Hi Guys
I recently emailed a tube vendor with the question as to wether i could use 6550's or kt 88's in these 2 guitar amps I am building. The a,mps are based on Blackie Paganos "Tubesville " 40Watt class A design. In this diesign a pair of 6550's with a 300 ohm Cathode resistor are specified.
Now the very helpful chap basically said I shouldnt use kt88's unless the amp is capable of 100 watts, and that I should stick to KT88's. Here is the quote
"The KT88 range for guitar applications is usually avoided unless one has some amp that can push to their designed level of 100watts per 2 tubes."
I was under the (mis)understanding that the KT88 and 6550 were in fact similar if not identical and that one was an american manufacture of a european tube.
So my question is can i use a kt88 in place of a 6550 or are they in fact distinct tubes
Many thanks
Nick
I recently emailed a tube vendor with the question as to wether i could use 6550's or kt 88's in these 2 guitar amps I am building. The a,mps are based on Blackie Paganos "Tubesville " 40Watt class A design. In this diesign a pair of 6550's with a 300 ohm Cathode resistor are specified.
Now the very helpful chap basically said I shouldnt use kt88's unless the amp is capable of 100 watts, and that I should stick to KT88's. Here is the quote
"The KT88 range for guitar applications is usually avoided unless one has some amp that can push to their designed level of 100watts per 2 tubes."
I was under the (mis)understanding that the KT88 and 6550 were in fact similar if not identical and that one was an american manufacture of a european tube.
So my question is can i use a kt88 in place of a 6550 or are they in fact distinct tubes
Many thanks
Nick
Here is a good initial write-up:
http://en.wikipedia.org/wiki/KT88
In my personal experience, KT88s are an upgrade to 6550s but the bias characteristics are a bit different. Therefore one shouldn't blindly substitute a KT88 in a 6550 circuit without verifying that the plate current is still within the capability and design of the circuit. A rebias may be required and it may not.
But personally I prefer the sonic characteristics of most KT88s over most 6550s. And for me, the KT88 which outperform all others on a sonic and voltage capacity basis is the JJ. I have had lesser KT88s arc at 650V but the JJ has yet to let me down. YMMV
http://en.wikipedia.org/wiki/KT88
In my personal experience, KT88s are an upgrade to 6550s but the bias characteristics are a bit different. Therefore one shouldn't blindly substitute a KT88 in a 6550 circuit without verifying that the plate current is still within the capability and design of the circuit. A rebias may be required and it may not.
But personally I prefer the sonic characteristics of most KT88s over most 6550s. And for me, the KT88 which outperform all others on a sonic and voltage capacity basis is the JJ. I have had lesser KT88s arc at 650V but the JJ has yet to let me down. YMMV
Thanks
Hi There
Does the kt88 draw more current that the 6550 everything being equal?
The amp I am building uses the hammond 1650N OPT with a rating of 60Watts.
My raison de etre for subbing the kt88 over the 6550 is more durability(electrically) than outright power... Given it will end up in a teenagers hands.
The PT can supply 300mA and i have 2 gz34's in parallel for current handling.
Thanks
Nick
Hi There
Does the kt88 draw more current that the 6550 everything being equal?
The amp I am building uses the hammond 1650N OPT with a rating of 60Watts.
My raison de etre for subbing the kt88 over the 6550 is more durability(electrically) than outright power... Given it will end up in a teenagers hands.
The PT can supply 300mA and i have 2 gz34's in parallel for current handling.
Thanks
Nick
I've experimented with this recently in some guitar amps. I used the Electro Harmonix tubes their 6550, KT88, 6CA7 and the JJ E34L. I experimented between the KT88 and 6550 in a Marshall 50 watt head I was repairing and I found that they didn't bias all that differently than one another BUT I did have to make a slight resistor change when switching from the Chinese 6550's that were in the amp when it came in. I don't really think this is significant I just had to rescale the pot to get the idle current where it needed to be for good result. This circuit does not stress either tube at all and either one sounded fine. I put in the 6550's and sent it out the door since it's a master volume design I don't think that the power amp's tone is all that significant to the sound of the amp when played at most normal levels.
