London Power JCM 800

[WholeToneMusic]

CS,
When CBS bought Fender they changed the circuits. Look up the Fender Twin circuit from 1972, It is a combo cathode bias/ bias balance circuit. The bias balance adjusts the balance between the output tubes. So it is not a Fixed/Cathode combo bias like you mentioned. I wouldn't recommend it.

[WholeToneMusic]

You could measure the resistance of the pot at it's ideal idle, take the pot out and put in a power resistor to switch between the cathode and fixed bias. Cathode bias demands more current so you might want to look into your power transformers available current to make sure your not loading it down to much.

[Colonel_Sanders]

Peteko, I found some info on the sound leakage you mentioned earlier in this thread. TUT4, p5-7.

"We do find a quirk in the bootsrap MV... There can be signal feedthrough at the 'zero' setting. This happens beacause the impedances are so high and the signal feeding the pot is quite high, too. The signal can electrostatically couple across the wiring and the pot itself. One solution is to use a lower-value pot so that the bootstrapped impedance will be closer to the stock value."

Wholetone, thanks again for the info. I'm using the hammond 372JX which I've heard as ample current for most applications, but I'll look up the specs.

And another question: Is it possible to use TUT 2 fig. 5-18/p.3-23 with the london power JCM800 bias supply? I'll whip up a schematic for those without the books after lunch...

Thanks,
C_S

[WholeToneMusic]

Hammond

The Hammond T-372-JX has ample current for your needs.

[Colonel_Sanders]

Cheers Wholetone!

Attached a couple of quick schematics...

The original circuit (bias_1) from TUT2 has a single 150K/220K 1W resistor, a reverse biased IN4007 and a 15k resistor in series, with 2 x 10uF 160V caps to ground either side of the 15K, before the 47K and 22K pot to ground.

I'd to use the Zener shunt bias (bias_2) from the london power JCM 800 with this circuit. Will it work without modification? I have no idea and am extremely confused

Thanks,
C_S

[WholeToneMusic]

Hello C_S

I have not seen the entire moddified JCM800 schematic in TUT2. Your best bet would be to email KOC. You can go to londonpower.com to email him, He is very quick to return emails and super helpful.

[KevinOConnor]

Hi Guys

Yes, this is an old thread but there is some serious misinformation here.

The zener in the bias supply is 100V-1W and that's all it needs to be. It is only there to clamp the voltage across the bias filter caps when the bias-set network is disconnected or not built yet. Once the whole circuit is built, the zener never comes on.

The zener has no effect on plate voltages or anything else. It does not effect the bias voltage either with the bias-set network in place.

In a shunt regulator using a zener diode, the zener is not "supplying current". Rather, it is clamping voltage by passing whatever current the load does not take from the current source feeding both of them.

Have fun