200 Watt SVT? How to do it?

[BassmanPaul]

I'm still leery of just pulling out two tubes as Kevin O'Connor (London Power) suggested, so I asked him if it's harmful to the output transformer, and he said it is not harmful and to check their FAQ which reads as follows:



So...

Does the SVT use a fixed-bias output stage, or is it cathode biased?

(I'm still waiting for *my* amp tech's response, since he's the one who's going to have to fix it when blow it up.)

P.S. I'm sick of all this amplifier crap. I'm gonna take up the drums. At least then if something breaks, you can see it.

The SVT like most tube musical instrument amplifiers is fixed bias.

If I may offer MY response, Just 'coz someone writes a book or even several books and has a web site, THAT doesn't mean the man is correct. Operating a BASS amplifier at a high volume without the correct load is dangerous to that amplifier. Tubes do not like being operated on anything but their designated plate impedance.

Paul

 

[edbass]

If I may offer MY response, Just 'coz someone writes a book or even several books and has a web site, THAT doesn't mean the man is correct.

In theory I agree with this logic. However, in this particular case, I think it does mean he's correct.

 

[Jim Campbell]

I'm definitely asking a specific question. How to run a legit SVT at 200 Watts.

The earth is littered with 200 watt all-tube amps. I have two of them right now and will buy many more before I'm carried feet-first out of my house.

But it's a valid criticism. Why spend all this energy trying to break a decent SVT when I could just go buy a Traynor?

Well... because I'm asking HOW to do it. Not SHOULD I do it.

I will probably wind up with a Reeves 225, or Morgan MP200 or some 100% custom boutique amp.

But the question will still haunt me... How to run a SVT safely at 200 Watts.

if i may......a lot of resources were expended to design and build the svt's,and they do what they do very well.....it may be possible to throttle them back without damage,i don't know,but i would think that svt components were chosen with with a specific intent,and altering the design without the same amount of research and development as the original design required might not be the best approach....

certainly a 200 watt svt would be nice,and i think even a 120-150 watter would be plenty for most venues,but to get there i believe the beast would be best if designed from the ground up,or at least reverse engineered by guys with the skill and resources to do it right

 

[JimmyM]

The SVT like most tube musical instrument amplifiers is fixed bias.

paul, my old friend, you're way more knowledgeable than i'll ever be, but the svt is adjustable bias.

 

[ehque]

Uh, you want less power out of your amp?

Why don't you just play all 300 Watts, but use a less sensitive cabinet? Or even, face one cab into the wall or something. I'm sure with the top cab facing you, and the bottom cab facing into the wall, your 300 watts will sound pretty much like 200 watts.

 

[fu22ba55]

i don't know,but i would think that svt components were chosen with with a specific intent,and altering the design without the same amount of research and development as the original design required might not be the best approach....

+10

I just got off the phone with my amp tech, and had a great discussion on this topic. (He's a veteran Class-A tube amp builder, and I've never been able to bring him something he couldn't fix.)

He basically echoed the above quote. He thinks modding an SVT DOWN to 200ish watts is a bad, bad idea. (Not to mention sacrilegious.) There are lots of ways to do it, but they are all "restrictive". Limiting power, adding resistance, etc...

(He also threw up in his mouth a little bit when I showed him the London Power power-scaling circuit. He says he thinks it would work, but he doesn't like the approach.)

He thinks the correct approach is to take a V4-B, mod the unit to accept KT-120s, and squeeze the hell out of it for 180-220 watts.

He says the mods would be 100% reversible, he wants to try it with the original transformers first, and if we don't get what we want, we can get bigger transformers made.

So, it's better to push a modded V4-B past its intended wattage than to attempt to restrict an SVT.

(It's easier to put a V8 in an AMC Pacer than to try and slow down a Peterbuilt with an anchor and a parachute.)

 

[edbass]

paul, my old friend, you're way more knowledgeable than i'll ever be, but the svt is adjustable bias.

Sorry Jimmy, but he's right. The vast majority of production amps are "fixed bias", however that just means the bias point is "fixed" externally the tubes.
A lot of amps (like your SVT) have a trim pot that makes this fixed point adjustable. The other bias method for a tube amp is using a cathode bias, in which the tube sets it's own bias point; as in a "Class A" amp.

 

[Jim Campbell]

+10

I just got off the phone with my amp tech, and had a great discussion on this topic. (He's a veteran Class-A tube amp builder, and I've never been able to bring him something he couldn't fix.)

He basically echoed the above quote. He thinks modding an SVT DOWN to 200ish watts is a bad, bad idea. (Not to mention sacrilegious.) There are lots of ways to do it, but they are all "restrictive". Limiting power, adding resistance, etc...

(He also threw up in his mouth a little bit when I showed him the London Power power-scaling circuit. He says he thinks it would work, but he doesn't like the approach.)

He thinks the correct approach is to take a V4-B, mod the unit to accept KT-120s, and squeeze the hell out of it for 180-220 watts.

He says the mods would be 100% reversible, he wants to try it with the original transformers first, and if we don't get what we want, we can get bigger transformers made.

So, it's better to push a modded V4-B past its intended wattage than to attempt to restrict an SVT.

