, they were literally within a thousandth of an inch of shorting. Other than that, they are good tubes, and how the designers got the significantly taller cathodes to heat as well with the same 1.6A heaters as the shorter counterparts like 6550 etc. is still a mystery. But these KT120s have a fantastically high zero bias plate current, I think I remember 550-600 or more mA at 300V screen potential on my Tek 570 VT curve tracer! The KT88/6550 tubes are lucky to go to 450-500 mA. The other interesting thing about these KT120s is that they take a very long time to conduct when they are turned on, and also for the glass to get hot at AB1 bias, compared with the 6550s, and they also take a lot longer to cool. Must be the physics of the larger tubes.
Hope this helps.

Bob"

[Xplorer]

cooool ! Thank s to Bob !

this would be the best kind of modern 6550 quad then ?

http://www.ebay.com/itm/6550-tube-Svetl ... 4ae1b5f77f

[daveweyer]

Just don't forget that you can get NOS Tung Sol 6550s for about the same price. I'd still try those first. We used to buy them at Yale Radio for about $3.50 each, ha!

[Xplorer]

sure ! if i can find a trustable NOS tung sol 6550 quad at a decent price, ok
but those i saw on eBay, i'm not able tu judge.
for example this : http://www.ebay.com/itm/Tung-Sol-6550-V ... 3a9d7436d6

[daveweyer]

Forget those on Ebay, they're gassy, almost burned out. look for a grey ring around the flashing to tell.

[JimiJames]

=C= 's are nice and keep things tight. A little warmer I like with the Sovtek's
I have not tried in Marshall's, though.

[neikeel]

I have a matched quad of Winged C 6550 I was going to use on my SIR amp but quite liked the EL34s so never got round to it. I have a JTM45/100 project on the 'back burner' with my virtually new GEC KT66s to drop in when finished, maybe give the 6550 a whirl in there too

[theactor19]

Anyone have sound clips with the 6550s and a strat?

[bill bokey]

Yep there are a few on my soundcloud : https://soundcloud.com/castle-made-of-s ... -superlead