Hola- Bogen CHB35A
Hope all is good neighbors! I'm excited to start a new project and this seemed to be the community where I was most likely to find direction.
So hello I'm Mastaphat. I'll be your noob for the day.
I have a soldering station and I have experience building jumper cables and replacing the parts in guitars and basses but I won't pretend to understand the science or theory as to why things are wired the way they are. I just have blind faith and wiring diagrams...Hey! I've gotten by this far.
Last week I was gifted this Bogen CHB35A. It doesn't seem to be modified but all the tubes are missing and I have no idea what kind of shape it's actually in but it doesn't seem terrible.
I want to build something sonically AKIN to Fender 5e3 or 5f6a. I don't play country but I have a very country/jazz tone. Hence the 5e3.
Preferably 35-45 watts or as close as I can get to 45 out of the stock parts.(assuming they're still good). But only 1 channel and with a gain control. If all I had was a 1 gain, 1 volume and 1 tone control I'd be happy. A standby switch wouldn't hurt.
How feasible is this?
Should I post about this in a different subforum?
Hope all is good neighbors! I'm excited to start a new project and this seemed to be the community where I was most likely to find direction.
So hello I'm Mastaphat. I'll be your noob for the day.
I have a soldering station and I have experience building jumper cables and replacing the parts in guitars and basses but I won't pretend to understand the science or theory as to why things are wired the way they are. I just have blind faith and wiring diagrams...Hey! I've gotten by this far.
Last week I was gifted this Bogen CHB35A. It doesn't seem to be modified but all the tubes are missing and I have no idea what kind of shape it's actually in but it doesn't seem terrible.
I want to build something sonically AKIN to Fender 5e3 or 5f6a. I don't play country but I have a very country/jazz tone. Hence the 5e3.
Preferably 35-45 watts or as close as I can get to 45 out of the stock parts.(assuming they're still good). But only 1 channel and with a gain control. If all I had was a 1 gain, 1 volume and 1 tone control I'd be happy. A standby switch wouldn't hurt.
How feasible is this?
Should I post about this in a different subforum?
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Moderately feasible, but probably not a beginner project.
Were it mine, I'd power it up carefully to test that nothing is terribly messed up. After a visual inspection, I suggest using Paul Ruby's first-power up steps (Paul Ruby Amplifiers), but include a light-bulb limiter (http://www.geofex.com/article_folders/spo_test.htm) inline with the power cord to avoid catastrophic failures. If it tests out as functional, then run a guitar into the the MIC#1 channel see if you like the the tone when driving a guitar cabinet. In it's stock form, the CHB35A is quite a bit more powerful than a 5E3, but you might get away with a robust set of 6V6 tubes and some adjustments to the bias circuit. If you don't mind the extra power, however, you could probably tweak the circuit to voice it as desired without a lot of major surgery.
Stph
Were it mine, I'd power it up carefully to test that nothing is terribly messed up. After a visual inspection, I suggest using Paul Ruby's first-power up steps (Paul Ruby Amplifiers), but include a light-bulb limiter (http://www.geofex.com/article_folders/spo_test.htm) inline with the power cord to avoid catastrophic failures. If it tests out as functional, then run a guitar into the the MIC#1 channel see if you like the the tone when driving a guitar cabinet. In it's stock form, the CHB35A is quite a bit more powerful than a 5E3, but you might get away with a robust set of 6V6 tubes and some adjustments to the bias circuit. If you don't mind the extra power, however, you could probably tweak the circuit to voice it as desired without a lot of major surgery.
Stph
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No reason to change to 6V6's, they are remaking the 7868's. The 6C4 splitter is not something you pick up at the corner music shop but it is easy enough to get in the mail. The same goes for the 6EU7's although if one were inclined it could be changed to a 12AX7 by rewiring the socket. You will be wanting to rewire the input to be a cathode bias rather than self bias stage. This involves adding a cathode resistor with bypass cap and then changing the plate resistor.
