Power tube type change

[Granny Gremlin]

So I have a Garnet Sessionman head which I absolutely love (pair of 6CA7/EL34s).  It's just not got quite enough headroom for some live situations I find myself in.... so I figured I would try running it with KT88s (I'd have my tech bias it properly etc), BUT, when I asked him about it he warned me that 6CA7s can take a much higher voltage than KT88s and he's seen problems when someone tries to sub them in with plate voltages in excess of 450-500 or so.  My session man is running at 520V so he's recommending against it.

So wondering about a second opinion on that.  Anybody?

Otherwise, aside from getting a different amp (not in the cards right now), is there anything I can do?  The amp has old 6CA7s in it now. 

[amptech]

I have two dynaco mk3 mono amps in my control room, they run nicely at near 500 volts.
I recall the design maximum for the original genalex KT88´s is 800V, as is the case with
the Mullard EL34 and I guess the Sylvania 6CA7.

No current production tubes, whether it is EL34 or KT88, will be comfy close to 800 volts, some not even 500 - 
but I cannot see why a current production KT88 should do worse than ditto EL34.
I have had failed EH KT88´s replaced, but cannot say if it was bad tubes or just worn out.

I have used the Svetlana C KT88 in two units, they have done fine for about 3 years.
I have a couple of matched Genalex sets which I used a couple of months too (for testing) and they
worked fine too - but there was not much difference.

I have heard some home build audio guys complain about crappy chinese KT88´s, but have never tested them.
I rely on Svetlana C. I think they are closer to their datasheets than other, they might be different to dial in
but last long. They are the Thomastik´s of tubes!

As a footnote, and I´m sure you are aware, both tubes and powersupply (lytics) should be checked if they run properly.
There are many amps thought to eat tubes that really just have lytics (or other components) running too close to maximum voltage, and therefore have unpleasant sounds. But then, the Garnets are high quality designs..

EDIT:
..and of course, it depends on the design of the amp... Your tech might have referred to datasheets of the tube (KT88) you wanted to use. As an example, there might have been screen voltage that is the issue. Older KT88´s have higher
(600v) maximum screen voltage, current KT88`s around 400(?) or so; current EL34 is 500V I think.

[clankenstein]

i would be wondering if the output transformer is up to it.however if it is kt90s will probably do the job.they are  kt88s with a larger glass envelope and greater plate dissapation.electro harmonix do them. you might want to check max volts though.stuff i googled suggested max is 600v triode and 750v pentode.

[Highlander]

Don't mess with something you like the sound of; just get more of it...

[FrankieTbird]

Your guy must have been confusing KT88's with KT66.  KT-88's are much more robust than EL34's.  Go ahead and try them.  If you're looking for more headroom, I think you'll like the results.  6550's would be fine to use also.

[Granny Gremlin]

The output transformer is a huge and overspec'd Hammond (like amptech said, Garnets are solid) - not a problem. I will have to change the tube retainers out though (bear traps) - that's no big deal. 

I was gonna use JJs; I find them consistently reliable and good-sounding.

Quite sure me tech said plate V not screen (he said 520V wouldn't be an issue there - the amp has them both at 520). 

The tech wasn't referring to any data sheets or schems for the tubes or amp just talking generally. I showed him the schem for the power (amp and supply) sections of the amp after he told me that over 500V on the plate would be an issue, but a lot of other people that go to him use lower quality tubes so he may be going off that experience.

I've had the thing gone over so I know the caps are fine (no tonal issues - sounds great at jam and in the studio, just live I have to push it too hard sometimes, still sounds good, but it's further into the dirt zone than I would like) - like I said the pair of 6CA7s are ancient (can't remember if I have ever changed them, I doubt it because I probably would have used EL34s as 6CA7s, until recently, were harder to find, unless you went NOS).  I did have the phase splitter/driver tube go recently - sorted that one out myself (amp sounded  fine but 1/4 the volume; all 4 preamp channels affected equally so I though power tubes initially because they're so old but that wasn't the case - sounds right now.... though I am preparing for those power tubes to die any time now due to age, which is why this whole KT88 thing came to the fore).  I know that that (driver tube going) can be a sign of other issues, but the thing has been piping along happily for 2 months since so I am not concerned.  Funny thing is that nearly the same thing happened to my other (smaller - 6L6) Garnet around the same time.  They are around the same vintage, but I suspect the smaller one has been used more heavily (had to change the power tubes a few years ago).

