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I am building an 18 watt sIII v6, my first amp build and am interested to know if anyone has implemented, or has comments on, the 'Mod for DC-cooupled cathode followers' that Merlin Blencowe suggests on his website?

He states "It is very common to see a DC coupled cathode follower in a guitar amp, often before a tone stack. The traditional arrangement suffers from the problem that on start up, the grid will immediately rise to HT potential while the cathode is still cold, at ground potential. It is not unknown for this to cause arcing between the electrodes, and rapid destruction of the valve, even when a standby switch is used.." and then goes on to recommend putting either a diode + resistor or neon lamp between grid and cathode. (http://www.valvewizard.co.uk/dccf.html)

I only have experience with solid-state rectifier amps and would appreciate any thoughts or input.

Thanks

-Dale

Hi Dale

This mod has been implemented into most of the popular build schematics over at AX84.com. Since I have built one of those amps, I have used it. What I can tell you is that it most likely does the intended job but is otherwise transparent from a tone standpoint. I didn't use it in my 18W build and .. so far so good. I think the point Merlin makes is that there will be the potential for arcing in the CF during power up and by adding this small mod it may help the tube last a while longer. However, it is one of those things that is difficult to quantify in terms of proof that it makes a difference. At the end of the day I don't think it can hurt to add it to any amp that uses a CF.

When considering mods to the CF, don't overlook the others discussed in Merlin's preamp book. These include lowering the cathode load resistor and/or using a grid stopper of significant size to promote soft clipping, compression and touch sensitivity.

Dave

Hi Dave,

Thanks for the reply and advice.

I do have Merlin's preamp book and per your suggestions looked at his other mods for "... lowering the cathode load resistor and/or using a grid stopper of significant size to promote soft clipping, compression and touch sensitivity."

The sIII v6 I am building uses a 56k cathode resistor for the V2B cathode follower stage, which appears to fall within Merlin's recommendation: "The effect can be increased by lowering the follower's cathode load resistor, to say, 68k, 56k or even 47k". However, his 'preferred version' includes a 100R grid stopper in front of the cathode follower as well as the 47k resistor and diode for protection. He goes on to say "… the added resistors and diode are protections components and do not affect tonality...)".

What does the 100R grid stopper do in this case? My understanding is that grid stoppers are there to create a very high frequency low-pass filter in conjunction with the input capacitance of the triode to eliminate picking up radio signals, but the ones I usually see in front of 12AX7s are much higher like 68k.

-Dale

I cant see a 100R grid resistor doing very much in this position. I wouldn't bother IMO.

_________________
Stephen
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....I'm not completely sure why the 100R grid stopper in the example that Merlin uses. He mentions in his book that it's there to prevent the chance of oscillation. However, he goes on to discuss the fact that the use of a substantially larger grid stopper on a DC coupled CF offers a great opportunity to promote soft clipping without loss of treble.