Low wattage tube head + poweramp - why not?

[el murdoque]

A while back I thought about what would happen if one were to build a small and light tube amp with just enough output to drive a cab at home and match that with a powerful class D unit. So basically, you get the tube sparkle from the preamp, plus if you so desire, the compression from the powertubes, basically all the tonal magic. At home, you set that amp on top of your practice cab and delve into your sonic bliss. When you play out, you snag up that tiny poweramp and drive your desired tone into the big cabs, reinforced by raw class D power.

For me, it's always been the compression of the powertubes, set just to the brink of breaking up that did the sound. I run my pre section fairly clean.

So the choice is between a class D amp that delivers gig-worthy levels of power while fitting into your gigbag but lacks the tubey mojo --- or a big ass tube head that either needs a second person to be carried for longer (SVT) or one of those super fancy models that can be carried alone, but cost way above two grand (mesa prodigy), even used.

Now after reading a bit about the Ampeg Pf-20, I found that this amp can run without a load and can send a DI signal from the preamp (as usual) but also one from the output transformer. Should such a signal, driven into a quality class D poweramp, not sound a lot like a cranked tube amp (which it basically is) ?

 

[Warwick Qc418]

yes, pretty much. I tried it with my Ashdown lb30 which also has a DI from output transformer but since the lb30 requires a load, it wasn't really practical.

 

[Wasnex]

A while back I thought about what would happen if one were to build a small and light tube amp with just enough output to drive a cab at home and match that with a powerful class D unit. So basically, you get the tube sparkle from the preamp, plus if you so desire, the compression from the powertubes, basically all the tonal magic. At home, you set that amp on top of your practice cab and delve into your sonic bliss. When you play out, you snag up that tiny poweramp and drive your desired tone into the big cabs, reinforced by raw class D power.

Baer prototyped an amp with similar design goals. The Baer Valkyrie had a low power push pull tube amp that could be inserted into the signal path. From the Valkyrie manual:

"Engaging the VALKYRIE's Split/Drive™ tube drive circuit routes the all solid state signal coming from the
preamp through our innovative variable tube drive section. This unique design is the result of many months
of research and development with one clear goal in mind. Deliver all the dynamics, harmonic overtones and
growl that only genuine tubes can bring to your sound, while preserving maximum clarity in the low end.

This is not the typical single 12AX7 design found in most tube hybrid bass preamps. Our Split/Drive™circuit
utilizes a separate dual tube, push-pull amplifier circuit complete with an output transformer and a few tricks
up its sleeves. The result is a tube section that combines the lively response you expect from real tubes with
a tight, punchy low end that never loses focus and definition, even at extreme volumes"

The reviews of the amp were very favorable, but I think only 10 were made. Too bad as I was pretty interested in the Valkyrie at one time.

I see no reason why you couldn't run a small amp like an Ashdown CTM30/LB30 into some sort of "power soak" with a built in load, like an Aracom PRX150-DAG, and use the signal to drive a class D amp. Keep in mind that part of the sound of a tube amp is derived from how its output section interacts with the reactive load of your speakers. In my experience, despite being one of the best attenuators available, the Aracom does have some impact on the tone because it loads the amp a bit differently than connecting speakers directly to the output. You should also be aware that the DAG lists for $875 and weighs 17lbs.

 

[el murdoque]

yes, pretty much. I tried it with my Ashdown lb30 which also has a DI from output transformer but since the lb30 requires a load, it wasn't really practical.

So you had to drive a cab with the ashdown and another with a poweramp? How did that sound?

 

[Wasnex]

Now after reading a bit about the Ampeg Pf-20, I found that this amp can run without a load and can send a DI signal from the preamp (as usual) but also one from the output transformer. Should such a signal, driven into a quality class D poweramp, not sound a lot like a cranked tube amp (which it basically is) ?

It appears the PF20 is designed to do exactly what you are asking for. On p. 7, the manual (see attached) says you can use the "Transformer Balanced Line Out" to drive a power amp. Per the block diagram on p. 13, it appears the amp has a built in dummy load and the "Transformer Balanced Line Out" is a signal taken off the output transformer, which is actively balanced using a solid state amplifier rather than being transformer balanced.

As I mentioned in my previous post, some of the magic of a tube amp comes from from the interaction of the speakers and the output section. Basically you should be able to use the PF20 as described, but it may sound and feel a little different than plugging straight into a speaker.

 

[Warwick Qc418]

So you had to drive a cab with the ashdown and another with a poweramp? How did that sound?

It was in a rock band with heavy drummer so I still had to crank the LB30 and it distort easily when cranked. That time it was a bit too much. I think it would work but my setup was not optimal. I'd need a small cab for the LB30 and a big one for the poweramp, not 50/50.

 

[Boot Soul]

Your idea has been offered in the, unfortunately long-discontinued, Hughes and Kettner BATT/Basssmaster. It was a 5 Watt tube amp that could be used as a stand-alone practice amp, and also had a post-OT line out for connecting a power amp. There are some positive reviews here on TB, but I have never tried one or heard one.

BATT / Bass Master - Hughes and Kettner Micro Distortion Valve Amps

 

[Kro]

It appears the PF20 is designed to do exactly what you are asking for.

This was the first thing that came to my mind when I read the thread title as well.

It's an idea I've toyed with on more than one occasion. I don't see any reason why it wouldn't work fantastically. Am I missing anything?

 

[Mushroo]

I can confirm from experience that the PF20T sounds great through a class D power amp. I haven't had a chance to gig with this setup yet, but it's worked well in my bedroom testing.

