The Amp Garage

Looks like the plot above came from Blencowe's load line plotter. Link for those who want to play (scroll to "downloads" at the bottom of the page): http://www.valvewizard.co.uk/links.html I think Merlin's load line plotter is intended for generic common cathode stages, which gives a good approxima...

Thanks! It's very nice graph which clearly demostrates the rather heavy AC load seen by V1a when only one side is driven. Now the jumpered inputs make a lot of sense - higher gain and a lot more headroom.

martin manning wrote:Btw, I've changed my thinking on this input stage again. Watch this space...

Looking forward to it, I'm still trying to wrap my head around the load line when only one side is driven.

90k is where I had to set the following stage load (Rl) to get the AC load line to appear in about the right place. Since the software doesn't model this configuration, I had to fool it. I worked out a method to solve this graphically, but I haven't drawn it up yet. So do you mean that you adjust R...

Triode A's plate current increases, but triode B's plate current decreases because triode B's plate voltage must follow triode A's, but its grid and cathode voltages remain fixed. That makes the load resistance seen by triode A appear to go down, making the AC load line much steeper. The chart is f...

martin manning wrote:I think 4k based on anode and screen both at 470V, 30W Pa max for 6L6GC.

In which case, the output power would be much higher than the "25W" rating. Anyway, I'm guessing the real impedance is probably between 4k-8k...

Really rough estimation, R = V^2/P = 470^2/25 = 8.8k.

Theashe wrote:If you plan on using LTSpice for simulating tube stages as well (and you should at least try it), you should get models from Duncan Amps:

Unfortunately, very few of the Duncan's tube models are accurate, and there is not much point to simulate the heaters...

wow... Just download the program from Linear, don't know what's the hassle...

You want simple... try PartsSim.

I assume that the higher voltage is acceptable if it's under idle conditions and there's minimal current flow? Yes, there have been many tests (and actual products) that use very high voltages - well above the datasheet limits without issue, even over long period of time, e.g., the Musical Referenc...

Slightly OT, but it has often been said that high plate voltage makes the amp sound hard, I have not build anything over 450V, so have no clue if that is true or not. What's your experience?

No need for AB2, 28W is no sweat in AB1, Modjeski even managed to get 35W from a pair long ago - the ingrediants are high plate voltage and high Ra-a.

Taking a closer look at the photos that you posted, I think the neon bulb is not used as a DC level shift (see my post above) but it's there to clamp the grid-to-cathode voltage - to protect the tube during initial power on, since it is direct-coupled to the first stage. So it is not really in the s...

Not sure how to read the resistor code on these... Any idea?