Tube preamp heads: Gimmick??

[Wasnex]

Most other brands don't find a need to clamp this. They haven't found a need to do some special protection on the input to the power amp section. It would probably be more complex than diodes.

Here's an old post that you may find relevant.

...Also, whenever you push a tube power amp to such state you have other stuff happening as well: The screen grid current draw of pentode and tetrode output tubes suddenly ramps up (as the tube goes to griod conduction) and this will often modulate the screen voltage of the tube, which in turn results to gain compression within the tube. Plate voltage will also drop so the clipping threshold reduces even further.

Also, whenever one tube "deep saturates" in a push-pull amp the other will "cut off". Cutting off the current flow in a circuit that is transformer coupled to reactive load will create transient ringing, which then add high voltage "spikes" to corners of the clipped wave, corners of the crossover clipping, and corners of wave clipped by OT saturation. Basically to everywhere where current flow suddenly halts. In poorly designed systems these high voltage surges may even arc through and damage the amplifier itself.

SVT, on the other hand, is an interesting case because its output stage design tries to minimize many of these effects: The screen voltage is somewhat regulated to prevent screen current from modulating it, and the power tubes are driven by DC coupled low source impedance high current amplifiers (cathode followers) that actually handle driving power tube grids to conduction rather nicely. (For same reason there is not much of the aforementioned bias shift during excessive overdrive). Additionally the input has diode clipper to "clamp" input voltages below certain level so that it becomes somewhat impossible to drive stages of the power amp to "deep" saturation.

SVT power amp is a very good example of VERY different design goals and practices than what you usually encounter in generic tube guitar/bass amps.

 

[seamonkey]

I have no doubt that the diodes change the overload characteristics of the SVT in a way that people find desirable. That doesn't mean distortion was the original intent of the circuit. I believe some of the reason for abandoning the 6146 based output section has to do with screen related failure when the amp was pushed into hard overload. I believe the screens in 6550 based amps are able to take more abuse. The diodes could be a hold over that was originally designed as protection for the 6146, but was retained because people like the way the 6550 output section overloads with the diodes in place. Or the diodes may still be intended primarily for protection. To be clear, an intent of the original designer was to make a robust amp that would withstand the rigors of professional (ab)use.

From what I have seen most users don't play their SVTs at maximum output levels, so the diodes should not typically have significant impact on the sound. It is easy enough to take some oscilloscope readings to determine the output power levels at which the clipper circuit becomes active. Until readings are taken, it's all supposition.

The Trace Elliot V4, V6, and V8 all have clipper circuits after their master volume control. The V4 and V6 are known primarily for clean power and have relatively low gain preamps. The V8 is also known for clean power, but it has a high gain circuit (V2) that can be switched into the preamp for OD. The clipper in the V8 is between V5a and V5B. The master volume controls the gain of V5a. The amp is setup to produce high gain OD in the preamp that is independent of master volume settings and clipper activation. If distortion was the intent of the clipper it should be before the master volume. Consider V2 in Marshall JCM 800 and JMP amps introduces asymmetric clipping into the signal well before the output section begins to clip.

I have seen posts from multiple techs, who suggest the drivers in V-Types provide insufficient signal to push the output section to full potential. This could be related to the clipper circuits, however all models produce lots of clean power with no aural indication of a functioning clipper, and these amps begin to compress long before any evidence of OD becomes perceivable. Additionally, all of the bench tests I have seen where power was measured showed the amps exceeding their stated power specs.

You must have some of these measurements.
Where are they?

Even the new lower power ampegs have these diodes.
Funny that other tube amp makers haven't been copying this low cost trick to add a little protection. It's not patented.
The good news is people can buy a Rat Tale cable now and get the same protection for their own brand of amp. And get some classic diode tone at the same time.

 

[Bassbeater]

Here's an old post that you may find relevant.

To this thread?

 

[Wasnex]

You must have some of these measurements.
Where are they?

Even the new lower power ampegs have these diodes.
Funny that other tube amp makers haven't been copying this low cost trick to add a little protection. It's not patented.
The good news is people can buy a Rat Tale cable now and get the same protection for their own brand of amp. And get some classic diode tone at the same time.

If I owned an oscilloscope I would happily provide the results. How bout you?

Sorry but a Rat Tale cable is not going to make an amp sound like an SVT and it will not be calibrated to track with the overload characteristics of any particular amp. I have several OD pedals that are clipper based. None of them produce the sound of a driven SVT.

The V4 schematics I have seen do not have the diode clippers. I haven't played a V4 but many say it sounds a lot like an SVT, with the exception that the SVT power amp has a boost in the low mids that is absent from the V4 power amp.

 

[seamonkey]

If I owned an oscilloscope I would happily provide the results. How bout you?

Sorry but a Rat Tale cable is not going to make an amp sound like an SVT and it will not be calibrated to track with the overload characteristics of any particular amp. I have several OD pedals that are clipper based. None of them produce the sound of a driven SVT.

The V4 schematics I have seen do not have the diode clippers. I haven't played a V4 but many say it sounds a lot like an SVT, with the exception that the SVT power amp has a boost in the low mids that is absent from the V4 power amp.

The diodes are outside the feedback, there's no "tracking of overload" it's strictly passive.
The SVT preamp must be a bit attenuator. In any other circuit that would use clamping diodes you'll see many stacked. They just aren't used because they are essentially useless.

Good news is IK MultiMedia Ampeg SVT has Certified models of the Ampeg amps. Certified by actual Ampeg engineers. You can try it yourself to see and hear the clipping.

Diode clipping is well know to be a "warm", "soft clipping" used in many heads, pedals, and modelers. Overdriven Solid-State is the roots of Rock.

You can have the last word - because iggy

 

[Wasnex]

You can have the last word - because iggy

OK....have a nice day.

 

[Wasnex]

To this thread?

Yes

 

[DavidEdenAria]

I owned an SWR SuperRedhead and rolled tubes in that thing several times. Zero difference in tone.

Same for the Eden Metro combo I had.

I forget which one, but one of the tubes in the SVT 6 Pro head made a ton of difference.

And V2 on the Mesa Walkabout totally changes the tone depending on the tube.

So, It depends.....

Same with my Eden Metro, changing the 12ax7 made little to no difference....it likely could make a difference in noise if the existing were to get noisy?

I save the tube rolling for my older guitar amps with simple circuits.

 

[kat888]

Those music man amps were only made for short period of time, and they are hybrids. I can’t think of any other significant amp that used solid state pre and tube power amp.

I played with a guitarist in the 80s who had a 'Rockit' combo. NZ made with Mesa Boogie styling. SS preamp, 130W valve power amp into an RCF 15" speaker. Oh the horror, the horror!

Not a bass amp and by no means 'significant'...

 

[Arthur U. Poon]

Anyone that understands this better than me, please correct me if I'm wrong.
But it seems to me that the whole deal of putting 1 to 3 tubes in the preamp section of a class D amp like many manufacturers are doing, is little more than a gimmick.

I've tested a few of these tube preamp head's compared to just solid state head's with no tubes at all and I haven't found much of a difference.

Recently I tried out the GK MB500 Fusion (3 tubes in preamp) against the Aguilar TH500 which has no tubes and I found the GK didn't do much for me but damn the TH500 was meaty, ballsy & Tubey.

Furthermore I see some companies trying to further mislead possibly unaware consumers by lighting the tubes with LEDs to further enhance the "Tube Experience".
One of these companies is Bugera but I'm not surprised after seeing their far fetched wattage ratings that they claim as well.

Do you guys agree or are there class D head's with preamp tubes that really get a more tube like sound.
I've tried the Markbass heads of this type and also was not dazzled enough to understand paying more for this feature.

Thanks.

I haven't tried the Fusion 500, but I own a TH500, and I agree with you, it sounds pretty darn tube-like. I've owned a few different GK heads over the years, and to my ears they can sound warmer than what I typically expect from a solid state amp.

I use a Demeter HBP-1 tube/solid state preamp in my rack setup. It has a single 12AX7 tube in it's circuitry, but to my ears it doesn't epitomize the sound I expect from tubes. My other preamp is a Jack Read Purity tube preamp, and IMO it's tone is far more tubey than my Demeter. IMO my HBP-1's tone leans more towards a solid state tone.

Of the hybrid amps I've tried, the Ampeg SVT4-Pro sounds the closest to an all tube amp to me, but it's still different than an all tube SVT.

 

[Arthur U. Poon]

Same with my Eden Metro, changing the 12ax7 made little to no difference....it likely could make a difference in noise if the existing were to get noisy?

I save the tube rolling for my older guitar amps with simple circuits.

I haven't tried the Eden Navigator preamp, but I experimented with a few different NOS 12AX7's in my Demeter HBP-1 preamp, and I couldn't hear much, if any difference from the EH 12AX7 that it came with.

 

[chadds]

Back in the day some hybrids had little more than a tube lighting up.
These days I use class “what ever” with a Monique and it’s as they say “liquid Jesus dipped in butter.”
To be clear there is circuitry available that can deliver a lot of “tubish ness” in other forms that aren’t tubes.

 

[Arthur U. Poon]

....Good news is IK MultiMedia Ampeg SVT has Certified models of the Ampeg amps. Certified by actual Ampeg engineers.

Interesting.

 

[Korladis]

I'd omit the part about tube preamps for a few very select cases. The old Musicman amps, for instance, were a tube power section with a solid state preamp and I thing by anyone's definition these would be considered a tube amp. Definitely a rare bird though.

Some old Peavey guitar amps, too. The Mace ones. Ridiculously high power for a guitar amp.

While technically correct based on the notion that one component uses tubes and the other uses transistors, I think you're the only person I've ever seen refer to them as anything but tube amps since the power section was tube.


Which is why I referred to them as rare birds. The only other one I can think of were those Crank guitar amps that were popular in metal circles in the mid-00s.

A guitarist in a band I used to be in had an old Peavey Mace. 150 watt tube power section. Solid state preamp. Coupled with its oversized 4x12, it was one of the loudest guitar amps I've ever heard.

He's pushing that "clipping diodes" claptrap again, eh? About once a year he starts in with that until someone sets him straight. Then he waits a while before he drops it again because I guess he thinks we've all forgotten about it.

Stay classy, San Diego.

That junk again?

I remember watching one of the resident engineers thoroughly debunk that BS a couple years back.

If I owned an oscilloscope I would happily provide the results. How bout you?

Sorry but a Rat Tale cable is not going to make an amp sound like an SVT and it will not be calibrated to track with the overload characteristics of any particular amp. I have several OD pedals that are clipper based. None of them produce the sound of a driven SVT.

The V4 schematics I have seen do not have the diode clippers. I haven't played a V4 but many say it sounds a lot like an SVT, with the exception that the SVT power amp has a boost in the low mids that is absent from the V4 power amp.

They do sound similar yes. But not the same. That low mid boost might be why. The V4 sounds a bit more aggressive but not as thick, to my ears.

 

[Wasnex]

That junk again?

I remember watching one of the resident engineers thoroughly debunk that BS a couple years back.

IMHO, the word of an old Ampeg engineer is good enough for me. See following quote:

The diodes are there.

But they don't cause distortion, they are really there to prevent excess overdriving. The amp will distort before the diodes conduct.

What they do is to smooth out the distortion somewhat.

 

[Korladis]

IMHO, the word of an old Ampeg engineer is good enough for me. See following quote:

Exactly. But Seamonkey keeps insisting that they're some kind of fuzz circuit.

 

[Arthur U. Poon]

Good news is IK MultiMedia Ampeg SVT has Certified models of the Ampeg amps. Certified by actual Ampeg engineers

Hmmm. I checked it's advertisement on YouTube. Looks like it's not applicable to a live rig though.

And even if it did, I still seriously doubt it would be 100% identical to running an all tube amp.

With all due respect.

 

[JimmyM]

Hmmm. I checked it's advertisement on YouTube. Looks like it's not applicable to a live rig though.

And even if it did, I still seriously doubt it would be 100% identical to running an all tube amp.

With all due respect.

It's not. It's not bad, but come on! The only ones who say it's the same thing are people who never plugged into a tube Ampeg in their lives.

 

[xk49w]

. . . Furthermore I see some companies trying to further mislead possibly unaware consumers by lighting the tubes with LEDs to further enhance the "Tube Experience".
One of these companies is Bugera but I'm not surprised after seeing their far fetched wattage ratings that they claim as well . . .

I suppose it is possible they are using LED biasing and the bling was a convenient freebie.

 

[seamonkey]

Hmmm. I checked it's advertisement on YouTube. Looks like it's not applicable to a live rig though.

And even if it did, I still seriously doubt it would be 100% identical to running an all tube amp.

With all due respect.

It and many others have been used live for a long time. Keyboards for years have been using computers live for decades. They may be Embedded in a box but many are just PCs or tablets these days.

Many of the modelers sound like many tube and solid-state amps. There are a few that the original amplifier makers certify. They put their brand name on them. They get past their bias by actually listening. They all have parameters to tweak as one amp sounds different than another on the floor, usually just a tweak away from each other.

Survival of the fittest is defined as reproductive success. Modeling, emulation, simulation are all natural selection heading in this way.