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Old 30th September 2008, 12:53 PM   #1
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Default KT88 and the 6550

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
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Old 30th September 2008, 01:31 PM   #2
Rodango is offline Rodango  United States
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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
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Old 30th September 2008, 01:52 PM   #3
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Default 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
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Old 30th September 2008, 02:09 PM   #4
Rodango is offline Rodango  United States
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Default Re: Thanks

Quote:
Originally posted by duderduderini
Hi There
Does the kt88 draw more current that the 6550 everything being equal?
In most circuits, yes. The KT88 typically needs greater bias voltage than a 6550 does in the same circuit to have the same plate current as the 6550 would. But the KT88 should be allowed greater current, and the circuit should be capable of it.
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Old 1st October 2008, 06:11 PM   #5
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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.
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Old 1st October 2008, 10:30 PM   #6
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Default 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
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Old 1st October 2008, 11:06 PM   #7
cerrem is offline cerrem  United States
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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
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Old 1st October 2008, 11:58 PM   #8
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Quote:
Originally posted by cerrem
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....
Agreed

Quote:
Todays tube makers are taking the liberty of "re-defining" the original charcteristics of some of the classic valves....
That's got to be the understatement of the year.
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Old 2nd October 2008, 01:32 PM   #9
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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.
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Old 2nd October 2008, 02:04 PM   #10
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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.
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