OT load/mismatch question

[GlideOn]

I have been tweaking my Single Ended Parallel for a few weeks now, recently took to a tech for final rundown and while it sound great, it's making a bit less power than I'd like around 15w with a pair of EL34s

I didn't think much of it at the time, but I simply thought to select a low enough OT as I'd be running in parallel, so I currently have a 2.5k OT primary with 4ohm and 8ohm taps.

I have a ln OT rated for 600v @ 300ma.

Configured in full-wace bridge diodes I get 300/0/300 to get 420v raw B+.

-5v ish for Choke.

So I have around 415v B+.

Shared 5k Screen Dropper and additional 1k Screen Resistors at each tube (pins 6 to 4, Marshall style).

I do not have a B+ dropper other then the 5H Choke and the 5K, but that's a separate junction, not in series with the B+ line.

Biased a bit cold @ 42ma each power tube.


Quick math:

415 ^ 2 / 25w = 6689

Parallel or divided in half:

3444.5 or let's just say:

3445 total Power section load.

I'm about 945 over the OT primary.

Not quite ideal, but also not terrible.

Problem is that my Screen dissipation is too high, like 15w where it needs to be 8w.

Tech says it's not ideal, but the shared 5K/5W Screen Dropper may be lowering the voltage under load enough to safeguard it. He recommended beefing it up to 10w though which I am already in process of doing.

My tech has gone dark past few days and I'd otherwise ask him (I think he's on vacation, lol), but could I mitigate this problem all together by just plugging into the 4ohm load?

I'm not quite sure if this is accurate, but I would be halving the load again and getting under the OT primary load instead of over.

Then cap off the 8ohm tap and just parallel the 4ohm for ability to use addl speaker cab without penalty?

Is this a good idea or am I have I taken a wrong turn here, better to use a higher load OT?

[shooter]

Screen dissipation is too high, like 15w where it needs to be 8w.

that's PER tube?

I currently have a 2.5k OT primary

I'm about 945 over the OT primary.

so you'll end up ~~ 800 under..... 3445/2
experimenting shouldn't make a difference just to see if it makes you sonically happy but long haul you want all your #'s at or below about 105% dissipation.

it's making a bit less power than I'd like around 15w with a pair of EL34s

sounds where you want it, just nowt power?
how many speakers have you tried, a well matched speaker will make the "audience" think the amp is 20-30% "louder" than the actual math

[GlideOn]

that's PER tube?

Yes, est 15w per Screen per tube under full load. Again, could be dynamic and not nominal. Or use something in realm of 2.2k/20w screens to drop about 200v, which is silly.

so you'll end up ~~ 800 under..... 3445/2
experimenting shouldn't make a difference just to see if it makes you sonically happy but long haul you want all your #'s at or below about 105% dissipation.

Precisely. Stay beneath that safe class A 100% dissipation or 95% conservatively. Should be entirely possible with the PT voltage and 30w rated OT thanks to the Toroid profile (they really are awesome transformers !)

Sonic impacts aside, I'd prefer to be under load and cathode bias for power and headroom accordingly rather than at the B+ or Screens.

how many speakers have you tried, a well matched speaker will make the "audience" think the amp is 20-30% "louder" than the actual math

I've tried it in my 2x12 8ohm cab with Mojotone British speakers and a 4x12 8ohm in my garage with WGS British speakers. There's expectedly a greater wall of sound but prominently a great wallop of bass response and moderate compression around noon on the Master Volume. Still impressive and bass really fills out the range for what it is, but I'd say it's barely above drumming level, un-mic'd.

It's otherwise housed in a repurposed Mesa MK4 combo shell with the stock C90 8ohm speaker. 99db, pretty efficient already.

[Merlin]

Long story short, your B+ is very high for SE operation, forcing you into a corner where you can't really expect much better than 15W.
Just look at the EL34 datasheet -it recommends 280V for 11W output, 2k load, cathode biased (so 22W into 1k load for two in parallel). If you were hoping for >20W clean, you shouldn't have started with >400V...
Reducing the load is a safe experiment, but it may only get you more distorted power. Try it.

[GlideOn]

Long story short, your B+ is very high for SE operation, forcing you into a corner where you can't really expect much better than 15W.
Just look at the EL34 datasheet -it recommends 265V for 11W output (so 22W for two in parallel). If you reduce the load impedance you may get more distorted power, so I guess if you're happy with that as a metric?
Your screen dissipation sounds crazy!

Too much voltage limits power?

Not familiar with that concept. Does it have more to do with PT or OT?

I could theoretically limit the B+ with some crazy dropping scheme.

Also, my AnTek OT does have an Ultralinear/Screen tap. Not sure if that would help?

[Merlin]

Too much voltage limits power?

