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Old 15th May 2004, 12:23 PM   #11
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Zeke,
Quote:
Can you specify exactly which “input capacitors and other tone-sucking components” I can get rid of?
I'm looking at the schematic you posted in Word Format, I don't have anything that reads AutoCad. The capacitors I am referring to are C1, 2, 9,10,11, 12,13 and resistors R13,14,15,16. Also the components associated with the aux input R17,18 and C14.

Quote:
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 amp’s power, voltages, tubes, etc.? I’m concerned, for example, that the Bogen shows voltages like 455v, 440v, 350v within the circuit while the 5F6A shows 430v, 385v, etc.
I wouldn't worry too much about the differences in voltages. The voltages on a Fender schematic are very much a "ballpark" figure anyway,with many schematics indicating a +/- 20% tolerance on voltage readings. The Bassman preamp will work over a fairly wide range of voltage.

Quote:
Should I implement a “standby” switch? Where should it go in the circuit?
The standby switch is more a convenience feature rather than an absolute necessity. I like to have one in my amps. I generally place the switch after the main filter caps as in the Fender AB763 schematics. See http://www1.korksoft.com/~schem/fend...b763_schem.pdf
for an example.

Quote:
What do you mean by “jumping the two channels for more preamp boost”?
The bassman has 4 inputs, 2 for each channel. You plug the guitar into the bright channel, input 1, then run a patch cable from input 2 to the normal channel input 1. Then you can crank both channels gain controls to overdrive the next stage. The two channels have a slightly different frequency response and you can use this effect as a tone control. For a greater difference between the two channels, check out the Marshall Plexi http://www1.korksoft.com/~schem/mars...1987_plexi.pdf
Here, Marshall uses separate cathode resistors for the input stages and a bypass capacitor on one of the 470k mixing resistors, as well as different cathode bypass capacitors and coupling capacitors for the two channels. Some folks find the plexi "bright" channel too bright and revert to a more bassman-like configuration. It all depends on the rest of the amp, speakers, guitar and personal preference. You could put in a switch to jump the channels and avoid having so many input jacks. I use a footswitch that connects the guitar signal to the second channel for a "lead boost". The first channel is always connected.

Quote:
Which ones are the power supply filter caps?
Usually the physically largest caps in the amp. You may have a multi-section cap, which is 2 or more caps in one large can. These can be hard to find these days and when you do find them, they tend to be expensive. If space permits, I use either radial or axial caps inside the chassis. You can leave the can cap in the amp if it looks cool to you, just disconnect it.

Quote:
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?
Yes, you can put this cap on both channels and make it switchable, like the AB763 era Fenders.
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Old 20th May 2004, 12:04 PM   #12
zeke is offline zeke  United States
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Thanks for all the great advice.
Now I have to process it all and get to work.
I'll post the results.
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Old 27th May 2004, 08:45 PM   #13
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Hi, I noticed this thread about converting a Bogen PA to a guitar amp, and I thought I'd ask a similar question: Can a PA amp be "converted" to be used as a "hi fi" amp? The reason I ask is because I recently aquired a pair of mono 10W Bogen Challenger PA amps. They have an RCA input labeled phono, and a screw-on type input labeled Mic. There are three pots:Volume for MIC, volume for phono, and Treble. There is a 3 pronged speaker socket on the back, as well as a standard screw terminal speaker output for 4, 6, and 8 ohm as well as one labeled 24V (?). Each amp uses two tubes, a 12AX7, and a 7868 (Sylvania). I hooked up a portable MD player and a Discman to the single RCA connecter on each amp labeled phono, and connected a pair of Paradigm Titan speakers. The amps I think sound pretty decent, but not especially sweet or special. The sound is pretty clear, maybe with a bit of low hummy buzz due to the fact that they may need new caps. If turned up much beyond the halfway mark on the volume, they distort a little, but only on loud, dense, or bassy music. The distortion is more like and overdriven sound on a guitar amp.

So I have a few questions for the people that know more about this type of thing than me:

What exactly is the difference between a PA amp and a "hi fi" amp?

I don't have a schematic for these, does anyone know where I could get one? Although they are very simple, I could probably draw my own schematic by tracing the wires.

Since the input is labeled phono, I assume this means it's for a turntable. But I plugged a Discman at full volume into it and it didn't seem to have a severely distorted effect like I would expect when you plug a line level input into a phono pre. Like I said, only slightly overdriven when at near max volume. Could amps like these have some sort of built in phono preamp? I have not tried plugging a turntable directly into it though.

I'd like to rebuild these and modify them if possible to be optimized for music listening as a mono block type set up with a preamp of some sort. Could I or should I disconnect the Mic input to improve sound or eliminate any unneeded parts? Maybe there are some resistor values to be changed or removed (or added) somewhere? Maybe with capacitors too? Of course this is assuming there is some difference in a PA amp and a hi fi amp.

Since I got these for very cheap, I don't expect to make anything super amazing out of them, but I figure there might be some things I could do to improve their sound a bit and use them as a small stereo for a small bedroom to be used at medium and low listening volumes. Even if I replaced a bunch of parts this would still cost me less than $100 for a little mono block tube stereo/bookshelf type system.

Last question: Am I crazy for thinking I could make a good sounding little mono block stereo out some junky little PA amps?

This is my first post in this forum, as I just registered, so I hope I haven't intruded on this discussion or anything. There was a lot of useful info here already on going from a PA to a guitar amp, so I thought someone might now how to go the other way. Thanks for any help anyone can give me.

Kevin
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Old 28th May 2004, 07:35 AM   #14
SHiFTY is offline SHiFTY  New Zealand
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Quote:
so I hope I haven't intruded on this discussion or anything.
Ban him! Nah, just kidding, but you should have started it in a new thread.

