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Old 19th January 2009, 02:04 AM   #1
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Default Simple SE and 6BG6GA's

I took a bit of time today to so some work on my 1 yr old Simple SE that I had been meaning to do. I figured I'd share my results in case anyone is interested.

The two 1500uf 50v caps were a bit too tall for my chassis so I had cut two holes in the bottom of the chassis for them to poke out from. I decided I'd rather that they didn't do that so I replaced them with a pair of Nichicon KG's of similar value (LKG1H152MESBBK). The lead spacing wasn't quite right, but I bent them to fit. They're also a bit too wide so I had to relocate the cathode bias resistors to the other side of the PCB. In any case I'm happy with the result.

Anyone ever heard of the KG's before? I've gotten KZ's and ES's, and know about FG's, but KG's? Is this a new line from Nichicon?

Anyway since I had to move the cathode bias resistors I figured I'd change the values from 560 to 430 ohms for my Philips/ECG 6BG6GA's since they're supposed to be able to dissipate 35w on the plates. 430 should push 'em good and also work well with KT88's if I ever get a pair.
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Old 19th January 2009, 02:13 AM   #2
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I also replaced the dead FREDS that I snapped out after they shorted.

So how's it going? Well nothing exploded! We'll actually my 6679 started to get crackly popish, so I replaced it. That's the 3rd 12AT7ish tube that I've gone through since I sparked the Simple SE up 12 months ago. I should check the heater voltage. The 6679's should be able to handle a bit of over voltage though.

In any case the 6BG6's have been in for about an hour with no red spots. None. They're happy as can be. I'm also running off the hexafreds which I would expect to put the voltage a wee bit over 450v for the B+.

Before I wrap up here, has anyone measured the voltages on the 12AT7? I'm just curious how hard they're being pushed.
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Old 19th January 2009, 02:26 AM   #3
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Quote:
Originally posted by whitelabrat
Before I wrap up here, has anyone measured the voltages on the 12AT7? I'm just curious how hard they're being pushed.
Pretty hard, I think. I haven't actually measured mine but I did build a model in LTspice. It thinks the plate voltage is going to be around 250VDC at idle. Running 10 mA (assuming the CCS is doing its job) will give a 2.5 watt dissipation per side. That's right at the rated limit for a 12AT7.
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Old 20th January 2009, 05:15 PM   #4
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Would one suppose that I could bring some stress off the 12AT7 by upping the value of R10 and R20? A goal may be to go from 10ma to 7ma? Or would that wreak havoc?
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Old 20th January 2009, 07:28 PM   #5
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Would one suppose that I could bring some stress off the 12AT7 by upping the value of R10 and R20?
I don't think so. It's the CCS that's running the show in this case. It'll just raise the plate voltage higher trying to maintain the 10 mA as best it can. For what it's worth, I seem to recall that George once mentioned somewhere the CCS used isn't particularly exact, and the 10 mA is really an approximate value. I think the actual current is slightly less.

I believe the correct way to adjust the idle current is by manipulating R19 and R29. I'm not sure which direction they need to go. I'm sure you could figure it out by consulting the data sheets for the IXCP10M45S. As you reduce the current, there will necessarily be a larger voltage drop across the CCS. I'm assuming the power dissipated in the CCS will remain the same (voltage drop is going up, but current is going down). If not, you may also need to adjust R14 and R24 to compensate.

Also keep in mind that reducing the idle current will result in a lower plate voltage on the 12AT7, and a smaller potential voltage swing coming out of it. I suspect you'll still have plenty of swing to fully drive the finals, but a full analysis is probably in order.
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Old 20th January 2009, 07:57 PM   #6
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I haven't actually measured mine but I did build a model in LTspice. It thinks the plate voltage is going to be around 250VDC at idle. Running 10 mA (assuming the CCS is doing its job) will give a 2.5 watt dissipation per side.
I started questioning where I got my own information. The 10 mA figure came from memory. I confirmed that I read the number here, on George's website.

The 250 volts came out of my LTspice simulation. Perhaps it has a crummy model for the 12AT7, or it just doesn't model realistically under the conditions presented by the CCS. Regardless, George's page (see above) indicates the plate is running close to 188 volts. That means the idle dissipation is quite a bit lower than I originally thought. I'd probably stop worrying about it at this point.
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Old 20th January 2009, 11:21 PM   #7
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Just curious, are you using an adapter to use the 6BG6GA?

Thanks!!
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Old 21st January 2009, 12:33 AM   #8
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Yep, that's the only way to do it unless you do point-to-point. It adds a good 1.5 inches to the height of the tube, so I can't put the cage on top any more. Some day I'll scratch up something for KT88's. The 6BG6's are so cheap and I like they way they sound so that probably won't happen anytime soon.
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Old 21st January 2009, 12:36 AM   #9
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OK, here is the deal. There are two things that contribute to the power dissipated in the 12AT7. As most know these are the voltage across the tube, and the current through it.

The current is forced by the CCS IC and that value is set by R13 and R23. The target current is 8 to 10 ma, which is generally obtained with a 330 ohm resistor. To lower the current raise the resistor value. Most of the 12AT7's that I have tried worked the best with currents from 5 to 10 mA. R19 and R29 are 1K ohm gate stopper resistors to prevent oscillation in the 10M45 chip.

The voltage across the 12AT7 is determined by its bias voltage since the current is fixed by the CCS chip. In this case the cathode resistor R10 and R20 will adjust the plate voltage. Lowering the resistor will reduce the plate voltage. I found that best results were obtained with a plate voltage of 175 to 200 volts.

R14 and R24 are there to reduce the dissipation in the CCS chips and shouldn't need adjusting. The only exception is when the board is used on low supply voltages (below 325 volts) to run tubes like 6V6, 6K6, 6Y6, or 6W6. In this case these resistors should be replaced with a wire jumper.
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Old 21st January 2009, 01:52 AM   #10
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Excellent info. Thanks for the post, George.
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