ClassicValveDesign Posted:

One of the simplest tube amplifiers anyone can build is the "spud", or single-tube-per-channel amplifier.

Spud amps are simplified with dual (or more) section tubes, especially when one section is a power section.

The 6FD7 is a dual dissimilar triode unit, like an ordinary TV vertical deflection tube (which it is), but is also unique in that it has a miniature 9-pin base despite being a large bottle envelope. It is also unique in that it has an amazingly efficient power amplifier section for a low-mu triode - upwards to 35%!

Vertical deflection TV tubes are excellent choices for audio, because they tend to be insanely linear and tolerate punishment and high voltage peaks and are very forgiving (ooops, did you hook that speaker up before powering it on? Is it the right impedance?).

In this current design I am using, which idles the tube cooler than the original right-at-10-watts (maximum Pa for the power triode), the efficiency is closer to 30%, with 2.5W RMS out at ~10% THD for a tad over 8W DC input.

The high gain section of the 6FD7 is *just* enough with a constant-current-source anode load to drive the amp to full output with 0.707V RMS input.

The amp has served me for 2.5 years now with only one tube set change.

The frequency response with the EDCOR XSE-15-8-5K transformers with the modest 3.5dB NFB is 17Hz to 22KHz at 1 watt and 24Hz to 19KHz at full output of 2.5 watts.

Cheers!

I have three questions regarding the schematic.
1.  U1 and U2, to me looks like is should be U1a and U1b. Is that correct
2.  In the PS section why is there a connection from the Heater circuit to the HV curcuit?
3.  In the amplifier section the 300v leg has 2 resistors shown before going to the collector. One is labeled with ** that indicates a trimmer. So I am confused is that a 360 ohm resistor and then a 200 ohm trim pot. That is what I think is happening there.

Thanks for the assistance. I have read that this is a great little SET.

cheers, Alan

I dont think it will work as shown. The pre and final supply lines are swapped. It will probably oscillate badly. I know because i did the same thing once.

Hi Alan,

Alan Blumenstock wrote:

Yes.


Alan Blumenstock wrote:

The heater lift is to eliminate hum coupling to the cathode from the AC heater.


Alan Blumenstock wrote:

The 360 ohm resistor labled "**" may be adjusted by you to give the 200V anode with approximately centre position on the wiper of the 200R trimmer. Each 6FD7 is different and in a box of 50 I bought, each one is SO different that the actual value of the 360 ohm resistor can be anywhere from 270 to 560 ohms.

Consistency was not Sylvania's goal (the brand of the box I bought) :-\


Bruce Vicknair wrote:

About a dozen of these have been made by DIY'ers that I know about (they contacted me) and none have had problems. My own has been in operation for four years and has performed flawlessly.

Here's why ot won't oscillate: the CCS on the gain stage anode provides an insane amount of ripple rejection throughout the audio band. It's very isolated at AC from any garbage on the rails.

Cheers!