Dual power switch in J5?

  • Forgive me if it is a crazy idea, but is possible to add a resistor (or resistor/cap combination) to V3 anode that would reduce anode voltage and obtain this way a very low power output, 0.5-1w? This resistor would be bypassed with a switch when full 5w power was required. Is this a proper way to obtain power amp distortion at very low volume? 5W is only half the volume of 50W amp! I love power distortion of J5 and I would like to have it at lower volume indeed.


    Best regards,
    Alvaro

  • Re: Dual power switch in J5?


    Quote from Vorbis


    Weber attenuator.


    Not the same ;D
    What I mean is running the power tube at less voltage and no other load than the original speaker. Arguably, this would maintain the power saturation feeling that load atteanuators are said to lack. I think that "Power scaling" devices of London Power company just reduce the output tubes voltage also, but they allow a continuous control of the amount of reduction. I would like to try a simply mod for reducing power tube voltage and see by myself if this works or not.


    Best regards.

  • Re: Dual power switch in J5?


    Hi Alvaro,
    this would work, if you use a power resistor large enough (e.g. for reducing Ua and g2 by about 100V you would need approximately a 2K/20W Resistor). But keep in mind that the operation point of the output tube is changing too (check the cathode current with the lower Ua and g2 voltage, you may need to change the cathode resistor too) and be aware, that the sound will change (the primary inductance of the OT is static and not matching the new operation point). But it's worth a try, especially if the sound is differing from just reducing g2 Voltage (Rockopa's Mod)...
    Regards,
    Martin

  • Re: Dual power switch in J5?


    Thank you, MAD.


    So, let's go a further step: what if we were able to make a variable voltage power supply source only for the V3 tube and output transformer TR1. The original design is 310V at power supply source, which is reduced to 303V at plate and 298V at screen. So, if we make a variable power supply source that would allow us to reduce V3 plate and screen voltage from 310V to 30V for example, do will we have also to modify original bias resistor/caps in any way? Or will be that way good to simple reduce V3 power?


    Best regards
    Alvaro

  • Re: Dual power switch in J5?


    Hi Alvaro,
    yes, you definitely have to align the cathode resistor to the changing plate/screen voltage, otherwise you amp will have either less output power or will burn the output tubes :o
    I don't know how Kevin O`Connor is doing that (Power Scaling), but I assume he's changing the negative bias voltage when changing the plate and screen voltage.
    Regards,
    Martin

  • Re: Dual power switch in J5?


    Hi MAD.


    I only have the volume I of Kevin O'Connor books but the power scalling is explained in the volume IV, so I cannot tell for sure. But from I could read in his web site (the more I read about this, the more I want to know ;)) only fixed biased amps require a special circuitry for adjusting bias and cathode biased amps do not need it.


    Well, J5 Mk1 EL84 V3 power tube seems to be cathode biased (there is a typical resistor/cap at the cathode at least). Although I am not sure whether the connection that V3 cathode has to the "center tap" of 7,1 volts transformer output created by R23 and R24 would mean that it would be not really cathode biased but fixed bias, (excuse me my ignorance :-\).


    There is a schematic for power scaling a 18 watt amp here: http://www.soultoneamps.com/schematics/18wpowerscaling.pdf .


    Best regards
    Alvaro

  • Re: Dual power switch in J5?


    Hi Alvaro,
    you're right, the J5 is cathode biased. The connection you're thinking about is just the ground reference for the heater, that`s not related to the biasing method. My last post (sometimes it`s a good idea to think more thoroughly before answering ;)) was valid, when you also plan to enlarge the plate voltage or thinking "hifi", but when you only want to reduce the plate voltage and don't care about an optimal operation point, it should work fine.
    Regards,
    Martin

  • Re: Dual power switch in J5?


    Hi again MAD:


    After studying a bit power scalling (as presented by Kevin O'Connor) I am convinced that there is no need (at least for me) to have an amp with continuous power reduction from 5 watts to the minimum possible, power scaling style. I would be glad with two power options: 5 watt (full power) and 0.5 watts, which will deliver just one half of the 5 watts sound volume. But I need that the tone of both versions was similar, of course. By the way, I friend of me has a Marshall JCM 800 and was really impressed when played the J5 amp. He liked its sound sooo much ;D. So I just want maintain this sound at 0.5 watts!


    I have to calculate the plate and screen voltages and cathode resistor value that would give 0.5 watts from EL84. Let's suppose I can get the proper values lowering voltages and modifying cathode resistor.


    Do you think that the sound,the overdrived sound of EL84 at 0.5 wats would be similar to the 5 watt full power with only plate and screen lower voltages and cathode resistor modification?


    The idea is to have a switch that will select 5 or 0.5 watts. With two circuits and a change of plate and grid voltages (and a way to have dual voltages, through a circuitry or even a new power transformer for the 0.5 watt version if required.


    Best regards
    Alvaro

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