This is my first post here. Yes -- I am a tubebnube but with all the obvious experts around I would like to ask ...
In a PP topology how important is a "matched" PI? Does it depend on the type of PI used (figger it does)? How much of the PI needs to be matched????? Tubes - coupling caps - resistors??? Whats the best way to get there??
For the matched PI tube - is there a good way to implement a "balancer" control to match unmatched sections?? Or would this better be done with a cathode CCS type circuit with like a LM371 or 350? (been spending time on the Hifi sites lately ).
I looked around for a thread on this issue but didn't see anything - if this has already been handled, please provide
It became important in guitar amps a few years ago. I don't think it's very important since a perfectly balanced tube won't run balanced since the circuit is not perfectly balanced. I have seen circuits to adjust the balance but I think you then replace it with a resistor. Can't hurt to balance unless there's some mojo from imbalance I don't know about.
You are probably best suited to just try a few different tubes in the PI spot.
There are ways to balance it but most times you can get a happy accident just by rolling a few tubes through the PI socket till you like the sound of one better than another one.
Dumbles have a trim pot on the board to adjust the PI balance, but it is my understanding that having it perfectly balanced does not yield the best tone. I think the way you dial it in is to plug in and play it and adjust it for the sweet spot.
Some modern Fender designs have a more balanced long-tailed PI such as the Blues DeVille. They use 2 resistors in series that add to about 100K as one of the plates and put the blocking cap output at that junction. If you wanted to try adjusting the balance this way, you could temporarily use a 100K pot as the plate resistor for one side of the PI and connect the wiper to the blocking cap, adjust for the best tone, then hardwire in two resistors by measuring the pot.
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With that type of inverter you can only balance either DC plate voltage
or AC signal balance. It becomes another tone issue. All push-pull technical
discussions state that it requires two equal singles of opposite phase.
The DC balance effects the quality of the inverters action (phase v.s. bandwidth)
and the AC effects relative contents of harmonic distortion. Somewhere between
the two is the tone your looking for.
A matched tube in a cathodyne PI is pointless (only one of the triodes is used for the inverting function)
A matched tube in a paraphrase PI is pointless (you'll never get the two stages matching so matter how hard you try, and even if you did, it defeats the whole purpose of that kind of PI, which is to dirty up the sound)
A matched tube is an LTP is mostly pointless (since the plates resistors are unbalanced a lot of the time anyhow)
As jjman said, matched tubes "became important a few years ago". I would add that it was especially as tube merchants had to think up new points of differentiation to flog tubes and hike up their margins.
Maybe in Hi Fi you want a matched PI tube, but is a geetar amp, its a waste of time. 2CW
tubeswell wrote:
A matched tube is an LTP is mostly pointless (since the plates resistors are unbalanced a lot of the time anyhow)
It is my understanding that the reason the two plate resistors are commonly 10K apart in values is because of the inherent unbalanced output of the LTPI.
The two different values are an attempt to get the signal levels somewhat close to each other.
When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal. http://www.aikenamps.com/LongTailPair.htm
but with all the obvious experts around I would like to ask ...
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