richwalters said:
Ahem... not this vendor. I test at 465 on the plate and screen on my "McShane" test/burn-in/match setup.
Also (minor rant here), since when does the fact that the external plate structure appears identical mean the tubes are identical? Look at an RCA 12AU7A and 12AX7A - see any difference in the plates? (no) And they sure are different. I could go on and on with examples, but remember the cathode chemistry, grid winding and wire material, etc. can all be different and not be visible externally.
Hi Jim,
I've usually purchased tubes matched and burned in from N*w S*ns*r, but lately this has not worked out too well for me. (And small quantity guys like me get reamed by the massive price increases on small quantity purchases) I think your additional level of screening, match and burn in under real world conditions is just what the doctor ordered.
I've usually purchased tubes matched and burned in from N*w S*ns*r, but lately this has not worked out too well for me. (And small quantity guys like me get reamed by the massive price increases on small quantity purchases) I think your additional level of screening, match and burn in under real world conditions is just what the doctor ordered.
Re: thsnks Rich for the schematic
Yup; the input tranny (audio band) is a flipped Sowter 3575X (the X version not on their homepage) as I specified the internal winding connects to be brought out to be paralled to become 2:1 step up. With values shown, the input Z is around 1500 Ohms as I'm using this with a low out Z preamp. Driving with a higher source Z will flatten the Q somewhat but as we are dealing with subsonic stuff, this doesn't really matter but the 3rd order slope is optimal around a 30 Ohm source Z typical with many solid state preamps. I found any other configuration of a HP filter, only brought up the noise floor which is against the objective. The cathode follower config is an obvious choice for lowest noise.
richj
duderduderini said:
Yup; the input tranny (audio band) is a flipped Sowter 3575X (the X version not on their homepage) as I specified the internal winding connects to be brought out to be paralled to become 2:1 step up. With values shown, the input Z is around 1500 Ohms as I'm using this with a low out Z preamp. Driving with a higher source Z will flatten the Q somewhat but as we are dealing with subsonic stuff, this doesn't really matter but the 3rd order slope is optimal around a 30 Ohm source Z typical with many solid state preamps. I found any other configuration of a HP filter, only brought up the noise floor which is against the objective. The cathode follower config is an obvious choice for lowest noise.
richj
kevinkr said:
Hi Kevin!
Thanks for the vote of confidence!
Their matching is pretty lousy on KT-88s, it is better on lower power tubes. I honestly have no idea what operating points they use for each tube. For my purposes, I try to select points that correspond with typical "real-world" applications - the 465 volts on the KT-88s and 6550s is typical of many amps operating on todays higher AC lines.
I use a 100K grid resistor on my (fixed-bias) test rig normally, but I've also tested all the New Sensor KT-88 and 6550 tubes I carry with 220K on the grids. Not a hint of trouble. In fact the only tube in that family that had any trouble with 220K/fixed bias was the SED 6550C. Which also is the hottest biasing 6550/KT88/KT90 tube out there. They often want to run bias current 50-75% above other typical KT-88/6550. A "cool" SED is about mid-range for the New Sensor current or GE NOS tubes (or for that matter Ei KT-90 as well).
As you well know on a Cit II the grid resistor is 100K, and I've used KT-88 Genalex and EH just fine in that amps. So why the spec is given as 50K is a mystery to me.
With that SED 6550C tube as an exception I find the typical KT-88 and 6550 bias about the same, and behave very nearly the same in most apps. I often find more bias variation within a given type than between the two different types (KT-88 & 6550). Tonally they are different though, no doubt.
hey guys-
as far as i can tell, the EH 6550 w/o the straight glass and EHkt88 are the same. the main difference in spec between 6550 and kt88 are the screen voltage ratings. not sure which amp you are building but i think that one has screens regulated to 300v by od3's. the main failure mode of current manufacture pentodes and tetrodes in guitar amps is screens getting lunched by overvoltage, less of a problem with a screen regulated amp. the SED/winged c 6550's are quite durable but dry sounding compared to the EH's to my ears. you should be able just select on sonics and not worry about longevity too much
as far as i can tell, the EH 6550 w/o the straight glass and EHkt88 are the same. the main difference in spec between 6550 and kt88 are the screen voltage ratings. not sure which amp you are building but i think that one has screens regulated to 300v by od3's. the main failure mode of current manufacture pentodes and tetrodes in guitar amps is screens getting lunched by overvoltage, less of a problem with a screen regulated amp. the SED/winged c 6550's are quite durable but dry sounding compared to the EH's to my ears. you should be able just select on sonics and not worry about longevity too much
Hey Blackie
Hi Blackie
Its Nick from Australia here and am building the Amps for my nephews.It is the tubesville T+A amp and yes I have followed your advice on the EH 6550 (Big bottle shape).I got some a week or so ago.
