starved power tubes?

[Alpha579]

Hey all,
I know you can starve the plate voltage of 12ax7s, but can u do a similar thing with power tubes eg 6v6's/6L6's/EL84's etc? what's the lowest voltage these will work on?
Thanks for any input?
Alex

[Roland]

I've run el34's and 6V6's at 180 VDC with no problems as a test once. Biasing can be a problem as it's not a linear change.Output power goes down dramtically. Just remember to not disturb the heater voltage if you run experiments.

[sir_modulus]

Starved output tubes don't sound at all like tubes. They sound just...bad. A good PT needs like 250 (in a 6V6GT) Volts to run well.

[Paul Marossy]

I think because the power tubes are in the output section, the inherent non-linearity that exist in some tubes would be greatly exaggerated. The end result would probably leave something to be desired. Finding a proper bias point at such low voltages would probably be difficult, too. Most tube data sheets don't even list operating conditions below 250V. I can't even imagine what kind of THD a EL34 would have at lower voltages - it's already at 10% at design conditions! And, was mentioned, the output power would be greatly diminished.

You can get away with it with preamp tubes in something like a Shaka Tube, but power tubes are a different story... You'd be better off trying some of these low voltage tubes that have been thrown around as of late. I have toyed with the idea of creating something that can run on 48V or so since I have several transformers lying around that I would like to use for something.

[cd]

There is absolutely no reason you can't run power tubes at lower voltages.  You just get less output power.  Which is highly desireable in some cases, where you want output tube distortion, but not the resulting volume (think 100W Plexi).  As with any output tube, if you change the B+ you have to adjust the tube bias as well, as well as the signal going into the output tube.  Since the output tube is distorting at a much lower level, less preamp gain is required.

This is the concept behind London Power amps (Power Scaling) and Maven Peal amps ("Sag" control) to get authentic power tube distortion, but without the volume, and without using an attenuator (all of which kill tone to some degree IMHO).

[Paul Marossy]

"There is absolutely no reason you can't run power tubes at lower voltages. You just get less output power."

This is true. But, how it sounds may be a different story. I would bet that below a certain range, it would not sound so good. I suppose much of that would matter on the power tube's characteristics. Someone correct me if I'm wrong, but I can't imagine an EL34 sounding good with a B+ voltage of 100V...

[Lonestarjohnny]

My tube book, RCA recieving manual give's a min. /Max. voltage the tube's were designed to operate on, this does not mean that you " Can't "
run them at other voltages, what it would do to tube's life is another thing, you can build it but and whether John J Public will buy it is 2 Different world's all together, My self I don't think you would gain anything at all by doing this, Just my own opinion, not the Holy Word,if you want low voltage tube's, there were many of these built, but guitar amp tube's are considered High Voltage device's.
JD

[Paul Marossy]

That's a good point, JD. Another thing you have to factor in is how many watts your tube is dissipating. Going over the max. rating is bad. I'm not sure how going the other way under affects tube life. I suppose if it's done right, the tube can still dissipate the right amount of watts at a lower voltage.

All I'm trying to say is that there are a lot of factors here. I don't see it being just as simple as lowering plate voltage and calling it good. If affects pretty much the whole amp. And how the tubes respond, who knows?!?

[Lonestarjohnny]

Paul, you got the bull by the horn,
:wink: Old Cowboy expression, that mean's you got the Rope on the Right end of the Steer !
JD

[Paul Marossy]

I'd hate to get the rope on the wrong end of the steer!  :shock:

[puretube]

I`m quite sure, power tubes will work at 0V plate voltage - just don`t expect them to work like they`re expected to work like.....

[cd]

This is true. But, how it sounds may be a different story. I would bet that below a certain range, it would not sound so good. I suppose much of that would matter on the power tube's characteristics. Someone correct me if I'm wrong, but I can't imagine an EL34 sounding good with a B+ voltage of 100V...

Actually it does sound good!  A buddy of mine has a Power Scaling kit installed in a 100W JCM800, and while it doesn't sound IDENTICAL (the amount of air pushed by a 4x12 @ 100W is not the same @ 1W) it's very, very close - closer than any external attenuator I've ever heard.  I don't know the exact voltage the tubes are running at when the power is dialled down, but I'd bet the house it's less than 100V.

