What are the best 6550/KT88 tubes for 2011?

[J M Fahey]

Well, as a general rule, tubes are heavily current limited, so the practical way to increase power is to increase voltage.
And, what many forget, to raise load impedance,because you can't go over the current limit.

[Leo_Gnardo]

Does anyone know anything about the china made KT100?
It is supposed to be a beefed up KT88?

I tried KT100 @ 12-15 years ago and no miracles here. KT100 were mentioned on Audio Asylum a couple years ago and although they look from the outside to be identical to Shuggie KT88's, the AA correspondent claimed them to be a re-labeling of a "sweep tube". Same pinout and bias voltage requirements as KT88. I didn't get any better power, and somewhat worse reliability than Shuggie or JJ KT88, after trying @ 20 tubes.

And KT120 - although some hi fi amps have recently been built with this tube, and others have used them to replace 6550 & KT88, haven't been a screaming success. The claim has been made on AA by techs-with-curvetracers that the KT120 curves aren't all that linear, therefore KT120 not so amenable to hi fi amp use and one even said it's a "solution in search of a problem".

[bob p]

Is that 300w a general term, or do you really get 300w output?

300W RMS is an FTC rating. A properly tuned SVT will make 300W without any problems.

If you start looking at the BS terms like "peak music power" the SVT will make even more.

[Leo_Gnardo]

300W RMS is an FTC rating. A properly tuned SVT will make 300W without any problems.

If you start looking at the BS terms like "peak music power" the SVT will make even more.

An important factor is being left out. At what distortion figure? Hi-fi and commercial PA amps usually rated at some fraction of one percent, MI amps at - who knows? 10% even 20% wouldn't be unusual. My rough rule-of-thumb is for every 5% increase in THD you can add 12% to the power rating. Guitarists love distortion, right? By leaving out the distortion factor, power figures can be inflated to impress those who would be impressed.

Early 70's before FTC got involved there were some outrageous claims made for power, from cheap home stereos to serious pro amps for PA. It was impossible to make an apples-to-apples comparison.

I'm going to start my own rating: PIMP, Peak Intermittent Music Power. Always an impressive figure. How much power the amp produces for a millisecond, just as it self-destructs. Oh whoops, that's already being done, some of those newer class D, G, H amps, ten thousand watts in a single rack space. To borrow a phrase, yeah, sure, right. Any way you look at it PIMP will sell amps - that's what it's all about.

[bob p]

And KT120 - although some hi fi amps have recently been built with this tube, and others have used them to replace 6550 & KT88, haven't been a screaming success. The claim has been made on AA by techs-with-curvetracers that the KT120 curves aren't all that linear, therefore KT120 not so amenable to hi fi amp use and one even said it's a "solution in search of a problem".

One of the well respected / conservative HiFi amp companies has convinced their customers that they "developed" the KT120 "in cooperation" with a tube manufacturer, in order to create an illusion of exclusivity and superiority over other amp manufacturers. The HiFi consumer marketplace is a strange crowd. They like to buy into ideas that promote "proprietary" designs and an air of exclusivity, and they're always willing to pay a serious price premium for the newest model. My friend was worried that his 55-watt 6550-based amp was now "obsolete." I told him not to worry, that it was far more likely that the company in question had just placed a stocking order for privately labelled tubes from a Chinese supplier, and had decided to let people start retelling stories about the great new tube design.

I agree that the KT120 isn't all it's cracked up to be.

What's most interesting about this KT120 situation is that with the advent of the "new design" KT120-based amps, the "obsolete" 6550-based HiFi amps have dropped precipitously in price on the used market. You know, it's the same old thing -- the HiFi guys want to pay through the nose for the newest/greatest thing, and nobody wants to own the "obsolete" model that's being replaced. I was able to buy my 6550-based 100W/channel amp on the used market for about 1/7 of the cost of the new KT120-based equivalent. It was like I was buying salvage gear at a surplus store.

[bob p]

An important factor is being left out. At what distortion figure?

I left it out on purpose, because the numbers are so fungible, and because if you really want an objective answer the 6550 data sheet prints the answer in black and white.

