Bogan DB-110

[mike60510]

I purchased a Bogan DB-110 PA for parts; however, I thought I would give it a try and see if I could get it to work. I’ve replaced the electrolytic and put in a set tubes with have no output, I’m not surprised. The source of my questions are about voltages at the transformer and out of the rectifier.

The amp has a 5Y3 rectifier. The schematic shows voltages into the rectifier should be 340 VAC and 375 VDC out. After replacing the electronics and new tubes, I measure actual voltages of 370 VAC into the rectifier and 490 VDC out. With the difference in voltage from the power transformer into the rectifier, would that explain the higher DC voltage out too?

I’ve attached a copy of the schematic. Thanks for any thoughts/comment/help in advance

[Chuck H]

Nope. There's something hinky going on. Is that 490V measured with tubes in place and drawing current?

[Chuck H]

Huh..? That amp has a 117V primary. No way it should be up to 370V even with modern wall voltages. But I just ran a sim on PSUDII and with 370VAC into the rectifier I get 385VDC out (approx) with your tube compliment loaded and 515VDC unloaded. I'm guessing your tubes are drawing very little current.

[mike60510]

Huh..? That amp has a 117V primary. No way it should be up to 370V even with modern wall voltages. But I just ran a sim on PSUDII and with 370VAC into the rectifier I get 385VDC out (approx) with your tube compliment loaded and 515VDC unloaded. I'm guessing your tubes are drawing very little current.

My mistake, thanks for the lesson. Putting all the tubes in makes a world of difference.

[Enzo]

Even if your high voltage was way off, like 100v high or 100v low, the amp would still make sound. To make sound, pretty much an amp just needs some voltage. It might sound best with a certain amount, but it makes sound with almost anything. SO if yours makes no sound, we look for something more fundamental.

Are ALL the tube heaters glowing? On all the 12AX7s, do pins 1 and 6 have high voltage? Do pins 3 and 8 have some small voltage?

The power tube cathodes - pin 8 - are biased by that 7 watt 300 ohm resistor. Should be roughly 25v there. Is there?

[mike60510]

Even if your high voltage was way off, like 100v high or 100v low, the amp would still make sound. To make sound, pretty much an amp just needs some voltage. It might sound best with a certain amount, but it makes sound with almost anything. SO if yours makes no sound, we look for something more fundamental.

Are ALL the tube heaters glowing? On all the 12AX7s, do pins 1 and 6 have high voltage? Do pins 3 and 8 have some small voltage?

The power tube cathodes - pin 8 - are biased by that 7 watt 300 ohm resistor. Should be roughly 25v there. Is there?

Thanks, no fire in the first two 12AX7s = good reason for no sound. Back to troubleshooting. Thanks for the information.

[mike60510]

[QUOTE=mike60510;481062]Thanks, no fire in the first two 12AX7s = good reason for no sound. Back to troubleshooting. Thanks for the information.[/

Got sound! I was wondering if anyone could offer some thoughts about this. The unit has a 2.7 ohm large wattage resistor in the tube heater wiring between the AF Amp and phase inverter. it does not show up in the schematic. With that resistor in the circuit, I get no power to the heater elements for the pre amp and AF amp tubes. I jumpered over the resistor and now I get power to the first two 12AX7's and sound.

Any thoughts on the purpose of the resistor?

Thanks again for the assistance.

[Chuck H]

It's in the schematic. R39 located near the transformer filament winding on the schem. I can't figure out a practical reason for it. It's value seems too high for minor voltage reduction and too low for any real isolation from the hum balance circuit. Maybe it's a hum abatement trick we haven't seen before? I think that resistor must be failed open because, well, it's in the amp, it's in the schematic and the amp is used. So it probably works in there.

Are all the filaments twisted?

What is your filament voltage?

With R39 jumpered, if your filament volts are in spec I might just rewire the preamp filaments off the PI tube and remove that resistor. But hold off for a sec in case someone pops in with a good reason for it's existence.

[mike60510]

It's in the schematic. R39 located near the transformer filament winding on the schem. I can't figure out a practical reason for it. It's value seems too high for minor voltage reduction and too low for any real isolation from the hum balance circuit. Maybe it's a hum abatement trick we haven't seen before? I think that resistor must be failed open because, well, it's in the amp, it's in the schematic and the amp is used. So it probably works in there.

Are all the filaments twisted?

What is your filament voltage?

With R39 jumpered, if your filament volts are in spec I might just rewire the preamp filaments off the PI tube and remove that resistor. But hold off for a sec in case someone pops in with a good reason for it's existence.

Yes, you are correct that is part of the hum balance circuit. Thanks, i see that now. Yes, the wires are twisted to the heater elements.

