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Thread: Bass Power Amp with GU50

  1. #36
    Senior Member woodyc's Avatar
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    Quote Originally Posted by es345 View Post
    ...2. The next time I will put a low Raa resistance to the specifications....
    I just thought of an alternative...

    fanamp.jpg

  2. #37
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    They are already mounted at the bottom
    Attached Thumbnails Attached Thumbnails bass-amp-fans-bottom-.jpg  

  3. #38
    Senior Member woodyc's Avatar
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    Quote Originally Posted by es345 View Post
    They are already mounted at the bottom
    Sneaky.

    But nice idea. Do your GU50 sockets have vent holes?

  4. #39
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    There is some distance between the socket and the glass, not much but enough for slow vent. You can see it at the beginning of the thread. The fans are low noise, with 1200 rpm (Slip Stream Slim 120 mm Case Fan, 12 mm)-

  5. #40
    Senior Member woodyc's Avatar
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    By the way, how do you figure the grid dissipation? I assume you get the rms current across the stopper but what about the voltage? Do you look at the positive blips and make an estimate?

    (You've inspired me to revisit an old design and shove some AB2 into it.)

  6. #41
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    I assume you are asking for G1.

    You find some scope measurement pictures and my rough estimation done during summer time this year when I was still optimizing. Please not that I have done the measurement in a sequence, the drive adjustment has varied a bit. But its ok for an estimation.

    - In the chosen overdrive situation the peak voltage at G1 is +27V (clipping picture)
    - the related current is about 25mA peak, estimated up to 25% of the period. That leads to an average power dissipation of 25mA*27V*0,25=~170mW . As said above I have limited the G1 voltage to be on the safe side for the G1 power dissipation. But it helps also in addition to limit the power disipation of G2 , not to forget.

    To explain the gridcurrent picture: the upper scope is showing the grid current, the lower scope is showing the output voltage at 3,9 Ohm. For the gridcurrent measurement I have separated the upper scope from ground. If you want to do that measurement, dont forget to use an isolation transformer for the scope measuring the voltage at the grid resistor. The whole scope is floating with the G1 driving voltage, in my case from +27v to about -140V during the measurement. Alternatively use a differential probe.

    Hans- Georg
    Attached Thumbnails Attached Thumbnails 20120805-6xgu50-clipping.jpg   20120810-grid1-current-clipping-small.jpg  

  7. #42
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    A small amendment:

    you may remember Steve Connors comment

    The physics of transformers won't allow you to have small size, low DC resistance and extended bass response all together. As far as I know, the 300PS OT is only specified down to 50Hz at full power
    Taking that into account I have discussed with my OT supplier to find another design compromise.
    This was the resulting spec. he agreed to.

    - Toroid, designed for 350W
    - same size as the former one ( 80mm height, diameter 165mm, max 167mm)
    - Raa 2K (changed from Raa 2K2).
    - full power at 50 Hz
    - primary coil resistance center to anode 18 Ohm. This is a much smaller value compared to the value of the first OPT (~70 Ohm)

    Here are the first measurements of possible clean power at 3,9 Ohm Resistor load.

    - 460 W at 46Hz or higher, the limitation is clipping. The upper frequency limit for this power is above 20 kHz.

    below 46Hz the first limitation is the magnetic saturation of the OPT
    - 400 W at 41Hz,
    - 250W at 34Hz,

    Thats an impressive result. Tomorrow I will do a test with speakers.
    12stringbassman likes this.

  8. #43
    Noodle of Reality Steve Conner's Avatar
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    I heard reports of a small earthquake in Germany, are you all right?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  9. #44
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    Quote Originally Posted by Steve Conner View Post
    I heard reports of a small earthquake in Germany, are you all right?
    Yes, we are.
    My hometown is 170km away of Hans-Georg's, but I could feel the rumble

    Cheers
    Matthias

  10. #45
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    speaker test done, I am still alive

    My subjective asessment: the sound seems to be a little more tighter, the high frequencies more transparent.

    Ok, I am fine with this machine now, I will start the next project (definitely not one with more power... )

    Hans- Georg

  11. #46
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    I couldnt resist, there is too much of an engineer in my DNA:

    Having spoken with my OT supplier we have made another try still fitting to the housing:
    Raa 2K, Rcu anode to center 50 Ohm, only 4 Ohm secondary to have more room for the primary winding.

    Here is the result: 400W down to 20 Hz, saturation now starting at ~16 Hz.
    This is a far better compromise. But nows its done, definitely.

    Attached is a small excel which illustrates the impact of the OT's winding copper resistance to the reachable output power.

    Hans- Georg

  12. #47
    Noodle of Reality Steve Conner's Avatar
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    Very impressive 8)
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  13. #48
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    I hadn't had a chance to say but yeah, that's awesome! I'm very impressed.

    I bought a 6k6 TF to use as 3k3 and this concept really bit me in the butt- I didn't get nearly the power I wanted because of the anode resistance.

    I think I'm going to have to make a really high voltage amp to use the transformer now...what a shame.

    Jamie

  14. #49
    Ted
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    Has the amp had much use?

    I'm just wondering how the characteristics of those GU50s drift with age.

    You mentioned earlier that they have to be matched as there is no individual bias control and the toroidal O/P transformer is not as tolerant of DC imbalance as an iron core.

