6550 / KT88 power-amp (vs. 6L6 or EL34)

idnotbe

Member
Messages
484
wonder the tonal characteristics of 6550 / KT88 power tube amp.

1. in distortion sound amps (marshall, wreck...) ,
what's the general characteristic of 6550 / KT88 comparing to EL34 ?

i like pre-amp distortion and i think power amp distortion is little bit muddy / unclear.
can KT88 be a good alternative for me?

2. in clean sound amps (fender, dumble...) ,
what's the general characteristic of 6550 / KT88 comparing to 6L6 ?

i feel 4 * 6L6 are loud enough and they give me enough headroom.
then, what's the pros / cons / character of 4 * KT88 comparing to 6L6?
 
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7,834
Wow, where to start... For starters the 6550 and the KT88 have the same Gm as the EL34, so they're quite dynamic. The 6550 could be likened to a hard vacuum 6L6 whereas the KT88 is a darker and stronger KT66, which is peakier and more compex sounding.

The EL34 is probably the narrowest of the medium glass octals. The KT88 is broader with less of a mid peak, the 6550 would be percieved as scooped. The EL34 being a true pentode sounds a bit rough and fluffy, even clean. The KT88 and the 6550 are way cleaner.

In an amp designed for EL34s or 6L6GCs you're likely not to achieve the full output of the big bottles or have them perform at negative, due to the fairly high bias feed resistors. Also your OT may not be designed to handle their max output so you're likely to run these on the colder side of things anyway.

So basically: you'd get more headroom, more lows and a broader sound and more detail with the big bottles. They can even sound a bit stiff or dead, particularly if biased cold.


A more direct replacement that's still capable of being biased at 32W is the KT77. It's brighter and middier than the big bottles and has the vacuum closer to the EL34. You could compare it to the love child of a Siemens/ RFT EL34 and a GE 6L6. Can come off as a bit tinny or hard if biased cold.

Then of course you have the fat bottle 6CA7, which is more like a miniaturised, straight sided 6550 w. lower output. EVH and The Who are known for having used these.
 

deeval

Silver Supporting Member
Messages
2,296
Funny as for Me Pre-Amp distortion can be more muddy, but I would suspect some pre-amp circuits can be good, as most Marshall vintage type have a great Power tube distortion when pushed, I love 6550,s for clean and distorted tones's when combined with a good Pre-amp it can make those 6550 sing real well, you need to try them to see how they do for your application, They have a sweet clean that has some very balanced tonal spectrum for my use, Now my Mesa amps have that Pre-Amp distortion but there circuit just seems to do well.Here is my Mesa King Snake with the combined Pre-amp and Power tube distortion 10 watt mode
 
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2,199
KT's to me are wide with a flat response. Your sound will be thicker. The KT amps I have don't really get into power clip unless it's 1983 and you're center stage at the US Festival..
 

Wyzard

Neither Here Nor There
Gold Supporting Member
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2,369
Then of course you have the fat bottle 6CA7, which is more like a miniaturised, straight sided 6550 w. lower output. EVH and The Who are known for having used these.

Just bought a pair of these in a Music Man amp - that's like two scores in one purchase afaic.

I have three mismatched fat 6CA7s sitting around waiting for dance partners, and am always looking to pick them up whenever I can at a reasonable price.
 

drbob1

Gold Supporting Member
Messages
30,224
Take the following with a grain of salt-the way an amp is set up will determine a LOT more how an amp is gonna function than just what kind of tubes it has in it. That said, in an amp that can run successfully with any of the octal power tubes (a Groove Tubes Dual 75 power amp) and some experience with different amps that use them:
EL34-the lowest clean overhead and smoothest breakup of the lot. Perfect for power tube breakup. Can be set up for up to 50w clean per pair of tubes ala the Dynakit stereo amps.
KT66-about the same output as the EL34, but a little harder and a little more bass.
6L6-slightly more output, slightly flatter overall EQ, slightly harder breakup
There's a bunch of "off brand" power tubes that fit in here somewhere: 7027s, 6973s etc
6550-can put out a LOT more clean power if the amp is set up for it. A pair can go from 50w (Marshall metal panel) to 75w (GT Dual75) to almost 100w (Ampeg). Breakup is harsh to my ears. Frequency response is pretty flat. Perfect tubes for what you want!
KT88s-about the same power output as the 6550, perhaps a bit more. EQ is maybe a bit more bass and maybe a bit more upper mid bark, sorta like a bigger, deeper EL34. Breakup is smoother than the 6550 but way up there volume-wise. No question you can get 100w out of a pair of these (my Sound City PA200 is 4 KT88s and it's the only amp I won't turn up all the way, even with ear plugs on, it's physically painful to stand in front of the 215, JBL loaded cab).

So, if I were looking for an amp for what you want, I'd find something that was designed for 6550s or KT88s that's running them at reasonably high voltage (or maybe 7027s)-an Ampeg V4 is ungodly clean and loud, Mesa Triple Rec or Coliseum, Hiwatt/Sound City/Marshall 200w, Fender 400PS (if you want REALLY clean).
 
