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Old 29th October 2013, 01:45 PM   #1
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Default Transformers Specs for a 2x KT88 Amp

Hi all,

I have good luck gettitng feedback on my projects in the forum, so I thought I'd throw my next project plan out there and see if you all agree with the direction I'm going.

I want to build a 2XKT88 amp, per the GEC data sheet. They have 2 design scenarios for a 100W tedrode mode and 70W UL mode... both with identical B+ (560vDC) and primary impedance requirements. As expected the screen supply in tetrode mode is about 300vDC. Maybe I'll have switchable modes, I definately want to experiment with both and see which I prefer. I figured I could get some transformers made specifically for the job, Heyboer came back with a resonable quote for the following:

OT:
100W
4.5k ct w/ UL taps at 40%
Anode current = .150A per side.
Secondary - 4/8/16 ohms preferably all usable via 1P3T switch w/ no rewiring

PT:
120vAC primary
395vAC (under .400A load) w/ CT (shooting for 560vDC under load w/ a full wave bridge rectifier)
5A at 6.3vAC heaters, or 3.5A at 6.3vAC and 1A at 12.6vAC


So I wanted to get some review and see if folks with more experience think this is the right direction regarding specs, if anyone knows of any caveats, or would make adjustments or additions given this is a one-off custom. For the record, I'll be deriving the Tetrode mode screen supply from the CT like this:

http://www.ozvalveamps.org/playmaster/pm117cct60w2.gif

Thanks to gingertube for tipping me off to this method in another thread.


Regards,
Brian
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Old 29th October 2013, 04:08 PM   #2
kevinkr is offline kevinkr  United States
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I probably wouldn't go too far over 500V on the plate with modern KT88..
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Old 29th October 2013, 05:42 PM   #3
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OK, that's fair. Running under 500vDC makes the power supply filtering a bit simpler as well... not sure what the tolerances are on the standard JJ can caps for voltage. I guess I should stack them with a voltage divider regardless just to be safe.


How about the arrangement in the link for getting 1/2B+? I can't for the life of me get my head around how it works and I've never seen it anywhere else. I'd like to hear from someone that has seen it and knows it works well (ie: is stable and low impedance).



Brian
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Old 29th October 2013, 11:40 PM   #4
tikiroo is offline tikiroo  Australia
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The arrangement for 1/2 B+ is just like the dual rail power supply for a S/S amp except the ground is in a different place. So instead of getting +/- 190V, you get 190 and 380V.
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Old 30th October 2013, 03:23 AM   #5
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You will need to series connect capacitors to handle that voltage, remember that if the B+ is 560V when running then it is likely to be around 660V (100V higher) at switch on before the tubes warm up and start drawing current. I'd be looking at series connectiong 2 off 350 or 400V rated caps. The standard JJ Caps are rated at 500V.

If you use that power supply arrangement then put a HV fuse (say 500mA) where it shows the standby switch.

Just finished rebuilding the power supplies of a 1965 Goldentone 60W Bassmaster which uses the same output tubes and transformers as the circuit you linked. I used a full bridge rectifier on the bias supply and suggest you do the same.

Cheers,
Ian

Last edited by gingertube; 30th October 2013 at 03:29 AM.
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Old 31st October 2013, 04:52 PM   #6
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Gingertube,



Noted! JJ makes some 220uF 385vDC caps that I can put in series straight off of the supply. I'll use a third to filter the screen supply.

It looks like I'll need to add a Bias voltage winding as well due to the B+ arrangement correct? No way to tap off like in a marshall, right?

What is the benefit of doing full wave vs. half wave in the bias supply? Is it related to the output tubes? I'm going to use KT88s not 6DQ6 (never even heard of them before this...)


Thanks,
Brian
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Old 31st October 2013, 04:55 PM   #7
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tikiroo,


Thanks for puttng it that way, makes some sense now how the voltage 'shifts' more positive. What I still don't get is how you have access to the 1/2B+ DC rail at the CT. Is the diode bridge forcing the entire transformer winding to float around 1/2B+?



Thanks,
Brian
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Old 3rd November 2013, 08:51 PM   #8
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Brian,
With a bridge across the full secondary like the posted example you will see that one end of the full winding is always clamped to 0V via a diode (the bridge negative to 0V link). The end changes with the half cycle of the AC signal. The other end of the winding is at full positive DC voltage and the CT will be at half that full winding voltage and always positive.

Cheers,
Ian
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Old 4th November 2013, 05:50 PM   #9
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Quote:
Originally Posted by kevinkr View Post
I probably wouldn't go too far over 500V on the plate with modern KT88..
Decades ago having built several KT88 amps around 50-100W+; I found the 50W versions with a slightly lower B+ sounded nearly as loud as the 100W which at 500V is an awkward voltage. So I specifically made amps around the 460V B+; the single 500V electrolytic is an advantage, against 2 staggered in series and the o/p tranny with a slightly lower A-A Z of 4Kohms; I ended up close to the GEC 65-70W version.
The attractiveness of 460V B+ is a smaller power transformer; and brings in the 6550 group, so one can compare tubes.
Only a hint; amp builders like myself and other users, often find out the hard way when todays tubes don't seem to last long when run with dissipations higher than 2/3 of specified plate ratings.
At present, I am putting the E Harmonix KT90 though it's paces, which I find is quite stable (currently 2000 hrs) being run at 600V at 80mA quies which is 48W plus a bit for the screen.

Those of us who remember the original KT88; this OEM tube in it's 1950's hayday could take 600V at 50mA easily; that's 30+ Watts sitting for 10,000hrs before replacement.

rich
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Old 11th November 2013, 08:02 AM   #10
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Rich, can you , or anybody else recommend operating points for current Tung-Sol 6550's?
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