Bogen M330A Tube PA

[FarisElek]

I picked up a Bogen M330A tube P.A. amp sometime ago that doesn't work.
This amp looks like it was kept in a plastic bag for the last 50 years. It looks really nice and clean on the inside. I'd love to use this pre to get great sounds on dynamic mics for drums, or gritty vocals. I'd like it to be relatively easy to drive into distortion, but maybe not as easy as it would be with 2 12AX7's in the input. I know it's not going to be a RED47, AMPEX 601, v72, or any of the other HIFI tube pre's I love, but anything you think I can do to help this guy be a nice little character machine, please help a guy out.

Somethings I'm looking into:
1. Rewire 6EU7 tubes for V1 to be 12AT7 and V2 to be 12AX7. Will the 12AT7 be a suitable replacement in V1? Is there anything I should do to optimize the circuit?

2. Rewire the 7868 power tubes to be 7591. There are the same tube in a different bottle, much cheaper, and I already have some. I'm trying to not to spend any money on this project. I've also got some really nice 6v6 tubes on hand, but I don't know if the inverter will properly drive them, how to optimize the schematic for them, if they will drive the current output transformer, or how to figure any of this out.

3. I'm going to wire the WMT-1 out, which was originally meant to be used with their special rca adapator transformer which is a 10k/600 to be a line level out for telephone communications I guess. I'm going to wire this directly to an Edcore WSM 15k-600 (because I already have one) to be a balanced XLR out. Anything I should change since it's a 15k and not a 10k, or will this work out fine?

4. I'm going to add a 200/50k transformer to Mic 1 I have from left over from an RCA BA2C I built. I was going to build another but decided not to. It looks like originally you could buy a bogen tm-200 transformer that plugs into X1, X2, and X3 that has a similar primary and secondary. I hear apparently those transformers are not great, which is fine because I'm not trying to buy anything other than what I have already on hand.

5. I'm going to install a switch connected big 8r resistor along the ground and 8 ohm out I can switch back and forth to easily switch from being an amplifier to just a DI/Mic pre

6. Install grounded cable.

7. PSU seems decent. Would a choke be possible add on?

8. What do y'all think of the eq? Can anything be done to make it tastier?


Thank you,
Ryan

 

[FarisElek]

I got this guy up and running and honestly, it's quite sweet sounding. It distorts pretty easily but I really enjoy the type of distortion. A couple things I've noticed is plate voltage starts at 50v on startup and then drops to around 20v. What could cause this? Also the 7591's get really hot. Besides that it's just scratchy pots even after cleaning. Might just check some new pots in there.

 

[FarisElek]

All my previous problems are golden.

Biggest thing I'm having trouble with is that the buzzing gets loud when I touch the microphone. It's pretty much where I want the noise as long as I don't touch the mic. What is up with that? I've already tried using alligator clips to try creating more of a star ground with no improvement. I have, however, hear improvement by moving channel 2's volume to full gain and channel 3's to lowest gain. Are there pots kaputz?

Thanks,
ryan

 

[FarisElek]

Oops. Wired XLR wrong. Very decently silent. Anyway, I'm moving on to modifactions to make this work well for guitar/bass as well as a mic. Any recs?

 

[FarisElek]

Well I just copied the Fender schematic of 1m resistor from tip to ground, ground to tip when jack is removed, and a 68k to the input of the 12ax7 and it workers perfectly. This unit is decently quiet, sounds great on guitar, bass, and vocals. The tone stack is completely fine. I can record through an amp and through the line out simultaneously and it sounds great plugged into a speaker. I don't know why these circuits get so much hate. The "ugly" distortion is it's own thing that I'm kind of into.

ANYWAY:
The remote outs all have insane voltage on them. Is it okay to remove the resistor that connect them to circuit to prevent any accidental touching? I assume it's okay but I figured I'd see if any of y'all can let me know for sure.

 

[ccaudle]

I don't have anything to add regarding your questions, but I would like to know where you found the schematic. I have an old Bogen PA amp that I would like to turn into a guitar amp, and I am planning on just tracing out the circuit since I don't have schematics, but it would be nice to have printed schematics to start with to save me that time.

