Suggestions for a low power PP power amp

[Klaatuboradanikto]

Gary, Kool stuff! If you wouldn't mind a power supply drawing that would be awesome! Have you built one of these for yourself then? Sounds like you have. Thanks a million!
Klaatu

[mooreamps]

Gary, Kool stuff! If you wouldn't mind a power supply drawing that would be awesome! Have you built one of these for yourself then? Sounds like you have. Thanks a million!
Klaatu

No, I've not built one these myself.. Like I said, this is just off the top of my head, but it should be electrically correct.. The voltages depicted are estimates...But do use the values of the chokes as drawn, "very important".... Good Luck....

-g

[Steve Conner]

Cool design, Gary. I can't see any reason why it wouldn't work. But do you have a Hammond part number for the 60H choke?

The PI and power stage have been combined into what is basically a self-split power amp, as used in the AX84 Firefly and Mark Lavelle's old Decimator

If wanting to build a tiny pentode amp, the EF184/6EJ7 might be worth a try. It's not in current production, but there are lots of cheap NOS ones. The only small pentode I know of in current production is the EF86.

[redelephant]

Well, since you want to get the 'tone' of a 6V6/6L6 amp, and use current production tubes, I certainly can't help you much and Gary's suggestion is very interesting, but to comment on the use of 6dj8s:
(Oh,btw, if you can use NOS tubes you have all kinds of cool and crazy options...such as 12DT8 as preamp and 12AV5 as power tube, both types NOS for cheap. Actually that's not really low power stuff, so how 'bout the 11MS8 High-Mu Triode / Beam Power Tube like this:http://www.nj7p.org/Tube4.php?tube=11MS8 ? Can be had for little money I think.)

I'm actually working on a 48v power-amp using some 6dj8 (ecc88) triodes I had lying around. I have 4 of them each dissipating about 0.5W per triode halve (according to my probably misguided calculations), so about 4W in total... Sort of hard to find an output tranny though if go for the single ended approach because of the ridiculously low impedance I need (although edcor appears to have some cheap ones up my alley). Theres also the fact that I need an 8 ohm speaker, which I don't own... So I might just go the p-p approach so I can steal the 16 ohm speaker from my existing amps The 6dj8 just seemed like a good candidate because of the relatively steep grid lines so you can jam a lot of current through it at low voltages (plus I have a ton of them). I haven't really seen any other designs use them however...

I use them a lot, great for lower voltage stuff. Tho 48volts a bit low even for these, but what the hell, it'll work as a milliwatt amp You can try a 10W Edcor with 1700primary and 16ohm sec, into 8ohms. Perhaps using just the 40% tap and not the whole primary, for even less ratio? Might work ok?
Here's a similar project using 6dj8 as input/driver tube, and 3x5687s in parallel as output, per channel, in my V-8 workshop amp:

[mooreamps]

Cool design, Gary. I can't see any reason why it wouldn't work. But do you have a Hammond part number for the 60H choke?

Device Part Number
Hammond 60 H 155C
Hammond 150 H 156C
Output transformer 125a
Power Transformer 369AX 40 Va 250V C.T. @ 115ma. 6.3V C.T. @ 2A


Use this to drive a 4x12, and you will see what the electronics is really capable of..


-g

[Klaatuboradanikto]

Cheers all around! Good discussion and lots of creative idea's. I can't think of anything else to bring up at the moment. Thanks ALL!
Klaatu

[redelephant]

Hey Gary. Have you tried increasing the Iq-LED by placing a CCS or a resistor from LED-anode to b+? or is the current coming thru the tube enough for steady LED voltage and low noise?
Many years ago I tried LEDs in the cathodes and didnt like it much, but I wonder if it might have been the low plate current that didnt get the LED on enough. Ive been wanting to try a ccs to b+ to see if that gives me that loving feeling with LED bias. Just not enough time. Do you thinl it will have any merit?
Thnx.

[Klaatuboradanikto]

As usual I spoke too soon. Nuts! So here's my question: All things being equal what would you expect the tonal and feel differences to be between a EF86 vs. 6S_7 vs. 12AU7/BH7? Anyone care to throw their two sense in on that one?
Klaatu

[exclamationmark]

I use them a lot, great for lower voltage stuff. Tho 48volts a bit low even for these, but what the hell, it'll work as a milliwatt amp You can try a 10W Edcor with 1700primary and 16ohm sec, into 8ohms. Perhaps using just the 40% tap and not the whole primary, for even less ratio? Might work ok?

Actually I was planning to use that very transformer because it comes pretty close to the ideal load line. Only problem is, is that I don't actually own an 8 ohm speaker! There's a 5W 15k to 600 ohm transformer they sell that should give me 400 Ohms with a 16 ohm load too.

I've sort of figured out that a 4k load on ONE triode half would give me the ideal load and dissipate roughly 0.4-0.5W, so I'm thinking 8 triode halves in parallel (4 tubes) would give me an ideal impedance of 500 Ohm. Which isn't too far off 400 ohms. I'm assuming cause of the ridiculously low impedance it'll clip/amplify rather asymmetrically, which could be a nice thing.

[mooreamps]

As usual I spoke too soon. Nuts! So here's my question: All things being equal what would you expect the tonal and feel differences to be between a EF86 vs. 6S_7 vs. 12AU7/BH7? Anyone care to throw their two sense in on that one?
Klaatu

I believe it's the tube on the first gain stage that provides the primary contribution of the tone of the amp... The selection of the 6SL7 was selected based on your first post.. Then , since this was going to become an octal based amplifier, which will give you a nice "fat" sound, I selected the 6SN7's in the power amp since you need to have a tube that can sink a lot of current in which to energize the primary coil of the O/T.... Don't worry, my thinking is ; "all tube amps sound better than S/S."!

-g

[mooreamps]

Hey Gary. Have you tried increasing the Iq-LED by placing a CCS or a resistor from LED-anode to b+? or is the current coming thru the tube enough for steady LED voltage and low noise?
Many years ago I tried LEDs in the cathodes and didnt like it much, but I wonder if it might have been the low plate current that didnt get the LED on enough. Ive been wanting to try a ccs to b+ to see if that gives me that loving feeling with LED bias. Just not enough time. Do you thinl it will have any merit?
Thnx.

I already do something like that in my electronic gain control. But, even when I built my first 10 amp, well when I took a VJ and added a 6V6 to it, I've always tossed out all the cathode resistors and replaced them with fixed bias. I've also did this mod on Fender combo's and big Marshall Heads. Now, that being said, I also now do something like that on my preamp gain switch, which is not depicted on any of these drawings...Secondly, I use the UL lamps instead of the regular LED's.. Their forward voltage drop a 1 ma seems to work pretty good enough...

-g

[kg]

Only problem is, is that I don't actually own an 8 ohm speaker! .

don't worry: nobody does, really.