London Power JCM 800

[kg]

Yeah, 1 x 1W 100V Zener is on the schematic. I'm not really up with zeners, but do 2 50V's in series = 100V Zener?

yes. i would actually prefer to string multiple zeners together because they tend to dissipate a lot of heat.

[WholeToneMusic]

I just re-read the page in the Torres book and he recommends to not drop the power supply by more than 50 volts in a Class AB amp with Zeners. Zeners in series are additive, Yes you could use 2- 50 volt zeners, Another alternative would be to get a Power Transformer with Dual B+ windings and switch between them. Mercury Magnetics has them. That gets pricey though.

[Colonel_Sanders]

I just re-read the page in the Torres book and he recommends to not drop the power supply by more than 50 volts in a Class AB amp with Zeners.

Does it say why? The more I read the more I want to switch to the 103 bias circuit.

Cheers for the Zener info. Its hard to find a 100V around here.

C_S

[Enzo]

Just thinking aloud here, I don't really know, but when chunking out part of the waveform with a zener, is not the resulting wave sent to the rectifier going to be choppy? Not a smooth sine wave? The more you chop out of it the more distorted the wave becomes, so potential noisy power? Or am I just rationalizing?

[kg]

Just thinking aloud here, I don't really know, but when chunking out part of the waveform with a zener, is not the resulting wave sent to the rectifier going to be choppy? Not a smooth sine wave? The more you chop out of it the more distorted the wave becomes, so potential noisy power? Or am I just rationalizing?

hey enzo!

once you get past the rectifiers, i think for the vast majority of power supplies it's already chopped/chunky/noisy.

if dropping rail voltage is your main goal you can put the zener anywhere you want.

ken

[WholeToneMusic]

Zeners

The part # for the 100 v Zener is NTE 5285AK, (leave the K off if you do not want reverse polarity) or for the 50V Zeners NTE 5275AK.

You can hook the reverse polarity zener to the CT of the power transformer and bolt the zener to the chassis. This way the voltage gets dropped at the PT and you have a chassis heat sink.

[WholeToneMusic]

He doesn't give an exact answer to as why, The 100 volt drop may just affect the plate, screen grid and bias voltage too much and take it out of it's operating range? Just a thought.

[Colonel_Sanders]

All these problems and the circuits in a book for beginners? Makes me question the other circuits I'm using and don't understand :-).

From what I read, I guess switching the zener regulated bias to the 103 would affect the other voltages in the amp and I'd have to re-design the power supply? Interested in what Peteko has done though.

Thanks for the Zener info wholetone.

[peteko]

Bias Supply Fixes

Hi everyone,

First of all, I apologize for not following up on this amp. I got it to work a while back and I should have done better to give more info on that, especially since a bunch of people helped in getting this fixed.

Ok, first I lowered the two 100k 1W between the PT and the cathodes of the 1N4007's, to 68K 2W. Next, had to decrease the 15k 1W in series with the Zener. The value I'm using is 1.5K 2W. This is all to increase Zener current.

Finally, I increased the 6.8k Zener shunt resistor to 10k, so the load would draw a little less current. Originally 6.25k (four bias pots) in series with 6.8k (shunt resistor) draw 7.66mA (100/13050). 100/(6.25k+10k) = 6.15mA.

These changes give enough of a negative bias range.

The amp sounds great. Really meaty tone, I don't have a microphone that can do it justice, but I might figure something out and post a link to a sample or two. What can I say except its 'the' Les Paul through a Marshall tone.

I want to thank everyone again for their help. I really appreciate it.

[Colonel_Sanders]

Cheers Peteko! I'm going to try it out this week.

[WholeToneMusic]

Your welcome Colonel, I am not familiar with the 103 method but the Zener method at the CT of the PS is effective for dropping the Voltage of the entire amp.

[Colonel_Sanders]

Ah, the Zener drops the B+ voltage? That makes sense.

I have another question regarding this amp... Is it possible to incorporate a cathode bias switch while keeping the individual bias pots? I've got the TUT books and I've been searching a lot, but the only way method I can find is to have a single bias pot for a push/pull pair.

Thanks,
C_S

[WholeToneMusic]

The zener drops the entire power supply of the amp when placed in line with the Center Tap of the Power Transformer. CS, Fender used a combo bias in the SilverFace era and the tone was horrible. That was one of the many problems with the Silver Face circuit.

There are amps that have a bias pot for each individual tube which may be a better route to go.

[mykey]

Yeah, 1 x 1W 100V Zener is on the schematic. I'm not really up with zeners, but do 2 50V's in series = 100V Zener?

yes.

[Colonel_Sanders]

Wholetone, what model are the silver face amps? Did a quick search and couldn't find a full schematic. By combo bias, I assume you mean fixed AND cathode bias? I was thinking of having a 'one or the other' switch.

Thanks,
C_S