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NewYorkDave

    New York (Hudson Valley)
Small-signal pentodes in a mini power amp
« on: February 28, 2006, 12:18:16 PM »
So, we're all aware of the practice of building low-watt tube guitar amps to attempt to capture some of that cranked-amp sound at low volume. The usual approaches include single-ended outputs using a small power pentode or beam power tetrode (the Champ and all its descendants) or a pair of small-signal triodes as a mini push-pull output section. Most of these designs use no global feedback. But that cranked-amp sound we're usually trying to emulate generally originates from push-pull pentode or BPT output sections with global negative feedback. To this end, I wonder: had anyone tried using small-signal pentodes in a push-pull output?


edanderson

    atlanta
Small-signal pentodes in a mini power amp
« Reply #1 on: February 28, 2006, 03:29:11 PM »
this guy uses a pair of 6ak6 in pp for a 2 watt guitar amp:

http://maza.fi/gear.php

tho he does mention he usually straps them as triodes.

ed

NewYorkDave

    New York (Hudson Valley)
Small-signal pentodes in a mini power amp
« Reply #2 on: February 28, 2006, 03:41:11 PM »
That's actually an itty-bitty power output pentode, not a small signal pentode, but it's a cool idea nonethless. Thanks.

I recall another eensy pentode in that class, 6CA5. I had an MTI "tubes" pedal that used one, along with a small output transformer and a dinky speaker as the "reactive load!" It sounded absolutely horrible, but I don't necessarily blame that on the 6CA5.

Small-signal pentodes in a mini power amp
« Reply #3 on: February 28, 2006, 05:51:24 PM »
Quote from: "NewYorkDave"
That's actually an itty-bitty power output pentode, not a small signal pentode, but it's a cool idea nonethless. Thanks.


Like a 12BY7, 6AU6 or an EF86; what other obscure and unlikely candiates can you think of Dave?
"The pen is mightier than a flamin' bag of poop."  --Dr. Bartholomew Simpson

Larrchild

    Fort Lauderdale, Florida
Small-signal pentodes in a mini power amp
« Reply #4 on: February 28, 2006, 07:28:57 PM »
A 6L6 has an Rp of like 25kohms. A 6AU6 has an Rp of 1-1.5 mohms.

I wonder if the output transformer primary would be the limiting object for maximum power xfer and freq response?

adamasd

    Duluth MN
Small-signal pentodes in a mini power amp
« Reply #5 on: February 28, 2006, 08:21:46 PM »
I have played around with using the 6DB6 like this. It seems perfectly happy running with a random 6V6 push pull transformer. I got it makeing sound and it sounded alright, but I never really tried to make it sound good. I just did it since the rp was close to that a 6V6 and I was bored. It got forgotten about while working on other projects and has not been thought about in months. Maybe a good starting place.

adam
so there

Small-signal pentodes in a mini power amp
« Reply #6 on: February 28, 2006, 09:39:34 PM »
6AK5?  I have a gob of those.  rf, if & video amps count or no?
"The pen is mightier than a flamin' bag of poop."  --Dr. Bartholomew Simpson

vibrolax

    Rochester, NY USA
Small-signal pentodes in a mini power amp
« Reply #7 on: February 28, 2006, 09:40:00 PM »
Mark Lavelle, one of my guitar amp building buddies, built this self-split push pull amp with three 6AU6's.

http://www.harmonicappliances.com/pentotron/pentotron.html

Before I built the following SE 6AK6 amp, I breadboarded a few attempts at making a spud amp with a single triode-pentode.  Neither the 6AW8 nor 6EB8 produced interesting results.  Not enough preamp gain with a single the mu 70/mu 100 triode stage, and difficulties with oscillation at full volume that I couldn't quite figure out.

Here's the pentode -> teensy power pentode amp that rocks my socks:

http://www.frontiernet.net/~jff/pipsqueak_pentode/Pentode_DIY_guitar_amp.html

I've posted on it before.

NewYorkDave

    New York (Hudson Valley)
Small-signal pentodes in a mini power amp
« Reply #8 on: February 28, 2006, 09:50:06 PM »
I had to use Google to figure out what you meant by "spud amp."

