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  • Bogen PH 10 questions

    Using a Bogen PH10, pp6v6 monoblock as a power amp with a old tonelab modeler in front. It works great really warm and clear…except for two minor things that I wonder if they could be changed or should I live with it?

    The primary thing, there is a constant back ground static (not hum), not particularly loud but significantly more than my other tube amps, its ear fatiguing bothersome. The static is constant, occurs without the modeler plugged in; its like a floor threshold constant static. Is it a resistor (or lack of one) or a ground issue? My other old amps are dead quiet compared to this thing…what is it and can anything be done?

    Second, it seems that it doesn’t have that much volume/loudness for a pp 6v6…I’m thinking like 10-18 watts; even with the modeler’s line level out being cranked, it is fairly loud but should be louder. So is there some resistor or is it just set up for a phono and needs to be changed. It looks like to me the input goes right to the volume pot and then to the phase inverter. Also the transformer is a replacement; its a Stancor A-3870, it works fine but is there another secondary tap that could be used or a bigger OP…any recommendations?

    I continue to be a noob, this monoblock seems to be pretty straight forward. Anyway just investigating if relatively minor changes could be done that I could learn and do or is it bench paying time. The gut shot pic shows the prior owner did some work; I’ve a grounded cord, put in a fuse and removed the wires to the power outlet socket. Any opinions and info is much appreciated, Thanks!

    schematic pic

    https://picasaweb.google.com/lh/phot...eat=directlink

    gut shot

    https://picasaweb.google.com/lh/phot...eat=directlink

  • #2
    Be sure that old can cap isn't in the circuit anymore!

    With regards to the volume-problem, it might be because the signal coming from your digital unit isn't loud enough. That mono block is just a phase-inverter and a power amplifier. You'd might want to stick some kind of amplifying stage in front of that, to get a bit more voltage than the 6SL7 can deliver by itself. Be sure to plug in a big ol' power resistor, and measure the power output now and compare it to the results after.

    YM2C

    Comment


    • #3
      Thanks, yes the old can cap is not in the circuit. So to find a preamp.

      forgot to link a pic of topside

      https://picasaweb.google.com/lh/phot...eat=directlink

      Comment


      • #4
        No expert here but a couple of observations:

        Some of the hiss that you're hearing may be "shot noise" caused by carbon comp resistors.If this is so,a change to metal film/oxides or carbon films may help.It looks like some of the resistors in your amp have already been changed to carbon films. R.G. Keen has a really good article on his Geofex page explaining where to use different types of resistors to reduce noise while retaining carbon comp "vibe" if you're interested in that.

        The phase splitter in your amp is of an older type called a paraphase.These have an inherent imbalance due to the fact that the signal for the second triode is taken from the amplified output of the first triode.If you're looking for more gain a LTP (long-tailed pair) may be more appropriate.Something to research.

        There are two NFB (negative feedback) loops I see originating in the OT secondary.The first is the one from the 8 ohm tap going to the cathode of the phase inverter. The second also originates on the 8 ohm tap and goes through the tone circuit to the output of the first triode in the 6SL7.This not a typical tone circuit for a guitar amp and you may want to consider modifying it to something more conventional.Both of these loops affect the gain and tone of your amp.

        Greekie's suggestion of adding a gain stage may be a very good one.


        Like I said, no expert here but I hope you find this useful.............................. Steven

        Comment


        • #5
          Neat little amp! You need to measure every resistor in the circuit. Any that are too high, out of tolerance shoud be replaced. I would suspect that there is one somewhere that is causing the crackle and also lowering the gain. Does the crackle go away if you remove the 6SL7?
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !

          Comment


          • #6
            I will go through the resistors. I pulled the 6sl7 and it was dead quiet, no static or crackle...so does this mean that the static is isolated to the resistors on the 6sl7? At this point I'm not going to do a pre amp, or change the PI or the negative feedback because the volume is fine for noodling at home and besides its way beyond my understanding. Thank you all for your patience.
            Tim

            Comment


            • #7
              Originally posted by wrathfuldeity View Post
              I will go through the resistors. I pulled the 6sl7 and it was dead quiet, no static or crackle...so does this mean that the static is isolated to the resistors on the 6sl7?
              There is a strong possibility that the cause is a resistor or capacitor connected to the 6SL7 or the 6SL7 itself. Remember you should discharge the filter capacitors before attempting to measure any resistors. Search the forum if you don't know how to do this.

              Check the 250 ohm and 500 ohm resistors between the 5Y3 and the output transformer. They could limit the power. This amp is a conservative design intended to operate for many years. Unlike guitar amps which abuse tubes for maximum output. Probably only about 10 watts output. Be sure to use the approiate output tap for your speaker.
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !

              Comment


              • #8
                Loudthud, Thanks! Got a couple of things on the summer honey do list but this will have to be worked around.

                Comment


                • #9
                  I checked the carbon comp resistors around the 6sl7 and one was high and another about there, so replaced them all; static is quite abit less...yea!!! I will now check the resistors between the 5y3gt and the output transformer...any recommendations for changes to these? Thanks!

                  Comment

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