Single Ended EL84 tube amp plate voltage thoughts...

[Mike Sulzer]

He did mention "no tone stack" in the first post.

SE amps won't clip evenly anyway, no matter what you do. To minimize the odd non-square distortion that happens to an SE power stage, the tube bias is usually targeted at 100% in order to give the 'most' class A power available. I'm guessing at idle the bias voltage for your EL84 is about -11.. -14vdc. So clean power starts to run out by the time the grid is fed 20..25vpp signal. You can easily get that from the anode of a single triode stage, from hot pickups or a pedal output. If there's a lossy tone stack after the input stage (you haven't said) then a second stage will be needed to 'recover' the signal.

[kevtronic]

Lots of info and advice. Cool deal. I'll be going over it all and hopefully tomorrow I can put it to use. For the mean time I've attached a schematic I whipped up real quick of the amp I'm working on. I also attached some schematics of other EL84 SE amps that I got the bright idea of checking out while going to the chiropractor today.

EL84 SE (kevtronic): EL84 SE (kevtronic).pdf
Kalamazoo Model 1 kalamazoo_model-1.pdf
Epiphone Valve Junior (versions 1-3) Stock Epiphone Valve Junior Ver. 1-3.pdf
Vox AC4TV Vox_AC4TV.pdf

[eschertron]

He did mention "no tone stack" in the first post.

Aww, Mike! That was yesterday

But to come to my own defense, there are a few small SE amps out there (champion 600 RI comes to mind) that have a FMV tonestack built in, but with zero panel controls. A good -20dB or more between stages. That was the image that got caught in my brain throughout this thread.

[eschertron]

EL84 SE (kevtronic): [ATTACH=CONFIG]43512[/ATTACH]

Nice! OK, I'll bite: what's with the OT? Surely not a PP model with a mock 50% ultralinear tap? Let us know how it sounds, I've been advised against trannys without an air gap for SE operation.

[aside] the part number on the print doesn't seem to be for a contemporary model. Google gave me nuthin' [/aside]

edit: One thing I see is the placement of the fuse. I think modern practice has it between the wall and the on/off switch, assuming the switch is on the high side. But I could be in the weeds again...

[kevtronic]

Nice! OK, I'll bite: what's with the OT? Surely not a PP model with a mock 50% ultralinear tap? Let us know how it sounds, I've been advised against trannys without an air gap for SE operation.

[aside] the part number on the print doesn't seem to be for a contemporary model. Google gave me nuthin' [/aside]

edit: One thing I see is the placement of the fuse. I think modern practice has it between the wall and the on/off switch, assuming the switch is on the high side. But I could be in the weeds again...

The chassis, power transformer, and output transformer are from an old newcomb class A chassis. The OT was originally used as shown in the schematic I uploaded. But, I did add a resistor to drop the the voltage on the screen a few votes below the plate. The OT works fine without using the screen tap on the OT and wiring up using just two of the wires, but it's there and sufficed for who knows how long, so, I'm using it for now. It's like a 50's chassis if I remember correctly. The can cap, amazingly, was fairly close to specs and the old carbon resistors were close, so I just kind of tweaked it and ran it until the can cap gave out. So, I gutted it and I am doing a home brew on the chassis, OT, and PT. Most all of the old chassis I've used pretty much have to be gutted from the get go, this one was had some life left and I wanted to run it as long as I could pretty much stock (tweaked for guitar a bit).

[R.G.]

There's an old sin I've heard referred to as Tlo g"listening with your eyes". I can't quite pick out whether you're accidentally getting involved with this or not.

You talk about wanting maximum clean headroom, then talk about this in terms of the rotation of the volume pot. If that's what you're doing, you're listening with your eyes. Where the volume control is set has not much to do with the output level or headroom of an amp.

An EL84 is a pretty deterministic device. You can look up the EL84 datasheet and find specs on what the suggested maximums are. Getting the maximum headroom out of one will involve giving it the largest power supply voltage and bias point that it can stand without burning up. The data sheet almost certainly has a suggested SE alignment for this. When you have the EL84 set up this way THAT'S ALL THE OUTPUT POWER YOU GET. By definition - you set it up that way.

With the output tube set up to give max power, you need to look at what the biggest clean signal you can put in to the grid without driving the output to more than its max clean output power. Tubes make calculating this easy - grid drive beyond 0V grid to cathode will be distorted, so the maximum clean INPUT signal the EL84 can take is a peak value equal to its cathode bias voltage; anything else will give you grid clipping.

So for maximum clean output, there is a maximum clean input to the EL84; that happens to be in the range of 11-12V peak as I remember for cathode biased IIRC. For maximum clean headroom, your 12AX7 that drives the grid or the EL84 needs to produce this much or a hair more. Anything else just produces the clipping and distortion you say you don't want by saying "maximum clean headroom".

12AX7 gain stages typically produce gains of 10 to 30. So you arrange your 12AX7 stages so that with the volume control maxed, you get just about the maximum grid signal to the output tube for clean output. Since two gain stages of 30 give you a possible gain of 900, and the grid signal for max clean output on an EL84 is in the range of 10V (roughly), two full-out 12AAX7 stages will produce max clean output at 10V/900 or around 11mV. That's about 1/10 of a single coil output. So design your 12AX7 stages with less gain, or as was suggested before in this thread, use stock stages, and resistor pad them DOWN til you're no longer overdriving the output tube at a volume control of "10"

Volume control numbers are purely arbitrarily, right? This is the great lesson Spinal Tap taught us.

