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Edcor PP 60w OPT
Edcor PP 60w OPT
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Old 23rd February 2018, 11:10 PM   #11
Gungnir is offline Gungnir  United States
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How do they sound?
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Old 23rd February 2018, 11:42 PM   #12
Tubelab_com is offline Tubelab_com  United States
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10K would be 2.5K per tube if you ran them class A. That's about 20 watts from a pair of 300B's. Maybe a bit more if you ran A2.
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Old 23rd February 2018, 11:53 PM   #13
Gungnir is offline Gungnir  United States
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That would be plenty of power. I have never heard any pp triode amps before, but seems like it would be the best of both worlds (SE and PP).
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Old 24th February 2018, 02:35 AM   #14
danFrank is offline danFrank  United States
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Edcor PP 60w OPT
807s would work well with your 10K 60 watt output transformers. Look at page 3:

http://www.mif.pg.gda.pl/homepages/f.../084/8/807.pdf

Use a 220vac secondary transformer as a voltage doubler configuration for the 300 and 600 volts B+
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Old 24th February 2018, 02:45 AM   #15
Gungnir is offline Gungnir  United States
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The 807s look like a good candidate, but not sure if I can use the pair of power transformers I have as they are only 150ma each.
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Old 24th February 2018, 02:47 AM   #16
Gungnir is offline Gungnir  United States
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What about 801a?
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Old 24th February 2018, 05:36 AM   #17
PRR is offline PRR  United States
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Quote:
Originally Posted by Gungnir View Post
.....the pair of power transformers I have as they are only 150ma each.
The zero-signal current is suggested 37mA. Doing Hi-Fi, especially with hot speakers, the average heat in the PTs will be far closer to 40mA than to 154mA.

That 37mA idle looks cold to me. I might aim nearer 30mA/tube (~~65mA per pair). That's still far under your 150mA spec.

I'd also look to 6550/KT88, being kinda a larger 6L6/807. The 6550 has a suggestion for ~600V ~5K load at 100W out. De-loading to 10K suggests a bit over half that, which is fit for your OT rating. However if these are Heaths you wonder about 0.9A vs 1.6A heater demand. OTOH to get ~~600V in the Heath PT you will be Silicon rectifier, avoiding considerable bottle-rectifier filament load.

For nearly all audio power pentodes, 600V on plates with 10K (even 5K) loading suggests running Screens WAY below full plate voltage. 6550 suggests ~300Vg2 for 5K, so even 200V will work at 10K. This means much less G1 bias and *swing*, easing the driver's work. However you see that G2 current can increase a lot on peaks, so it really wants a dedicated supply, not a saggy/hot resistor voltage divider.
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Old 24th February 2018, 12:05 PM   #18
Gungnir is offline Gungnir  United States
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The zero-signal current is suggested 37mA. Doing Hi-Fi, especially with hot speakers, the average heat in the PTs will be far closer to 40mA than to 154mA.

That 37mA idle looks cold to me. I might aim nearer 30mA/tube (~~65mA per pair). That's still far under your 150mA spec.

I'd also look to 6550/KT88, being kinda a larger 6L6/807. The 6550 has a suggestion for ~600V ~5K load at 100W out. De-loading to 10K suggests a bit over half that, which is fit for your OT rating. However if these are Heaths you wonder about 0.9A vs 1.6A heater demand. OTOH to get ~~600V in the Heath PT you will be Silicon rectifier, avoiding considerable bottle-rectifier filament load.

For nearly all audio power pentodes, 600V on plates with 10K (even 5K) loading suggests running Screens WAY below full plate voltage. 6550 suggests ~300Vg2 for 5K, so even 200V will work at 10K. This means much less G1 bias and *swing*, easing the driver's work. However you see that G2 current can increase a lot on peaks, so it really wants a dedicated supply, not a saggy/hot resistor voltage divider.
Lots of good info here, much appreciated! The power transformers are Edcor 425-0-425 with bias tap and 6.3v heater taps. I purchased them off eBay a while back for a really good price, same with the OPT. I will probably make these into mono blocks on hammond steel chassis.

I appreciate everyone's advice and sorry for some of the dumb questions as I am not an engineer; most of my electronics experience is from my time in the Marine Corps, the rest I just "figure out" on my own.
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Old 24th February 2018, 12:47 PM   #19
Tubelab_com is offline Tubelab_com  United States
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Quote:
I have never heard any pp triode amps before, but seems like it would be the best of both worlds (SE and PP).
I built a P-P 300B amp about 12 years ago and I liked it a lot. It keeps a good bit of the SE 300B "magic" while adding the dynamics that only a P-P amp can provide. It blew up during a thunderstorm power surge when I lived in Florida (the lightning capital of the US). I saved it, and have been waffling between repairing it and designing a new one. I have learned a lot in the years since I built that one.

The OPT's that I have are 6600 ohms, which requires class AB operation with 300B's. The old amp made 28 watts in AB1. If I were to redesign it, I would run AB2 for a little more power and better dynamics. I also have a bunch of 307A tubes which work a lot like 300B's when triode wired, and didn't cost me anything, so I may use those.
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Old 24th February 2018, 01:30 PM   #20
thedecline is offline thedecline  Canada
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Have you decided on a schematic yet?

I too want to build a PP amp designed for hard rock/metal but that doesn't seem to be a very common design consideration for most tube amps out there and I'm not smart enough to design my own. I don't have the transformer restrictions you do however.
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