redplating 1959 w larmar

[raiken]

measured between CT and either plate I got the following voltages of the secondary when 10.4v was applied to the primary:
16: 1.98v
8:1.41v
4:1.00v

I'm not quite sure what you meant the way you phrased that. What I meant was this:

(1) Apply the signal between the CT and one side, measure the voltage at the 16 ohm tap
(2) Apply the signal between the CT and the other side, measure the voltage at the 16 ohm tap
(3) Apply the signal between the two sides, measure the voltage at the 16 ohm tap

If (1) and (2) is what you did, and you got the same 1.98V result on the 16 ohm tap for both, then your tranny has a 441 ohm load on each side, which would correspond to a 1.76K plate-to-plate load. If so, you should then measure around 0.99V on the 16 ohm secondary when applying your signal from plate to plate.

RA

[Roe]

...

Really stupid question, but that power adapter that you're using has an AC output, not a DC output correct?

yes, its a 9v ac adapter (to be used with line6 dl4).

[Roe]

measured between CT and either plate I got the following voltages of the secondary when 10.4v was applied to the primary:
16: 1.98v
8:1.41v
4:1.00v

I'm not quite sure what you meant the way you phrased that. What I meant was this:

(1) Apply the signal between the CT and one side, measure the voltage at the 16 ohm tap
(2) Apply the signal between the CT and the other side, measure the voltage at the 16 ohm tap
(3) Apply the signal between the two sides, measure the voltage at the 16 ohm tap

If (1) and (2) is what you did, and you got the same 1.98V result on the 16 ohm tap for both, then your tranny has a 441 ohm load on each side, which would correspond to a 1.76K plate-to-plate load. If so, you should then measure around 0.99V on the 16 ohm secondary when applying your signal from plate to plate.

RA

This is strange. I did 1 and 2 and both gave me 1.98v. but then 3 also gave the same voltage (2.02) at first but next time I checked it measured 0.98v
I doublechecked and plate to plate gave .98v at 16ohms (and .7 at 8 and .5 at 4)
the first measurement of 3 must have been wrong. I don't understand how it could have happen

[raiken]

measured between CT and either plate I got the following voltages of the secondary when 10.4v was applied to the primary:
16: 1.98v
8:1.41v
4:1.00v

I'm not quite sure what you meant the way you phrased that. What I meant was this:

(1) Apply the signal between the CT and one side, measure the voltage at the 16 ohm tap
(2) Apply the signal between the CT and the other side, measure the voltage at the 16 ohm tap
(3) Apply the signal between the two sides, measure the voltage at the 16 ohm tap

If (1) and (2) is what you did, and you got the same 1.98V result on the 16 ohm tap for both, then your tranny has a 441 ohm load on each side, which would correspond to a 1.76K plate-to-plate load. If so, you should then measure around 0.99V on the 16 ohm secondary when applying your signal from plate to plate.

RA

This is strange. I did 1 and 2 and both gave me 1.98v. but then 3 also gave the same voltage (2.02) at first but next time I checked it measured 0.98v
the first measurement of 3 must have been wrong

Okay, then that says your OT is fine, and it is right around the 1.7K they claim it to be, and each side is well-balanced.

Again, I stand by my original diagnosis.

RA

[Roe]

...
This is strange. I did 1 and 2 and both gave me 1.98v. but then 3 also gave the same voltage (2.02) at first but next time I checked it measured 0.98v
the first measurement of 3 must have been wrong

Okay, then that says your OT is fine, and it is right around the 1.7K they claim it to be, and each side is well-balanced.

Again, I stand by my original diagnosis.

RA

thanks a lot Randall! I'll experiment more tomorrow. Its getting late here in Norway - good night

the reading that were off were probably caused by ac being applied not only to plate but also another pin on the socket

[flemingmras]

Again, I stand by my original diagnosis.

RA

And that is "too low of a plate load for the plate voltage, causing the tubes to exceed their dissipation at some point in the swing for the class of operation they're running in?"

Now my question is...why was Marshall building these things with transformers that would allow them to do that?

Also, just so I'm straight on my transformer math, since the plate - center tap is 1/2 the turns ratio of the plate - plate, the impedance ratio of 1/2 the primary is roughly about 1/4 that of the full primary due to the impedance ratio being the square of the turns/voltage ratio?