(It's easier to put a V8 in an AMC Pacer than to try and slow down a Peterbuilt with an anchor and a parachute.)

that sounds pricey...i see reasonably priced v4b's often and i wonder if 2 stock units/aby box would serve you better ,allow more flexibility for small or larger rooms,a back up amp,and the opportunity to cannibalize parts in a pinch.....jimmy used to use a v4b with a fridge and iirc said it was plenty loud

 

[fu22ba55]

that sounds pricey...i see reasonably priced v4b's often and i wonder if 2 stock units/aby box would serve you better

There's no doubt two V4Bs would be a good solution.

I guess price depends upon how much I pay for a V4B, and whether or not we could keep the stock transformers. The transformers are the expensive bit.

I'd have to compare the mod cost vs. having a custom amp built from scratch.

 

[sanfordandsonny]

Have you weighed the advantages of a v4's sensitivity vs the v4b's ultra low.
I don't usually use the ultra low in the SVT because of what it does to the midrange
And the v4 and v4b and the svt all have their eq points in the same place.
Thank you for the heads up on not destroying my "lifer" amp.

 

[fu22ba55]

Have you weighed the advantages of a v4's sensitivity vs the v4b's ultra low.
I don't usually use the ultra low in the SVT because of what it does to the midrange
And the v4 and v4b and the svt all have their eq points in the same place.
Thank you for the heads up on not destroying my "lifer" amp.

I'm not a huge fan of the ultra-low when given the opportunity (my V9 has no ultra-low), so no big loss there...

What does the sensitivity do? Is it just a variable pad that lets you determine how hard you whack the input stage?

 

[BassmanPaul]

paul, my old friend, you're way more knowledgeable than i'll ever be, but the svt is adjustable bias.

It's STILL a fixed bias amp Jimmy. The bias is a provided third supply sometimes known as the "C" supply. This dates back to the old radio days when batteries supplied the power. The heaters were driven by the A battery. High voltage by the B battery, thus the B+ that we use today, and the bias was the C battery. The fact the the supply is variable has no relevance. I suppose you could call it a variable fixed bias amp if you wish.

Paul

 

[poorbassist15]

+10

I just got off the phone with my amp tech, and had a great discussion on this topic. (He's a veteran Class-A tube amp builder, and I've never been able to bring him something he couldn't fix.)

He basically echoed the above quote. He thinks modding an SVT DOWN to 200ish watts is a bad, bad idea. (Not to mention sacrilegious.) There are lots of ways to do it, but they are all "restrictive". Limiting power, adding resistance, etc...

(He also threw up in his mouth a little bit when I showed him the London Power power-scaling circuit. He says he thinks it would work, but he doesn't like the approach.)

He thinks the correct approach is to take a V4-B, mod the unit to accept KT-120s, and squeeze the hell out of it for 180-220 watts.

He says the mods would be 100% reversible, he wants to try it with the original transformers first, and if we don't get what we want, we can get bigger transformers made.

So, it's better to push a modded V4-B past its intended wattage than to attempt to restrict an SVT.

(It's easier to put a V8 in an AMC Pacer than to try and slow down a Peterbuilt with an anchor and a parachute.)

A search of other forums and Google yields this about the KT-120:
"That's a big damn tube."

Jut get a Peavey VB-2 if you need 200 watts.

 

[sanfordandsonny]

whacking the input so worth it over the ultra low
and if you get a v4 or vt22 with a broken reverb it's even cheaper!!!!!

 

[JimmyM]

It's STILL a fixed bias amp Jimmy. The bias is a provided third supply sometimes known as the "C" supply. This dates back to the old radio days when batteries supplied the power. The heaters were driven by the A battery. High voltage by the B battery, thus the B+ that we use today, and the bias was the C battery. The fact the the supply is variable has no relevance. I suppose you could call it a variable fixed bias amp if you wish.

i so don't get that, paul. you can adjust the bias with trimpots but it's fixed? wish i understood. can you try using dumb guy language?

 

[UncleFluffy]

i so don't get that, paul. you can adjust the bias with trimpots but it's fixed? wish i understood. can you try using dumb guy language?

From the user's point of view it's adjustable, but from the amp's point of view, it's fixed. You can change the bias - but the amp can't change it itself.

[Invalid or Expired Link Removed]

 

[JimmyM]

someone's got to streamline these terms to say what they mean and mean what they say!

 

[UncleFluffy]

someone's got to streamline these terms to say what they mean and mean what they say!

Sounds like a plan. I'll fix electronics if you fix music theory ...

 

[JimmyM]

Sounds like a plan. I'll fix electronics if you fix music theory ...

ha! i'll get right on it!

 

[Dave M]

i so don't get that, paul. you can adjust the bias with trimpots but it's fixed? wish i understood. can you try using dumb guy language?

In a fixed bias arrangement the bias voltage is a fixed value applied to the tube.
This fixed amount of voltage can be varied/adjusted by use of a trim pot. Or, you can do it like Mesa does and use a non-adjustable arrangement of this "fixed" voltage.

Fixed bias can, and 95% of the time IS, adjustable. Just because Mesa does not provide an adjustment control (read: trimmer) does not mean that they are the only ones with a fixed bias arrangement. Ampeg, Fender, Marshall, etc., amps that have a fixed bias arrangement also have trimmers to allow adjusting this voltage.
The other option is cathode biased, and you probably don't want that in a modern bass amp.

Make sense? I think it's a terminology thing?