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Old 17th September 2006, 12:25 PM   #1
Dusk is offline Dusk  United States
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Smile All Tube Stereo power amplifier

Hi
I would want to build a all-tube stereo power amplifier 50watts per-side. the problem is how can i use only one channel and have the other channel switched off....is there a circuit that will do this......i have been told that you can't have a tube amplifier on with no load on the output......i only have one speaker cab now.
please help....THANKS DUSK
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Old 17th September 2006, 02:17 PM   #2
lineup is offline lineup  Sweden
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Default Re: All Tube Stereo power amplifier

Quote:
Originally posted by Dusk

1.
I would want to build a all-tube stereo power amplifier 50watts per-side.

2.
the problem is
how can i use only one channel and have the other channel switched off
....is there a circuit that will do this......

3.
i have been told that you can't have a tube amplifier on with no load on the output
......i only have one speaker cab now.
1.
I say you could try to build an easier project.
- all tubes
- two channel, stereo power
- 50 RMS Watt per channel ( into what? 4 or 8 Ohms )
.... this is TOO MUCH, even for most any very experienced tube builders
But is not impossible, almost nothing is.

2.
You should not be needing circuit for this.
A simple On/Off switch would do nice.

3.
Maybe there is some situation / some amplifier, where you need to have load connected all the time.
Load may be used as a part of the circuit.
But amplifiers shouldnt be built that way.
In in 95% they are, and you can connect / disconnect load
and circuit will work nice in both situations.

Now, before connect / disconnect we should always
turn amplifier Power Off.
But this is another thing.




TO3 Active PowerResistor pictures!

lineup the clever man
... got circuits you dont have, you cant figure out
and some poor never ever will get to know ...


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Old 17th September 2006, 04:40 PM   #3
Dusk is offline Dusk  United States
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Smile All Tube Stereo Power Amplifier

Hi LINEUP
Iam sorry i should have mention that both 50watt channel will be fed from the same power supply as well as sharing the same heater supply. So you mean that all i need is a switch to turn off one channel's power while iam using the other? It seem so simple....iam no amp GURU.......JUST A GUITAR PLAYER....DUSK
the amplifiers will feed 4and8 ohm speaker load.
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Old 17th September 2006, 04:55 PM   #4
Bare is offline Bare  Canada
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hendrix land? yer in Coquitlam BC ?
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Old 17th September 2006, 04:55 PM   #5
Klimon is offline Klimon  Belgium
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Hi Dusk,

It seems to be the consensus that most tube amps don't mind when they are powered up without a load; the decware zen for one gets it's output trafo's burned down in this situation -- I never found an explanation why some amps always need a load and some don't.

A simple solution - if you want to be safe - is to use a 4/6/8 ohms power resistor, mounted on a heatsink or metal strip to increase power dissipation. Just install a switch between the speaker output and the dummy load.

edit: if you're running both channels from one power supply you can't disconnect one signal channel; the voltages on the other channel will rise dramatically and probably exceed the components max. specs and destroy a few components.

Best,

Simon
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Old 17th September 2006, 06:38 PM   #6
Dusk is offline Dusk  United States
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Smile 50watt tube stereo power amplifier

Hi Klimon
This sound like what i am looking for ......many thanks for the info
...DUSK
oh, Bare......not quite.....peace.....DUSK.....crosstown traffic......ha,ha....i can see you had your fun...................yea.
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Old 17th September 2006, 08:53 PM   #7
rdf is offline rdf  Canada
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Quote:
Originally posted by Klimon
Hi Dusk,
It seems to be the consensus that most tube amps don't mind when they are powered up without a load; the decware zen for one gets it's output trafo's burned down in this situation -- I never found an explanation why some amps always need a load and some don't.
Simon
Hi Simon. Does the Zen fry without an iput signal? Sounds like oscillation to me if so. There's a good thread on the consequences of running a tube amp without a load somewhere on this site. Essentially, as I understand it, without a secondary load the voltage across the OPT primary skyrockets and can exceed the wire's insulation breakdown.
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Old 17th September 2006, 09:05 PM   #8
lineup is offline lineup  Sweden
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Quote:
Originally posted by rdf
...as I understand it, without a secondary load the voltage across the OPT primary skyrockets
and can exceed the wire's insulation breakdown.
yes,
the load on a secondary winding or the absence of load
will effect the impedance on primary winding

the suggested 'dummy load', 4 / 8 Ohm, conntrolled by a 2-way switch
is a suitable and good solution
if amplifier will need this

99% of solid state transistor output amplifier, will not need this.
A major difference between Valve amps with OPT, output transformer,
and transistor based power amps with 'normal' transformerless output.


lineup - prefer OTL, tube amps without output trafo - but here we are talking only preamps
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Old 18th September 2006, 08:08 AM   #9
lndm is offline lndm  Australia
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Default Re: Re: All Tube Stereo power amplifier

Quote:
Originally posted by lineup
1.
I say you could try to build an easier project.
- all tubes
- two channel, stereo power
- 50 RMS Watt per channel ( into what? 4 or 8 Ohms )
.... this is TOO MUCH, even for most any very experienced tube builders
The first serious all valve power amplifier I built was just that. Go for it I say, if you feel you want to


I often switch amps on the bench without a load. I know in theory I shouldn't. I was taught that this was only a problem when a signal was present, and it only affects some amps. There are always oscillations though, sometimes. I haven't had a problem with it.

Though an 8 ohm dummy load would be ideal, I would use something a little higher if there was signal present, to save energy and ease up on the power supply.

Of course, if the inactive channel has its input shorted, you can use a moderately powered 8 ohm resistor as it won't have anything significant to dissipate.
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Old 18th September 2006, 08:44 AM   #10
lineup is offline lineup  Sweden
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Quote:
1.
I say you could try to build an easier project.
- all tubes
- two channel, stereo power
- 50 RMS Watt per channel ( into what? 4 or 8 Ohms )
.... this is TOO MUCH, even for most any very experienced tube builders
But is not impossible, almost nothing is.
one line was missing, for a correct quotation

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