Next up I tried the KT88's in a Sovtek Mig 60 amp in place of the JJ E34L's I have used in the amp for many years. They seem to have a lot of mid range push and they are very punchy which was a good improvement. This amp runs the tubes at over 600 volts B+ so I avoided the 6550. Then I tried the 6CA7's and that's what we're going to stick with for now. They have a nice sound that's an interesting cross between the E34L and the KT88 with most of the punch but a bit softer roll into overdrive mode, very impressive.
All in all I've been pleased with the Electro Harmonix output tubes. I didn't care much for their 12AX7 but the power tubes seem to sound good and they are reliable. For whatever reason I think they are different than the older Sovtek branded tubes even though they are made in the same factory.
I would experiment because having a tube that's capable of more power doesn't necessarily hurt anything in the circuit and you may or may not like the sound.
Next up I tried the KT88's in a Sovtek Mig 60 amp in place of the JJ E34L's I have used in the amp for many years. They seem to have a lot of mid range push and they are very punchy which was a good improvement. This amp runs the tubes at over 600 volts B+ so I avoided the 6550. Then I tried the 6CA7's and that's what we're going to stick with for now. They have a nice sound that's an interesting cross between the E34L and the KT88 with most of the punch but a bit softer roll into overdrive mode, very impressive.
All in all I've been pleased with the Electro Harmonix output tubes. I didn't care much for their 12AX7 but the power tubes seem to sound good and they are reliable. For whatever reason I think they are different than the older Sovtek branded tubes even though they are made in the same factory.
I would experiment because having a tube that's capable of more power doesn't necessarily hurt anything in the circuit and you may or may not like the sound.
Thanks Dave
Thanks for that.
I suppose the real reason as I said for considering the 88's over the 6550 is that they seem to be a beefier tube and therefore given the same b+ will be more durable and thats only my theory.
Blackie recommends eh 6550's in this so I might just go for this. The PT is a 360-0-360 with tube rectification going into a .05uF cap as its first cap so i imagine that b+ will be around 450V dc.
The vendor recommends sed 6550's as they have the hardest vacuum according to him but several , including blackie and yourself, like the EH's
Thanks
Nick
Thanks for that.
I suppose the real reason as I said for considering the 88's over the 6550 is that they seem to be a beefier tube and therefore given the same b+ will be more durable and thats only my theory.
Blackie recommends eh 6550's in this so I might just go for this. The PT is a 360-0-360 with tube rectification going into a .05uF cap as its first cap so i imagine that b+ will be around 450V dc.
The vendor recommends sed 6550's as they have the hardest vacuum according to him but several , including blackie and yourself, like the EH's
Thanks
Nick
The original 6550A and the original KT88 are identical with respect to thier specs...
They handle the same voltages, they have the the same transconductance and plate characteristics....
They do have different tonal charcteristics, but that is another story... They are a direct substitute unless someone can show otherwise in terms of actual data sheets, not opinions...
There will be process variation in which the biasing point will vary, which is normal... isolated measurents are not really valid....
As for the valves they are making today..... Then I wouldn't be surprised if there were differences.... Todays tube makers are taking the liberty of "re-defining" the original charcteristics of some of the classic valves....
The power output charcteristics of an amplifer are governed by plate load, transconductance of the valves and the applied screen voltages...not to mention the limt of the rail voltage before clipping.....
Having two KT88 valves in a amplifier will not automatically assume you will put out 100W......
Guitar amps are 90% of the time in SQUARE WAVE mode.... So your pentodes are really behaving like triodes.... You are hitting the B+ rail voltage after it droops at full power output......
So regardless of what bloody valve or transconductance you have in there you will have a 50% Duty-Cycle Square-Wave that is hitting the drooped rail...... How far you droop is a function of the plate-load slamming into the TRIODE region of the valve characteristics...which is very close for most pentodes regardless of valve type....
WHat I am trying to say is your max square wave clipped output power is governed by the amp circuit, not so much by the valve type.... However, your maximum CLEAN sine-wave output power level is a function of transconductance for the same given amplifer.... This will always be less than the square wave output power, so if your amp is safe enough to crank then it is safe enough for whatver your max clean output power will be...
Chris
They handle the same voltages, they have the the same transconductance and plate characteristics....
They do have different tonal charcteristics, but that is another story... They are a direct substitute unless someone can show otherwise in terms of actual data sheets, not opinions...
There will be process variation in which the biasing point will vary, which is normal... isolated measurents are not really valid....
As for the valves they are making today..... Then I wouldn't be surprised if there were differences.... Todays tube makers are taking the liberty of "re-defining" the original charcteristics of some of the classic valves....
