Making A Guitar Amp Out Of A Broken 1960s PA Amp (Bogen CHB50)
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- Опубліковано 2 лют 2023
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First video on this amp: • Powering Up A Forgotte...
Need tools? Here's a video showing what I use (with links!): • Must-Have Tools For Wo...
Additional resources for those of you trying to fix one yourself:
• Bogen CHB-50 Tube Ampl...
• Modifying a Bogen CHB-... - Наука та технологія
That tube you are worried about is the phase inverter tube. It is what is responsible for sending the positive side to one output tube and the negative to the other, for the push-pull output. A problem with that tube may be the cause for the asymmetrical clipping you are seeing. It could also be that the resistors going to the grid of each of those tubes are out of spec. I would check the value of all of those resistors and replace accordingly.
As you probably know, the 6C4 tube is the phase splitter tube for the power tubes. It doesn't add any amplification to the power tube grids, it just splits the phase of the signal for the push-pull setup of the power tubes. The tone control tube provides the amplified signal to the 6C4.
The crackling noise that you were talking about may be from dirty tube socket connectors, or it could be from an old carbon composition resistor that's going bad. Those things can be hard to trace to the source. Also, possibly one of the preamp tubes is noisy, that happens sometimes. The background hum is very difficult to get rid of completly with a tube amp...
As for the distortion imbalance on the power tubes that you see in the oscilloscope, it's likely that the tube bias is out of balance. Some amps have a bias balance control, and some don't. According to the schematic, the CHB-50 does have a bias balance control. You can probably adjust the balance to get a more symmetrical onset of distortion using the oscilloscope to adjust it. That may also lower the background hum when the amp is running. It seems that no matter how well matched a pair of power tubes are, they're never exact, and the bias balance helps with that.
I've got a Bogen chb35a I had converted to guitar 🎸 circuitry 15 years ago and it's a great sounding tube amp, using 2 7868 output tubes for 35 watts 👌👍👏
Wow, back in the early 70's I had a Bogan CHS 100. It was my beat up amp for 5 years and it got me out of lots of problems. I litteraly beat the shit out of that amp, yet it contined to get me out of issues when I needed it.
Funny thing was, I sounded realy freakin good ! I did a show one ( My Band ) and we needed another amp being we were outdoors in a wide open field. I used it for side fills. We mixed the sidefills with vocals, guitars and bass and bass drum. It worked great !
I sold it sometime in early 80"s but It was working well then.
Really nice Ten Years After. Thank you.
I had a guitar amp back in 1970 smaller but similar to this one to plug my red hollow body electric guitar into. Nice that you are starting to get appropriate sponsors. Very detailed video.
A pair of 6L6GC's should put out at least 30-35 watts with 450 volts on the plates. Also, the clipping shown on the 'scope should be more closely even on the upper and lower peaks. To see if the tubes are balanced, obtain two resistors of between 10 to 50 ohms, within 5% of each other, 2 watts, and connect one of each in series with the cathode (pin 8) of each 6L6GC output tube. With the amplifier on and warmed up, measure the voltage across each resistor. The current through each tube will be the measured voltage divided by the resistor value. For 6L6GC tubes, this should be between .035 and .045 Amperes. They should also be nearly equal. Also, check the voltages on both 12AX7 and the 6C4 stages. Something may be out of range here. Of particular concern is the 6C4 phase splitter, since its cathode is about 100 volts above ground, and any heater-to cathode leakage in this tube will throw things way off balance. Finally, use a couple of 2.2K resistors to mix left and right stereo channels into one mono channel. tying them directly together may cause severe distortion as their differences fight each other.
Enjoyed seeing the tube vs solid state comparison via the oscilloscope. Thanks!
Good job on the amp. I recently had the same trip. Still messing around with different circuits with it. Learning a lot, and having fun.
When I’m soldering point to point, I always remove the tube before soldering the socket. I had the unfortunate incident of the heat travelling up the pin and ruining the vacuum on the tube. A trap for young players!
I really appreciate the fact that you not only diagnose and repair audio equipment, but that you also know how to PLAY a musical instrument and feed that into your audio components. I admire your full-spectrum talent! I’d say that makes you an audio Renaissance Man! Very well done, indeed!