The volume controls should be changed to resistive divider rather than the signal coming into the wipper. Maybe change one of the 0.010 uF coupling caps to a higher value and have more bass on that channel. Use the 470k resistors after the pots to form a mixer for the two channels then directly into the next stage rather than the master volume. The next stage is fixed bias, might be ok for PA but maybe too stiff for guitar. A 1.5k resistor and with 100k resistor on the plate. Same goes with the following stage. The tone section could be used for guitar with some part changes or scrap and go to a Fender tone stack, either the Blackface or Bassman circuit. For the tone controls as is, Traynor used it if memory serves me.
The NFB may need to be adjusted to taste. The filter caps should probably be replaced by now. You really should know more than place-part-here. A good place to start is this.
http://ax84.rru.com/media/ax84_m35.pdf
http://ax84.rru.com/media/ax84_m37.pdf
Then watch some Uncle Doug's amp vids online.
http://makearadio.com/schematics/images/bogen-chb35a-6.jpg
The volume controls should be changed to resistive divider rather than the signal coming into the wipper. Maybe change one of the 0.010 uF coupling caps to a higher value and have more bass on that channel. Use the 470k resistors after the pots to form a mixer for the two channels then directly into the next stage rather than the master volume. The next stage is fixed bias, might be ok for PA but maybe too stiff for guitar. A 1.5k resistor and with 100k resistor on the plate. Same goes with the following stage. The tone section could be used for guitar with some part changes or scrap and go to a Fender tone stack, either the Blackface or Bassman circuit. For the tone controls as is, Traynor used it if memory serves me.
The NFB may need to be adjusted to taste. The filter caps should probably be replaced by now. You really should know more than place-part-here. A good place to start is this.
http://ax84.rru.com/media/ax84_m35.pdf
http://ax84.rru.com/media/ax84_m37.pdf
Then watch some Uncle Doug's amp vids online.
http://makearadio.com/schematics/images/bogen-chb35a-6.jpg
I recommend the updated AX84 P1-eX guide because it corrects a number of errors: http://ax84.com/p1x/p1-ex-theory.pdf
Stph
Stph
Nice. Thanks to everyone for all of your advice. I posted this on r/diytubes too. One person there mentioned the output being a ¿voltage doubler?. Sounded as if that might be uncommon for a guitar amp will that require some extra attention/modification? I have a small collection of tubes, mostly preamp, but should I go ahead and plan to buy the remainder of the tubes this amp requires before I dive in?
Here's a link to some photos if you care to look. Bogen CHB3A - Album on Imgur
Here's a link to some photos if you care to look. Bogen CHB3A - Album on Imgur
> the output being a ¿voltage doubler?. Sounded as if that might be uncommon for a guitar amp
Matter of fashion and history. And relative costs of diodes and production changes. MANY 1950s TV sets used voltage-doubler supplies, and the plan worked fine. Bogen knew what they were doing. Leave the power supply alone.
Matter of fashion and history. And relative costs of diodes and production changes. MANY 1950s TV sets used voltage-doubler supplies, and the plan worked fine. Bogen knew what they were doing. Leave the power supply alone.
I was a member of the Aussie Guitar Gearheads forum for a while, and I learned that a number of vintage Australian valve guitar amps used voltage doubler power supplies.
The main reason for this was the type of valves used for the output devices. Those Aussie amps used (relatively) newer valves, sometimes called TV sweep tubes, and those newer valves required much less voltage on the screen grid than on the anode.
Using a voltage doubler was an easy way to provide this: the anodes got the doubled voltage, while the screen grids got half that.
-Gnobuddy
I just finished reading the pdf suggested by Printer2 and stephen_keller. It was very informative. Thanks guys. Uncle Doug helped a lot too. I watched his series on the pre and power sections. Funny thing I thought the name Uncle Doug was familiar. Turns out I watched his video on reconing speakers sometime in the past couple of years. I've reconed a p12q, c12k and a Frankenstein g12H thing. Pairs nicely with my Delta Blues.
Anyways after reading the pdf and taking a few pages of notes I referred back to the Bogen schematic posted by Printer2. A few things are confusing me.
1. The cathode as well as the plate has a 240v charge. Why would that be?
2. The upper secondary winding of the PT has a lone diode putting off a -23v. Any hints?
Regardless I plan on reading what Paul Ruby has to say and adding a light bulb limiter as stephen_keller suggests. If it works I'll just add-in only what's necessary to play guitar and go from there. If I have to overhaul it just to get it to work I'll probably plan to turn the preamp into something akin to either the 5e3 or A 5f6A. Either way I'd have to go with only one channel and one input, gut the mag and aux inputs as well as move the tone controls. I may do all this anyway in the long term but ideally I'd like to take this slowly, maybe play a little RnB and really get to know 'er.
