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Old 21st January 2009, 03:01 AM   #11
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Old 7th February 2009, 07:56 PM   #12
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Default Simple SE driver tube thread

OK, so armed with some of the helpful information that George clarified above, I took a few measurements on my Simple SE. I also finally got around to trying out one of the odd "12AZ7N" tubes that I've got around.

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

The 12AZ7 has (or at least, is supposed to have) virtually identical electrical characteristics as compared to the 12AT7. The differences are in a more voracious appetite for filament current, a much sturdier tolerance for H-K voltage, and subtle variations in the inter-electrode capacitances. Well, my 12AZ7N doesn't seem to behave much like a 12AT7 at all. My first clue was when I popped it into the emissions tester and it immediately pegged the meter. By my guess, it tests out to over 240% the emission of a real 12AT7.

First, I started with a Telefunken ECC81 in my Simple SE. I measured plate voltages of 215/224 and cathode voltages of 1.88/1.89. The cathode voltage can be used to calculate the idle current - ~8.5 mA. This agrees with the ~85 volt drop across R14/R24. The CCS is dropping the voltage from 400 volts down to the 220 volts seen at the plate, for a total of 1.5 watts dissipated in the CCS.

Next, I stuck my 12AZ7N into the driver socket. Cathode voltages remain identical, indicating the idle current is the same and the CCS is doing its job. The plate voltages are quite a bit lower - measured at 142/157 volts each side. I calculate each CCS is dissipating 2.2 watts in this case.

The informal listening test will be conducted shortly. Does anyone have any comments regarding these particular tubes, the lower than normal plate voltages, or the power handling capabilities of the CCS?
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Old 7th February 2009, 11:03 PM   #13
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Does anyone have any comments regarding these particular tubes, the lower than normal plate voltages, or the power handling capabilities of the CCS?
The lower plate voltage is likely due to the different characteristics of these tubes. It can be adjusted by changing the value of the cathode resistor. If it sounds OK this is probably unnecessary. The CCS chip is rated for 35 watts on an infinite heat sink. That little heat sink is good for around 2 watts. Run the amp for a while, unplug it, then touch the heat sink. If bad language ensues it is too hot. The cure is a bigger heat sink, or a higher value for R14/R24. My guess is that it will be OK. I have a trimmer pot for the cathode resistor in my test amp so that I can plug in just about any tube.

I checked when you posted about this tube and I don't have any 12AZ7, and my favorite tube seller doasn't either. I have been sorting tubes for the past 6 weekends. I have gone through at least 15,000 tubes, finding all sorts of 9 pin dual triodes, no 12AZ7! I will be gone next weekend, but when I return I have a big bunch of tubes to "test".
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Old 8th February 2009, 03:10 AM   #14
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Originally posted by tubelab.com
I checked when you posted about this tube and I don't have any 12AZ7, and my favorite tube seller doesn't either. I have been sorting tubes for the past 6 weekends. I have gone through at least 15,000 tubes, finding all sorts of 9 pin dual triodes, no 12AZ7! I will be gone next weekend, but when I return I have a big bunch of tubes to "test".
Say the word and I'll send you one of these 12AZ7N. I've got a bunch of them. No need to send it back, either. Just let me/us know if you learn anything about it.
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Old 11th April 2009, 12:34 AM   #15
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A quick update to this thread, although perhaps a bit off topic:

One note about the 6679s, and just about all my other 12AT7s is that they were all ebay iffy cheapos. Thats why they didn't last very long. Can't trust those ebay guys. In any case I recently got two sleeves of untouched 6679s and everything is doing great.

I finally got around to plunking in a pair of EH KT88's. These are really nice especially with the big OPTs. I can't wait to get my 15in Tannoy HPD's repaired.

One thing about the KT88's is that the Hammond 274BX really started buzzing (mechanically) like mad. I couldn't take it anymore so I ripped out the 274BX and plopped in a 373BX that I pulled off another amp. No buzzing. The only downside is the B+ dropped from 450v to 400v as expected. The 430ohm cathode bias resistor is too much for the job so I paralleled a 1.5k resistor on the other side of the board to get a total of 330ohms. Looking good.

I also stuck in a 0.1uF orange drop along with the 80uF film cap to help smush some of the solid state rectifier switching noise. I don't think it made a difference, but it's not noticeable when music is playing.

Looking forward to some of Tubelab's new boards. Pssst, one vote for a 6aq5 Simple P-P.
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Old 11th April 2009, 03:13 AM   #16
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Originally posted by whitelabrat
One thing about the KT88's is that the Hammond 274BX really started buzzing (mechanically) like mad. I couldn't take it anymore so I ripped out the 274BX and plopped in a 373BX that I pulled off another amp. No buzzing. The only downside is the B+ dropped from 450v to 400v as expected.
I didn't have any previous experience with the Hammond power transformers, but I ended up buying the 374BX for mine based on collected reports that the 300 series were mechanically more stable than the 200 series. I haven't been disappointed with my 374BX. It's 99.999% buzz and hum free. You can almost feel a slight humming if you place your fingers on it, but there's nothing audible.

With regards to the 373BX, I'd figure a 400 volt B+ would be a good thing if you intended to run mostly EL34 and 6L6 type tubes. Keep your output transformers closer to 3K (rather than 5K, which probably suits the higher B+ better) and you should be golden.
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Old 15th April 2009, 06:31 AM   #17
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Default 6BG6GA Lifespan

I am curious to hear how long your tubes last in the Simple SE. I bought a quad (6GB6GA)a couple years back with hopes of buying a cache of them because they were so cheap. About the time the seller raised his price $2 a tube two of the four tubes er ... a ... stopped transconducting. Anybody else have this trouble? These are the only power tubes I've had give up the ghost in over five years of "casual" listening. Maybe just my bad luck?
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Old 15th April 2009, 01:55 PM   #18
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I assume you mean the Phillips-ECG 6GB6GA's? I've had a pair in mine for not much more than half a year, so I can't vouch for their lifespan. I'm guessing I was pushing them at about 66ma and then later 79ma at about 450v without any fuss which I find impressive. I don't get a lot of listening time either, maybe a couple of hours of time on the weekend so I'm not really putting a lot of time on them.

I've since switched to EH KT88's, but I'll likely go back to the 6GB6GA's once the KT88's give up.

I did try some old 6GB6G's. They turned red within about a minute or so, and I wouldn't expect those to last very long without tuning things down quite a lot.
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Old 25th October 2011, 04:46 PM   #19
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So how is the life span of these Philips/ECG 6BG6GA in SSE? And how do they sound? Someone at audio asylum was complaining about their top end performance. Is it true?

I know I'm posting on 2.5 year old thread but I'm thinking about trying them in my SSE.
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