renips
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Are kt88/6550 the same tube? I see a lot of 6550 tubes with /kt88. What gives. Are they the same tube?
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6550/kt88
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MartyStrat54
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No, they are not the same tube. They are both 35W dissipation tubes with big glass, but they are constructed differently.
The 6550 was developed by Tung-Sol in 1955 originally for use as a servo amplifier. It was used in audio equipment before semiconductor power amplifiers came into use and continues in production today. It is often found in valve-driven bass amplifiers such as Marshall and Ampeg guitar amplifiers and the Leslie speaker system.
Based on the 6L6 vacuum tube, the 6550 was designed to have higher output power and better stability. It operates with a plate voltage of 600VDC, a screen voltage of 400VDC, and plate dissipation of 35 watts. The KT88, KT90, and 6550, although not identical, are often interchangeable, dependent on external circuit parameters. The 6550's glass envelope was originally wider in the middle than at the top and bottom, but a straight-sided design was later introduced by GE and Philips.
The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has similar ratings to the American 6550 which was designed for use as a servo amplifier. It is one of the largest tubes in its class and can handle significantly higher plate voltages than similar tubes, up to 800 volts. A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with 2.5% total harmonic distortion or up to about 50W at low distortion in hi-fi applications.
The KT88 was introduced by GEC in 1956 as a larger variant of the KT66. It was manufactured in the U.K. by the MOV (Marconi Osram Valve) subsidiary of G.E.C, also labeled as IEC/Mullard, and, in the U.S., Genelex Gold Lion.
The KT88 is more powerful than a 6550 and I think it is not as sterile sounding as a 6550. The KT88 was designed as an audio tube and a 6550 was designed as a servo amplifier tube.
The 6550 was developed by Tung-Sol in 1955 originally for use as a servo amplifier. It was used in audio equipment before semiconductor power amplifiers came into use and continues in production today. It is often found in valve-driven bass amplifiers such as Marshall and Ampeg guitar amplifiers and the Leslie speaker system.
Based on the 6L6 vacuum tube, the 6550 was designed to have higher output power and better stability. It operates with a plate voltage of 600VDC, a screen voltage of 400VDC, and plate dissipation of 35 watts. The KT88, KT90, and 6550, although not identical, are often interchangeable, dependent on external circuit parameters. The 6550's glass envelope was originally wider in the middle than at the top and bottom, but a straight-sided design was later introduced by GE and Philips.
The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has similar ratings to the American 6550 which was designed for use as a servo amplifier. It is one of the largest tubes in its class and can handle significantly higher plate voltages than similar tubes, up to 800 volts. A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with 2.5% total harmonic distortion or up to about 50W at low distortion in hi-fi applications.
The KT88 was introduced by GEC in 1956 as a larger variant of the KT66. It was manufactured in the U.K. by the MOV (Marconi Osram Valve) subsidiary of G.E.C, also labeled as IEC/Mullard, and, in the U.S., Genelex Gold Lion.
The KT88 is more powerful than a 6550 and I think it is not as sterile sounding as a 6550. The KT88 was designed as an audio tube and a 6550 was designed as a servo amplifier tube.
Marshall Mann
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Well, kind of.
With out going into great detail, they are in the same family, just as a 6L6 and a KT-66, or a 6CA7 and an EL-34 are. They are similar in construction and can be interchangeable, but tonally they can be worlds apart.
Basically In the 50's and 60's the US and Europe where constantly one-upping each other in tube designs. So when the Europeans developed the KT-88, the US answered with the 6550.
With out going into great detail, they are in the same family, just as a 6L6 and a KT-66, or a 6CA7 and an EL-34 are. They are similar in construction and can be interchangeable, but tonally they can be worlds apart.
Basically In the 50's and 60's the US and Europe where constantly one-upping each other in tube designs. So when the Europeans developed the KT-88, the US answered with the 6550.
MartyStrat54
Well-Known Member
The 6550 came out in 1955 and the KT88 came out in 1956.
They are only interchangeable in certain situations. Max plate voltage on a KT88 is 800 and only 600 for a 6550. Also, the screen voltage on a 6550 is only good for 400.
