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Another good reason to build amps with driver stage.

Discussion in 'Home Brew' started by ElectronTubesRule, Sep 10, 2011.

  1. ElectronTubesRule

    ElectronTubesRule Active Member

    Sep 6, 2011
    Likes Received:
    So I took a boat load of Bendryl to combat my 4 days of no sleep. It occurred to me while sleeping that if you build a tube amp with a driver up front then you can use cheaper parts. By cheaper I am not talking about their quality or tolerance rather their ratings for power handling.

    I think the old guys had some good idea. Often a set of 811's driving a pair of 813's....Why????? Well today if you build a 4000watt Pep amp the air vari caps and all the other front end tuning parts have to be able to handle the voltage both RF and DC.Why? Because the input tuning is directly connected to the driver's which in today world of 10,000 watt dissipation tubes with handles and such might mean you need some serious power handling capacity.

    Mostof the early driver set up's and some sweep tube non-rf specific driver's can work happily at 350-750 volts. If the driver's are in the 40-100 watt range no matter if it is a one tube or two tube set up you only need parts that can handle the DC/AC and RF power in the driver section for your tuned input. Thismeans you donot need $600 var. vac. caps or monster two foot long 25KV air caps etc........You can use light weight tiny air variables and do the exact same job. Your switch's can be regular switch's you do not have to wory that much about arcing in the band switch and even if you did something lame like have full voltage going through your meter's and switch's we are talking about 350-750volts not 2500-5000volts!!! Very Very Unlikely that voltage in the 350-750 range is going to skin across a plastic meter cover and zap you. It can but it is far less likely then it is with 2500++++ Volts.

    So this would reduce the cost of the input tuning circuit and it would keep extremely high voltage ac/dc/rf totaly out of the tuning section and front end of the amp with out overly complex circuit design. Their are some old glass tubes that put out 100 watt's with 350 volts on the plate and 1.65 watt's of input. The coils could be made lighter and cheaper useing less copper and less silver to plate that copper.

    On top of that a transistor radio with 100 watt's of output potential will more easily match with lower swr and less chance of mismatch feeding back with enough energy to heat it up and blow it the lower it's output is. SWR and impedance match changes with the power output of the transistor. I have checked this with meters and as I turned up the power on my radio the SWR and impedance changed with output power.Maybe it does not change but the meter's ability to read it changes I do not know which is the case????

    I am guessing that even with a bad input match as in Joe the Idiot bumps the tune and load knob that a radio able to put out 100 watt's RMS that is only driving 1-4 watt's into this mismatch could do so all day long and not suffer a failure assuming that it was not locked down continuously transmitting. If the above was repeated but this time Joe Idiot had his radio putting out 65 watt's to drive his amp to the legal limit that the mismatch would cause damage to the output transistor in his radio in less then a days time.

    I have not given much thought yet to what happens int he output tubes but I am thinking most of the bad stuff will take place in the driver tube with little of it being passed on. So I was wondering if you could select driver's with the idea they would act as a filter or conditioner for the signal you want to pass into the output tubes to reduce the chance of arcing,flash over, parasitics and such......Probably not but with so many low power tubes to chose from it might be worth a thought. Someone far smarter then me on this subject should give it some thought though since we seem to have a greater rate of arcing, flash over, parasitics and oscillation then they seem to have had 20-30 years ago and their has to be a reason??? I can not help but wonder if it is because of the fact we are driving tube amps directly from transistorized outputs from the radios with no intermediate tube of lower power and gain to act as conditioner or matching device.

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