I noticed that the bass reflex speaker enclosure was very good, but you need to do a couple of minor mods to it, first remove the coaxial driver and, using a trim router, round over the outside surface of the woofer cutout, this will eliminate sound wave defraction around the woofer edges at the front of the baffle, second, do the same thing around the bass port to eliminate port noise/defraction as well, it will sound a lot better, and the sharp edges will look better rounded over as well ! As far as hum/buzzing goes, try shielding those input jacks and running the shielded cables away from any wires that carry d/c or a/c current !, again, great video 😊
Really enjoyed watching this repair/restore. Informative, interesting, engaging. Well done 👍
Great video!, as I mentioned in my previous comments, I just would plug in the audio stereo cables and use the two front controls to mix the right and left signals together, that way I was able to 'play ' with the left and right mix !, this mixer amp is great for you as a guitar player, you can mix music and guitar, or do an accompliment mix with a keyboard track, or drums track !!😊
Oooo, live guitar intro - nice one bud !
i would like to hear some hot pickups electric through this :) and a proper guitar cab.. and perhaps a boost :D
Excellent job!
Great post about vintage 6l6 amp.
Thanks.
for the background noise suspect the anode load resistors on the preamp valves .
Yep, that was the noise on my amp. A 47K 2 watt resistor going to the plate of the 1st AF tube.
Good job 👍
The 604s sounds wonderful with the guitar playing. Careful, those 604's only handle about 30w. Check the grounding solder joints, and replace the pots, make sure there is a grid resistor (1meg) between the wipers and grids.
dirty pots can be the culprit too, as well as tube bias.....cool video
I had 2 of them in stereo with a stereo preamp sounded good
Found this channel the other day and I'm hooked your content is amazing and I aspire to learn as much as possible from your videos. I happen to have a Pioneer SX 1980 that I rescued from a moving job and it's complete but quite rough maybe we could figure something out to get it on a video? I would absolutely love to get it working again. Hope to hear from you.
What a find! Those things are worth big money!
AH👍👍 It didn't work before but now it does. Is it perfect?, probably not but it's way better than before and you still have parts coming. The only thing left to replace are some resistors and a tube, I'd say it's almost a done deal. Maybe use some DeoxiT.... Have fun playing "A walk in the park". Great video, thanks.
That was pretty damn cool.
Seriously! If we all knew how easy it is to make amplifiers, (relatively), we’d of held on to all the old tube amps that used to be everywhere- I’ve had so many old tube amps that I mindlessly discarded- uhhhg- Time Machine please
Excellent video! Probably 60 cycle hum coming from main.
Love your videos, and your passion.
I think PA amplifieirs were rated at peak power, not RMS which the DVM is reading. Also distortion was a lot higher, perhaps 3 to 10 percent THD. I had a carbon Allen Bradley style resistor with made a sound like what you are hearing that was noisy feeding the B+ to the first AF triode of a 12AU7A. Dirty tube sockets can produce that noise as well, move the tubes around and listen for the crunchies and watch the waveform. SRT Amplification already said what I was thinking, so I won't repeat that wise advise.
I wanted to add, a cold solder joint can be noisy like what you are hearing as well.
Interesting enough, I get all my tube amp parts from Antique Electronics, it's very seldom that they don't have something I need for a project amp.
Bogan amps are a good platform for converting to a guitar amp. There seems to be a lot of them available still on the secondary market. My personal favorite for conversion is the CHB-35. I always change all the power supply capacitors. With the CHB-35, I usually take out the big 9 pin sockets for the 7868s, install octal sockets, and use 7591As. I tend to replace a lot of the old carbon comp resistors with carbon film ones, or sometimes metal film ones. Also, I usually rewire the preamp tube sockets to use the 12AX7 tube instead of the 6EU7. The 12AX7 is a common guitar amp preamp tube. Of course, that all takes more work...
Unsoldering parts in a Bogan amp is a pain in the butt!
In your experience did the PAs have 220ohm plate resistors? If so do you change them to the common 100ohm and rebias?
@@davidsimpson3380 Often enough, the first input stage circuit of the 6EU7 will have a 470Kohm plate resistor, the rest of the preamp stages tend to have 220Kohm plate resistors.