With that resistor in the circuit, i don't get more than 3.1 volts across the heater elements of the pre-amp and AF amp tubes. With it out of the circuit I get 6 volts. Sounds better too, no static. With the resistor in circuit I adjusted the hum balance pot and didn't notice any significant voltage change at the heater elements

Thanks again

[Chuck H]

The 3.1V is measured to ground? Probably half the winding referenced to the false CT formed by the hum balance control. I think that resistor is failed open. I don't see how R39 is part of the hum control. It's not part of it's obvious purpose anyway. Good that you don't get voltage changes when using the hum control. That would be a problem.

[Enzo]

Since the resistor goes to the CT of the tube heaters, and 4,5 are wired to X that resistor is in series with the two tube heaters. I figure it drops a volt and a half. Now sure why.

There may be a rationale for that in this as a PA amplifier, but as a guitar amplifier, there is no reason you can't simply shunt that resistor and have all 12AX7s running on straight 6.3vAC.

And we really should check it for open before we do that just for science.


R39 is part of the hum control in the same sense the tube heaters are part of it. I don't see how it would abate any hum, other than to lower the emissions of the cathodes.

[mike60510]

Since the resistor goes to the CT of the tube heaters, and 4,5 are wired to X that resistor is in series with the two tube heaters. I figure it drops a volt and a half. Now sure why.

There may be a rationale for that in this as a PA amplifier, but as a guitar amplifier, there is no reason you can't simply shunt that resistor and have all 12AX7s running on straight 6.3vAC.

And we really should check it for open before we do that just for science.


R39 is part of the hum control in the same sense the tube heaters are part of it. I don't see how it would abate any hum, other than to lower the emissions of the cathodes.

I was wondering if anyone had any suggestions on where to put the tone stack and volume controls. Currently, the volume is prior to the second 12AX7 and the tone and bass are as between the two stages of the second 12XA7.

Thanks again for the assistance.

[Chuck H]

There is a lot about the amp isn't typical of guitar amps. Where different controls are located is different with models of guitar amp. Fenders put the tone stack AND volume control between stages one and two ALL within the first preamp tube. Marshalls typically put the volume only between stages one and two, buffer with a cathode follower and THEN employ the tone stack right before the phase inverter. There really isn't an absolute best place for any given control for that reason. And we aren't working with a guitar amp, yet. If you don't like the way it sounds, adjusts or behaves at different gain levels then you'll need to tell us what you don't like about what it's doing and what you would like it to do instead.

[mike60510]

There is a lot about the amp isn't typical of guitar amps. Where different controls are located is different with models of guitar amp. Fenders put the tone stack AND volume control between stages one and two ALL within the first preamp tube. Marshalls typically put the volume only between stages one and two, buffer with a cathode follower and THEN employ the tone stack right before the phase inverter. There really isn't an absolute best place for any given control for that reason. And we aren't working with a guitar amp, yet. If you don't like the way it sounds, adjusts or behaves at different gain levels then you'll need to tell us what you don't like about what it's doing and what you would like it to do instead.

Thanks for the advise. I was reading some blogs on-line about the location of the tone and volume controls and that spurred my questions. After looking at numerous schematics, the 0.1 uF coupling caps are a bit too high for a guitar amp and I was planning on changing those with a smaller value, 0.01 uF or something in that range.

Thanks again, its a fun process. I realize I have a large learning curve.

[Chuck H]

Ok, well, let me expand a little on what I was talking about.

There will always be some "expert", opinionated engineer or designer saying that some one way is "best" or some other way is wrong. But WRT guitar amps that isn't necessarily true. Don't drink the KoolAid offered by any of these people. Guitar amps are signal processors, not reference amplifiers. They create tone as well as amplify it. That is, they don't simply make things louder, they shape and distort the input signal, sometimes in unique ways, from input to speaker. Since there are many different "topographies" for guitar amp designs it's usually best to play a bunch of amps and pick a type that suits you as a player. If you're building an amp it might be a good idea to consider a favorite amp and/or players whose work you like and research those designs to see how those amps circuits are arranged. That would apply to a tear down/rebuild of something like your Bogen. The other approach would be to just start with the amp as it is and try to manipulate component values in situ to get closer to how you would like it to sound. This may or may not get you where you want to go because the circuit arrangement may be too different from other amps that actually do what you like. So...

Maybe consider what sort of tone you would like from this project. Players that sound like that or an amp you've had that you really like. Mention it here and you can count on design suggestions that apply specifically to YOUR amp project and no one else's guitar amp facts. Which, when it comes to where to put a tone stack or volume control, are possibly just personal ideals or opinions and not facts at all.