Are KT88's Interchangeable with EL34's

[hdiddy]

Are these the big bottle 34's and can they be used interchangeably with a bias adjustment and no resistor change?
Thaks
H

[2 Loud 4 You]

I know KT-88's can interchange with 6550's but never head of them switching with EL-34's. My UL runs KT-88's and VHT makes no mention of EL-34's as an alternate tube. As a matter of fact they make essentially the same amp as an EL-34 version in case people preferred that sound. Hopefully someone with more tube knowledge can shed more light on the subject.

[Madsman]

It depends entirely on the amp, but generally speaking, no.

[Brian Wallace]

Yes, you can run KT88's in place of EL34's. This is what Marshall did with the Park's. Same transformers, just stuff KT88's in and bias. :RoCkIn

[Madsman]

I was under the impression that it required a resistor swap too. Also, wouldn't it depend on how the amp is typically biased? I think the original question is one of "can I put KT88s in my amp, and just re-bias it and go, or is there other work to do?" I will defer to you on this... but I was under the impression that KT88s could get ya in some trouble if you don't do everything the amp needs in the swap.

[Brian Wallace]

Here is a picture of a park with stock 50W transformers using KT88's. It appears that the 220K bias feed resistor has been dropped to 100K and nothing else has been altered.

http://marstran.com/69%20362.htm

[noctilux1]

Brian, do you have a preferance of KT88's for an old Park 50 watt PA head? Other than GEC.

also: love the look of that Abaddon head

[bluesaxe]

KT88's are not big bottled EL34's, they are a different type of tube (tetrode) than EL34 (6CA7) (pentode). However they share the same pinout and can be used in EL34's applications if the rest of the circuit is designed to accomodate them.

I've designed a HiFi amp that uses both interchangeably. This amp is cathode biased, and doesn't even require a cathode resistor change to swap the tubes. However, It is important that the bias feed resistors be dropped to 100k if you want to run the KT88's close to their design ratings when using fixed bias. KT88's (and 6550's) can't deal with the high grid resistance from the 220k resistor typically found in an EL34 bias circuit and the tubes could be destroyed.

The other important thing to remember is that a properly biased KT88 will need signicantly more bias current than an EL34, due to the higher (42 watts) dissipation rating. EL34's typically are speced at 25 watts. This means that your output transformer power transformer and bias circuit need to be up to the task. You can, if you wish, run the KT88's colder at EL34 dissipation ratings, but they most likely won't sound their best.

KT88's will draw slightly more heater current (1.6A vs 1.5A), but this shouldn't be a problem for most amps.

For more info, here's a link to the Genelex KT88 data sheets

http://www.tubezone.net/pdf/kt88new.pdf

[vibroverbus]

The other important thing to remember is that a properly biased KT88 will need signicantly more bias current than an EL34, due to the higher (42 watts) dissipation rating.

I know 'bias current' is widely used but I think it can (or at least should) be confusing to newbies, at least those trying to think in proper terms - what you/we really mean of course is "KT88 will draw significantly more idle current". Strictly speaking 'bias current' should describe what current is drawn from the bias supply circuit, and obviously should be negligible most of the time.

[Brian Wallace]

The other important thing to remember is that a properly biased KT88 will need signicantly more bias current than an EL34, due to the higher (42 watts) dissipation rating. EL34's typically are speced at 25 watts. This means that your output transformer power transformer and bias circuit need to be up to the task.

This is why you only see the KT88 used with the 50W transformers. The 1202-118 has a higher current capability than the JTM45 1202-55 PT, the 1202-55 would be stressed with KT88's in it. This allows the KT88 to be biased in it's proper range of operation within this amp. Another PT used with the KT88 was the 1202-133.

noctilux1,

I don't have a favorite KT88 as I don't use them in my builds. Thanks for the compliment on the Abaddon amp. That's a lace wood front that is stained purple. :AOK

[bluesaxe]

I know 'bias current' is widely used but I think it can (or at least should) be confusing to newbies, at least those trying to think in proper terms - what you/we really mean of course is "KT88 will draw significantly more idle current". Strictly speaking 'bias current' should describe what current is drawn from the bias supply circuit, and obviously should be negligible most of the time.

You are correct - "idle current" is a better term. Idle current is controlled by adjusting the bias voltage on a fixed bias amp. The potentially different bias voltage needed to adjust the idle current of a KT88 tube used in an EL34 amp may require modification of the bias supply to get the KT88 into the correct range.

[Joe F]

hey hdiddy - basically, no.

What amp are you talking about?

Also, if it's more headroom you want, try JJ E34L's or JJ KT77's first (you'll need to rebias) before going KT88.

[Roe]

a nice new kt88 is the genalex "RI"

[Swarty]

As far as specs go, KT88s, 6550s, 7027As, EL37s and KT66s are much closer to a 6L6GC than an EL34.

[RussB]

let the old threads die

[zombiwoof]

hey hdiddy - basically, no.

What amp are you talking about?

Also, if it's more headroom you want, try JJ E34L's or JJ KT77's first (you'll need to rebias) before going KT88.

Or 6CA7!

Al