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Old 21st February 2011, 09:10 AM   #1
oddboy is offline oddboy  United States
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Default Simple SE with KT120s

Hi, new guy here. I have ordered a Simple SE board for a future build.
My previous experience has been to build several amps, aikido preamps, and rebuilding Dynaco amps.
Before ordering the PT and the OTs I need some advice concerning getting the max wattage out of a pair of KT120 tubes.

Possible fixes for obtaining a higher B+ for the KT120s:
High voltage motor run caps in the power section
SS rectification
Larger values for R14,R24 to cut the voltage down to the 12at7

Any ideas would be welcome.
Thanks
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Old 21st February 2011, 01:02 PM   #2
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Quote:
Originally Posted by oddboy View Post
Possible fixes for obtaining a higher B+ for the KT120s:
High voltage motor run caps in the power section
Don't forget to watch the voltage rating of the coupling cap (C11/C21). At startup when the tubes are cold and no current is flowing, you might get full B+ voltage across the coupling cap if you are using solid state rectification.

Hopefully someone (George?) can chime in and suggest a maximum reasonable voltage rating for the circuit board itself. There has to be some limit based on the distance between the traces.
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Old 21st February 2011, 04:24 PM   #3
rknize is offline rknize  United States
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I would love to know what a pair of those can really do (and for how long). Also watch the PIV of your rectifier and eventually the max voltage of the CCS will become a problem. I know George has abused the SSE PCB plenty, so I imagine there isn't much to worry about there.

There is a practical limit to that the octal tube base can handle as well. I haven't found the plate curves, but does the SSE driver even have enough swing?
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Old 1st March 2011, 03:52 AM   #4
oddboy is offline oddboy  United States
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I ordered the big Edcor OT's and a PT to supply B+ ~500v 250ma, the first build will have motor run caps,600v coupling caps and a separate filament transformer.

Possible choices for a second high powered version:
ICPX10M90S for a CCS (digikey stocks them).
PT 450-0-450 300ma
Choke 5H To 10H 300ma

The Tim Taylor version is just a thought for now.

Thanks for the ideas.
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Old 1st March 2011, 05:40 AM   #5
tinitus is offline tinitus  Europe
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google Leven KT120

different design, so I dont know if this will be of any use
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Old 1st March 2011, 05:48 AM   #6
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One of the big criticisms I've heard of the 120 is it's need for a small grid leak, make sure you read up on the (horrible) KT-120 datasheet to find it.
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Old 13th March 2011, 03:06 PM   #7
w5jag is offline w5jag  United States
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Quote:
.... and suggest a maximum reasonable voltage rating for the circuit board itself. There has to be some limit based on the distance between the traces.
I've ran mine in the middle 500's with generic ceramic chinese sockets without any issue.

Quote:
... Possible fixes for obtaining a higher B+ for the KT120s:
How high do you want to go?

The only data sheet I have seen claims it can take 650 vdc on the plate and screens in triode mode, 850 on the plate as a pentode (beam tetrode). That sounds optimistic to me, but I've ran that new Tung Sol "6L6 STR" in the 500's, so maybe it's true.

If you go there, choose your sockets carefully, because arcs across pins are possible at that voltage. Once an arc path starts, it's hard to undo and typically parts need to be replaced. Higher voltages can also arc out at you, so even more care than normal is prudent.

With enough power supply, I'll bet 20 - 25 watts single ended pentode or ultra linear is possible with these. I'd like to play with some and may pick up a pair at some point. I'm sure a lot of people will be interested in how your amp turns out.

Win W5JAG
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Old 13th March 2011, 05:09 PM   #8
w5jag is offline w5jag  United States
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Forgot to mention, as you push your B+ beyond the published values for the Simple SE, the power tube cathode voltages will go up fast.

Seems like when I was running mine in the mid 500's, I was seeing cathode voltages approaching 60. You will need a higher max voltage on the cathode bypass caps; 100 volts might be OK, 160 volts would be better.

Even if they don't EXPLODE , a venting electrolytic cap stinks like crazy and makes a pretty big oily mess on everything around it.

Win W5JAG
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Old 13th March 2011, 08:34 PM   #9
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Quote:
Hopefully someone (George?) can chime in and suggest a maximum reasonable voltage rating for the circuit board itself. There has to be some limit based on the distance between the traces.
I have pushed my SSE board to about 540 volts using EH KT88's. I didn't leave it there for long. I still had 500 volt Panasonic electrolytics in it and I know what happens when they blow. Some possible isues:

The green screw connectors are typically conservatively rated at 250 VAC between adjacent terminals, although I have seen some with higher ratings. This is the reason for the unconnected center pin on the HV winding of the power transformer.

I doubt that any current production 5AR4 would survivs startup with this much voltage. Most of the fast SS rectifiers we use are 1200 PIV which is marginal. 1600 volts is needed here.

The octal output tubes have an inherent weakness. Pin 3 plate is next to pin 2 heater. Under high voltage and mismatched load or overdrive conditions an arc can jump from pin 3 to pin 2. This can blow stuff up. The white ceramic Chinese sockets seem to be OK. Stay away from anything plastic or phenolic.

The CCS is probably not an issue. They are rated for 450 volts and I have gone to 500 on several occasions. The 12AT7 will eat 100 to 150 volts and the 10 K resistor eats another 100 volts so the CCS only sees 250 or so.

THe cathode bypass caps and the cathode resistors on the output tubes will lilely need upgrading. Attempting to run 6DQ6's in my board left me with capacitor goo all over the place. The cathode voltage was around 60 volts which was causing the cathode resistors to dissipate about 7 watts. One of them died letting the voltage rise until the cap exploded. 10 watt resistors and 100 volt caps are probably needed.

The OPT's themselves will see peak voltages of twice the B+. Make sure that they can handle it. It's one thing to fry something cheap, but buying a new OPT and setting it on fire doesn't make for a good DIY experience.

I don't know what your experience level is, but I would get the board working at a reasonable voltage level using a common audio tube first. Then install the KT120's, run them for a while, and if they prove stable, crank up the voltage. I have been doing this for a long time and thats what I would do. It is wise to experiment with only one variable at a time. New tube, new board, extreme voltage level, all at the same time, not a good idea. If it blows up, what went wrong?
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Old 14th March 2011, 04:46 AM   #10
oddboy is offline oddboy  United States
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Thanks for the replys. I am building a normal Simple SE per the manual. As George stated I will build the amp stock and then push the limits one step at a time.
Gary
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