[J M Fahey]

Fully agree and add: I went even further and suggested plain using just *one* triode.
Still waiting for the power output at clipping and drive signal needed to reach that ... with a single triode.

[R.G.]

I actually built an amp similar to what the OP is suggesting. I used the chassis out of a Magnavox console hifi that I found in an antique shop. It was actually set up for the 6AQ5, a reasonably close "little brother" to the EL84/6BQ5. With two 12AX7s, it had enough gain to drive the output tube completely crazy, and that was with dividers between three of the four triode stages. The triodes let me have the triodes do most of the clipping/limiting, and only a little of the SE output stage contribution. It was one of the sweetest amp overdrives I've ever heard. But it was definitely a distortion amp.

[kevtronic]

Fully agree and add: I went even further and suggested plain using just *one* triode.
Still waiting for the power output at clipping and drive signal needed to reach that ... with a single triode.

The original set up was with a single 6av6 triode, but I decided to switch it over to 12ax7. I get I can use a single triode to run it pretty clean, even cranked, as that was how the amp originally was.

All great info provide all through this thread and I knew I'd get lots of it here

But, the two main reasons I originally started the post was whether I should stick with 250 volts on the plates (specs) or push beyond like others I have read about. Once that is done and set I can move on to the rest type thing. It does seem acceptable to many to push the plates 300-350 on an SE El84, so I think I will go that route to see what happens. Also, the fact of a very distorted wav form I was seeing on the o-scope, which just seemed to be too over the top when cranking the amp. But, I can see no one seems to think much of it, so I guess that was just the result of the tube being driven extra hard, and me thinking about to much, I guess, lol. Cool deal, that's what I'm looking for. Others opinions, thoughts, and experiences...

There is more than enough here for me to chew on and much of what you have said reinforced what I've read else where. I should be able to get back to the amp this afternoon. I'll do my best to post info on it. But heads up a head of time I'm not the pro you know when it comes to forums and social media, I suck at it basically - lol...

Thanks again, man - GREAT FORUM HERE...

[eschertron]

Just a thought: You have enough gain in the design as posted to use the input as a microphone-level input. An extra jack and a couple mixing resistors, and you could tie in at the second stage grid with an instrument-level injection point. It would give you some additional gain choice, plus a cool little PA amp to take to coffeehouse gigs.

[Chuck H]

Another thing I'll mention is that by padding down the screen with a 5.1k resistor you will be mitigating excess screen current that DOES happen when the grid is driven too much. Which you can certainly do with the series triodes. That high resistance will be unnecessary when the grid drive is brought down to a lower level and you'll get much better dynamics and more power from the el84. I'd still keep a resistor there, but I might lower it's value to 1k (only after the grid drive is brought down though). 330V is about max for a single ended el84 so I think your 310V should be fine. And I want to reiterate that you DO want to be able to clip the amp at least a little because there may be times when you aren't pushing a full guitar signal into it, but you still want maximum clean output. In order to amplify a lower level signal to full output you need more gain than is necessary for a typical guitar with the volume knob full up. This adds versatility to the amp at the expense of having the amps volume set lower than all the way up for max clean with a typical guitar. That may seem confusing, but it's been explained a couple of other times in the thread. I can understand not wanting the amp to break up at something like 2 on the volume control. It's aesthetically unpleasant. But as mentioned, it IS arbitrary. If the amp is putting out full clean power that's all the clean head room it has even if you adjust the circuit so the amp doesn't clip until set to 10. The only REAL problem with that much gain, as I mentioned, is driving the power tube into over dissipating the screen grid. You're pretty safe with that 5.1k resistor to the screen. But I also mentioned that you can get more and better performance from the amp by reducing both the amount of gain and that resistance value. So THAT would be the real reason to lower the gain. And of course that will also place the point of clipping at a higher number on the volume knob.

[R.G.]

Remember that "headroom" is a misnomer as it's used in the guitar world. "Headroom" is actually something like "how big a transient signal over my normal signal level can I pass through fairly cleanly".

If you want biggest clean signal possible - and that condition gets you the maximum possible headroom - you generally want all the power supply voltage the outputs can stand, and with the stages all biased dead in the middle of their linear swing.

So your answer to the power supply voltage question is - use the bigger power supply voltage, as you're thinking. Back it down a little if you're burning out tubes in getting (mostly) symmetrical clipping levels.

[Chuck H]

Using your drawing and not having the amp on my bench... This was my idea. "I" would start here:

[J M Fahey]

You may also borrow a tried and true idea, used by none the less than Marshall and Mesa Boogie
Dig and read (you may also post them here for comments) Mesa Boogie Mark 1 and Marshall JCM800.
Both are based on earlier simpler amps, the MB is basically a Twin Reverb channel, the marshall a somewhat modded Plexi, and both have an extra 12AX7 triode added at the front end.
Funny thing is that both amps sport *separate* jacks, the first one going through the added triode, so meant to scream, and the second one *fully* bypassing it and going straight into the next triode grid, for "normal" operation.
You might do the same, so you can both sell your cake and eat it.
Check the way they creatively use the jack internal leaf switch and precautions they take against popping and leaving some triode grid non referenced to ground.

[kevtronic]

Just a thought: You have enough gain in the design as posted to use the input as a microphone-level input. An extra jack and a couple mixing resistors, and you could tie in at the second stage grid with an instrument-level injection point. It would give you some additional gain choice, plus a cool little PA amp to take to coffeehouse gigs.

Cool thought on the little tube PA. Great idea actually...