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previous: Phil Is there a big difference in voltag... -- 6/18/2000 9:04 PM

Re: Vibro Champ (single ended 6v6) questions?

6/20/2000 7:46 PM
MBSetzer	Re: Vibro Champ (single ended 6v6) questions? Measuring procedure should be about the same, only the readings will probably be different You would benefit by looking at the schematic for your amp, if it is blackface, the one with a 2-prong power cord and 350VDC on the plate would be it. For a silverface, typical schematic will have a 3-prong power cord and about 420VDC on the plate. The other components themselves are virtually the same for all Vibrochamps except for the power transformer which runs at higher voltage in the SF. Like it says on the older drawings, typical voltages are listed plus or minus 20percent, I would not worry unless the amp is outside that range. Champs are cathode-biased, the amount of current at idle (and resulting operating point for signal handling) is dependent on the actual value of the stock 470ohm cathode resistor. There is no separate source of negative voltage for biasing the grid more negative than a grounded cathode would be, instead the grid is grounded through its attached 220K resistor, and the cathode voltage rises above ground as the tube conducts when the high voltage is applied to the plate. The bias either way is the difference between these readings at the socket pins for grid & cathode. The idle current would be changed by replacing the stock 470ohm with a resistor having a different value. Lower ohms will allow more idle current and the tube will run hotter, higher ohms and the tube will run cooler. You wouldn't want to change it much, and the next standard value of 510ohms is about the only thing I would try if you were really unhappy with the tone of a true 470ohm 2Watt or higher rated resistor. If the stock resistor is a 2Watt and still close to 470ohms, I leave it there, and find a tube that will perform well in the original stock circuit. There is something about a Champ that makes it different than other Fenders, since Champs are single-ended, they only have one power tube that has to handle the complete signal. Some tubeheads will bias tubes in single-ended Class A circuits for 100percent of rated maximum heat dissipation. This really does require the tube to run much hotter at idle than players are accustomed to when the same tube type would be used in the vast majority of amps which are push-pull, where a pair of tubes share the power output. Also, in a single-end Class A amp like a Champ, the current through the 6V6 is highest during idle, and stock these amps will usually run hot enough at idle to make the plate of the 6V6 glow slightly, which might be alarming with other amps. These amps have always done that, and even if it was not intentional by Leo Fender, loads of them still have the original tubes working. If you biased them so there is no glowing at idle, NOS 6V6 would probably last 60years instead of 30, that would be a drag if you never had the opportunity to change the tube and experience the various tones from different brands, especially NOS, like the amp was designed for. When you start playing though, the plate cools to more moderate temperatures as the idling heat is transferred to the output transformer when signal is being amplified. The screen however is called on to dissipate more current (heat) when driven than idling, so an additional resistor to limit the stress on the sometimes fragile screen is advisable if you are really going to crank it. OTOH, with a regular push-pull circuit, the tubes are usually only biased from 50percent to 70percent of their rated heat dissipation at idle. As signal is applied, both the tubes and the OPT increase in temperature, so the time to check these for glowing plates is when really playing loud. I love my Vibrochamps, it should only be necessary to turn down the intensity to cancel the vibrato, you should be able to leave the speed setting where you like it. A working footswitch allows you to keep both knobs set, you knew that already These are antique phono plugs & jacks, and good contact must be made to effectively short the inner conductor to the outer (ground), to silence the vibrato with the footswitch. Make sure the unplugged jack has a low ohm connection between the inner & outer conductors when the footswitch is in the on position first before you think the amp needs work. Then make sure there is good contact for both the conductors, lots of times the outer blades of the phono plug will separate enough with age & use that there is no contact with the chassis jack. Or if the surfaces are too corroded it might just need to be polished up a little or use contact cleaner. Also make sure both small tubes are 12AX7 types. Sometimes if a different tube like 12AU7 or 12AT7 is used in either the preamp (first tube) or vibrato (second tube), or both, it will affect the vibrato for the worse. Nothing permanent, you just can enjoy the alternate preamp tube tones without the vibrato being as familar or useful as with originally intended tubes. Hope this helps, Mike

6/20/2000 7:46 PM

MBSetzer

Re: Vibro Champ (single ended 6v6) questions?

