6BQ5/EL84 tube buffer questions

[rafjr00]

Hello everyone,

I have been doing quite a bit of reading on building a simply tube based buffer to experiment with. There is a great and at times confusing amount of information on this subject.
Easy Cathode Followers (Page 2): March's Circuit of the Month
Please see the Easy Pentode Circuit section.
If I add a 170v power source is this a functional circuit? Also would it be possible to wire in a switch to go from pentode to triode?

I have a Denon PMA-900V, I would like to use this in the processor loop so that I can take it in and out of the audio circuit.

I would appreciate any help and opinions.

[rotaspec]

I'm fairly new to tubes, but why would you want to use a power tube like the 6BQ5/EL84 for this?
Surely a small signal pentode or triode would do fine, and save you a lot in the power supply section?
Do a search on "cathode follower" or "tube buffer" here; you should get plenty of results.

Gary

[aardvarkash10]

kinda depends what your source is and what you are buffering to.

for buffering a line signal before an amp (ss or hs) the EL84 is overkill imho - there are plenty of smaller cheaper signal level tubes that would fit the bill and would operate closer to their optimum operating point - just one twin triode tube would do it for two channels.

In answer to the question from the op - yep, it will work. I haven't done the math so I couldn't say whether is it optimal.

[rafjr00]

Gary and Aard,

I believe that both of you are right in that the EL84 is overkill, the idea came from the link I provided. That is why I asked about being able to switch it from pentode to triode with a switch, as I would open up my experimentation options. I do not have my heart set on this tube, but I do like trying things that people "shake their heads" at. Aard as I stated in the first post I would like to use this in the processor loop of my PMA-900V which has a input of 150mV/30K ohm.
Please let me know what information you need and I will do my best to get it.
I am not looking for the "best" sound, as a matter of fact I am in search of tube distortion albeit the good kind. I am not a purist, I am simply looking to advance my very limited knowledge, have a little fun and to say "look what I made."
Thank you to both for your comments so far and please feel free to offer more.
Riki

[rafjr00]

What is the difference between a buffer and a single input/output preamp?

[Miles Prower]

I don't like it. If you want to supply a pentode's screen from an active decoupler, a transistor would be a far better choice for that since the drain or collector is a good deal more insensitive to AC on the rail than any triode.

Secondly, if you're going to use a pentode for a cathode follower, the screen voltage must follow the cathode voltage, otherwise, it's not a pentode anymore. That means either an independent, floating screen supply or a series dropping resistor with capacitor coupling to the cathode. Pentode cathode followers are one case where you don't want to stiffen the screen supply.

If the screen-to-DC ground voltage is fixed, then Vsg moves with the cathode voltage. That's not what you want (it's more like ultralinear operation then). Vsg has to move with the cathode to make Vsg a constant.

[aardvarkash10]

A buffer does exactly that - its buffers the output of a device (like a CD player) from the input of the following device (like a power amp). Usually, it is used to ensure a very light load is presented to a source while providing a low Z (and so by implication high current) input to the following device.

They normally do not amplify or alter the signal, just increase its ability to feed a heavy load.

A pre-amp typically amplifies the signal, and may ALSO (but not always) have a low Zout

OK, I'd go with a simple 12A*7 cathode follower buffer.

Easy to build, a wide range of easiliy sourced tubes with the same pin-out for tube-rolling, only one tube needed (appeals to the el cheapo in me!), simple to understand.

Miles is much more onto the technicalities than I, but for a simple to read and practical start, go to valvewizard's page on cathode followers. All the calcs you need are there. Bear in mind he is primarily interested in guitars, so mild distortion is his forte!

[rafjr00]

Whoa, that went so far over my head, I don't even think I saw it. :-)
What is the active decoupler in the provided circuit? The circuit is made up of resistors and capacitors, I thought those were passive devices? The screen or is it a grid, closest to the plate is connected to the cathode.
I think I have a good bit more reading to do.

[rafjr00]

So, since I am using the processor loop I don't "need" a buffer for its intended propose? The work for this applacation because they do not provide any gain/amplification.
As for the Valve Wizard site, AC or DC coupled?
Can I build the circuit shown on Valve Wizard's DC coupled Cathode Follower and start experimenting?
Riki

[aardvarkash10]

yes rick - build one. AC coupled is easier for you and no, as long as you use reasonable values of resistance and capacitance, it will do no damage.

They are simple (20 minutes soldering) to build, and easy to design. Try it - draw up what you think will work, then come back in and ask for positive critique.

Don't forget the power supply - you'll need 250-300V if you are wanting to use the 12A*7 series. You don't need a lot of current though - between 2 and 10mA per channel, so you can get away with a reallllly titchy transformer and lightweight filtering.

If you want to design a power supply go to Duncan's Amp Pages and download the PSUD-II power supply design software. My 10 year old can use this and it makes psu design a doddle. Also good for your learning.

Oh, and its free.

By now you should have a grip on my design imperatives - cheap, easy, cheap, simple to build, cheap, accessable and cheap.