The power output charcteristics of an amplifer are governed by plate load, transconductance of the valves and the applied screen voltages...not to mention the limt of the rail voltage before clipping.....
Having two KT88 valves in a amplifier will not automatically assume you will put out 100W......
Guitar amps are 90% of the time in SQUARE WAVE mode.... So your pentodes are really behaving like triodes.... You are hitting the B+ rail voltage after it droops at full power output......
So regardless of what bloody valve or transconductance you have in there you will have a 50% Duty-Cycle Square-Wave that is hitting the drooped rail...... How far you droop is a function of the plate-load slamming into the TRIODE region of the valve characteristics...which is very close for most pentodes regardless of valve type....
WHat I am trying to say is your max square wave clipped output power is governed by the amp circuit, not so much by the valve type.... However, your maximum CLEAN sine-wave output power level is a function of transconductance for the same given amplifer.... This will always be less than the square wave output power, so if your amp is safe enough to crank then it is safe enough for whatver your max clean output power will be...
Chris
Todays tube makers are taking the liberty of "re-defining" the original charcteristics of some of the classic valves....
This would easily explain the slight differences I found in the bias points between a Chinese 6550 and a Russian version in the same circuit. It wasn't a huge change that I had to make but just a resistor change to get the bias pot into the correct range.
This would easily explain the slight differences I found in the bias points between a Chinese 6550 and a Russian version in the same circuit. It wasn't a huge change that I had to make but just a resistor change to get the bias pot into the correct range.
I bought a bunch of "pre auditioned" Russian tubes last year. There were several Electro Harmonix 6550's and KT88's. I set out to match these up into pairs. In two cases the best match was a EH 6550 paired with an EH KT88. They were matched for similar bias voltage at a given current (80 ma at 400 volts) and transconductance under the same conditions. I am convinced that the EH 6550 and the EH KT88 are the same tube.
They deliver the best bass and are the best tubes for loud rock or techno in a Simple SE. One of those matched pairs of "dissimilar" tubes has been in my SSE since I built it and they hold up well, but have lost some of their "blue glow" appeal.
They deliver the best bass and are the best tubes for loud rock or techno in a Simple SE. One of those matched pairs of "dissimilar" tubes has been in my SSE since I built it and they hold up well, but have lost some of their "blue glow" appeal.
Looking at eh 6550 vs kt88
Hi Tubelab
Thanks for that. After studying the pictures of the 6550 vs the kt88 and assuming the bottle was/is the same shape, I was almost sure that the plate structure of the kt88's looked closer to the outside of the glass envelope than the 6550. Hence the assumption that the kt88 might be more durable.
Given your comments though, I am apt to believe your theory cause lets face it, on paper the two tubes are the same al la CEREM's post so why on earth would you change anything other than the ink?
It seems though that EH gets the nod as a brand so I might go for them.
Who makes the mesa boogie st glass shape 6550's?
Many thanks
Nick
Hi Tubelab
Thanks for that. After studying the pictures of the 6550 vs the kt88 and assuming the bottle was/is the same shape, I was almost sure that the plate structure of the kt88's looked closer to the outside of the glass envelope than the 6550. Hence the assumption that the kt88 might be more durable.
Given your comments though, I am apt to believe your theory cause lets face it, on paper the two tubes are the same al la CEREM's post so why on earth would you change anything other than the ink?
It seems though that EH gets the nod as a brand so I might go for them.
Who makes the mesa boogie st glass shape 6550's?
Many thanks
Nick
I haven't tried the Tungsol 6550 reissue in guitar amplifiers, but I have had good luck with it in other applications - it seems very consistent and a slight step up from the EH stuff..
The KT88EH and 6550EH internal structures appear very similar to me as well, perhaps identical although I currently have no samples to compare.
I used to prefer JJ 6550 to pretty anything in this class, but I have had some explode in Citation IIs in recent years and no longer recommend them.
Cerrem makes a good point, afaik the 6550A and KT88 are fairly interchangeable, however the original KT88 specification has slightly higher overall plate and screen voltage ratings. The 6550 (not A!) however has slightly lower ratings in some of the vintage data sheets I have at home. (Mainly PD) In practice it seems like the vintage KT88 will dissipate slightly more power safely than will any typical 6550A. (Modern ones are probably identical to each other at least in the case of current Expo-Pul production.)
