Valve Arts KT66 or GE 6550's for Superbass?

[Hendrixnut07]

Never really liked EL-34's and my Metro Superbass needs new tubes/rebias. After researching EL34 alternatives, Ive narrowed it down to the Valve Art KT66 or GE 6550's.

If it wasnt for the bias drift issue with KT66's and the fact there more likely to go south faster(not all but some) than NOS I would go with KT66's no questions asked. What I dont see is paying for a quad of chinese KT66's that maybe lasts me 6 months, when for a little more I can get NOS 6550's that will last forever.

Anybody running KT66's or 6550's in their Marshall Superlead/Superbass?

[HEAVIER THAN HELL]

I personally like 6550s ALOT, they're great sounding valves for Marshalls.

I've recently used some Svetlana 6550s and they were a really nice sounding 6550, definately up there with some nice NOS, not disgraced by my GE 6550s.

I'd say the Svetlana 6550s are more in the GEC camp than GE though, they have a BIG deep bass end that's solid as a rock with a really punchy tone.

I want to try the current CHinese 6550s that're made on the old GE tooling. They're supposed to be really nice.

[Hendrixnut07]

Thanks, how are the 6550 cleans? BIG bottom end and loud clean headroom what about the breakup?

[PCollen]

Never really liked EL-34's and my Metro Superbass needs new tubes/rebias. After researching EL34 alternatives, Ive narrowed it down to the Valve Art KT66 or GE 6550's.

If it wasnt for the bias drift issue with KT66's and the fact there more likely to go south faster(not all but some) than NOS I would go with KT66's no questions asked. What I dont see is paying for a quad of chinese KT66's that maybe lasts me 6 months, when for a little more I can get NOS 6550's that will last forever.

Anybody running KT66's or 6550's in their Marshall Superlead/Superbass?

I run Shuguang KT66's in my JTM45, which are the same tube as the Valve Arts. I run a low plate voltage, about 405 Vdc, and have them biased at 44mA Ik (about 17 watts) . I've had them in for over a year, they still sound great, and I experience no bias drift.

[jfhudak]

Never really liked EL-34's and my Metro Superbass needs new tubes/rebias. After researching EL34 alternatives, Ive narrowed it down to the Valve Art KT66 or GE 6550's.

If it wasnt for the bias drift issue with KT66's and the fact there more likely to go south faster(not all but some) than NOS I would go with KT66's no questions asked. What I dont see is paying for a quad of chinese KT66's that maybe lasts me 6 months, when for a little more I can get NOS 6550's that will last forever.

Anybody running KT66's or 6550's in their Marshall Superlead/Superbass?

I run Shuguang KT66's in my JTM45, which are the same tube as the Valve Arts. I run a low plate voltage, about 405 Vdc, and have them biased at 44mA Ik (about 17 watts) . I've had them in for over a year, they still sound great, and I experience no bias drift.

Word. I've had very good luck with my Shuguangs, though I am using their version of a 6CA7. I've noticed in the last week or so that they don't like a hot bias. Mine at least seem to prefer about 60% or a little lower.

[Hendrixnut07]

Back on this boat, thinking about GE 6550's for my Metro SB. A friend of mine says he has em in a '68 Plexi and gets that '' Royal Albert Hall '69 '' tone say Stone Free at 8:18 -

http://www.youtube.com/watch?v=Oglj3C2xevs" onclick="window.open(this.href);return false;

The percussive low end attack is what I want I dont like EL-34's at all pushed mushy to my ears IMO.

[Billy Batz]

Thats exactly why I like 6550s. Percussive low end. In fact if you do a search of 'percussive low end' youll probably see my posts come up a lot in regards to 6550s. The one quality they have that seem to be completely unique is that the lows are big and deep but at the same time they dont fart or boom the hell out of you they stay tight. British tubes sound compressed and rubbery. 6550s are open even moreso then 6L6s. Its something I like for strats but for humbuckers I prefer the british tubes in marshalls.

[Hendrixnut07]

Thats exactly why I like 6550s. Percussive low end. In fact if you do a search of 'percussive low end' youll probably see my posts come up a lot in regards to 6550s. The one quality they have that seem to be completely unique is that the lows are big and deep but at the same time they dont fart or boom the hell out of you they stay tight. British tubes sound compressed and rubbery. 6550s are open even moreso then 6L6s. Its something I like for strats but for humbuckers I prefer the british tubes in marshalls.

