SB-12 Fliptop 7868 Bias Question

[pintail78]

Juat retubed a SB-12 w 7868's, the schematic says it should be at 80 ma for the pair and at 475 volts on the plate that puts the amp at 38 watts for the pair! The 7868's say they have a 19 watt max dissipation, I am wondering how they can run that high unless Im missing something here. Right now I biased it out at 30 watts for thr pair until i could figure this out. Help anyone!

1 more thing, the amp is dead quiet with the guitar plugged into it, but hums really badly with nothing plugged in, any ideas?..........

[thehoj]

That does seem high.
I've got a guitar amp running a pair of 7868's, and I've got them biased for 70% of max plate dissipation.

My B+ is a bit higher than 475, but if you calculate it out for 475v you want about 28ma from cathode to ground on your power tubes.

70% of 19w is 13.3w
I = 13.3 / 475
I = 28ma

[RWood]

Also, Ampeg's 80mA was assuming 420 volts on the plates, not 475.
But that is still 88% of max; pretty high for grid-bias. I agree with Hoj to dial it down to 28mA per tube and use your ears to fine tune it from there.

As for the input jack, check that it is a switched jack and that there is good contact between the hot pin and the switch pin when there's no plug inserted. use contact cleaner and emory cloth or a nail file. Then check the switch pin's connection to ground. What's happening there is that with nothing plugged in, the signal is grounded out and thus silent.

[pintail78]

Cool, i ended up at 470 volts on the plate, I had to add a 20K resistor to be able to drop down to 28ma, now its balanced and biased right at 70%. The input jack has a broken blade on it, so ill fix it.

BTW it doesn't have reverb but the portaflex cab gives it an airy feel such that it doesn't really need it for guitar. Its a great guitar amp. Thanks for your help!

[Dave Curtis, dB AudioTech]

As for the input jack, check that it is a switched jack and that there is good contact between the hot pin and the switch pin when there's no plug inserted. use contact cleaner and emory cloth or a nail file.

Don't use anything abrasive on any switching contacts; you'll remove the plating and they'll soon corrode. You want to burnish them, not sand or file them. Replace "emory cloth" with a new dollar bill, and lose the nail file, or get a contact burnisher.

[Enzo]

Where are these bias specs? I looked through my Ampeg files and I don't see bias current levels on the schematics.

[Ted]

Enzo - I've got one of these amps in for repair and it has the 80 mA figure on the bottom left of the schematic (identified 5-66, bottom right).

[soundmasterg]

The RCA manual says a pair of 7868's into a 6600 ohm load, fixed bias, 450v plate, 400v screen can do 44 watts RMS. I would assume that is optimal conditions with a large output transformer too. My project Bogen CHB100 with 4 7868's push-pull parallel, 480v plate, 435v screen and a smaller than optimal OT is putting out 58 watts RMS and 105 watts peak. Your 30 watts doesn't sound too far off from what they should be if your OT is on the smallish side of things.


greg

[Enzo]

Yeah, but his 30 watts is idle dissipation, not audio power output.

[soundmasterg]

Yeah, but his 30 watts is idle dissipation, not audio power output.

Ah sorry, I missed that part.

greg

[imaradiostar]

Bogen CHB100 with 4 7868's push-pull parallel, 480v plate, 435v screen and a smaller than optimal OT is putting out 58 watts RMS and 105 watts peak.

RMS or peak? Please explain the difference- I don't understand how it can be both. Is peak distorted and RMS is a sine wave?

jamie

[soundmasterg]

RMS measurement is taken when the scope shows a clean sine wave...so you turn up until you see flat tops and back off a bit from there. Peak is full up, so yes it is distorted at that point. Since Bogen called this amp a CHB 100, I would guess they were advertising the power of the amp with the peak rating rather than the RMS rating.

Greg

[imaradiostar]

Makes sense. I've always measured based on the sine output- the most usable number for clean audio. I guess it's good to have a measurement of "peak" output since guitar amps are often used with distorted signals or pushed till the output section distorts.

jamie