There are sound samples, an install report, and links to FAQs here:

http://bruceclement.com/music/psk.htm

[Alpha579]

Those sound samples are amazing!!! Does that 17vdc sample mean the plates are actually running on 17vdc, or was that a reading elsewhere?

[Paul Marossy]

Well, that's some interesting stuff. You learn something new everyday.  
Kevin O'Connor isn't the average DIY tweaker, though. As that article states, the power scaling gizmo isn't perfect, and it is a lot of work to install.

[Alpha579]

Im not sure i9f im graspping this here...If those sound samples sound so good, why dont people design tube amps that run off lower voltages to get that sound at lower volume levels?

[Roland]

Sir_modulus is right also. There is a change in the load resistance of the tube when the B+ is lowered. This could result in an unacceptable tone or at the least very low headroom. The OT might not like the impedance mismatch.

It could be the power scaling devices somehow compensate for this load resistance change and the tonal problems associated with it. I can see how such a device could be a challenge to design especially if it will have to work with differant power tubes all of which will have varying impedance changes as the voltage changes.

Another consideration could be the plate current. At lower voltages there could be problems if your transformer can't handle the load increase. Of course there is probably a voltage level where it would drop.

[Luap]

Alpha,
1. Popular high gain sounds are hard to achieve with low voltage
2. There is no reason except engineers like to design stuff for optimal (known good) operation, from maximum amplification to longest tube life. ...as someone stated earlier, there is not a whole lot of data on starved operation, but most data suggests it safe to try.

How open minded are you?
-There is no harm in trying this for yourself as 0volts/0 current are well within the permissible range of any tube!

Doug Hammond a few years back used to have a dual pentode distortion pedal. It's not on his site any more (maybe it was too grindy?). It was based on the BK butler pedal but substituted 2x6ak6 in place of the 12ax7

I've built a couple of amps with 35V B+ (4x9v batteries). The sound is very similar to a cranked fender PP, breakup is on the round (slightly farty) side.. Frequency response shifts to the low end so the treble never gets too shrill.  

Some of the HV tubes I've tried at low voltage

6ak6:
I used this tube for the first amp I ever built, "The Whisper". it has a very 6L6 like tone on low B+

6au6:
This tube is pin compatible with the 6ak6, when popped into the whisper it is very easy to drive it into blocking distortion. In another amp I built, I'm running them PP in triode mode at around 130v, they sound really sweet.

6bm8:
I built my "spud too" with this triode and pentode, it has the tightest/cleanest output of the three. After some tweaking the amp now has warm/clean/balance tone. Sounds great behind my outboard reverb!

FYI-The sopht amp has really good high end for a low voltage amp might want give that one a whirl. It's really cheap to build if you use a 70Vline transformer for the OT. ($3-7)

Give low volts a try it's really fun!

Luap

[Alpha579]

yeh, i want to get some low voltage tubes like the sopht to try out, they sound cool. I wonder what kind of volume you could get if u used a 30v@3a power supply, and used 4 12K5's in a push pull...could get quite loud...

[Luap]

Volume is one of the first things you give up when you're working with low voltage. The OT tranforms voltage swing to current swing and if you are running low voltage there just isn't much swing!

[Paul Marossy]

"Im not sure if im grasping this here...If those sound samples sound so good, why dont people design tube amps that run off lower voltages to get that sound at lower volume levels?"

You probably won't find a commercially manufactured amp that is built like that. If you do, it'll be a very expensive boutique amp type thing. That's why I built my Firefly, for example. This amp is somewhere around 1/2 watt or so and you can get cranked amp tones at pretty low volume levels. But, even this amp uses a B+ voltage that is nearly 300V.

"Doug Hammond a few years back used to have a dual pentode distortion pedal. It's not on his site any more (maybe it was too grindy?). It was based on the BK butler pedal but substituted 2x6ak6 in place of the 12ax7."

If you are referring to the Pentode Driver, that's still on Doug's site, I believe.