Anyway, did you ever try the 6BM8? I used that in my little 2-watt amp.

I s'pose I could use a pair along with one 12AX7 to make a decent p-p guitar amp, but that'd be just more power that I'd have to lose in the attenuator. Even the 2-watt amp shown above is too loud for my apartment without an attenuator. I once used it (through a 4x12) to jam with live drums and had no problems with being heard.

Larrchild

    Fort Lauderdale, Florida
Small-signal pentodes in a mini power amp
« Reply #9 on: February 28, 2006, 10:04:13 PM »
On the road in PA they have these foam lined roadcases with Marshall cabs and sm-57 elements mounted in em. They would use em for the PA feed, and the backline was a semi-prop.

Maybe containing the decibels in a box?

Meanwhile, I'll look for more low Rp pentodes!


Small-signal pentodes in a mini power amp
« Reply #10 on: February 28, 2006, 10:12:02 PM »
"The pen is mightier than a flamin' bag of poop."  --Dr. Bartholomew Simpson

Larrchild

    Fort Lauderdale, Florida
Small-signal pentodes in a mini power amp
« Reply #11 on: February 28, 2006, 10:19:25 PM »
Phase inverter scares me.   Hammond 125b can do 25k ohms tho=)

CJ

    California
Small-signal pentodes in a mini power amp
« Reply #12 on: February 28, 2006, 10:45:02 PM »
Well, I always wanted to try 10 or 20 6AQ5's in a parallel push pull.
Anyone done that?
If I can't fix it, I can fix it so nobody else can!
Frank's- www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar- http://tdsl.duncanamps.com/schematics.php

Larrchild

    Fort Lauderdale, Florida
Small-signal pentodes in a mini power amp
« Reply #13 on: February 28, 2006, 10:50:43 PM »
Hearing Aid pentodes, dude.  Build a little 1/8th scale fender twin  just to get in the pimp gallery.

http://www.cs.uu.nl/~gerard/RadioCorner/Sets/BlaOmni.htm

vibrolax

    Rochester, NY USA
Small-signal pentodes in a mini power amp
« Reply #14 on: February 28, 2006, 11:27:14 PM »
The triode section in a single 6MB8 doesn't have enough gain for a guitar amp, if you want any distortion at all.  I believe Univox made at least one amp with a 6BM8 driver/power tube.

Mark Lavelle made this design using pair of ECL86's for the phase splitter/push-pull power stage of an amp using a 6AU6 or EF86 for the preamp.

http://www.harmonicappliances.com/images/hwy86_50pct.jpg

I've thought about trying a TV triode-pentode using the pentode for gain stage, and triode for power amp, but none of the triode sections I've seen look promising.  Something 12AU7-like might be OK

Another possibility is using a compactron with a triode/beam power tube,
as in:

http://www.angelfire.com/vt/audio/compactron.html

Personally, I think a 6AK6 has just the right amount of beef for bedroom shredding.

Jon

Small-signal pentodes in a mini power amp
« Reply #15 on: February 28, 2006, 11:31:15 PM »
Quote from: "CJ"
Well, I always wanted to try 10 or 20 6AQ5's in a parallel push pull.
Anyone done that?


Stop teasing Dave.
"The pen is mightier than a flamin' bag of poop."  --Dr. Bartholomew Simpson

skipwave

    Los Angeles
Small-signal pentodes in a mini power amp
« Reply #16 on: March 01, 2006, 12:51:10 PM »
I was just going to chime in are mention that the Mullard Tube Circuits book I just got has a plan using an EF86 preamp stage and an ECL82 in a little 7-watt amp. Then I saw that the preferred sub for the ECL82 is the 6BM8.

http://nj7p.org/Tube1.php?tube=ecl82
Quote from: PRR
Now, maybe you don't, or shouldn't, grab the ribbon for far-harpsichord, nor the hot condenser for snare-kissing... yet often we do.