It does in SE because it forces you to bias at low current (100% of max dissipation or whatever), which leaves you in a corner, because then it's hard to choose a load that offers decent current swing in both directions. Low load? You hit cut-off too soon. Higher load? You hit grid clipping too soon.
I'm guessing you biased to about 60mA per tube, which offers a theoretical RMS current swing of 42mA. That suggests a theoretical 9W into the load, or 18W for two tubes. Except reality isn't perfectly efficient, so 15W doesn't surprise me.

[GlideOn]

Too much voltage limits power?

It does in SE because it forces you to bias at low current (100% of max dissipation or whatever), which leaves you in a corner, because then it's hard to choose a load that offers decent current swing in both directions. Low load? You hit cut-off too soon. Higher load? You hit grid clipping too soon.

I see. Over or under this I still get side effects.

To be clearer though - is the problem more plate voltage or screen voltage?

Screens are a problem atm, for sure.

[shooter]

Screens are a problem atm, for sure.

yep, but it seems there's something missing, G2 shouldn't be at 200% when plate is at 100%
 
post your schematic so I can learn visually, I suck at parsing participles 


what is your cathode R's value and the VDC at the cathode, NO signal?

[GlideOn]

Screens are a problem atm, for sure.

yep, but it seems there's something missing, G2 shouldn't be at 200% when plate is at 100%
 
post your schematic so I can learn visually, I suck at parsing participles 


what is your cathode R's value and the VDC at the cathode, NO signal?

Not quite sure what you mean by plate at 100%

Isn't Class A plate max 450v? Or 800v?

I have 390r/100uf on each cathode currently.

Beyond that I'm afraid that where my ability to fathom is at its' limit.

I've painstakingly made up physical layout in DIY creator, if that helps.

Schematic I'm working on. Closest to a JMP in preamp but Single Ended Parallel in power section.

[Merlin]

To be clearer though - is the problem more plate voltage or screen voltage?

Screen voltage for the most part. But as you said, dropping a lot of screen voltage is kinda wasteful. But this is a guitar amp, so does a few extra watts of clean power really matter?

[shooter]

Not quite sure what you mean by plate at 100%

dissipation in watts. it's not "max plate volts" that determines the tubes dissipation, it's a given current at a given voltage.  sorta what Merlin was referring to when he said 

which leaves you in a corner,

the cathode vdc /390 ohms yields the tubes current, (plate + screen current)
you indicated in the OP it was ~42mA per tube so each tubes cathode VDC should be ~ the same since you have 1 390 ohm Resistor for each tube.  ohm says .042 * 390 should be about ~~16.4vdc

[GlideOn]

To be clearer though - is the problem more plate voltage or screen voltage?

Screen voltage for the most part. But as you said, dropping a lot of screen voltage is kinda wasteful. But this is a guitar amp, so does a few extra watts of clean power really matter?

True, it matters to the point of being audible in a mix I suppose. At least edge of breakup cleans. Any more than that is overkill for my needs.

If I had a higher Z load in the form of a different 5k OT, could I better manage load and have greater breathing room to adjust? I can drop voltage, that doesn't seems to be an impossibility.

If purely to have the ability to plugin a second or parallel output jack.and still be under load, that would be nice so even if I have only 15w it can be spread out among multiple speaker cabs to be heard.

[Merlin]

A higher load impedance might buy a little more clean output power. But I think it would be splitting hairs, since loudspeaker impedance is higher than nominal at most frequencies anyway! You'd probably get more actual loudness just but using multiple / bigger / more sensitive speakers. A lower load will buy you more distorted output power.

[GlideOn]

This was a bit of my benchmark and inspiration:

https://www.silktone.org/store/p18/parallel.html

Allegedly, these get 25w from a pair of KT66.

Custom transformers and speakers. Actually lighter than mine somehow using EI types, even with Toroids I couldn't quite manage the same.

Very noticably juicy, warm harmonic overtones, plus that same bass response I wonder if is an attribute that comes with this type of design.

Mine so far does sound very similar, but perhaps a bit tighter, sharper and brighter.

[shooter]

perhaps a bit tighter, sharper and brighter.

the guitarist I use to evaluate my builds made this comment;
"You can't cheat playing through your amps, you will HEAR every missed note, every mistake, you have to actually play well"
also said once the pedal board is dialed in, he can cheat AND sound good doing it 


I gave up on all the tubes from 1 to 4 in parallel moved to the KT88 running between 90-105 max dissipation, 400-420vdc at plate 40% UL tap for G2, dial the bias as close to "center" as sounds good allows, drive with enough G1 VAC to push into compression/clipping.


class A is about harmonic content more than the OD tone

[GlideOn]

perhaps a bit tighter, sharper and brighter.

the guitarist I use to evaluate my builds made this comment;
"You can't cheat playing through your amps, you will HEAR every missed note, every mistake, you have to actually play well"
also said once the pedal board is dialed in, he can cheat AND sound good doing it 


I gave up on all the tubes from 1 to 4 in parallel moved to the KT88 running between 90-105 max dissipation, 400-420vdc at plate 40% UL tap for G2, dial the bias as close to "center" as sounds good allows, drive with enough G1 VAC to push into compression/clipping.


class A is about harmonic content more than the OD tone

You make a good point, and happens I haven't really fooled around with pedals so maybe I can try some boosting and EQing to 'fake' some more volume.

