Screen resistors and plate voltage

[krokit]

This is a more of a curiosity than anything else so..

Replaced the screen resistors in my 2203 today all good no worries.. Then I checked the plate voltage and it was 466, so I re-biased. Funny thing is, I checked the plate voltage a few days ago and it was 480! Presumably this is a side effect of the new screen resistors?

Cheers

 

[Jonathan Wilder]

This is a more of a curiosity than anything else so..

Replaced the screen resistors in my 2203 today all good no worries.. Then I checked the plate voltage and it was 466, so I re-biased. Funny thing is, I checked the plate voltage a few days ago and it was 480! Presumably this is a side effect of the new screen resistors?

Cheers

No...it's a result of the rebias.

Plate voltage and plate current are out of phase with each other. When one goes up the other comes down and vice versa.

My question is...why was the rebias needed?

 

[krokit]

No...it's a result of the rebias.

Plate voltage and plate current are out of phase with each other. When one goes up the other comes down and vice versa.

My question is...why was the rebias needed?

Thanks Jon. When I checked the plate voltage about a week ago it showed 480, so I set the bias to 36ma. Today I replaced the screen resistors and out of curiosity checked the plate voltage again, which this time read 466, so I tweaked the bias up to 37ma. I presumed it was the new screen resistors, or is it normal for the plate voltage to vary like this?

Cheers

 

[Joey Voltage]

Thanks Jon. When I checked the plate voltage about a week ago it showed 480, so I set the bias to 36ma. Today I replaced the screen resistors and out of curiosity checked the plate voltage again, which this time read 466, so I tweaked the bias up to 37ma. I presumed it was the new screen resistors, or is it normal for the plate voltage to vary like this?

Cheers

What he was getting at was that that 1Ma worth of plate current difference is mousenuts, (slightly colder than 70% maybe preferrable anyway) and you wouldn't have had to do any rebiasing. he might have been testing you a bit

I personally understand that you were just following a bias protocall, and if you don't have a vast knowledge or even intermediate, you did the RIGHT thing. plate voltage will vary depending on more than one factor. The screen resistor change wouldn't be likely

 

[Jonathan Wilder]

Thanks Jon. When I checked the plate voltage about a week ago it showed 480, so I set the bias to 36ma. Today I replaced the screen resistors and out of curiosity checked the plate voltage again, which this time read 466, so I tweaked the bias up to 37ma. I presumed it was the new screen resistors, or is it normal for the plate voltage to vary like this?

Cheers

Well actually...

What I was getting at was that with the bias pot set full cold, the plate voltage will read higher than it will with the bias pot set to the "optimum bias" setting. As you rotate the bias pot to increase plate current, plate voltage will do the opposite...decrease in value...due to the DC resistance of the OT primary dropping more voltage across it as more current is pulled through it.

It's a "handoff" action. In a series circuit, the sum of all voltage drops will always equal the total supply voltage. This means that the more voltage that is dropped across the OT primary, the less voltage will be dropped across the tube plate and cathode to keep the sum of the two voltage drops equal to the total supply voltage, hence the lower plate voltage reading when you biased it hotter. The hotter you bias, the more the OT primary DCR will pull down the plate voltage.

Another factor that comes into play is the fact that the supply voltage is unregulated. Depending on how stiff the supply is will determine how much the total supply voltage itself will drop when a given load is placed on it. This causes the total supply voltage to drop some when drawing more current from it, which will change the sum of the voltage drops within the series branch as well.

 

[Joey Voltage]

Well actually...

What I was getting at was that with the bias pot set full cold, the plate voltage will read higher than it will with the bias pot set to the "optimum bias" setting. As you rotate the bias pot to increase plate current, plate voltage will do the opposite...decrease in value...due to the DC resistance of the OT primary dropping more voltage across it as more current is pulled through it.

It's a "handoff" action. In a series circuit, the sum of all voltage drops will always equal the total supply voltage. This means that the more voltage that is dropped across the OT primary, the less voltage will be dropped across the tube plate and cathode to keep the sum of the two voltage drops equal to the total supply voltage, hence the lower plate voltage reading when you biased it hotter. The hotter you bias, the more the OT primary DCR will pull down the plate voltage.

