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|1st October 2004, 07:17 PM||#1|
Join Date: Apr 2004
Location: Northeast U.S.
Help with amp rebuild
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'd really appreciate some sage advice.
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.?
|2nd October 2004, 07:44 AM||#4|
Join Date: Sep 2004
Location: Lansing, Michigan
Hi Zeke, from the Great Lakes
Lotsa guys make old Bogens into guitar amps, you can too.
The standby switch looks like it would work OK, but I wonder if it is necessary. If you want a standby, is there just no real estate for separate switches? Unless you have it already, rotary switches that will be happy switching both the AC mains and the HV are probably more expensive and maybe less reliable than toggles. But your circuit looks fine. This is a small amp, so a standby isn't really necessary. Things like cathode stripping are more a matter for larger stuff.
I don't think your boost will do anything. It just puts two stages in parallel. That might be interesting if you wanted to handle more currrent or something, but it won't affect the gain any more than standing on one leg or two would affect your heighth. Wiring the output of the channel 1 tube to the input of the channel 2 tube gets you an extra gain stage. ANother trick would be to separate the two cathodes in the first tube. Put a 1500 ohm resistor to ground from each cathode in stead of the 820. The bypass cap has a large affect on gain in a stage as well as on its freq response. Try it with and without that cap and see for yourself.
Leave the bypass cap at the 250uf on one channel but knock it down to maybe a 1uf or even a .68uf or something on the other. Now the two have different gain and response. Now your switch does a litle more.
The way the old plexis worked was the low gain jack went into one of the inputs like you have, but the high gain jack went into the other stage which then fed the low gain stage. Ther was an extra stage for the high gain jack. The low gain jack had a cutout contact to eliminate any noise drifting in from the high gain stage.
The thing at V5-9 is a stability network. It helps prevent oscillation and parasitics. if you removed it, the amp might still work OK, or it might become unstable.
Why not experiment with the NFB resistor. Listen to the difference with it in and out. Then think about what you might want to do with the difference.
Those litle plate resistors in the output are also there for stability, and depending on their type might even serve as fuses.
Forming caps? I never bother, but other guys swear it must be done. It never hurts. Especially on older caps.
Mylar and other film caps have no polarity, unlike the electrolytics. SOme caps are basically a strip of metalized foil rolled up. There is something called "outside foil" which means the wire lead on one end is connected inside to the foil side on the outer side of the roll. SOme guys make a case that the outer foil should always go to the end closer to ground. In high freq circuits like short wave radios, this might matter. It makes darn little difference here. On film caps, if there is a line or stripe around one end, that means outer foil. Just stick them in any way you like. I like to make all my components face the same direction when I build or repair something, jusy so it looks professional.
You can replace a 40-20-10-10 witha 40-20-20-20. Don't go over 40 for the first two caps though - the ones not in the can.
Now the thing about can caps was that for the manufacturer of the amp they were convenient. Today they are stupidly expensive. if your existing cap is OK, leave it. If it is bad, just replace it with four individual caps - much cheaper. They don't all have to be in one place together.
The line out might work. What is the signal level at that point? If it is 20VAC, it won't be very useful. You could pad it down with a couple resistors though if that happens.
The way you draw the line out won't work though. You have a grounding contact wired in. That means if nothing is plugged into that jack, it grounds the signal line - which is right across your volume control. Eliminate the grounding contact there. Your line out is also a power amp in as wired.
Tube amps MUST have a speaker load if they are producing any output. To run th e preamp out only, you need to either disable the outputs - a switch in the grounded leg of the balance pot would do it. Or remove the signal from the power amp. One way wopuild be break the line between the volume wiper and the followong grid and ground that grid. A switch or relay that connected that grid to either gounf or the volume control would work.
Replacing R45 in the bias supply with a 10k resistor and a 5k or 10k pot in series makes the bias adjustable.
Those 7868 tubes are relics of a bygone age. If someone has started making them again, I don't know about it. If they are not in good shape, I might suggest resocketing for a more common tube type.
|2nd October 2004, 12:20 PM||#6|
Join Date: Mar 2003
|2nd October 2004, 12:37 PM||#7|
Join Date: Oct 2002
Blog Entries: 2
A few miscellaneous comments:
Forming caps: Don't even THINK about hooking up a power supply electrolytic without forming it, especially if it's more than a year old. You might get away with it 9 times out of 10, but that 10th time... let's just say that Russian Roulette is not a particularly safe game to play. Exploding caps can cause (and have done so- I've seen it) severe injury, and sometimes worse. Forming is easy and essentially free, so why not be safe and do it?
Standby circuits: To do these properly takes some planning. You need to consider cathode stripping verus cathode poisoning.
Open circuit loads: It is possible to have an insulation failure on an output transformer if you run the amp at high levels without a load. Sometimes, those high level occur without you feeding signal to the amp as the effective plate load rises and the amp begins to oscillate. OPTs are expensive, so it's not worth taking chances. Always have some sort of load connected with power on. And it's good practice to have a 100 ohm 5W resistor permanently soldered across the output, just in case.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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