Bogen DB130 power amp conversion

Discussion in 'Tube Audio' started by ab0ez, Oct 16, 2008.

  1. ab0ez

    ab0ez AB0EZ

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    Would any of you folks out there like me to post a thread on rebuilding and converting a pair of Bogen DB130 integrated amps into pure power amps?

    These are great amps. Put a pair of 6FW5's in the output sockets and they are even better.
     

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    Last edited: Jul 11, 2012
  2. ab0ez

    ab0ez AB0EZ

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    To rehash, the Bogen DB130 integrated amp was made back in the glory days of mono systems. In it's original form it used a pair of 6AV5 tubes running about 680 volts on the plates. The screen voltage was provided via an adjustable tube regulated circuit.

    It's fairly easy to remove or bypass the preamp sections on these amps and turn them in to really nice power amps. With a few mods they are outstanding. Great bass, great highs, and quiet enough with a pair of KHorns with a pair of T-350 tweeters that you have to get within a foot of one to hear any ambient hiss. The adjustable dampening circuit works very well.

    Throw in a pair of 6FW5 output tubes, set the screens for about 210 volts and prepare to listen to some music!

    Anybody interested in the details?

    Here's a few teaser pics of a gold face DB-130 partway through the modification process. I suppose it's a lot more than a modification since the design is changed by more than 50%, but I digress. This amp features 6CG3 half wave damper diodes for the power supply vs the original twin 5Y3 full wave rectifiers, a 6CG7 front end and the output has been converted to use 6GV5 (that will be documented in another post) output tubes. As you can see, the amp has also been re-capped and the preamp sections removed. We now have a pure and basic power amp with one whale of a power supply.
     

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    Last edited: Jul 11, 2012
  3. Tinkerbelle

    Tinkerbelle Everything in Moderation ------- Hudson Valley, NY Super Mod Subscriber

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    I think it would be a very interesting read. Bogen amps seem to be often overlooked. I've read good reviews on the DB-130 and occasionally thought of picking up a pair.
     
  4. ab0ez

    ab0ez AB0EZ

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    The Bogen DB130's, like most amps, came in a few different configurations during their production run. There were also two different face styles.

    Some had twin 5Y3 rectifiers (my favorite type of 130 to modify), a 5U4 rectifier mounted vertically on some and horizontaly on others, and some with a 5AR4 rectifier.

    There were other changes and improvements to the bias circuits and so forth as well.

    The pictures shown are the two different faceplate styles. The gold face amps have the twin 5Y3 rectifiers and the two silver amps use the 5AR4 rectifier.

    The amp was spec'ed with a frequency response of 15-30,000Hz +/- .5dB, 35 watts output with a tone burst peak power of 100 watts. Harmonic distortion was listed as .3% at 35 watts. Intermod disortion was listed as less than 1.5% at 35 watts. These were definately hi-fi amps!
     

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    Last edited: Jul 12, 2012
  5. ab0ez

    ab0ez AB0EZ

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    To refurb, rebuild and modify the 130's into operation as a pure power amp, a few rules need to be set down.

    1) These amps use uncommonly high plate voltages. The secondary on the power transformer is in the neighborhood of 1000 volts center-tapped. ALWAYS remember that and use common sense when working.

    2) All the tube sockets need replaced. Yes, it is a pain, but I have had more than one amplifier drive me insane with weird symptoms only to discover the cause was a bad tube socket. Not only do the contacts go bad, but on some sockets the dielectric material breaks down.

    3) The old selenium rectifier in the bias circuit absolutely must be replaced! Contact Jim McShane and order some of his fast recovery diodes. Mount the new diode and 100uF 100WVDC cap on a terminal board and you're done.

    4) Install some test point jacks for measuring the screen voltage and the plate voltage. This is much safe than trying to place alligator clips on the underside of the amp. You can also put in a test jack for the bias voltage, but since the plate current is really set by adjusting the screen voltage this isn't totally necessary.

    5) The electrolytics will need replaced as will most of the old carbon comp resistors, especially plate load resistors and the 10 ohm bias current balancing resistors.

    6) The original power cord must go. The insulation on the individual wires gets brittle, not worth the risk.
     
    Last edited: Mar 4, 2009
  6. ab0ez

    ab0ez AB0EZ

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    Here are a couple of pictures of a gold-face amp in original condition, over 50 years old. You can see the old wax covered caps, the selenium rectifier, and the general layout.
     

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    Last edited: Jul 12, 2012
  7. ab0ez

    ab0ez AB0EZ

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    The first place I always start with an amp rebuild is with the power supply. After all, when it comes down to it, an amplifier in nothing more than a modulated power supply and can only be as good as it's power supply!

    The reason the DB130 with dual rectifiers is my favorite version is because with it you can reconfigure the power supply to use 6AU4, 6AX4, 6CG3 or other damper diodes. You can either run the 6 volt filaments for the damper diodes off of the original 5VAC tap or you can install a separate 6 volt transformer for them. I've done it both ways and, believe it or not, it only makes about 1 volt difference on the B+.

    Damper diodes are tough. They have a low voltage drop, which helps lower the power supply impedance. They also have a very slow voltage ramp as they warm up. Think of it as a built-in soft start. They were also designed to provide gobs (that's a technical term) of current.

    For this project, I chose to use the 6CG3 damper diode. Why? Because I had the ceramic compactron sockets and the 6CG3 tubes!

