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PostPosted: 06 Jul 2017, 15:01 
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Hi Everyone, This could go on the main Poddwatt thread, but it might get lost there. Sometimes I just build something because I have the parts. Such is the case with this amp. It is a variation of the basic Poddwatt. I had some tubes I wanted to try (12A6 pentodes). They are low power “metal” tubes designed originally for automotive amps. In many ways it is similar to 6AQ5. Anyhow the amp uses the boosted grid voltage of later model Poddwatts and the DC heater supply (AC ought to be OK though). Otherwise it is rather similar. Idle current is 31 ma per tube to stay within dissipation ratings. The driver is my favorite 12SL7. Total heater power is just under one amp at 12 volts. I used inexpensive parts where possible. All coupling and most bypass caps are general purpose Solens. The build is not critical but care must be taken to insure pin one on each of the power tubes is either connected to the chassis (I did this) or the signal ground (probably work OK, but I don’t recommend it). This is to prevent the possibility of accidental shock if there is an internal tube failure. Actual performance was a lot better than I expected. Response was down 0.1db at 40HZ and 32KHZ at 1 watt (yes that is 0.1db). It was only down about 10 db at 80KHZ. The bottom end was -3db at 14HZ. At that frequency distortion was clearly evident though. Response was clean at 20HZ. S/N was a bit lower than many of my projects at -77dbv, still quite acceptable. Power output was just over 2.5 watts RMS into 4 ohms. I didn’t measure distortion, but the sound was quite nice. I estimate less than 1% at 1 watt going to around 4% at 2 watts. For a cheap amp it is not to bad IMO. Tubes, caps and trannies are lower cost than what I generally use. I would expect you could duplicate it for under $200 easily.

Edit: use 200uf /350 volt caps for the first two in the power supply.

Good Listening
Bruce
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PostPosted: 06 Jul 2017, 23:19 
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Metal tubes are sexy 8-)

Great work, Bruce! :thumbsup:

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PostPosted: 07 Jul 2017, 05:49 
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Not only sexy, but often a lot cheaper. I have used metal tubes in my MAD Stealth, though the 12A6 behaved a bit odd so I use glasstubes wwhere the glass is tinted black (inside).

And tawking of cheap, using 12V-tubes in place of 6V also may reduce expenses. Check the prices for 6SN7 and 12SN7. Note that some 12-tubes aren't the same as 6V. I think 25L6 isn't a 25V-version of 6L6.

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PostPosted: 07 Jul 2017, 11:56 
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Hi Correct, the 25L6, 35L6 and 50L6 are not the same as the 6L6. They are low power output tubes for early radios and phonos that used line voltage heater strings. In performance those tubes are more like 6AQ5 than anything else I have run into. Actually pretty similar to the 12A6. There are also 7 pin versions...25C5, 35C5 and 50C5. I believe there was even 117C5 version. Lots of strange tubes back then.

Good listening
Bruce

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PostPosted: 16 Jul 2017, 14:33 
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Hey, what about a 12SC7 (metal) instead of 12SL7 for the driver?
That would result in an all-metal design, which would be cool.
They are very cheap too.

They look fairly similar. Mu is about the same.
Different pinout.

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PostPosted: 16 Jul 2017, 16:57 
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Hi, yes they would work. But I have a whole bunch of the 12SL7s.

Good listening
Bruce

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PostPosted: 17 Jul 2017, 17:54 
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Some what hard to use a common cathode twin triode in an srpp topology


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PostPosted: 17 Jul 2017, 19:03 
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Well, you could use a common cathode as a see-saw splitter and have a Futterman topology ;)

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PostPosted: 17 Jul 2017, 20:10 
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Oops, that will teach me to look at the whole spec sheet. I suppose I could cheat and use a FET for the upper triode (nope).

Good listening
Bruce

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