Another factor that comes into play is the fact that the supply voltage is unregulated. Depending on how stiff the supply is will determine how much the total supply voltage itself will drop when a given load is placed on it. This causes the total supply voltage to drop some when drawing more current from it, which will change the sum of the voltage drops within the series branch as well.

Well Actually Jon, Thats more of an explaination of why the voltage will drop, not necessarily asking him the test question of why he had to rebias when he noticed it dropped, which is of course "it will be close enough", and well with in desired range assuming the higher "lower load" voltages as your divider (wow yikes! that wording sounds like a paradox). the technicality of it is that he did bias it slightly more correctly, when he took the second measurement, however, it is almost always unneeded, and slightly colder bias is perfectly fine with in reason, and very unnoticeable to a point

If he did the right thing, and set the bias supply to reflect maximum negative voltage before turning the pot when doing the initial calculations, your explaination would account for the voltage drop. How ever many damn well know that is not always the case.


to the OP, in very basic Laymans terms just so you are following, what he explained to you is that the higher the current being drawn from the supply feeding the circuit (ie the higher the idle plate current/bias current of the output valves in this example), the lower the voltage of the supply will be pulled down(not always the case, but thats far removed from this discussion at least). when you biased the valves to reflect your desired plate current, there was a heavier load on it than before, and thus the voltage dropped.

In other words, this IS normal

 

[krokit]

What he was getting at was that that 1Ma worth of plate current difference is mousenuts, (slightly colder than 70% maybe preferrable anyway) and you wouldn't have had to do any rebiasing. he might have been testing you a bit

I personally understand that you were : just following a bias protocall, and if you don't have a vast knowledge or even intermediate, you did the RIGHT thing. plate voltage will vary depending on more than one factor. The screen resistor change wouldn't be likely

Thanks Joey. I tried it a little colder, in fact I dropped it down 1ma at a time, plugged back in had a blast, down 1ma had a blast ect, all the way down to 30ma. But 37.5 seemed to hit the spot, with my batterd ears anyway

Well actually...

What I was getting at was that with the bias pot set full cold, the plate voltage will read higher than it will with the bias pot set to the "optimum bias" setting. As you rotate the bias pot to increase plate current, plate voltage will do the opposite...decrease in value...due to the DC resistance of the OT primary dropping more voltage across it as more current is pulled through it.

It's a "handoff" action. In a series circuit, the sum of all voltage drops will always equal the total supply voltage. This means that the more voltage that is dropped across the OT primary, the less voltage will be dropped across the tube plate and cathode to keep the sum of the two voltage drops equal to the total supply voltage, hence the lower plate voltage reading when you biased it hotter. The hotter you bias, the more the OT primary DCR will pull down the plate voltage.

Another factor that comes into play is the fact that the supply voltage is unregulated. Depending on how stiff the supply is will determine how much the total supply voltage itself will drop when a given load is placed on it. This causes the total supply voltage to drop some when drawing more current from it, which will change the sum of the voltage drops within the series branch as well.

Thanks Jon. I thought as much!

Kiddin!

Cheers for explaining man, I'll have to read it a few times though!

 

[cagamp1]

Lots on this topic. I'm believing your 2203 has EL34's, not 6550's.

Changing your screen resistors with the same value should not have changed the anode voltage. I didn't see mention of a non-stock value.

You should always note your supply (wall/outlet) voltage along with your anode and bias voltages. They are always relative to one another. If your wall volatage drops the rest will follow suit. Several volts change at the wall will make the difference you are describing.

 

[big dooley]

You should always note your supply (wall/outlet) voltage along with your anode and bias voltages. They are always relative to one another. If your wall volatage drops the rest will follow suit. Several volts change at the wall will make the difference you are describing.

you can say that again... i once biased my amp pretty hot at 220Vac, noting went wrong, until i had i gig in a place which had 230Vac coming out of the wall... i decided to open up the amp and check things out... it was way too hot for comfort, so i backed off the bias... since then i'm running the amp with a variac set at the point where my heatervoltages are in spec (which is 240Vac)

in your case going from 480Vdc to 466Vdc can certainly be caused by a voltage drop...
let's say you had 125Vac out of the wall which dropped to 121Vac... i wouldn't be surprised if that was the case

 

[wkcchampion]

I love these threads, they allow me to learn!

