Using 6V6 or el84 in the preamp section...

[bruno]

Anyone here using a 6V6 or el84 in preamp, as an extra gain stage, would yield any results worth experimenting with, something like:

triod - tonestack - triode - gain control -power tube gainstage - pi - output section

[HotBluePlates]

It would be a waste of a tube.

What are you mainly trying to do with tubes in the preamp? Amplify voltages, so that you can build up the signal from the small guitar signal to enough to drive the phase inverter (or driver tube) and then ultimately drive the output tube grids. So you're looking for voltage amplification, which usually corresponds to mu in triodes.

What is your least-used common triode? Probably the 12AU7, right? That's because we often want to develop a large output voltage swing (much more than we could use) to either allow for loss in a tonestack or through a volume control and to allow easy overdriving of a following triode stage to cause it to distort. It might take a 4v peak-to-peak input signal to slap a 12AX7 silly; if another 12AX7 was driving that, and had a gain of 50 (which is typical), then that first 12AX7 only needs 4/50 = 80mV peak-to-peak to cause the later 12AX7 to puke. Since our guitar signal is bigger than that, you have room to add a tone circuit and a volume control. If you used a 12AU7, with a stage gain of 10, it would take 5 times as much input signal (400mV peak-to-peak) to achieve the same result. And you know from experience that a 12AU7 seems like it stays clean forever and just barely edges into distortion.

So what's the triode gain of a 6V6? It's 9.8, so as a first guess stage gain might be 5. Not too good. But pentodes usually have their stage gain quoted as Gm * load resistance, and the Gm for a 6V6 might come in around 5000 micromhos, so you'd think just slapping a big resistor on the plate will get a lot of gain. The problem is that Gm is dependant on plate current, with higher plate current often correlating to higher Gm. But even a 5k plate load resistor limits maximum 6V6 current to less than 6mA. We can get usable output if that were 5k of impedance but very little resistance, as is the case in an output transformer.

So the problem is that the output tubes function well as output tubes, but they do not function well as voltage amplifiers, and not in small-signal stages like we use in preamps. It's like needing a phillips head screwdriver and being handed a hacksaw.

[bruno]

Thanks hotblueplates, that was a great explanation!

[FYL]

It would be a waste of a tube.

Some Japanese audiophiles waste a couple of 845's and half a ton of iron for a buffer...



 

[PRR]

> even a 5k plate load resistor limits maximum 6V6 current to less than 6mA.

60mA. More likely 30mA (set the plate halfway up a 300V supply).

Which opens a different question. Preamps need CLEAN power. At 1mA-5mA, you can clean-up with 10K and 20uFd. But at 30mA you need a much smaller resistor, say 1K (30V loss). Then for equal cleaning you need a larger cap, perhaps 200uFd. Big!

Yeah, you can do a choke, or active filter/regulator, etc. You can do many things when cost (and weight!) is not an object. The 845 SE buffer that FYL cites can put out about as much power as a Champ (tho not intended to deliver "power"), yet must weigh and heat-up more than an SVT.

Commercial designs rarely do anything this crazy.

In "practical" design, you figure what you need. A guitar preamp puts 50V signal into about 250K load (tone or volume network). 50V/250K is 0.25mA signal current. The tube standing current should be much larger, 2 to 10 times larger. 0.5mA to 2.5mA. The usual 12AX7 stages with 300V supply run 1mA to 2mA, are plenty fine. A 12AU7 can pass over 10mA, but why would we want that? The cost of increased current ability is decreased gain, and guitar-amps need lots of gain in a small package.

BTW: one of the early radar amplifiers DID use 6V6 as an input "preamp" tube. It was really a differential amplifier to cancel heater-voltage DC drift. I think G1 drew current from ground, and signal went to G2. Then the effective input signal was Vg2-Vg1, both on the same cathode, so heater and cathode variations cancelled out. We'd normally use a "twin" triode, but these were new, are not always perfect twins, and may have been in short supply during the war, whereas 6V6 were a glut.

[Tiny_Daddy]

There's another problem: Power tubes with their large internal parts can be microphonic. Not as noticeable for output stages but a curse if there is lots of gain after them.

[tubeswell]

But you could build in a 6G15 circuit onto the front end of your amp and put that 6V6 or EL84 to good use as a reverb driver with a 5k Pr 10-15W SE OT driving the pan. Nice sounding verb

[FYL]

Fender tried that briefly with the Vibro-King, a '90s design by Bruce Zinky.

OTOH, the late Gar Gillies - one of the true greats - used a small power amp (basically a tweedy SE) as a front end/effect with his Garnet Herzog and H-Zog.

[tubesornothing]

So I wonder if you can get the distortion benefits of 6V6/EL84 in the preamp - that's the only thing that comes to my mind.  I suppose if a EF86 or other small signal pentode has the same distortion, it'd be a waste.

[steven]

I put a tweed champ in the middle of a low-power twin. Triode -> 6V6 -> Champ OT -> dummy load -> triode recovery stage. Fred Nachbaur did it in his Dogzilla, and Guy Hedrick did it in his amps. It works, but I'm not happy with it yet (still tweaking it) and I was restrained by the physical size and layout of the 5E8 chassis. I should say most of the reasons I'm not happy with it have nothing to do with the champ, but with the main amp. I can make it sound like a champ turned up all the way, but really really loud. Or not loud at all. I don't know if I would do it again, but I probably will, in a reasonably-sized chassis, just so I can prove to myself it can be done right.

steven

[HotBluePlates]

> even a 5k plate load resistor limits maximum 6V6 current to less than 6mA.

