What is the story on impedance matching ot's and tube's? it seems to be what ever and a lot of schematics never mention what impedance used so how do you decide what is right and wrong and how does a pair vs a quad fit in.
Todd
OT's and stupid ? of the day
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Re: OT's and stupid ? of the day
Common question. Needs sticky! To start, look at the data sheet for the power tube. Find the specs on class AB push pull pair and look for the plate to plate impedance for the specified plate voltage.
I build and repair tube amps. http://amps.monkeymatic.com
Re: OT's and stupid ? of the day
thats fine but i dont see a whole lot of people following That like a pair of el 84 book says what 8k but spits and voxs use 4k This is what im talking seems as though the guitar people throw the book out. And was reading a thread here on a champ and the builder was asked whether he was using the 5k primary or the 8k??????
Re: OT's and stupid ? of the day
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Last edited by matt h on Fri Mar 27, 2015 3:36 pm, edited 1 time in total.
Re: OT's and stupid ? of the day
Half the "optimum" primary impedance of a P-P pair for a P-P quad.
Generally speaking, you can operate with a 2X mis-match with little risk to tubes and OT. These systems are pretty robust.
Mismatching speaker loads to multitaped OT secondaries also provides another "lever" to better match primary loads on the power tubes.
Cheers,
Dave O.
Generally speaking, you can operate with a 2X mis-match with little risk to tubes and OT. These systems are pretty robust.
Mismatching speaker loads to multitaped OT secondaries also provides another "lever" to better match primary loads on the power tubes.
Cheers,
Dave O.
Re: OT's and stupid ? of the day
The valvewizard page nicely encapsulates the issues, constraints and process http://www.valvewizard.co.uk/pp.html
When that's understood, have a play with http://bmamps.com/Tech_tds.html
It's fantastic!
Select the tube, operating mode and voltages (these being the level at full load, which will be lower than the static value), then tinker with the load impedance so as to keep the red max dissipation line to the left of purple curve denoting the tube's limiting plate dissipation.
A certain degree of crossover (ie a section of the red line to the right of the purple curve) may be tolerated.
I ran a test recently using my limited selection of various power tube types in a regular '50W' guitar amp, varying the secondary load value to determine the primary impedance that gave the most power output.
I'll get around the fully documenting the results sometime but the conclusion was that around 5k was a good match for 6L6, EL34 & KT66.
The 6L6 included 'used but good' Winged C and NOS JAN Philips 6L6WGB, so it's not a 'golden age made tubes are better' thing.
The DrZ EZG50 uses a 5k5 OT.
When that's understood, have a play with http://bmamps.com/Tech_tds.html
It's fantastic!
Select the tube, operating mode and voltages (these being the level at full load, which will be lower than the static value), then tinker with the load impedance so as to keep the red max dissipation line to the left of purple curve denoting the tube's limiting plate dissipation.
A certain degree of crossover (ie a section of the red line to the right of the purple curve) may be tolerated.
I ran a test recently using my limited selection of various power tube types in a regular '50W' guitar amp, varying the secondary load value to determine the primary impedance that gave the most power output.
I'll get around the fully documenting the results sometime but the conclusion was that around 5k was a good match for 6L6, EL34 & KT66.
The 6L6 included 'used but good' Winged C and NOS JAN Philips 6L6WGB, so it's not a 'golden age made tubes are better' thing.
The DrZ EZG50 uses a 5k5 OT.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him!
Re: OT's and stupid ? of the day
thanks I dont do well with tech reading. going to have to read that about 300 times. Thanks.
Re: OT's and stupid ? of the day
Let's try more simply. Each tube has a range of the highest voltage it can withstand without failing in some way, and the highest current it can allow through without damaging the tube and dramatically shortening its life. Get outside those and the tube has a vastly shortened life, perhaps only seconds depending on how far out.
Within that range, there is a maximum power (that is, internally generated heat) that the tube can get out through that glass envelope without melting things and dying.
Under those limits the tube will live a long productive life, but to make the most of that "productive" thing, we need it to put out the most current times voltage going OUT of the plate without exceeding the power generated inside the tube.
In the Golden Age, they would generate the plate voltage-current characteristics like in the datasheet for a new tube, then guess what the most power out point might be based on history. This was a guess because power tubes don't have a nice, sharp point where their maximum current is. It's a soft, slushy round corner, so the true "most output" high current swing is somewhere there, but hard to pin down exactly.
Then they would build up a test rig, put some of the new tubes in it, and have it feed a sine wave into a dummy load resistor on the secondary of an output transformer. The dummy load resistor was either a hacked array of series/parallel resistors or something like a 100W pot (don't laugh - I've seen 100W and 200W pots). They would measure the voltage/current into the load resistor before noticeable clipping, and then change the load resistance and measure again.
Knowing the impedance ratio of the transformer and the variable resistance of the load, you can generate any plate to plate loading you like for the tubes, and you can measure the power actually delivered to the load.
I have seen plots like this from the Golden Age, and I could probably dredge one up if needed. What this shows is a curve of power out versus plate to plate load impedance.
This is nearly always a very broad, soft curve with a mild maximum of power out versus plate to plate load. And that's how you pick the maximum power point for plate to plate loading for a pair of tubes.
That's the first half of the story.
You can do the same variable-loading thing but measure the output distortion per load condition. When you do this, you get a broad, soft minimum in distortion at some loading.
Invariably, the lowest-distortion plate to plate load is not the same as the highest-power plate to plate load.
The hifi people like OTs that load them for lowest distortion, the guitar amp folks like the highest power point.
Within that range, there is a maximum power (that is, internally generated heat) that the tube can get out through that glass envelope without melting things and dying.
Under those limits the tube will live a long productive life, but to make the most of that "productive" thing, we need it to put out the most current times voltage going OUT of the plate without exceeding the power generated inside the tube.
In the Golden Age, they would generate the plate voltage-current characteristics like in the datasheet for a new tube, then guess what the most power out point might be based on history. This was a guess because power tubes don't have a nice, sharp point where their maximum current is. It's a soft, slushy round corner, so the true "most output" high current swing is somewhere there, but hard to pin down exactly.
Then they would build up a test rig, put some of the new tubes in it, and have it feed a sine wave into a dummy load resistor on the secondary of an output transformer. The dummy load resistor was either a hacked array of series/parallel resistors or something like a 100W pot (don't laugh - I've seen 100W and 200W pots). They would measure the voltage/current into the load resistor before noticeable clipping, and then change the load resistance and measure again.
Knowing the impedance ratio of the transformer and the variable resistance of the load, you can generate any plate to plate loading you like for the tubes, and you can measure the power actually delivered to the load.
I have seen plots like this from the Golden Age, and I could probably dredge one up if needed. What this shows is a curve of power out versus plate to plate load impedance.
This is nearly always a very broad, soft curve with a mild maximum of power out versus plate to plate load. And that's how you pick the maximum power point for plate to plate loading for a pair of tubes.
That's the first half of the story.
You can do the same variable-loading thing but measure the output distortion per load condition. When you do this, you get a broad, soft minimum in distortion at some loading.
Invariably, the lowest-distortion plate to plate load is not the same as the highest-power plate to plate load.
The hifi people like OTs that load them for lowest distortion, the guitar amp folks like the highest power point.