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71A Tube Buffer Preamp

99.4.72.198
Posted on September 15, 2009 at 12:57:48
chrismercurio
Audiophile

Posts: 52
Joined: August 4, 2008



All,

I'm looking for help and opinions on the attached schematic. The following is based on lots of reading and a little bit of algebra. It is admittedly derivative of work by others. Will it work?

Thank you in advance,

Chris

If it was me, I'd do this., posted on September 16, 2009 at 09:25:10
JLH
Audiophile

Posts: 639
Joined: June 25, 2000

I think this is a well thought out project. Any parts that are not really required are removed and the parts that are there, perform more than one function. I have not heard this unit, but can't imagine it sounding anything but good.

Rgs, JLH

RE: If it was me, I'd do this., posted on September 16, 2009 at 13:53:18
chrismercurio
Audiophile

Posts: 52
Joined: August 4, 2008
 In all fairness, mine is almost as simple as Doug's. If I counted all of the pieces used in a generic CCS, I'm sure I'm using less parts. Since I haven't experimented with a CCS vs bias resistor and cathode bypass cap I cannot comment on the sound difference.

Thanks,

Chris

CCS on plate, posted on September 16, 2009 at 17:11:31
nc
Audiophile

Posts: 315
Location: San Francisco, CA
Joined: November 19, 2003
 will solve your excess voltage issue.

Pretty much what my Royal is...., posted on September 16, 2009 at 05:59:59
Gordon Rankin
Manufacturer

Posts: 2247
Joined: June 9, 2000
 Chris,

My Royal Preamplifier is almost exactly your design sans the TL404. I use a 6V4 rectifier and a custom loading choke and a custom inverting MQ 10K:10K transformer.

But I use the P&G volume control and don't use the TL404 as the 71A has little gain and it would be all lost pretty quickly in the TL404.

Even the 600 ohm tap would then only give you an overall gain (excluding the loss on the input transformer) of 1.02 or litterally a buffer.

Thanks
Gordon
J. Gordon Rankin
Gordon,, posted on September 16, 2009 at 17:28:54
nc
Audiophile

Posts: 315
Location: San Francisco, CA
Joined: November 19, 2003
 Using the same schematic as discussed here, have you tried 10Y/801A in a linestage with lower (less than 260V) voltage?

I read a comment of yours saying that you like to run 801A at 33ma - 40ma, as higher current results in less Rp, but that was in the context of an output type loading a 5K or 10K output transformer.

What?, posted on September 17, 2009 at 05:38:22
Gordon Rankin
Manufacturer

Posts: 2247
Joined: June 9, 2000
 nc,

I have never used the 801 so I don't know where these comments come from.

The 10 operating points in the manuals are weak. I remember when Larry Moore was making that 10 squared amp and he called and we were talking and he was using book operating points for everything.

I said look at the disapation and the book operating points. They are just silly.... for the output you should be pushing the 10 to higher currents that lead to lower Rp. Which he did..

I would not consider the 10 for a preamp as there is too much noise in the heater.

The 71A is a perfect tube for a preamplifer.

Thanks
Gordon
J. Gordon Rankin
RE: Pretty much what my Royal is...., posted on September 16, 2009 at 08:50:42
chrismercurio
Audiophile

Posts: 52
Joined: August 4, 2008
 Gordon,

Thank you for taking the time to comment. It was (obviously) heavily inspired by your work and many of the posts you have made in the board about proper implementation of the 71a in a buffer or low gain preamp. My interest in the 404 came after hearing the Doc B's spud headphone amp.

I see 2 options based on your post...

1. Turn it into a buffer preamp and replace the TL-404 with the B7-5K and keep the operating points roughly the same.

2. Leave the TL-404 and raise the operating point of the 71a closer to the max operating point of 180v and still have a moderately low gain preamp with a powerful enough headphone amplifier. This is the more complicated and more expensive option.

Now the questions:

1. If I left the operating points the same, is there a more elegant way to lower the voltage than the big bleeder resistor at the output of the rectifier tube?

2. What are the cons of running the 71a at 180v instead of 80v?

Thanks!

Chris

RE: Pretty much what my Royal is...., posted on September 17, 2009 at 05:42:50
Gordon Rankin
Manufacturer

Posts: 2247
Joined: June 9, 2000
 Chris,

I would not rule out the TL404 for a buffer. Either way, if you use the B7 or the TL it's still a buffer as it would not have any gain.

This really does not matter what voltage you run it at it's still pretty much the same gain.

I run them pretty low because it works for my circuit. You will have to try this out and find out what sounds best for you.

