SO... I have this pre-amp from NAD from the 80's. It puts out 16vrms from its HI RCA, 8vrms from norm. I believe. Way plenty for an EL84, so why not drive the tube directly from the output?. A couple coupling caps, some chassis punches, binding posts, blah blah blah , slap on a power transformer and some outputs... and VOILA! an Integrated EL84. Like A monkey on the back of the NAD. Have I just smoked too much or is this a second chance for an 80's preamp that will otherwise play center-stage in a dark closet under some photo albums? Never seen a mutant pre with a tube output stage growing out of the top of the chassis. Any comments? Has anyone tried this? Have I been awake too long?
It'll work fine: the drive requirements of an EL84 are trivial. If you search, you'll find EL84 spud amp designs out there.
Do you mean the NAD doesnt have enough &@$$% (current) to drive the grid of an EL84 in single ended UL, or Pen-toad? Or not enough voltage gain? It's easy enough to throw any sorts of tube front-end on the EL-84, but my question is WHY (or why is it a bad idea) can I not drive the grid of the EL84 directly from the NAD output?
too slow..
too slow..
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The NAD has plenty of current drive for an EL84, even in triode, but it's real marginal on voltage. Yes, it will make it, technically, but you may be way out at the nonlinear ends of the swing to get full modulation - and with a SE EL84, even in pentode, you are going to need full modulation.
Aloha,
Poinz
AudioTropic
Aloha,
Poinz
AudioTropic
I'll go further. 😀 You can go to the bank on the "sand" preamp having a low O/P impedance. Couple the 16 VRMS O/P to a 1:2+2 step up phase splitter trafo. That gives you enough Volts to drive PP triode wired EL34s. Power O/P will be quite adequate for mating to 91 dB. speakers. A $50 AnTek AN-4T360 is quite adequate for powering a quad of EL34s.
Now THATS what i'm talkin about! I want to go easy on the weight factor on the top of this chassis though as the chassis for my tube projects are usually a bit stouter. I was thinking S.E. because i have a collection of 5K:8OHM transformers from Japanese R2R's. Cool, Thank for the confidence boost, I ill post pics of the spawn...
How are you going to handle the mandatory NFB in pentode mode? A triode wired EL84 is good for about 2 W. Do you have the appropriate high efficiency speakers?
SE with the "iron" you have is fine, but use triode wired 7591As, which are as easy to drive as EL84s. NFB will not be needed. Put the PSU in an outboard box and connect with an umbilical.
The sky is the limit.. I am a huge Fostex fan, FE206, FF225k, and two sets each of RM760 and RM780 co-axial studio monitors. I have a quad of Baldwin 7591's but I have a Soft spot for the 6BQ5's. I'm not against GNFB if used properly. My goal here is 4-10 watts of love just because.
The only other point is that most sand amps have a heck of a lot of internal circuitry in them. For me the beauty of a tube amp is the small number of active stages to screw things up.
(I'd rather not mess the signal than mess it and then try and correct it later with science).
That being the case, by the time you've put in a PSU for your output tubes, your output trannies and interstages (if you take the PP option suggested) the question isn't so much "can you?" as "why would you?".
90% of the work to make a functional amp has already been done. Seems to me that you'd get a better result going the whole hog than frankensteining the NAD.
(I'd rather not mess the signal than mess it and then try and correct it later with science).
That being the case, by the time you've put in a PSU for your output tubes, your output trannies and interstages (if you take the PP option suggested) the question isn't so much "can you?" as "why would you?".
90% of the work to make a functional amp has already been done. Seems to me that you'd get a better result going the whole hog than frankensteining the NAD.
Just looking on the WEB, how about the high voltage Op amps and line drivers. Some swing 30V. That would seem like a heck of a phase splitter for a small PP. Just a thought as I don't know any better.
McTavish,
I see a way clear to use the R2R O/P trafos and "12" W. multi-grid power O/P tubes. Members of the 6V6 family also work with the "iron" and they draw less heater current than the EL84. 😉
The O/P trafos have be set up to invert the signal. That way, GNFB can be applied to the tube's cathode. Full pentode mode will yield approx. 5 WPC. Regulate g2 B+ with LR8 regulator ICs.
The inexpensive way to get a decent B+ rail is by employing a Greinacher ("full wave") voltage doubler. A Triad N-68X isolation trafo from Allied is low cost and quite up to the needs of this project. Perhaps a "wall wart" for heater power.
I see a way clear to use the R2R O/P trafos and "12" W. multi-grid power O/P tubes. Members of the 6V6 family also work with the "iron" and they draw less heater current than the EL84. 😉
The O/P trafos have be set up to invert the signal. That way, GNFB can be applied to the tube's cathode. Full pentode mode will yield approx. 5 WPC. Regulate g2 B+ with LR8 regulator ICs.
The inexpensive way to get a decent B+ rail is by employing a Greinacher ("full wave") voltage doubler. A Triad N-68X isolation trafo from Allied is low cost and quite up to the needs of this project. Perhaps a "wall wart" for heater power.
That's like bringing a knife to a gun fight. Some of the National LME chips will swing almost +/-100V and deliver enough current to drive A2 or AB2.
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