    If you don't want to convert to damper diodes, you can also simply change the rectifier tubes to 5V4's. They use the same filament voltage and current (5V @ 2 amps) as the 5Y3, have a higher current capability, and a lower voltage drop as well as a slow high voltage ramp-up. No wiring change is needed, it's a drop in replacement. Do not use 5U4's, they draw 3 amps of filament current each and will make the power transformer run quite hot.

    Here's the much simplified power supply schematic with all unnecessary preamp sections removed. The power supply capacitance is significantly upgraded as is the safety margins. In the original configuration, the 33K resistor used for dropping the B+ for the driver section was only an 8 watt resistor. This resistor is pretty much fried on every 130 I've ever seen and sometimes gave off so much heat so as to blacken the metal of the chassis above the resistor. So, I replace them with 33k 50 watt wire-wound resistors. They aren't that much bigger physically and they run quite cool now! I also change out the 15k 2 watt resistors to a 15k 10 watt wire-wound resistors. Again, we have increased the safety margin and lowered the operating temps of these parts.
     

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    Last edited: Jul 12, 2012
  8. ab0ez

    ab0ez AB0EZ

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    Shown in the attached pictures are the original selenium bias rectifier stack and filter capacitor. This MUST be replaced. Selenium rectifiers do not age well and when one goes (and they will), not only does it smell worse than the night after a hard-boiled egg and beer party, the fumes are toxic.


    As previously stated, contact Jim McShane (you can google his name and it will point you to his website) and order some of his 600 volt 3amp fast/soft recovery diodes. They are 50 cents each, so order 10 or 20 to have some on hand. Mount the diode and a 100uF 100WVDC capacitor on a terminal strip (observing the correct polarity on both!) and not only do you have a safe bias rectifier, you have a better one!
     
    Last edited: Mar 4, 2009
  9. ab0ez

    ab0ez AB0EZ

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    Here is a shot of the two rectifier sockets after the conversion to the 6AU4 damper diode tubes. Pins 7 an 8 are the filament, pin 5 is the plate (HV from the transformer) and pin 3 is the cathode (rectified DC). Place a jumper from pin 3 to pin 3 on the two sockets and that's about it. Reconnect the wire that goes to the first filter capacitor to pin 3 of either tube socket. In this case, I placed the wire on pin 3 of the upper tube socket.
     
    Last edited: Mar 4, 2009
  10. Tinkerbelle

    Tinkerbelle Everything in Moderation ------- Hudson Valley, NY Super Mod Subscriber

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    Very interesting thread, thank you for the posts.

    Is much of this also applicable to the DB-230 (the 6AV5 version)?
     
  11. Alex6969

    Alex6969 Jonny B. Goode

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    :lurk: Very nice little project.
     
  12. ab0ez

    ab0ez AB0EZ

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    I'm not sure about the DB230. If you have the schematic, please email it to me at ab0ez at comcast dot net I'll take a look at it and tell you if the circuits are similar.

    Cheers!
     
  13. Tinkerbelle

    Tinkerbelle Everything in Moderation ------- Hudson Valley, NY Super Mod Subscriber

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    OK, thanks, I just thought that you might already know having worked the DB-130. The only DB-230 schematic I've seen is the 6DW5 version, not the 6AV5, which may be the DB-230A, but not sure.

    If very similar it might be easier to pick up a stereo DB-230 vs two mono DB-130 units.
     
  14. bricktop

    bricktop Couch woofer

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    AK user jon_s should have a DB230? - 6AV5 schematic. It may even be up here on AK somewhere as I think I scanned it for him and posted it.
     
  15. bricktop

    bricktop Couch woofer

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  16. Tinkerbelle

    Tinkerbelle Everything in Moderation ------- Hudson Valley, NY Super Mod Subscriber

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  17. jon_s

    jon_s AK Subscriber Subscriber

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    DB230 uses lower plate voltage than the 130, so basically half as powerful. Overall though, the amplifier design is quite unique. It is a clean-sounding amp, and the phono stage is great. I should take some under-the-hood shots on that amp, since mine is restored (and listed in Barter Town ;)).

    I also measured it recently - I get about 0.5% THD at 1W, but that's through the preamp stages and tone controls and filters, and with an 0.08% THD source (bricktop's old HP rig) - the power amplifier should be quite a bit cleaner on its own.

    The biggest flaw of the amp is the ridiculous stereo/mono switch that bridges the amp channels at their inputs AND output transformer secondaries. I'm sure that seemed "fancy" at the time, but it's a recipe for stability problems, and scares the snot out of me. I disconnected all of this.
     
  18. ab0ez

    ab0ez AB0EZ

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    I know what you mean. I just finished rebuilding an Eico HF-87 for a friend and one of the first things on the chopping list was taking that ridiculous stereo/mono switch out of the circuit!
     
  19. ab0ez

    ab0ez AB0EZ

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    Been a busy last 7 days or so. I had Lasik done on my eyes last Friday and am thrilled with the results!

    Back to the DB-130. Here's 2 shots of the faceplate without and with flash after cleaning. I used some spray foaming type bathroom cleaner and paper towels. The towels were black when I was done, but it shined up really nice! Sure looked a lot nicer minus the 50 years worth of dirt and grime.
     
    Last edited: Mar 4, 2009
  20. ab0ez

    ab0ez AB0EZ

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    Tinkerbelle,

    Looking at the schematic provided via the link above, you should definitely change out the selenium bias rectifier (M2) and the dual selenium filament voltage rectifier (M1) with some of Jim McShane's diodes. In spite of the lower plate voltage on the 230, most of my post about the DB-130 can be adapted to the DB-230. I just might have to pick up one of these for myself and see what I can do with it!
     

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