• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Simple AB2 PP-amplifier with OP-amp driver

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#21
I wrote earlier:
With EL84 the max. power would be some 15 W while with 6P15P it is 20 W.
Click to expand...

Today I had a little time to test the amplifier with EL84 (actually 6P14P-EV).
I was surprised because I got 20 W (as triode) before clipping. The OPT was 6k6 Toroidy.
The main reason seems to be that my output tubes needed only 12.5 V bias and therefore the OP-amp has more drive headroom as I estimated, or actually LT Spice.

I try to get time to do full tests with 6P14P-EV during weekend.
 
#24
GoatGuy said:
Pardon me if this comment is like a fâhrt in a flower show, but what is the point of using a high-gain op-amp (100× / +40 dB, before GNFB) to completely 'tame' the output valve transfer curves to the point of being flat-line linear?
GoatGuy ✓
Click to expand...

Hi there Goat Guy,

Isn't that flat-line linearity that make big direct heated triodes so popular with SE topology? Or is it that the load lines are near vertical and not horizontal (like a pentode)?

I have recently discovered anode to grid local feedback and I simulated in LTSpice pentodes with perfect diagonal load lines (similar to SIT or VFET transistors).

I have only built and Atmasphere type OTL but I wanna give it a go with a SE amp and I can't chose between triode or pentode since I do not know what to expect sound wise.

From you experience... what orientation should the load lines have (near vertical like triodes, diagonal like SIT or horizontal like pentodes) for best sound when loading a SE output transformer?

Regards,
Silviu
 
#30
This is an interesting thread. Over twenty years ago I went to an auction and came home with a car full of tubes, including a huge lot of NOS Philips PL83 which seem to be pretty much the same thing as 6P15P. I thought I had gotten rid of them years ago but I found dozens of them hiding at the bottom of a box just the other day.
I also have this pair of small toroid transformers with a set of windings that allows them to be used as PP OPTs with 15% cathode feedback (CFB), reflecting 5,6k @ 8R. I've tested them with a pair of triode wired 6L6s and found them good for about 5-6W down to 30Hz or so.
I also have another pair of larger toroids that would be very suitable for PSU duty with windings for +300V, -150V and +15V with enough current capacity to heat both input and power tubes.

The limiting factor here would be the power rating of my output transformers, but it should be enough if I run the tubes in class A1 instead of AB2.
5,6k feels a bit low for a pair of PL83, but I believe CFB makes tubes act as if they were lower mu, lower Rp devices.
 
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#31
A little bit of breadboarding usually tells the truth, which was also the case this time: PL83 is not the best choice if you want a lot of power in PP triode class A1. No big surprise here, but I had hoped for a bit more than 1,8W at 270V B+ with 165R cathode resistors and 5,6k p-p. Rewiring the output tubes to pentode mode raises the power to 5W, which is the limit for what my OPTs can handle @ 30Hz. Technically I get 15% UL operation "for free" as the screen grids are referenced to ground while the cathodes move around 15% of the plate swing due to the CFB.
 

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