Ampeg SVT preamp question

[Newbietube]

Can anyone tell me what the supply voltage to the pre amp should be for the svt? I'm going by the shematic on schematics heaven. My plan is to build the pre amp first, then if finances/time allow, put it together with a 100-150 w power amp.
Also does anyone know where in the uk I could get one of the tone circuit inductors?
Cheers Just

[dhuebert]

Why do you want an SVT preamp?

Don

[Newbietube]

I was impressed by my friends SVT and also having used a hartke amp head for a number of years , wanted something less complicated but with more valve tone. I have also almost given up on using any of the graphic functions.
Is there a problem with the svt? Or is there a pre amp thats as good with similar tone shaping? The other consideration was the Mesa bass 400 pre amp.
Any input from experienced builders welcome.
Cheers Just

[dhuebert]

Kevin O'Conner http://www.londonpower.com/ has nothing good to say about SVT. As for me, everytime I hear one I am very impressed. My only concern is the great complexity of it and the esoteric components. For the novice I would recommend starting much more simply. There are my projects here that are very simple to build and yet challenging in their own way. I just look at that SVT schematic and think: I don't even understand this nevermind trying to build it successfully. This has been said many times, but layout is absolutely critical to these things and the more complex they are the more difficult it is to get it right. The last thing you want is to spend tens of hours building only to have a buzz you can't get rid of. Believe me, I speak from experience. I've had a couple of builds that you don't hear about here because I've abandoned them due to a problem I couldn't solve.

I don't want to put you off but I can't stress enough: for the new builder, start simple.

BTW the toolbox amp you see in my current avatar took a year of development and construction. It still has some problems associated with layout that can only be solved by dismantling it and starting over from scratch. The owner is thrilled with it but does use a graphic eq pedal for his upright bass.

Don

[Newbietube]

Thanks for the advice, I'll consider something simpler. Maybe it'll be the mesa bass 400 preamp seeing as it has only two valves. I'll have a good look at your projects too.
Cheers Just

[cartoonweirdo]

As far as a simple preamp with legendary ampeg tone, what about the B-15?

[soundmasterg]

Kevin O' Conner said that the SVT preamp was overly complicated, but he didn't trash them otherwise. It is all in his book "The Ultimate Tone volume 3". Its a fine sounding preamp, and channel 2 on the SVT is very similar to the B15.


Greg

[dhuebert]

On rereading my post I might have been more diplomatic about it.

I spoke to him on the phone recently because I wanted the schematic for his bass preamp project and ended up buying one of his books to get it. I wish I had kept my money. Anyways, on the phone he was pretty blunt about the SVT. As for me: I have heard many SVTs and have always been impressed. It may be psycho-acoustics, but whenever I hear a bass amp I don't like it turns out to be solid state. But to try to build an SVT as your first project sounds like a poor idea to me. I have been an electronic technologist working professionally since 1984 and it took considerable trial and error to get my designs working right.

Don

[Newbietube]

Fair enough, I have taken in all the mentioned points and have also started reading a few techie books on tubes. I have found a simpler design (mesa bass 400) which only uses two 12ax7's and only has a simple passive tone circuit. Bearing in mind I only want to construct a pre amp at the moment, how critical is it that the supply voltage is exactly the same as specified on the schematic? The transformer I have in mind is slightly short on voltage which after rectification would leave me about 20volts dc short of the spec. It would appear there are many conflicting opinions written regarding pre amp circuits (and tubes generally)so on one hand it would seem ok and on the other I could be hitting a dead end(and dont want to waste time and money). So really I'm asking this question of those whose experience is the last word on these things.
Thanks everyone for your advice so far.
Cheers Just

[soundmasterg]

I haven't talked to KOC on the phone about the SVT, so I can only go by what he printed about it. I've talked to him on the phone about other things and he has been very helpful over the years and has some great products and ideas.

Regarding the voltages....

Tube circuits are pretty much 20% tolerance so while having the voltage off by 20 volts may change the sound, the circuit will still function and you'll get an idea of what the real circuit sounds like. Changing the voltages up generally results in more gain and more high frequency content, where as changing the voltage down gives less gain and more breakup because there is less headroom and the circuit distorts sooner. Basically less voltage will give a browner tone. You can vary the dropping resistors quite liberally to tweak it to what you want.

Have fun!

greg

[dhuebert]

The transformer I have in mind is slightly short on voltage which after rectification would leave me about 20volts dc short of the spec

250 +/- 20 is perfectly acceptable. If the preamp you're looking at has a 12AX7 with a 100K plate resistor and a 1.5K cathode resistor and 250 volts B+ then it is all boiler plate. These things are very tolerant of variations in values. If your quiescent plate voltage is somewhere near half the B+ then you're good. Because the triodes are cathode biased, it takes alot to upset them. Don't be afraid to experiment with values.

Have you seen duncanamps tone stack calculator? Have a look at the tone stack you're considering and see if you like what it does in simulation.

I prototype things like this:



and when I'm satisfied I'll find a chassis and build it like this:



Don

[Newbietube]

Hi all after everyones input I'm still at the planning stage as I still havent bought any of the bits yet let alone started construction. My next question is power supply related. What sort of value capacitors should I use, seeing as I'm only going to be driving a pre amp for the time being? Obviously I wont need them to be as big as for a full power amp though I appreciate the supply will need to be nice and smooth.
Cheers Just

[Cygnus X1]

I'm in the same boat as you, Newbietube.
I want to build a low watt, high gain tube amp, but have never built an amp before.
I went ahead and bought the parts and pieces needed for a 5E3 tweed Deluxe, although that isn't even close to the sound I'm after.
Why?
To get some building experience.
So, for aroud 225 US dollars, I can easily get 4 or 500 for it, and continue.
And I learned a lot, even though it's a simple circuit.
I suggest doing something similar.

[EWBrown]

I'm not all that savvy about the finer points of guitar / bass amp designs, but I'd venture a SWAG that a tone control on a bass amp is pretty much irrelevant, due to the lower octave and less high freq response, and I'd just go with volume and use the controls on the bass itself for any tone "tailoring". This also goes along with the keep the signal path simple concept...

If any tone control is used, it could be a simple cap and pot to ground, between the first and second VA stage just to roll off the HF response.

/ed B

[soundmasterg]

If you've ever played a bass amp, you would notice that the tone controls are very interactive on some, and effect a large change in the sound on just about all of them. The Ampeg SVT uses an inductor for the midrange control so it either actively boosts or cuts, and the lows and highs are also widely effecting the tone. On the old Sunn 200S/2000S etc., the tones also make a large difference in the sound even though they are not active on that amp. The tone controls are voiced to affect the proper frequencies for bass, and they work just as well as a guitar amp's tone controls.

Greg