NAD - Oops. I. Did. It. Again. (bogen content)

[DankStar]

well, I may have righted a wrong, I dunno yet:

- I used to have a bogen CHB-20A old tube pa I had converted into a guitar head and it resulted in a really sick fuzz tone when cranked. sold it like a dum dum a few years back.

looked like this:

Been keeping and eye peeled and found one for a good price this weekend. Gotta wait for shipment now.

I'm hoping it sounds like my old one once I get new 1/4-inch ins and outs and a grounded power cord put on it, but we'll see.

Tube compliment is: one 6EU7 and two 6GW8. must be the 6Gw8's that give the nasty (but yummy to these ears) OD.

My name is dankstar, and I'm addicted to old tube PAs.

 

[TwilightOdyssey]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Sweet!

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Sweet!

I hope so; if it doesn't do the crazy fuzz like my old one I'll probably put it back on the bay - I have enough ones that can do clean mild/warm OD.

 

[trevorus]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Dude, I have one of these too! Mine has the 7868 power section. I turned it into a Matchless Spitfire front end, but I need to replace the output tubes. They are OOOOLLLLDDD, and distort into squarish clipping pretty fast.

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Dude, I have one of these too! Mine has the 7868 power section. I turned it into a Matchless Spitfire front end, but I need to replace the output tubes. They are OOOOLLLLDDD, and distort into squarish clipping pretty fast.

that's awesome. yours must be the higher wattage 35 (or higher). luckily they started re-issuing new 7868s again a couple years back. I might hit you up for some questions on the light mods I have planned if you don't mind - might try to save a few duckets and do it myself since my mods are so basic.

 

[MetalManiac]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

I have a Bogen CHB-100 which is a 100 watt amp with the 4x7868's which that was converted by a tech I knew. I'll post pics of Guts and everything..stay tuned..one problem with it is one tube is pulsing..not redlining, just pulsing, so i need to get it checked. Im gonna put it on the block for sale.

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

I'll post pics of Guts and everything..stay tuned..

that'd be great - close ups of the ins and out and where the 3-prong power connects inside would be appreciated (if not for my use, then the guy I'm gonna take it to).

 

[Rich_S]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

I have a spare CHB-20A on the shelf, that I'm going to convert either to an AC15 or a Spitfire one of these days.

My 18 Watter combo started as a CHB-20A, from this:

to this:

CHB-20A trivia:

• The ECL86 output tubes are clever little beasts. They comprise an EL84 and half a 12AX7 in one tube. Because the guts are smashed in so tightly, they can't handle as much power as a full-on EL84, so my 18 Watter clone is actually only about a "13 Watter". In most ECL86 amps, the tubes are wired up so that a pair of them are both the phase inverter (where most amps use a 12AX7) and the output stage.
• Bogen got the CHB-20A's 20 watt ratting by running a ridiculously high B+ voltage; somewhere around 425 IIRC. I had to install a string of 10 or 11 10-volt zener diodes in mine to get the B+ down to a "normal" B+ for an 18 Watter. Obviously, Bogen caed about hitting the 20-watt mark in the specs more than they cared about tube life. Replacing tubes, after all, was the responsibilty of the owner, not the manufacturer. For PA applications, the cold, sterile sound from the high voltage wouldn't matter much. Guitarists, we hate it.
• The CHB-20A has a weird and little-known phase inverter. I've seen it described as a see-saw. Anyway, it's not the normal long-tailed pair, nor the concertina, theat we usually see in guitar amps. Mine's now a LTP, since it's been rewired to an 18 Watter circuit.
• The CHB-20A uses a voltage-doubler power supply. A tricky arrangement of caps and diodes gives them something like 440 volts out of a 220 volt transformer. As far as I can tell, they do this because it's cheap to manufacture - they get the most out of an inexpensive transformer, taxing it to its limits. No center tap needed, either, so they save a few more pennies there.
• As much as they were mass-produced, consumer-grade amps, designed to be made cheaply, these Bogens were BUILT. I had a heckuva time un-wiring mine when I did the conversion. Every wire was inserted through the teminal, wrapped around it, and then soldered. Getting them un-soldered and removed was a beeotch.

I <3 Bogens.

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Cool rich - I think I stumbled upon your 13 watt schematics on photobucket and some other forum when I was googling for schematics and other info.

 

[Rich_S]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Yeah, any time someone Googles "Bogen CHB-20A", they come up with me.

 

[trevorus]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

I'll have to take a pic of my rats nest I put inside the Bogen. It was pretty much a 50 dollar test bed. Might do a 7591a octal, since they are electrically the same as the 7868.

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

got the bogen friday - took it in the shop today (might have it by later today!)

here's a gut shot:

Most of these old PAs need grounded power cables as part of the conversion (or at least I like to have them), but this one was designed with one. Just needed 1/4-inch ins and outs. Well, crossing my fingers no caps are bad, but everything looked good visually, and he was able to generate test tones through a cab.