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logdrum

Member
Messages
2,123
The best way to experiment with power tubes is to get a single ended amp like the UniValve or BiValve. 6550 in guitar amps if you are going for pre-amp distortion but for linear power tube distortion KT-88. Both you have to push hard to get there and they are loud. I do play clean, so for the THD I only use the 6550 or KT88. Surprisingly the cheap Chinese Valve Art is the best 6550 for me. Broad spectrum but has a smooth top-end and I have an ANOS TungSol which is too mellow compared to the Chinese. I also have the Ampeg VL-503 and that maps sounds best with 6550. My tech told me not to put KT-88 on the 503 but supposedly okay with the 502. On that amp EL-34's are a bit narrow sonic but some people like it that way.


For bass Mesa Boogie Prodigy 4:88 is 250 watts at 4 ohms only 4 KT88 with Mesa's new Silicon Steel transformer. The Bass 400 (not plus) on 6-6550 is probably 275 at 4 ohms and so is the Ampeg, SVT-CL is 300 watts at 2 ohms and 4 ohms also also 6 X 6550. I don't play the big amps anymore but I will also not sell them as log as I own my place in NM.
 

logdrum

Member
Messages
2,123
Funny as for Me Pre-Amp distortion can be more muddy, but I would suspect some pre-amp circuits can be good, as most Marshall vintage type have a great Power tube distortion when pushed, I love 6550,s for clean and distorted tones's when combined with a good Pre-amp it can make those 6550 sing real well, you need to try them to see how they do for your application, They have a sweet clean that has some very balanced tonal spectrum for my use, Now my Mesa amps have that Pre-Amp distortion but there circuit just seems to do well.Here is my Mesa King Snake with the combined Pre-amp and Power tube distortion 10 watt mode


I thought the KingSnake used 6L6 only. Did you mod it to take 6550s?
 
Messages
7,863
Tubes are actually neutral sounding. It is how the amp is designed to bring out certain sounds out of various tubes. Many amps that use tubes are not used for distortion or guitars.

So how they are going to sound has more to do with what amp and other gear you are using.
 

drbob1

Gold Supporting Member
Messages
30,224
Tubes are actually neutral sounding. It is how the amp is designed to bring out certain sounds out of various tubes. Many amps that use tubes are not used for distortion or guitars.

So how they are going to sound has more to do with what amp and other gear you are using.

While there is some truth to this it is NOT the whole story, IMHO. Different classes of tubes, at the very least, are going to respond to inputs that reach their non-linear regions differently. So, in a completely clean environment ANY tube can be made to be audiophile grade "transparent" (see my comment about the Dynakit with EL34s), but when pushed or abused they are going to sound different from each other, even within a certain tube type.
 
Messages
7,834
Second me to dr Bob. More power and headroom tend to provide a more linear response and then each brand tend to have their respective resonant peaks that make them behave slightly differently.

Then in ultra linear operation and at nominal/ expected conditions of operation they sound more neutral than in a crude guitar-amp, for sure. Then again even the hifi buffs tend to favour some brands and types over others.

Some of this could be attributed to the shape and size of the bottle and the hardness of vacuum these offer. A skinny bottle is always going to lose out against a coke-bottle in therms of headroom, although their ratings may be similar or identical. So although a KT77 and a tall bottle 6L6GC may both be beyond 30W max, the KT is bound to compress more prior to clipping than the 6L6. Hence it's recommended that the KT be biased as had it been 25W although it would stand up to 32W, the KT66s is 30W max.

Then of course the ruggedness of the metal parts and grids does their part. One reason for down rating KTs I believe is they didn't hold up so well beyond 600V plate voltage, although at lower dissipation they can take it - the higher B+ does technically make the valves put out more than their theroretical values.
 

drbob1

Gold Supporting Member
Messages
30,224
Yeah the 66s and 77s you might have to baby a bit, but the KT88s (and 90s, 120s, 150s...) seem to be much more durable at high plate voltages. In fact, Origin Electronics in Chicago is building hi-fi amps with the KT150s now, I'd love to see them build a "clean" guitar amp with a pair of those-putting out 300w! Or, 450w if you push them into distortion. Imagine that thru a pair of 412s with say EV12Ms playing downtuned doom music. That'd be about 139 dB at 1 meter, or 130 dB at the front of the stage!
 

idnotbe

Member
Messages
484
thank you for all valuable info.

because i am dumb for tubes, the below is the note for the tube compatibility. (to refer in the future again and again)

6550 - kt88
5881 - 6l6 - kt66
6ca7 - el34 - kt77
 

sg~guy

Member
Messages
681
-my 2203KK with big bottled tone monster's, GOLD LION KT88's,.. Still has the Marshall mid-honk, I have come to the conclusion big glass = big dynamic tone, this theory goes for pre amp tubes as well... You guys can keep your skinny tubes!!
 

bolero

Member
Messages
492
I was using 6550's in an old jtm100 for a while, it sure sounded great with a strat!

also played a les paul into a cranked Ampeg V4B, it was glorious...one of the best cranked LP sounds ever

it was pretty loud though
 




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