I've become so busy the last few months and wasn't able to upload as much as I intended to. So here's what's in the pipeline for the coming year.
 |
| My one and only... |
 |
| The JP757A - a John Piro (re)creation, amongst others... |
 |
| Revisit MM/MI phono cartridges |
 |
| American Classics |
 |
| Japanese Idler Drive(s) |
 |
| '68 G401 |
 |
| DIY tube DAC |
 |
| Tonearms are like violin bows |
 |
| my camera adventures |
Happy Holidays and a Great New Year to Everyone!
Hi Joseph. I have been reading your interesting blog for many years now. I built a very early version of your 2A3 SE amplifier, the one that uses two sections of a 6SL7 in parallel as a simple driver. It sounds good, but I have seen that you continued to experiment with different driver circuits: Type 76 driving a 6SN7, or 6SN7 driving another 6SN7.
ReplyDeleteCan you please tell me from of all the different driver circuits you tried, which one is the BEST? I will guess that it is Type 76 driving one triode of a 6SN7? Am I right?
My speakers are 95 dB FOSTEX full range drivers mounted in a back-loaded horn. My room is quite small, so 2.5 Watts per channel is enough to provide very good volume.
Thank you for your advice.
Dennis
Prescott, Arizona
USA
Hi Dennis,
DeleteAfter being in this hobby since the 80s, I've learned that there's no such thing as THE best. There are just way too many variables, synergy between components, type of music...BUT the greatest of them all is personal taste.
The 76/(1/2)6SN7 input/driver is the most sophisticated technically. It has the widest bandwidth and can squeeze the most power out of the 2A3. To my ears it offers the most extension in the frequency extremes. The 6SL7 goes against the established rules of audio due to the Miller effect, but it is very musical to my ears. I still use that topology for my SE45 and SE46 amps. Somewhere in between is the Radiotron SE2A3 using a pentode (6C6 or equivalent) input/driver, which is a reasonable compromise between the two.
In my man cave, the 76/6SN7>2A3 sounds better driving the Altec 755A in Silbatone cabs than the 6SL7 or 6C6 fronted 2A3. As I type, I'm listening to the Altec 753As driven by a Radiotron 2A3 stereo amp. Through the years, I grew to appreciate the synergistic match the Radiotron 2A3 amp has with the 2-way Altec, especially as it evolved to the Altec 753A.
Since you're quite handy, I urge you to build both the 76/6SN7 and Radiotron 2A3 circuits and let your ears decide what is best in the context of your system, room and musical taste.
Thanks for the kind words and all the best in the New Year!
JE
Thank you for the reply, Joseph. It's nice to communicate after so many years of reading your posts!
DeleteI once measured the parallel 6SL7/2A3 amplifier and it started to roll off at 19 kHz (-1dB) - and that was no doubt due to the Miller effect you mentioned. Since at my age (65) I cannot hear much over 12 kHz, I didn't find that to be a problem. Incidentally, my OPTs were taken from an old Sansui tube receiver, which I took apart, separated the "I" and "E" laminations - and then put an paper air gap between them! Now THAT is being an audio cheapskate!
I also find your 6SL7 driver to be overall musical, but the amplifier does have a fair amount of upper vocal "glare" or "shout" that could be the driver, the "homemade" transformers, or it just could be the amp going into clipping, as my speakers are only 95 dB efficient. I'm leaning toward the driver just not being robust enough. BTW, I'm using all great NOS tubes, as I'm very fortunate to have a large collection: NOS Sylvania JAN (1940s) 6SL7 and 1940s NOS RCA 2A3s.
I'm curious if you ever tried running one channel in one of the 6SN7 triodes, the the opposite channel in the other triode? This would give me a 3-tube driver counting the 2 Type 76s. I tried to research if there are adverse crosstalk effects, and the best I found is that due to capacitive coupling, the isolation is approx -50 dB at 1 kHz, dropping to ~-30 dB at 10 kHz. That's still a fair bit of isolation, but...if you think "sharing" one tube for two channels is a bad idea, I'll punch two extra socket holes in the chassis and use two separate 6SN7s just to be sure.
All the best and Happy New Year to you as well, kind sir and brother of the audio constructor's community!
Dennis
Prescott, Arizona
USA
Upper midrange glare has never been an issue with the 6SL7 input/driver stage in the context of my system. Neither do I think it's your PP OPTs reconfigured for SE because I used PP OPTs with no air-gap in the original Simple 45/2A3 article.
DeleteI have very limited experience with Fostex drivers but it is known in the DIY community that some models have upper midrange glare. Maybe there's a notch EQ/filter circuit for your particular driver? That's probably the best solution.
Back in the day 6SN7s were cheap so using 2 x 6SN7s was an aesthetic decision. I just wired them so that when I interchange tubes between channels the other half is used.
JE
Very CLEVER to wire your 6SN7s to use the opposite triode!!! Just swap them when they get weak!
DeleteYou make a good point about the FOSTEX drivers perhaps being some upper mid glare. Since I'm running 100% digital sources, I can experiment with DSP EQ to drop that region a dB or two and see if she smooths out! Thanks for your insights!