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Old 20th December 2011, 04:24 AM   #1
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Default EL84 SPP Edcor OPT choices

I am in the planning stages of an EL84 SPP build and need some advice.
I like Edcor iron. It has served me well on SSE and TSE builds. The XPWR008-120 seems to be the most popular choice for power.

The question is what to use for OPTs.
I have three on my list-
CXPP25-MS-7.6K
CXPP25-MS-8K/23%
CXPP50-MS-7.6K
Any others from Edcor to consider?

Is the 50 watt too big? George indicates that bigger iron is better up to a point. What is that point? I would appreciate any feedback as to the merits and/or drawbacks of these choices and any other important considerations.
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Old 20th December 2011, 04:47 PM   #2
akimmet is offline akimmet  United States
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I used personally used CXPP25-MS-7.6K, but those run your output tubes harder than an 8k primary.
That decreases your choices to only JJ for new production tubes. I haven't tried the Chinese offerings, but the other Electro Harmonix and most of the other brands owned by NEW SENSOR will melt down relativity quickly. If you run 8k primaries and use a 285-0-285 transformer, then almost any output tube will last a long time.

25W is already overkill, 50W will only soak up more power.
Here are some other good choices.
CXPP25-MS-8K (40% UL taps)
GXPP15-8-8K (the bare minimum in power handling no multi-taps)
The Dynaclone transformers have a great reputation as well, but are $20 more Z565-48 17.5(35W) Watt Triode USA Dyna Clone Output Transformer for Dynaco MADE IN USA
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Old 21st December 2011, 06:13 PM   #3
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Thanks, that's good information. I had planned on starting with the JJ tubes but would not want to be limited to them.

Is it possible to run the EH EL84's with a 5U4/5R4 and lowering C1 value with the 300-0-300 transformer?
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Old 21st December 2011, 06:50 PM   #4
chrisb is offline chrisb  Canada
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Quote:
Originally Posted by surface tension View Post
Thanks, that's good information. I had planned on starting with the JJ tubes but would not want to be limited to them.

Is it possible to run the EH EL84's with a 5U4/5R4 and lowering C1 value with the 300-0-300 transformer?
probably an appropriate caution to keep the input C low on 5R / 5U4 with any tube type, following with choke(s) doesn't hurt either



the JJ's & NOS Sylvania 12AT7 were nice as initial set in my own, but the Psvane 6BQ5 and 12AT7s transformed this "little" amp into something rather special - and before you gag at the prices, you could certainly several times spendy-er with cherry-picked NOS Telefunken, Siemens, Tungsram, Mazda etc


George: did you ever get those FH3 fired up?
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Old 22nd December 2011, 05:34 AM   #5
akimmet is offline akimmet  United States
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You only need to drop plate voltages to 300V. On my amp with 7.6k OPT primaries and a 300-0-300 power transformer(that puts out 320V) I get about 330V across my tubes. Probably just enough in my case to push EH tubes over the edge.

I tried a 5U4 in mine; it works, I just didn't like the voltage sag (6V vs 0.5V).
You can stick with the 300-0-300 transformer. You just may need to increase R1 to 200ohm 15W. Lowering the value of C1 may help drop some voltage as well.
You may not even need to change anything at all. just measure your cathode to plate voltage at idle with the JJ tubes in and see what you get.
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