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FDP Forum / Amp Mods, Repairs, and Projects / Question about SF champ bias .


LA , Calif

I try my best
Oct 6th, 2016 07:34 PM   Edit   Profile   Print Topic   Search Topic

Since my line voltage here is constantly jumping from 117 to 120 by the time I can read one voltage across one resister a second later the plate is 359 then 370 it's difficult enough.

I read the cathode resistor at 679 ohm and the drop across is 25 volt 359 plate end up with 13.21 watts.

I do the same instead read across the 6V6 plate pin 3 to cathode pin 8 offers 332 actual plate end up with 12.22 watts.

My question is since I think lowering the cathode resistor will lower the plate at leas that's what I see on old paper I recorded on a 73 SF Champ. Since I am after later breakup and use a JJ 6V6s rated at 14 watts if I do lower the cathode resistor from 679 actual to 616 ohm actual a 53 ohm diff is there any way of calculating by my present voltage drop where the 616 ohm will take it? Or should I leave well enough alone? Trouble is all the 5 watt wire wound jump from 500 to 680 to 820 so basically you need to build a stack of them to get close been there done that.

Also does the UF valve of the bypass cap make any diff in tone I have a 25 UF 100 volt now what will double that do ? I've read nothing to more bottom end . Is it similar to a bypass cap value on a 12ax7 25 uf 50 volt instead of a 10 UF where the 25 UF passes more low's .

(This message was last edited by catnineblue at 09:40 PM, Oct 6th, 2016)



Oct 7th, 2016 01:08 PM   Edit   Profile   Print Topic   Search Topic

With the unstable mains and saggy HT, there are too many variables to make calculation feasible.

When the mains is high, what is the heater voltage?

If 6.6Vac or less, I suggest to reduce the HT by adding a resistor between the rectifier and reservoir cap, as cathode bias that's too cold of centre tends to sound thin.

Cathode bypass operates in the same manner for power and pre amp stages.
The bypass cap may be of a lower than expected value in order to limit the amp's bass response, thereby reducing the tendency of the little speaker / cab to fart when subject to loud bassy signals.


LA , Calif

I try my best
Oct 8th, 2016 03:57 PM   Edit   Profile   Print Topic   Search Topic

pdf64, I used a Weber PT . It states 6.3 VAC @ 4 amp . Even at 119 VAC line it reads 6.3 VAC on the heaters. I've read you should not exceed this with most 6V6 GT tubes.

As far as the 6V6GT cathode bypass cap right now it's 25 UF @ 160 volt. I have a ten inch speaker not the 8 " fender one and this is a copy of a SF champ circuit I built. I did increase the 2UF cap on the 12ax7's second gain stage to 25UF and that boosted the bottom end. Since the 10" Jensen C10R speaker does not fart out I thought I would parallel a 25 UF 50 volt cap to it giving me 50UF @ 50 volts hoping it would increase the bottom end a bit more.As far as the voltage across the bypass cap I read 25.5 VDC so 50 Volt is plenty. It's either that or a speaker like the Jensen Jet Falcon 10" that states tight fat bass and smooth highs rather than the bright highs the C10R offers. I have a Weber Sig 10S ceramic looks like the same design cone and 1" voice coil just a larger magnet but the Weber is more bright than the Jensen.

In the 73 SF champ I had the stock 8" fender alnico speaker farted out so I replaced it with a Weber Sig 8S alnico and it had a solid bottom end. Yet right now speakers cost money I don't have.

(This message was last edited by catnineblue at 05:59 PM, Oct 8th, 2016)



Oct 9th, 2016 05:37 AM   Edit   Profile   Print Topic   Search Topic

These 'student grade' amp circuits were probably designed to accommodate the lower spec speakers available at that time. So yes, with beefy modern speakers, it's reasonable to increase the amp's bass capability, if desired.

Note that the power tube cathode voltage will likely rise when the amp is being overdriven. Hopefully a 50V rating on its bypass cap is sufficient to accommodate that rise, but it may be worth checking.

FDP Forum / Amp Mods, Repairs, and Projects / Question about SF champ bias .

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