• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

KT88: EH vs. Winged C

I designed a KT88 PP amp and used EH KT88s. I love them. They seem to be able to take high voltages well and sound great. Recently, I have been looking at 'Winged C' Svetlana KT88s as well. I wanted to try them out, but wanted to get some feedback before I blow a bunch of money on them.

In particular, I was wondering if anyone had used these in higher voltage applications (>500V). I like the way they look, but they do cost more than the EHs, and I'd like to know if that additional cost will translate into additional reliability or at least the same reliability as the EHs.

I would be particularly interested if there is anyone out there who has dealt with a large number of these tubes and has a good idea of how statistically reliable they are. (e.g. we get 1 return for every 50 that we sell)

Oh yeah, and do they sound better?

Thanks in advance,

SS
 
Sofar have clocked up 4750 hrs on an event meter with winged C in an amp with quiescent current of 80mA per tube at 265V B+ (22W quies and 32W full load dissip ). This may seem lowish but I run most of my tubes with low B+ in parallel pairs and high quiescent current.
Nothing unusual with the quiescent side has drifted over time and I might have to wait a long MTBF.

However, on my other amp which runs at 530V B+, the KT88's "C" gm's can vary considerably if unmatched and I would double check the gm so they all track sim signal current. I find them well-made with thicker glass than NOS GEC and can accept a hefty tap test. Make sure not to strain the screens as I have lost a couple on hook-ups. >500V ballpoint does seem to be an itchy UL operating voltage and your post isn't unusual.
My constructional philosophy is to fit lower-rated screen resistors to act as fuses.


richj
 
Sofar have clocked up 4750 hrs on an event meter with winged C in an amp with quiescent current of 80mA per tube at 265V B+ (22W quies and 32W full load dissip ). This may seem lowish but I run most of my tubes with low B+ in parallel pairs and high quiescent current.
Nothing unusual with the quiescent side has drifted over time and I might have to wait a long MTBF.

Hi rich,

just curious why the lower B+ voltage and higher bias current if you mind?

Cheers Michael
 
Hi rich,

just curious why the lower B+ voltage and higher bias current if you mind?

Cheers Michael
Biassing so Running near closer to class A operation gives much better HF response with lower THD at 10Khz. YOu will find alot of yesterday tubes where run with higher Iquies (they were designed for harder operation). I like ¦¦ pairs, it makes o/p tranny design easier.
Hope this helps.

BTW; I've haven't noticed any difference using cryo treated tubes compared to those produced by normal manufacturing processes. It' a gag.



richy
 
I designed a KT88 PP amp and used EH KT88s. I love them. They seem to be able to take high voltages well and sound great. Recently, I have been looking at 'Winged C' Svetlana KT88s as well. I wanted to try them out, but wanted to get some feedback before I blow a bunch of money on them.

In particular, I was wondering if anyone had used these in higher voltage applications (>500V). I like the way they look, but they do cost more than the EHs, and I'd like to know if that additional cost will translate into additional reliability or at least the same reliability as the EHs.

I would be particularly interested if there is anyone out there who has dealt with a large number of these tubes and has a good idea of how statistically reliable they are. (e.g. we get 1 return for every 50 that we sell)

Oh yeah, and do they sound better?

Thanks in advance,

SS

I know everyone has their own opinion on tone quality, but I find the SED (made in St. Petersburg) KT-88 unlistenable. It has no top end, it has muddy, one-note bass. I think it's awful. I don't even carry it for sale.

I like the KT-88 EH - I wish it had a tad more bass, but it's quite nice - especially for the price.
 
+500V for some newish brands is unknown terroritory: vacuum isn't as hard as it used to be and various other cut short issues related to facilate production runs- biassing gets more critical esp as myself who like to run tubes fairly hard. It's easy to ruin the screens of a pair of Svetlana KT88 (black circle with C in middle) by pushing them too hard at 550V was exactly what happened by overdrive but thats just myself.; These BTW start to show reddish anodes at 530V @ 60mA Iquies and a tad more, the Iquies
seems to vary and unsettle as if a lower grid leak is required. Behave like 6550's ? they nearly all look the same construction.
JimMcShane mentions the EH versions seen a better bet. I'd agree with him.More robust.

r):
richy
 
Biassing so Running near closer to class A operation gives much better HF response with lower THD at 10Khz. YOu will find alot of yesterday tubes where run with higher Iquies (they were designed for harder operation). I like ¦¦ pairs, it makes o/p tranny design easier.
Hope this helps.

BTW; I've haven't noticed any difference using cryo treated tubes compared to those produced by normal manufacturing processes. It' a gag.

richy

That's an interesting question where the better bandwidth comes from I guess you run your tubes together with different type of transformer with much lower input impedance then "typically", these transformers with low impedance from the very little I understand and have seen seem to have better bandwidth due to the not so horrendous turns ratio difference between primary and secondary, well this is just my speculation what's your take on that?

btw about the lower B+ and higher Iquis, do you still maintain the similar power through the tube so they don't run too hot?

I hope I will do well with a pair of KT88 in OPS for my upcoming project...
cryo.. yeah not my cup of tee either.

Cheers Michael
 
That's an interesting question where the better bandwidth comes from I guess you run your tubes together with different type of transformer with much lower input impedance then "typically", these transformers with low impedance from the very little I understand and have seen seem to have better bandwidth due to the not so horrendous turns ratio difference between primary and secondary, well this is just my speculation what's your take on that?

btw about the lower B+ and higher Iquies, do you still maintain the similar power through the tube so they don't run too hot?


Cheers Michael

The output transformer in bottle amps is the achilles heel. (looking at squarewave) As frequency rises, so do the parasitic losses, and the tubes have to dump this extra reactance and it's nasty. Remember Miller effect in the o/p stage: Je higher B+, the higher the effective tube Cag product = Miller current with rising frequency and voltage; so that's a definite argument for working at lower impedances, higher currents and lower B+ with a lower Z o/p tranny.
Remember less Cu wire, the only thing changed is the turns ratio (fewer pri turns) and to take higher current, the iron size is the same for a high Z specimen for the same o/p power. The tubes can be biassed a bit more to the flatter side with higher current. With high B+ one looks towards the often cramped right side of the graphs and you will see negative grid bias also rockets. Lower B+ generally implies lower anode heating power and longevity.
The emphasis on lower B+ and paralleled stages is now focussed on the driver stage to be able to shunt Miller effect as grids are now parallelled. A low Z diff Williamson stage works wonders here with a CCs in the common cathode stage.

The argument for parallel pairs isn't so popular with MI; as more tubes have to be adjusted when replaced in situ; but for HIFI go for it.
A clue to Miller effect is to look at the Mosfet loss equation model.The effect of the voltage is the prime culprit.

richy

richy
 
Hey, Cyro, something I actualy can contribute to.
Through my bladesmithing I came across the question whether cyro treatment would improve the quality of steel.
It has been found, that very shortly after the heat-treatment of the metal cyroing can help, since it reduces rests of austenit, making the steel less likely to break. If performed to late it does nothing though.
I don't know in which way this affects tubes, since the metal either is already heat treated industrialy (which would be pretty close to ideal) or not heattreated at all; it shurly has no effect whatsoever if performed days, months or years after the manufactoring of the tube. The time frame is about two hours after heat treatment before the rest austenit stabilizes.
 
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Where I live the weather does the cryo for me....no difference
 

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