 

[krokit]

Lots on this topic. I'm believing your 2203 has EL34's, not 6550's.

Changing your screen resistors with the same value should not have changed the anode voltage. I didn't see mention of a non-stock value.

You should always note your supply (wall/outlet) voltage along with your anode and bias voltages. They are always relative to one another. If your wall volatage drops the rest will follow suit. Several volts change at the wall will make the difference you are describing.

Great info, thanks cagamp

I love these threads, they allow me to learn!

Absolutely, I love this forum!

 

[Jonathan Wilder]

In the case of valve amplifiers, wall voltage does in fact cause a much bigger variance in the HT voltage than the variance that occurs on the primary side of it. This is because step up transformers are basically simple voltage amplifiers. They have a voltage gain that is equal to the turns ratio of the transformer.

Lets say you have a transformer with a 3:1 turns ratio (3 turns on the secondary for every 1 turn of the primary). This means that the voltage on the secondary will be 3x higher than the voltage coming in on the primary, which translates to a voltage gain of 3. In this particular example, this also means that any variance in primary voltage will cause a corresponding variance in secondary voltage that is 3x higher than the voltage variance on the primary.

Basically it can be expressed as a percentage. A 10% variance in wall voltage will cause a corresponding 10% variance in HT voltage. Since the HT voltage is a larger value than the wall voltage, a 10% variance in HT voltage is a much bigger change than a 10% variance in wall voltage.

 

[Marshall Mann]

Good stuff guys!

Man I miss watching Jon and Joey spar with each other!

 

[kbhe]

Electric Politicians ! Left coast verses East coast. I wonder what the electrons think about this?

 

[Adwex]

Good stuff guys!

Man I miss watching Jon and Joey spar with each other!

Discussion good, sparring bad.

We've already got a top dog around here, we don't need another.
If anyone wants to fight, here I am.

 

[tresmarshallz]

I wonder what the electrons think about this?

my JMP electrons think Joey is The Marshall Whisperer, a natural marshallmanship practitioner

 

[Adwex]

my JMP electrons think Joey is The Marshall Whisperer, a natural marshallmanship practitioner

There are several extremely knowledgeable folk around here, Joey is just one of them.

Jon Wilder (Wilder Amplification), Lane Sparber, and JCMJMP are also quite capable.

Then there are a bunch of other very helpful people too numerous to mention with a diverse wealth of knowledge and experience on many various topics.

 

[Joey Voltage]

Okay, this sounds like a deja vu response I recently gave here, but forgot who and where. Now without complicating this matter more than it needs to be, because it really isn't complicated. I will put this into very easily understandable terms for anybody interested.

First, I'm not going to get into a stupid semantics debate about what is, and isn't an "amplifier", but I would never consider a transformer one. A transformer is just that, a transformer, hence it's name, it can transform both voltage and current . it can be seen that a choke isn't a transformer, it isn't transforming anything. (and no! I do not want to hear but it can transform more ripple into less), but I digress.

once you realize, and understand that a transformer works on the principle of simple ratios, it is easy to understand how just a few more, (or less) volts from the mains on the primary, can yield sometimes very large changes in the secondary-again this is dictated by a ratio, and you will see why shortly

The turns ratio (typically denoted as N) is dictated by a really remedial formula, it is simply the number of turns of the secondary, divided by the number of turns on the primary-thats it.

