Really low parts could el84 pp design for a beginner?

[nightanole]

Ok i got a guy at work who wants to rebuild a gutted fisher stereo el84pp amp. All it has left is the sockets and transformers. so each side has 3 sockets, 2 for the el84's and one for a 9pin 12ax7 or something. The output transformers are 8k and the b+ is 320v unloaded.

He wants to modernize it with low gain(running off an older nad cd player) and run it in pentode mode for 15-18 watts to run some 85db book shelfs.

I have all the parts for a diyparadise simple el84 pp with a lm317 in the tail for the power stage, but i got nothing for the driver stage, and i dont know how to enable a feedback loop, which i know he will need since he will need since he wants pentode mode.

Any help with a simple driver stage with feedback (do they made local feedback output stages)? I think he would be happy with a el84 for the driver since it would mean the same tube for everything.

[Eli Duttman]

Putting a LM317 in the O/P stage tail forces Class "A" operation. So, expect only 10 WPC. A yield of 15 WPC is about right from Class "AB" pentode mode. Either way, background music is what he'll get with those 85 dB. speakers. Per Paul Joppa's 102 dB. rule, 85 dB. speakers need at least 60 WPC, for serious listening.

Assuming the limited power yield is OK, there's more than 1 way to skin this cat. Naturally, I'm partial to things I've been associated with. An "El Cheapo" variant, with "fixed" bias pentode mode O/P tubes, should be satisfactory. The 12AT7 splitter/driver will require ZVN0545A source follower buffering. By any chance, does the Fisher power trafo have a bias winding or tap?

[nightanole]

Hmm i have a lm317 setup from diyaudioprojects that uses a 300v b+ instead of 210 b+ in pentode mode. i just assumed it was putting out 15 watts instead of 7watts due to the higher b+ and pentode move vs stock ul mode. Oh well.

http://diyparadise.com/buildel84b.html
Thats not 15 watt in pentode mode? Again he doesnt have a feed back loop, but he uses ul or triode mode so its not needed as much.

Yes the power transformer has a 2 wire bias out that is 40v.

[Eli Duttman]

Thats not 15 watt in pentode mode?

Repeat, the tail CCS forces Class "A" operation. Theory tells us that 50% efficiency is max. for Class "A". Pentodes are pretty darned good real world and will deliver 40% efficiency, in Class "A". If each tube's plate is dissipating the max. allowable 12.5 W., the amp delivers approx. 10 W. To get 15 WPC, the net efficiency must be 60%. Theory states 78.5% is max. in Class "B". A Class "AB" operating point, which is necessary for decent sound, must deliver lower efficiency. Getting 15 WPC and decent sound from a PP pair of EL84s requires considerable care.

The separate 40 V. winding is GOOD news. That winding can be bridge rectified to provide B- ('T7 tail CCS) and C- (EL84 control grids).

BTW, full pentode mode "finals" work best in combination with regulated g2 B+. A Maida regulator will take care of that item, quite nicely.

How did Fisher rectify the B+: FWCT, bridge, or Greinacher doubler? Was the "donor" an integrated amp, with a phono section and tone controls?

[planet10]

Originally Posted by Eli Duttman

Repeat, the tail CCS forces Class "A" operation.

A common CCS -- and i was told only works in triode (Allen Wright). A pair, as in Baby Huey, allows AB operation.

dave

[nightanole]

LOL i dont want to start another argument thread. I just wanted to get a simple amp going. If its going to take more wiring and parts to get another 5 watts its not worth it. thats not even 2db.

Still on topic of a pentode setup with some feed back so he doesnt get too sloppy of output.

The b+ is FWCT, i was planning on converting it to SS Since the whole unit is tubeless right now. No the amp has no controls, just 2 rca in's.

[Eli Duttman]

Snubbed UF4007s will do quite well to rectify the B+. Keep the 1st filter cap. size reasonably small, to prevent trafo overheating, and install a Triad C-24X choke between the 1st cap. and the reservoir part.

Why bother with a CCS in the O/P stage tail, to begin with? Believe me, between infrasonic filtering at the amp's I/Ps and the GNFB circuitry, you'll do just fine. Remember, Fisher built with good "iron".

[nightanole]

El cheapo looks good. I got the negative bias supply, But can the ccs be replaced my something simpler?

YH's Lazy Man Constant Current Source
LM317 / LM338 / LM350 Voltage and Current Regulator Calculators

And do i need a cap at the input?

And what if i dont need a volume pot, or just have left over 50k's or something. I can just omit it and not change any other values right?

[mashaffer]

Why not keep it simple and use something like a 12AT7 or 6N1P as a CC VAS and cathodyne PI with feedback from OPT secondary to VAS cathode? The only really tricky part I see is getting a stable amp with gNFB without a scope but you are going to need some kind of FB with pentode mode. The other option would be to use a LTP PI and plate to plate FB over the EL84s which won't have as much stability concern as global. For LTP you could go CCS as mentioned or add a small PT and some Si diodes for a negative rail and use a big resistor.

I think that you would have to abuse the EL84s a bit to get 18 hi-fi watts out of it but a dozen should be doable and adequate for a reasonable size room.

[nightanole]

Ok that first sentence was waaaaay over my head.

This is only going to be full tube amp #4 for me, and #3 is still in production and waiting on edcor (COME ON I ORDERED MID FEB!!!)