When I rebuild the preamp to use 12AX7s, I generally follow what Fender did, using 100Kohm plate resistors. Usually thetone control tube is already a 12AX7, I change the tone capacitors and resistors to resemble a late Ampeg tone circuit, which usually has 220Kohm plate resistors on that tube. You don't have to change the tone circuits at all to convert one of these to a guitar amp, it's just that the bass and treble control frequencies are set up in the Bogan for a more all purpose generic sound control. That's not a bad thing, it's just not specific to the range of as guitar.
@@amberyooper Thank-you for the detailed reply. I've been converting a LaFayette pa-35 which is similar. I put bypass caps on one of the gain stages and the amp was very gainy. I realized the gain was from the plate/resistors/12ax7 biasing, so I put the bypass caps on a switch and had wondered about changing the plate resistors/biasing scheme for the 12ax7s but the amp has a lot of wonderful sounds so I'll probably leave them for now. I wrestled with the 37v bias tap and switched the selenium rectifier for a 1N4007 and rebiased. It's quite a great little amp. Sounds like a JTM 45 to me, I may build a combo cab for it. Thanks for sharing your adventures! ......... also, the power transformer puts out 375v and plate voltages are now about 477vdc @38ma after rebiasing, what kind of plate voltages are you seeing?
@@davidsimpson3380 It seems that Bogan set up their amps to run the main voltage at somewhere between 450 volts and 480 volts for a lot of their amps, depending on the power tubes used in the amp. The CHB-50 schematic that I have shows a voltage of 440 volts on the plates of the power tubes and 420 volts on the screen grids. Those are just average voltage readings for amps made back in 1960s, based on a wall voltage of 110 volts. The wall voltage is higher now, so the voltage in the amp will also be higher. The 6L6GC power tubes will handle the voltage in your amp with no problem.
Speaking of the Lafayette amp, I rebuilt a Lafayette PE-30 a couple of years ago for a friend. That turned out to be a pretty nice amp.
@@amberyooper Thanks for the information amberyooper :) The Lafayette is turning out quite nicely and after making a bias probe I've got it dialed in to about 445v at the plates. Used a large 10 watt resistor between B+ 1 and the screens. Happy amping !
That thing was a total mess inside. Do a total restomod and properly set the components on a circuit board and give the covers a proper wash and make it pretty.
I think that's rated 50W input. 50W output was a lot back in the 60s even for solid state amps. I doubt many modern tube amps output that much. A small amount of wattage can translate to a lot of sound. Check out Pearl Acoustics' "How loud is 1 Watt (with an 87db Loudspeaker)" video about input power vs. sound pressure. ua-cam.com/video/vaiEDYB5c9M/v-deo.html . It looked like only the positive side was distorting at 10V output, so you maybe could get a little more if that was fixed. Is there some imbalance somewhere? It's a push-pull amp, but you replaced both amp tubes. Maybe one of the 100K resistors is out of spec?
6l6gc's have a plate dissipation of 30 watts. That does not mean you Will get 30 watts of audio per tube. And that is with cathode bias. Grid 1 bias for a push pull cirquit is 70 percent of 30 watts. Depending on how much the power supply is capable of putting out in Milliamps and high voltage.
Question. How are the output tubes wired up in that amp? Parallel Class A. Push Pull Class A B.
How are the tubes biased by cathode or grid number one? Depending on type of biasing you may be able to get more than 12 watts of audio output but don't go over Maximum plate dissipation with Cathode biasing. With class A B go no higher than 70% of Maximum rating for those 6L6GC.
Read more about tube biasing. I'll bet your a smart young man. I guess I could say you already know. Thanks for the video hear on Utube. Hope this helps. In grid1 biasing for class AB the negative voltage, make it very reliable and be careful. Or they will go warp drive into redplating especially in class A B. Glad to see a young man interested in tube amplification. Hope to see you down the road again here on UA-cam. Dwight.
..i'd Love to change the world, ..but I don't know what to do, ..
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Do you not have an electric guitar at least for testing? Get a cheap single could equipped guitar to test your work, dude
If no one has mentioned this yet, you’re going to kill yourself by touching components like this. Are you sure those caps are drained? If yes, you didn’t mention that or show how you did it. Use a chopstick to point to things instead of your finger. You can always tell when someone hasn’t had electronics training (or they’re dumb) when they touch components and circuit boards.
Good job 👍