A tube amp that will drive 2 ohms or less...

[poopydoopynoopy]

Hello!

I am a proud ribbon speaker owner/builder, and I have done a lot of reading in this section.
Very very beautiful works are done here !

I don't really see it mentioned though what the minimum speaker impedance should be for a tube amp. Perhaps a dumb question, but I have never used or worked with tubes yet...

I am looking for at least 20w rms into around 2 ohms worst case, 5 ohms nominal.
Can you talented folks point me in a direction which may lead me to the correct parts to build monoblocks/multi-channel tube amps ?
I have read the Boozhound pages so far.....
Thank You for sharing all these works!

paul

 

[Wavebourn]

This one will do the job:

http://wavebourn.com/forum/download.php?id=123&f=7

Why?

It has a special driver to drive low input resistance of an output stage tubes surrounded by parallel feedback by voltage.

Continuous RMS is 80W per channels. Designed to drive Magneplans.

 

[DougL]

I am looking for at least 20w rms into around 2 ohms worst case, 5 ohms nominal.

I would probably use a Push pull KT88 / 6550 amp, Triode Class A on the 4 ohm taps and call it good.
Triodes are relatively forgiving about output Z, although distortion does rise.
Alternately, a transformer with a 2 or 3 ohm tap could be custom wound.

HTH

Doug

 

[Miles Prower]

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Best to get new speeks. Any speeks with impedances of two ohms or less are made for solid state amps so that the marketing department can brag about the awesome lotsawatts their amps can pump out. Of course, most solid state amps sound so hideous anyway that it makes little difference, but such low impedances are a major sonic compromise.

Doug Self complained about this very thing:

These diagrams were generated by SPICE, plotting incremental output gain against output voltage, with load resistance stepped from 16 to 2 Ohms, which I hope is the lowest impedance that feckless loudspeaker designers will throw at us.

Distortion in Power Amps

Needless to say, that diagram (Fig. 17) showed increasing distortion as Zl went down.

"I have never used or worked with tubes yet..."

The A Number One rule you need to learn is that VTs are high voltage / low current devices. That means Hi-Z devices. They do not like operating into Lo-Z loads. That's what OPTs are for. However, it's hard enough designing and manufacturing OPTs that can match an 8R speaker load into a final that wants to see Zl > 1000R. A 2R load: fugeddaboudat. A VT OTL would require an insane number of paralleled VTs to drive that sucker. The heater power alone could probably warm a good sized room.

VT amps need to work into more sensitive speeks with well mannered impedance v. frequency characteristics. There often isn't enough power to get much past the cross over networks solid state amps need to keep from blowing tweeters or even mid-ranges from all the high order harmonics these things pump out (a 100W, MOSFET amp I once had took out both the tweeters and midranges in a set of Technics speeks when Mr. Roommate turned the volume up too high) and you also don't have the milli-ohm Zo's that these carbon fibre or ceramic subwoofer cones attached to heavy voice coils need to keep 'em under control.

Insensitive, inefficient speeks also serve to mask all that solid state nastiness.

 

[Wavebourn]

Dough Self complained, but I instead of complaining designed a tube amp that can drive them easily...
Even my previous one, without local NFB around output stage, drove speakers with small inefficient drivers and complex crossovers nicely during last BAF festival...

 

[a.wayne]

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Best to get new speeks. Any speeks with impedances of two ohms or less are made for solid state amps so that the marketing department can brag about the awesome lotsawatts their amps can pump out. Of course, most solid state amps sound so hideous anyway that it makes little difference, but such low impedances are a major sonic compromise.

Doug Self complained about this very thing:



Needless to say, that diagram (Fig. 17) showed increasing distortion as Zl went down.

"I have never used or worked with tubes yet..."

The A Number One rule you need to learn is that VTs are high voltage / low current devices. That means Hi-Z devices. They do not like operating into Lo-Z loads. That's what OPTs are for. However, it's hard enough designing and manufacturing OPTs that can match an 8R speaker load into a final that wants to see Zl > 1000R. A 2R load: fugeddaboudat. A VT OTL would require an insane number of paralleled VTs to drive that sucker. The heater power alone could probably warm a good sized room.

VT amps need to work into more sensitive speeks with well mannered impedance v. frequency characteristics. There often isn't enough power to get much past the cross over networks solid state amps need to keep from blowing tweeters or even mid-ranges from all the high order harmonics these things pump out (a 100W, MOSFET amp I once had took out both the tweeters and midranges in a set of Technics speeks when Mr. Roommate turned the volume up too high) and you also don't have the milli-ohm Zo's that these carbon fibre or ceramic subwoofer cones attached to heavy voice coils need to keep 'em under control.

Insensitive, inefficient speeks also serve to mask all that solid state nastiness.

................... What hevar ....Mr 10% thd ...

 

[Allen Wright]

Driving low impedance loads is a breeze with a good tube amp - that has a output transformer ***MATCHED*** to the required speaker impedance.

Just today I completed my dpa150 amps using 2 pairs of KT88 per monoblock, and measured 150 watts into ONE OHM!

But that's what this particular amp is designed to do from the ground up - drive one ohm Apogee Scintillas. And that's without any negative loop feedback!

When someone wants one for higher impedance, we will just rewire the windings on the output traffo to match whatever they want.

Tube amps are generally comfortable with +/- 50% load variation around the design impedance, but go too low and they'll distort, go too high and they'll normally still sound nice, but lose power.

Regards, Allen (Vacuum State)

 

[Andy Bartha]

I agree with Allan. Some time ago I built a couple of mono blocks with a pair of EL 509s with outputs wound for 2 ohms. It was basically a knock off of the Rozenblit 150 amp from some years ago. Works great and drives the low impedance speakers great. The speakers are about 3 ohms.