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Old 2nd February 2009, 12:56 PM   #1
colinB is offline colinB  United Kingdom
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Question Tube Buffer – question?

Some background first: Over the past couple of years I have on and off built tube amps from schematics taken from the web. Initially I built a 5687 pre-amp and a EL34 triode connected SE amp attached to a FE167 full range speaker with reasonable success. I used this set-up in my garage for a few months using a cheap MP3 player as the source and it all worked great. Then over the next few months I really got addicted and I built more power amps, Darling DC coupled, Bevois Valley EL84PP and finally a 6EM7 DC coupled amp.

Then I decided to move some of the kit into the house and that’s when the problems started. When I set-up the preamp + any of the power amps with my CD player (Naim CD3) as source I had way too much gain (at 8-9 o’clock volume was way too high). So I built a 12B4 pre-amp to replace the 5687 pre. This was slightly better but sadly I found that I still had too much output (at 9 o’clock on volume pot.)

I then tried just connecting the CD player through a 100K volume pot to the power amp but the sound was a bit ‘flat’ & lifeless.

After some searching on the internet I came across this article that seems to offer a potential solution to my problems.

http://www.tubecad.com/october99/page8.html

Based on the article it looks like I can adjust the gain using the ratio of resistors (R1 & R2) to suit my set-up.

I have a schematic from the web using a D3A tube connected as triode: B+ ~270V, Ra=5K6, R1 = 100k, R2 = 150k, Rk = 82R (no bypass cap) Rl = 1M & C1 = 0.1uF

However, the real question though is what is the recommended/best way to connect the volume pot (100K) and buffer?

#1: CDP > Buffer (anode follower) > Vol Pot > Power amp > Loudspeaker or

#2: CDP > Vol Pot > Buffer (anode follower) > power amp > Loudspeaker

Any help is appreciated.

Cheers
Colin
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Old 2nd February 2009, 03:33 PM   #2
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Why would you use an anode follower (and an expensive tube!)for this application instead of the more generally recommended cathode follower?
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Old 2nd February 2009, 08:14 PM   #3
colinB is offline colinB  United Kingdom
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Hi,

Quote:
Why would you use an anode follower (and an expensive tube!)for this application instead of the more generally recommended cathode follower?
I wanted to use the anode follower as I could achieve a small amount of gain by adjusting R2, whereas with a cathode follower, as I understand it, gain is usually <1.

I take your point about the d3a tube though but as I already have these tubes available I'm good to go with these.

I guess the answer is build the anode follower try the vol pot in front of the buffer then switch to the other option and decide which suits best. I guess I was hoping some-one would already have experience of both options and could save me some time.

Cheers
Colin.
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Old 2nd February 2009, 09:12 PM   #4
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OK, well then, just a few comments.

1) IMHO, never build a preamp with a 0.1 uF coupling cap and a 1M output resistor, it's basically unusable except for with a homebuilt high input impedance amp. Even then 0.1 uF is really small.

2) With those values you listed, gain is 0.42 by my calcs. A cathode follower doesn't look so bad

3) With those values, I get about a 1400 output Z. I wouldn't just swap that 100K pot you plan to try on the input into that position.
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Old 3rd February 2009, 08:46 AM   #5
colinB is offline colinB  United Kingdom
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Hi leadbelly

Thanks for sparing the time to go through the calculations. It looks like I'm going in the wrong direction with this one. I think I'd better start doing some background reading on cathode followers.

A quick search on the forums here as turned up plenty of reading so I'll start with these.


Cheers
Colin.
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Old 3rd February 2009, 09:50 PM   #6
colinB is offline colinB  United Kingdom
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Default D3a Linestage build

Hi,

I didn’t have a great deal to do to-day so I out of curiosity I went ahead and built the D3A anode follower as described in post #1. It worked but probably not correctly.
I’ve attached a pdf file that shows the oscilloscope traces for 6Vpk-pk 10kHz square wave input signal and the corresponding output from the buffer circuit and output measured after volume pot at different settings of volume pot.
Strange thing is the buffer circuit didn’t appear to invert phase of signal and also circuit produce gain. This doesn’t fit with calculations carried out by leadbelly.
As I don’t really understand how these circuits work I’m unable to interpret the data from the oscilloscope traces although it looks like as the volume pot is increased the square wave is less well defined – probably a really bad thing.
Anyway, I guess I’ll just move onto a cathode follower now – it seems to be the recommended way forward. On the other hand I suppose ought to spend more time ready Morgan Jones instead of building more amps!
Cheers,
Colin.
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File Type: zip d3a linestage_1.zip (99.5 KB, 111 views)
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Old 3rd February 2009, 10:18 PM   #7
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How did you zip the file? WinZip can't unzip it for me.
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Old 3rd February 2009, 10:22 PM   #8
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Nor me either.
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Old 4th February 2009, 07:57 AM   #9
colinB is offline colinB  United Kingdom
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Hi,

I used Winzip 12 to compress the file - downloaded from here

winzip.

Perhaps I set the compression to the wrong setting? I found it quite difficult to get the file size down to the below the size limit set for posting here.

I'm at work now so I can't re-post it until tonight as the file is saved on my home PC only

Sorry about that!

Cheers
Colin.
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Old 4th February 2009, 10:27 AM   #10
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How's the CF turn out? Me too having problem with too much gain and have spare 12B4. Thx
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