 

[emrr]

I don't have anything to add regarding your questions, but I would like to know where you found the schematic. I have an old Bogen PA amp that I would like to turn into a guitar amp, and I am planning on just tracing out the circuit since I don't have schematics, but it would be nice to have printed schematics to start with to save me that time.

SAMS Photofact would be probable first source on anything Bogen, if it's not pasted in the amp chassis.

https://www.samswebsite.com/en/photofact/search/index/or/samsid/os/0/brand/BOGEN

 

[abbey road d enfer]

ANYWAY:
The remote outs all have insane voltage on them. Is it okay to remove the resistor that connect them to circuit to prevent any accidental touching? I assume it's okay but I figured I'd see if any of y'all can let me know for sure.

Well, these outputs are so-called "constant voltage". Their nominal level is 70V rms. Unless you have dedicated PA systems, you should not use them.
The "normal" outputs are those on the xfmr barrier strips, which also appear on pin 2 of the "speaker sockets".

 

[FarisElek]

Well, these outputs are so-called "constant voltage". Their nominal level is 70V rms. Unless you have dedicated PA systems, you should not use them.
The "normal" outputs are those on the xfmr barrier strips, which also appear on pin 2 of the "speaker sockets".

Oh yeah for sure. I meant the remote control outs. They are super high voltage, like 325v ish, have to double check as I last checked when the unit wasn't working. I believe the 470k resistors to the xtr strip might actually be necessary to the circuit so ill have to mount some internal isolating pins to get them off the back panel. Seems so dangerous to have those freely exposed on the back.

 

[abbey road d enfer]

Seems so dangerous to have those freely exposed on the back.

Not really "dangerous", since the current there is limited to about 0.5mA. I must admit it wouldn't pass any modern safety standard, though.

 

[Hubbub]

FWIW the remote control outs are really ins. The remote control unit used high value pots wired as a variable resistors to ground. Low resistance killed the voltage to the preamp tube and muted the source. Add resistance and the plate voltage rises, giving more gain. Worked OK for PA vocal mixing, and was pretty insensitive to how long the cable was.

 

[FarisElek]

Would it be advantageous to put the master volume after V2B instead of before? Would that make distortion worse though? Also, would I get more clean gain If I tied 2 of the inputs together? I thought maybe I could tie inputs 1 and 2 to input 3 and make their connection switchable as like a boost channel of sorts.

Also, could I make the b+ for the 12ax7 higher? Say 100v? 50 just seems very low and they seem very cold.

 

[abbey road d enfer]

Would it be advantageous to put the master volume after V2B instead of before?

You'd run the risk of clipping V2b. In addition it would improperly load the White EQ.

Also, would I get more clean gain If I tied 2 of the inputs together? Do you mean in cascade or in parallels?

Parallels would bring a marginal improvement in noise performance.
Cascade would probably make it a fuzz-box.

Also, could I make the b+ for the 12ax7 higher? Say 100v? 50 just seems very low and they seem very cold.

You would need to completely reconfigure the bias arrangement.
See how the input tubes have no cathode resistor?

 

[FarisElek]

You'd run the risk of clipping V2b. In addition it would improperly load the White EQ.

Parallels would bring a marginal improvement in noise performance.
Cascade would probably make it a fuzz-box.

You would need to completely reconfigure the bias arrangement.
See how the input tubes have no cathode resistor?

ah yeah, grid leak bias. It's something I'm not super familiar with to be honest. Is there Nothing I can do to extend the clean gain of V1 and V2? I can still raise the voltage to the plates and adjust the grid leak bias, cant i?

 

[abbey road d enfer]

ah yeah, grid leak bias. It's something I'm not super familiar with to be honest. Is there Nothing I can do to extend the clean gain of V1 and V2? I can still raise the voltage to the plates and adjust the grid leak bias, cant i?

Well, you would have to increase the grid resistors. It takes some trial-and-error to get it right. Then you just would decrease R4 and R5.