Using old Bogen PA amp for guitar

[cjb92861]

Sonically speaking, what’s the downside of not converting a Bogen CHB-100 P.A. amp to a “guitar amp,” but still using it with a guitar?

I was considering just adding ¼” input and output jacks and running it into a speaker cabinet for a clean tone. I’m not looking for break-up/overdrive, just a clean signal. If I want distortion I’ll tweak the signal with my Line6 POD.

(I plan on installing a grounded plug too...)

Thanks in advance.

 

[6942]

Webster PA is my guitar amp

I found a working 1969 Webster Pa amp (6V6 PP - 12 watts) at a local yard sale.
I replaced the can caps/filter caps, but that's pretty much it.
It has an Amperex Bugle Boy EF86 front end and quite a large output transformer.
Works great as a clean amp, though at higher volumes, it can sound pretty gruff (in a nice way) through a 1960 Jensen P12R speaker.

Steve

 

[bdgregory]

It works fine (I've done it), but it's a little bland. However, if put a pedal in front of it and it can be really special. I also have a CHB-50 and CHB-35 which I have used more than the CHB-100. I have a modeling preamp/pedal I use and it sounds amazing - I can get crystal cleans, trashy grunge, great reverb/effects, and anything else you can imagine.

I've also converted a CHB-50 to guitar use (pulled inputs and other non-essential circuits out, reconfigured gain stages, changed the tone stack. I'm not satisfied with the results there yet and continue to fiddle with it. For this reason I recommend the first approach unless you really want to get your hands dirty.

 

[cjb92861]

Thanks!

 

[vanman]

help: problems after changing output tubes

this is my first post. I'm a guitar player and have started converting old PA amps into guitar amps. My current project is a Bogen challenger chb-35.
It was working fine with the old RCA 7868 tubes. I put in a pair of used sylvania tubes. the amp started crackling, with one tube flashing blue with the crackles. amp then lost volume and tone. put the original tubes back in and amp did the same thing. don't know what happened. amp works fine for a couple of minutes and then crackles and loses tone and volume. could this be the filter caps? haven't changed them yet, waiting for parts. Plate voltage on the 7868's is 490 volts. schematic calls for 450 volts. thinking this is because the line voltage is higher now then back then. any help would be appreciated. thanks.
other changes to the amp:
made the first stage cathode biased,
changes in the tone stack, still working on this,
was going to change the PI from split load to LTPI, change the 6C4 to either a 12AX7 or a 6j6 which would fit in the existing socket,
will move the master volume, mayber after PI?
lower plate voltages to specs? how?

soo much fun.

 

[Gunny]

with the power on, check the back terminal strip for high voltage DC. Stupid design fault. Disconnect anything that's 'hot' from that strip.

I've cascaded the tube stages in that amp a couple of times. Sounds OK.

 

[vanman]

yeah, I already disconnected that. tried to remove all unecessary stuff. I also just cascaded the first 2 stages. not sure if it will stay that way. I used the mag switch with cascade on one side and parallel on the other. not quite working right yet. What do you think about the problems in my power stage?
thanks.

 

[vanman]

since I'm new here. wanted to ask. should I post this as a new thread to get more responses?
thanks, paul.

 

[Dr.Ow!]

Vanman-

I have a homebrew Club Deluxe with a 117v pt. The B+ was of course too high and I needed to get the line volts down for something usable. While deliberating pro's and con's of VVR or a variac I stumbled into mentions of 'poormans variac'. That's what I've been using and works great and is as simple as can be. It is the same as the light bulb limiter. I use 2 bulbs so I can play with bulb combos giving a great range. Currently I am using 2x100w and nets the exact heater voltages. I have browned it down with 100w and 60w also.
The most important thing to pay attention to with old and lower V pts is to do whatever needed to get the heater voltages in spec or near to it. Then go from there to modify the B+ and rail volts as/if necc.
There could be some downside to this approach but I haven't found any mention. Before cheap-ish Variacs were available this was a common homebuilder radio/ham buff method to control V's, as far as I've heard anyway.

 

[Rich_S]

My main (only) amp started as a Bogen CHB20A PA head, but I converted it to an 18 Watter Lite IIB (see 18Watt.com for description and schematic. This was basically a complete rewire. 12AX7 input stage, MArshall EQ, LPT phase inverter. It's really only a 13-Watter, since it retains the ECL86 output tubes of the Bogen.

Vanman: Bogen ran these amps at ridiculous B+ voltages. For one thing, they didn't care about guitar tone - they wanted to get as much power out of these amps as they could. As-is, they sound tight and sterile for guitar. Also, they didn't care about the reduced tube life. Back then, everybody just accepted that tubes had to be changed. Tubes wearing out was the user's problem, not Bogen's.

That CHB35 is, what, 35 years old now? It needs filter caps, no question. If they haven't gone yet, they will soon.

Disconnecting unused sections of the amp is going to drive the already-high B+ even higher.

In my amp, the B+ was nearly 450 volts - way high for a little EL84 wannabe. To get it down to normal Marshall levels (~325 volts) , I put a string of 11 10-volt zeners diodes in series with the DC supply, between the first filter cap/voltage doubler and the B+ line to the output stage. Pic of the zener string attached.

