Bogen CHB 20a question

[Seegs]

i have had this amp for a couple months, and in all honestly it sounds great when cranked and loud very nice break up. But it really doesn't react well with any pedal i have used in front of it and i think it's just time for me to move on. i don't have much into this amp but was wondering if there would be something i could turn it into? or be able to re use some of the parts out of it and chassis. i have looked and saw that others were using other bogen amps into different amps But with this one having 6gw8 tubes it might not work out well. I have also moved the MV to later in the circuit to allow for more gain control though i like it it just isn't doing it for me.

would it be worth making this into something else?

[birt]

i would leave the power amp as it is (those are really nice tubes, also for guitar amps) and change the preamp to any single tube preamp you like. fender, ampeg, vox..

http://www.emailtom.org/images/bogen-chb20a.jpg

[DummyLoad]

bias V1A like V1B. bypass master V (R5) and clip out aux1/2 input level mix (C14/R22 to R5). R3/C3 should also be bypassed. bypass (clip out) C1 input cap and add 33K between R1 (1M) and Pin5 (g1) - hint, look at fender AB763 input - only use one jack.

--DL

[Seegs]

ok, so what i have done is remove the aux completely, and by removing the MV , adding the resistor and by passing the rest what will this do to the circuit exactly. I am new to this and have been teaching myself as i go ( i do understand basic electricity well ) so what this looks like it will do is allow both pre amp tube/6gw8 to be driven more? and the carry that to the power amp side.

personally i would like this amp a whole lot more if it would react to pedals better. going to give this a shot tomorrow and see how it works out

[PRR]

> doesn't react well with any pedal

That's the first stage. No need to change the rest of it. If the first stage is Bogen's classic grounded-cathode with 5Meg grid-leak, change to a Fender-like cathode resistor bias.

[Seegs]

change to a Fender-like cathode resistor bias.

would you happen to have any were i could look up a schematic. i am not good enough yet to just be able to look at a circuit and understand exactly the values of resistors and caps and what to move were. ( unless this is what was explained earlier then got it)

That's the first stage

ok,  that's good to know hope to get this figured out tomorrow

[Willabe]

would you happen to have any were i could look up a schematic.

Sure! Look in our host Dougs library of schematics.

It can be found on the forum home page about 1/2 way down or on the forum topics page at the very bottom.

Here's the link;


http://www.el34world.com/charts/Schematics1.php


                
                   Brad      

[gldtp99]

Hi--- you've already gotten great advice---i have several of these CHB20A's but i haven't messed with them for a few yrs.
In stock preamp form they have early breakup, especially with humbuckers, because of the design of the early part of the preamp---V1a: the first gain stage.
Converting over to Fender AB763 V1a specs should yield a general purpose amplifier that would accept pedals in a more conventional manner.
You could pull the transformers and use them to build another completely different amp----- but this is a lot of work----- other output tubes could be used with the CHB20A iron----- i'd use JJ 6V6's because some versions of CHB20A's use a PT that puts out B+ voltage that's on the high side for most ECL86 and EL84 type tubes----- the JJ 6V6's will handle the B+ with no problems if you have one of the higher voltage versions.
If you do this be sure you understand how to set up the Voltage Doubler power supply---it's a little different than most small tube guitar amps------also, the CHB20A is fixed bias and has a capacitor coupled bias supply circuit that's also a little different than the usual Fender/Marshall way of doing things----let us know what you come up with----- i love to see what others do with these old CHB series Bogens...........................gldtp99

[Seegs]

alright, so i did the mods listed above, and i must say it takes pedals a little bit better, well a lot better now. but i am having a grounding issue. nice hum going on so i will have to figure that out.  with that being said the clean headroom of this amp is little to zero which is fine with me. it does seem there is a sweet zone were i have a decently clean signal and then when using pedals the react a lot better.

I will also admit this amp has a killer overdriven sound just on it's own and if i didn't play more then one style of music and maybe only played blues or 80's rock music i would be happy. but sadly it's not so.

[Seegs]

not to beat a dead horse, But i cannot get this thing to what i want. I am still relatively new to building and modding amps but that is why i came here to try and learn as much as i can. i can read all i want but until i do it i normally don't get it completely.  That being said i want to know what have you guys made out of these amps. i really love the ac15 sound and was wondering if the Iron on out this could be used to build a PTP turret style build.  or if changing the circut more on this bogen i could get a cleaner sound maybe by lowering the B+ voltage ?  this thing just breaks up really fast and it seems like the pedals just push that and aren't really doing anything. which would be the preamp as stated above.

my main question now would be if at all possible is there anything left to do on getting this more tame or would it be more worth it to just start over or just buy a different amp?

