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Bass Power Amp with GU50

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  • #16
    Hey glad to hear this amp has carried on into some further design. I won't lie this amp helped me implement my own mosfet driver system, I just have one question about ZD3 and ZD4, are those there to limit the signal swing out of the phase inverter? also the positive voltage to the mosfet followers is only 30 volts is that to limit the amount of positive swing available to drive the tubes grids up? In my amp that rail is around 70V, wonder if i should drop it down?

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    • #17
      You are right, ZD3/4 are limiting the signal swing. R9a/R10a in series of the Z diodes leads to somewhat more smooth clipping.
      I wanted to be sure not to kill G1 of the powertubes in case of heavy overdrive. If you have 70V at the drain of the mosfets, don't worry. The only difference is a little bit more power dissipation at the mosfets.

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      • #18
        You are right, ZD3/4 are limiting the signal swing. R9a/R10a in series of the Z diodes leads to somewhat more smooth clipping.
        I wanted to be sure not to kill G1 of the powertubes in case of heavy overdrive. If you have 70V at the drain of the mosfets, don't worry. The only difference is a little bit more power dissipation at the mosfets.

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        • #19
          ok good i figured that was what the zener arrangement was for

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          • #20
            Here is the last part of the design

            - preamp
            - housing

            Hans- Georg
            Attached Files

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            • #21
              Steve - if you want half a dozen GU50s PM me.

              Hans - do you have specifications for your transformers that you'd be prepared to share?

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              • #22
                This is what I have ordered (the web page of the company is mentioned above in the thread)

                Both are Toroid types

                OT

                - designed for 350W
                - frequency response from 20 Hz to 20kHz at full power 350W (-3 db limit)
                - Raa 2,2 kOhm; designed for Ub=950 at idle Mode
                - secondary 4 and 8 Ohm

                PT

                Secondary:

                W1 320V 0,1A Preamp+PI
                W2 220V 0,7A Ub Rail 3
                W3 220V 0,7A Ub Rail 2
                W4 240V 0,8A Ub Rail 1 + G2 supply
                W5 100V 0,1A Bias and PI CCS suppy
                W6/W7 6,3V 4A Heater GU50 and MosFET Stage supply
                W8 6,3V 4A Heater Preamp +PI


                If you want to add margin to the GU50 Heater increase W6/W7 to 5A each.
                If you want separate supply for the MosFet stage add W9: 24V 0,2A

                Hans- Georg

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                • #23
                  Hi Hans-Georg. Looks like a fun project. I'm having trouble understanding your whole driver/signal limiting strategy. Did AB2 not work out?
                  Last edited by woodyc; 12-11-2012, 06:57 AM.

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                  • #24
                    AB2 is working out very well. But you need to take care not to overload G1 of the power tubes as the Mosfets are basically not limiting the current. So you have to control the maximum voltage. What is helpful and what I have implemented is a "smooth clipping" strategy . Some more background you can find here. Grid stoppers with AB2.

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                    • #25
                      I read this thread on DIYAudio: KT88 AB2 Experiences - diyAudio

                      Ouch! It confirmed my fears of the worst that could go wrong when driving new production tubes into AB2. GU50 are of course not new production tubes, so they're unlikely to have gold-plated grids that could be damaged.

                      My interest in this project is that I have the OPT and driver transformer from a Fender 300PS, and the PT from a FT-DX560 sitting around looking for a use.
                      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                      • #26
                        Reading that thread: really "impressive" to see what can happen to a G1.
                        One observation: in the discussed configuration the KT88 are used in pseudotriode mode. In beam pentode mode at the moment of very low anode voltage the high G2 voltage is still "motivating" the electrons to flow rather to the anode than to G1 . This it might protect G1.

                        Using my bassamp now since some month the powerstage is quite stable, no drift or loss of power. Obviously I dont play at the power limit but what is amazing is the clean headroom available. (some motivation to start a project )
                        Last edited by es345; 12-11-2012, 06:04 PM. Reason: typo

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                        • #27
                          Originally posted by es345 View Post
                          AB2 is working out very well...
                          Well I was looking at some GU50 characteristics for VG2=250 and it looks like you could get about 310W with a 900V B+, a primary of around 3500 Ohms and no grid current. Since you have 50 more volts on the screens, it seems like something very close to 350W should be possible with no grid current. If you are getting anything from grid current it must only be a small fraction of a dB?

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                          • #28
                            taking your diagram you have a saturation voltage at the anode of about 150V.
                            If you have a OT with a coil resistance center to anode of about 70 Ohm (this is my case) you will get about 270 W output, not more. (see the diagram below)

                            Now my case:
                            If you look to the beginning of the thread you will find that I have reached 300W without AB2.With AB2 it is now somewhere between 350 and 360, so I just count it for 350W.
                            This is ~0,7 db and that is what I have expected.

                            Why not more?
                            AB2 helps you in bringing down the saturation voltage, something like 50V. If you have high anode voltage the possible gain is limited.

                            An example:
                            - UB=900, Uanode min =150V >> U peak at OT, center to anode=750V
                            - now you go to AB2 reducing Uanode min to 100V >> Upeak at OT center to anode= 800V
                            this is about 0,56db more.

                            So is it worth to do?

                            For me the decision was yes, because I definitely haven't any crossover distortion (Prio 1) and i have some watt more output (Prio 2). And not to forget: It was a great fun to develop and learn something about AB2.
                            Attached Files

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                            • #29
                              Very impressive build! Would it be possible for you to share your Excel worksheet? Looks like a handy tool to have...

                              TIA,
                              Jaz

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                              • #30
                                you can find it here Smartest way to 200W? - Page 2

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