High-ohm grid resistor in tube (pre)amp design

On a sunny day (Thu, 12 Nov 2009 12:05:16 -0800 (PST)) it happened John Walliker wrote in :

perhaps

state amp and high

of those old tube things,

If you can describe it, then you can program it.

You understand wrong. With quiche, real men eat strawberry-spinach salads ;-)

No transformers in the signal path.

As for "instruments", I designed and built a 400W guitar amplifier once-upon-a-time ;-)

I can still design with tubes, though I now limit myself to 1KV and upwards stuff. Why waste all that power? ...Jim Thompson

Like all engineering, it's what gets the job done in the most effective way and a transformer is a perfectly respectable component to use for such an application, especially considering it's history of getting the job done well at low cost. It even adds weight to the box, so the roadies think it's been properly screwed together :-). Such stuff matters and you build the product, imperfections and all, to sell into that market. I got into micros in the late 70's, so you can see where my head was going. There's only so much you can do with a class ab amplifier and it was time to move on.

If you could accurately model a Marshall 100 or Watkins AC30 or similar and then dsp the resulting signal chain, you can bet that some muso's would still say they could still tell the difference and to be honest, they probably could :-)...

Regards,

Chris

Grid current causes the coupling cap to become charged and when the signal stops it will bias off the following triode stage until it discharges.

Something best avoided in a guitar amp.

..... Phil

What's a 'Watkins AC30'? The well-known AC30 certainly wasn't one of Charlie Watkins's creations.

Chris(a different one)tofire

The requisite DSP techniques hadn't been rummaged by 1978. WEM carried on making the 'Copycat' with a tape loop and an AC induction motor long after bucket-brigade devices had become available, and then started using them after the rest of the industry had gone digital. Line 6, for one, have made a reasonable job of simulating 'valve sound' in DSP, but that's only my personal opinion. As noted in another thread in this Usenet group, it's believed that Line 6 upsample before applying a non-linear function (e.g. tanh) so the highest of the resulting harmonics don't create noticeable enharmonic aliases when they beat with the sampling frequency.

Chris(tofire)

PS: of course that should have been 'Copicat'.

"christofire"

Much better preforming but more expensive units existed, like the Klempt and the famous Binson Ecorec - the latter using a rim driven drum which eliminated most of the horrible problems experienced with tape loops.

There are still no readily available digital units that produce the same kinds of sound effects possible with these mechanical units.

.... Phil

I've seen your photo of the (futuristic?) 120 mm drum elsewhere but I've never had the opportunity to examine one in detail. Was the rim of the drum coated with oxide in some way like tape, or was it just a steel drum?

Chris

"christofire"

1. A machined aluminium ring of about 10mm square cross section and 120 mm in diameter.

2. A thin, *continuous* steel band about 7mm wide, shrink fitted onto the above ring.

1. A pressed steel disk that held a 12 mm steel centre shaft and the above ring.

The surface speed of the steel band was about 410 mm per second or 16 ips.

.... Phil

Amazingly complicated, but it did the job so effectively. It's a shame the Boss RE-20 has feedback from all the delays instead of just the longest one, otherwise it could have made a good job of the Meazzi.

Chris

Quite right, memory fades. The AC30 was by Vox, from the 60's initially and later reintroduced by the company in the late 70's. 4 x EL84 in the output stage, iirc...

Regards,

Chris

OK, I'll get a noise gate allright -- I believe it's called a "killfile", and I'm testing it with your messages right now. OK, it seems to work pretty well. Thanks once again for your helpful insights.

Best regards,

Richard Rasker

"Richard Rasker is a TROLL "

.... Phil

or semiconductors.

There 'are' tube based op amps. Not very common anymore, but the first op amps were all vacuum tube based.