Increasing clean headroom in vintage SVT?

[okabass]

I As sbd mentioned before - antiparallel diodes in the poweramp are the magic ingredient of the trademark svt tone. These can be removed by your tech really easily. The sound then is cleaner, but also lacks of the typicall svt grind.
If the power tubes start to clip, then it can sound really harsh. These diodes can be replaced back very easy.

HI
Sorry, that's common BS. SVT power amp needs around 300mV for full power. The pi diodes 1N456 or 1N3070 (similar to 1N4148) start to clamp around 700mV or so (the threshold voltage). So, the power amp distorts much earlier than the diodes start to distort. The diodes are merely a safeguard limiter.

 

[pickles]

If you want a similar tone but a little cleaner/clearer and with more headroom, you could try an SVP-CL with a Crest CA6/9.

 

[ariwax]

Don't muck around with your tubes. You can destroy your tone.Run it into a THD Hot Plate and then into a SS power amp. That's how I get my little V4BH to put out 2000 watts of clean headroom, at 8 ohms. I am told it can be heard several blocks away, and I can dial in as much or as little saturation as I want from the pre AND the power sections. Those little Hot Plates are a miracle, and cheaply gotten on the evilBay.

 

[Psycho Bass Guy]

Don't muck around with your tubes. You can destroy your tone.Run it into a THD Hot Plate and then into a SS power amp. That's how I get my little V4BH to put out 2000 watts of clean headroom, at 8 ohms. I am told it can be heard several blocks away, and I can dial in as much or as little saturation as I want from the pre AND the power sections. Those little Hot Plates are a miracle, and cheaply gotten on the evilBay.

No; they're a great way topossibly destroy an otherwise killer amp and defintely kill a set of output tubes. I've commented on this several times before. If you value your amp, do NOT use a load attenuator.

 

[Joth]

Whoa, I replaced all the diodes in my SVT, and I put IN4007s in place of those two being mentioned, what did I mess up?

 

[ariwax]

Psycho Bass Guy, the Hot Plates are rated as safely tolerating 185 watts of output. It's true that it wouldn't be a good idea with a 300-watt SVT if you ran it flat-out, but as far as impedance matching, the THD's are far superior to the coterie of Power Soak/Air Brake boxes out there. The amp just sees it as a 4 or 8-ohm cabinet. You would be far more likely to blow out the resistor bulb on the Hot Plate than blow out your speaker tubes. And the THD is frequency-compensated, which has given me zero tone suck and plenty of saturation, and I can even get some sweet feedback if I lean into my amp. (Though I don't make a habit of this.)

And of course you go through power tubes more quickly with an attenuator if you're running them flat out all the time. Don't turn them all the way up, and they will last longer.

 

[Energy]

You want clean headroom? Get a HIWATT!

Just kidding.

The ways to increase clean headroom were already mentioned:
Check BIAS.
Replace tubes.
Try KT88 instead of 6550.
There's one more: try a GE 5751 preamp tube @ V1.

 

[Danm]

or KT88's

 

[Energy]

or KT88's

May I politely direct your attention to my last post, the one directly above yours:

Try KT88 instead of 6550.

 

[Psycho Bass Guy]

Psycho Bass Guy, the Hot Plates are rated as safely tolerating 185 watts of output. It's true that it wouldn't be a good idea with a 300-watt SVT if you ran it flat-out, but as far as impedance matching, the THD's are far superior to the coterie of Power Soak/Air Brake boxes out there. The amp just sees it as a 4 or 8-ohm cabinet. You would be far more likely to blow out the resistor bulb on the Hot Plate than blow out your speaker tubes. And the THD is frequency-compensated, which has given me zero tone suck and plenty of saturation, and I can even get some sweet feedback if I lean into my amp. (Though I don't make a habit of this.)

And of course you go through power tubes more quickly with an attenuator if you're running them flat out all the time. Don't turn them all the way up, and they will last longer.

