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bereanbill 23rd May 2009 01:47 AM

SE 6V6 cathode resistor value
Me again. Still looking into a nice SE amp to build. I was planning a 6V6 powered model for a while...sort of a mutated Fender Champ with an extra gain stage and MV. While looking through some schematics I noticed that Fender used a 470 ohm cathode resistor on all of thier SE Champs, while the VM amp I have...also SE 6V6 uses a 300 ohm. Quite a difference, not? They both run the plate at about 350V and use a single 12AX7 in the preamp. Why such a difference in cathode resistors? The VM is from an old reel to reel player and is way more gainy than the Champ ( is gainy even a word ? I guess it is now !!) :)

flysig 23rd May 2009 02:49 PM

A quick look at the 6V6 datasheet shows about 74mA of anode current, which through a 300 ohm resistor gives you a grid bias of -22v. That looks about right for a typical class AB1 push pull guitar amp. Going to the higher resistor would take it to seemingly too much bias of around -34v. At that much bias it is in cutoff and the amp won't be class AB1.

The answer is probably that it is a hi-fi design vs a guitar design.

Funker 23rd May 2009 04:21 PM

Hi flysig,

refer to the Fender champ schematic . They use a high supply voltage of about 360V for the Champ. This operation point is not given in the tube data sheet. See link below!

The original Fender diagram show you a 470 Ohm kathode resistor. the kath. Voltage is 19V and resulting 40mA kath. current . The data specs of the 6V6 says max. Plate dissipation 12W and with 40mA you will be close to the limit. Assume 4mA for the screen left 36 mA Plate current. The Plate voltage is 350V and 19V at the kath . So the Plate turn 11,9W into heat.

To run a SE Amp with 70mA Plate current at abt 330V between Plate / kath dissipate 23 W and ruin the tube in few minutes . The 70mA in the data specs is valid for a PP amp with 2 tubes !

In my little homebrew amp there is also a 6V6 in the final. I run it with 420V supply voltage . 420 minus 30V at the opt winding and another 30Volts at the kath resistor give 32mA Plate current . The kath. resister is 910 Ohms .

If I would need much more current I should use a 6L6 in the final.

To find out which kath resistor sounds best. Replace the kath resistor against a potmeter in series with the low limit value resistor to avoid tube overload. Play the amp and adjust to the best sounding setting. By the way , SE audio amps can only run in class A.

regards from Hamburg

bereanbill 23rd May 2009 07:38 PM

Thanks Flysig & Funker ( sounds like a law firm ). I am using the PT from the VM amp so I will try out the 300 ohm rK. I like the tone and the crunch of the VM so I have gutted a small SS combo to use the chassis and cabinet. My weekend project. :D

dsavitsk 23rd May 2009 08:40 PM

This might prove helpful in answering your questions:

flysig 23rd May 2009 10:39 PM


Originally posted by Funker
Hi flysig,

refer to the Fender champ schematic . They use a high supply voltage of about 360V for the Champ. This operation point is not given in the tube data sheet. See link below!


Es tun mir leid! I missed that this is a single-ended design. Thank you for the link to tubedata, which is something that I desperately needed.

Funker 24th May 2009 02:42 PM

Hi flysig,

after the first time you blow up your circuit design you enter the next level :D .

So here is my own design, I added a distortion stage to it . The Amp give 5 Watts. A friend of mine use it for rehearsal and small Pup gigs. The chassis and the enclosure is from a scraped chinese Trasistoramp which had a horrible sound.

The Kath Resistor must be 910Ohms , in my drawing you see an 680 Ohm which is a wrong value The drawing was first for an 4 watter EL84 with 300V Supply . I used the same drawing and so I forget to change some values.

regards from Hamburg


jjman 24th May 2009 05:11 PM

The schematic for my '74 (Vibro) Champ shows 24v on the 470ohm cathode resistor and ~418v on the plate. That comes out to about 18watts :bigeyes:

I changed it to a 750ohm and it idles around 12.5watts now.

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