1954 Bogen VP17X Bass Control

[Sylvaniac]

Hello, I see these old tube Bogens are popular for rebuilding into guitar amps. Mine is as of yet stock, save for a recap to get it working. It's sounding great. But whereas the treble control works fine for both the phono and mic, the bass control works only for phono and does nothing for mic. I'm wondering if this is just part of the design? Could this be some kinda mod?

I should note, this is part of a Bogen record player (Lenco B50-16) system, such as was used by square dance callers. I'm thinking, maybe if a mod was done to the bass, it was perhaps done so that the caller could to boost the bass on the phono music without boosting the bass of his/her voice?

I'm also working on a 1955 Bogen J330. The treble and bass controls work equally for both phono and mic. Hmmm...

Schematics are a little daunting to me yet but I can post them as well as photos.

Thank you.

[Chuck H]

Speculating the possible cause for the bass only working on the one channel is a pedantic exercise without a schematic. I hope you can find one. It's likely that either the two channels are not joined prior to the bass control or there is a problem with the channel that doesn't adjust bass. If you are confident in tracing circuits you could draw your own schematic of what you have. That would at least give us something to work with. Otherwise we will need a factory schematic to determine if there is a problem or any modifications were done. The likelihood that a member here has acute memory regarding that amp are possible, but not probable. So we're probably going to need to work from scratch. And that means having a circuit to look at.

[Sylvaniac]

Speculating the possible cause for the bass only working on the one channel is a pedantic exercise without a schematic. I hope you can find one. It's likely that either the two channels are not joined prior to the bass control or there is a problem with the channel that doesn't adjust bass. If you are confident in tracing circuits you could draw your own schematic of what you have. That would at least give us something to work with. Otherwise we will need a factory schematic to determine if there is a problem or any modifications were done. The likelihood that a member here has acute memory regarding that amp are possible, but not probable. So we're probably going to need to work from scratch. And that means having a circuit to look at.

Hi, Chuck, thank you. I'll post the Photofact and some pictures and comments later today.

[Sylvaniac]

1954 Bogen VP17X Bass Control

Speculating the possible cause for the bass only working on the one channel is a pedantic exercise without a schematic. I hope you can find one. It's likely that either the two channels are not joined prior to the bass control or there is a problem with the channel that doesn't adjust bass. If you are confident in tracing circuits you could draw your own schematic of what you have. That would at least give us something to work with. Otherwise we will need a factory schematic to determine if there is a problem or any modifications were done. The likelihood that a member here has acute memory regarding that amp are possible, but not probable. So we're probably going to need to work from scratch. And that means having a circuit to look at.

OK, Chuck... In the Photofact photo, you see R20 (20K) going from bass pot wiper, up into the circuit somewhere. In my amp, not only is R20 a 56K but it connects from bass pot wiper to the outer lug of the phono pot. Very difficult but I tried to capture that in the photos. Note, in the schematics, there is NO R20 or any resistor on the wiper of the bass pot. It all looks totally contradictory to the schematic. But I've not mastered the interpretation of schematics at all.

[Jazz P Bass]

From what I can ascertain from the schematic, Mic input does not have a Bass Control.

[g1]

I thought the bass control would work on whatever is at the grid of V2, so should affect both channels?

[Sylvaniac]

I thought the bass control would work on whatever is at the grid of V2, so should affect both channels?

Hi, I think you might have something there. I may wire the resistor in question as directed in the schematic. If I'm interpreting correctly, that resistor (R20) goes from phono pot wiper to grid of 6SL7 on to grid of V2 as you point out. Maybe that'll do it.

[Sylvaniac]

From what I can ascertain from the schematic, Mic input does not have a Bass Control.

Hi, yes it looks like that. But it also looks like someone rewired R20. According to schematic, it goes from wiper of the treble control to the grid pin 4 of the 6SL7. But I'm confused because in the Photofact photo, R20 is connected to bass control wiper to the phono volume control. That photo contradicts the schematic, right? Or am I misinterpreting again?

[g1]

I would completely ignore the layout photo. It will just mess you up. You can wire things different ways and get the same results. Or there may be changes. The schematic shows R20 going to the treble pot.
I would suggest you print a copy of the schematic and draw in the changes you have found around the bass control.
Don't make any changes unless you have notes that enable you to change it back to exactly what you started with.

[Sylvaniac]

I would completely ignore the layout photo. It will just mess you up. You can wire things different ways and get the same results. Or there may be changes. The schematic shows R20 going to the treble pot.
I would suggest you print a copy of the schematic and draw in the changes you have found around the bass control.
Don't make any changes unless you have notes that enable you to change it back to exactly what you started with.

That's very prudent advice, thank you. You know, I just looked it over and thought better of diving in just yet. I noticed other differences from the schematic. And there are more resistors with values that conflict with those noted in the schematic. I bought this mainly to play 78's on. And the bass works great for phono. So I can live with this, but not if it isn't correct. OK, so here's maybe a clue: As we know, R20 goes from bass pot wiper to treble pot outer lug. I tried jumpering that lug to the outer lug of the mic pot. Now the bass pot did affect the mic pot. Hard to describe this... When I maxed the bass pot, fully clockwise, it gave the same bass response as before (when the bass pot was not jumpered to mic pot); however, when I decreased bass, counterclockwise, it rolled off bass.

OK, I have a same-era Bogen J330 I'm working on. It's a stand alone amp not part of a record playing system. Notice the bass and treble controls. At max they are "normal" (very even and flat sounding). But notice you can only "decrease" the bass and treble response; can't boost it beyond flat ("normal"). And this function is working properly and on all channels, mikes and phono. So now I'm thinking the VP17X is wired differently because it's part of a record player system, where you can, in fact, boost bass and roll off treble for playing records, but where you only have flat frequency response for the mic channel. Another way of trying to understand if this is by design is: since these systems were popular among square dance callers, it might not have been practical to have one bass boosting pot both for the voice and the music, as the music would sound nice and bass-punchy; however, with that much added bass the voice would be boomy and muddy.

Alas, if only I could find a user manual.

[Jazz P Bass]

I thought the bass control would work on whatever is at the grid of V2, so should affect both channels?

Ahh, I see that now.
Thanks G-One.

[Sylvaniac]

This just in! I have located a Bogen VP17X instruction manual. Bass control does not affect mic. Controls are working as they should. Regarding R20 possible rewiring, I think this could just have been a matter of schematic revisions that were never published, as we know happened commonly back then.