Comments Sought on 1st Scratch Design: Parallel SE EL34 Amp

[Alexo]

Well here it is, please excuse my poor Microsoft Paint skills!
Anybody see anything weird, wrong or dangerous? That cathode follower stage with clipping diodes will be on a push/pull switch to be taken out of circuit and will probably require some tweaking, though I have prototyped it and it sounded pretty cool.
So basically it's a 5F2 front end driving parallel el34's in a single-ended configuration. Maybe I will need negative feedback but I would like to avoid it if possible.
Everything look ok? Power supply? OT impedance and wattage? Bias? Power resistor ratings?
Thanks!!!!
-Alex

[loverocker]

Methinks you need a grid leak resistor for the EL34 grids (and one for the first triode, too), and probably separate grid stoppers for each EL34.

And I'd be tempted to start with lower cathode caps than 25uF in the preamp. Your bass may be too flabby as-is (esp with no NFB). Maybe 1-3uF.

And to avoid motorboating, you'll need another decoupling stage in the power suppy instead of running 4 triodes from a single node.

Great Paint skillz

[rjgtr]

Goto shcematic Heaven and look at the schematic for the Gibson Gibsonette. It is parallel 6V6s, but the configuration should still work fine with a change in the EL34 cathode resistor.

[phsyconoodler]

If you add a gain stage to the 5F2A it will make those EL-34's scream.
Fargen makes an amp similar but with a full tone stack.I don't think that's necessary,but it is versatile.

[Alexo]

OK... revision 2!
I took your suggestions, also ditched my weird drive circuit for more of an AC30 front end as the extra gain stage... maybe it's gonna be a little weird as I tend to think of the AC30 as rather jangly and EL34's as rather thick, not totally sure how they wil mesh but I wanted to try an EF86 and I am hoping they will sort of balance each other out. Is that choke rated for high enough current?
Any and all comments welcome!
Thanks for the comments thus far!

[JoeV]

I pop a couple screen resistors on the power tubes of all my builds for longer and safer running.

[Alexo]

Duly noted, 470 ohm 2 watt, you think?
That's just a guess, please forgive me if I am way off.

[zaphod_phil]

Usually 1k for EL34s. One for each tube, soldered right ionto the tube socket pin. Same way for the grid block resistors.

A few more things - you will have some crazy high gain levels in this amp. Layout will be critical not to get a howling, screeming, whistling monster! I would also suggest changing the the second stage's cathode resistor to 5k without a bypass cap, to give you compressed clipping and reduce some gain at the same time. Then change the last preamp stage's cathode resistor to something around 680 ohms to keep the overdrive tone crunchy. You will also need to reduce the preamp cathode caps (1st & 3rd stage) to 1uF or even 0.68uF, to reduce mush in the overdriven tone.

If that approach still proves too difficult to manage gain-wise, you could alternatively, move the volume and tone control between the 1st and 2nd preamp stages, and turn the 3rd stage into a DC-coupled cathode follower, which will provide some nice soft clipping.

And BTW, your power supply has an inbuilt contradiction. You're using a vacuum rectifier, which is usually done for sag reasons, and followed it with a choke which helps prevent sag! In any case, you get almost no sag with single-ended amps anyway, so you may as well just use silicon rectifiers. Furthermore, with your current PSU design, you will get a pretty low voltage out. This is because you have the vacuum rectifier, which itself will reduce some voltage, and then you have a choke input filter after it. You will need to place a cap in front of the choke to raise the voltage back up. You may also replace the choke with 150 ohm 10W resistor, as your choke will need to be capable of handling around 200mA.

[JoeV]

What Phil said. 1K/2W minimum. Good luck, I can't wait to hear clips.

[Alexo]

Thanks guys! Those comments are invaluable.

In regards to the power supply, maybe I'm trying too hard to avoid the problems with my last build - a little 5C1. It had too high a voltage and a frustrating little hum that won't go away, hence the choke and the tube and all that. What if I nixed the choke and used diodes, would hum be an issue going straight into the OT off of that first filter cap node? I've seen amps that do this and amps that don't... I would rather not have 5 stages of filtering because as things are, the filtering can be neatly taken care of by a single JJ cap can but I do not want to compromise the filtering just to avoid an extra cap and I know SE amps are more prone to the hums.

For the preamp stage, just conceptually, you are proposing to reduce the 2nd stage's gain to get more controllable overdrive and increase the third's to get more drive in the output tubes? Sound like good ideas to me. As do the cathode cap changes, I will probably throw some switches on there to use different values too.

