DeathTrap400™ : : A Pretty Damn Good 300B Amp

[Ed LaFontaine]

tomchr,

pm sent before post 40 was up. That is the set I am interested in.

Thanks

[pso]

Hi,

This amp can be driven to clipping with CD player, or is necessary a preamp?

[tomchr]

With transformer coupled input, the input voltage needed to get to clipping is 6~6.5 V RMS. With capacitor coupled input, the voltage required will be 3 dB less - so roughly 4.5 V RMS. A CD player provides about 2 V RMS.

With my 87 dB/W*m efficient speakers, the amp can be driven to ear splitting SPLs in a 16 m^2 living room using a CD player. To get to clipping with a CD player input, you'll need a preamp with a gain of about 10 dB (3x). This with transformer coupled input.
With a cap coupled input, you need roughly 7 dB of gain to get to clipping.

~Tom

[pso]

Seems that for a small or medium sized room, with a CD player is enough to get a good sound pressure, thanks.

[JoshK]

Originally Posted by tomchr

Folks,

The input tubes were biased as follows: 12BH7A: Vgk = -11.2 V; Ia = 12 mA. ECC99: Vgk = -6.0 V; Ia = 12 mA. 6N6P: Vgk = -6.2 V; Ia = 15 mA.
The cathode followers were running at 4.4 mA. For the cathode followers, Vak was roughly: 12BH7A: 230 V; ECC99: 125 V; 6N6P: 170 V. These voltages were chosen for optimal THD.

~Tom

I am lost here. Why would you run the CF at 1/3 the Ia as the driver stage? Doesn't that completely defeat the purpose of the CF, or atleast most of it? CF does give you lower Zout, but its also suppose to be the driver of current into the load. If you bias at 1/3 the driver stage, you have only 1/3 of the current into the load to deliver. Usually not what most designers strive for.

[tomchr]

Originally Posted by JoshK

Why would you run the CF at 1/3 the Ia as the driver stage? Doesn't that completely defeat the purpose of the CF, or atleast most of it?

Driving a 300B is quite tricky - hence, the need for a cathode follower in the first place. However, the 4.5 mA biasing the cathode follower in my design is plenty.

Actually, you want the current in the cathode follower to be low as this results in lower distortion of the overall amp. This may seem a bit counter-intuitive, but described quite well by Morgan Jones in "Valve Amplifiers", 4th ed. The THD of the cathode follower is quite low to start with but increased by the lower bias current. As the harmonics produced by the cathode follower are 180 degrees out of phase with those produced by the preceding common cathode amplifier stage, the harmonics end up canceling, thus, lowering the THD of the overall amplifier.

The final design was the result of much simulation to get close followed by experimentation to minimize the THD of the overall amplifier.

~Tom

[tomchr]

Folks,

The Schematic & Bill of Materials .pdf for the power supply board is attached.

I have identified one minor issue in the bias supply. With the values shown in the schematic and my less than optimal prototype setup, the Q1 tends to oscillate. Currently, I have fixed this by adding a 10 nF, 50 V ceramic cap connected from the gate to source (pin 1 to pin 3) on Q1. A more permanent solution will probably be a ferrite bead placed on the gate lead of Q1. Something like a Bourns FB43-110 or FB73-110 will probably work.
Symptoms of the oscillation include the output voltage changing dramatically as the tubes in the amplifier warm up and the load on the bias regulator increases.

As stated, it is likely that this oscillation is caused by the two feet of wire I have connecting Q1 of the power supply board to the load. But I think in the interest of reliability, I'll recommend adding the ferrite bead. I'll need to verify the fix first, though.

Under proper working conditions, the output voltage of the bias supply will change a few 100s of mV from no load to full load (~10 mA).

~Tom

[tomchr]

On the positive side, the Driver Board works flawlessly in my setup.

[pso]

You listen to various styles of music, or one in particular?

[tomchr]

Mostly classic rock (Dire Straits, Mark Knopfler, Pink Floyd, etc.) but also more vocal-ish music like Nora Jones, Brandi Carlile, and Celine Dion and jazz (Stan Getz). Every now and then, I'll spin a classical CD. Love Mozart's Requiem.

My default test albums include (in no particular order):
- Dire Straits, "Brothers in Arms"
- Dire Straits, "On Every Street"
- Celine Dion, "Falling Into You"
- Eric Clapton, "Unplugged"
- Mark Knopfler & Chet Atkins, "Neck & Neck"
- Pink Floyd, "The Dark Side of the Moon"
- Pink Floyd, "The Wall"
- Pink Floyd, "The Final Cut"
- Sting, "Mercury Falling"
- Simon Preston, "Bach Organ Works", specifically, Tocata & Fuge in D minor, BWV 565
- Stan Getz, "Cafe Montmartre"

A well designed amp will play all kinds of music well. I don't buy into the idea that you need an amp designed for a particular genre. Feel free to agree or disagree, but that's my philosophy.

~Tom