...anyway, looking at the spec sheet seems like a good idea, but am I reading this right - the JJ KT88 is supposed to handle up to 800V on the plate and 600V screen? 

http://free-hosting.infodiv.com/audiotube/power_tetrode/power_tetrode0034/power_tetrode0034-01.html

Sovtek says 800V max too....

Looking at the JJ 6CA7 datasheet I see the same limiting values (and therefore no reason why KT88s wouldn't be OK in place of my 6CA7s, but I ain't no expert).

http://www.jjtubes.eu/image/data/powertubes/datasheet/6CA7-data-sheet.jpg

Don't mess with something you like the sound of; just get more of it...

There is no more to be had; that's my problem.

Your guy must have been confusing KT88's with KT66.  KT-88's are much more robust than EL34's.  Go ahead and try them.  If you're looking for more headroom, I think you'll like the results.

I told him the reason was lack of headroom so I doubt he would have confused KT88s with 66s but it's possible, old hippie and all that. ... actually looking at general specs online that does seem likely (max plate V for KT66 listed at 550V).  I'll double-check that with him.  Thanks!

[amptech]

The amp has old 6CA7s in it now.

Just an idea, I´d try a fresh set of EL34´s and see if that does the trick. Tubes do wear out, you know

And for headroom (You can of course try the KT88´s, I´m just thinking other possibilities) you might ask your tech
about installing a less aggressive driver tube. He´d have to check the PI/driver arrangement in this particular amp,
but sometimes a low gain tube might tame the amp. Still, a pair of EL34´s will never give you lots of clean headroom at practice levels with a band - at least not in a typical 50w design. But nor will a pair of KT88´s in the same circuit!

On the other hand, take a circuit designed for two KT88´s delivering 60-80 watts a pair and
dial in two good EL34´s - and you might end up with something else!
A customer came to me with 3 of these dynaco MK3 60w mono amps once. He gave me two of them to restore and use in my studio, if i built a bass preamp into the 3rd unit - he wanted a practice amp to go with a home made, very effective 4x12 cab. I designed (put together, that is) a gain stage with an ECC82 tube, feeding the 6AN8 and driving two EL34´s in the output stage (instead of the KT88´s) The Dyna´s are designed for clean, flat punch, and the dirt only arrives at full volume. I recently got a call from an engineer who did the sound at a show he played, ordering a similar build as soon as he found a Dyna chassis! It´s a very simple design, but works fantastic for bass. You need an ultra linear tapped tranny, though.

I think i missed my point here, but in my case here, the same circuit provided much the same characteristics for both a pair of EL34´s and a pair of KT88´s. I merely suggest that taming the driver/preamp might do the trick

[Happy Face]

Now this is all second hand and prolly useless.

I just sold my pride and joy hand wired tube amp. For some good or not so good reasons.

When it arrived, the buyer (who paid a lot) was miffed that one of the four KT88s was clearly on its last legs. I wasn't aware of that so I sent him my (gently used, or so I was told) backup set, which I'd got in a trade for a couple of used GE 6550s. He said that one of that set was pretty microphonic.

The only reason I bring this up is that the tubes in question were Winged Cs. And the buyer's tech commented that, in his experience,  Svetlana Winged Cs are prone to going microphonic.

I have no opinion on the mater, but I thought it was interesting since I paid up for those tubes both in cash and trade. 

[amptech]

I have never had that kind of troubles with WC tubes. I have installed them on every amp I have serviced since 2004, unless the customer asks for a specific brand or WC does not make that tube. I take it you mean loose electrodes when you say microphonic output tubes?

I have had a couple of tubes with unstable electrodes from Svetlana branded Reflector tubes, but that´s something else.
Those tubes were used, so I cannot judge them because of that - no tube lasts forever. Just because I don´t use/rely on the many relector brands, doesn´t mean they don´t make tubes that sounds good or lasts long.

[Granny Gremlin]

Just an idea, I´d try a fresh set of EL34´s and see if that does the trick. Tubes do wear out, you know

Yeah - I have been waiting for these to die.  I know what a set of power tubes on the way out sound like; they might not be fresh but they have plenty of life in them still.  All I have laying about is a likewise used pair of JJ E34Ls, so not sure that will tell me anything useful to swap out (and of coarse, it would need to be rebiased; I am not up to that).  I got them in a large lot trade; figured it would be good to have some emergency spares.  I could pop the 6CA7s out and try them on the tube tester at this local shop I like (tested the E34Ls soon after getting them - definately well used, but still OK).