N= secondary turns/primary turns

so if you have 10 turns on the primary, and 100 turns on the secondary you have a ratio of 10:1

now much like ohms law, this formula can also be rearranged and morphed in many ways to find what you need. now that you know that you have a 10:1 ratio figured you know about what voltage to expect on the secondary for any given primary with this equation.

secondary voltage = primary voltage*turns ratio

so if you have a 120v wall ac on the primary, and we have a 10:1 ratio, we would expect a secondary of 1200Vac

Now say you want to know the turns ratio of a transformer, but have no clue how many turns are on the primary or secondary..... chances are unless you designed it you wont, simples use voltages of the secondary and primary. again just morph the equation

N= secondary voltages/Primary voltage

so if you have 120Vac on your primary, and 1200Vac on the secondary

N= secondary voltage/primary voltage

1200V/120Vac = 10 or 10:1 every one gets you 10.

Now it should be pretty obvious why the actual wall vac can have a noticeable effect, but if it isn't this should put it to bed.

say for example, instead of the theoretical 120V wall vac, we actually have a real life 130vac, and say we use that same transformer that has this 10:1 ratio.

we now have 1300vac at the secondary, which is 100V higher than before. the ratio. but we knew this, the ratio 10:1 dictated it. 10V difference on one end bought us 100V on the other.

This should put the issue to bed!

 

[tresmarshallz]

There are several extremely knowledgeable folk around here, Joey is just one of them.

Jon Wilder (Wilder Amplification), Lane Sparber, and JCMJMP are also quite capable.

Then there are a bunch of other very helpful people too numerous to mention with a diverse wealth of knowledge and experience on many various topics.

+1000, I agree, I was trying to inject some humor into the discussion...and just speaking as to my personal experience as he's the only one I've had work on my amps.
The expertise here among all the experienced ones is stunning...and I am regularly stunned.

 

[Adwex]

Okay, this sounds like a deja vu response I recently gave here, but forgot who and where. Now without complicating this matter more than it needs to be, because it really isn't complicated. I will put this into very easily understandable terms for anybody interested.

First, I'm not going to get into a stupid semantics debate about what is, and isn't an "amplifier", but I would never consider a transformer one. A transformer is just that, a transformer, hence it's name, it can transform both voltage and current . it can be seen that a choke isn't a transformer, it isn't transforming anything. (and no! I do not want to hear but it can transform more ripple into less), but I digress.

once you realize, and understand that a transformer works on the principle of simple ratios, it is easy to understand how just a few more, (or less) volts from the mains on the primary, can yield sometimes very large changes in the secondary-again this is dictated by a ratio, and you will see why shortly

The turns ratio (typically denoted as N) is dictated by a really remedial formula, it is simply the number of turns of the secondary, divided by the number of turns on the primary-thats it.

N= secondary turns/primary turns

so if you have 10 turns on the primary, and 100 turns on the secondary you have a ratio of 10:1

now much like ohms law, this formula can also be rearranged and morphed in many ways to find what you need. now that you know that you have a 10:1 ratio figured you know about what voltage to expect on the secondary for any given primary with this equation.

secondary voltage = primary voltage*turns ratio

so if you have a 120v wall ac on the primary, and we have a 10:1 ratio, we would expect a secondary of 1200Vac

Now say you want to know the turns ratio of a transformer, but have no clue how many turns are on the primary or secondary..... chances are unless you designed it you wont, simples use voltages of the secondary and primary. again just morph the equation

N= secondary voltages/Primary voltage

so if you have 120Vac on your primary, and 1200Vac on the secondary

N= secondary voltage/primary voltage

1200V/120Vac = 10 or 10:1 every one gets you 10.

Now it should be pretty obvious why the actual wall vac can have a noticeable effect, but if it isn't this should put it to bed.

say for example, instead of the theoretical 120V wall vac, we actually have a real life 130vac, and say we use that same transformer that has this 10:1 ratio.

we now have 1300vac at the secondary, which is 100V higher than before. the ratio. but we knew this, the ratio 10:1 dictated it. 10V difference on one end bought us 100V on the other.

This should put the issue to bed!

The issue was already in bed, Jon tucked it in and kissed it goodnight. It didn't need to be woken up and fed again.

Allow me to provide the stupid semantics...perhaps "amplifier" was a misleading word. A transformer however can certainly be considered a voltage "multiplier" though.

zzzzzzzzzz......