Thanks
Ben

[jazbo8]

It can be done, but I think Tubenit mentioned it somewhere, it might easier to start from scratch than trying to use the existing CHB circuit topology. Here is an example for your reference: http://emailtom.org/2011/05/05/bogen-chb-20a-rebuild-project/

Jaz

[Seegs]

The bogen is back, still messing with it. mods so far are fender style pre amp, removed the aux, moved the MV to after the pre amp.this really opened up the amp for me being able to keep the volume down but still get a decent overdrive out of the pre amp without anything pushing it. i have found this to really let the amp come to life a little more. responds much better to pedals and is still dead quite when on. i am pretty impressed with this amp which i have less then 50$ into.

my final question would be. as far as the ecl86 tubes go which i understand is a 12ax7 and a el84 in a sense. this amp definitely has it's own kind of tone. what kind of speakers would you guys pair with this amp? i am thinking about going with a alnico blue and a g12 h. i haven't found much info on what this amp pairs with well and don't have the budget to spend hundreds of dollars to try different speakers over and over right now. and i have been playing with the idea of making this into a combo amp so we will see how that goes

also would it be possible to add a reverb unit to this or should i just rely on a pedal?

Things i want to add, stand by switch, cab selector switch ( if i go with a head unit), and re route the on light to a jewel style and mount it.

[jazbo8]

Speakers make a huge difference to the sound, much more than the amp itself, so depends on what kind of music you play and your tonal preference is... It's hard to recommend a speaker, but head on over to Emience's website and listen to some clips, and you may find something to your liking.

The chassis is practically empty, so there is a lot room for you to add a reverb or tremolo circuit (assuming the power transformer can support the extra current, but Bogen's pretty well known for its beefy PT, so I think it can be done).

Jaz

[smackoj]

Here is another way to 'skin the cat' on your Bogen. Do you have a speaker you normally play thru with your other amps and guitars? Ok, keep that speaker and try one possibly two other spkrs. (can't try too many cuz you'll need a 2nd mortgage!) If you still don't hear the sound you want, consider trading out the Bogen's stock OT for a nice aftermarket OT. Aside from Mercury Mag and some over-hyped stuff on fleebay, there are lots of economical OT's on the market, old and new. I gutted and built up a similar Bogen to yours (mod CHA-20). I couldn't get a likeable sound from it so early in the process I bought an Alan Amps OT which improved it a great deal. I have had to tweak the preamp design a couple times but I now have a really solid, good sounding gig amp head.

 

[Seegs]

and it's back.. so i tried to use a fender bandmaster preamp design.. and here is the problem i now have.. i have checked everything about 300 times and cannot figure out why this is happening.

when i have the volume pot under 1/8th volume it acts fine gets decently loud but as soon as i start to turn the knob that volume starts to decrees then by the time it's all the way up the volume is off. and in between 1/4 and full open the sound is almost none existent.

like i've said i have looked at every connection every cap every resitor.. i just don't understand it. i must be missing something. it's like as soon as the tube would start to breakup it just goes away.

[PRR]

The coupling cap feeding th volume control may be leaking. Power-up, *carefully* check for DC voltage on the volume pot lugs.

[Seegs]

i'm getting about 1.4 volts across the lugs.. i hooked up the volume pot with the 2 terminals backwards and fixed that. but now i get a consant volume. after 1/8 turn it gets as loud as it will ( very quite) and even with the MV all the way up and the mic volume up i am getting nothing. going to check all the caps right now.

[Seegs]

alright i've checked everything i printed the schematic, followed everything highlighted as i went.. checked the voltage.. i'm running about 20v above the specs right now.

the only thing i can think is that the capacitors off of pins 4 and 9 of the preamp are in backwards. or is feeding voltage to the MV. i'm still kind of lost. granted i am new to this but i do understand what is going on and how these circuits work to a degree. but this isn't making much sense i must be missing something

thanks for the help

- i was kind of in a rush last night.. this is my only amp and i had to play in the morning.. which didn't happen ha ha

EDIT: after some more checking it seems that i have been getting voltage back from the power tubes to the volume pot. which i don't exactly understand fully. i have been following the conversion posted earlier.

[Seegs]

ok i have rebuilt the bogen, and from what i have done i don't know why i didn't just build somthing else but it's cool. took some tweaking and getting it just right but the tone is good, and i def had a leaky cap before because now the caps hold a charge correctly and i have to drain them for the first time. the sound is much better, i would say thicker and more defined. but i pretty sure i have a microphonic tube. i can tap it and hear it from the speaker. and i am getting some distortion or break up on hard strums on low volumes or just really early break up.

But overall it's a sweet little amp. going to swap out my speakers for these rola's and see how they sound with it.  hopefully be able to upload a decent sound clip once i get everything fixed. and buttoned up correctly.