The Dark Arts of Direct Coupled Cathode Follower Drivers!

[mooreamps]

Ok, I have a question... So, the SVT amp is able to put some juice into a loudspeaker.. But, what does this type of amplifier sound like ? Is it something for a "Who" cover band ? or an amp for the Heavy Metal players ?

-g

[balijukka]

I believe the Ampeq circuit is there to provide a safe biasing and buffered signal for those 3 tubes at each side. Not to feed current for grid conduction.

jukka

[Steve Conner]

Ok, I have a question... So, the SVT amp is able to put some juice into a loudspeaker.. But, what does this type of amplifier sound like ? Is it something for a "Who" cover band ? or an amp for the Heavy Metal players ?

The SVT is the standard rock'n'roll bass amp since the 70s. Big, loud, muddy, rumbling bass. I used to play bass for one metal guitarist who referred to the SVT sound as "a f***ing giant hamster fart". Many metal bassists nowadays seem to use big solid-state rigs to get a clean, dynamic sound that contrasts the shredding guitars.

It's not quite as dirty as the Geezer Butler or Noel Redding school of bass would like. They use the bass like a rhythm guitar. You can get away with that in a trio.

I've played a few of the reissue SVTs, and the rackmount one, but never an original one or the really new SVT-CL reissue. They all sounded similar. I think a lot of the mojo is in the 8x10 cabinet with its crappy 32 ohm speakers.

I think the cathode followers in the SVT are there mainly to drive the bias feed resistors, which have to be a low value to avoid thermal runaway of all those paralleled tubes.

[kg]

i agree with jukka and steve: more of a grid leak resistance/stability issue than grid current/power issue.

[soundmasterg]

also, i found a copy of a post i made to the "old ampage" eons ago about the SVT circuit and my own thoughts on it:

Thanks for posting that Ken! I remember seeing it years ago but didn't copy and save it for reference, so now that its back I can. Great to see you posting again....now if only the rest of the old-timers would come back like Mike Soldano, Andy Marshall, etc.

Greg

[J M Fahey]

Well, really, driving the grid with a "more positive" voltage will make that tube be able to pass more current than before.
The fact that when said voltage is more positive than the cathode, that diode starts to pass current and its impedance lowers *a lot* is an unfortunate side effect, but let's not forget that the *voltage* is still the controlling parameter.
The amount of current is not that relevant per se, we are not talking "Beta" here.
As MerlinB hinted and Thermionic Scott completed, to be able to use that extra current you need to lower proportionately that load impedance; as was said here "rise the load line angle", said in a graphical way.
[OFF]
As a side note, I can proudly say that in Argentina I'm known as "the SVT killer"; *many*of the of the biggest Rock artists here have *sold* their SVTs to buy my "B300" heads.
Yes, SS heads.
How come?
They have a limiter, of course, *but* it's unlike all others.
It does not obsessively keep the amp clean as most (all?) do, it allows normally about 10% distortion, which grows to , say, up to 30% when slapping, tapping, or even plain old overdriving.
It growls, Musicians say that "the Fahey gets angry if you push it" or that "it puts a biker leather jacket over its tuxedo".
Another factor is that its damping factor is 1 (generator impedance around 4 ohms).
They keep their original 8x10"s though, or use 2xEVM15L in a *large* box.
Oh well, I got carried away, this topic was about a tube trick.

[Steve Conner]

Ooh, "SVT Killer", that's fighting talk!

Hartley Peavey says something similar in one of his White Papers. Apparently when Peavey developed their DDT system, they found that if it was set up to remove all clipping, it made musical instrument amps sound too dull. They had to slow down the response time of the DDT circuit to let the power amp clip heavily on attacks.

My own experiments bear that out too.

[mooreamps]

Ooh, "SVT Killer", that's fighting talk!

Hartley Peavey says something similar in one of his White Papers. Apparently when Peavey developed their DDT system, they found that if it was set up to remove all clipping, it made musical instrument amps sound too dull. They had to slow down the response time of the DDT circuit to let the power amp clip heavily on attacks.

My own experiments bear that out too.

Well, if I could get a couple hundred watts or so out of my 813 project, running two of them in p/p, it might be a set-up for bass in a small club or venue. I'd have to do an 8x10 cab instead of the 4x15 cab for something like that...

-g

[J M Fahey]

Hartley Peavey says something similar in one of his White Papers. .... they found that if it was set up to remove all clipping, it made musical instrument amps sound too dull. They had to slow down the response time of the DDT circuit to let the power amp clip heavily on attacks.

My own experiments bear that out too.

Well, then I must guess that both Hartley and Steve are quite smart guys
Here's a couple pictures of bands playing in *large* places (our "Luna Park" boxing stadium, seating 12000 and "River Plate" soccer stadium seating 48000) where the bands (Almafuerte and La Renga) switched to Faheys after selling theis SVTs. To be more precise, La Renga *is* sponsored by Ampeg, so they hide their rack behind a cardboard guitar case, "by chance" left exactly in front of it. They use their free Ampegs as extra power amps, driven from mine.
Oh well, Marketing and recording contracts !!
"Hidden" rack-mounted tube preamp Fahey


B300 heads (replacing an SVT Classic) , tube (blue) and SS (black) preamps, all with SS "angry" poweramps, driving a mix of 8x10" Ampeg and 2x15" Cerwin-Vega.

This guy has a real power complex: 4 Fahey B300 driving 4 8x10" Ampegs.
Under menace of lynching he actually used only 2.

[balijukka]

You should create a small and fast amp that can do anything. Call it Messi.

jukka