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Cheap Forum Amp by Captainbeefheart


MEH Synergy

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For a hoot I tried an EL84 triode strapped as the follower circuit instead of the 6SN7, it does gain some power. 17.3 watts output at 5% THD. Not a huge ordeal, just need to make the power supply a little more beefy for the extra current.

 

FWIW These models aren't great at accuracy for grid current A2 mode. It's ballpark enough and extremely accurate for non grid current operation but from what I have measured in reality is always much better. It will most likely be 20 watts or more at 5% THD.

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37 minutes ago, captainbeefheart said:

For a hoot I tried an EL84 triode strapped as the follower circuit instead of the 6SN7, it does gain some power. 17.3 watts output at 5% THD. Not a huge ordeal, just need to make the power supply a little more beefy for the extra current.

 

FWIW These models aren't great at accuracy for grid current A2 mode. It's ballpark enough and extremely accurate for non grid current operation but from what I have measured in reality is always much better. It will most likely be 20 watts or more at 5% THD.

The EL84 follower would be a great alternate build for some members. I’d keep it as an alternative.

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12 hours ago, Curious_George said:

What about a 6BX7?

 

I will have to check to see if I have some around to try. The 6BX7 is great for common cathode stage with the output at the plate due to the very low plate impedance, like with a Williamson circuit driving the power tubes.  For a follower stage  it doesn't gain much in regard to transconductance vs 6SN7 which will determine cathode impedance. I have several very high gm tubes that I have experimented with followers but some have stability issues. One such tube is a 12HL7 which doubles the gm of a 6BQ5 at 21mA/V or 21,000uMhos. That can give us a follower stage with an output impedance  of 47 ohms!! The 6BQ5 works pretty good with an output impedance around 100 ohms, it is still in production which is also a bonus.

 

When I build this circuit I will be able to see the grid drive performance better vs simulation as the models don't really work great for grid current. If the 6BQ5 is much better vs 6SN7 parallel which will have similar gm I will go with it.

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1 hour ago, captainbeefheart said:

 

I will have to check to see if I have some around to try. The 6BX7 is great for common cathode stage with the output at the plate due to the very low plate impedance, like with a Williamson circuit driving the power tubes.  For a follower stage  it doesn't gain much in regard to transconductance vs 6SN7 which will determine cathode impedance. I have several very high gm tubes that I have experimented with followers but some have stability issues. One such tube is a 12HL7 which doubles the gm of a 6BQ5 at 21mA/V or 21,000uMhos. That can give us a follower stage with an output impedance  of 47 ohms!! The 6BQ5 works pretty good with an output impedance around 100 ohms, it is still in production which is also a bonus.

 

When I build this circuit I will be able to see the grid drive performance better vs simulation as the models don't really work great for grid current. If the 6BQ5 is much better vs 6SN7 parallel which will have similar gm I will go with it.

 

You're right, I forgot about the necessity to use tubes that are in production. In order to keep the cost down, the 6SN7 is probably the best choice in your circuit. It is a great all around tube!

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15 hours ago, Curious_George said:

What is the input sensitivity? Looks like you will be able to use this connected directly to a source. 
 

 

 

Gain is +20db, with 1Vrms input you will reach 12.5 watts output. Another reason why I was going to conservatively rate the amplifier as a 12 watt amplifier, that and distortion is at 1% at 12 watts. People can enjoy the amplifier directly from a source without a preamplifier. From experience with real world grid current drive vs simulation it's tough for me to tell the exact output at 5% THD but I am willing to bet it will be at least 20 watts. It's my experience that the majority of sources will be at least 1Vrms or 1.4Vpeak which means those will never clip the amp, and of course many sources especially today's high end CD players or DAC's use higher rail voltages for the opamps, usually at least +/- 5V easily outputting 2Vrms and of course if you use a preamp on average they can swing even greater output pushing the amps boundaries to maximum of around 20 watts.

 

1.26Vrms will get you to 20 watts which should be doable from most sources.

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On 1/25/2022 at 1:24 PM, captainbeefheart said:

This is a Klipsch community and I know we do not need much power to get great sound, regardless spending $200 I think is easier to sell to the wife compared to $350

Yes. Most of us have efficient Klipsch speakers. I think you're on to something. I find it very interesting. Especially at $200. Sounds like fun.

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