And for headroom (You can of course try the KT88´s, I´m just thinking other possibilities) you might ask your tech
about installing a less aggressive driver tube. He´d have to check the PI/driver arrangement in this particular amp,
but sometimes a low gain tube might tame the amp. Still, a pair of EL34´s will never give you lots of clean headroom at practice levels with a band - at least not in a typical 50w design. But nor will a pair of KT88´s in the same circuit!

My amp is actually a PA/Vocal head (4 Hi Z inputs and a mixer section) so it is pretty clean (and can be gain staged nicely due to individual pre channel volumes plus master).  I am only slightly hairy at recording/practice volume (i.e. when I want to be) - I have never cranked the thing all the way (except when I had that driver tube die) but live I do have to push it a little further into dirty territory than I prefer.  Also, wouldn't the KT88s be giving just less than double the power if biased properly in the same circuit?  That alone should go a long way to solving my problem.

I merely suggest that taming the driver/preamp might do the trick

I do have a 12AT7 or 2 laying about (I think the stock driver is a 12AX7), I could try that, but I think I did that before and it wasn't enough gain.... but I may be confusing things with the reverb driver (my other Garnet has a sweeter/lusher reverb, despite a smaller tank so I think I played around with that).

[amptech]

  Also, wouldn't the KT88s be giving just less than double the power if biased properly in the same circuit?  That alone should go a long way to solving my problem.

I can´t see that happening - but I´m not familiar with the circuit. I don´t know how much power it delivers. Yes, a 2xKT88 design can deliver twice the power (or just less, as you say) but a ´clean´ EL34 might be designed for an output of 25, 35, 45 w (just an example) and you can have a power/output tranny designed to go with that power.
Usually, you can drop EL34 in a KT88 cct, not vice versa because you will not have enough negative bias available and not enough iron to handle the power.

Ask if he´s got a test set (used) KT88´s and try! Put in a 1R1%1W resistor between cathode/earth and see how it works - but don´t be amazed if each tube runs at 100-150mA, you then need enough negative bias to tame them.

That dynaco MK3 example I used, I have the notes somewhere, but I think it put out 40W with EL34´s and 50W with KT88´s, at a point wich there was just a tad clipping. I think they are designed for 50 clean watts with KT88´s at 480V anode voltage. Different circuit designs = different results, but do not expect twice the power. 

[Granny Gremlin]

I see; thanks!

[Psycho Bass Guy]

I've been very busy, but the issue in this case is not tube performance, but rather the ability of the power transformer to supply the necessary current to the tubes' heater elements. KT88/6550's require 1.6 amps of heater current and EL34/6CA7's require 1.5. A tenth of an ampere multiplied by two is a close to a quarter of an amp, a pretty substantial difference. Depending on the quality of the power transformer, this could be no big deal or it could cause catastrophic failure. The unscientific method to check is to perform the swap and feel the power transformer. It if has a noticeable difference in temperature at all, if it gets warm, it's probably a bad idea.

BTW, in regards to KT88/6550 voltage ratings, be sure and look into the specs for the 6550A, which is what all modern "6550's" are based on. And should you happen to run across true vintage Tungsol coke-bottle 6550's, they'll handle much more than their ratings. The only relatively fragile member of the 6550 family is the original RCA small bottle 6550, which is where all the low ratings for vintage 6550's come from because RCA wrote the tube manuals. 800 vp(late) and 600vs(creen grid) is the rating for both the KT88 and 6550A.

I am using Sovtek branded KT88's, which are now sold as "Tungsol" 6550's in my Trace Elliot VR400 at 728 vp and vs (ultralinear OT) and have gigged the snot out of them playing extremely loud with no problems and I had an RCA small bottle 6550 in an old SVT I bought that was working just fine at 702 vp and 487 vs. It was probably the last of the original sextet as this was a Linden, NJ-made SVT and the entire output section was hodgepodge of different brands. Since Linden was so close to the RCA factory in Red Bank, NJ, they used RCA tubes.  When production shifted to Magnavox in Jefferson City, TN in 1972, Magnavox installed Tungsol 6550's in them until the GE 6550A became available.