Measuring procedure should be about the same, only the readings will probably be different

You would benefit by looking at the schematic for your amp, if it is blackface, the one with a 2-prong power cord and 350VDC on the plate would be it. For a silverface, typical schematic will have a 3-prong power cord and about 420VDC on the plate. The other components themselves are virtually the same for all Vibrochamps except for the power transformer which runs at higher voltage in the SF. Like it says on the older drawings, typical voltages are listed plus or minus 20percent, I would not worry unless the amp is outside that range.

Champs are cathode-biased, the amount of current at idle (and resulting operating point for signal handling) is dependent on the actual value of the stock *470ohm* cathode resistor. There is no separate source of negative voltage for biasing the grid more negative than a grounded cathode would be, instead the grid is grounded through its attached 220K resistor, and the cathode voltage rises above ground as the tube conducts when the high voltage is applied to the plate. The *bias* either way is the difference between these readings at the socket pins for grid & cathode.

The idle current would be changed by replacing the stock 470ohm with a resistor having a different value. Lower ohms will allow more idle current and the tube will run hotter, higher ohms and the tube will run cooler. You wouldn't want to change it much, and the next standard value of 510ohms is about the only thing I would try if you were really unhappy with the tone of a true 470ohm 2Watt or higher rated resistor. If the stock resistor is a 2Watt and still close to 470ohms, I leave it there, and find a tube that will perform well in the original stock circuit.

There is something about a Champ that makes it different than other Fenders, since Champs are single-ended, they only have one power tube that has to handle the complete signal. Some tubeheads will bias tubes in single-ended Class A circuits for 100percent of rated maximum heat dissipation. This really does require the tube to run much hotter at idle than players are accustomed to when the same tube type would be used in the vast majority of amps which are push-pull, where a pair of tubes share the power output.

Also, in a single-end Class A amp like a Champ, the current through the 6V6 is highest during idle, and stock these amps will usually run hot enough at idle to make the plate of the 6V6 glow slightly, which might be alarming with other amps. These amps have always done that, and even if it was not intentional by Leo Fender, loads of them still have the original tubes working. If you biased them so there is no glowing at idle, NOS 6V6 would probably last 60years instead of 30, that would be a drag if you never had the opportunity to change the tube and experience the various tones from different brands, especially NOS, like the amp was designed for. When you start playing though, the plate cools to more moderate temperatures as the idling heat is transferred to the output transformer when signal is being amplified. The screen however is called on to dissipate more current (heat) when driven than idling, so an additional resistor to limit the stress on the sometimes fragile screen is advisable if you are really going to crank it.

OTOH, with a regular push-pull circuit, the tubes are usually only biased from 50percent to 70percent of their rated heat dissipation at idle. As signal is applied, both the tubes and the OPT increase in temperature, so the time to check these for glowing plates is when really playing loud.

I love my Vibrochamps, it should only be necessary to turn down the intensity to cancel the vibrato, you should be able to leave the speed setting where you like it. A working footswitch allows you to keep both knobs set, you knew that already

These are antique phono plugs & jacks, and good contact must be made to effectively short the inner conductor to the outer (ground), to silence the vibrato with the footswitch. Make sure the unplugged jack has a low ohm connection between the inner & outer conductors when the footswitch is in the on position first before you think the amp needs work. Then make sure there is good contact for both the conductors, lots of times the outer blades of the phono plug will separate enough with age & use that there is no contact with the chassis jack. Or if the surfaces are too corroded it might just need to be polished up a little or use contact cleaner. Also make sure both small tubes are 12AX7 types. Sometimes if a different tube like 12AU7 or 12AT7 is used in either the preamp (first tube) or vibrato (second tube), or both, it will affect the vibrato for the worse. Nothing permanent, you just can enjoy the alternate preamp tube tones without the vibrato being as familar or useful as with originally intended tubes.

Hope this helps,
Mike