Connecting Guitar Tube Amp to External Transformer for 70 Volt Line Audio System

Discussion in 'The Projects Forum' started by jon baceb, Apr 14, 2015.

  1. jon baceb

    Thread Starter New Member

    Apr 14, 2015
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    How do I get the following 70.7 line audio distribution system to work simply with tube guitar amps without causing a catastrophic transformer failure? Do I need to place a high wattage resistor somewhere in the circuit to dissipate heat and to avoid high undamped flyback peaks?

    I would like to use a large well-insulated crawl space to house speaker cabinets and microphones. The source tube amplifiers (various 50 watt amps), mic preamps and the Digital Audio Workstation will be in another room over 70 feet away. I will be running 16 gauge speaker wire to the cabinet(s).

    This whole idea came about because I found a schematic of the Newcomb Pathfinder PA tube amp that a couple of people have been converting to use as a guitar amp. The schematic for the Pathfinder below shows 70v and 25v output options along with the regular low impedance low voltage outputs.

    Initially I could only find autotransformers through an internet search. However, after doing an extensive search I found that Edcor manufactures single phase transformers for both amplifier output and speaker distribution. The Atlas Sound AT-100 100 watt 70volt attenuator is a popular product but I cannot find a schematic of the device online. What is drawn on the schematic is drawn by me and not confirmed.

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  2. #12

    Expert

    Nov 30, 2010
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    I'm not all that good with 70 volt systems, but I can tell you a tube amp hates an open circuit. Just don't turn it on without a speaker connected? That seems like the obvious answer. What gremlins might sneak into 100 feet of wire is beyond me.

    Has anybody here done 70 volt distribution systems?
     
  3. JDT

    Well-Known Member

    Feb 12, 2009
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    85
    With valve (tube) amplifiers, the valves (tubes) can be damaged when the load is open or very high impedance. This can happen when the volume control switch is set to off or low level. It is possible to get high voltage flash-over in the valves and/or damage to the primary of the output transformer.

    One way to overcome this is to have a resistor load on the secondary of the output transformer. This wastes power, though.

    Of course there are new-fangled devices called transistors around now!
     
  4. jon baceb

    Thread Starter New Member

    Apr 14, 2015
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    Where would I place the transistor(s) in the circuit as to not waste power and what value/type do I need to get this to work? Once I get an idea of what to do, I'll upload a revised drawing.
     
  5. #12

    Expert

    Nov 30, 2010
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    He didn't mean you should install some transistors, he meant you should buy a transistor amplifier in the first place. They don't get all sparky when there is no load.
     
  6. jon baceb

    Thread Starter New Member

    Apr 14, 2015
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    0
    I'm using a solid state power amplifier in my setup now. My tube amps are plugged into a load box that I built with a 150 watt 8 ohm resistor for a load and with a tap for a balanced signal that goes into a ss amplifier. It works great but I want to try something new.
    I know vintage equipment can get tempermental. That's the reason I posted this thread. With all the circuitry and experts out there, there has to be some stable solution that will keep tube amps happy and eliminate the need for a second solid state power amp.
     
  7. #12

    Expert

    Nov 30, 2010
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    All we have so far is, "Don't turn it on without a speaker attached." Is that some kind of contradiction to your intent?
     
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