diy guitar tube amp help.

[rickl]

I'm using an HP 200C to test my tube gear. It gets kind of freaky at higher frequencies but it works fine for a nice 1k sine wave.

rick

[PRR]

> this was built in 1939

If it is truly pre-WWII, it is quasi-collectable. If you brought it to my door, I'd give $25 as-is. That isn't a lot but it is five times more than I paid for my last two (1950s) H-P 200s put together.

H-P's first sale was to Disney, for the Fantasia road-show. The multi-channel sound was switched from a lesser number of film-tracks to many speakers with coded tones. That was goofy; but my Langevin broadcast console had a shelf and wires made expressly for an H-P 200AB. They were everywhere.

While it is a True Classic, "as seen in" all old audio-lab photos, the world is full of them. Good working uncalibrated 200ABs from lab-gear refurbishers sell for $50-$75. As noted, eBay rarely gets $30 for one.

> If I remember correctly, this oscillator has a 600 ohm output.

The 200AB is rated nearly 1 watt in 600 ohms. I ferget what the 200B is; there was one rated for a little more power at less bandwidth.

The 6K6 is a smaller and more linear ancestor of stuff like 6V6. It is biased for a good 8 watts of output, but loaded very hi-Z to swamp winding inductance. Possibly not the best way to do it, but before GOOD oscillators like the 200 came along it was hard to do good empirical study of transformer high frequency action.

You can load a 200AB with 150 ohms and it hardly sags. The output impedance is 50-100 ohms. It has enough grunt, trimmed well below clipping, to smoke 1 watt resistors.

There is no tap to get anything like speaker impedance. This is the ultimate Line Amp, not a speaker driver.

A common 25V P.A. speaker transformer will convert its output to speaker impedance, but with typically awful response and distortion.

An array of a dozen 8 ohm speakers can be wired 100 ohms and work directly. Build a "Marshall Stack" with 8 or 12 cheap six-inch speakers four to a cabinet. Or find some money, get eight hot 16-ohm 8-inchers, even a Fender Twin won't laugh at your power chords.

The output stage is worked at unity voltage gain (25V in, 25V out). You'd have to cut-up the oscillator stage to get enough gain to do a guitar.

Oh, wait. Guitar speakers are happy with zero feedback. Snip the wire from the 6SN7 cathode to the OPT and ground it. Input sensitivity will be ~1V.

> The 6K6 tubes could also be used in a guitar amp. Find a schematic that uses 6V6s.

Here we get into the taste of the tone. Fender guitar (and Gibson) evolved around the less-linear 6V6 class of tubes. It is possible that a more-linear 6F6 won't "sing" on the clean electric guitar. Of course it will play, and many styles may work fine with a very clean low-level tone, and of course it will break-up at "11" as nasty as any other tube, so it may be sweet IF the impedance can be cured and some more gain made-up.

[bear]

Hey NewPaltzWonder!

I sent you an email... to the address that ur account is set up on.
I presume you don't read that account?

Shoot me an email, I'm in ur area...

_-_-bear

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