LTP bias Q

[Chuck H]

Some here may know that I've long been a proponent of the Paul Ruby zener mod to minimize grid loading on el84 amps. The PR mod uses a pair of zeners to clip the PI negative swing a volt or so above the bias voltage. This minimizes coupling cap charging and allows a period of time for them to discharge while the power tubes are in cutoff. Brilliant! But since I use this circuit in "my" designs I've always wanted my own solution to the problem. So I was thinking...

Couldn't you just bias the PI cold to cutoff one volt over the bias voltage and achieve the same thing? This just seems too easy and I've never heard it proposed so I figure there may be a reason.

Thoughts?

TIA

[teemuk]

Still my morning cup of coffee in hand so excuse me if I'm not thinking clearly yet but....

...wouldn't asymmetric clipping of the PI, coupled to capacitive coupling of signal to power tube grids, as usual cause shift in DC offset at the grids (bias).


Someone thinking more clearly help me follow this through....

[The Dude]

My first "wonder" about doing it that way would be consistency of preamp tubes. I wouldn't think bias would be similar enough across a range of tubes and it would warrant rebiasing when tubes are changed and even as they age. With the PR mod, this isn't an issue as it's at the output. You can swap tubes at will without worry of a circuit readjustment. Just thinking out loud. I certainly haven't considered all possibilities.

[pdf64]

You're probably already aware of it, but in http://www.valvewizard.co.uk/acltp.html Merlin mentions:-

Biasing: Choose a bias point to taste. Hot-to-centre biasing will encourage the power valves to distort more, before the phase inverter does, while colder biasing will reveal more phase-inverter distortion, tipping the tonal balance in favour of the preamp.

I've not properly thought it through or trialed it yet.

[Chuck H]

You're probably already aware of it, but in The Valve Wizard -Long Tail Pair Merlin mentions:-

Biasing: Choose a bias point to taste. Hot-to-centre biasing will encourage the power valves to distort more, before the phase inverter does, while colder biasing will reveal more phase-inverter distortion, tipping the tonal balance in favour of the preamp.

I've not properly thought it through or trialed it yet.

Hmmm... I can't get my head around that either. Merlin's statement there would certainly apply to class A amps, but I can't see how it would always be the case with AB bias. I don't have the book (I know... Shame on me). Perhaps it's clear about things in context.

[Chuck H]

My first "wonder" about doing it that way would be consistency of preamp tubes. I wouldn't think bias would be similar enough across a range of tubes and it would warrant rebiasing when tubes are changed and even as they age. With the PR mod, this isn't an issue as it's at the output. You can swap tubes at will without worry of a circuit readjustment. Just thinking out loud. I certainly haven't considered all possibilities.

Good point. It wouldn't be a big deal to rebias the PI whenever that tube is changed. It might confuse some poor tech though if this circuit landed on their bench without knowledge of what I was up to.

I think it may be possible to fudge on the side of caution. Maybe bias so that MOST preamp tubes used as an LTP PI will only cut off after MOST power tubes are in cutoff and still get MOST of the benefits of reduced grid loading. The PR circuit is indeed already perfectly suited for this cause. Just bouncing my "what if"s off the higher minds.

[Chuck H]

Still my morning cup of coffee in hand so excuse me if I'm not thinking clearly yet but....

...wouldn't asymmetric clipping of the PI, coupled to capacitive coupling of signal to power tube grids, as usual cause shift in DC offset at the grids (bias).


Someone thinking more clearly help me follow this through....

Hmmm... I guess if the PI were MOSTLY in cutoff this could happen. I don't think it would be that extreme to cause an offset that shifts the bias. Thinking about so called "cold clipping" stages used in many preamps now. No bias voltage shows up on the following stage grid in these circuits. I'm not seeing why the PI should be that different. Of course it IS different. Different operating impedances and the grids of power tubes are more likely to try to conduct when the tube is clipping. Such differences might be lynch pins to this idea. No way for "me" to know without feedback from someone that DOES know or testing it on the bench.

[Dave H]

Hmmm... I can't get my head around that either. Merlin's statement there would certainly apply to class A amps, but I can't see how it would always be the case with AB bias. I don't have the book (I know... Shame on me). Perhaps it's clear about things in context.

