I recently acquired a Maxi-Matcher 2 valve tester.
Been playing around with my batch of valves from a reputable American tube store. I bought a couple of 5881's as matched.
Plug 4 of them into my matcher and check for both plate current (Ip) and Transconductance (mu).
These are the readings I get.
V1: 15,9mA/ 2,32mu
V2: 17,8mA/ 2,41mu
V3: 17,5mA/ 2,38mu
V4: 18mA/ 2,35mu
Now, are these considered matched or not?
What tolerance between Ip and mu is acceptable?
Any help will be appreciated.
Been playing around with my batch of valves from a reputable American tube store. I bought a couple of 5881's as matched.
Plug 4 of them into my matcher and check for both plate current (Ip) and Transconductance (mu).
These are the readings I get.
V1: 15,9mA/ 2,32mu
V2: 17,8mA/ 2,41mu
V3: 17,5mA/ 2,38mu
V4: 18mA/ 2,35mu
Now, are these considered matched or not?
What tolerance between Ip and mu is acceptable?
Any help will be appreciated.
My AVO valve tester booklet reckons within 5% is considered a good match. 1% is excellent and 10% is not good. For current at the set grid bias.
I have tested some 6L6GCs and in a batch of 24 valves, I only could match 2 Quads, then three sets of pairs. One was way out but matched another JJ I had knocking around from a while ago. Still don't like JJ!
I have tested some 6L6GCs and in a batch of 24 valves, I only could match 2 Quads, then three sets of pairs. One was way out but matched another JJ I had knocking around from a while ago. Still don't like JJ!
Enzo just replied on another forum and reckons 10% is good enough.
It depends at what the seller matches at.
It depends at what the seller matches at.
It is, at the end of the day, down to how deep your pockets are and how much hum rejection you require.
10% maybe good enough for some but not up to standard for my customers.
10% maybe good enough for some but not up to standard for my customers.
I can only say that i would not sell these as a quad. 2 and 3 _might_ qualify as a pair.
Any two from 2, 3, and 4 would be a good pair - unless the circuit was unusually fussy.
1 and 3 are a pair, if 10% is good enough.
1 and 3 are a pair, if 10% is good enough.
I think that the plate current you used is fairly low. That's why the transconductance value is low too. Typical idle current for 5881 is some 50 mA. Now you have one-third of that.
What supply and bias voltage did you use ?
You maybe get more truthful values with higher current.
What supply and bias voltage did you use ?
You maybe get more truthful values with higher current.
I find that I can have tubes that look matched in a high voltage lower current circuit typical in high power class AB amps, but then I put them in a class A amp with lower voltage and higher current circuit and see some variance.
Basically you should match tubes for whatever circuit they are going to be used in. If it's a tetrode/pentode anode current is more dependent on screen voltage than plate voltage.
Basically you should match tubes for whatever circuit they are going to be used in. If it's a tetrode/pentode anode current is more dependent on screen voltage than plate voltage.
Matched valves will drift apart as they age. Better to design circuits so they don't need close matching. Of course, those brought up on simulations may be disconcerted to find that real circuits don't quite work like they think and real valves are not usually matched to 0.1%.
Hardly, that's a pretty narrow view. In general, push-pull circuits will have better performance (sans feedback) if the tubes are more closely matched. However, one should always provide some level of balance adjustment to compensate.
For SET designs that have zero feedback (as part of the design) you pretty much have to match tubes in sets for the amps if you want simple things to be the same between the channels. Things like gain, distortion, output noise, output power and frequency response. I don't consider it bad design doing things this way, just a different approach, which also requires a different approach to fitting tubes to them. If done properly the benefits are substantial.
Regards, KM
So, a circuit like one used in the Dynaco st35 or sca35 which required a MQ of el84 would be in that category of amplifiers? Many of the classic tube amps which were cathode biased required MQ of output tubes. the funny thing for me , is that i seem to like the sound from the cathode biased amps more than the fixed bias ones which require less matching because they have a wider range of output tube adjustment.
I have been mostly using the LM317 for keeping the bias in my cathode biased amps even for push pull ones. You don't need closely matched tubes and i don't hear any sonic penalty. The only caveat besides the voltage limits is the temperature limits. best regards,
I have been mostly using the LM317 for keeping the bias in my cathode biased amps even for push pull ones. You don't need closely matched tubes and i don't hear any sonic penalty. The only caveat besides the voltage limits is the temperature limits. best regards,
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The Dynaco ST35 and SCA35 were strictly budget designs, period. Using a single bias resistor for 4 output tubes is just being cheap, you can't justify it any other way. But that was the primary goal, make a cheap amplifier that could be sold at a low cost. Nothing wrong from a marketing view, but that's it.
Using a CCS for biasing multiple tubes does not negate the need for a closely matched set of tubes. You simply limit the total current flow, nothing more. Using poorly matched tubes results in unequal current flow per tube... the total current simply gets divided based on how close each tube is to specification. The end result is poor matching, gain and unequal DC current balance in the output transformer. Performance suffers, period.
Regards, KM
His idle currents are within 10%, but as tested his mu were within 2%. Where I first saw the question was in the context of guitar amps, where ultimate fidelity is not the goal. And as someone mentioned above, these are idle specs, and not taken at real world loads and voltages.
Finally a good post.
Cathode bias doesn't do anything good. Indeed, the tubes biased this way have local feedback which interferes with triode feedback and GNF.
Cathode bias will bias at different current depending on the Z of the speakers connected, which will in turn move the B+ point out of the ideal range.
Disconnected speakers will shorten the life of cathode bias tubes very fast, which isn't happening in grid bias.
As I understand Mu is an indication of how well two or more valves with similar Ip and Mu will match at different plate currents.
Gm, as one of the parameters used at the matching, is the "sensitivity" of the tube, the more Gm the more it will amplify a given signal.'
For tubes used in a push-pull amp, it's imperative that the two tubes amplify the signal
equally. NFB ( Negative feedback), if used, will reduce the inbalance, but best is if
bith tubes amplify equally.
For tubes used in a push-pull amp, it's imperative that the two tubes amplify the signal
equally. NFB ( Negative feedback), if used, will reduce the inbalance, but best is if
bith tubes amplify equally.
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