TT21 Vs KT88

[GK]

Started sorting through my valve collection on the weekend. Found a dozen or so untouched NOS TT21's and couple of KT88's.
What's the difference between the two? The specs seem to be pretty much identical but the internal construction of the TT21 looks a bit more elaborate.

Also found 60+ untouched NOS 12E1's so far. Anyone here ever done a big parallel push-pull with a heap of these before?

[Shoog]

Internally the TT21 and the KT88 should be identical. The TT21 is designed for higher voltage transmitter duty where the duty cycle is likely to be shorter. I wouldn't use them any differently to KT88's in audio otherwise you are likely to end up frying them.

They fetch very good money on Ebay because they are extremely rare. I would advise selling them (if they can be tested) and buying some more KT88's.

Shoog

[GK]

Originally posted by Shoog
Internally the TT21 and the KT88 should be identical. The TT21 is designed for higher voltage transmitter duty where the duty cycle is likely to be shorter. I wouldn't use them any differently to KT88's in audio otherwise you are likely to end up frying them.

They fetch very good money on Ebay because they are extremely rare. I would advise selling them (if they can be tested) and buying some more KT88's.

Shoog

OK, thanks. The difference in internal construction is probably mostly due to the fact that the TT21's and KT88's are different brands.
The TT21's are still in their original boxes, having never been opened, but I can test them no problem.

I've only sorted half way through my collection so far, so I'll likely find a few more. I'll look up what they fetch on ebay.

Found about a dozen NOS boxed U19's as well. Started drooling, hoping that they were big triodes suitable for an audio amp, but they turned out to be just big boring diodes

Cheers,
Glen

[SHiFTY]

Heh I'm keen, I am building a TT21 amp, but I only have 4, always keen for spares! I can't send you a PM as yours is disabled- but please message me me if interested!

[Tony]

hi,

i am interested in the 12E1's, how much for a dozen? please hit my email button if you please..


thank you,

tony

[GK]

Sorry, 12E1's are not for sale (projects planned) and I haven't decided the fate of the TT21's yet.

[Tony]

that's okey, the TT21 has a g2 rating of 600volts, anode dissipation of 45watts and g1g2 mu of 8....


wow, the 813 has a mu of 8 like wise.....

so at plate of 600volts and plate diss of 45w you can run about 75 mA of triode.....with a grid bias of -48......

[Shoog]

I wouldn't subject the TT21 to this abuse. They are exactly the same as the KT88 but with the top cap to prevent flash over at the base. They are designed for transmitter duty - which is intermittent and therefore the average plate diss is going to be a lot less than 45W.

Infact I can think of absolutely no reason to use them over a KT88 - unless I was building a TRANSMITTER.

Shoog

[Tony]

yes, 37.5watt is the CCS rating for this tube, still substantial....

[kstagger]

supposedly the TT21 was pumped down harder than the KT88, but I haven't found any verification of that. The higher B+ rating just comes from the plate cap connection - no chance of 'arc-over' with the octal socket.

I found this information in the past while googling for another rare GEC tube:

"Incidentally, my colleague Mike Forsyth and I "invented" the TT21 series transmitting valves which are based on the KT88 audio beam tetrode. We were ardent amateur radio geeks in those days, and the 807 was an icon, but low-ish in power. 150w was the governments limit. The 813 was far too big. The GEC equivalent 807 was the KT8C, a puny 1930's job. We realised the KT88's internal capacitances were quite low, and we persuaded the girls on the production line for the U19 rectifier (a top-cap anode job with a similar glass envelope) to put through 6 KT88s with a top-cap anode wire. It only "cost" us two nights each at Hammersmith Odeon, and as desirable 18yr. olds, was rather well worth it!). We approached the company station G3--- guys with these 6 for advice, and before we knew it, it was up, up and away. Thanks, Pat Cundy, and George Jessop. "

-from http://books.dreambook.com/elecstuff/electric.html