This just goes to show how important it is to match your cab impedance to the correct tap. I'm sure if you biased WAY cold you could get away with mismatching, but why bother?

[Mars Hall]

Now my question is...why was Marshall building these things with transformers that would allow them to do that?

Does this mismatch apply to 50 watters as well? I plan on purchasing a bass spec 50 watt Metro kit in the future. Will I need to swap the PT for one with lower voltage?

[raiken]

Does this mismatch apply to 50 watters as well? I plan on purchasing a bass spec 50 watt Metro kit in the future. Will I need to swap the PT for one with lower voltage?

How much voltage do you get out of the PT? I don't think any 50 watters run near 480V...my '72 ali panel head sitting behind me only runs 410V on the plates (and sags to about 380V at heavy overdrive), with a 3.4K primary. I think the highest 50W voltage I recall seeing was a '69 plexi that had 445V on the plates. I didn't measure the plate-to-plate impedance on that one, unfortunately, but it was probably 3.4K as well.

Randall Aiken

[raiken]

And that is "too low of a plate load for the plate voltage, causing the tubes to exceed their dissipation at some point in the swing for the class of operation they're running in?"

Yep.

Now my question is...why was Marshall building these things with transformers that would allow them to do that?

Just a guess, but I'll bet they took the 3.4K value straight off the Telefunken datasheet for 425V operation, and figured that would be the correct impedance for any plate voltage. Same as every other amp builder since then.

Also, just so I'm straight on my transformer math, since the plate - center tap is 1/2 the turns ratio of the plate - plate, the impedance ratio of 1/2 the primary is roughly about 1/4 that of the full primary due to the impedance ratio being the square of the turns/voltage ratio?

Correct.

This just goes to show how important it is to match your cab impedance to the correct tap. I'm sure if you biased WAY cold you could get away with mismatching, but why bother?

Not necessarily! If you have the wrong reflected plate load impedance (too low), there is no bias current you can use, even down to zero mA plate current/full class B operation, that will keep you from exceeding the maximum plate dissipation at some point in the power range of the amp. You have to either change the plate-to-plate impedance of the transformer or lower the plate voltage. By the way, the max dissipation point will not occur at full clip output - it will occur somewhere around midscale power levels. The tubes will actually run cooler at full power than they do somewhere in between.

Randall Aiken

[novosibir]

Now my question is...why was Marshall building these things with transformers that would allow them to do that?

Just a guess, but I'll bet they took the 3.4K value straight off the Telefunken datasheet for 425V operation, and figured that would be the correct impedance for any plate voltage. Same as every other amp builder since then.

I hate to disagree with you , but there's at least one exception

My custom wound DINO OT's are (and ever since were) 2.18K primary (operating w/ 470...480V on the plates)
And my custom wound British Purist OT's are 2.03K primary (operating w/ 450...460V on the plates)

By principle they're something like 'primarily overwound' Dagnall/Marshall OT's - and because a overwind needs more iron, the lams of my OT's are 1.75 inches instead of 1.5 inches.

As for the input resistor to the PI I'm even using values between 47...100K, depending on the amp.

In the DINO I'm using NOS RFT's from 70's production, in the British Purist either =(C)= or NOS Mullard Xf2's (extra charge) and I've never had a redplating issue. Although I still have a common pre screen resistor after the choke, before it goes to the independent screen resistors of each output tube.

Larry

[raiken]

I hate to disagree with you , but there's at least one exception

My custom wound DINO OT's are (and ever since were) 2.18K primary (operating w/ 470...480V on the plates)
And my custom wound British Purist OT's are 2.03K primary (operating w/ 450...460V on the plates)

By principle they're something like 'primarily overwound' Dagnall/Marshall OT's - and because a overwind needs more iron, the lams of my OT's are 1.75 inches instead of 1.5 inches.

As for the input resistor I'm even using values between 47...100K, depending on the amp.

Larry

And that's probably why your customers aren't complaining about redplating.
Although, even 2.03K is a bit low for 480V, unless it sags sufficiently at full drive (which I'm sure it does, unless you use a very stiff supply).

BTW, I use 4K for my 50W heads, which results in a bit lower power, but I also run 470 ohm screen grid resistors, which bumps it back up a bit to account for the lower plate/screen voltages I run (typically 420V or so).

