Bogen M330A Tube PA Amp for Guitar
I recently acquired a 1965? BOGEN M330A tube PA amplifier during clean-out of a storage room at work. Itís in perfect perfect condition. I don't think it was ever even plugged in. The AC cord was still rolled up with a string tied around it and an inspection tag attached to it. If it will help, I can list all the features, inputs, outputs, controls, etc. I can also send photos. For tubes, it has 2x 7868s, 2x 6EU7s and 1x 7247. I did a controlled power-up through a variac and everything seemed fine. Then I brought it to Urban Antique Radio in CT for an assessment. Mike checked it out and we plugged a CD player into it and it sounded great!
Since I can't afford a "real" tube guitar amplifier, I was hoping to use the Bogen. I can play my guitar through one of the MAG-TAPE inputs and it sounds so beautiful (the amp, not my playing). I found the following sites (a few years old) and they sound like exactly what I want to do but Iím a mechanical engineer, not electrical, so there is not enough detail for me to actually do anything. Also, I know these things can kill you so I donít want to fool around with it before I learn what to do.
ďConverting Integrated/PA Tube Amps into Guitar AmpsĒ by Joe Jasniewski
ďNotes on Converting PA Amps to Guitar AmpsĒ author unknown
THE POINT: If I supply the schematic and manual, will someone please give me specific instructions for how to re-wire it to achieve what is described in Mr. Jasniewskiís paper? Iím handy with a soldering iron and meter and Iím respectful of the potential hazards (no pun intended) so Iím sure I can do the work if someone will tell me what to do.
If you post the schematic, I'm sure we could come up with something.
You could first try tweaking the tone stack, as mentioned in the articles. If you go with the Baxandall / James tone controls, try the James circuit in Duncans Tone stack calculator.
If this gets you the sound you want, great, you are done.
I tried this on my first pa amp conversion about 10 years ago. I was hoping to use the stock circuitry as much as possible. While the clean sound was pretty good, the overdriven sound was a bit harsh, and it just didn't respond the way it should. I eventually tore out all the ciruitry and went with the tried and true Fender AB763 topology, and never looked back. The Pa amp sounded like a really good Fender. I have done 4 pa amp conversions over the years and have used both the 5F6A and AB763 topologies with success. The best sounding one has a modified 3 stage AB763 preamp into a 5F6A power amp, with presence control and post phase inverter master volume.
I am not familiar with the 7868s, and 7247. The 6EU7s are similar to 12AX7s and they are easily available, not too expensive. Don't know about the other tubes. I personally would use readily available tubes like 6L6 and 12AX7.
Thanks for your response.
I'll try to post the manual, including the schematic, (zipped pdf) separately for use in this discussion. I donít know what disclaimers are necessary but I bought it from MusicParts.com. Hopefully the manual will go through. If not, I can post the schematic I did in AutoCAD.
I appreciate your comments about the overdriven sound. This is exactly right for blues. As is, it sounds wonderfully clean and creamy from strong full bass to shimmering highs but Iíd like the option to dirty it up.
Iím anxious for your (and anyone elseís) comments on what I might do with this great old amp.
I guess the manual is too large. Here's a zipped Word file into which I'd pasted the AutoCAD drawing. (is there a simpler way?) It's missing the main power section because I haven't CADed it yet. If you need it, let me know.
Thanks for your seggestions.
Got the schematic. There are several issues with this circuit for guitar uses
Preamp V1A, V1B, V2A
6EU7s configured with "grid leak" biasing method, not a bad idea for its intended use, but not so good for a guitar amp. Fender did this in very early amps and quickly switched to the standard cathode bias scheme. You probably don't need 3 inputs either.
The next problem is the "mixing bus". The signal goes thru one volume control directly into another one. This is lame for guitar. That master volume needs to be later in the chain.
Next comes the tone stack. It's probably all wrong for guitar. You could redo the existing tone stack or put in a Fender/Marshall/Vox tone stack. Since you already have two 1 Meg pots, a Vox style tone control would work.
Now we get to the power amp section. This is a pretty standard output stage for a PA amp, but not optimal for guitar. Most likely there is too much Negative feedback. I'm not too crazy about those 15k plate resistors on the output tubes either. That's pretty bizarre, as are the output tubes themselves.
Overall, the circuit could be greatly improved for guitar amp purposes, but would require major rewiring. You could maybe get by with tweaking the tone controls, moving the master volume, and lowering the Negative FeedBack.
To me, though, this amp is a good candidate for 5F6A bassman / Marshall Plexi conversion.
Thanks. my naivete is showing.
Is it too much to ask you (or anyone else) to edit the schematic to show me exactly what to do? What to re-wire?, what parts to replace with what values?, etc. I can email the AutoCAD drawing if it would help.
Iím willing to research the topic and do the work myself, Iím not afraid of major re-wiring, but I just donít have the electronics knowledge to translate the information and diagrams specifically for use with my schematic, and make the best use of the existing parts.
