304TL SET schematic redesign. EML-20b Alternative? - diyAudio
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Old 30th December 2014, 05:41 PM   #1
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Default 304TL SET schematic redesign. EML-20b Alternative?

So I came across the attached schematic listed below here on diy, but would like to use a cheaper driver than the EML-20B ($500). I have ordered the 304tl's, 3b28's and their sockets. Heyboer will be making the output transformers.. From reading it appears that a 6sn7 may be an acceptable replacement for the 20b but I am not confident in properly sizing the components to make this a simple replacement.. particularly the chokes and capacitors.. I am open to other driver stage candidates if a better suited tube exists. Any gurus here that can assist?

EML-20A-20B-30A Data sheet. By Emission Labs.
http://www.radiostation.ru/tubes/6SN7.pdf

304TL based Class A
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Old 31st December 2014, 09:38 AM   #2
Gluca is offline Gluca  Italy
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801A. I built pretty much the same amp years ago (I used an input transformer to get some more gain) ... It is still working.
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Old 1st January 2015, 05:28 AM   #3
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Thanks for the response Gluca, I was hoping to hear from you, due to your input in the other 304 thread. I am a little hesitant to use an input transformer, would rather go with a tube. I came across the el84, and its big brother the 7189a which can handle the current and voltage while providing the close to the same gain when run in triode mode http://frank.pocnet.net/sheets/135/7/7189A.pdf

Thoughts?

Thanks!
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Old 1st January 2015, 06:27 AM   #4
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Out of curiosity, what is Heyboer charging for the output transformers? I've known of them for years, but never done any business with them.
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Old 1st January 2015, 10:28 AM   #5
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Quote:
Originally Posted by europorsche914 View Post
Thanks for the response Gluca, I was hoping to hear from you, due to your input in the other 304 thread. I am a little hesitant to use an input transformer, would rather go with a tube. I came across the el84, and its big brother the 7189a which can handle the current and voltage while providing the close to the same gain when run in triode mode http://frank.pocnet.net/sheets/135/7/7189A.pdf

Thoughts?

Thanks!
EL84 would have a short life at 400V, and like many IDHTs, the triode curves are not-so-linear at high voltage.

The KT66 is probably the best IDHT for linearity, but if you need more gain, perhaps EL34 could be used.

Still, the EML20 is purpose-designed for this position, and any substitute will be a compromise in terms of linearity for the gain.
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Old 1st January 2015, 02:42 PM   #6
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Heyboer quoted me $90/ with a 4 week lead time. Yes I understand that the el84 is not able to handle the power but the 7189a should be able to, at least from what I'm seeing on the spec sheet. I'll take a look at the kt66 and see if any of its variants can do what I am looking for. On a side note, the original poster with this schematic was getting a lot of feedback in regards to improvements that could be made in the schematic. This was the last version of that process before he lost interest. Does anyone see anything that may need tweaking? I'll have to get psud installed anyway so I appreciate everyone's input as I familiarize myself with this design. I am looking to do a build thread once I settle on the parts list.

Thanks!
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Old 1st January 2015, 04:31 PM   #7
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You might consider the 3C24/24G as a relatively inexpensive alternate to the EML 20A/B - that's what I did when the AVVT AV20s became unavailable.
They sound great, but be forewarned that their filaments draw considerably more current vs the EML 20 A/B.
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Old 2nd January 2015, 05:03 PM   #8
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I took a look at the 3C24/G and it appears to have a number of attractive qualities. Can someone confirm my suspicions, to achieve 42 ma plate current @ 400v, I will have to run this tube with ~8v of bias? Are there any other changes that will be required to the schematic to besides increasing the NIMH battery voltage and higher filament voltage/current? What is an acceptable grid choke size?

Spec Sheet: http://www.mif.pg.gda.pl/homepages/f...114/h/HK24.pdf
or, http://www.r-type.org/pdfs/3c24.pdf

Thanks!
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Old 2nd January 2015, 05:57 PM   #9
Gluca is offline Gluca  Italy
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I'd rather use an input transformer instead of a grid choke.

400V/42mA looks to be in positive grid area ... you'll need some power/current to drive its grid. i fear you should run it at higher plate voltages.
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Old 2nd January 2015, 06:19 PM   #10
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At 400 vpg and using an R(k) of 910 ohms, the 3C24 drew approx 6.5 mA.

Last edited by dowdylama; 2nd January 2015 at 06:26 PM. Reason: clarity
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