Thanks
Gordon
J. Gordon Rankin
TL-404 is an autoformer, posted on September 15, 2009 at 17:16:43
mwiebe
Audiophile

Posts: 182
Joined: November 3, 2001
Contributor
  Since: January 5, 2006
 Chris,
The way you have the output wired will not work.
Matt

RE: TL-404 is an autoformer, posted on September 15, 2009 at 18:42:29
chrismercurio
Audiophile

Posts: 52
Joined: August 4, 2008
 Can you be slightly more specific? Is it because the shape I used isn't an autoformer?

Thanks,

Chris

RE: TL-404 is an autoformer, posted on September 15, 2009 at 19:24:13
mwiebe
Audiophile

Posts: 182
Joined: November 3, 2001
Contributor
  Since: January 5, 2006
 Chris,

That is not the schematic symbol for an autoformer, use the symbol of a choke if you have it. It's important to get it right visually because it reminds you there is no isolation of any kind in an autoformer. There is just one continuous wire from top (B+) to ground.

For this reason never put the parallel feed blocking cap between the autoformer and ground as all the taps will be at the B+ DC voltage. In your case your headphone jack would be 80Vs over ground if you moved the parallel feed blocking cap to the ground leg. Moving the parallel feed blocking cap to the ground leg in your drawing could lead you to think this was okay.


I've never run a 71a this low on current but it would probably work fine. However you are using the wrong power transformer for this circuit. I realize it may be handy but you have dropped quite a bit of money on the TL404 and your circuit deserves a power transformer of much lower voltage.

I would think sizing the circuit to fit this power transformer would also be a nice sounding option.
Matt
RE: 71A Tube Buffer Preamp, posted on September 15, 2009 at 13:55:16
nullspace
Audiophile

Posts: 203
Location: Philly, PA
Joined: June 18, 2004
 Hi Chris,

If Rk and Ck are hooked up to one end of the filaments, I believe the other end ought to be floating and not attached to signal ground. I've seen schematics where, when using DC, Rk and Ck are attached to a SPDT switch and swapped back and forth from one side of the filament to the other to save on wear and tear. Also, if it were me, I'd bump up the size of Ck to >=47uF, but that's going strictly by a rule of thumb: Ck>=gm/30.

As you have the PS drawn, the B+ dropping resistor will be ginormous and regulation not that great. I would suggest putting the LC section first and the RC section second. And, since you have the voltage available, maybe also try the 135Vpk, -27Vgk, 17.3mA operating point.

Good call on the SLA battery, and if you want a steady voltage you will need to have a sizeable battery as you've noted in the schematic -- I wouldn't go any smaller than 4.5aH. An alternative might be to use a 12V battery and some sort of SS current regulation. VT52.com has many fine examples.

I fiddled with Voltsecond's worksheet and your 4.7uF parafeed cap looks to be a good choice.

I suspect that it will be a very nice sounding preamp.

Regards,
John

RE: 71A Tube Buffer Preamp, posted on September 15, 2009 at 13:14:45
nc
Audiophile

Posts: 315
Location: San Francisco, CA
Joined: November 19, 2003
 Why do you need to drop voltage to 80V? Otherwise, it should "work".

RE: 71A Tube Buffer Preamp, posted on September 15, 2009 at 13:34:37
chrismercurio
Audiophile

Posts: 52
Joined: August 4, 2008
 My calculations for the cathode capacitor and the bias resistor were based on the plate voltage = 90v. Since it isn't an amplifier and I was looking for something simple that provided close to unity gain, I don't need to run it any hotter than that. Should it be run closer to the 180v max?

Chris

RE: 71A Tube Buffer Preamp, posted on September 15, 2009 at 13:49:46
nc
Audiophile

Posts: 315
Location: San Francisco, CA
Joined: November 19, 2003
 But you are adding complexity by dropping voltage; and certainly if you have been to the tubediy forum, you are adding DCR and you know how, right or wrong, DCR is the keyword over there. I think you may want to ask around and see what is the best sounding op point, and just use that.

Also, you may want to search "filament bias" over at tubediy. I have seen DIYers use "filament bias" for this circuit.

RE: 71A Tube Buffer Preamp, posted on September 15, 2009 at 18:51:27
chrismercurio
Audiophile

Posts: 52
Joined: August 4, 2008
 I can't find a "best" operating point. Gordon Rankin recommended 75-80v @ 10mA here:

http://db.audioasylum.com/cgi/m.mpl?forum=set&n=46444&highlight=71a+Gordon+Rankin&r=

Since he uses the 71a in a commercial product, and has/had MANY reputable products to his name over the last 18 years he is in my mind a very credible source. I'm sure there are more elegant power supply solutions than the ham fisted method that I came up with for dropping that much voltage.

Best,

Chris

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