Anyways after reading the pdf and taking a few pages of notes I referred back to the Bogen schematic posted by Printer2. A few things are confusing me.
1. The cathode as well as the plate has a 240v charge. Why would that be?
2. The upper secondary winding of the PT has a lone diode putting off a -23v. Any hints?
Regardless I plan on reading what Paul Ruby has to say and adding a light bulb limiter as stephen_keller suggests. If it works I'll just add-in only what's necessary to play guitar and go from there. If I have to overhaul it just to get it to work I'll probably plan to turn the preamp into something akin to either the 5e3 or A 5f6A. Either way I'd have to go with only one channel and one input, gut the mag and aux inputs as well as move the tone controls. I may do all this anyway in the long term but ideally I'd like to take this slowly, maybe play a little RnB and really get to know 'er.
I assume you are talking about V2A. I'm not sure why Bogen would run that stage in parallel with a fixed resistance. I'm guessing it is an attempt to semi-fix the cathode bias of that stage. The 220K resistor and the 680 ohm resistor (along with the tube an its load resistor) form a divider that sets the cathode voltage of that triode at roughly .8 volts. It is not fixed because it will still vary slightly with the signal swing, but it might be more stable than without the parallel resistance. Seems like a bypass capacitor would have been be more stable choice, but maybe those were too expensive at the time.
That is the negative bias supply for the output tubes.
Stph
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Thanks for clearing that up.
Regretably, it'll be another week or more before I have enough time and money to build the light bulb limiter and to get a decent multimeter. So I'll have to quell my excitement for the moment.
It seems that spending ~$50 for a used Fluke is the way to go. So that's my current plan. I have an old analog meter from radio shack but it's been banged up and unused for years in the bottom of a box.
In the meantime, have a 40w Weller soldering iron but only one tip that I've used forever. Will this do or should I plan to invest in my soldering equipment as well? Here is a link to what I currently own:
WELLER Station, Solder, 5-40 W - 5JH84'|'WLC100 - Grainger
The ax-p1 pdf was a good read. I'm eager to further my understanding of amps/circuits so I'm open to suggestions for what I should read and learn from next.
Regretably, it'll be another week or more before I have enough time and money to build the light bulb limiter and to get a decent multimeter. So I'll have to quell my excitement for the moment.
It seems that spending ~$50 for a used Fluke is the way to go. So that's my current plan. I have an old analog meter from radio shack but it's been banged up and unused for years in the bottom of a box.
In the meantime, have a 40w Weller soldering iron but only one tip that I've used forever. Will this do or should I plan to invest in my soldering equipment as well? Here is a link to what I currently own:
WELLER Station, Solder, 5-40 W - 5JH84'|'WLC100 - Grainger
The ax-p1 pdf was a good read. I'm eager to further my understanding of amps/circuits so I'm open to suggestions for what I should read and learn from next.
240V is going "toward" the cathode, yes. But via a Voltage Divider.
You MUST understand Voltage Dividers before all else!!
The divider is 220,000 and 680. So voltage is reduced more than 200:1. A fine calculation says 0.74V before the 12AX7 comes into play. Not "240V".
We can estimate the voltage dropped across a 12AX7 with 0.74V bias, but current in the tube will lift the 0.74V somewhat. So a bit of circular recursion (try try again) is required. Complicated by 12AX7 being fairly non-linear on the large-scale. I guessed some values from too much experience.
Then I remembered Bogen always posted voltages. While hard to squint in this scan, they agree within a ballpark.
Attachments
Again, Voltage Divider!!
The main B+ is 450V from a Voltage Doubler (which is no real relation to a Voltage Divider). Using a non-doubler rectifier we expect 225V DC. *But* there is a 56k:15k resistor divider, so about 1/4th, so say 55V. But the 39k after the divider is another load. And because the diode conducts in bursts while the 39k sucks all the time, this is not a simple voltage divider. While it can be calculated, the usual design is to find a similar plan, steal it, and then tinker to fit.
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