Suggestions for a low power PP power amp

[Klaatuboradanikto]

Gentlemen, I'm new to building and am looking to make a low watt amp. I'm planning on using some early Fender pre-amp design. What I'm wanting for the power amp is to make a push pull using smaller tubes i.e. 12BH7, 12AX7, 6SL7, 6SN7, etc. My goal is to achieve the same basic tone (or as close as possible) to a 6V6, 6L6, or 5881 sound. I realize there are compromises here.

From what I've seen on other threads using a design like the Firefly / Moonlight as a jumping off point, it can sound a bit more SE than one might expect. SE is definitely not the sound I'm after. One builder suggested using 50c5/50b5s for a more accurate sound (or so I gathered) but I believe this to no longer be in production. That said I prefer to try and use current production tubes. Can someone please help straighten out my brain?!

P.S. Recommendations for the PI are appreciated too!

P.S.S. Since "The Tone" is the goal I think I would even consider something like a 6AQ5 or 6BQ5 tube as used on Gibson Skylarks. These produced around 5 watts and I could always dump the unwanted juice. However, presumably going with the smaller tubes would mean the overall build would have a smaller footprint as well which would be good.

[mooreamps]

Then I would use a pair of the "octal" preamp tubes for the P/I, like the good sounding but rarely used 6SL7, and then feed that directly into the O/T.... Hope this helps...

-g

[Klaatuboradanikto]

Thanks for the input Mooreamps. I'm trying to understand what you're saying but not confident I'm following. What I interpret you to say here is you recommend a pair of 6SL7's for the power tubes as well as a 6SL7 for the PI. That said I'm thinking that you would be shaking your head "No" right now. Can you please confirm or clarify? Much appreciated! Thanks, Klaatu

P.S. Obviously I'm trying to create a mini Tweed amp. Of the DeLuxe or Harvard sort. Would you suggest to copy their PI's as well or due to the miniaturization would you recommend something else to off-set against the fact that different tubes are being used?

P.S.S. Perhaps you could shed a bit of light on why you think the 6SL7 to be a better option too just for my edification. Thanks bunches!

[mooreamps]

Well, the title of the thread is low power P/P amp, and the 6SL7 is an octal triode, as the 6V6 is an octal pentode... Just seems to me you'd get about a watt using the 6SL7's in the phase inverter, I'm guessing in the LTP config, and then just have that drive the output transformer... Obviously you'd want primary input impedance upwards of about's 22 K, which I believe can be had in a hammond O/T...

-g

[Steve Conner]

Gary is suggesting leaving out the power amp stage altogether. Or turning the phase inverter stage into a self-split power amp, which is the same thing. There is an AX84 design like that.

Considerations:

Triodes overdrive differently, and damp the speaker differently than pentodes and beam tetrodes. If you're concerned about that, don't use triodes. A pair of 6AQ5s running at reduced voltage might be a nice starting point.

Human ears are non-linear, the perceived tonal balance of sound depends on how loud it is, so in order to sound like a big, loud amp, a small, quiet one must measure quite differently. The physics of this are summed up in the Fletcher-Munson curves, and the bottom line is that there is no point in striving to make an exact scale model of your favourite big amp, because it would sound quite unlike it by the mere fact of being quieter.

[mooreamps]

Gary is suggesting leaving out the power amp stage altogether. Or turning the phase inverter stage into a self-split power amp, which is the same thing. There is an AX84 design like that.

Steve, he said low power.. I may have to put up a drawing of what I was thinking of.. Not sure if it's along the same lines as something you saw somewhere else..

My thought was to use a 6SL7 or perhaps a 6SN7 ; in a A/C or D/C LTP ; driving one of these as the O/T, with the primary impedance set to abouts 22K..Do this in stereo, and it might make a cute little IPod amp...

http://www.hammondmfg.com/125a.htm



-g

[Klaatuboradanikto]

Steve and Gary, both idea's are very interesting to me so I appreciate your chiming in.

Steve, I'll look up your PI idea on AX84. I was also starting to entertain the 6AQ5 idea so I'll give that more thought as well.

Gary, A drawing would be greatly appreciated if you have the time. I was talking to a guru of mine about this last night kicking around the 6SN7 and 6SL7 idea. His thought was to use a 12A_7 LTP for the PI and for the power amp was to use 2 - 6SL7's, bind the triodes in each tube together to make a mini power tube and use these to create a PP power amp. He recommended the 6SL7 because he just felt the 6SN7 didn't have enough oomph to be dynamic enough for a good output tube. Now I'll admit I'm too much of a novice still to really know which is the right way to go here.

So to clarify Gary, I think I hear you saying to use a 6S_7 tube for the PI and one 6SL7 for the power tube utilizing each triode in that tube to create the PP. Correct?

