I have 10 NOS Mullard Xf2 El34s. I can get 3 pairs to be matched within 4 ma in my Express clone at around 400vdc and the slight mismatch at 3 or 4ma sound best.
The other 2 pairs are off by 6 and 8 Ma and don't sound as good IMO. Ideally having two bias pots would rectify this but I'm not moding my vintage amps.
The question is does higher plate voltage usually cause more drift between tubes or is there really no correlation?
Just trying to see if there is a scheme before I try them, mix and match, in amps that have 390 to 450dc.
It took over 2 hours to go through the 10 I had in my Express let them warm up, test and swap! Thanks
Selecting matched tubes based on voltage
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RockinRocket
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Stevem
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Re: Selecting matched tubes based on voltage
It’s not just the idle current that makes for tonal and response differences, it’s how the tubes current draw changes in response to different frequency’s as higher signal drive levels are applied to the tube.
It might interest you to know that Groovetubes matches the many brands of output tubes they sell at 3 different frequency inputs before they give them a 1 to 10 rating in regards to clipping and power output.
These test frequencies are 400 hz, 1500 hz and 4000 hz.
Here’s a picture of a test out chart from the 4.1 issue of the Groovetubes book.
The chart list the test out of pairs of tubes with a number 5 rating and shows the rms wattage output for the tubes tested a these 3 different frequency’s.
Note worthy is the last test of the original GE made output tubes listed as GT 6L6 OS ( old stock ) and the power output and clipping levels between these tubes given a number 1 rating , and a 10 rating.
All of these tubes used where tested in a Groovtubes power amp that they used to make and had a wide adjustment range that could be made to its bias setting so it could easily accept most any output tube.
You should also note that it’s not just how high the plate voltage is that will call for a needed change in the bias voltage, but the amount of screen voltage has a bigger effect then the plate voltage assuming you have a wide enough range of bias voltage available.
It might interest you to know that Groovetubes matches the many brands of output tubes they sell at 3 different frequency inputs before they give them a 1 to 10 rating in regards to clipping and power output.
These test frequencies are 400 hz, 1500 hz and 4000 hz.
Here’s a picture of a test out chart from the 4.1 issue of the Groovetubes book.
The chart list the test out of pairs of tubes with a number 5 rating and shows the rms wattage output for the tubes tested a these 3 different frequency’s.
Note worthy is the last test of the original GE made output tubes listed as GT 6L6 OS ( old stock ) and the power output and clipping levels between these tubes given a number 1 rating , and a 10 rating.
All of these tubes used where tested in a Groovtubes power amp that they used to make and had a wide adjustment range that could be made to its bias setting so it could easily accept most any output tube.
You should also note that it’s not just how high the plate voltage is that will call for a needed change in the bias voltage, but the amount of screen voltage has a bigger effect then the plate voltage assuming you have a wide enough range of bias voltage available.
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Selecting matched tubes based on voltage
Being a push-pull, matching for gm is also important not to have a semiwave more amplified than the other.
Well, you may want some, but not too much different.
Well, you may want some, but not too much different.
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Stevem
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Re: Selecting matched tubes based on voltage
Like you have been finding out in terms of a bit of mismatch is that this allows the output stage when drivin hard to pass some even order harmonics along with the odd order ones.
Perfectly matched outputs will however cancel more 120 Hz power supply hum, but with good first node filters I have never found that minuscule level of added hum by an imbalance a issue.
Perfectly matched outputs will however cancel more 120 Hz power supply hum, but with good first node filters I have never found that minuscule level of added hum by an imbalance a issue.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Selecting matched tubes based on voltage
If the two tubes have a different gm, even harmonics will be higher than odd harmonics starting from the lowest signal, not when pushed hard.
Re: Selecting matched tubes based on voltage
Several years ago I built a primitive tube tester, basically a guitar amp-like power section. As I recall, it put out B+ around 250VDC, which seemed perfect for the pile of 6V6 and 6AQ5 tubes I have. I wired sockets for both types. It has a pot that allowed bias adjustment with a decent sweep. My thinking was that I can select 3 points on the bias voltage. This will cause plate and screen voltage to vary (and by extension, cathode voltage, too.) If you take all the voltage readings, you can calc for dissipation and gm. I honestly don't know how valid this method is for tube testing. I'm thinking it's better than nothing, it showed me which tubes were strong vs. weak, and it suggested what matched reasonably well. I reasoned, since I had 3 data points, there was at least a method to the madness. This is something I did with a derelict chassis, spare PT, and other used parts. It's just a thought. If you have adjustable bias voltage in that amp, you might give it a try. Just remember not to allow the bias voltage to go higher than what the tube is meant to run at.
Edit: Having thought about this all day, I realized, no calc is needed. Just compare the voltage readings. Look for the curve to be reasonably similar.
Edit: Having thought about this all day, I realized, no calc is needed. Just compare the voltage readings. Look for the curve to be reasonably similar.
Re: Selecting matched tubes based on voltage
You can find a signal generator and a simple used oscilloscope for the price of a one of the NOS EL34 you have bought.
That will give you the possibility to better match the tubes not only now, but for the future as well.
And by matching I not only mean match the characteristics of a pair/quartet of tubes, but match the tubes to the circuit you want to mount them on.
That will give you the possibility to better match the tubes not only now, but for the future as well.
And by matching I not only mean match the characteristics of a pair/quartet of tubes, but match the tubes to the circuit you want to mount them on.