Let me pick your vacuum tube brains....

John_C

John_C

formerly Skeksis268
Dec 30, 2008
3,455
1
36
Coventry, UK
www.myspace.com
Schematic image: http://db.tt/kqmgs7a
or if you'd prefer a PDF: http://dl.dropbox.com/u/910836/FA 60 Schematic.pdf

am i right in thinking that the second valve stage (marked V3-B) here is in fact biased self bias, controlled by the gain control (RV1)? Doesn't make much sense to me

Or am i completely missing something here? I've set up a simulation of the clean channel in LTSpice and i'm getting all sorts of horrific crap and the source of the problem seems to be something to do with the bias of that valve.
 
looks like a fake-schematic, like the official ENGL-schematics.
C4 should go to the grid of V3b, and there is at least one missing coupling-cap at v3b-anode. but its possible that there are more components missing.

regards, khe
 
I can't see anything wrong with that stage, is just cathode biased (or self-biased as you say).
The filter before it is the classic high shelv + bright cap bypassing the upper half of the gain control, so nothing wrong with that either.

What bugs me is V1-A, I don't understand why did they bother polarizing that stage, since its completely unused...
Stupid question: is it possible that you've made the mistake of connecting the pin 6 of V1-A with R8 in your Spice circuit? Looking at the schematic, they cross each other but they're not connected.
 
yeah i have no idea about V1-A, but i can confirm that it's like that on the actual circuit. (NB the codes in red on the image just represent the actual labelling on the board)

I have pin 1 attached to R8 and C4 attached to R10 etc. but the two wires are not connected.
 
You must have some other mistake then, maybe posting a screenshot of the Spice circuit would help.

Also, another stupid note: remember that you should still take into consideration the block formed by R12-RV6-C10-ecc. , since it is part of the load of V1-B even if you're using the clean channel, so it's going to have its influence on the overall sound (this is not going to fix your issue, anyway).
 
here's a pic of the schematic as it is at the moment
Captureoftestinprogress.PNG


the NH in front of the 12AX7 just means i'm ignoring the heating

I really have no idea what's going on, i get this:
Capturetrace.PNG

Red is 10 times the top channel output, green is at the grid input of U4 and blue is just above C5 (names from LTSpice schematic)
 
The schematics looks correct... what's up with C18? Disconnected?
Where did you put the voltage meter for the red signal, exactly? You may want to check the voltage over R11 too, to check if the U4 output makes sense.
The green signal looks correct, the second stage has a small cathode res and it's easily going into grid current limiting (upgoing cycle chopped off in your image).

hah, just realised that the tone controls on channel 2 are a 100% copy of a marshall 1959
Click to expand...
Yeah, this happens in a shitload of amps actually, but in this case it is going to respond completely different because of R28 = 100k... WTF? They probably didn't want to load the tonestack driver too much, but it is really unusual... interesting though.
Also, R31 with C23 forms an high shelv filter, like Mesa does on the Rectifier on Modern mode, but they use a pot, while here R31 is fixed.
You may consider putting a 100k or 250k pot instead of R31, if you're looking for some mods. The whole tonestack is voiced a bit dark because of R28 and a touch more highs could be better...
 
Hmm no negative feedback? I think it would benefit from a mod there. Add a depth control and a presence control.
 
I left c18 disconnected as there was no value given for it on the schematic. Just checked on the actual board and there's holes marked out for it, but it's not there.

The red signal is taken from the output of the 1st channel marked ch1out
 
Thanks for the suggestions, i'm gonna get a fully working model going, mess around with some mods on there (i have a small amount of electronics knowledge from my physics degree, but all the valve stuff is pretty new to me) before i move on to the physical amp

Any ideas as to what's going on with that red signal?
 
Problem solved, to an extent. Turns out that the bass control was turned down, which ofc meant that a 40Hz test signal was pretty much only gonna show harmonics, hence the wierd output.
 
That's why I never use pots in LTspice, I always just throw 2 resistors in their place. Allows me to 'see' what they are set at a bit better.
 
haha shit these simulations take a long time, just playing a DI through it to see what i get and to see if it even closely approximates the real thing. Currently working at 2ms of audio per second..........this is gonna take a while :p
 
Yep those tube calculations are what really does it.

When you get better at it, you can actually figure out which components on the schem don't really do anything tone wise in the simulation, so by removing those pieces you can drastically make it faster.

But it's so cool how real the end result sounds. Too bad spice wasn't super fast, then amp sims would sound 99% better.
 
OT: where do you get the vacuum tube libraries? I don't know much about how these programs work when it comes to getting more than the stock components.
 
http://www.duncanamps.com/spicemodels.html for the models, they seem to behave quite convincingly.


Wolfeman said:
Yep those tube calculations are what really does it.

When you get better at it, you can actually figure out which components on the schem don't really do anything tone wise in the simulation, so by removing those pieces you can drastically make it faster.

But it's so cool how real the end result sounds. Too bad spice wasn't super fast, then amp sims would sound 99% better.
Click to expand...

The end result sounded.........shit but real :p and also there seems to be waaaay more gain available than in the real amp.
 
skeksis268 said:
The end result sounded.........shit but real :p and also there seems to be waaaay more gain available than in the real amp.
Click to expand...

Well you put it in front of a cab sim right? It will sound horrible if you don't.

You probably have the voltages set too high, that will increase the gain like crazy. You really need to open up the amp while it's running and under load, and see what the tubes are getting.
 
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