PRR

    Maine USA
Small-signal pentodes in a mini power amp
« Reply #17 on: March 01, 2006, 10:11:22 PM »
> A 6L6 has an Rp of like 25kohms. A 6AU6 has an Rp of 1-1.5 mohms.
> the rp was close to that a 6V6

In Pentodes, the plate resistance is utterly meaningless in audio applications, resistance-coupled OR power amplifier. We just can't use it.

It's like the Collector Resistance of a BJT. We might estimate that for a DC voltage amp, but in audio and Power use it just does not matter, and is rarely mentioned.

> I wonder if the output transformer primary would be the limiting object for maximum power xfer and freq response?

Not really. The transformer is shunted by Rp||Rl (and the ratio). Triodes are transformer-friendly: their low Rp will maintain bass even when inductive reactance falls below load impedance. In Pentodes, Rp is always far higher than Rl, and huge changes in Rp make no difference to the transformer. We design the tranny for Rl, ignore Rp.

Instead of Plate Resistance, in Pentodes it is often useful to know the Screen Impedance. This is equivalent to the plate resistance of the Pentode when triode-wired. If you don't know that, look for the G1-G2 Amplification Factor (usually between 4 and 40, 10 for many audio power pentodes) and the Cathode Resistance 1/Gm. For 6L6 it will be about 2K; good loads will generally be somewhat higher, say 3K to 5K.

> 6AK6 ... 6CA5

These represent two compromises. 6AK6 likes fairly high plate voltage and very low plate current. This means very low heater power, but requires hi-Z loading. The 6CA5 has a much bigger heater demand, but that allows low load impedances. Low-Z windings are easier to do well/cheap than hi-Z windings. So do you want to buy fat heater iron or fancy output iron?

Low power, cost, heat is not a new goal. The kings of this field, in the 1-2Watt range, are the AC/DC radio tubes. 50L6 is not a different 6L6, but is a lot like a cross between original 6L6 and EL34 but scaled-down pint-size. And the "L6" name looks good on guitar-amp bottles. If you want something smaller, 35C5 is a small but mighty tube. Tubes like 35C5 exist at 6V heater. Might be fun to scale a Fender Twin down to 1/10th power: change 450V to 150V or 100V, keep almost everything else the same.

As a general solution to getting "tone" without neighbors playing rhythm on the wall: vary the B+. NYD, you have a variable DC supply(?): wire the little tubes and start ramping from +150V down to +20V. You may find that +50V is plenty loud for the bedroom, even in recording. And at the few-Watt level, you can use a big MOSFET to control the B+ with a pot.

> push-pull pentode or BPT output sections with global negative feedback.

Very light feedback. Maybe 3dB-6dB at nominal load. What it mostly does is control the uncontrolled rise of gain at bass-resonance when a loudspeaker is driven by pentodes. You can also let it be uncontrolled, but that suggests a different speaker choice, and as you say the light-feedback plan has been awful popular.

> Not enough preamp gain

One bottle is hard. There is a 12-pin TV IF tube that might go there.

In two bottles: 60FX5 is king of high-gain 1-Watt tubes. You can build a stereo record player with just a coupla these. A very high output ceramic needle makes almost enuff signal to saturate the output. There is even a 12V version, and I think I've seen it stocked somewhere.

You still need gain of 100 plus tonestack loss. This either means a high-gain pentode and low-loss stack, or good old 12AX7 2-stage preamp. Fender's Champ explored this decades ago. He had most-all the same tubes you have, and motivation to think hard about minimal amps; you probably won't find much that is good that he didn't do before you.

One other idea: bringing the pickup up to a couple volts is not a "tone" job, but it is a noise problem. Hide a 5532 in there. Gain about 10. That can drive a triode into overload, cover some tonestack loss, and still smack a power pentode.

AMZ-FX

    south Louisiana
Small-signal pentodes in a mini power amp
« Reply #18 on: March 02, 2006, 07:52:40 AM »
The late Fred Nachbaur has already been there...  see his Lil 4x4 design at:

http://www.dogstar.dantimax.dk/tubestuf/4x4intro.htm

regards, Jack
Free Schematics and Projects - http://www.muzique.com/
My Guitar Effects Blog - http://www.muzique.com/news/
Follow me on Twitter - http://twitter.com/#!/AmzFx


 

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