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Thread: PI voltage Divider - Variations in similar amps...

  1. #1
    Member HaroldBrooks's Avatar
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    PI voltage Divider - Variations in similar amps...

    I own two vintage Valco amps built several years apart (1941 vs. 1949) and the circuits are very close to one another. One thing I don't get is how the voltage divider used to feed the second grid of the PI tube is quite different from one to the other.

    The tubes are the same in both amps as are many of the voltages, caps, and resistors. the plate voltages for the PI tube are nearly identical in both amps, but one amp divides the plate voltage from the first section of the 6SC7 PI tube using a 240k and 7k resistor, the other uses a 240k and 11k resistor.

    If both amps are very similar with very similar plate voltages, wouldn't one PI tube be "Out of Balance" in a manner of speaking from one schematic to the other ?

    I realize balance in not all that important in guitar amps, but I was just wondering if there was something critical I may be overlooking in both designs.

    The plate resistors in the National model 50 schematic are not correct to my original PI plate resistors, My model 50 uses 100k resistors and they have never been tinkered with. My amp is also running a 5Y3 rectifier, not an '80', and this is correct as per many others I've seen of the same model and year. So both amps have the same value plate resistors.

    Is there something else critical I am missing ?




    I would like to understand how a balance in the PI tube is achieved, or if there is an acceptable range of imbalance that is allowed ? Even so, why change one way or the other ?

    Thanks for any help !

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    Last edited by HaroldBrooks; 04-30-2019 at 07:35 AM.

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    Supporting Member loudthud's Avatar
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    The number 50 amp has lower plate resistors so each side of the PI has lower gain. It needs more signal from the divider to balance the signal on the lower output tube.

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    Member HaroldBrooks's Avatar
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    Sorry, not sure if I made it clear but the original PI plate resistors in my model 50 are 100k, not 60k as you see in the early schematic,

    So both amps have the same value PI plate resistors when they were built originally, and different values for the paired voltage divider going into the second grid.

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    Maybe your amp has a build error?
    Bear in mind that the unbypassed cathodes of the paraphase and power tube balanced stages will introduce negative feedback, which I think will tend to reduce any imbalance.

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    In order to get equal amplitude but opposite phase signals at the PI plates, the second PI triode grid signal must have the same amplitude as the first one. The voltage divider neutralizes the gain of the first triode. As a plate resistor of 100k means more gain than 60k, the divider ratio needs to be adjusted to compensate.

    Edit: Just noticed that Loudthud said the same.

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    Member HaroldBrooks's Avatar
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    Thanks for all the responses !

    I do feel it might be the case that Valco just switched the PI plate resistors of my amp from the 60k listed on the schematic, to the later versions using 100k without changing the original values of the voltage divider resistors.

    One thing regarding the Model 50 amp, when pushed into clipping it is the absolute most compression I have from any of my amps, that is the signal becomes compressed and remains clear with out too much intermodulation distortion. It feels like I am running a compressor pedal before the input, but actually much better and more natural in response. I wished I knew what caused that specifically, but it's probably a very complex and interrelated system effect including voltage sag, and I will need to leave it for a future understanding.

    Both amps sound and feel fabulous to play, and no complaints at the moment. Just want to understand the limits of what was done regarding Valco's changes, but pdf64 I think spotted it as an error, and so it may be, but one that still works for that amp because of negative feedback. I would like to know how far you can 'push' this type of thing, and what would be the sonic effect of doing so (imbalance) and I may just try that on my own, as an experiment regarding the extremums.

    I still have a lot to learn, but with you guys helping me out, it's a certainty !

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    Quote Originally Posted by HaroldBrooks View Post
    ...One thing regarding the Model 50 amp, when pushed into clipping it is the absolute most compression I have from any of my amps, that is the signal becomes compressed and remains clear...
    I guess that the HT may be fairly saggy, as the field coil feeds the whole amp? eg what's the Vdc at the OT CT, at idle and then fully overdriven?
    Another factor may be the unbypassed, shared power tube cathode; below high power levels, even with some imbalance the opposing polarity signals at the cathode will tend to cancel out. But at high power levels, especially overdriven, cathode degeneration will come into play. So as input signal level rises, it may tend to reach mild overdrive but then be held there (kinda like a compressor) and the signal level would have to increase a lot before the depth of overdrive increased noticeably.
    You now going to advise that you bypassed the cathode resistor

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    I did add a 220uf bypass cap for the power tubes, but literally just a couple of days ago !

    Can't say it changed the high natural amp compression any, still great just like before.

    I think you nailed it with the HT sagging. I checked the voltage drop through the field coil, and it does drop a lot more when fully overdriven. Also, this amp in particular has a different FC speaker than my other very old Valco amps. This one has a Jensen 10GRS, whilst the others have 10" Rola field coil speakers.

    Not saying the Jensen is necessarily better, but it is a tad different, and may be influencing the circuit differently, and in this case positively by adding compression, but this only a hunch.

    I will need to "discover" the source of this eventually, as it's a super cool 'effect' built right into this amp, one that none of about a dozen other amps I own has, and if I could tune one of my other Field Coil speaker Valco amps to have that quality of natural compression on demand, it would make me content form months on end.

    By the way, this amp (Model 50) needed an entire cap job and a few new resistors when I purchased it, but that compression was there even in the initial deteriorated state, right up until now with the new caps and plate resistors, so I believe it's fully independent of the health or values of those components.

    Than leaves the HT voltage changes (as you suggested) or something else I can't begin to figure out (Eddy current and magnetization changes in the field coil ?)... But hopefully in time I will ! You just gave me the idea to measure the voltage drop across my Supro Supreme FC, and compare it to the model 50, both at an idle and at full song high gain. I will do that and let you know what I find. I might one day swap speakers and see what that does to either amp.

    Thanks for your help pdf64 !

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    Last edited by HaroldBrooks; 05-02-2019 at 04:36 AM.

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