It already sounds noticably better than my other push/pull amps, just something juicier happening especially with low mids and bass.

I DO have an Ultralinear/Screen tap on my OT, could I tap that to limit the screen current?

[Merlin]

I DO have an Ultralinear/Screen tap on my OT, could I tap that to limit the screen current?

That should reduce screen dissipation at high output levels, yes.

[GlideOn]

I DO have an Ultralinear/Screen tap on my OT, could I tap that to limit the screen current?

That should reduce screen dissipation at high output levels, yes.

Okay, it could benefit from a little less distortion and compression high up on the Master Volume. Gets quite bouncy and saggy up there almost like a tube rec.

Less screen dissipation = more available cathode bias, no?

Does it actually help to improve output power though?

I know thread was specifically about output impedance, it's all kind of related to balancing the power section for maximum power.

[shooter]

no

UL tapped G2 is more about controlling under dynamic conditions.

related to balancing the power section for maximum power.

If by "power" you mean audio power, that happens when you can "swing" the biggest signal, that happens when the "bias" is centered.
The more the "unbalanced" the less one side will "contribute" to audio power, since it's, well cut off.

[pdf64]

no

UL tapped G2 is more about controlling under dynamic conditions.

related to balancing the power section for maximum power.

If by "power" you mean audio power, that happens when you can "swing" the biggest signal, that happens when the "bias" is centered.
The more the "unbalanced" the less one side will "contribute" to audio power, since it's, well cut off.

Are you thinking of a push pull output stage?
I think this thread is still in regard of parallel SE.

[pdf64]

...
Less screen dissipation = more available cathode bias, no?

Does it actually help to improve output power though?
 ...

As shooter noted, UL affects screen dissipation under dynamic conditions, whereas your reference to bias might indicate you're thinking of the idle condition?

Compared to regular pentode operation, UL will tend to reduce audio power output, all else being equal.

[shooter]

Are you thinking of a push pull output stage?I think this thread is still in regard of parallel SE.

i might not have worded it well 


if you're only getting 70% of a signal through an SE amp because the cathode/B+ combination is such that 1/2 the waveform is compressed/clipped (operating closer to class A/B) output signal power will be less.
if it's "center biased" (Class A) you swing both halves to their full "potential"

[Merlin]

Less screen dissipation = more available cathode bias, no?
Does it actually help to improve output power though?

Bias is the same either way since the screen voltage is practically the same either way.
Power will be a bit lower in UL, but it will be 'cleaner' power.

[GlideOn]

I suppose there's no free lunch, nor was I expecting this to be an easy process to compute.

Honestly, I took a page from @shooter and tried clean boost pedals to "fake" some volume and the results pleasantly surprised me. Tried this weekend with mic'd drummer and I'm happy to say it passed the test of having just enough headroom, namely thanks to having some fullness in the bass and punchy tone from stout transformers that it lacked previously.

Secondly, I figured my V4 and V5 power tubes are fully bypassed @ 100uf, which is very thick and punchy with intermediate volumes, but gets a bit wooly up high. I made my master volume a Push/Pull to series jumper a 10nf into my 22nf PI coupling cap stage and get 6.8nf value. Felt like I can run the Master up a bit higher to get a little extra clean headroom by reducing gain/distortion in the bass. Pretty happy with the compromise there.

I also think my 5k Shared Screen Dropper is actually saving my Screens from melting - so it will stay for now. Power tubes do get hot after a few hours so looking into building a fan to help keep things cool. I am well within the ballpark of tones and EQ options and faking a volume boost with a Clean Boost pedal works incredibly well.

I am mentally exhausted over this amp to be honest. It's been about 6 months of tinkering and I'm ready to put it back into the chassis for good. It simply sounds amazing and I'm relieved it can get over a mic'd drummer having sampled it this past weekend it does it, if but barely. It will get plenty loud, but it's enough for clubs and practices. If I ever use it for a larger venue I'm sure every amp will be mic'd up regardless.

Thank you all for the tips and insights, I've learned that it's all the sum-of-parts, higher voltage PT doesn't always equal higher power and try the simple things and get off your test bench from time to time to play in love situations, even if the amp has to be "naked" for a bit with proper supervision it is immensely useful to trial run these things.