Want to build a triode amp using 2A3 or 300B

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Want to build a triode amp using 2A3 or 300B

Postby hilldweller » Sat Oct 26, 2013 5:57 pm

Hi all, I am wanting to build a stereo amp using triode output tubes and I would like to get about 35 watts or so per channel. I can not seem to find a good circuit/schematic for an amplifier of this type. Can anyone out there guide me?
I will also build single driver full range speakers for this, possibly a horn type labyrinth cabinet. I am at a loss for knowledge about this and would appreciate any advice. I have been relentlessly searching for sonic excellence without spending two arms and two legs. I appreciate anybody's help.
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Re: Want to build a triode amp using 2A3 or 300B

Postby Geek » Sat Oct 26, 2013 10:19 pm

SE or PP?

Tube choice suggests SE, but power suggests PP.

hilldweller wrote:Hi all, I am wanting to build a stereo amp using triode output tubes and I would like to get about 35 watts or so per channel.


Hope you have a BIG budget, because you're looking at about 12 x 2A3's per channel and custom iron all around if SE.

300B in PP can do this.

I can not seem to find a good circuit/schematic for an amplifier of this type. Can anyone out there guide me?


Because designers will have discovered that triode transmitter is better for that power level.

I will also build single driver full range speakers for this, possibly a horn type labyrinth cabinet.


Then you don't need anywhere near 35W.

Maybe some clarification with what you want to accomplish before cementing in tube types or power levels?

Cheers!
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Re: Want to build a triode amp using 2A3 or 300B

Postby hilldweller » Sun Oct 27, 2013 4:44 am

Geek wrote:SE or PP?

Tube choice suggests SE, but power suggests PP.

hilldweller wrote:Hi all, I am wanting to build a stereo amp using triode output tubes and I would like to get about 35 watts or so per channel.


Hope you have a BIG budget, because you're looking at about 12 x 2A3's per channel and custom iron all around if SE.

300B in PP can do this.

I can not seem to find a good circuit/schematic for an amplifier of this type. Can anyone out there guide me?


Because designers will have discovered that triode transmitter is better for that power level.

I will also build single driver full range speakers for this, possibly a horn type labyrinth cabinet.


Then you don't need anywhere near 35W.

Maybe some clarification with what you want to accomplish before cementing in tube types or power levels?

Cheers!

I have read that some of these types of triodes have a plate dissipation of 40 watts.
http://www.tubedepot.com/gl-px300b.html
is this not the case?
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Re: Want to build a triode amp using 2A3 or 300B

Postby Geek » Sun Oct 27, 2013 5:16 am

Yes, but plate dissipation isn't output power.

In SE mode, a 300B can deliver about 9 watts.

In PP they will do 20 in class-A to 30W a "hot" class-AB.

Cheers!
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Re: Want to build a triode amp using 2A3 or 300B

Postby hilldweller » Sun Oct 27, 2013 8:02 am

Thanks for the know.
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Re: Want to build a triode amp using 2A3 or 300B

Postby SDS-PAGE » Sun Oct 27, 2013 4:15 pm

I have read that some of these types of triodes have a plate dissipation of 40 watts.
http://www.tubedepot.com/gl-px300b.html
is this not the case?


SE designs are not particularly efficient, power wise. 35 watts in SE is asking a lot. Even if you paralleled 300Bs, the most you will get is 18 watts. My recommendation is to invest in efficient pairs of speakers (97 dB/W/m or greater) for a SE amp in either tubes. The 45 is an excellent alternative for speakers that are 100+ db/W/m like some of the Klipsch heritage series speakers like Cornwalls, La Scalas, K-Horns, Belles, etc.

As far as 2A3/300B circuits, I like JE Labs, WE91A, Loftin-White, and Sun Audio circuits.

You might want to consider 6C33C (around $60 pair) in SE mode for very respectable 15 watts:

http://www.mi.ru/~tube-art/6c33_amplifiers.htm
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Re: Want to build a triode amp using 2A3 or 300B

Postby Gingertube » Sun Oct 27, 2013 10:07 pm

4 off 300Bs in Parallel Push Pull will get you close to 40 Watts. I'm about to do something like that myself.
Why - coz I have these Output Trannies which are currently being driven by 4 off triode strapped EL34 but deserve something better - http://www.plitron.com/wp-content/uploads/P4006-01.pdf
The trick for decent parallel push pull triodes is a "GRUNTY" driver stage to drive the Miller Capacitance of the Output Tubes to high frequency without slew rate limiting.

Math:
mu 300B = 3.85
Cg1 = 17 pF
Cg1a = 7.7 pF

Capacitance at grid 1 each 300B = 17 + Miller Capacitance = 17 +(3.85 x 7.5) = 46 pF
2 in parallel + wiring strays - call it 100pF to drive on each side of the push pull

The 300B grid will bias at around -80V which means that we need to swing 80 Volts peak (160V peak to peak) signal to the grid.

Depending on which data sheet you read the Plitron -3dB roll off is either 187kHz or 217kHz.

That means we want to drive 80V peak into 100pF to a frequency of at least 200kHz (say half this for ordinary EI Output Trannies)

Slew Rate = 2 pi F x V peak / 10^6 Volts per microsecond = 2 pi x 200,000 x 80/10^6 = 100 V/us

Signal Current = Slewrate (V/us) x Capacitance (uF) = 100 x 0.0001 Amps = 10mA.
That is the signal current required so the idle current on the driver stage would want to be say twice that = 20mA, maybe a differential amp made up from triode strapped EL84 or 6V6 or similar.

This should tell you that this sort of amp is not trivial to design but taking a BFI (Brute Force and Ignorance) approach is not a bad way to tackle it.

If you don't want to get involved with slew rates etc there is another way to look at the problem:
200kHz means a signal time constant of 1/(2 pi F) = 0.8 us

Sampling theory tells us that for a signal to settle to 99.7% ( i.e. 0.3% distortion) of its input value takes 5 timeconstants, that is in this instance we want the grid drive 5 timeconstants to be 0.8 us so actual timeconstant = 0.16 us
This timeconstant will be the output impedance of the driver x the 100pF grid capacitance.

Thus Driver Output Z = 0.16 us / 100pF = 1600 Ohms (max)

Cheers,
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