1977 Marshall 2203 High Plate Voltage!!

[cluster]

Hello all, So I recently acquired a 1977 2203 in very fine condition. The only thing done to it was someone replaced the 220k bias feed resistors with 100K running 4x EL34s. When I checked the amp's plate voltage I got a jaw dropping 524VDC. Is this normal for this vintage of amp. It is the Canadian version with the lay down power transformer and metal toggle switches instead of the plastic butterfly ones.

The other thing I noticed is that although the amp is mostly quiet (no hum or oscillations), it does have this kind of scratchy pot noise (without moving the pots). Maybe like little pops. Irregular scratchy noise that gets louder with the volume. I tried changing tubes but with little effect. I'm wondering if this is a problem with the plate resistors in the preamp stage? I understand that this early plate resistors could be noisy. Any idea what it could be? Thanks in advance...

Joseph

[Mick Bailey]

Is the plate voltage measured with the tubes in place, and is the amp correctly biased?

Whenever I'm looking at plate voltage I always check the heater voltage to make sure this is at or close to 6.3v. If I see high plate voltage but the heaters are OK then I don't worry too much. If I see high plate voltage coupled with high heater voltage I then investigate the PT to make sure it's correct for the mains voltage.

A good EL34 will take much more plate voltage - it's high screen voltage as well as high plate voltage that can be a problem.

To check where the noise is coming from remove each of the preamp tubes in turn starting at the first one and see where it stops.

[Enzo]

I don't consider 524v jaw dropping. With a 500v target, it is only 5% off. Look at the EL34 data sheet, they are made for much higher voltage.

[cluster]

I don't consider 524v jaw dropping. With a 500v target, it is only 5% off. Look at the EL34 data sheet, they are made for much higher voltage.

thanks. i'm just use to 485-495VDC on modern marshalls. my mid 70's Hiwatt has 525 on the plates. my early 71 suoerleads also have 525VDC. i thought by 77 marshall had dropped the plate voltage on superleads to ~490VDC,

[Chuck H]

I think Canada has stayed with 115V mains voltage as compared to other areas that slowly rose from 110V to between 120V and 125V now like here in the U.S. If the Canadian amp does have a PT specific for 115V and you're now putting 120V+ into it then that could account for the extra voltage. Like Mick mentioned, check filament voltage to see if the overage is roughly the same % and check to see that the amp is not biased colder than typical as this too would increase standing voltage on the plates. Oh, and also as per mick, pull tubes to isolate the crackle.

[Enzo]

Look at Music Man amps. Over 700 volts.

[bob p]

Hello all, So I recently acquired a 1977 2203 in very fine condition. The only thing done to it was someone replaced the 220k bias feed resistors with 100K running 4x EL34s. When I checked the amp's plate voltage I got a jaw dropping 524VDC. Is this normal for this vintage of amp. It is the Canadian version with the lay down power transformer and metal toggle switches instead of the plastic butterfly ones.

Hey Joe,

This kind of problem is impossible to answer if you don't provide us with all of the necessary information up front. It's as if you provide one data point, and then ask someone what the correct line would be that passes through it. The answer is that an infinite number of lines can be drawn through a single point, and a huge number of correct answers could be given in response to your voltage question, the result being that it may take a while to zero in on the correct answer to your problem.

Whenever anyone asks voltage questions, I always give the same response: For us to answer your question, the answers are most likely to come fast when you create a complete voltage table for all of the tube pins in the amp, along with referencing the line voltage at the input to the amp. Complete data like that allows us to take an accurate survey of what's going on.

If you provide all of the necessary data, one of many of the regulars here can likely give you a definitive answer in a single post. If you don't provide all of the necessary data, you'll get a number of accurate answers, but they may not zero in immediately on solving your problem. More often than not, you'll end up being asked to provide more information, and that draws out the process of getting your problem solved.

This subject comes up so often that I wonder if it would be useful to have a sticky post on how to present voltage data when you're asking for help with an amp. I think it'd streamline the whole process if people provided more data up front.