 

[scantron81]

I have a CH35. Fantastic head. Sounds like nothing else. Breaks up really well with my Avatar 2x12 Celestion Greenback cab.

 

[bdgregory]

In my amp, the B+ was nearly 450 volts - way high for a little EL84 wannabe. To get it down to normal Marshall levels (~325 volts) , I put a string of 11 10-volt zeners diodes in series with the DC supply, between the first filter cap/voltage doubler and the B+ line to the output stage. Pic of the zener string attached.

Rich_S - for us naivete's, can you elaborate on the use of the string of Zeners? Why that rather than a power resistor or choke? Also, is there a consistent voltage drop across each Zener regardless of specific type?

thanks,

 

[Rich_S]

Zeners are a special type of diode that break down at a certain, fixed reverse voltage. Unlike normal recifiers diodes, zener are supposed to be reverse-biased and are supposed to break down, thereby providing their magical fixed voltage drop. So, a 10-volt zener will have 10 volts across it, and that 10 volts won't change with the load current going through it.

By contrast, a big power resistor will drop voltage, but in proportion to the current flowing through it. With no guitar signal, the resistor will drop a certain amount, and that voltage drop will increase with load as you play the guitar louder, making the amp saggy. Some sag in a giutar amp can be good, but the decoupling resistors between power supply stages usually provides an amount that sounds "right", dropping large amounts of B+ across a resistor would probably make the amp too saggy.

People often put zeners in the CT leg of a power transformer to adjust its output a bit, but the CHB20A doesn't use a center-tapped PT, it uses that voltage-doubling power supply (another high-production-volume cost saving measure I assume). So, the only way to drop 110 volts off my B+ was brute force - run B+ straight through a bunch of zeners to drop it. It's better to use (11) 10-volt zeners than one big one because the combined string handles more power (11 x 1watt). A single 100-volt, 10-watter zener would probably do the trick, but that's an uncommon and expensive piece. 10-volt, 1-watt zeners are easy to find, adn dirt-cheap. Also, using zeners in 10-volt increments make it easy to tweak the final voltage. If it's a bit off, just add or subtract diodes until you have it right.

 

[bdgregory]

thanks Rich, this is very helpful . . .

 

[vanman]

Rich,
thanks for the info on the Zener diodes. At one point I put in a 1K 5W resistor and it did drop the voltages to spec, but distorted the output. Don't really understand why. I'm learning by trial and error, seems like a lot of error..... oh well. I was thinking about putting resistors on each different leg of the power supply to drop each one, ie. supply for preamp tubes, power tubes, etc.
Do the zener's have polarity? I might be misunderstanding the placement of the diodes. Looks like you have the red wire from the PT at one end, and then the other should connect before the filter cap section and bias supply take-off?
I did replace the filter caps and it resolved my problems with the output tubes.
The amp seems like it has potential to be a great sounding guitar amp.

 

[vanman]

resistors: carbon comp or carbon film

I have been using carbon comp resistors but noticed that in a lot of high end amps they have these pretty blue ones which I think are carbon film. should I be using these instead?
thanks.

 

[6942]

I use 1% Metal Film resistors.
VERY tight tolerance, and they do not drift off spec with age.
Some people claim to hear sonic differences, with different types of resistors?
I cannot, but it's your $$$, thus it's your call.


Steve

 

[Rich_S]

Here's the PS section of my Marshall clone, showing where the zeners go, adn their orientation. Keep in mind this is stripped down from the original; the Bogen started with a fixed-bais supply for the output stage that's been removed from my amp (replaced with a cathode-bias RC pair per the Marshall design).

I just checked my other CHB schematics - the 35, 50, and 100 all have the same type of voltage-doubler supply. Some values may be different for the bigger amps, but the topology is the same.

 

[vanman]

thanks rich. very helpful. I will look for the diodes and try it. I am also thinking of using the Marshall 18 watt circuit . at least for the front end, since this a 35 watt, 7868 amp. was also thinking of the marshall 1959 circuit. not sure yet. or else a hodgepodge. am going to put a LTPI, maybe use a 12AU7 since specs seem to match the stock 6c4, but dual triode, and a PPI master volume. I am using this amp as a learning tool to try different things.

 

[Rich_S]

Since the CHB35 has three 9-pin sockets, I would make it a plexi-style amp. Once you get the sockets wired for 12-volt dual triodes, you can swap in whatever 12A_7 tube sound best to you.

A lot of guys in the DIY workd build what they call a 6V6 Plexi - thinking octal 6V6's sound more like a baby EL34 that the similarly-named EL84s do. I haven't built a 6V6 Plexi, but plan to some day. They are reputed to sound really good, if you're looking for that vintage-Marshall-at-reasonable-volume thang.

I think the CHB35 is a good candidate for that, even though it has those weird output tubes.

Of course, you don't HAVE to convert it to an existing guitar amp. Some people get good guitar tone out of lightly tweaked Bogens, but I think that's generally the earlier CHA series - they had more "normal" tubes and reasonable operating voltages.