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Old 25th November 2008, 06:35 AM   #1
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Default Wanted. Simple EL34 Push Pull Design

Hi All

Looking for a simple EL34 Push pull design. I have spent hours searching the net (and this forum) but not yet found anything suitable.

Background is that in late 1992 I purchased a kit to make a directly coupled hybrid transister valve amp. This was featured in Sep 1992 edition of ETI (Electronics Today International) and was designed by a Jeff Macaulay. A kit was available for what was then GBP 170 (at lot of money to me at the time when I was a poor student).

I do not understand more than the the basics of electrical theory but am able to follow a schematic and completed the kit without difficulty. The result was disappointing to say the least. Later additions of the magazine slated the design but following the suggested modifications of other readers I was able to get it working and was reasonably happy with the sound quality. I followed this kit with a Maplin valve pre-amp and the two have served me well all the years in-between. Recently the amp has stopped working due to a fault in the power supply and I have decided that if I go to the trouble of fixing it I would also like to rebuild it to a better design re-using the EL34s and the output tranformers and the rebuilt power supply.

The design uses two EL34s per channel in push/pull configeration but the output transformers do not have ultralinear taps (?triode mode).

I am looking for a circuit therefore which can re-use these valves and the output transformers. I do not want to use a transistor first stage / phase splitter as in the original. All the designs on the internet I can find use either ultralinear configeration or are single ended.

I do have some ECC83, ECC82 and ECF82 valves which I could utilise in the first stages to save a few pounds but this is not mandatory.

Sorry but I do not currently have a scanner and so cannot post the circuit design of the original.

My introduction to valve amplification has been somewhat troubled and drawn out, seemingly by a poor design but I would like to give it another go so to speak.

Any help appreciated. If nothing is forthcoming I will probably have to give up an build a GainClone!

Regards
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Old 25th November 2008, 04:56 PM   #2
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This is as simple as a pp design can get: http://s136.photobucket.com/albums/q...ntcomplete.png

Im listening to it quite loud as i type this and it sounds awesome.

Just skip the ultralinear and replace the screen resistors whith 220ohm ones and tie them to B+, then bias to 70mA combined for both tubes.
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Old 25th November 2008, 05:09 PM   #3
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My next project is going to be Poindexter's EL34 music machine amp. Triode PP EL34's around 16 watts or so IIRC. I have seen two versions of this design, one with 6SN7's and one with 6GK5's.
The 6GK5 schem has a CCS (which is the design I am planning on building) Output transformers either 5Kohm or 6Kohm.

If your existing chassis uses 9 pin driver tube sockets hopefully some of the experts here can chime in with other recommendations since the 6GK5's are 7 pin and the 6SN7's are octal.

Poinz' write-up indicates that 9 pin front end tubes were an earlier incarnation of this, you may want to email him and ask.
His email is on his website at www.audiotropic.net

All of the comments I have heard about this deisgn have been very positive.

Here is the link to the 6SN7 version:

http://www.audiotropic.net/Projects/ampEL34.html

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Old 26th November 2008, 04:58 AM   #4
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What about the EICO HIF 87, Audio Innovations 500, Leak, or Marantz 5 and lastly there is a really simpel one posted by Eric Hsieh using just 2 ECC83 and 2 EL34s.
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Old 26th November 2008, 07:24 AM   #5
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If you want a dynamic sounding simple amp without NFB you will have to get rid of the terribly unlinear ECC82. The investment in one 6DJ8/E88CC per channel should be no problem. The same guy that did the AI series 500 is behind this one.

With the right OPT this one will deliver over 10W ClassA with below 1% THD.

If going only Class A you can save money by skipping the EL34 cathode cap.

If you want to try other OPTs it can be adopted to be used with a 115+115:6//6V 50VA mains toroid as OPT. The EL34 cathode circuit will then need some sand to get equal current through both tubes.

By the way, do you now the impedances and other data of your OPT?

Sensitivity is on the low side but I suppose you have preamp? If not, it could easily be constructed using a single 6H30 together with two Lundahl LL1544A.
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Old 26th November 2008, 12:46 PM   #6
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The Audio Tropic design is not without problems. It's basically a Mullard 5-20 with an SRPP substituted for the input tube and the grid stoppers removed. There are a lot of ways of improving it, especially with regard to distortion and source impedance. The SRPP has its niche (high voltage swings into reactive loads), but there seems to be a fascination with it that transcends the actual virtues. The reality is that the input stage needs only swing a few volts, high gain there is a virtue, and for those conditions, a simple triode voltage amplifier (though less sexy and exotic) will be quieter and more linear.

If it were me and I were restricted to published and proven designs (rather than designing my own), I'd go pentode and use the Curcio ST70 design published in Glass Audio (volume 1, number 1, page 1).
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Old 26th November 2008, 01:21 PM   #7
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SY, I think Jack asked for "Simple" ! Do you happen to know any data for the OPT?

Curcios design was elegant, but as far from simple as you can get.
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Old 26th November 2008, 01:30 PM   #8
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The circuit is not ultra simple, but it's all done on PCBs, easy assembly. Curcio used a 4k3:8 OPT, but it's not like that's terribly critical given the wild impedance variation with frequency that most speakers present.
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Old 26th November 2008, 02:15 PM   #9
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Checked and the Curcio is a lot of money these days.!

Also remember Jack lives inside EU in GB. This totally rules out the Curcio pricewise.

Some might like the cascodes, I dont in this applicaton. Tried them both in ST-70 and Mk-IV many years ago when Curcio was in fashion.

But I love them as input-stages in my guitar-amps !
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Old 26th November 2008, 04:33 PM   #10
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Quote:
Originally posted by SY
The Audio Tropic design is not without problems. It's basically a Mullard 5-20 with an SRPP substituted for the input tube and the grid stoppers removed. There are a lot of ways of improving it, especially with regard to distortion and source impedance. The SRPP has its niche (high voltage swings into reactive loads), but there seems to be a fascination with it that transcends the actual virtues. The reality is that the input stage needs only swing a few volts, high gain there is a virtue, and for those conditions, a simple triode voltage amplifier (though less sexy and exotic) will be quieter and more linear.

If it were me and I were restricted to published and proven designs (rather than designing my own), I'd go pentode and use the Curcio ST70 design published in Glass Audio (volume 1, number 1, page 1).
SY: Poinz's 6GK5 schem is attached. Any comments/improvements on this design? I have a pr of hashimoto HW-60-5 (60 watts, 5K) Output tranformers, a quad of EL34's, KT88's, and 6550's and misc driver tubes so I am open to suggestions. I am comfortable designing the PS but I would like to use a published and proven design for the rest. I am looking for a triode design. (pretty much the same as the OP)
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