Why Does The Output Tube On My Vibro Champ Not Stay Lit?

[Darg1911]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Hmmmm. That may explain a lot. Pin 8 of that socket should have a resistor to ground. That's the cathode resistor and sets the bias. So, that's where your problem was? And is the resistor still there and is it still good?

Here's biasing info if/when you need it. Maximum plate dissipation for a 6V6 is 12 watts (if memory serves). If it's the bias that needs to be changed, it will require changing the cathode resistor of the 6v6. Schematics I found say it's 470 ohms (doesn't guarantee that it is). Put some numbers and measurements into a calculator. There are several to play with in the link below. They'll tell you where you are and where you need to be.


https://www.tedweber.com/bias-calc

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Running the amp with a 5881 and a 5V4 and an extra cab with my 10 inch 4 ohm speaker (total 2.6 Ohms) probably contributed to damaging the cathode resistor.

[Darg1911]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

I think that the stock resistors were 1 watt. Go with a higher wattage resistor there. 3 watt resistors are just as easy to find online and the voltage rating is usually higher on the 3 watt metal oxides which also helps.

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Thanks Darg. Would it also be wise to find a higher voltage rating capacitor for the cathode?

[Darg1911]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

I'm thinking wrong here. It's not the voltage, it's the current through that resistor. Still applies though in terms of using a higher wattage resistor. A lower voltage rated cap should still be fine. I think they are typically 25 volt caps in the schematics. Of course, the only negative to going with a higher voltage rated cap is size. But you shouldn't really have to go to a high voltage rated cap.

I'm running the hell out of the 6V6 in a SE amp. It's not a champ, but similar, based off an old Valco. There are two cathode bias resistors, switchable. One runs the tube at about 12 watts plate dissipation, the other runs it hot, hot, hot. Over 20 watts plate dissipation. It's not like a clean and dirty set up. It's more like a "normal" and nasty set-up. I didn't build the amp but I've tweaked it. I upgraded the OT and added a choke. It hummed like a SOB and I've got it so quiet with re-working some grounds and wiring, plus adding the choke, that it really shocked me (how relatively quiet it runs). At any rate, the 6V6 plate voltage is over 400 volts at 12 watt dissipation and around 370 volts at 20 watts dissipation. I haven't burnt through a 6V6 yet but they probably wont last very long. It's an interesting amp, it uses a 6SL7 as the driver/preamp tube rather than a 12ax7. Also has a rotary switch that switches the plate resistor and cathode resistor+bypass cap values on the 6SL7 which effectively changes that tubes bias. A couple positions of the switch are close to original spec values. A couple others essentially starve the plate, which makes for some nasty, but interesting tones. There was no bypass cap across the cathode bias resistor on the 6V6. I added one on a switch to switch it in or out. I have a small supply of NOS 6v6's for that amp. I think that there is one current production 6V6 that would probably survive the circuit but I don't think I'd trust current production tubes in general. At any rate, the tube is running outside of it's comfort zone. By a mile, some would argue. Although, I've seen plenty of reports from people with old Champs that claim that the 6V6 is running at 18 (or more) watts plate dissipation with relatively high plate voltages (probably because of more voltage at the wall these days). Overall point being that the 6V6 is a pretty tough tube. At least the old ones were. I get no red-plating. The trannies never get beyond just warm.

At any rate, it sounds like you are on your way to getting it back up and running. I wish I was more of a techie so I could be of greater help but I'm happy to be able to help even if it's just a little bit. Keep updating. I'm curious as to how it will all work out.

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Thanks Darg. What value cathode resistor will I need to continue running a 5881? My wife gave me the OK to purchase an Allen upgrade vibro champ power transformer. The original had been way too hot to the touch and smelled of near breakdown. I prefer 5881 over 6V6s. I'm after a really nice tone machine and I think I'm almost there.

[Darg1911]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Thanks Darg. What value cathode resistor will I need to continue running a 5881? My wife gave me the OK to purchase an Allen upgrade vibro champ power transformer. The original had been way too hot to the touch and smelled of near breakdown. I prefer 5881 over 6V6s. I'm after a really nice tone machine and I think I'm almost there.

I don't know. You'd have to run some numbers into that Weber bias calculator. I think max plate dissipation for a 5881 is 25 or 30 watts so I guess you could get around 10 watts or so if everything was optimal and your OT was up to the task. You should really read up on that tube in a SE amp. A lot depends on the plate voltage and the OT primary impedance and also what you want from the tube/amp. There is a bunch of stuff about load lines and linear operation that is beyond me. Unfortunately, I'm a follower, not an innovator when it comes to this stuff.

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Thanks Darg.

[hamerfan]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

I use a NOS Tung Sol 5881 in my 5F1 clone. I purchased a special Hammond tranny with different outputs. So i can use the 4 ohm with the 6v6 and the 8 ohm with the 5881. To bring the filiament current draw down i substituted the pilot light bulb with a led.

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Gentlemen..........I installed a 490 Ohm carbon comp resistor and a 20 uf 200 volt cap and the amp sounds excellent (using the 5881) kind of like a low power twin (perhaps running a little cold) The previous resistor was reading 606 Ohms out of the circuit when all was over-heating. I'll use pedals for dirt. I feel very little warmth on both transformers and I'll try a 6V6 later to see if there is still a red plating issue. I want to thank all of you who has replied and offered their help....you are a good group to hang out with here on this forum. I can only hope I can learn enough to help others as you guys do.

[jeremy]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

glad it seems to be running safely!

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

All is good.....6V6 is not red plating. I can effectively swap 6V6 to 5881 (probably not biased quite right but they both sound great)

[Darg1911]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

Happy to hear it's working now.

[Jeffblue]

Re: Why Does The Output Tube On My Vibro Champ Not Stay Lit?

I have to thank all you guys for your input, insight and sharing your experience and knowledge with me. It is always a blast to hang out with you guys on this forum. I have more confidence about working on my amps and for that I thank you. I may now have enough confidence to tackle a project that I have been unable to finish, a Mojotone Studio One.