M-O KT88
Va -------800V max
Vg2 -----600V max
gm ------11.5mA/V
Pa+G2-- 46W max /40W rated
GE 6550A
Va--------660V max
Vg2------440V max (450V UL)
gm-------11mA/V
Pa+G2---42W/42W
These are from the M-O and GE data sheets respectively and modern production may differ in significant ways.
The KT88EH and 6550EH internal structures appear very similar to me as well, perhaps identical although I currently have no samples to compare.
I used to prefer JJ 6550 to pretty anything in this class, but I have had some explode in Citation IIs in recent years and no longer recommend them.
Cerrem makes a good point, afaik the 6550A and KT88 are fairly interchangeable, however the original KT88 specification has slightly higher overall plate and screen voltage ratings. The 6550 (not A!) however has slightly lower ratings in some of the vintage data sheets I have at home. (Mainly PD) In practice it seems like the vintage KT88 will dissipate slightly more power safely than will any typical 6550A. (Modern ones are probably identical to each other at least in the case of current Expo-Pul production.)
M-O KT88
Va -------800V max
Vg2 -----600V max
gm ------11.5mA/V
Pa+G2-- 46W max /40W rated
GE 6550A
Va--------660V max
Vg2------440V max (450V UL)
gm-------11mA/V
Pa+G2---42W/42W
These are from the M-O and GE data sheets respectively and modern production may differ in significant ways.
My comments on the 6550 and KT88's being identical apply only to the current EH production.
There have been a lot of "stuff" called KT88's over the years. I still have about 25 early 1990's vintage Chinese "KT88's". They red plate at about 27 watts and will remind you that the Chinese invented fireworks. I would never use these at plate voltages over 400 volts. I won't even put them in a Simple SE where a $4 Chinese 6L6GC works well. I have some Sovtek "6550WA" from the same (or a little later) era that work great.
I saw a fireworks display inside one of the Chinese KT88's that was so violent that the glass cracked and stopped the fun, but I haven't seen one explode yet.
There have been a lot of "stuff" called KT88's over the years. I still have about 25 early 1990's vintage Chinese "KT88's". They red plate at about 27 watts and will remind you that the Chinese invented fireworks. I would never use these at plate voltages over 400 volts. I won't even put them in a Simple SE where a $4 Chinese 6L6GC works well. I have some Sovtek "6550WA" from the same (or a little later) era that work great.
I saw a fireworks display inside one of the Chinese KT88's that was so violent that the glass cracked and stopped the fun, but I haven't seen one explode yet.
I know the ratings are slightly higher on the KT88 than the 6550 but what are the differences really? Or should I say what WERE the differences back on the old tubes say a real GEC KT88 vs a Tungsol 6550? How were they made differently so they could handle 800 volts vs 600 or 660 as some 6550 data sheets say? Maybe the differences were pretty minor and mostly it was a testing/rating difference and not a great manufacturing difference....
I wonder.
I wonder.
DaveMcLain said:
The main differences is the permitted screen grid volts in UL mode and worst of all one can choose from many vendors of the 6550A;B,C,WA,WB grouping. Somewhat sim to KT88 but here there aren't so many variants about. The item to watchout is the value of the grid leak resistor, NOS 6550A stuff quotes 50KOhms max in fixed bias (which makes a distortionless drive much harder) whereas Newly Svets quote 100K. The vacuum on a NOS 6550A is softer than modern day stuff. This may account for sonic diferences.
The differences in dissipation are nit-picking.I find the New TungSols darned good performers and I use'em with 50K grid leaks.
All a bit odd as the 6550 is a sibling of the grand daddy 6L6.
My other comment is that the KT88's are quicker to settledown with specified 6.3 heater volts; I find the 6550 warm up slower and more heater sensitive. I run mine at 6.5 volts
richj
I will remember that next time I melt one! I usually just break the glass and cut away the plate since the tube is already dead. I have seen tubes where the screen grid wires are melted like a blown fuse. I have seen cathodes where the oxide coating has been burned off and the metal sleeve blackened.
I used to have a collection of tubes that I toasted in a spectacular manner, but I have lost or given them all away.
I have really tried to make a spectacular meltdown with some of my gassy 6V6's but the cathode is just not strong enough, it fries first.
I am not done with the 6BQ6's yet, they still work, ....... sort of.
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