1977 Marshall 2203 High Plate Voltage!!

[g1]

I don't think we should be advocating the current shunt method, as it's needlessly hazardous, and less accurate than the OT voltage drop/resistance method, the only benefit being an instant current reading.
A competent person might choose to play fast and loose with their own safety, but they are fully aware of the hazard and what to expect.
It seems a very bad idea to suggest it to a noob

I don't think any of us made the suggestion, or were really advocating it, he brought it up, asked about it, and asked about other methods:

So, I was going to try the "Shunt" method but I want to clarify how it's done. I have a really good Fluke (1000V rated) to use. Could someone here explain to me how to properly bias an amp using this method? If there is another method that I'm missing, please let me know. I don't have a scope, so that method is out. Any suggestions would be appreciated.

I agree that it's dangerous, and hesitate to be the one to bring it up if it's not already being discussed. But this is more of a tech forum, as opposed to more 'guitar' type forums where some forbid discussion of even going inside a tube amp.
So I think it's important to differentiate between someone who is already in their amp measuring plate voltage, and someone who wants to bias their amp using some rear panel test points.
And there's also that guy (much more reputable than I ) who has a section of his bias page on his website explaining shunt method.

[cluster]

Thanks for everyone's help. Oddly enough I also have a 1983 JCM800 (2203) Canadian Version with the metal switches, no 16 ohm tap. It's a pain because the B+ fuses are inside on the board. 500ma for each winding of the OT. Sounds different than this 1977 amp. 1977 has more headroom but the breakup on the JCM800 is nicer. Maybe its just the different tubes? Who knows? One can never have enough Marshall amps LOL..

Last question: when biasing using the dc resistance/voltage method across the CT, how does the current come into play? I mean, if we always choose 70% max dispassion of 25 watts = 17 watts and then dividing that by the plate voltage to arrive at the bias current at each tube. how does the current at each OT winding come into play?

[nsubulysses]

I'd say if it sounds good, it is.
100k will help with tube life , they say.

would anyone care to explain why some people might say this? just curious

(the 100K thing)

[nsubulysses]

Last question: when biasing using the dc resistance/voltage method across the CT, how does the current come into play? I mean, if we always choose 70% max dispassion of 25 watts = 17 watts and then dividing that by the plate voltage to arrive at the bias current at each tube. how does the current at each OT winding come into play?

70% max plate dissipation is not a number to always choose. It is just some talked about number. Anyway, that's a whole nother thread

measureing DC resistance across the OT primary windings allows you to indirectly measure current, by actually measuring voltage. Ohms law

for instance, OT primary winding measures 30 ohms and when amp is on and idling it measures 1.5VDC drop from OT CT to one of the plate windings -- 1.5V / 30 = .05A. Or 50mA

Say plate voltage is 530V, but don 't forget, if it's a 4 power tube amp you are measuring current draw for TWO tubes, so 50mA / 2 = an estimated 25mA per tube

.025 x 530 = 13.25W

if max plate dissipation for EL34 is 25W, 13.25 / 25 = 53%.

Slightly weak bias but really should be judged on if it sounds bad or has a visible crossover distortion on the oscilloscope screen. If it does not in either regard, the bias setting is just fine even thought it's not "70%"

[pdf64]

would anyone care to explain why some people might say this? just curious

(the 100K thing)

My understanding is that the grid current of a gassy tube (perhaps due to wear) will tend to increase, creating a voltage drop across the grid leak resistor so as to reduce the effective bias voltage. That tends to increase heat, more gas is released and current runaway can result.
A lower value of grid leak resistor can help to mitigate that effect.

70% max plate dissipation is not a number to always choose...

Yes. Aiken's rationale is that 70% is suitable guideline for 'most' amps http://www.aikenamps.com/index.php/i...why-70-percent
The page goes on to show how the very high HT and low load of an amp such as the one under discussion make it unsuitable for the guideline.
Hence in this case, the bias may best be set for cool idle conditions, or the load impedance increased.

[cluster]

70% max plate dissipation is not a number to always choose. It is just some talked about number. Anyway, that's a whole nother thread

measureing DC resistance across the OT primary windings allows you to indirectly measure current, by actually measuring voltage. Ohms law

for instance, OT primary winding measures 30 ohms and when amp is on and idling it measures 1.5VDC drop from OT CT to one of the plate windings -- 1.5V / 30 = .05A. Or 50mA

Say plate voltage is 530V, but don 't forget, if it's a 4 power tube amp you are measuring current draw for TWO tubes, so 50mA / 2 = an estimated 25mA per tube

.025 x 530 = 13.25W

if max plate dissipation for EL34 is 25W, 13.25 / 25 = 53%.

Slightly weak bias but really should be judged on if it sounds bad or has a visible crossover distortion on the oscilloscope screen. If it does not in either regard, the bias setting is just fine even thought it's not "70%"

Thanks for that explanation.

joseph