For example, whatís negative feedback? (I assume weíre not talking ebay;)
Thanks for your patience.
Negative Feedback is where a portion of the output signal is "fed back" to an earlier gain stage in the amplifer. It is ideally 180 degrees out of phase with the input signal and it does the following things:
- Makes the gain more predictable because it is relying on passive components to set the gain. Without NFB, the gain can vary as the tubes age or the line voltage fluctuates.
- Reduces harmonic distortion
- smooths out the frequency response
- Decreases the output impedence and improves damping factor.
In the Bogen, the NFB resistor is R42 which feeds some of the output signal back to the cathode of V3A. The gain of the power amp (everything to the right of the tone controls) is determined by the ratio of R42 to R34. By the way, R34 is indicated as 180k, this is really high for this resistor. I believe it should be more like 180 ohms. The more gain stages involved in the feedback loop, the greater the risk of instabilty (the amplifier becomes an oscillator).
You then need compensation capacitors to diddle with the frequency response and the phase response. In the Bogen this would be C25, which bleeds some high frequencies to ground. It was common for tube hi-fi and PA amps to use 20 - 30 db of feedback. In other words the voltage gain with feedback would be 1/10th to 1/30th the gain without feedback. Guitar amps typically use 6 - 15 db feedback (1/2 to 1/5 the open loop voltage gain) and a shorter feedback path. This provides a subjectively smoother overdrive tone. Some guitar amps dispense with NFB altogether. It's all a matter of personal preference but I generally prefer an NFB loop with a presense control.
On your amp, start by increasing R42 to 47k. The will decrease the ampunt of Negative Feedback. For now, leave the rest of the power amp section alone (everthing to the right of the tone controls)
Next, download the Fender Bassman 5F6-A schematic http://www1.korksoft.com/~schem/fend...5f6a_schem.pdf
Look at the 12AY7 on the Bassman. Wire up your V1A and B like the 12AY7. Get rid of all the funky input capacitors and other tone-sucking components in the signal path. If the bassman doesn't have it, you don't need it.
Wire up your V2A and B like the first 12AX7 in the bassman.
For the tone controls, if you want to keep the existing 1 Meg pots, try the Vox AC-30 tone control.
otherwise copy the bassman tone controls. If you can't accomodate a midrange control, you can replace the 25k pot with a 10k resistor.
Put the master volume control right after the tone controls. The wiper of the MV control goes to the grid of V3A. The "Top" of the MV pot goes to the wiper of the treble pot. The "Bottom" of the MV pot goes to ground.
You should be able to dial in clean and crunchy sounds from the preamp. Try jumping the two channels for more preamp boost. For the full-on overdriven tube amp sound, you need to push the output tubes into overdrive, by cranking up the master volume. With the preamp gain controls and the master volume, you should be able to get a good balance of preamp/poweramp overdrive characteristics.
You should also replace the power supply filter caps. The old ones tend to go bad at an unpredictable time. I like to replace all electrolytic caps and often the coupling caps to the power tube as these tend to get "leaky" and screw up the output tubes bias.
Just answered your Email. Sorry it took so long. I have converted many a Bogen PA to guitar amp and they really rock if done right.
The first thing you could decide on is what type of preamp you wish to clone. The poweramp section is fine as is unless you want to add a presence control. Check the link I sent you by Email. Oh,
here it is for any others with interest,
A Fender type preamp is great if that is what you want, they are also easy as pie to clone. With all those extra preamp input tubes not doing anything you could build just about anything. The more complicated though the more chance of
noise and other probs surprising you.
So what is your FAV guitar amp? Some of the easy ones are,
Let us know how it goes :)
Thanks Mike and adamamp for your comments. Iím all for keeping things as simple as possible but, as Einstein said, not more so. Iím currently revising the schematic to show the planned modifications. So far, Iím thinking Iíll implement the 5F6A pre-amp design and the AC-30 tone control as Mike suggests.
A few questions:
Can you specify exactly which ďinput capacitors and other tone-sucking componentsĒ I can get rid of?
Can I use the component values exactly as shown in the referenced (5F6A and AC30) schematics or do I have to change their values somehow to work with my specific ampís power, voltages, tubes, etc.? Iím concerned, for example, that the Bogen shows voltages like 455v, 440v, 350v within the circuit while the 5F6A shows 430v, 385v, etc.
By the way, Iíve revised and attached (zipped AutoCAD drawing) the as-built schematic to include the main power section. (Mike, youíre right, R34 is 180 ohms, not 180k.)
Should I implement a ďstandbyĒ switch? Where should it go in the circuit?
What do you mean by ďjumping the two channels for more preamp boostĒ?
Which ones are the power supply filter caps?
I noticed that the 5F6A has Bright and Normal channels and the only difference between them is that the Bright channel has an additional .0001 capacitor. Can I simple add a selector switch to include/exclude this capacitor on both my channels for selectable Bright/Normal tone?
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