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Old 5th April 2010, 01:52 PM   #381
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You can't hook the screens up to the UL taps because of the screen voltage limitation of most sweep tubes.....I don't think you could take a sweep tube and get the same amount of linearity using UL than you can with plate-to-grid feedback. Sweep tubes were never intended for audio.
Trust me I have blown enough tubes to realize that sweep tubes must run in pentode mode. Triode and UL are out with a very few exceptions like the 6AV5. Even those will make more power in pentode mode. The red board is capable of 1% distortion at 100 WPC output, I have seen it happen.

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Well I just received a bunch of 6HJ5s.
I ran those at 100 WPC for a while with no issues, but I think that I will settle at 60 to 80 WPC in my final design.

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I received a box of transformers for free a little while ago so
My collection of "free" transformers had grown to at least 1000 pounds probably a lot more. I went through the ones in my house yesterday and gave about 300 pounds to the scrap metal guy. As Bono would sing "still haven't found what i'm looking for". Of course that is hard to do when you don't know what it is yet.

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So, would there be any advantage to a design that does use the UL taps?
Probably not if sweep tubes are the chosen output tubes. If you were using KT88's or other audio pentodes, UL might be worth considering. If you have compatible sweep tubes, the red board is hard to beat.

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Pardon all the questions, but I'm still far short of having Morgan Jones committed to memory...
Hey, I have been at this for a long time, and the book of Morgan along with the RC-30 stay on my workbench. I don't even put them away anymore.
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Old 5th April 2010, 02:31 PM   #382
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Originally Posted by tubelab.com View Post
Hey, I have been at this for a long time, and the book of Morgan along with the RC-30 stay on my workbench. I don't even put them away anymore.
Ditto on the MJ. I need to get the 3rd edition, though it is kind of pricey for both books. There's no point in committing it to memory as long as you remember that you read it somewhere.
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Old 5th April 2010, 03:36 PM   #383
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Thanks to Russ and George for the UL explanations. Please understand I was not trying to "second guess" Pete's design (me?....hardly!). It's just that I'm still learning about the various vacuum tube circuit topologies. Can't wait to hear the red board sing!
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Old 6th April 2010, 01:05 PM   #384
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Has anyone tried this circuit with non sweep tubes? I have several of the Sylvania super 6BG6GA's. It has been stated that these are NOT high perveance sweep tubes but rather 6L6GC or 7027 with a top plate cap.

Any thoughts on using these tubes or any of the 6L6GC, EL34, KT88 or the ilk?
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Old 6th April 2010, 02:56 PM   #385
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As far as the general topology, I don't see why it wouldn't work. Obviously you'd want more plate voltage for the bigger tubes and definitely more screen voltage for all of them. The input/driver section of this board seems to be able to drive anything I've plugged into it, though I haven't explored what the limits are in terms of voltage swing.
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Old 6th April 2010, 04:59 PM   #386
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Originally Posted by tomlang View Post
Has anyone tried this circuit with non sweep tubes? I have several of the Sylvania super 6BG6GA's. It has been stated that these are NOT high perveance sweep tubes but rather 6L6GC or 7027 with a top plate cap.

Any thoughts on using these tubes or any of the 6L6GC, EL34, KT88 or the ilk?
hey-Hey!!!,
Those are fine 6L6GC tubes. No reason at all not to use them in this amp topology and I don't see any reason to change anything but the delivered grid bias( Pete's amp is fixed bias?; should get away with a twist of the bias pot).
cheers,
Douglas
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Old 6th April 2010, 05:36 PM   #387
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Has anyone tried this circuit with non sweep tubes?
A minor circuit modification will be needed to run 6L6 type tubes in this design. First there are the obvious PC board issues since you want to put an octal tube where a 12 pin socket resides. Some creative rewiring could be used here.

The second issue is the screen voltage supply. Pete runs the screens of the input tubes and the output tubes off of the same 150 volt supply. The typical octal audio tubes need more than 150 volts on their screens, usually 300 to 400 volts. The 6CB6 will not be happy with 300 volts on the screen and 140 volts or so on the plate. It should be possible to lift some resistors and run some flying wires to run the output tubes on a supply derived from the plate voltage source.
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Old 9th April 2010, 10:17 PM   #388
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Well I was trying to find a transformer for the bias supply and for whatever reason the transformer that would have worked I had didn't like me too much and let out the magic smoke. So I am still searching. Antek has a 50va dual 25v secondary transformer that I thought might work if I put the windings in series to get 50vac. Any opinions on that.
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Old 9th April 2010, 11:38 PM   #389
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That will work fine...overkill in terms of VA but the magic smoke should have a better chance of staying inside.
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Old 12th April 2010, 11:48 AM   #390
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hey-Hey!!!,
For bias and CCS negative rail service I like the $7, 25.2V/450 mA trans from Radio Shack. I use either a doubler or a FWB depending on the use; the half-rail voltage usually gets the CCS attached and full doubled voltage goes to bias. How that'd work on Pete's board is a bit of an open question.
cheers,
Douglas
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