Most of today's applications for either of these tubes is generally well below what they are designed for. In a Marshall amp for instance, the KT88's in a Kerry King model are biased at 25 watts, not 35 watts. This seems to be the case in most 100 watt guitar amps that are running 6550's or KT88's. They are being ran cold. I would love someone to come out with a properly designed KT88 amp where two tubes are safely putting out 100 watts and the tubes are biased properly.
They are only interchangeable in certain situations. Max plate voltage on a KT88 is 800 and only 600 for a 6550. Also, the screen voltage on a 6550 is only good for 400.
Most of today's applications for either of these tubes is generally well below what they are designed for. In a Marshall amp for instance, the KT88's in a Kerry King model are biased at 25 watts, not 35 watts. This seems to be the case in most 100 watt guitar amps that are running 6550's or KT88's. They are being ran cold. I would love someone to come out with a properly designed KT88 amp where two tubes are safely putting out 100 watts and the tubes are biased properly.
MajorNut1967
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Okay, here comes Mr. Asshole. Marty was correct the KT88 was introduced in 1956 by the GEC and manufactured by MOV (Marconi Osram Valve). It is a kinkless tetrode with a maximum plate dissipation of 35watts and a maximum G2 dissipation of 6watts. There is a misnomer that the KT88 can handle 800 V on the plates, the original spec and this is for the original tube design is: 600 V on the plate and 600 V on the G2. This is one reason the tubes were blowing up in the Marshall Major amplifier, which averaged 620 V on the plates.
And even though the KT 88 and the 6550 are basically both beam tetrode type power tubes and have similar connections they sound worlds apart. As Marty told you the 6550 was originally intended for military use as a servo driver and originally was not designed for AF usage.
And even though the KT 88 and the 6550 are basically both beam tetrode type power tubes and have similar connections they sound worlds apart. As Marty told you the 6550 was originally intended for military use as a servo driver and originally was not designed for AF usage.
MajorNut1967
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and still we all love you for that 
bennieusmaximus
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So if you have a 6550 amp like from the late '70's"JMP", can you run kt88's with no problem?
Marshall Mann
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My apologies to the OP, I got the dates backwards.
dread1
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bump
MajorNut1967
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dread1
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Could these be used in place of 6550's, without having to change screen resistors and such?
MartyStrat54
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Thanks for the pertinent info. I got the 800VDC Plate Voltage off of a reputable website (Wilkipedia). Just goes to show that you can't believe all of your sources.
MajorNut1967
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MartyStrat54
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That needs to be answered by one of the techs. Personally, I don't think they are a straight across swap. I think the bias resistors and the screen resistors need to be changed at the very least.
Jonathan Wilder
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If the amplifier was set up from the factory for 6550s, they can be interchanged with KT88s in most cases. Depending on the valve and the circuit you MAY have to change out a range resistor in the bias supply but that would be about the extent of it.
However, if an amp was set up from the factory for KT88s, they cannot always be interchanged with 6550s.
Case in point would be the Marshall Major. These are of an Ultra Linear design in which they derive their screen voltage directly from the OT primary winding, which is just a couple of volts below B+. The B+ in these amps is up around 560 loaded and 6550 screens are rated for a maximum of 450 volts whereas the KT88s can take 600V on their screens. In this scenario it would be a VERY bad idea to attempt to install 6550s.
While 6550s and KT88s can safely work into the same OT primary load, they cannot always operate at the same voltages. They can both run "dual rail" (i.e. screen voltage at roughly 1/2 the B+), but the 6550 can ONLY run dual rail if the B+ is above 450-500V whereas the KT88s can run Ultra Linear up to 600V.
dread1
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This would be one for the tech to decide. I will drop it off and have it looked at if I decide to swap. Thanks guys!
A thread resurrection... Thanks for the detailed explanation.
What else would need to be changed when going from KT88s to 6550s, if the voltage is too high? Is it the bias resistors (I think 150k for a 50W amp with 6550s). Is it also the bias supply dropping resistor (the one is series with the diode)? Thanks.
Jonathan Wilder
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They'll both run into the same load and they both have similar plate characteristic curves so the bias supply should not need to be altered. The main thing to watch for is that the screen voltage under load does not exceed 500V. If it does, it would not be a good idea to install 6550s in that amp unless you drop the power supply voltage.