One of the ECL86's tested pretty bad, but he fired it up and then retested and it had some life. Guess I need to keep my eyes peeled for a new/NOS one.

Super clean on the inside - he believed it hadn't been tampered with, outside of a 300V power tap (wire) someone had stuck on a cap to power something else. Quick desolder (or a snip, I didn't see, hehe) and that was gone.

 

[kevlar3000]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Old PAs are full of magic! I had to pass on a cheap Bogen recently, cash was tight.

well, I may have righted a wrong, I dunno yet:

- I used to have a bogen CHB-20A old tube pa I had converted into a guitar head and it resulted in a really sick fuzz tone when cranked. sold it like a dum dum a few years back.

looked like this:

Been keeping and eye peeled and found one for a good price this weekend. Gotta wait for shipment now.

I'm hoping it sounds like my old one once I get new 1/4-inch ins and outs and a grounded power cord put on it, but we'll see.

Tube compliment is: one 6EU7 and two 6GW8. must be the 6Gw8's that give the nasty (but yummy to these ears) OD.

My name is dankstar, and I'm addicted to old tube PAs.

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

thx for the input kevlar!

got this one back, and it sounds really nice - takes a lot longer (on the master volume knob) than my previous one to give up the goods on the fuzz sound though, as in super loud (I'm gonna have to wait til the house is clear to do a clip of it raging). with last one I had, I recall the preamp volume was able to control the fuzz more - now I'm thinking it must have been wired up differently with an extra gain stage or something. can't knock it though, it's very clear/loud, fun. sounds great with an od pedal too.

 

[TwilightOdyssey]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Wish I had the guts to try something like this.

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Wish I had the guts to try something like this.

for what it's worth, your playing style and these types of amps would blend really well. trick is always finding someone that doesn't want an arm and a leg for just doing some simple mods. guy I use is an former TV repair guy, but he also plays a lap steel and his son is a 70s rockin kinda guy (he got a kick out of the amp too, and played through it before I left and was pretty surprised).

 

[TwilightOdyssey]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

for what it's worth, your playing style and these types of amps would blend really well.

I figured as much. I may have to look into something like this. Always nice to add another flavour!

 

[Rich_S]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

Now DankStar's got me thinking I should get my spare CHB-20A working and leave it stock, rather than mod it into an AC15 or whatever. It would certainly be a lot less work.

 

[kevlar3000]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

thx for the input kevlar!

got this one back, and it sounds really nice - takes a lot longer (on the master volume knob) than my previous one to give up the goods on the fuzz sound though, as in super loud (I'm gonna have to wait til the house is clear to do a clip of it raging). with last one I had, I recall the preamp volume was able to control the fuzz more - now I'm thinking it must have been wired up differently with an extra gain stage or something. can't knock it though, it's very clear/loud, fun. sounds great with an od pedal too.

A simple tweak to try is the "Parkhead" mod off of the Plexi forums. Put a resisitor that is 1/2 the value of the volume pot ( or pots in the case of the PA) over the 2 hot lugs ( forgive if my terminology is wrong) on the pot. If inclined, you can also put different value caps, tied into the resistor, on each of the inputs. The resistor fattens up the amp, and gives it noticeably earlier break up, while the caps you add can effect both breakup and the sound of each input.

Super cheap, super quick, ect,,, I have done this with several of my PA heads. I imagine you could also have a lot of fun with a Y cable, or jumping channels too...

 

[DankStar]

Re: NAD - Oops. I. Did. It. Again. (bogen content)

A simple tweak to try is the "Parkhead" mod off of the Plexi forums. Put a resisitor that is 1/2 the value of the volume pot ( or pots in the case of the PA) over the 2 hot lugs ( forgive if my terminology is wrong) on the pot. If inclined, you can also put different value caps, tied into the resistor, on each of the inputs. The resistor fattens up the amp, and gives it noticeably earlier break up, while the caps you add can effect both breakup and the sound of each input.

Super cheap, super quick, ect,,, I have done this with several of my PA heads. I imagine you could also have a lot of fun with a Y cable, or jumping channels too...

these sound like cool mods (although the resister/cap thing is beyond me, I don't mess inside amps).

as it stands, I'm gonna ask my guy if:

- I can take out one power tube and get less power without damage being done (I've only seen that trick in amps with four power tubes where two are taken out)

or,

- if using an RCA "jumper" on both aux inputs will act as a cascading gain circuit by closing the circuit loop - if I'm even describing that correctly (otherwise, I may have him cascade it internally where the mic output feeds to the input of stage 2 of the preamp tube so that I have two gain knobs rather than one active and one unused). I think right now I'm only using 1/2 of the preamp's gain.

Like I said, the amps sounds great, it just needs to be pushed to its max to do what I know it can do (right now even if i dime the master & mic input vol, the preamp isn't fully pumping). I don't need to fine-tune eq settings and such, or "voice it for guitar" like a lot of peeps like. I just need it to deliver the fuzz at a lower volume.

part of the schem showing where the jumper would be: