Fender 5F1 Champ clone with triode mode
 

An FYI to anyone who wants to run their 5F1 in triode mode. I recently completed the build of a 5F1 Champ clone. Great sounding little amp! I decide to add a switch to run the 6V6 output tube in triode mode. I used a 100 ohm resistor on the screen, to a SPDT switch, to switch between the B+ (pentode mode) and the plate (triode mode). When in triode mode I had excessive hum. The original power supply does not have adequate filtering for triode mode, so on the front end of the power supply I installed an additional LC filter. 10uF 630V MP as the first capacitor after the rectifier & a 15H 75mA choke (Hammond 158L) to input of the original supply circuit. Problem solved, no more hum when running in triode mode. I'm not sure if 15H is needed, maybe 5 - 10H would do, but that is what I had on hand.

Hope this helps anyone who wants to modify their 5F1 for triode operation.

Scott

How would you say the sound changed?

I've done some champish circuits with the 1626 triode. Excellent cleans, I do believe the appropriate term would be 'beautiful'. Not for all styles of playing though.

I just finished the amp with the power supply mod today, so I really haven't been able to play it enough to make a determination. I'll have to agree that the appropriate term is "beautiful". The triode mode doesn't lower the output as much as I would have guessed, but it does sound great, lots of clean overtones at about 1/4 to 1/3 input attenuator setting. With heavy attack it really has some punch. In fact I'm amazed at the amps response to picking style. With the volume up over 1/2 it is just amazing, very bluesy. I'll keep you posted when I've had time to play it more. The best thing though is the added LC filter makes this baby almost dead quiet. I didn't build from a kit, but I did purchase the Triode Electronics 5F1 power & output transformer. I'm using NOS Raytheon 5Y3GT & 6V6GTA, and a Sovtek 12AX7LPS.

I have done just the same as you did with the same circuit. In the end I got rid of the switch; pentode gives me an aggressive sound that I really like whereas in triode mode it sounded too nice, too smooth (for me). The extra hum wasn't welcomed either and a choke changes the sound of the amp. I don't want to change the sound so it stays in pentode mode.

Cool. Something I didn't think about... I suppose a switching circuit could be implemented to take the additional LC filter completely out of the circuit for a stock 5F1.

Sound is more dynamic and more alive without a choke. It depends on the DC resistance of the choke - the less the better. It isn't a dramatic change though so a new switch might not be worth the trouble. These switches tend to fail over time so it's better if you simplify things.

There is no "sag" in a class A amp like the 5F1, so I don't see that choke, or no choke, makes any difference whatsoever except to provide better filtering and therefore better noise reduction. Switches fail over time when not used properly, so I don't see that as a factor.

It is not about seeing, it is about hearing. Theory meets practice and gets confused.

Touché, I'm with you on that.

Actually, I have nothing against chokes or switches. I don't need a choke because hum is pretty low in my amp, as for switches, it's just that I don't need them either. Before making the chassis I use them; bypass cap on/off, negative feedback on/off, triode/pentode on/off, but when I find the tone that I really like I get rid of the switches.