|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
76.204.95.187
All the potential output tubes I have on hand for P-P use are unmatched, but I'd like to use them if possible by applying a CCS and/or maybe gartering if needed.
In looking at a schematic for a 6AS7 push-pull amp, I see that the builder uses a full, cascoded CCS for each cathode using two DN2540s per tube section. Recently I saw a schematic for a 6CW5/EL86 push-pull that uses only a common CCS in the cathodes.
I think the DN2540n5 in the TO-220 package can handle up to 150 mA of current and dissipate up to 15W with a good heat sink. What are the considerations for using separate or common CCSs in the output stage?
To be honest, I'm just trying to keep things as simple as possible and keep component count down as well.
Thanks for any help in understanding this.
I found this post from diyAudio member "Shoog":
"If you use a single CCS for an output pair you need to include a current balancing pot to null current imbalances and minimise static core saturation.
"The better way is to have independent CCS for each cathode and bypass them seperately. This can be turned into a differential pair by connecting the two caps cathodes together and referencing the node to a 1meg resistor, which then goes to earth. This unfortunately puts your caps in series and so effectively halves their value - so a cap of twice the size is needed. Bypass the caps with film caps and bypass each valve cathode to the other with a film cap. This works very well and needs no rebalancing as the valves age."
I'm having some trouble picturing his suggestions.
Here are some links to examples:
Common CCS in a KT88 PP:
http://jastrid.xs4all.nl/Dalojan/Schema_KT88PP.html
Common CCS in an EL86 PP:
http://lagarto.web.infoseek.co.jp/el86pp.htm
Individual CCS in a 6AS7 PP:
http://www.triodeguy.com/Triodeguy%20PDF%20files/6AS7%20CURRENT%20BALANCED%20PP%20AMP%20SCH.pdf
Thanks for any of your recommendations,
--Jeff
Follow Ups:
I appreciate all the references and your experiences. Nothing beats well-matched tubes, but I'll play around with these ideas using the tubes I have on hand and see what happens.
Jeff
Here is a link to the odd blocks
they use a CCS for the KT88 output tubes
http://diyaudioprojects.com/Tubes/KT88-Push-Pull-Tube-Amp/
Jeff,
I've been using, for several years, a scheme similar to the KT88 post you refer. Mine uses 1 ohm resistors for measuring the individual currents and a low ohm pot for a small amount of balancing potential. I think it works great and sounds even better. If you can live with the class A power limitations that are imposed by this scheme then I think you'll like it. I do have several sets of output tubes and I check the tubes periodically to make sure they are matched fairly well. My tubes are pentodes wired for triode operation.
Best,
John
De gustibus non est disputandum
Read the bottom 4 posts in this thread and see what KevinC (Kevin Carter)has to say about using balance pots with a single CCS.
Cal
I appreciate the link. I guess I will do what I can for somewhat mismatched tubes.
I question what will happen if the tube is driven into Class B?
I've just used the K&K rig as a plate load in my pre with amazing results.
The Supertex seems to be the one of choice for power handling on top and a 2SK369 JFET on the bottom, which can give more detail. Naz used this setup for his CCS.
...so if you try to overdrive it into class B, you will just get clipping (well, maybe a watt or so of class A2, then clipping.) Not that I would be against this approach, it just depends what you want. Designing for class AB will allow more power, but probably requires at least a little global NFB for best sonics. Not that NFB is automatically a bad thing either...
JJ
How do Pentodes fair drawing grid current... if such were to occur?
Thanks,
W
Good question. I'm not an expert, but I think it depends a lot on the specific tube type and circuit. When triode connected, some pentodes may tolerate significant grid current without damage or excessive distortion; I seem to recall hearing that this is especially true of some 6L6 equivalents. This might still hold to some extent in ultralinear operation. In straight pentode operation, I would think the tube could hardly go into A2 or AB2 at all, because the Vg1 = 0 curve is already almost at zero plate voltage. I think there were some old Macintosh amps that were designed to go into AB2, but their OPTs had those special cathode windings, so again that was not exactly standard pentode operation.
I have done it this way with good success.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: