Tube preamp heads: Gimmick??

[Wasnex]

Wasn't the Acoustic 360/361 introduced around the same time?

I believe the 361 made about 170 watts. It was horn loaded for increased efficiency.

 

[bucephylus]

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.

Getting “a more tube like sound” is so subjective, that you are going to get a lot of different responses.

First, many of us did a lot of gigs over the years with tube preamps driving SS power amps in rack systems. There are some very fine tube preamps, such as Alembic and Demeter. As to whether such systems sound like “tubes” is debatable with no answer; because they don’t sound like an SVT. But, the point is there are great tube preamps.

Also, those rack mount power amps now are Class D. But the Class D power amps are not distinguishable from earlier designs, with the exception of maybe the Crest CA Series, which had some specific topology aspects.

So, sure, you can get great tone out of a Demeter tube pre/Class D hybrid. Do you get that same tone out of a less expensive and not as well designed Class D hybrid? Not necessarily; but that’s not the fault of the Class D module per se. Markbass does not = all the other amps out there.

It baffles me how TB’ers will spend $1500+ on a big tube amp and then compare it to a $500 Class D amp, and blow off the $1000 Class D better designs. Oh well, don’t expect that to change any time soon.

 

[DJ Bebop]

I believe the 361 made about 170 watts. It was horn loaded for increased efficiency.

Most of the amplifiers produced by ACC were solid-state, but a few models later in production were valve amps. The company is remembered in particular for its Acoustic 361 bass stack, consisting of an Acoustic 360 bass pre-amplifier and one or two Acoustic 361 W-bins, each featuring a built-in 200-watt RMS power amplifier and a rear-facing 18" Cerwin-Vegaloudspeaker.

 

[SJan3]

I believe the 361 made about 170 watts. It was horn loaded for increased efficiency.

Naa, I had one and I believe each cab had a 220watt power amp in the bottom. So, 2 361 bottoms delivered 440 watts thru 2 18" Cerwin Vegas. A potent rig by any standards!

 

[DigitalMan]

Yeah, as they say "All generalizations are false"

Cmon, you make things a million times worse when you exaggerate.

 

[Wasnex]

Getting “a more tube like sound” is so subjective, that you are going to get a lot of different responses...

...It baffles me how TB’ers will spend $1500+ on a big tube amp and then compare it to a $500 Class D amp, and blow off the $1000 Class D better designs. Oh well, don’t expect that to change any time soon.

My only experience with class D is the Epifani UL902C and Ampeg SVT 8Pro. When run clean the 902C has a very similar feel and tonal profile to a Sadowsky SA200. The SA200 take EQ far better IMHO.

The SVT 8Pro can be dialed in to sound a lot like a vintage SVT including getting a bit of grind with the OD circuit. The biggest difference is the low end. The SVT 8Pro Bass EQ control is voiced a bit lower than an all tube SVT. Per the 8Pro manual +17/-13db at 20hz and per the vintage SVT manual +/- 12db at 40hz. No surprise that it can be a challenge to get the lows on the 8Pro to sound exactly like a vintage SVT.

 

[Bassbeater]

Many of us remember how "zippery" digital effects units used to be. I think it's safe to assume that tube emulation and tube preamps in D-class will get better in general with time. When I bought my very heavy LH1000, the thing that kept me away from D Class was basically skepticism. I wasn't ready to believe that a black wifi router thingy was going to move my speakers correctly. They seem to be strong contenders though.

 

[seamonkey]

"During the 1960s, Jack plugged his EB-3 into a stack of Marshall amps (Marshall 4x12s with Marshall 100 heads). His desire for greater volume induced him to crank up the volume control, unwittingly producing the heavy, distorted tone that became so popular in the ensuing years. Later, Jack had instrument technician Dan Armstrong install a diode into the EB-3's wiring to produce the same effect without overdriving his amplifiers."

Great story! A good natural world observation, then followed up with experiment and more observation.

There are quite a few diodes built into the signal path of many heads. Someone was trying to claim they were some kind of protection circuit.
If so, Rat should be selling these diode cables as extra protection for heads
Either spin could be a big seller.

 

[Wasnex]

Naa, I had one and I believe each cab had a 220watt power amp in the bottom. So, 2 361 bottoms delivered 440 watts thru 2 18" Cerwin Vegas. A potent rig by any standards!

I have no doubt that a pair of 361s was loud!

Great story! A good natural world observation, then followed up with experiment and more observation.

There are quite a few diodes built into the signal path of many heads. Someone was trying to claim they were some kind of protection circuit.
If so, Rat should be selling these diode cables as extra protection for heads
Either spin could be a big seller.

Ampeg SVTs and Trace Elliot V-types have antiparallel diodes arranged in a clipper circuit in the back end of their preamps. I have frequently seen the function of these diodes as clippers designed to prevent massive transient overload in the output section. Whether this circuit is designed to provide overdrive or protection would be a function of the level the diodes clip at in relation to the output section. If the output section produces full power and goes into over drive before the clippers, the diodes function as primarily protection. If the clippers overload before the output section reaches full power, the intent is to produce distortion.

The clippers are on the first gain stage following the master volume in the V-Types and just before the phase inverter in vintage SVTs. Logically, if the intent of these diodes is to produce over drive I would expect to see them earlier in the preamp...surely they would at least be before the master volume.

 

[JimmyM]

Ampeg SVTs and Trace Elliot V-types have antiparallel diodes arranged in a clipper circuit in the back end of their preamps. I have frequently seen the function of these diodes as clippers designed to prevent massive transient overload in the output section. Whether this circuit is designed to provide overdrive or protection would be a function of the level the diodes clip at in relation to the output section. If the output section produces full power and goes into over drive before the clippers, the diodes function as primarily protection. If the clippers overload before the output section reaches full power, the intent is to produce distortion.

The clippers are on the first gain stage following the master volume in the V-Types and just before the phase inverter in vintage SVTs. Logically, if the intent of these diodes is to produce over drive I would expect to see them earlier in the preamp...surely they would at least be before the master volume.

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.

 

[Wasnex]

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.

 

[Jim C]

We all hear it differently:
I had an SVT 3 Pro as well as a 7 Pro; maybe a little tubey but not to my liking
SWR SM400 - No tube tone and I rolled a bunch of vintage tubes through that head before giving up; nice and clean with no HPF; a real speaker killer

Alembic - Kinda tubey but the limited EQ was not for me
Shuttle Max 9.2 - Kinda tubey; nice amp
SVT, V4B, and any Ampeg tube pre - Tube City
Walkabout - Oh yeah; that's the business right there and really responds well to Telefunken and Amperex (IME)
They only get sold because 300 watts is not enough headroom for many (myself included)

 

[fhm555]

Seamonkey is one of those rare individuals...a tube amp expert who has never used a tube amp in his life

I've had some tube amps but not enough to be able to give an opinion other than generally favorable, but I think amp choice is like a lot of other things, it's more about what you are comfortable with than it is chasing some quantifiable unit of relative greatness.

 

[JimmyM]

I've had some tube amps but not enough to be able to give an opinion other than generally favorable, but I think amp choice is like a lot of other things, it's more about what you are comfortable with than it is chasing some quantifiable unit of relative greatness.

Everything is a compromise when it comes to picking out gear. Need vs practicality vs available money.

 

[teemuk]

Ampeg SVTs and Trace Elliot V-types have antiparallel diodes arranged in a clipper circuit in the back end of their preamps. I have frequently seen the function of these diodes as clippers designed to prevent massive transient overload in the output section. Whether this circuit is designed to provide overdrive or protection would be a function of the level the diodes clip at in relation to the output section. If the output section produces full power and goes into over drive before the clippers, the diodes function as primarily protection. If the clippers overload before the output section reaches full power, the intent is to produce distortion.

If I recall correct, in SVT power amp overdrive and overdrive of the input diodes takes place about simultaneously. When power amp begins to clipping distort the diodes will also clipping limit the input signal, which - as mentioned - prevents harder overload of the output section.

I don't know what happens if you would remove those diodes but I suspect it would definitely make the output stage clip nastier than without them. Harder overload condition might aid grid conduction of the output stage (which is designed pretty robust in that regard, BTW) but that would lead to futher increase in screen current draw. Grid conduction usually causes screen current draw to increase rapidly, and the screen circuit was particularly one of the weak points and failure mechanisms of the early SVT design.

As far as I know, SVT design is derived from an "all-purpose" power amplifier aimed for industrial applications, such as driving electric motors. SVT was never intentionally designed as an audio amplifier but copied from an amplifier that drove a lathe motor. If you compare SVT design to typical tube -guitar- power amp you will notice it's a totally different kind of specimen and will feature very different characteristics regarding overall behavior under overdrive conditions.

 

[seamonkey]

If I recall correct, in SVT power amp overdrive and overdrive of the input diodes takes place about simultaneously. When power amp begins to clipping distort the diodes will also clipping limit the input signal, which - as mentioned - prevents harder overload of the output section.

I don't know what happens if you would remove those diodes but I suspect it would definitely make the output stage clip nastier than without them. Harder overload condition might aid grid conduction of the output stage (which is designed pretty robust in that regard, BTW) but that would lead to futher increase in screen current draw. Grid conduction usually causes screen current draw to increase rapidly, and the screen circuit was particularly one of the weak points and failure mechanisms of the early SVT design.

As far as I know, SVT design is derived from an "all-purpose" power amplifier aimed for industrial applications, such as driving electric motors. SVT was never intentionally designed as an audio amplifier but copied from an amplifier that drove a lathe motor. If you compare SVT design to typical tube -guitar- power amp you will notice it's a totally different kind of specimen and will feature very different characteristics regarding overall behavior under overdrive conditions.

One of the Ampeg brand owners had removed the diodes. Then they shoehorned them back in. It may just they got put in for something, then people started liking the extra clipping, when a redesign engineer said they weren't needed and removed them, then people wanted that clipping back. Every ampeg amp has these clipping diodes now. 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.

People can now just plugin a Rat Tale clipping diode cable into their effects loop, or between the pre-amp and amp to get the same diode effect.

Overdriven SS amp, or Fuzz, is still more popular and the root of Rock sound. Tube pre-amps didn't distort enough. Fuzz boxes came along and solved that. Later more tube stages added to try and put it all in the head - thus the "All tube" marketing. Some designers snuck in diodes into their all tube designs to help increase the distortion.

For single tube pre-amps, some designers take the dual triode and essentially wire it up as tow diodes for clipping. With a different curve and non-linearity than off the shelf silicon. They use opamps and other circuitry along with the tube to match levels. The diodes even tube ones show up in a feedback path sometimes.

I'd like to see more pre-amps and pedal methods. It's really where all the tone comes from. There is so much negative feedback in high power amps that same class of any devices act very much the same.

 

[two fingers]

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.....

 

[dukeorock]

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.

You can usually tell from the marketing blurbs whether it’s just a gimmick or not, I think.

I don’t think any generalization works.

Agreed!

 

[jnewmark]

We all hear it differently:
I had an SVT 3 Pro as well as a 7 Pro; maybe a little tubey but not to my liking
SWR SM400 - No tube tone and I rolled a bunch of vintage tubes through that head before giving up; nice and clean with no HPF; a real speaker killer

Alembic - Kinda tubey but the limited EQ was not for me
Shuttle Max 9.2 - Kinda tubey; nice amp
SVT, V4B, and any Ampeg tube pre - Tube City
Walkabout - Oh yeah; that's the business right there and really responds well to Telefunken and Amperex (IME)
They only get sold because 300 watts is not enough headroom for many (myself included)

The only other head I would add, no longer available, is the MusicMan HD 150. SS pre and 4 6L6 power tubes. As I've posted frequently, the closest to a tube amp I've ever played, without tubes being in the preamp.

 

[Wasnex]

I don't know what happens if you would remove those diodes but I suspect it would definitely make the output stage clip nastier than without them. Harder overload condition might aid grid conduction of the output stage (which is designed pretty robust in that regard, BTW) but that would lead to futher increase in screen current draw. Grid conduction usually causes screen current draw to increase rapidly, and the screen circuit was particularly one of the weak points and failure mechanisms of the early SVT design.

One of the Ampeg brand owners had removed the diodes. Then they shoehorned them back in. It may just they got put in for something, then people started liking the extra clipping, when a redesign engineer said they weren't needed and removed them, then people wanted that clipping back.

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.