I plan to order triple the caps and resistors I need, one set as they are in the schematic, one set at +100% value and another set at -50% just because I know I will have a lot of tweaking to do. I could sit here and do all the book learning till I'm gray in the face and carpel tunneled out and I still wouldn't feel ready to design an amp, or I can just jump in and give it my best shot and learn along the way. At least that's how I see it.

Thanks again, I will post some clips, hopefully they won't be of buzzes and squeals!

[ur12]

I have built a similar amp to what you propose. It is a 2 channel amp with built in channel switching. It has a octal pentode channel and a hi gain 12ax7 channel. These go into a Dual EL34 Paralleled Output stage through a custom designed SE Airgapped OT. Here is a couple of clips of the amp. The Octal channel has a kind of Clean Brad Paisley tone. The other clip is with both channels mixed together.

[zaphod_phil]

What if I nixed the choke and used diodes, would hum be an issue going straight into the OT off of that first filter cap node? I've seen amps that do this and amps that don't...

This is a *bad* idea for single-ended amps. PP amps cancel out a lot of DC ripple coming onto the power tube anodes, but SE power amps can't do that, which is why so many Champs suffer from hum. So you do need to have a Pi filter before the OT in your case. Say a couple of 47uF caps with either a choke or a something like a 150 ohm 10W resistor between them.

And with a parallel-SE amp you should wire the heaters of the two power tubes anti-phase, to help cancel heater hum in them.

[Alexo]

Wow those sound great! Smokin' playin' too!
I'd be happy if my amp had some of that, I just love that growly responsiveness of EL34's...
How did you spec your output transformer? I'm thinking of either getting the monster 2.3K SE OT from MM which will be overkill as Paul told me it can easily handle 50 watts (!) or the cheaper good ol' Universal SE for it's multiple output taps... and cheaperishness.

[Alexo]

This is a *bad* idea for single-ended amps. PP amps cancel out a lot of DC ripple coming onto the power tube anodes, but SE power amps can't do that, which is why so many Champs suffer from hum. So you do need to have a Pi filter before the OT in your case. Say a couple of 47uF caps with either a choke or a something like a 150 ohm 10W resistor between them.

And with a parallel-SE amp you should wire the heaters of the two power tubes anti-phase, to help cancel heater hum in them.

Thanks again, Phil! You have saved me countless headaches and solder fumes already.

[zaphod_phil]

....or the cheaper good ol' Universal SE for it's multiple output taps... and cheaperishness.

A Hammond 125GSE will handle two EL34s or 6L6s just fine.

[ur12]

Wow those sound great! Smokin' playin' too!
I'd be happy if my amp had some of that, I just love that growly responsiveness of EL34's...
How did you spec your output transformer? I'm thinking of either getting the monster 2.3K SE OT from MM which will be overkill as Paul told me it can easily handle 50 watts (!) or the cheaper good ol' Universal SE for it's multiple output taps... and cheaperishness.

I found one more I forgot I had. Just thought you might want to hear a SE Paralleled EL34 amp. This is a LP using te Neck PU.

If it were me I would go with the 2.3k MM. Yes it is over kill but unlike a PP amp you got current running through the OT at ALL times and any extra over specs you get will help to keep the OT lasting a long time. Don't under estimate the SE design. Your amp is going to be L-O-U-D!!!

[jimipage]

WOW, ur12 -- great playing AND great sounding amp!!
schematic? please?

[tarzanalog]

Bump for schematic?!?

[Mooreamps]

And BTW, your power supply has an inbuilt contradiction. You're using a vacuum rectifier, which is usually done for sag reasons, and followed it with a choke which helps prevent sag! In any case, you get almost no sag with single-ended amps anyway, so you may as well just use silicon rectifiers. Furthermore, with your current PSU design, you will get a pretty low voltage out. This is because you have the vacuum rectifier, which itself will reduce some voltage, and then you have a choke input filter after it. You will need to place a cap in front of the choke to raise the voltage back up. You may also replace the choke with 150 ohm 10W resistor, as your choke will need to be capable of handling around 200mA.

It's not a contradiction. The rectifier tube does not generate rf switching noise like solid state does, and is the other alternative if low power shottky can not be had. The series choke reduces the the surge current stress on the rec tube, and just what I would have done as it also makes the tone a little smoother in this configuration.

No wonder you guys struggle with this stuff. Ya, I'll bet pre-amp voicing would be an art form untill you figure out how to graduate from "power supplies".

-g

[Alexo]

ducks!