Thanks! I often hear the 6550's described as '' stiff '' and '' sterile '' in the cleans, do you find that to be the case? Also you see alot of '' 6550 to EL-34 '' bias conversion threads. I play strats too and love that percussive low end Jimi gets on later shows such as RAH '69.

Which 6550 do you find is sweeter and not harsh sounding?

Tungsol
RCA
GE
Sylvania

[Billy Batz]

Its a personal thing. I can definitely see where people can think them stiff or even sterile and I certainly do with LPs. But it works for me with strats. I think most people would prefer that more rubbery, saturated british strat tone. I really prefer a more open american feel with strats.

I havent used any NOS. Of the new production Ive tried the Shugs and the TungSol RIs and I think the TS sound sweeter.

I dont bother with the bias conversion stuff. I mean you may have to to get the bias voltage in the correct range. Actually I think your going to definitely have to change a resistor there because the bias current with 6550s is way higher so the voltage will need to be lower then you can probably get as is. I have a custom bias control so I have a huge range. As far as the bias splitters I dont bother changing them to 150k or 100k but I do think it may be a small improvement in performance. You may get a cleaner response from them.

[Roe]

the new tung sol kt66 sound a bit like a 6l6gc or a 7581. some have even said it is like a 6550.

[HARLEYIII]

My metro 12 series superbass likes valve Art 6550s, but Im fixin to try kt88's in it like my reissue 1959 has. I like the way the kt88's (EH, but all I could afford)sound in it better than the valve art 6550's.

[doc10039]

I am new and just finished my Plexi Superbass build and I am also looking at 6550's for the output tubes?. Which resistor in the bias circuit would need to be changed to obtain the correct -VDC ? Also would changing the bias splitter resistors to 100K help with performance with the 6550's? I am going to use the amp for bass and want to get the best out of it.

Thanks

Doc

[shakti]

Resurrecting this ages-old thread. I see that some people here have used 6550s in their SLs without changing the 220k bias splitters, with no issues. I'm trying to figure out if I would do any damage to 6550s by running them with 220k splitters?

And someone mentioned "improved performance" with 150k or 100k...improved in what way?

[neikeel]

There's no simple answer, the situation is too complex and you can't watch onto the grid leak resistors isolated from the rest of the circuitry.

Go lower with the value of the grid leak resistors, then you're cutting bass response due to another time constant with the previous coupling cap. And keep in mind, that the grid leak resistors AC-wise are in parallel to the PI's plate resistors - so reducing the value of the grid leak resistors is reducing the PI tube's gain.

Go higher with the value of the grid leak resistors, then you'll get blocking distortion, when you drive the power tubes into saturation - or even already before saturation, when you have output tubes with a bad vacuum.

This especially with 100W amps, where each two tubes are sharing only one grid leak resistor - where the grid leak current of two tubes has to flow through only one resistor.

After the data sheet of i.e. an EL34, the max. grid leak resistor's value is 700K - but only then, when NOT operating in saturation!

For two EL34's this means, that max is 350K (two 700K's in parallel). But you have to add to the grid leak resistor's value the value of the swamp resistors and the entire resistance through the bias supply (47K or 56K & 25K trim pot)!

So each two output tubes in a 100W Marshall with a grid leak resistor's value of 220K actually are seeing about 300K to (real) ground! 600K each tube - not far away from the max. 700K for clean, not saturated operating!

And you now see, that's impossible, to tell a range like 'from xK to yK', where within your amp wouldt work properly.

Larry

and

Since the 6550 are more prone to draw grid current than the EL34, you at least have to lower it to 150K, to avoid the clamping effect of the bias voltage and to avoid blocking distortion, when the power stage is driven into saturation.

To be still more 'on the safe side' I'd recommend to lower it to 100K or even 82K! To balance the bass lost in this case, you can increase the coupling caps to .047 (.047/100K is the same RC time constant than .022/220K) or maybe try .1

Larry

Is what I remembered!

[shakti]

Thanks Neil! What would this forum be without you (or Larry, for that matter)?

This is a very useful and technical explanation. I first have to decide which amp to put them in - either a 12 series or a 69-style SL with the higher filtering. I'll see if I can get away with 150k, if only for the reason I have plenty of 150k Iskras and nowhere else to use those. FWIW, none of the purported Hendrix amps from the era show any signs of having changed the 220ks, and I am currently quite convinced that he did use 6550s extensively during '69 and possibly '70. Not that it proves anything, and it would be better to have the tubes operating safely and properly.