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Old 14th November 2012, 12:54 PM   #1
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Default choosing input valve

I want to build a single ended amp with EL 34's as the output tube. I can not decide whether to use 6sn7 or 12ax7 as the input tubes. I have schematics for both. I've also noticed some manufacturers favour one or the other, such as Icon Audio (in the UK ) use 6sn7. Unison Research use 12au7 or 12ax7.
So my question really is, "does one sound better than the other?"
They are similar in price and readily available (in the UK) so that is not an issue.
I will be using the amp with an Arcam CD17 and Kef IQ5 speakers.
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Old 14th November 2012, 01:36 PM   #2
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just build an amp to where they have both sockets and a/b it.

Its all about personal choice. They both add a little bit of colour in there own way some people like one or the other.

I like octals, but that is because it is easier for me to solder because I have arthritis.
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Old 14th November 2012, 01:39 PM   #3
DF96 is offline DF96  England
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Properly used, neither will add colour but simply amplify the input signal. How much voltage gain do you need from input to the EL34 grid? 6SN7 or ECC8212AU7 will give you about 15; ECC83/12AX7 about 70-80. Do you plan to use negative feedback?
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Old 14th November 2012, 02:06 PM   #4
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Sorry that colour is a bad word this decade/century. Maybe personality would be a better word.
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Old 14th November 2012, 03:17 PM   #5
DF96 is offline DF96  England
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Better to have valves and circuits which add nothing. 'Personality' is fine in a guitar amp but undesirable in a music reproduction system.
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Old 14th November 2012, 03:48 PM   #6
rmb is offline rmb  United States
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Dudleyperkins: Avoid use of the 12AX7 in a power amplifier. You don't need the high mu triode there. I would suggest using a 12AT7 (6201) input, a 12BH7 phase splitter, and a 12AU7 (5814) low gain driver direct coupled to the EL34's. I used this to replace the PWB in a Dyna stereo 70 and it cleaned up the sound immensely. I built the new board on a small piece of aluminum so it would directly fit in place of the Dyna PWB, (Equivalent military versions of tubes in parenthesis.) I didn't like the way the 7199 in the Dyna circuit affected the audio. If you like pentode input, try a EF86 (6267).

The 6SN7 is a rugged medium-mu triode, and can drive most of the standard push pull output tubes (6L6GC, EL34, 6550, etc.) operating in AB1 faithfully. You can improve audio quality by direct coupling stages and minimizing the use of plate coupling capacitors. If you modify a stereo 70, all the voltages are there including a negative bias supply. The 6SN7 can handle higher plate voltage and dissipation compared to a 12AU7. In brief, you want plenty of clean voltage swing at the output tube grids while the lower level tubes are operating well within the linear region of the operating curves.

I am assuming you know something about amplifier design...some of the old tube and output transformer manuals (i.e. Acrosound, UTC) have some good schematics.
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Old 14th November 2012, 04:41 PM   #7
kevinkr is offline kevinkr  United States
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The OP is talking about building an SE amplifier.. The 6SN7 is an excellent choice based on its reasonably low rp and very good linearity. The 12AX7A will have some difficulty driving the miller capacitance of an EL34 should triode operation be intended even as an option.

The additional gain of a 12AX7A would allow for the application of some global feedback due to the higher gain if a single stage driver was contemplated. (Roughly 6 - 10dB depending on the required input sensitivity)

Another tube well worth considering would be the 5842 if you can get your hands on a few. Very linear, and with a low rp (<2K) that allows them to drive a capacitive load effectively.
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Old 14th November 2012, 05:08 PM   #8
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Some comments, if I may.

Don't use the 12AT7 in SE circuitry, unless it's CCS loaded. It's a matter of too much 2nd harmonic distortion.

Kevin is spot on about the 12AX7 being a wimp, with next to no drive capability. That fact can be dealt with easily enough. Use a DC coupled ZVN0545A source follower as shown in the phono preamp schematic I've uploaded.

The 5842 and the somewhat similar Russian 6С45П (6s45p) can be quite useful, BUT they can drive you crazy with parasitic oscillation. My jest is that those types come close to oscillating stored in the cardboard box. Carbon composition stoppers of substantial value on each grid connection, a 100 Ω carbon film stopper on the plate connection, and ferrite beads on the heater wires are in order. The "in box oscillators" were engineered for use in grounded grid topology at RF.
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Old 15th November 2012, 12:52 AM   #9
sherpa is offline sherpa  United States
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What about using a 6SL7 for a SE amp? Shannon from DIYTube already has one designed called the Clementine. I have one that i absolutely love. Check them out .... diytube.com :: Index
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Old 15th November 2012, 01:43 AM   #10
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What about using a 6SL7 for a SE amp?
The triodes in a 6SL7 are very closely related to those in a 12AX7. They are high μ, high RP, and very low gm. Those characteristics make them poor drivers of substantial Miller capacitance "finals". A pentode wired O/P tube, with its small CMiller, can be OK, but any sort of triode is out.
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