60mA. More likely 30mA (set the plate halfway up a 300V supply).

Damn... I *did* forget a zero...

That's still a lot of power being sucked up for no gain. Because after all, it is a power tube.

Some Japanese audiophiles waste a couple of 845's and half a ton of iron for a buffer...

No one probably checked, but that's $1000-1500 for a pair of tubes... all to add a volume control between your preamp and power amp. And that's not counting the half-ton of iron mentioned. And if you read the news on the japanese economy, you'll see that whoever is actually using this must either be richer than god or insane. Or both.

But you could build in a 6G15 circuit onto the front end of your amp and put that 6V6 or EL84 to good use as a reverb driver with a 5k Pr 10-15W SE OT driving the pan.

OTOH, the late Gar Gillies - one of the true greats - used a small power amp (basically a tweedy SE) as a front end/effect with his Garnet Herzog and H-Zog.

I put a tweed champ in the middle of a low-power twin. Triode -> 6V6 -> Champ OT -> dummy load -> triode recovery stage. Fred Nachbaur did it in his Dogzilla, and Guy Hedrick did it in his amps.

I'd argue that each of these answers a question not asked, at least in my mind (maybe I'm wrong, though).

The primary idea is not whether an output tube can be physically placed within a preamp circuit, but used as a preamp tube. In each of these cases, it is being used as an output tube, but in the case of the Herzog and other like it, that output power is being wasted off to leave a preamp-level voltage, usually for input into another amp. Okay, so that other amp may be built on the same chassis...

So we've all heard American Woman. And we know the Herzog (basically a Champ, knocked down to preamp level and injected into the input jack of another amp) can produce some cool saturated distortion tones. For my dollar, I'd rather build the Herzog and be able to remove the dummy load resistor with a switch and use a regular speaker in its place, as the simple amp that it is.

And the standalone reverb idea is also using the tube as a power amp to drive the reverb tank. But I'd also point out that you can buy NOS 6K6's for less than an EL84 or 6V6. My knee-jerk reaction against all this is probably due to being in the Army: spending more money, more effort, wasting power and achieving a result that could be more easily and cheaply done another way (perhaps even done better) smacks of what we call the "Good Idea Fairy". That when someone in the military (usually an officer) has a "great idea" about how to do something that is more onerous, more wasteful, more disagreeable than a way it's already done, and yet produces no better result.

I could probably get behind all this more if we resurrected some TV tubes that no one wants (most could be bought for $0.10 each or less), and build a functional and good-sounding amp.

[PRR]

Sakuma says "I opened the reference books every day. And then, that old interstage transformer asked me:
'Sakuma, who do you build this amplifier for?
Do you make the amplifier to get praise from electronics teachers?'
Since that day, I have made amplifiers for my own pleasure as an amateur".


Mr. Kei Ikeda, the top authority among Japanese audiophiles said "People should not talk badly about Sakuma's methods. He finds ideas from deeper inspiration than we can imagine".

[HotBluePlates]

I should go find an interstage transformer to teach me electronics.

But if I slip up and talk to the one LooseChange pulled out of an old Gibson, it might be disgruntled and unbalance my phases.

The japanese can sometimes be counted on to take a good idea waaaaaay too far.

[FYL]

Sakuma systems are - were? - used in the Concorde bar/restaurant.

http://www10.big.or.jp/~dh/codo/tennai/tennai.html

[PRR]

> ...something that seems wrong in the PS schematic  12AB5 pp amp 02

Yes, the plate and G2 feeds have been reversed.
 
> will only create clean sounds

At a glance, you need more than one triode stage to get from guitar to 6V6-like power tubes. See 12AX7 Champs and DeLuxes.

It seems to need 11.5Vrms at 12AB5 grids, V1A has gain near 50, it needs a full 0.23V to reach full output, and more to get "not clean" output. While a guitarist "can" make 0.2V peaks, amps are usually scaled to require less than 0.05V (50mV) to reach clipping.

> some of the unwanted result can be due to the output arrangement

It is "tame", "hi-fidelity". Guitarists usually don't use a lot of NFB. It will be a little more gain and a little less tame with the cathode windings taken out. But it still won't be a "normal guitar amp". It needs more gain.

> 12ab5 looks like a 6aq5 with 12 volt filaments

It's apparently a 6V6 made for a small bottle, and 12V, but the heater consumption is 10% less.

> The switching supply can pull a load it can't start. Any suggestions on how to minimize the inrush current?

What's the inrush current for a good cap and a square-wave? About infinity. Insert some resistance. How much simple loss can you stand, 20V? Then 20V/80mA= 270 ohms 5W resistor from rectifier to first cap.

That still gives about 1 Amp start-surge. Several dozen Amps back at the 12V section. If it still won't start, you may have to switch in 1K for starting, and use a relay to reduce to 100 ohms once the tubes pull current (a handy delay factor).

Still, it "should" start computers and such, which are cap-input. Maybe that H-bridge you took out did some current limiting? Also maybe your wires are "too good". A car cigar outlet has several feet of the skinniest wire, you may be doing "the right thing" with a short length of fat wire, which may be "too good".

[mackie2]

ALL--

I have built an 811A SE, where I used a power tube to couple and bias the output  tube--Used a 12B4 to develop +24 v bias , on cathode, for 811A grid. The amp had 15 watts out with a good SE sound.

Old Altec amps used 811's in PP--for theatre Amps--many moons ago.

Mackie2