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In Reply to: RE: your favorite driver tube for 300B posted by cboy168 on November 20, 2009 at 15:50:28
Timely question.I have been looking into designing something similar, though I will be using Lucus' Black Art Audio transformers over here in Aust. I am very glad he is winding again.
I have considered a few options and read various sources of info ad nauseam - analysis paralysis would be appropriate... though "analysis" may be somewhat too generous in my case. I digress.
The high transconductance tubes and pentodes-as-triodes seem to appeal technically, yet folks seem to try them, then move on after a while. Specifically regarding the 417A, it reputedly performs more to most people's liking at 10mA bias, resulting in higher output impedance, consequently affecting i/s transformer suitability/choice. Recently I have read comments from folks who seem to be i/s transformer coupling devotees that do not like high-gain high-transconductance driver tubes.
It seems that many folks settle at the 'SN7 cascode and 'SL7 arrangements; however, I would guess that these people are not really into experimentation and/or are looking for a specific type of presentation. There are exceptions, for example Steve Brown, who has tried many configurations and is well respected, coming back to the 6SN7 recently and enjoying it.
The AV20 and AV30 (Edit: that is old AVVT verbiage - current manufacture is EML 20A/B or 30A) series tubes look nice; however, they are available from only one manufacturer, have only moderate output impedance for i/s transformer coupled driver duty and, depending on your system config, will likely require an extra stage of gain.
For not big $$$, you could use the 5687, 7044, 7119, 6N6p, ecc99, 6H30 - similar gain and lower output impedance than the EML 20 series, though they are not DHT. These seem like they would be solid drivers - they are some folks' faves - and you should be able to find a flavour you like amongst that group. Also, if you take Paul's advice and run a separate power supply for the driver, you could have the multiple transformer taps to allow you to run different voltages to the driver stage, allowing you to give the EML 20 a crack if you wished.
There are also the trioded output pentodes to consider: 6V6, 6L6, and EL34 etc. which many folks have used and also like very much. Hell, if WAVAC uses them... Low output impedance, nice gain for a power tube(though lower than the EML 20), available from a variety of sources of current production, though are probably higher distortion than something like a 45 (which may not matter to great degree), which brings me to what I would really like to do.
My current thinking - although likely not final and certainly NOT absolute - is that if you wish to gain a decent enough portion of the potential of i/s transformer coupling to justify its inherent expense, it is worth doing well. The definition of "well" and specific recipes will vary between different folks, but a dedicated high-ish current/low output impedance driver and well-designed i/s transformer is an important part of my recipe. Two well-designed stages may be 'better' than one pretty decent stage. Currently, I am considering the 45, though its low-ish gain limits my 1st gain stage options a little, though that may not be a big deal.
As you would know, the correct (is there such a thing?) decision for you will depend on a number of ‘technical’ and other factors such as whether you are wanting to use the amp as a test bed for further experimentation, cost, heat, space availability, appearance, component availability, technical ability, artistic orientation/ability…
Cheers
R
"As long as we have any intention to be right... we should be wary. So long as words have the slightest ego attachment, they are dishonest."
Charlotte Joko Beck
Edits: 11/21/09Follow Ups:
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