Help needed for OTL Headphone Amp design

Hi,

This is the schematic diagram of a headphone amplifier I would like to build. It was taken from the web, originally designed for 10 paralleled 5687 tubes, with -200V bias and -13V on cathodes and 400Vpp drive. I have just adapted it for a single 6080WC with the 2 sections paralleled, to get an output impedance of about 100 ohms.
Not sure on the correct bias and drive (actually -120V, -152 according to the -32V cathodes, and 240Vpp drive).
Probably the drive needed is much less, allowing to use only 1 section of the 6CG7 (1 section per channel), instead of the actual cascade topology.
Need help. Any advice?

Thanks,
Val

:roll:

> Need help. Any advice?

Don't do it.

Seriously: "inverted" mode is abnormal. It works OK for sensing a very-high voltage into a low-current meter. It generally is not good for "power".

In this case: 6080 Grids are not rated for any power or current at ALL. Now, since your plate dissipation is zero, your grid voltage low, you might get away with it. Or might not.

I suspect you have over-estimated the input (plate) drive needed. For normal Mu of 2 or 3, and 32V cathode-grid, I'd expect cut-off to be -64V or -96V, and class A bias to be around 0.6 times that or 40V-60V.

> Not sure on the correct bias and drive

Inverted mode is a mystery because it sucks. The wise old men never charted such a silly mode, nor rated tubes for such operation. Steve Bench has played with inverted. But I think it is just a toy.

The published plan has way too much heater-cathode voltage on the 6CG7.

> get an output impedance of about 100 ohms.

You can get ~100Ω from a single 6080 section in simple cathode follower. Parts cost will be much less, power draw will not be much higher.

Thanks for the quick reply.

The idea of building a cathode follower output stage was already in my mind. Will try to redesign the unit and post the schem.

Thanks again,

Val

:?

2xECC88/6DJ8 for each channel.

PCB layout:

http://gyraf.dk/tmp/Tube_headphoneamp.pdf

Jakob E.

Hey Jakob

What is the lowest headphone impedance it can drive optimaly.
Anyone has sugestion for a tube phone amp that can drive anything from 32 ohm with minimal distortion? Of course without output transformer.

chrissugar

I haven't used it with lower than 300 Ohms headphones - that's the lowest we have in the studio. Those it drives happily.

Jakob E.

Thanks for the update.

I was trying to use my 6080WC's, so will work on this mod design.

Respect,

Val

:idea:

Over at www.headwize.com they have a 6AS7 OTL amp that sounds alot like what you are looking for. 6AS7s and 6080s I very often see swapped in power amp, how close they actualy are and how well the 6080 willl actualy function in that circuit i can not say for sure though. Its a nice simple little design, I played around with it once just as an excuse to try out a couple 6AS7s I had just gotten, sounded nice, seemed to run well, but it did not live long since I had more important use for the power transformer.

adam

> 6AS7s and 6080s I very often see swapped

They are the same tube, sold on different markets. Technically the 6080 is better tested, but all 6AS7/6080 uses were heavy-duty, so there are no cheap 6AS7/6080 tubes.

There does seem to be some difference in different production lines. The 6AS7/6080 tube was made in 3 or 4 factories over the years. All meet the high-current part of the spec-sheet, which is what matters in power-regulator use. But small differences in grid windings mean small differences down at low currents. And a hard-worked audio amp may get down there, and distort different one tube to another. Steve Bench has identified two general "groups" of 6AS7/6080s.

But tube sellers, even RCA and GE, didn't make every type of the 10,000+ types they sold. Especially limited-market tubes like 6AS7/6080. They often rebadged tubes made by other factories. It is hard to tell what 6AS7/6080 you have without a curve-tracer. Fortunately if you run them at high current (which sounds better) it hardly matters.

well since we are talking about 6AS7s anyways, generaly i see people run them quite hard, a fair ways out of the published specs. Is this just people abuseing them since they are so cheap and easy to get these days, or were the published specs abit on the conservative side for this tube.

adam