DIY 6c45 Amplifier

Few days ago we looked at 6C45 vacuum tube. Today, let’s DIY 6c45 amplifier!

The 6c45 tube can be used in pre-amplifier, driver and power stage. For this round, this will be a minimalist 1W ~ 2W single ended triode integrated amplifier with only 1 stage!!! Usually an integrated tube amplifier will have at least 2-stage but if one can tolerate the low output power; this will be the way to go!

Parts count is as low as it can get! It could be made even simpler but I’ve added a few components to enhance the sound by lowering the noise floor and strengthening the low frequency response. Those steps are necessary as this DIY 6c45 amplifier output power is limited and 6c45 likes to work or oscillate at high frequency and those steps are necessary to keep it well behaved!

6c45 single ended triode amplifier

Above is the schematic for the “complicated” minimalist DIY 6c45 amplifier. Here are some explanations to the amplifier.

This is a single stage integrated amplifier in parafeed configuration. 6c45 tube gets the input directly from the source (media player), amplifies it, and drivers the loudspeaker via the parafeed output transformer.

A 50H plate choke is used as a load to the 6c45 tube, with 5uf / 400Vdc film & foil capacitor as coupling capacitor to the parafeed output transformer. Parafeeding gives the system a much better bass response. If one dislikes the idea of capacitor in the signal path, substitute it with an ordinary air-gapped transformer, replacing the 50H plate choke.

A Lundahl LL7902 input transformer has been added to the input stage. One can configure it to 1:1 or 1:2. The input transformer has 2 functions – isolate the amplifier from the signal source to prevent ground noise and ground loop being coupled to this ultra sensitive 6c45 tube; provide more gain to the system when it is configured to 1:2.

On the cathode side, a 20R resistor is connected in series with a 100R VR/potentiometer. The potentiometer is used to change the bias point. Once you found THE bias you like, measure the resistance value and swap it with a single resistor. This will give you better sound / connection since the potentiometer is removed!

*** One end of the potentiometer must be left floating / unconnected! Refer schematic! ***

Here are a few operating points that you can try. Do note that we’re exceeding the specifications here. Try at your own risk!

150Vdc plate, 50mA, -1.0V grid, 20 Ohm cathode resistor
170Vdc plate, 46mA, -1.5V grid, 32 Ohm cathode resistor
200Vdc plate, 38mA, -2.0V grid, 57 Ohm cathode resistor (used by us, like!)
215Vdc plate, 36mA, -2.5V grid, 70 Ohm cathode resistor

Please do not worry if the plate voltage, current, grid voltage, and cathode resistor vary with regards with the above values. It is common for the 6c45 tubes to be mismatched between samples to samples. Try the points and let us know which one suits your system and taste!

Tips:

Provide a source with low output impedance to realize the low distortion and high bandwidth capability of the LL7902 transformer – garbage in, garbage out.
For optimum termination for best square wave response, terminate the LL7902 transformer secondary with 5kR resistor in series with 1.3nF capacitor (might not be needed).
Connect the output of the volume pot to 1, 3, 6 and 9 grid pins of the tube and add a 50R ~ 500R resistor (carbon composition) in series with each of the pin. Connect both the cathode pin 4 & 5 to the cathode resistor. This prevents high frequency oscillations that sometimes people relate to steely, brash highs or highs sibilance.
Use mono stepped attenuator for each left and right channel to match the gain as 6c45 tubes mismatch is very common between samples.

We use the cathode potentiometer to play with various grid bias points. How can we play with various plate voltages? Simple! Design the power transformer with various taps so that you can switch from one tap to another to get different resultant voltage. Or, switch between capacitor input versus choke input filter to get 2 resultant voltages, as follow:

capacitor input power supply

With capacitor input power supply as above, you get 200Vdc.

choke input power supply

With a flip of switch, disconnect the 20uF capacitor and you will get a choke input power supply. With choke input, you get ~150Vdc. Simple isn’t it?

That’s it on the design side!

The simpler version of this DIY 6c45 amplifier (no input transformer, no bias / operating point adjustment) has been in trial with several pair of loudspeakers, 88dB/w/m 2-way B&W 601S2 bookshelf, 92dB/w/m Tannoy System 800 studio monitor, and 94dB/w/m Foxtex FE167E in TQWT enclosure. Among the 3, it matches the best with Fostex FE167E giving enough (realistic) volume and drive strength to power a small size room. Strains are heard though if you push it to the limits.

The traits that are common among 3 setups are transparency, immediacy and realism. For simple vocals, jazz, instrumentals and less demanding music at medium listening volume, it will not disappoint you!

There is a better tube that would work even better – EC8020, but it is made of unobtainium and therefore we couldn’t report back how it would sound in real world. If one day pair of EC8020 drops from the sky, we would for sure report it here! Meanwhile, enjoy this DIY 6c45 amplifier! I simply love it!

Ken