Critique my Pspice and simple tube amp design?

[Tonefishin]

I'm trying to learn about tube amp design and I've been trying to learn Pspice at the same time. I figure Pspice is a good tool to help me understand what is happening to the signal as it works its way through the amp.

I've been looking for a very simple schematic to break-me-in on Pspice and since I've also recently built a Firefly amp (design by Doug H on AX84), I decided to try to model the clean side of that amp. I got the tube models from a library put together based upon the Norm Koren site, and I'm still trying to figure out how to do the transformers. I'm using a straight DC power supply for B+.

I know I have something wrong in my model because it outputs only noise...no surprise, hahaha. So, if you're willing to take a peak at it and critique it I'd really appreciate it. I tried representing the pickup as a 1khz 1V dc voltage source with 7k resistance.

Also, if you know some of the quirks of these Pspice tube models or know of better ones, I'd appreciate anything you could share. If you happen to have a simple working Pspice tube amp project file you'd be willing to share, I'd be so very thankful.

Here's the link to the schematic and noise output trace:

http://i146.photobucket.com/albums/r...ch-n-Trace.jpg

Thnaks in advance for any help. Cheers, Jim

[Enzo]

garbage in, garbage out.

Look at your circuit. I hope the amp is not wired like the schematic. The schematic won't work, and won't pass signal. The output will be only whatever inherent noise ther is in V2b and V3.

1v is way too hot for a guitar signal. A tenth of a volt is more like it. But if you want to just characterise the circuit, then go ahead with 1v.

V2a looks OK. But after that things get crazy. R9 feeds the grid of V2b, but it is connected to ground. Not a lot of signal in ground. Perhaps R9 was meant to connect to the top end of R2? Or perhaps you intended another resistpr between R2 and ground?

V3 has no B+!!! Perhaps the primary of the transformer was to have a center tap connected to B+?

Correct those major flaws and see what it does.

[Tonefishin]

garbage in, garbage out.

Look at your circuit. I hope the amp is not wired like the schematic. The schematic won't work, and won't pass signal. The output will be only whatever inherent noise ther is in V2b and V3.

1v is way too hot for a guitar signal. A tenth of a volt is more like it. But if you want to just characterise the circuit, then go ahead with 1v.

V2a looks OK. But after that things get crazy. R9 feeds the grid of V2b, but it is connected to ground. Not a lot of signal in ground. Perhaps R9 was meant to connect to the top end of R2? Or perhaps you intended another resistpr between R2 and ground?

V3 has no B+!!! Perhaps the primary of the transformer was to have a center tap connected to B+?

Correct those major flaws and see what it does.

Yes, GarbInGarbOut is so true! Thanks for the critique!

I've been puzzled by the guitar pickup output level. I've dug up pickup specs that say 1.5 to 2 V rms for pickups and I have seen others say it is at 0.1V

Yes, the R9 to ground thing is a big mistake stemming from that I originally put in a variable resistor and then switched it to a set resistor (since I couldn't figure out how to make the variable one work...but I forgot to remove the ground connection from the variable resistor....big booboo on my part!

As far as that darned output Xfrmr, I still haven't figured out how to do that. It should be setup for push-pull w/ a CT...I'll get that worked out.

Thank You again for spending the time to look at this. Jim

[cbarrow7625]

For the CT transformer just try using 2 transformers. Wire the primaries in series and then the secondaries in series. The series junction of the primaries is now your CT. Ignore the series junction of the secondaries.

Good Luck.

Chris

[Tonefishin]

For the CT transformer just try using 2 transformers. Wire the primaries in series and then the secondaries in series. The series junction of the primaries is now your CT. Ignore the series junction of the secondaries.

Good Luck.

Chris

Hey thanks Chris! I'm doing that now. As soon as I get the K-linear coupling stuff (assigning L1, L2, etc. column values) figured out I'll give it another try.

[Tonefishin]

For the CT transformer just try using 2 transformers. Wire the primaries in series and then the secondaries in series. The series junction of the primaries is now your CT. Ignore the series junction of the secondaries.

Good Luck.

Chris

So in this case, maybe I don't need the K-linear for coupling?

[Tonefishin]

Here's the schematic and trace w/ the fixes. Still something goofy?

http://i146.photobucket.com/albums/r...h-n-TraceB.jpg

Any comments appreciated...jim

[gmoon]

You've forgotten the grounded signal path between V2a and V2b, as Enzo noted previously...

[Tonefishin]

You've forgotten the grounded signal path between V2a and V2b, as Enzo noted previously...

Yes, thanks. I had corrected that but forgot to post the new schematic...sorry.

[cbarrow7625]

The signal is still going to ground between the two stages. Move the 470k resistor to the TOP of the 500k resistor. Right now you have it connected to the BOTTOM of the 500k resistor, grounded!

The 500k resistor will give you the ground reference you need for the second tube stage.

What's with the 1k resistor on one leg of the OT secondary?

Chris

[Enzo]

I also wonder what R3 is for?

[Tonefishin]

Here's the updated schematic and trace. Looks pretty good...I think!?!

http://i146.photobucket.com/albums/r...race-Worki.jpg

....and thanks for the comments.


Enzo/Chris, R3 was necessary due to an error in PSPice without it. Pspice made the recommendation, actually. Here's what it said "Voltage source and/or inductor loop involving L2_TX2
You may break the loop by adding a series resistance"

[Tonefishin]

Anyone know why there is no gain through the first triode?

[mooreamps]

Anyone know why there is no gain through the first triode?

Yes. The bottom of R2 should be at ground, thus giving the first gain stage a plate load. Then ; take the signal {for the next gain stage } at the tap between C5 and the top of R2.

-g

[Tonefishin]

Yes. The bottom of R2 should be at ground, thus giving the first gain stage a plate load. Then ; take the signal {for the next gain stage } at the tap between C5 and the top of R2.

-g

Thanks! So many of my troubles come from trying to deal with that variable resistor by replacing it with a fixed one (and that I'm green). Now I can't wait to get back home to the model to fix it. Will update then.