I stand by my comments. You've obviously fallen for some sales language. The only difference between a Hot Plate and any other kind of power attenuator is its build quality. Using light bulbs as voltage soaks is an old tech trick for measuring output stability. BUT when used in conjunction with an actual reactive speaker load, seriously overstresses the output section. All "frequency compensated" means is that there is a shallow low pass filter to attenuate highs to simulate a speaker's natural high end rolloff. And last, the biggest no-no you've reccomended is to use a 300 watt amps with an attenuator rated for just over half that.

Power output is not governed solely by volume knob settings- it's a 300 watt amp period- failing to 'crank' it doesn't mean that it's going to be putting out any less than 300 watts with a strong enough input, which becomes impossible to judge sonically through an attenuator. It's an irresponsible suggestion in this case.

I'm glad it works for you with your 100 watt V4BH that is probably running Sovtek 5881's which have a LOT more operational headroom in your amp than ANY kind of 6550 or KT88 would in an SVT, and quite frankly, a vintage SVT should be on par volume-wise with MOST 2000 watt ss amps anyway.

 

[ariwax]

True, not a good idea with the SVT, as it is 300 watts. But wattage issues aside, I fail to see how using a resistor array on an amp (likesay, my 100-watter) would stress the power section any more than a voice coil would. The bulb is not the impedance engine on the THD, it's just the high-end rolloff/hiss gate (which you can switch off). The actual resistance comes from the guts of the Hot Plate - a reactive and dissipative network of inductors, resistors and capacitors - not just a resistor filament. Not all attenuators are alike.

Of note, aside from just "build quality", the Hot Plate does have two switches which boost (or rather, restore) high and low end to the sound, bringing it as close to true as my ears can hear - one other feature which makes it superior to other attenuators I've run across.

 

[mattvon]

Would this be a good time to mention I played a HUGE room with my SVT this past weekend and fell in love with it all over again?

The secret? Bright input. Did the truck perfectly. Actually, I played the second set with the regular input and it sounded even better.

Thanks to all who posted--rest assured, I'll be changing nothing on my amp.

 

[Psycho Bass Guy]

I fail to see how using a resistor array on an amp (likesay, my 100-watter) would stress the power section any more than a voice coil would.

Instead of putting power across a voice coil, your amplifier is dumping most of its output as heat into the attentuator. That in itself is not damaging, BUT prolonged high output use into an attenuator is not a good idea because you're continuously running the amp above its operational limits; to get power section compression, you HAVE to exceed the limits of the output section. Basically, you're redlining a Ferrari engine into a tractor tansmission. Something's eventually going to give.

 

[ariwax]

I see. It's not the attenuator per se, it's the saturation of the power tubes themselves. Thanks for the explanation.

For my purposes, it seems to make a lot of sense. I like a warm clean tone with a little grit - gives the sound a bit of a voice and helps me cut through a mix. But in order to achieve stage volumes with my V4BH I need to clip it more than I'd like. (Though it sounds great distorted, it's just not the most versatile tone for what I do.) So I load it down to line level with the THD and run it through a 2000 watt, 17-lb solid-state power amp with a high-low crossover. The Ampeg doesn't seem to mind - so far, anyway.

 

[asad137]

Instead of putting power across a voice coil, your amplifier is dumping most of its output as heat into the attentuator. That in itself is not damaging, BUT prolonged high output use into an attenuator is not a good idea because you're continuously running the amp above its operational limits; to get power section compression, you HAVE to exceed the limits of the output section. Basically, you're redlining a Ferrari engine into a tractor tansmission. Something's eventually going to give.

Uh, that doesn't make any sense.

When you crank up an amplifier when connected to a speaker cabinet, and get power section compression, it's fine. So why is it suddenly different if you have a resistor and inductor network that simulates a reactive speaker load?

Asad

 

[Psycho Bass Guy]

Uh, that doesn't make any sense.

When you crank up an amplifier when connected to a speaker cabinet, and get power section compression, it's fine. So why is it suddenly different if you have a resistor and inductor network that simulates a reactive speaker load?

Asad

With a speaker, you can HEAR the amp being pushed far too hard and will turn it down, not so with an attenuator. There are other factors as well. Not every attenuator is made equal, and for MOST of them, the way they accomplish voltage reduction does all kinds of nasty things to an amp that a speaker never could.