Quote:
What exactly is the difference between a PA amp and a "hi fi" amp?
Often PA amps are designed fairly conservatively, so they can be left on for 20 years and not die. No kidding. But many of the best amps (example: LEAK TL12 point one) were used for PA but were also very good for hifi.

In my opinion, you would be best off stripping back all of the old components and building a "modern" design. You only need to change passive components like caps and resistors, build a much simpler design with perhaps 10 components per channel. If the amps already work, you're halfway there.

The 7868 is a bit of an oddball, but it is easy to drive. You could possibly chuck in a EL84 instead, but it has a different pinout.

If your 7868s are a bit weak, there will not be too much power. The amp should go fairly loud before distorting (depending on speakers of course.) If they seem to have a fair bit of grunt then the 7868s are probably ok.

I would adapt a SE 12AX7- EL84 schematic. There are plenty on the net. You could probably get away with 1/2 12AX7 (its a dual triode in one glass) into a 7868. Very conventional but will work just fine.

If you have a digi cam, post photos of the top and the wiring, you can attache them to your posts.

IMHO, you have a good little project there, well worth doing.
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Old 28th May 2004, 11:51 PM   #15
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Thanks for your advice ShiFtY. It's encouraging to know that these little amps would be worth the trouble to rewire and rebuild. Since the 7876s are hard to come by it seems, and are pricey, I think the sensible thing to do, as you suggested, is to adapt a 12AX7 EL84 design to what I already have. But I don't have the experience to know what would be appropriate. I'll try attaching some pics of the bottom wiring and top layout, and if you have a suggestion or a link to a suitable design, that would be great.

I'm not a complete novice, I know how to solder and not electrocute myself inside a tube amp, but this is my first experimental rebuild, so I need help finding a design. A parts list would be a big plus as well. Again, thanks for the help on this.

Kevin
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Old 28th May 2004, 11:52 PM   #16
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Heres the bottom pic, I don't know how to include more than one pic in a post, sorry.
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Old 22nd July 2004, 10:52 PM   #17
zeke is offline zeke  United States
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Default Bogen M330a for Guitar

Hi,

Thanks for all the suggestions.

Here’s the Bogen M330a schematic I modified to implement the Fender 5F6A pre-amp and tone control. It’s a zipped WORD document with an embedded AutoCAD drawing. When you zoom in close the resolution get choppy but it should be clear enough to see what I’ve done.

Any comments or suggestions for improvements?

Do I have the 6EU7 stages (V2A and V2B) wired correctly between them?
Attached Files
File Type: zip bogen m330a mods no power section 22july2004 b&w.zip (88.5 KB, 122 views)
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Old 23rd July 2004, 04:00 PM   #18
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Zeke,
I noticed a couple of errors in the schematic
  • V1A & B Cathode resistor is 320 ohms - should be 820 ohms
  • The cathode cap is a 6v device. That's what is on the Fender schematic. However, you should use at least a 25v device here.
  • Volume control pots are wired backwards - wipers should go to 270K mixing resistors. Optionally a 100pf cap can be added accross the ungrouded pins of one of the pots for a "bright" channel.
  • The 470k resistor on the Aux input should go directly to pin 5 of V1B
  • V2A Cathode resistor is 320K ohms - should be 820 ohms
  • V3A Cathode resistor is 22K ohms - should be 2.2k
  • V3B Cathode resistor is 820K ohms - should be 820 ohms
V3A could be improved by changing the operating point. Change the plate resistor to 100k, cathode resistor to 1.5k.
This will
  • Increase the current through the tube
  • Lower noise
  • Improve input overload margin
  • Increase voltage swing at the plate.
  • Lower the impedence (a little)
  • Be more in line with the Fender amps that used a similar type of circuit.
  • Probably sound better. But maybe not!
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Old 24th July 2004, 01:29 PM   #19
zeke is offline zeke  United States
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Thanks, some typos and some just plain wrong. This forum is great!
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Old 30th September 2004, 08:03 PM   #20
zeke is offline zeke  United States
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Hi.

I'm the guy re-doing an old Bogen M330A PA tube amp for guitar use.
I think I'm ready to start wiring but I'd like a sanity check and I have some questions.

I'm attaching two pdf's of the circuit. One is the power supply circuit, the other is the rest of the circuit.
The inputs, preamp and tone controls are basically Fender Bassman 5F6A.
The power amp and supply section are the original Bogen layout.

What do you think of the "standby" approach? (3-pos. 2-circuit rotary switch) Will my scheme work?

How do I do a "preamp out" circuit? As shown, won't it leave the amp "unloaded"?

Does the "Boost" switch make sense?

The 1.5k resistor and two caps between V5-9 and ground are wired into the unit but are not on the original Bogen schematic. What are they for? Do I need them?

What would happen if I omit the 47k NFB resistor (R214)? I've seen other similar schematics without it. Should I replace it with a pot? What value?

What are R53 and R54 for? Again, other similar schematics don't have them? Should I omit them?

All the passive components are new (Antique Electronic Supply). Orange drop and Atom caps, metal film resistors, including a new 4-gang multi capacitor aluminum can thingy (C33A, B, C, D). Do any of these caps have to be "formed"?

Some caps (e.g. orange drops) aren't marked with polarity. Does that mean it doesn't matter and I can install them in either direction?

The original values for C33A-B-C-D are 40-20-210-10 mf respectively but the closest equal or larger values I could find in one aluminum can are 40-20-20-20 mf. What difference wil this make?

Any other comments, observations, corrections, etc.?
Thanks.
Attached Files
File Type: pdf bogen m330a schem mods nopowersupply 30sep04.pdf (99.9 KB, 149 views)
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