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Old 10th September 2012, 04:58 PM   #1
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Default KT88 & 6550 Pentode power output?

I have another thread going about converting an old Bogen amp to Higher Fi

Anyway, I tested the output iron to be around 4.5K A-A.

Looking at the datasheets, minus the benefits of using 8 X EL34s lower distortion, etc., the KT88 and 6550 do not look too bad in pentode with 300v on the screens and 600 on the plates. They claim around 100W at 3%. Both look to be a fairly ideal match for this output iron primary impedance as well.

Has anyone ever driven them at 6-650V B+ with regulated screens to get this kind of power? I usually see 450-500V for 60-70W AB1 UL. Of course this is UL.

Thanks!

Blair
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Old 10th September 2012, 10:32 PM   #2
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When using sep supply for the screens, I design so the screen volts ramps up after the anode. If not, quickest way to ruin and strain good tubes.

I remain sceptical regarding longevity, with todays 6550 versions for anything over 500V B+ whatever the configuration.

(If someone is remaking the orig RCA "A" spec'd version then I'm interested). This 6550A version was a hound in front of the others.
I've alot of the Tung-sol new edit 6550 version which are going soggy after 2000hrs in modest UL conditions. I see nothing other than good reputable maker KT88's or EH KT90's which I am currently using.
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Old 10th September 2012, 10:40 PM   #3
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Hi Rich,

So, the new production KT88s will handle the voltage? You mentioned EH. Is that your preferred "flavor"?

How do you slow the screen ramp? Delay circuit?

Thanks!

Blair
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Old 11th September 2012, 12:15 AM   #4
EssB is offline EssB  United Kingdom
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The Ampeg SVT used 3 pairs of 6550 in pentode with B+ about 660V and 300-350V screens to get 300W, similar conditions to those you are proposing so you could trawl the forums to find out what tubes will survive
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Old 11th September 2012, 06:36 AM   #5
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Quote:
Originally Posted by deicide67 View Post
Hi Rich,

So, the new production KT88s will handle the voltage? You mentioned EH. Is that your preferred "flavor"?

How do you slow the screen ramp? Delay circuit?

Thanks!

Blair
On doesn't need much of a delay; or can be same time as rising anode volts ...Remember in Tetrode running, the o/p stage thd is influenced by the quality of the screen supply, and for cheapness a small current limiting resistor is often fitted to each screen to limit dissipation when the anode swings lower than the screen volts on signal. Beware of continuous ratings.
Many earlier amp models omitted it altogether and on a hard thrash it doesn't take much to get glowing screens. The only very brief mention of this is in Morgan Jones Valve amp 4th edit (earlier version) p.510.

Over the decades, Power supply performances have improved, and my preferred tack for tetrode screens is a low impedance current limited SS regulator with a specifically set current limit. Burying one's head in the sand, yes.

I find the EH robust; the straight envelopes often mistaken for disguised
EL34's. Expect 20% more output power when subs for 88's and the heater current is up a notch too.

richy

Last edited by richwalters; 11th September 2012 at 06:37 AM. Reason: error
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Old 11th September 2012, 11:41 AM   #6
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Thank you Richie!

What about a simple CL-60 on the mains line of the screen transformer? It will slow it down and "ramp" it up to voltage.

Do I need a current limiting device on the screens? I've seen a few schematics where the screen supply is just a beefy PS separate from the B+ supply. Isn't the idea to make it super stiff?

I was planning on using an old Triad that is 410V into a FWB into a beefy CLCRC to drive my screens and driver stages.

Using two tubes vs. four (EL34s) will help me keep things simpler, and the B+ current needed will be lower allowing me to use a single choke per channel.

I will try to draw up the output stage for a look over if you do not mind.

Thank you!

Blair
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Old 11th September 2012, 01:36 PM   #7
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Something like this? Sorry, I do not know the symbol for a CL

Click the image to open in full size.
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Old 3rd October 2012, 12:20 PM   #8
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I am almost finished with my chassis, so I will start gathering my PS parts. Will the proposed PS schematic above work to delay the screens for a few seconds while the rest comes up?

With 650V on the plates into a 2.25K load using KT88s, what screen voltage should I aim for?

Thanks,

Blair
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Old 3rd October 2012, 08:27 PM   #9
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when using kt-88's, look into the Ip 's of the different brands.....I wouldn't assume that the power tranny can support 8 hi Ip kt-88's. 8417's have pretty low Ip's and are closer to el-34's and low Ip russian 6550a's . On the KT-88 side, sevelanta's are low Ip. Asian kt-88's tend to be hi Ip.
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Old 3rd October 2012, 09:04 PM   #10
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Thanks!

This helps. I'm only using 4 KT88s. The power tranny should be able to handle that right?

Blair
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