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SVT LTSpice Model (geek content) and a question for users

fdeck

fdeck

Supporting Member
Commercial User
Mar 20, 2004
18,674
21,402
Madison WI
sites.google.com
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HPF Technology LLC
#1
Okay for all of you amp geeks out there, I entered the SVT preamp circuit into LTSpice, an electronic simulation program.

Invalid Link Removed

Unpack the zip file and double click on the ".asc" file after installing LTSpice from Link Removed

I am indebted to Duncan's Amp Pages for the 12AX7 triode model, and used Joe Piazza's SVT schematic as my starting point.

Now a question for all SVT users: Which channel do you use, and how do you set your tone switches & knobs? I am particularly interested in how folks use the Mid control. Assume that I have ulterior motives. :D
 
#6
What am I supposed to do with this LTSpice program, F? I see you can click on different parts of the circuit and graphs come up, but I can't figure it out.

BTW, good luck on building your Ampeg emulating pre...I'll be happy to beta test it and I might actually send it back, even if it works ;)
 
#7
Cool spice model.
Are you going to add in the output section?

Some things to keep in mind are component variances
Tubes are ±20% as I think some of the original carbon composite resistors were.
The other fun thing you can do is show wear like on old caps loosing capacitance. When you drop the values on the supply caps the amps response output changes dramatically.

A gentler introduction to schematics and circuits for these old amps is Peavey Revalver. It looks to me that Peavey (Alien Connection) figured out a way to speed up the spice to realtime.

I can guess - every audio engineer seems to want to do a project with tubes. A right a passage. Sounds like Fdeck it heading for a pre-amp :)
 
#10
JimmyM said:
What am I supposed to do with this LTSpice program, F? I see you can click on different parts of the circuit and graphs come up, but I can't figure it out.

BTW, good luck on building your Ampeg emulating pre...I'll be happy to beta test it and I might actually send it back, even if it works ;)
Click to expand...

I'll have to figure out how I can provide a gentle tutorial. Maybe a collection of screen shots would do the trick. LTSpice is "user friendly" only as far as any engineering software ever is.

I've looked at a bunch of amp schematics online, and most of them are pretty simple. For instance, the bass / treble circuit in the SVT and other Ampeg amps is the circuit published by James in 1948. But that SVT midrange control is unique.

I drew up that schematic, mainly to confirm my guess that the midrange circuit is just a basic active mid. Similar circuits are published, though typically with op amps, to illustrate how a graphic EQ works. But in real EQ's, with rare exceptions, the inductor is replaced by a simulated inductor circuit.

Indeed I am playing around with a new preamp design, but I have decided not to target any specific preamp for emulation. So far my project exists entirely inside LTSpice and my mind, and I don't want to embarrass myself by sharing a circuit that turns out not to work. However, when I get my first prototype built and working, I will put the schematic and some other details online. I want this to be functional, with decent tone, and something that a DIY'er can tackle.
 
#11
As friendly as any engineering software? I guess I can pull it off my computer now. I'll never be able to understand it. It's true...you can't teach an old dog new tricks ;)

Anyway, hope it turns out well for you.
 
#12
fdeck said:
I'll have to figure out how I can provide a gentle tutorial. Maybe a collection of screen shots would do the trick. LTSpice is "user friendly" only as far as any engineering software ever is.
Click to expand...

Aww, it only took me a few hundred hours or so to get anything useful beyond schematic drawing. ;)

Thanks for sharing that file. I haven't had time to really dig into it yet, but I will eventually. And good luck with the new preamp. I'll be posting my latest DIY build soon -- just sending off the front panel tomorrow for etching and when I get that back it'll be done.
 
#13
Passinwind said:
Aww, it only took me a few hundred hours or so to get anything useful beyond schematic drawing. ;)

Thanks for sharing that file. I haven't had time to really dig into it yet, but I will eventually. And good luck with the new preamp. I'll be posting my latest DIY build soon -- just sending off the front panel tomorrow for etching and when I get that back it'll be done.
Click to expand...

How are you getting the panel done?

Are you offering to show us your etchings?
 
#14
fdeck said:
How are you getting the panel done?
Click to expand...

Front Panel Express in Seattle. I finally built something that feels worth the expenditure to me. It's actually an engraving/in-filling process, as you probably know, but calling it 'etching' leads to:

Are you offering to show us your etchings?

I'm easy, but I'm not cheap! :cool:

Here are some specs that I measured this morning:

Link Removed
 
#15
Passinwind said:
Front Panel Express in Seattle. I finally built something that feels worth the expenditure to me. It's actually an engraving/in-filling process, as you probably know, but calling it 'etching' leads to:

Are you offering to show us your etchings?

I'm easy, but I'm not cheap! :cool:

Here are some specs that I measured this morning:

Link Removed
Click to expand...

Nice!
 
#16
Ok, apologies for reviving a REALLY old threat, but it's still number two or so in my Google search results :D. The LTSpice schematic is flawed, in the sense that the Ampeg SVT uses the 12DW7 instead of only the 12AX7. The 12DW7, as many of you might know, is half a 12AX7 and half a 12AU7 in one glass envelop. The latter having totally different characteristics (plate impedance, amp. factor, headroom, etc.). The schematic will therefor NOT give an accurate simulation. Substituting U1, U2 and U6 for a 12AU7 will do the trick though.
 

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