The bottom line is that the SVT puts out 300W of RMS power. I intentionally left out the part about how it can be measured to put out 700W if you use creative measurement schemes. More on that later.^*

if you look at a 6550 data sheet the distortion figure for a pair of 6550 operating at 600V into a 5k load in AB1 yields 100W @ 3.0%. That's clean by MI standards. The SVT runs the voltages 10% higher. It has no trouble making 300W.

^* If you're willing to let the distortion numbers climb, and/or take measurements that involve music-like signal pulses, then the numbers can go much higher. I've heard people say that they can get 600W - 700W "peak music power" out of an SVT. I've never tried such measurements. I guess that what they are calling "PMP" is what you're referring to as "PIMP." I like your acronym.

Realistically speaking, PIMP explains why in the real world you need a speaker cabinet with a 600-700W rating for use with an SVT. When someone cranks an SVT they tend to destroy lesser cabinets, either with peak power (over excursion) or with RMS power (thermal failure). Both ratings are relevant when the power numbers are this high -- the SVT is famous for destroying speakers using both failure modes.

The bottom line is that the SVT is a monster of an amp. Such a monster that people who are used to 50W guitar amps may have trouble wrapping their head around what a monster it really is.

One thing that's interesting about this is that bass players who are used to 1kW SS amps often find it hard to believe how loud that 300W tube amp is by comparison. I'm going to stop before we get into the discussion about tube watts vs. solid state watts.

[g1]

300W RMS is an FTC rating. A properly tuned SVT will make 300W without any problems.

Back in the nineteen hundreds , measuring 325 watts at onset of clipping was routine for SVT. I can't say what kind of results modern tubes would give, but 300W should be no problem.
The schematic rates the output at 330W: "The load resistor should be able to dissipate the full power of the amplifier, or 330 watts".

An important factor is being left out. At what distortion figure? Hi-fi and commercial PA amps usually rated at some fraction of one percent, MI amps at - who knows?

Most of the Fender (newish) mention 5% THD (ex. '65 Twin (reissue) 85W at 5%THD with 1khz signal).
I like to measure clean power so I consider 5% THD fudging but that's just me. At one time when I had access to THD analyzer I clipped an amp to 5% and it was fairly clipped. The arguement is that 5% is where it starts to sound noticeable, so up to that point is ok to consider full power. For me, the onset of clipping is full power, not some amount of squared off signal. That's why I don't agree with measuring power with a DMM and using your ears to guess when it's clipping, not very accurate.
So like Leo said, running up the THD figure can really add to power ratings. Here's a quote from the Amplified parts website (Guitar Speaker Power Handling | Amplified Parts)
re: JCM800 2203"Typical power at clipping, measured at 1kHz, average distortion 4% 115 watts RMS into 4, 8, 16 ohms. Typical output power at 10% distortion 170 watts into 4 ohms."

[soundmasterg]

The Blueline SVT's with 6146's will put out 330 watts RMS for sure. The 6550 based Magnavox-era SVT's are more like 300W or slightly less in my experience. The MTI-era SVT's are a mixed bag....the very early ones like the one I used to have had leftover Magnavox transformers with slightly higher voltages than the later Japanese transformers and these will put out hte same power as the Magnavox era SVT's. The Japanese transformer MTI SVT's are around 240W RMS from what I have seen....but of course this all depends on where you decide the sine wave is clean on your scope....if you err on the cleaner side of things then you get less power. If you allow a little distortion in the mix, then you get more power. The reissue SVT's I haven't seen on a scope so I can't comment on their output capabilities.

Greg

[imaradiostar]

I like this thread. I hope it keeps rolling and I don't mind that it's a zombie thread.

FWIW, I have a pair of kt120's and they're capable of more than the 6550's in my possession. I've run them at 4k to 6k6 and voltages around 700 volts and they'll happily make 100 clean watts in that range without breathing hard. To me that's the benefit. I've used them ultralinear at 700 volts and they seemed OK with it. I don't know that I'd send an amp out the door that way but it's still pretty neat. When I say clean, I mean CLEAN, like under 1% distortion measured with my HP 339A.

I'll probably end up having a transformer wound for them when I get a chance and will likely get about 120 to 150 watts out of the pair. I plan to use plate voltages around 800 volts in that case. Screen voltages will be substantially lower.

Jamie