Big Bottles With 600V on the screens

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JD0x0
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Big Bottles With 600V on the screens

Post by JD0x0 »

Anyone have some good recommendations for big bottle 6550/KT88/KT120 tubes that can handle ~600V on the screens. I recently got an old AIMS amp that was heavily modified. The PO was running KT88's (I'm told you shouldn't run screens that high on them, while other data sheets show 600V as the max (that's still high to be sitting at max, IMO) and about 5 minutes (if that) into testing it one of the Gold Lion KT88's went into melt down. The info sheets sent with the amp say it's running about 600V on the screen (with a screen resistors)
I always have the option of dropping the screen voltage a bit, but I'd like to see what I have as far as tube options @ 600V on the screens, as that was apparently the original design on the amp. They were trying to pull 120W from a duo of 6550's, which I believe are OK with 600V on the screens, if they're good old Military grade tubes.
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tubeswell
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Re: Big Bottles With 600V on the screens

Post by tubeswell »

6550s and KT88 never do well with 600V on the screens IMO, especially modern production ones. If you've got 600V on the plates, you want the screens to be between 300 to 400, but definitely under 400. And you also probably want a 12AU7 buffer/driver stage to ensure sufficient current for driving the grids (like an Ampeg SVT).
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JD0x0
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Re: Big Bottles With 600V on the screens

Post by JD0x0 »

tubeswell wrote: Sat Dec 02, 2017 3:03 am 6550s and KT88 never do well with 600V on the screens IMO, especially modern production ones. If you've got 600V on the plates, you want the screens to be between 300 to 400, but definitely under 400. And you also probably want a 12AU7 buffer/driver stage to ensure sufficient current for driving the grids (like an Ampeg SVT).
Yup, it already has the 12AU7 driver stage. The amp was originally built when military tubes were still popular and in production and likely why it was running at such specs. (Remember Leo ran many 6V6's past their spec, as well, and some of those tubes are probably still running in their original amps.)

http://www.audiomatica.com/tubes/6550.htm
Screen Voltage (max)
600
V
http://www.audiomatica.com/tubes/kt88.htm
Screen Voltage (max)
600
V
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Kagliostro
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Re: Big Bottles With 600V on the screens

Post by Kagliostro »

EL34 is declared to 800v on Plate and 500v on G2 BUT ..... as far I can know those indications are reliable only on OLD tubes

new production is prone to melt with that voltages

http://www.mif.pg.gda.pl/homepages/fran ... e/EL34.pdf

I suppose the same thing for KT88 & 6550

K
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roberto
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Re: Big Bottles With 600V on the screens

Post by roberto »

You have to search the datasheet of the tube you are using.
You cannot repair a Chevrolet Corvette C7 with the manual of a C1.

This is one of the latests, look how much lower the voltages are:
https://newsensor.com/pdf/sovtek/6550we-sovtek.pdf
pdf64
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Re: Big Bottles With 600V on the screens

Post by pdf64 »

Wow, that seems past the point at which describing them as 6550 becomes mis-selling, as they're unsuitable for any form of grid current operation, eg AB2, due to the limit on positive G1 voltage being 0.
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Re: Big Bottles With 600V on the screens

Post by Stevem »

Sounds like your only hope is KT120 tubes, as I think the amp may have been made for 8514 tubes which your just not gonna find any more.
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didit
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Re: Big Bottles With 600V on the screens

Post by didit »

Stevem wrote: Sat Dec 02, 2017 12:17 pm Sounds like your only hope is KT120 tubes, as I think the amp may have been made for 8514 tubes which your just not gonna find any more.
Hello Steve --

Did you mean 8417 - https://frank.pocnet.net/sheets/168/8/8417.pdf?

Best .. Ian
Last edited by didit on Sat Dec 02, 2017 8:33 pm, edited 1 time in total.
JD0x0
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Re: Big Bottles With 600V on the screens

Post by JD0x0 »

Stevem wrote: Sat Dec 02, 2017 12:17 pm Sounds like your only hope is KT120 tubes, as I think the amp may have been made for 8514 tubes which your just not gonna find any more.
The 120's were what I was looking at as well. If I'm not mistaken, though, the amp was originally designed and running 6550's.

This is what I pulled up on the 8417's
8417 Beam Power Tube
(Vfil=6.3V @ 1.6A, Power output=35 watts)
(Vplate max=660V, Vscreen max=500VDC)
sounds like that's too low for the Screen V. The old datasheets for the 6550's claim the screen max is 600V
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pdf64
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Re: Big Bottles With 600V on the screens

Post by pdf64 »

Are the power tubes in a UL arrangement?
That's easier on the power tube screens, though your experience with the GLs may indicate that it's still not easy enough for at least current production.
I think that Ruby have a KT88 that's supposed to cope with high voltages.
But JJ KT88 have worked out well for me, better for me than KT120 which had a short life, in a nearly 600V HT old UL PA amp that I re-purposed for bass.
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didit
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Re: Big Bottles With 600V on the screens

Post by didit »

EH KT90 seems closest in-production option. Based on data, won't bias or have the sensitivity of the 8417. Empirical experience would be cool.

Best .. Ian

http://www.thetubestore.com/lib/thetube ... kt90eh.pdf
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JD0x0
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Re: Big Bottles With 600V on the screens

Post by JD0x0 »

Looks to be an underlying issue as well. I was able to get sound out of the amp again, but the rightmost KT88-98 will begin redplating within seconds, even with the Bias pot turned down and the signal sound weak and dirty.. Seems like a separate issue and likely why the previous tubes blew. The KT88-98 spec sheet says 600V max screen, so shouldn't light up that quickly.
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pdf64
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Re: Big Bottles With 600V on the screens

Post by pdf64 »

Sounds like you're not using a light bulb limiter.
They are invaluable with this type of issue.
A tube that's redplated should be thought of as being damaged / unfit for service (especially in such extreme conditions), unless proven otherwise.
As you may appreciate, a light bulb limiter can save a lot of time and expense
stephenl
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Re: Big Bottles With 600V on the screens

Post by stephenl »

Just a thought...

You could change the rectifier to a FWB and then use the CT for a 300v screen supply instead of grounding it. Or does this model use a voltage doubler? In that case you already have a 300v supply as well.
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Phil_S
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Re: Big Bottles With 600V on the screens

Post by Phil_S »

I'm having a bit of difficulty with what I think is the premise of the question: what tube and apply 600V to the screens? The responses all seem to indicate pretty clearly that 600V on the screens likely isn't a condition that will work or if it does, it will be difficult to make it work. My amateur reaction is, if it was mine, I'd do something about knocking down the screen voltage to 300V. This will allow you to choose your tube from among the various possibilities, and you won't be frying any transformers. I imagine there is more than one way to alter the screen voltage by conventional and tested ways.

You say the amp is heavily modded. So, what's one more mod, especially one that makes the amp operate safely?
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