The whole kt88 and 6550 thing just perplexed me a tad as they seem awfully similar in most ratings except the screen grid .
I am also building PSE 6550/kt88's for my home power amps which is partly why I started the thread and I figured that if a kt88 had higher ratings over a 6550 in the volts area, then it might be more durable in those lovin teenage hands in the guitar amp situation.. The regulating of screen grids with the 0D3's is of course clever to say the least, not to mention obvious now that you mention it.
I will let you know how I go once I build the amps and maybe send you a few pics
Regards
Nick
Hi Blackie
Its Nick from Australia here and am building the Amps for my nephews.It is the tubesville T+A amp and yes I have followed your advice on the EH 6550 (Big bottle shape).I got some a week or so ago.
The whole kt88 and 6550 thing just perplexed me a tad as they seem awfully similar in most ratings except the screen grid .
I am also building PSE 6550/kt88's for my home power amps which is partly why I started the thread and I figured that if a kt88 had higher ratings over a 6550 in the volts area, then it might be more durable in those lovin teenage hands in the guitar amp situation.. The regulating of screen grids with the 0D3's is of course clever to say the least, not to mention obvious now that you mention it.
I will let you know how I go once I build the amps and maybe send you a few pics
Regards
Nick
the eh's wear quite well in this amp, it was something i built in an old bassman chassis, ear tuned, and documented afterwards as i was pleased with it. it was backline at the Continental for years and really held up.
the classis operating points for 6550's call for a low screen voltage and i feel they sound better that way. i used that trick a lot in 6550 designs, sometimes mounting the od3's visible thru the chassis to double as a pilot light (pics on my website)
at any rate quite a few guitar amp and hi-fi designs exceed screen voltage ratings and this definitely is the biggest cause of premature failure, along with impedance mismatch on the secondary and hammering on the amp into a funky load
the classis operating points for 6550's call for a low screen voltage and i feel they sound better that way. i used that trick a lot in 6550 designs, sometimes mounting the od3's visible thru the chassis to double as a pilot light (pics on my website)
at any rate quite a few guitar amp and hi-fi designs exceed screen voltage ratings and this definitely is the biggest cause of premature failure, along with impedance mismatch on the secondary and hammering on the amp into a funky load
SpreadSpectrum said:
I think it is the worst sounding tube I've ever heard - and I'm not exaggerating. It has a rolled off top end, tubby bass, and other than in a system that is screaming hot on the top end I can't understand why anyone would use it.
Since I wouldn't use it in my own gear I won't carry it - that's my policy.
Could I convince you to save a few more $$ and get the Genalex reissues? They are worlds apart from a SED. A much better choice IMHO...
As most are referring to new manufacture replacements of the 6550 and KT88, I really can't give any opinion as I don't have any of these. I did purchase a quantity of the early Chinese made stuff in the late 80's. These were junk and over 80% were useless.
My brother built a pair of 50-watt mono-blocks about a year ago and used new issue Tung-Sol 6550 tubes. They biased well, delivered a solid 50 watts and sounded okay. They have floated around a bit since new requiring re-balancing, both AC and DC. As a result, I don't consider them that stable in the long term.... but if you're only option is to buy new (as in you need quantities over time) then your options are limited.
One minor spec difference between original 6550 and KT-88 was the filament current, the 6550 rated at 1.6 amps and the KT-88 rated at 1.8 amps. I doubt this pertains to any new manufacture variants.
GE made a 6550A in a simple cylindrical bulb, not as pretty as the old Tung-Sol "coke bottle" ST-glass, but these were quite rugged and could handle the higher voltages without issue. I used many matched pairs when I was doing some rebuilds and modifications on the old Dyna Mk-III amps. I even used them in many ST-70 rebuilds as real EL-34 tubes were either unattainable or priced far out of reach for the person who sent the amp in for rebuild. 15-20 years later they're still holding up. I think some places still have some stock on these. They would cost more but chances are they would hold up longer and without serious drift.
Regards, KM
My brother built a pair of 50-watt mono-blocks about a year ago and used new issue Tung-Sol 6550 tubes. They biased well, delivered a solid 50 watts and sounded okay. They have floated around a bit since new requiring re-balancing, both AC and DC. As a result, I don't consider them that stable in the long term.... but if you're only option is to buy new (as in you need quantities over time) then your options are limited.
One minor spec difference between original 6550 and KT-88 was the filament current, the 6550 rated at 1.6 amps and the KT-88 rated at 1.8 amps. I doubt this pertains to any new manufacture variants.
GE made a 6550A in a simple cylindrical bulb, not as pretty as the old Tung-Sol "coke bottle" ST-glass, but these were quite rugged and could handle the higher voltages without issue. I used many matched pairs when I was doing some rebuilds and modifications on the old Dyna Mk-III amps. I even used them in many ST-70 rebuilds as real EL-34 tubes were either unattainable or priced far out of reach for the person who sent the amp in for rebuild. 15-20 years later they're still holding up. I think some places still have some stock on these. They would cost more but chances are they would hold up longer and without serious drift.