Although, it should be mentioned, there are plenty of Talkbassers happily gigging with PF20T and PF50T's. So you might not even need the additional power amp, unless your band is loud or you play big rooms.

 

[Gravedigger Dav]

A while back I thought about what would happen if one were to build a small and light tube amp with just enough output to drive a cab at home and match that with a powerful class D unit. So basically, you get the tube sparkle from the preamp, plus if you so desire, the compression from the powertubes, basically all the tonal magic. At home, you set that amp on top of your practice cab and delve into your sonic bliss. When you play out, you snag up that tiny poweramp and drive your desired tone into the big cabs, reinforced by raw class D power.

For me, it's always been the compression of the powertubes, set just to the brink of breaking up that did the sound. I run my pre section fairly clean.

So the choice is between a class D amp that delivers gig-worthy levels of power while fitting into your gigbag but lacks the tubey mojo --- or a big ass tube head that either needs a second person to be carried for longer (SVT) or one of those super fancy models that can be carried alone, but cost way above two grand (mesa prodigy), even used.

Now after reading a bit about the Ampeg Pf-20, I found that this amp can run without a load and can send a DI signal from the preamp (as usual) but also one from the output transformer. Should such a signal, driven into a quality class D poweramp, not sound a lot like a cranked tube amp (which it basically is) ?

There is an amp that does what you are trying to achieve. Have a look at the Fender Bassman 500. It is a tube preamp with a class D power amp. Fender builds really nice compression into that amp. The power section is the same as the Rumble 500.
I like the idea you have, but are you talking about using just the preamp of the small amp to drive the Class D? If you are thinking of putting the speaker out of the amp into the Class D, that will not work.

 

[Kro]

I like the idea you have, but are you talking about using just the preamp of the small amp to drive the Class D? If you are thinking of putting the speaker out of the amp into the Class D, that will not work.

Nope, the PF20t and PF50T have transformer balanced line outs, so you can take a feed directly from the power section. There are probably other amps that do that, and speaker-level DIs as well, but those Portaflex models are probably the current poster children for that type of setup.

Both pre AND power section tone can go straight into the power amp of your choosing.

 

[el murdoque]

There is an amp that does what you are trying to achieve. Have a look at the Fender Bassman 500. It is a tube preamp with a class D power amp. Fender builds really nice compression into that amp. The power section is the same as the Rumble 500.
I like the idea you have, but are you talking about using just the preamp of the small amp to drive the Class D? If you are thinking of putting the speaker out of the amp into the Class D, that will not work.

The Bassman 500 is just a standard hybrid amp with 2 12ax7 tubes in the preamp and a class D poweramp.

What I'm talking about is the feature which is implemented in the Ampeg PF-20, the Hughes&Kettner Bassmaster and (with aforementioned limitations) in the Ashdown LB30.
A reugulated line level output from the output transformer of the powertubes.

 

[el murdoque]

Your idea has been offered in the, unfortunately long-discontinued, Hughes and Kettner BATT/Basssmaster. It was a 5 Watt tube amp that could be used as a stand-alone practice amp, and also had a post-OT line out for connecting a power amp. There are some positive reviews here on TB, but I have never tried one or heard one.

BATT / Bass Master - Hughes and Kettner Micro Distortion Valve Amps

I'm very aware of the existence of the Bassmaster. Living in Germany, the used market usually has one or two on offer, they go at around €180,-
This design is not an all-tube design however, and it is not as beautifully executed as in the PF-20.
I might pull the trigger on a Bassmaster, just to see how it works.

 

[el murdoque]

View attachment 2849616

I like the looks of that amp very much. I just wish they'd have rigged it with oldschool toggle switches on the front for power & standby, though.

 

[jw23mind]

I just wired my approx 144W RMS tube amp to have a slave output, voltage dividing down from the OT(an old Mesa D-180 heavily reworked by me into just a power amp). I put this in the effects loop of my Class D VT Bass 500 head. The tube amp is loaded by 1/2 of my 410, the Class D by the other half. Calibrated the slave signal such that all 4 drivers receive equal power. Effectively have doubled to 288W RMS.

 

[beans-on-toast]

What you are proposing works very well. People were running a tube preamp into a solid state PA power amp back in the 80's. That's when I first tried it. They were also using tube amps with a dummy load with line level output into the PA amp. A good way to have high power and the contribution of tubes.

Those Portaflex amps make doing this easy. Another good approach is to use a Torpedo Live load box: Two notes Audio Engineering - Torpedo Live.

 

[tchristian]

. . . or a big ass tube head that either needs a second person to be carried for longer (SVT) or one of those super fancy models that can be carried alone, but cost way above two grand (mesa prodigy), even used.

I carry my Traynor YBA-300 with one hand. The cost new was nowhere near 2 grand. The cost used is well under a grand. It delivers full-on rich tube goodness. The company has a solid reputation. (I have to use 2 hands to lift it onto a cabinet. Oh darn. ) YMMV of course.

 

[Munjibunga]

Or you could just buy a Demeter.

 

[groovaholic]

The SWR Interstellar Overdrive also would fulfill this application.

 

[bonin in the boneyard]

Check out the Frenzel Mini Bassman:

frenzeltubeamps.com - Mini Bassman

From the specs:

"Line Out - post output transformer LINE OUT with LINE OUT LEVEL control for recording or driving a power amp, Auto switched internal load to simulate speaker if speaker is not plugged in. Inject great tone directly into the PA system with the Speaker Simulated Direct Interface."

16lbs, with other cabinet options available. Very reasonably priced for what you get. No personal experience with them, though, but maybe someday.