Marshall JCM 800 Lead series 2205, help please!

[gtrplayr1976]

You must have the older one on the bench ?

[Appaloosa]

thanks, I guess I probably just feared for the worst first. The master Vol. does work now. The Values I get on V1 now are: P#1 231, P#2 0, P#3 1, P#6 85, P#70, P#8 0, and P#9 0.

What should I get? so I obviously have a bad resistor, cap, or diode in this section? I have tried different 12AX7 and got the same results

thanks!

[Appaloosa]

i Assume so, its small, fuzzy, and hard to read...further adding to my problems. Maybe I can figure out how to get a bic lighter to work so i can burn it.

[gtrplayr1976]

AC ,or DC ?

[Chuck H]

It seems you have a short to ground on pin 8. This would cause the low voltage reading on pin 6. Either a miswire or a shorted component on the pin 8 circuit.

[Appaloosa]

DC

[gtrplayr1976]

You may have this , but though I would post it just in case.

[Appaloosa]

thanks, I will investigate this section carefully now. if it were to work correctly what is the typical voltage I should see on these pins?

[Appaloosa]

thank you, that is helpful.

[gtrplayr1976]

I would think if V2 is working the voltages would be similar ? Someone correct me if this is wrong.

[jmaf]

thanks, I will investigate this section carefully now. if it were to work correctly what is the typical voltage I should see on these pins?

On pin 6 you should be looking at more than 180 VDC.

Is D1 ok? It may have clamped to ground keeping your cathode at 0 vdc. As Chuck H pointed out the zero volt reading on V1B's cathode(pin 8) is not right.

[Chuck H]

Well now, That's an unusual circuit. With a 470k plate load and a 1n4007 oriented cathode to ground there is only about 1V of bias and a fairly large drop across that plate resistor. I'll guess it should be about 100V. That 10k resistor is just there to support the diode and provide bias in the event that the diode goes open. But you should definitely have about one volt on pin 8. If your amp has this circuit I'll wager it's either miswired or there is a short in the resistor or diode.

[jmaf]

Well now, That's an unusual circuit. With a 470k plate load and a 1n4007 oriented cathode to ground there is only about 1V of bias and a fairly large drop across that plate resistor. I'll guess it should be about 100V. That 10k resistor is just there to support the diode and provide bias in the event that the diode goes open. But you should definitely have about one volt on pin 8. If your amp has this circuit I'll wager it's either miswired or there is a short in the resistor or diode.

That diode is non-linear, especially noisy at low voltages and totally not suited for that job. I've run tons of audio experiments using 1n4007's and they always made a noisy mess. Marshall wanted to clamp the cathode down to a diode drop for some reason and this was the job they pulled off.

[Chuck H]

Marshall wanted to clamp the cathode down to a diode drop for some reason and this was the job they pulled off.

That's correct... They pulled it off.

Since I don't use diodes in this application I would have no idea why a 1n4007 should be any more noisy or less linear than any other diode. Isn't a volt a volt the diode world over? What specs would I look for in the data sheets that let me know how noisy or non linear a diode will be when used only for it's foreward voltage drop? I plan to experiment with this some in future projects.

[jmaf]

That's correct... They pulled it off.

Since I don't use diodes in this application I would have no idea why a 1n4007 should be any more noisy or less linear than any other diode. Isn't a volt a volt the diode world over? What specs would I look for in the data sheets that let me know how noisy or non linear a diode will be when used only for it's foreward voltage drop? I plan to experiment with this some in future projects.

Unfortunately things are not so black on white when it comes to diodes. If you just use them on power supplies and power applications, you'll never notice, but as soon as you try to use them for effects pedals and signal level stuff, you start noticing their limitations.

To be able to accept greater reverse voltages and the high forward current than transistors and similar N-P semiconductors, 1n4007 diodes and other similar power rectifiers are built in such a way that they must trade some linearity and low noise specs for power handling capability(thicker N P regions, etc).

Germanium diodes for instance are less noisy, drop only 0.3v and leak less, but are only able to handle small signals. 1n4007's leak a lot! When you rectify a perfect sine wave signal, you can see the pulses below zero volts on a scope, and the pulses are normally distorted too, with spikes and noise all over. LED's drop voltages from 1.3V to 3V! So not all diodes are 1v the world over, they vary as much as they vary in brand, model and manufacturing process.

This tiny leakage and noisy trait doesn't matter in power supplies, where 1n4007's are meant to be used, just filter it with an as large as possible reservoir cap and nobody will notice the millivolts leakage and the noisy rectification.

I don't know why Marshall did this, it looks like terrible engineering to me, unless I'm completely missing something....