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Low Voltage (60V) Stereo Tube Amplifier for Dummies (2+2W)
Low Voltage (60V) Stereo Tube Amplifier for Dummies (2+2W)
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Old 27th November 2014, 01:25 PM   #1
pilovis is offline pilovis  Italy
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Default Low Voltage (60V) Stereo Tube Amplifier for Dummies (2+2W)

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6 Vcc - I=> 0.5 A
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24+24Vac - P=> 48 VA
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NOTE: 600 ohm audio output transformers can be replaced with 220/24Vac 48 VA (or 120/12 Vac) line transformers.

This is a safe, cheap and funny tube amplifier for dummies to practice, you can safely use 6SL7, 6SN7 or ECC81,ECC82,ECC83,ECC85,ECC88,6FQ7 (caution: different pinout) without risking anything, just check the correct pinout of the model you want to use and adapt it to this schematic.
The same for the tetrode, you can try 27GB5, CV10955, CV10725, PL500 (all have the same pinout) or PL36 (different pinout) and listen to the differences.

Note: it is obviously possible to use AC voltage to supply heaters, I choosed DC to limit hum noise at a very very low level (not audible), if you choose to use AC voltage, remember to twist wires togheter.

Starting from the point that an electrical device to be safe must stay below 60 Vdc or 48 Vac of supply voltage, I developped this project choosing 6SL7 (6HC9 russian) as a driver stage because, despite the very low anode voltage and after a lot of testing with other double triodes, I found it to be the best solution for my ears
I also choosed PL504 because it is an indestructible tube, it works well at 50V, withstands permanent short circuit on the output in this schematic and it is very easy to find on Ebay.

The output transformers should be in the range between 600 and 800 ohm, but you can safely use trasformers with 1k or more, the only risk is a little more distortion and lower audio volume, you can also use line transformers with the right winding voltage ratio (10:1) and enough steel core to avoid DC saturation (DC saturation is quite low at this voltage).

The sound is great, try to believe

A lot of times I found people that guiding principle was: "it shouldn't work" or "it is not theoretically correct", instead I found that some weird solutions worked very well, obviously it depends on what you want to obtain, if you are looking for an High End Hi-Fi amplifier, this is not the right project for you
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Last edited by pilovis; 3rd December 2014 at 04:52 PM.
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Old 27th November 2014, 02:25 PM   #2
djn is offline djn  United States
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That looks like a great weekend project. How does it sound???
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Old 27th November 2014, 03:30 PM   #3
azazello is offline azazello  Bulgaria
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IMO, ECC88 will sound better with low +U, that is small for 6SL7....what is U on the plates of 6SL7?
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Projects: SE GK71, SE 12BQ6, PP KT88, OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,...
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Old 27th November 2014, 03:47 PM   #4
Mosquito is offline Mosquito  Argentina
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Pilovis, from where are you getting the 120VAC input for the heaters wall wart (isn't Monferrato al 230VAC zone?)
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Old 27th November 2014, 04:01 PM   #5
azazello is offline azazello  Bulgaria
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IMO, I doubt that U heaters every PL504 are the sames, if You connected them in series.
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Projects: SE GK71, SE 12BQ6, PP KT88, OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,...
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Old 27th November 2014, 04:17 PM   #6
pilovis is offline pilovis  Italy
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Va PL504 = 53V
Vk PL504 = 3V

Va 6SL7 = 36 V
Vk 6 SL7 = 0.4 V

I choosed the 6SL7 because, despite the low anode voltage, it sounds better than ecc88, ecc83, ecc81, ecc82 or 6SN7 (all tested).

The sound is great, try to believe

PL504 is designed to allow heaters to be connected in series.

Last edited by pilovis; 27th November 2014 at 04:26 PM.
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Old 27th November 2014, 05:18 PM   #7
artosalo is offline artosalo  Finland
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Quote:
Originally Posted by azazello View Post
IMO, I doubt that U heaters every PL504 are the sames, if You connected them in series.
P-series (300mA) tubes are especially designed to be operated at series connection.

Quote:
IMO, ECC88 will sound better...
Quote:
I choosed the 6SL7 because... it sounds better than ecc88
Isn't this quite typical outcome of this kind of discussion ??

Last edited by artosalo; 27th November 2014 at 05:29 PM.
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Old 27th November 2014, 07:18 PM   #8
Mosquito is offline Mosquito  Argentina
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there are construction details that make the SL7 tube react better than others to such extremely low vgk biasing condition?
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Old 27th November 2014, 07:22 PM   #9
djgibson51 is offline djgibson51  New Zealand
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Good one
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Old 27th November 2014, 07:45 PM   #10
jean michel is offline jean michel  France
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hello ,

What gain with which bandwidth ? On my, j have OPT of 3 K, the assembly is in pen mode triode!
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