Are you thinking about the bias point of the output tubes? I think Merlin is referring to the bias point of the LTP.

[Chuck H]

I'm talking about the LTP. My thinking (probably not as good as Merlin's ) is that even biased cold, but still with enough negative swing to drive the power tubes properly into cutoff, there shouldn't be a reduction in signal at the power tube grids. I can see where a colder bias on the PI probably requires more input voltage to the PI. Also not usually a problem with most guitar amp designs.

I'm actually thinking almost opposite of the text above. If the PI is biased cold, but doesn't clip the negative swing until after the power tubes are in cutoff, that condition should allow for MORE positive swing from the PI to drive the power tubes into saturation. Provided there is adequate drive to the PI input to compensate it's colder bias condition.

[Dave H]

I'm talking about the LTP.

Right, it was your mention of class AB that fooled me

The LTP has far more output swing available than is required to drive the EL84s to saturation. You could limit the output of the LTP to say 18V peak by reducing the value of the EL84 grid leak resistors and still not reduce its gain too much.

I haven't noticed the problem myself. I probably don't drive the amp hard enough. It's either that or I like the sound of crossover distortion.

[g1]

As far as what Merlin said, I interpreted it as being either center-biased PI or not, forget hot or cold.
Center biased gives you max symmetrical output, so PI clips last (or later than PI biased off-center). And clipping being either cut-off or saturated.

[nickb]

Some here may know that I've long been a proponent of the Paul Ruby zener mod to minimize grid loading on el84 amps. The PR mod uses a pair of zeners to clip the PI negative swing a volt or so above the bias voltage. This minimizes coupling cap charging and allows a period of time for them to discharge while the power tubes are in cutoff. Brilliant! But since I use this circuit in "my" designs I've always wanted my own solution to the problem. So I was thinking...

Couldn't you just bias the PI cold to cutoff one volt over the bias voltage and achieve the same thing? This just seems too easy and I've never heard it proposed so I figure there may be a reason.

Thoughts?

TIA

Well you idea does work and I commend you for your original thinking. As someone else already said, the problem will be setting up the PI bias and worse, keeping it there. Probably a little help from some feedback and (gasp) opamps will solve that.

Zener's looking better now, huh?

[potatofarmer]

A couple thoughts -

Cutoff clipping is very gradual because of the island effect so it will be very hard indeed to get the PI to "stop" at a certain voltage.

Since we're dealing with a phase inverter when one power tube grid is going to be getting +X volts of signal, the other is going to be seeing -X volts. Since we're worried about "+x" not being greater than "Vbias+1", instead of just tweaking the bias point, it might be easier to pick a different loadline where the maximum voltage swing to either output is roughly "vbias+1". You could start by using smaller plate resistors on the PI and then smaller grid leaks on the power tube grids.

I believe Merlin wrote extensively about the overdrive effects of PI bias in the second edition of his preamp book, giving the nod to center-to-cold PI bias for improved blocking response, but I'll have to double check that...

[nickb]

Another thought...

At the and of the day it pretty much reduces to a diode clipper that clips just greater than 2x the bias. You could achieve the same effect by just having appropriately smaller series zeners on the input to the PI. There is the effect of negative feedback to consider and that makes me think that before the PI is a better place.

[R.G.]

If I missed something in my scan of the thread, forgive me. There is a problem lurking here that's fundamental to the idea, I think.

Biasing the signal for clipping one side or another at the PI necessarily is separated from the power tube grids by the coupling caps. The coupling caps wash out any DC level changes, so what goes to the grids is always volt-second centered and pure AC. Grid blocking is an effect caused by the grid acting like a diode in that it conducts DC pulses out of the incoming coupling caps on positive peaks and shifting the DC offset on the coupling caps. Feeding the coupling caps an asymmetrically clipped signal to counteract this grid conduction doesn't help the grid-blocking DC shift because all the DC is removed. All it can do is to shift the overall size of the signal fed to the grids, and that will cause some reduction in grid blocking just because the signal's smaller.

So (just speculating without simulating it) I think that you can't do anything on the PI side of the caps that couple to the output grids that help any more than making the peak to peak signal smaller. The caps' DC removal washes out any advantages you thought you were getting.

I think...