  15. #50
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    I am using the machine 2-3 times a week for about 3 hours each day. I didn't observe any drift of the GU50. They seem to be very stable - as expected for military equipment. I am doing regular measurements about one time a month (test pin for each tube is at the rear side of the amp)

    Regarding DC imbalance: one time I have measured at one tube only a 10th of the idle current. The reason was a bad soldering point at the plate (shame on me). Interesting: I havent recognized that during play. I see at least 2 possible reasons: 1. I didn't use all the power or 2. I have some problems with my ears . By the way: after fixing the tube was still ok.

    Some words about my "matching process": I have bought some 50 tubes for an acceptable price, measured them and sorted them into "sixpacks" with closest measurement results, e.g 1x21mA,3x22mA, 2x23mA. Then take 1x21mA and 2x22mA at one side, the other three at the second side. Adjust the bias and balance, thats it. No big science and the result is ok.

    Hans- Georg

  16. #51
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    I was asked to share the latest schematic. Here it is.
    Attached Thumbnails Attached Thumbnails bassamp-400-rel-3d-pre-amp-.jpg   bassamp-400-rel-3d-power-amp-.jpg   bassamp-400-rel-3d-power-supply-.jpg  

  17. #52
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    Hey good to see that this amp is going strong. I have implemented many of your strategies into my own ~200 watt amp with a kt88 quad. I should post my schem up here too, I haven't done much measurement with it to see if I am getting the power I hoped to but I don't think it would have been as operational as it is without referencing your GU50 amp.

  18. #53
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    Some status update including updated schematic:

    the machine is running quite well, the GU50's are extremely stable, no drift at all.

    I have built in 2 additional features:

    1. Limiter/compressor

    This is a passive one around a VTL5C1. Basically it is a volume controlled attenuator just in front of the master volume. It is in the schematic of the preamp (right half). You can see the compressor working indicated with the LED in the mid of the knob second from right hand (amp picture). I have used a pot with clearshaft illuminated via LED and have inserted a Fresnel lense in the center of the knob to pass the light (knob picture).

    2. Cathode current display on the back beside the bias adjustment pots

    This is a typical gimmick of an engineer : you can select to show the cathode current of each power tube, the average current of group 1, group2, and of all 6 power tubes. As I have external access to bias and bias balance the adjustment can be done now on the fly. Schematic is attached in a separate page.

    Hans- Georg

    PS: Still working on the labeling of the front plane....
    Attached Thumbnails Attached Thumbnails bass-400-rel-4a-pre-amp-.png   bass-400-rel-4a-power-amp-.png   bass-400-rel-4a-ccm-.png   bass-400-rel-4a-power-supply-.png   20140902-bass-400-compressor-small.jpg   20140904-illuminated-pot-clearshaft-knob-fresnel-lense-insert-1-.jpg   20140916-cathode-current-display-small.jpg  


  19. #54
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    Very nice project. Do I understand correctly that there is no STANDBY switch in the amp?
    Can you share some details on the output transformer (was it changed to toroid)?

    Mark

  20. #55
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    Quote Originally Posted by MarkusBass View Post
    Very nice project. Do I understand correctly that there is no STANDBY switch in the amp?
    Indeed there is no standby switch. I personally don't miss it.

    Can you share some details on the output transformer (was it changed to toroid)?
    PT and OT are toroids from the beginning. That was the only way to limit the total weight of the amp to ~22Kg.

    These are the main specs of the OT:
    * Raa = 2K
    * RL = 4 Ohm
    * -3db limit at 20Hz and 20kHz when running with output power of 350W
    * size 165 x 80 mm + 2mm (max)
    It was designed by this company Ringkerntransformator, Transformatoren, Ringkerntransformatoren | Ringkern, Ringkern-Transformator, 100V-▄bertrager - BADEL Elektronik GmbH and manufactured by their Polish company in Świdnica Transformator du?ej mocy, toroidalny, sieciowy, na zamˇwienie, zamiennik - BADEL Elektronik Sp. z o.o.

    Hans- Georg

  21. #56
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    I'm not familiar with toroid output transformers. Has it the interleaved windings, or any special solutions were applied? Did the high supply voltage influenced somehow the design?

    Mark

  22. #57
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    Well , I have set the requirements and they have done the design. I forgot to mention that inside the specs I have also put the supply voltage of 950V.
    Unfortunately I cannot help you with more details about the implementation. Nevertheless the result fits to my requirements:

    400W at 20 Hz, thats better than expected
    400W at 10 kHz
    200W at 20 kHz, (-3 dB)

    Hans- Georg

  23. #58
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    Hi Hans Georg,

    I know it's an old thread, but this is exactly what I would like to build, except I'm looking for an octet instead of an hextet of GU50. I've already contacted a company for the toroidal transformers, that's polish as well but a competitor of the one you linked here.

    The specs are close to yours, I will paraphrase them here like yours:

    OT
    -3 dB at 20 Hz and 20 kHz at 450 Wrms;
    Raa 2 kOhm with separate 30% UL and 10% CFB;
    B+ 900 V idle (not dropping below 850 at full power);
    4 and 8 Ohm taps

    PT
    300V 100 mA Preamp+PI
    420V 1A Ub Rail 3
    24V 200 mA MosFET Stage supply
    220V 1,2A Ub Rail 1 + G2 supply
    200V 100 mA Bias and PI CCS suppy
    2x 6,3V 7A Heater GU50
    6,3V 3A Heater pre +PI (dc elevated)

    Do you have any suggestion on the design?

    Thank you in advance.
    Roberto

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