Filiment heater voltage

Trem Abuser · Post by **Trem Abuser** » Sun Dec 05, 2004 12:24 am

Appreciate the warning.

I do not screw around with my safety and well being. If I'm working on the amp, its not just turned off; its completely unplugged from the outlet. And I only have one hand in the amp, never both at once.

The only time my hand is in the amp when the amp is on is when I'm measuring voltages, and I'm very very careful when doing this.

I have no interest in injury or worse from an amplifier.

rjgtr · Post by **rjgtr** » Wed Dec 08, 2004 3:34 pm

So did you try it?

Trem Abuser · Post by **Trem Abuser** » Wed Dec 08, 2004 4:24 pm

This issue came up because I put together one of those "bias rite" kits from Ted Weber. I was testing it out on my Fender Champ and I did not know if I had put the bias rite together properly, as I wasn't sure what numbers I should be seeing from the amp.

Trying it out on my silverface Champ, I got a current of 34.8 mA of current with the bias rite. Measuring the voltage directly (no bias rite), I got 375 volts from pin 3 of the 6V6 tube to ground. I was trying to determine if these numbers sound about right, or whether I did not put the bias rite together properly.

Thanks,
Scott

rjgtr · Post by **rjgtr** » Wed Dec 08, 2004 5:26 pm

Yes That sounds about right.

I've been building a single ended, Class A, two channel amp and have been hip deep in spec sheets lately. Making sure everything is within spec. Funny thing is there's almost no common single ended amp running 6V6s within spec.

Trem Abuser · Post by **Trem Abuser** » Thu Dec 09, 2004 1:50 pm

Thanks for the help on the bias current.

On your amp, are your two channels like a bright and normal, or did you add an additional gain stage to one of the channels?

Do most single ended designs run too much current on the 6V6? Or is it a voltage issue?

rjgtr · Post by **rjgtr** » Thu Dec 09, 2004 10:31 pm

Well, cathode biased amps don't actually put the full voltage on the tube. It hovers above ground a little. For instance, in an amp running the tube at 375V if you measure from the cathode to ground it will usually read in the 15-20ish Volt range. So the tube only sees 355 V if the cathode reads 20V. Plus, the specs for the 6V6 aren't the real limits of the tube. Otherwise, the 415V a Deluxe Reverb uses would fry any 6V6.

Fortunately, most of the old tube manufacturers under spec'd tubes. As Leo Fender learned you could abuse them a lot more. As long as the tube is biased right (no pun intended) it will work unless you really abuse it.

My design is very simple. I'm using a SS recto, a 6V6 and two 12ax7s. Since the amp is Single Ended, there is no need for a phase inverter. I'm not looking for volume so I didn't need more than enough tubes to get the job done. The clean channel uses half of V1 and half of V2 for a classic Champ style sound. There are a few changes, like a Deluxe Reverb/Silverface Champ style tone circuit and bright cap with a high value (to keep it from sounding harsh). I use the complete set of gain stages for the Lead channel, which is sort of like a DSL or Rivera style circuit. Of course it is PTP and has some tweeks. The hardest part of the design was getting the layout right and "dialing" in the feature set. I treated the Lead channel sort of like a modded plexi/JCM800, with each of the gain stages in V2 subbing for the full V2 and V3 in a normal amp.

It's been fun doing it. It started out as me wanting a channel switching low watt amp. After Christmas I plan on doing another one with 50 watts (and a Triode switch for less) and maybe a Cathode/Fixed bias switch. I like the preamp, but I've got ideas of ways to expand it a little. My other future project is a rack preamp that uses a very low watt power tube for direct recording or headphone use.

Funny, at the end of the day, I'm still going to use one of George's kits to upgrade my 1987x RI, because as useful as channel switching is, I still love the purity of a one channel amp.

Trem Abuser · Post by **Trem Abuser** » Fri Dec 10, 2004 4:42 pm

Your project sounds intriguing. Should be good fun.

I'm assuming your amp is cathode-biased (no negative feedback circuit). Do you like the sound of cathode-biased amps? While I appreciate the Champ's tone, and its fun, it doesn't quite have the hutzpah I am seeking. Even with the "Torres Champ Kit" installed.

Of course, the 6 or so watts it produces might be a part of that.