[cluster]

Hey Joe,

This kind of problem is impossible to answer if you don't provide us with all of the necessary information up front. It's as if you provide one data point, and then ask someone what the correct line would be that passes through it. The answer is that an infinite number of lines can be drawn through a single point, and a huge number of correct answers could be given in response to your voltage question, the result being that it may take a while to zero in on the correct answer to your problem.

Whenever anyone asks voltage questions, I always give the same response: For us to answer your question, the answers are most likely to come fast when you create a complete voltage table for all of the tube pins in the amp, along with referencing the line voltage at the input to the amp. Complete data like that allows us to take an accurate survey of what's going on.

If you provide all of the necessary data, one of many of the regulars here can likely give you a definitive answer in a single post. If you don't provide all of the necessary data, you'll get a number of accurate answers, but they may not zero in immediately on solving your problem. More often than not, you'll end up being asked to provide more information, and that draws out the process of getting your problem solved.

This subject comes up so often that I wonder if it would be useful to have a sticky post on how to present voltage data when you're asking for help with an amp. I think it'd streamline the whole process if people provided more data up front.

Thanks for that. I'll try to provide more info next time i'm inside that amp. Appreciate the response. Cheers.

[cluster]

I think Canada has stayed with 115V mains voltage as compared to other areas that slowly rose from 110V to between 120V and 125V now like here in the U.S. If the Canadian amp does have a PT specific for 115V and you're now putting 120V+ into it then that could account for the extra voltage. Like Mick mentioned, check filament voltage to see if the overage is roughly the same % and check to see that the amp is not biased colder than typical as this too would increase standing voltage on the plates. Oh, and also as per mick, pull tubes to isolate the crackle.

Thanks for that response. So an amp that is under biased will have higher plate voltage??

[Jazz P Bass]

Yes.
A lower bias will result in less current draw from the supply, so the voltages will rise.

Being that the supply is not 'regulated', the voltages will raise & lower depending on line voltage & current draw.

[g1]

I think Canada has stayed with 115V mains voltage as compared to other areas that slowly rose from 110V to between 120V and 125V now like here in the U.S.

We're up to about 125V even 126V at times. Fluctuates across the country though. We are hydro-electric rich in my province.

Thanks for that response. So an amp that is under biased will have higher plate voltage??

As far as under or over biased terminology, it is much less ambiguous to refer to 'idle current'. Some people mean the opposite of what you expect when referring to 'under-biased'. Referring to idle current takes away any confusion.
As idle current increases, plate voltage supply tends to decrease.

[Chuck H]

We're up to about 125V even 126V at times. Fluctuates across the country though. We are hydro-electric rich in my province.

And aren't the salmon happy for THAT!?! Same in western Washington.

I made an assumption that Canada held out longer than some because Hammond (my favorite transformers ) stayed with 115V primaries for a loooong time after everyone in the US had gone to 120V primaries.

[bob p]

I thought that everywhere in NA they kept the grid at the same voltage, so regions could exchange power when needed.

Which brings to mind -- does anyone know the reason that the utility companies want the grid to be 125 instead of 110?

[Enzo]

Problem is that the grid voltage is nominal. In a way like speaker impedance. Impedance is nominal at a given freq, but we all know over audio the impedance varies greatly. The grid relies on it being some voltage at the source, but at the end of a long rural run, the voltage will be lower. In hot areas in August, on a summer evening, all those air conditioners and TV sets can drag the voltage down to the level of brown outs. Most stuff doesn't care a lot if the mains is high, so they can set out at the power source a bit high so that at the other ends of distribution, the drops don't accumulate enough to be excessive.


The real trick to the grid is keeping the phase aligned.

[cluster]

Posting a picture of the main's PT. It's a lay down but with a twist. It has what appears like a dermal sensor coming off the winding. I've heard of these before. can any comment on what these PTs were about??