Intuitively I think Steve has a point regarding whole triode vs. pentode vs. tetrode thing but I'm really committed to experimenting here. I also want to acknowledge I know goofy stuff goes on with ears, emulation and such so it's all about getting as close as I can to the goal. The worst thing that can happen is I'll have several iterations and learn something along the way.

Again, thanks both for the excellent debate. If you have anything further to add I'd greatly appreciate the input.

Klaatu

[mooreamps]

Gary, A drawing would be greatly appreciated if you have the time.........
Klaatu

"ok"......This is just off the top of my head.... Should be pedal friendly for more front end gain...

Correction.... change 1 K 3 watt to 470 ohms 1 watt.....

-g

[macdillard]

Go over to AX84, go into the Associate projects, and take a look at the AX84 4-4-0 Studio. Use that PI and output power section and tie it to the pre amp you want to use..I think the 4-4-0 is about 1 watt.

[exclamationmark]

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...

The Blackstar HT-5 uses a 12BH7 in p-p I believe to make 5 watts (plus sounds killer :P). There's also a myriad of 12au7 amps out there too. I don't know what kind of sound you are looking for (ie, fender clean 6l6 or peavey 6505 6l6) but I'd say definately try and find a 12BH7 schematic. Only thing is the grid voltage goes down to something like -40v, so you need significant drive from your PI. Really any double triode valve that can pass a good amount of current can easily produce 5 watts in P-P, but I couldn't tell you how they would sound in reference to a pentode. There's also EF86 designs out there too!

[Steve Conner]

Exclamationmark, it might be worth trying a small mains transformer, you can get them from most electronics dealers with dual 120V primaries and a choice of secondary voltages. They aren't so good for OTs in normal tube circuits because they saturate with only 120V at 50Hz, but for your low voltage application they seem more promising.

I've tested a few: the potted E-I type rolled off at a dismal 400Hz, but a small toroid was still going strong at 100kHz. I've seen hi-fi guys use larger toroids as OPTs for low impedance tubes like the 6C33.

[Klaatuboradanikto]

Whew! More good idea's! Thanks guys!

I'm digging in deeper to the AX84 site. I'll look for your suggestion Macdillard. Good one!

I've been toying with the 12BH7 tube and will probably build something up around this so that I can easily swap variations in and out. That should be interesting. I'll look into the EF86 as a possibility. I had thought about this previously but threw it out thinking it wouldn't have the right character of what I'm looking for. Your nudge and Steve Conner's point about pentodes and beam tetrodes sounding different than triodes makes good sense though. Thanks Exclamationmark!

And that leads me to Steve. You raise a good point about pentodes and beam tetrodes. I'm just wondering if there is a tube out there that I'm over-looking that is like a 6AQ5 i.e. low power but is still in production? I don't think there is but I put the question out to anyone. I don't want to exceed 5 watts in a PP configuration as I've already inferred. And as previously stated I'm trying to find that classic tweed overdrive in this output design (6V6, 5581 or 6L6).

Gary!! Thanks for the drawing! Good on ya for that one! Think I'll have to build it up and see what she does.

The only bummer is if I can't narrow the list a bit I'm gonna have a lot of projects lined out on the bench! Sometimes the experimenting, fun as it is, drives me a bit nuts! So if anyone has anymore idea's on pairing things down or a better mouse trap, I'm all ears. Thanks all!

Klaatu

[mooreamps]

Gary!! Thanks for the drawing! Good on ya for that one! Think I'll have to build it up and see what she does.

Thank you... I was thinking, it might be a good one to start with since it only requires two tubes.. Then perhaps later on, you could build something else....You said in your first post you are new to building amps.. "We all were at point or another..." The first build is the most painful, but they get easier as you get a few of them under your belt... Good luck...

-g

[Klaatuboradanikto]

Gary, I'm curious about the absence of the PI? I'm not familiar with a PP not having one and I'm not seeing how this schematic operates without it. Or if it does have one I'm just not seeing it. Please forgive my ignorance. Can you describe or name what it is you're doing here so I can do more research and just educate myself more? Also, were you to use a tube PI what would it be? A 6S_7 LTP as you mentioned earlier?
Thanks,
Klaatu

[mooreamps]

The second pair of gain stages is the P/I... All I did was substitute the normal plate load resistors with the primary winding of the output transistor.. Also, I intentionally left out the values of the caps in the tone stack... I thought I'd leave that research to you.. :}

Also, use the choke tail as drawn.. It actually works quite lovely, and it only raises the cathodes about's 20 volts.. You can get that part from Hammond as well.. Also, unlike other designs, if you're expecting this layout to hiss like a snake, it won't.....But do isolate the input jack off the chassis and do the star grounding... That will keep the hum low... If you need a power supply drawing, let me know.. I'll draw one up.. You can use the power transformer from a Valve Junior.. You won't need a whole lot of current source for this...




-g