Bogen DB130 power amp conversion

[ab0ez]

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.

[ab0ez]

To recap, the Bogen DB130 integrated amp was made back inthe 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 adjustabe tube regulated.

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 if 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?

[Tinkerbelle]

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.

[ab0ez]

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!

[ab0ez]

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.

[ab0ez]

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.

[ab0ez]

The first place I always start with an amp rebuild is with the 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 or 6CG3 or other damper diodes. Because these use 6 volt filaments, you will have to add an extra 6 volt filament transformer, but since we are removing the preamp section, we will have extra room underneath the amp.

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 made to provide gobs of current.

For this project, I chose to use the 6AU4 damper diode. Why? Because I didn't have any ceramic compactron sockets in my junkbox for the 6CG3 or it's cousins!

If you don't want to convert to damper diodes, you can also change the rectifier tubes to 5R4'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. 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.

The attached picture shows the DB130 with the new ceramic rectifier and output tube sockets.

[ab0ez]

Shown in the attached files are the original schematic, specifications, and the new power supply schematic with 6AU4 damper diode rectifiers. Not shown in the new schematic are the filament connections or bleeder resistors.

The power supply filtering is greatly increased over that of the original.

If anybody would like the schematics, send me a PM. I have them for all the different variations of DB130.

[ab0ez]

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!

[ab0ez]

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.

[Tinkerbelle]

Very interesting thread, thank you for the posts.

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

[Alex6969]

Very nice little project.

[ab0ez]

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!

[Tinkerbelle]

Originally Posted by ab0ez

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!

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.

[bricktop]

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.