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Matchless Clone

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12/3/2003 3:42 PM

Matchless Clone

I’m starting to build my HC-30 clone and I have a couple of questions regarding the cathode resistors on the power tubes.

On the schematic I’m working with there is a single 62ohm resistor for Hi power (4 tubes) and the 62ohm in put in series with a 68ohm (for a total of 130ohm) for Low power.

In a previous thread “Cooling down a Bad Cat” there was talk of using larger resistors to get the tubes into the 12W dissipation range (down from 16W I think) but there was no mention of what those values would be.

Any suggestions ?

Also, why would the two resistors be different ? I expected the half power resistance to be exactly double the full power resistance.

12/3/2003 4:02 PM
Dirty	My suggestion is to use a common 4-6K resistor before the 100 Ohm screengrid resistors to bring down the screenvoltage! Then you should be fine with the stock values for the cathodes and have the platedissepation within safe values ! I do this on all my "Matchless" type of amps with double tubelife as resault Skål / Dirty http://www.swedenblue.com/dirty/dirty_amps.html

12/3/2003 6:00 PM
Bob DeRosa	Thanks for the reply Dirty, I was planning on increasing the screen resistors to get the screen voltage down to about 300v. Is a "common 4-6K resistor" the same as replacing the 100ohm screen resistors with ~1K ? So if I get the screen voltage down, I can keep the power tubes biased that hot ?

12/3/2003 9:51 PM

My DC-30 needed about 1.5k to get the screens under 300Vdc. I completely removed the 100ohm CC resistors and used 3W metal film.

I've never touched the Cathode resistors and my idle dissipation is about 14W per tube. I got JJ's from Bob@eurotubes.com that he rated "medium" and they seem to be just about right with the stock cicrcuit.

Amp sounds great and the tubes have lasted much longer than before I tamed those screens.

If you need any layout photos or advice from a genuine C-30 lemme know. I have a digital camera and can host/post photos.

I used the C-30 as a guide while building my Spitfire clone and it made everything so much easier and better!

You can also dramatically improve hiss, microphonics & noise over stock Matchless by using metal film resistors for the 68k & 1M input resistors.

12/3/2003 8:35 PM
Bob DeRosa	Thanks Joe, I've heard that going to 1k screen resistors can hurt the tone. What did you notice, if anything, by going to 1.5k? I'll be using all metal film resistors. You may get some benefit by replacing the plate resistors with metal film too. As far as layout help goes, thanks but no thanks. Making the layout for all of these clones I'm building is part of the fun for me

12/3/2003 9:58 PM

I´m running the screenvoltage through a 4W 4-6K wirewound resistor then split to two 100 Ohm 2W metalfilm-resistors, one for each screen.

Take a look at the Chieftain schem if you don´t understand.

I think this mod improves the touchresponce and clean headroom.

I´m using 1W and 2W metalfilm-resistors in all my amps! (wirewound for cathodes)

Skål / Dirty
http://www.swedenblue.com/dirty/dirty_amps.html

12/3/2003 10:15 PM
Joe CA	Hey dirty - you sniped me bud... grin Nice looking amps! Are you getting your cabs from Jamison or some other source? Yes, I see what you are doing with the large feed resistor to the 100ohm screens. I'm not sure why it would make much difference with EL84's but it's certainly something I might try.

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