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Old 30th December 2008, 04:16 AM   #1
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Default New SET Amp

Using a sweet tube that I'd really rather not make any more famous than it is. It's the square plate version made by GE, supposedly the most rugged and able to take well over 18-20W. I have all the parts except for the OPT's, which will come any day now. No boutique parts, but low-impedance Nichicon etc.

Here are the schematics. I should note that the OPT's I'm using are Edcor GXSE15-16-8K's run as 4K:8, which have a tap for ultra-linear.

What do you guys think?

Click the image to open in full size. Click the image to open in full size.

I am open to the possibility of using a noval pentode or something in the place of the SRPP, maybe in E-linear a la PakProtector
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Old 30th December 2008, 06:58 PM   #2
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The 6AV5 doesn't seem to be tough to drive. You could try a small pentode and feed the pentode screen off the cathode of the 6AV5. Personally, I would throw a choke or a current source on top of one of those 5965 triodes, or maybe even choose a tube with a lower mu.
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Old 30th December 2008, 07:41 PM   #3
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Really? How do you figure? I'm getting a gain of 36 with the 5965 SRPP, and the output tubes are biased at -50V, so that means I need almost 1.4V to max them out. Seems like I'm cutting it a bit close as it stands...

I'm interested in the pentode driver option though. Could you possibly link me to an example of a schematic where they implement something like that?
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Old 31st December 2008, 12:20 AM   #4
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Most everyone knows what I think about the **** (number withheld to protect the currently reasonable price), although I believe the must rugged of the breed are the Sylvania's (no glow at 25 watts). I was driving them with a CCS loaded 5842 buffered by a mosfet follower (PowerDrive) which gives a gain of about 40 (hacked Tubelab SE board). This is just barely enough, although I was running the **** a bit hotter which required a higher bias voltage allowing more drive. These tubes respond well to cathode feedback especially with the Edcor OPT's.

I have found some GE 6BQ6GA's that have the same guts inside them. They behave the same way as the **** except for the plate cap and pinout. The pinout is the same as the 6L6 except for the plate cap, so they can be stuffed into a 6L6 circuit with some serious rebiasing and a plate cap wired to pin 3. I put some of the AES 98 cent tubes into a Simple SE board which uses a CCS loaded 1/2 of a 12AT7 for a driver and cathode bias for the output tube. Plenty of gain, sounds great, 98 cents for the output tubes! Not all GE 6BQ6GA's have **** guts inside though.
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Old 31st December 2008, 12:46 AM   #5
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I just glanced at the datasheet, it looked quite easy to drive, but not under the operating conditions listed above. Sounds like a CCS and an AT7 is a good idea. For the pentode driver, have a look through the Mullard single ended EL84 schematics and you'll bump into it.
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Old 31st December 2008, 01:19 AM   #6
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Quote:
I just glanced at the datasheet
The datasheet mentions this pesky screen grid maximum rating of 175 volts. If we believed in datasheets these tubes would never work in triode or UL. Every tube that I tried will work to 275 volts and some will go to 350. When cranked this hard, you need lots of negative bias, and therefore lots of drive.

When you are misapplying a tube, you just "crank them up till they glow, then back up a little". Some serious experimenting is often required to find the sweet spot. Many tubes don't like it, but some do. My favorite sweep tube has a max screen rating of 275 volts and a plate dissipation rating of 40 watts. The right ones will work just fine at 450+ volts and 90+ watts in triode mode. Need about 100 volts of bias and a big bunch of drive though.
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Old 31st December 2008, 09:40 AM   #7
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Yeah, I ended up doing this when picking my operating point on an 829B push-pull amp I did a while back. Sometimes you can "assist" the tube in handling a bit more (special cooling).
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Old 19th January 2009, 03:18 AM   #8
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Got the thing running. It's funny, absolutely black background, no hum, buzz or noise or ANYTHING. It freaked me out at first. Lucky eh? Makes for a very nice amp for my college dorm room. Sound gets as loud as I generally need with 88dB speakers...

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Click the image to open in full size.

Click the image to open in full size.
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Old 19th January 2009, 05:57 AM   #9
JoshK is offline JoshK  Canada
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nice looking little amp. well done.
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Old 19th January 2009, 06:04 AM   #10
athos56 is offline athos56  United States
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Very Nice! Where did you get the case?
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