I suppose it all comes down to this: do you want to be "period correct" and copy a mistake, or do you want your tubes to last a long time? I hate to say it, but guitarists are a funny sort - I'll bet 99% of the time the answer is "I want it period correct, damn the dissipation, full steam ahead!".

Randall Aiken

[flemingmras]

Not necessarily! If you have the wrong reflected plate load impedance (too low), there is no bias current you can use, even down to zero mA plate current/full class B operation, that will keep you from exceeding the maximum plate dissipation at some point in the power range of the amp. You have to either change the plate-to-plate impedance of the transformer or lower the plate voltage. By the way, the max dissipation point will not occur at full clip output - it will occur somewhere around midscale power levels. The tubes will actually run cooler at full power than they do somewhere in between.

Randall Aiken

Yes this I understand due to voltage/current being out of phase. The most dissipation occurs during the actual up and down "swings"...more than likely at a point in the swing where voltage and current equal each other rather than at the peaks and zero crossings (somewhat of an oversimplification but you get my point).

Now throw in the fact that plate voltage was never consistent throughout the years with Marshall (the same transformer part number could have different HT secondary voltages from transformer to transformer). Right now I'm working on a '72 Super Lead with all original trannys right now that has a 430 volt plate voltage with tubes in and biased to about 70mA per side. However I've seen these amps with just over 500VDC B+ (usually around 510-515), yet they used the same plate-plate load NOR did they compensate the component values in the bias supply to work with the plate voltage that the PT provided. This '72 still had the 95VAC at the bias tap yet still used the 27K/15K/47K/25K pot arrangement that would've been used in a higher plate voltage 100 watter. As such a set of Winged =C= EL34s would only bias up to 56mA per pair with that resistor combination.

[novosibir]

Although, even 2.03K is a bit low for 480V, unless it sags sufficiently at full drive (which I'm sure it does, unless you use a very stiff supply).

It's the DINO with 470...480V on the plates and its OT is 2.18K
The 2.03K OT is in the British Purist w/450...460V on the plates.

BTW, I use 4K for my 50W heads, which results in a bit lower power, but I also run 470 ohm screen grid resistors, which bumps it back up a bit to account for the lower plate/screen voltages I run (typically 420V or so).

In the British Purist I have a common 220 ohms pre screen resistor and also 470 ohms independent screen resistors, but only when the amp is supplied with NOS Mullards. Supplied with =(C)= the independent screen Rs then are 1K as usual.

I suppose it all comes down to this: do you want to be "period correct" and copy a mistake, or do you want your tubes to last a long time?

And there's still another herd of 'nowadays amp builders', where it seems sometimes, as they only can build amps like drawing after numbers - or they always were been playing football, as they've teached math & physics at school?! Sorry, but I couldn't resist!

Larry

[flemingmras]

And that's probably why your customers aren't complaining about redplating.
Although, even 2.03K is a bit low for 480V, unless it sags sufficiently at full drive (which I'm sure it does, unless you use a very stiff supply).

BTW, I use 4K for my 50W heads, which results in a bit lower power, but I also run 470 ohm screen grid resistors, which bumps it back up a bit to account for the lower plate/screen voltages I run (typically 420V or so).

I suppose it all comes down to this: do you want to be "period correct" and copy a mistake, or do you want your tubes to last a long time? I hate to say it, but guitarists are a funny sort - I'll bet 99% of the time the answer is "I want it period correct, damn the dissipation, full steam ahead!".

Randall Aiken

The problem with guitarists is that they don't want limits set on their tone. They want hard and fast rules to keep their amps alive but if it's something that sets a limit on their tone they want nothing to do with it.

They don't realize that lots of times there are no "hard/fast rules" and different circumstances may warrant different practices. Without knowing what's going on in the circuit you have no way of knowing which circumstances warrant which practices and you end up confusing yourself in the end.

BTW Randall, by using a higher plate load while dropping the resistor on the screens, doesn't that tax the screens a bit?

[raiken]

It's the DINO with 470...480V on the plates and its OT is 2.18K
The 2.03K OT is in the British Purist w/450...460V on the plates.

Sorry, it's very late here (2:30AM, I should be in bed!), and I didn't read that correctly!

RA