Happy Vibro Champ

[shortcircuit]

I have a 79-80 silver face Vibro Champ with a Torres champ champ mod + , right now it has a 22uf/500v cap on the bias resistor(don't think this was part of the mod) , It's going to get a total cap job+ death cap removal . What is a happy bias on a Vibro Champ ?? I ran the numbers just to check before re-cap
Cath R =483ohm , Voltage drop across 26.80vdc , plate V 418vdc (just a note 26.62vdc across 330pf from pin5 to 8) came out .055ma dissipation 21.52w smokin hot for a 6v6 isn't it?? 14w is the data on a Electro Harmonix 6v6gt(that's what's in it) where is the happy range?? remember your in the major leagues , I'm in t-ball, get a lucky hit now and then , still run towards 3rd base.

[The Dude]

Check this:

Weber Bias Calculator

Edit: I do think that's a bit hot. I'd up the cathode resistor value a bit to get into a safer range. I'd also change the cathode bypass cap. It's already running on the edge of it's rated voltage. If you up the cathode resistor value, it will go higher yet, or is that the 500V rated cap you are talking about?

[mac dillard]

Fender ran the single ended cathode biased Champs really hot..

[The Dude]

They did, but line voltage is higher now than then. IMO, one should correct bias for the higher voltage. Also, make sure there is a 5Y3 installed. I have seen, on occasion, where someone installed a rectifier with a lower voltage drop.

[shortcircuit]

They did, but line voltage is higher now than then. IMO, one should correct bias for the higher voltage. Also, make sure there is a 5Y3 installed. I have seen, on occasion, where someone installed a rectifier with a lower voltage drop.

Sovtek 5y3gt , going back to post 2,yes cathode bias cap 500v 22uf , it's at 11.21uf now . replacing with 50v/25uf??

[mozz]

Get rid of that Sovtek 5y3. Some of those had practically no voltage drop. Get a good old USA 5y3, get about a 60 volt drop, it will knock your dissipation down some. I have heard, from rumors, that if you up the cathode resistor too high, you lose the sought-after tone.

Replace cathode cap with 50/50v or 50/100v.

[mac dillard]

Fender schematic shows 420 vdc to the OT so his voltages are not far off. Also shows 470 ohm cathode resistor. It also shows the cathode voltage. Best I can read in is 24 volts. If my math is right that is 20 watts diss. assuming very little drop through the OT.

[The Dude]

I haven't used Sovtek 5Y3, so I wasn't aware of the lack of voltage drop. Nice to know, mozz. I've heard the "rumors" regarding that "sought-after tone", also. But, I've upped the cathode resistor to 680 ohms myself in the shop and didn't notice any tonal change. I'm sure it's possible to go too far.

I'm not trying to argue with you, mac dillard, but I don't like running a 14 watt tube at 20 watts. Sure, Fender did it, but tubes were more durable then. If someone wants to do that, be my guest.

[Enzo]

Remember, a single ended amp like the Champ is by its nature a Class A amp, so it is normal to run the power tube at 90% or more. So yes, Champs run hot.

[mac dillard]

No argument. Just pointing out that his amp is just about on spec with Fender per the schematic.

[The Dude]

Understood, Enzo, but 90% would be about 26mA at 418V (if my math is correct). He's getting about 55mA. That seems extreme, at least to me.

[Enzo]

Wasn't aiming that at you, Dude, I was speaking in general. You know a lot of guys will say to themselves "HEY, this thing ought to be at 70%."

[mac dillard]

First thing I would do would be get rid of the Sovtek 5Y3 and go to a NOS 5Y3. And I agree with Dude. That is really hot, but it appears to be as designed.

[The Dude]

^^^^^^ Agree with replacing the 5Y3 first and reassessing. Another note: When you replace the cathode cap, leave the leads longer and keep it away from the cathode resistor. Heat is the enemy of electrolytics.

[shortcircuit]

Understood, Enzo, but 90% would be about 26mA at 418V (if my math is correct). He's getting about 55mA. That seems extreme, at least to me.

using actual voltage across pin 3 to 8 is 391vdc , I went pin 3 to grnd to get 418vdc. I still agree 20's are hot, I will bias down and see if I turn into claptone . going back to post 6 & 8 , I'll try these and check any difference in dissipation, just for a test. I'll be back