Regards, KM
NOS Stuff
I doubt any one would argue that NOS is better than any current production but for me I see they are reaching/fetching depressingly large sums of money and I refuse to go down that track. The stuff I have built not only requires the tubes you use but a stockpile of maybe 2 to 3 sets for when the old ones give up / blow up. In that scenario, snaring a quad of ge's is one thing but having a few sets (read another 8 of them) becomes prohibitive.
After having looked at the 300b costs I thought there has to be an alright new production tube out there that is durable and sounds good, hence the 6550/kt88 quest. My friend who built a pse gold lion reissue kt 88 has done a great job and it sounds nice, maybe not the fluidity of a 300B but certianly impressive nonetheless.
As for the guitar amps, well I want to make sure my nephews can get new tubes as the years pass so Hopefully the 6550 will be made for a while to come (try teaching gen X about stockpiling!!).
In summing I am compelled to use new production tubes as they are readily available and cost effective.
If anyone knows of NOS ge's at a good price email me quietly so no one else hears!! I wouldnt mind buying 12.
The genalex reiisue seems to have the goods thankfully with quite a strong vacuum, and I can attest to its ability to put you alongside the musician when listening to it.. full marks to New Sensor for having the cahoonas to build it.
Nick
I doubt any one would argue that NOS is better than any current production but for me I see they are reaching/fetching depressingly large sums of money and I refuse to go down that track. The stuff I have built not only requires the tubes you use but a stockpile of maybe 2 to 3 sets for when the old ones give up / blow up. In that scenario, snaring a quad of ge's is one thing but having a few sets (read another 8 of them) becomes prohibitive.
After having looked at the 300b costs I thought there has to be an alright new production tube out there that is durable and sounds good, hence the 6550/kt88 quest. My friend who built a pse gold lion reissue kt 88 has done a great job and it sounds nice, maybe not the fluidity of a 300B but certianly impressive nonetheless.
As for the guitar amps, well I want to make sure my nephews can get new tubes as the years pass so Hopefully the 6550 will be made for a while to come (try teaching gen X about stockpiling!!).
In summing I am compelled to use new production tubes as they are readily available and cost effective.
If anyone knows of NOS ge's at a good price email me quietly so no one else hears!! I wouldnt mind buying 12.
The genalex reiisue seems to have the goods thankfully with quite a strong vacuum, and I can attest to its ability to put you alongside the musician when listening to it.. full marks to New Sensor for having the cahoonas to build it.
Nick
That's a shame. It has some unique features that look like they would make a very rugged tube (screen grid radiator fins, ribbed plate structure).
I worry a little bit about the New Sensor tubes. It looks like the inside of the plate structure has a shiny surface (at least from the seam between halves of the plate). It seems like this would make it hard for the cathode/grid/screen to get rid of heat (since the inside of the plate won't readily absorb radiated heat due to its shiny surface).
I would love to do a tube autopsy and see if the inside of the plate really is reflective. However, all of my EH KT88s are practically new. I'm not that curious.
I have an AB2 capable amp and had planned on dissipating a tiny bit of power in the grid. I just don't know if that would be a good idea with the New Sensor plate design. If the grid starts glowing, 360 degrees of shiny mirrors around it won't help any.
More on the subject of the thread: I've always wondered if the only reason for the higher screen and plate voltage ratings of the KT88 was due to the upper mica disc in the KT88 shielding the other mica from the getter material. Less chance of getter material contaminating the mica and creating leakage paths. I can't imagine that there is anything too magical in the KT88.
But the Tung-Sol 6550s had the extra mica disc too...
The other 'main difference' is the grid bias voltage. If you are using fixed bias you will have to rebias when you swap between 6550 and KT88, and you may not have enough adjustment range to do it correctly. The result of wrong bias will be premature clipping in one direction or red plates or even fireworks in the other direction.
They are really NOT interchangeable.
They are really NOT interchangeable.
To an extent. They we're basically interchangeable in the 60's as long as you observed the voltage limits of the 6550 but these days the quality and specs vary a lot.
My personal experience says the new Tung-sol 6550 wears out faster than the unbranded Shuguang KT88, but then again I am beating the crap out of them...
I haven't built with fixed bias yet, and considering I use toroid power transformers as outputs running in class A as per the datasheet (485V, 94ma) getting proper balance is paramount. Nothing that a TL783 can't do
My personal experience says the new Tung-sol 6550 wears out faster than the unbranded Shuguang KT88, but then again I am beating the crap out of them...
I haven't built with fixed bias yet, and considering I use toroid power transformers as outputs running in class A as per the datasheet (485V, 94ma) getting proper balance is paramount. Nothing that a TL783 can't do
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