Bogen Challenger CH18 new project

[xtian]

New project day! Vintage 50s Challenger CH18 purchased on eBay for $65, including shipping. All original GE and RCA tubes (hope they're good!).

Paint should clean up nicely. Only missing one knob. Look at the nice salt crystals growing on top of the filter caps!

I think I'll strip it and convert to 5e3. Some other posts on the web talk about having to lower the voltage with Zeners, but we'll see. What do you think: try to get it working with minor tweaks?

[Phil_S]

Wow! I have not been slumming at eBait for a while. I look for this sort of deal whenever I'm there. $65 with tubes, iron, and cage is really quite a steal. The cage and chassis look fairly primo for a piece that age. Good for you!

Are you really hankering for a 5E3? I think there are other circuits that would live very nicely in that rig. I'm thinking about a Princeton or an 18W. I am not all that fond of the Deluxe, which is a very special kind of amp in its own way.

Good luck with it.

[rdjones]

A Bogen should have a bit of it's own kinda vibe to it.
I'd at least listen to it with minimal mods to get it up and running before gutting it completely.

I've put an input jack on several (not CH18) over the years to be used with guitar.
Look up "Free Form Guitar" from CTA's first LP.

Tack a filter cap in there and fire it up ...

reddog

[xtian]

Phil, I thought 18-watt designs, like the 18-watt Lite, were EL84 amps. Not true? What schem would you suggest?

Gotta build a bulb limiter.

Anyone know a good way to remove rivets?

[Cliff Schecht]

I advised a student this semester who wanted to use one of these Bogen's as a basis for a tube amp for his senior project. In the state that he got it in, the fuse holder was jumpered with a very thin piece of bare stranded (and slightly frayed) wire and was complete with 2-prong cord and 6PS .022uF death cap. Heck the entire amp was a mess, somebody did a poor job of trying to convert it to a guitar amp.

I was nervous enough advising a student on a tube amp and once I put the power transformer on a function generator to figure out the output voltage (nearly 850V RMS!), I got really nervous. He plugged it in in that sorry state a few times before I could warn him not to unfortunately..

The student ended up using the power and output transformer from the Bogen and built his own chassis to fit in a piece of crap Epiphone bass combo. The amp itself turned out about like how my first few amps turned out, a point-to-point mess, because he started running out of time to build the amp. Didn't sound too terrible but had an interesting distorted sound with some bloom (which he was aiming for). The power transformer, after a 5U4, was putting out over 550V so he used a zener to drop the B+ down some (don't remember but he was using JJ 6V6S which are pretty tough).

The best part is he ended up selling it for $1500 to some idiot. I wouldn't have paid $100 for it!

[Phil_S]

Phil, I thought 18-watt designs, like the 18-watt Lite, were EL84 amps. Not true? What schem would you suggest?

Gotta build a bulb limiter.

Anyone know a good way to remove rivets?

Build the 18W with 6V6's. It's been done before, so you don't have to concern yourself with being a heretic.

Rivets? Use a good center punch to keep the drill from drifting on initial contact. Just drill them out. You may need to start small, like a 1/16" and work your way up, but it is much better if you can go for the correct hole size without graduating the bit size.

The problem with rivets is, once they get loose, they turn with the drill, so the bit stops cutting. A slight angle and some pressure sometimes helps. If you can get them to the point where they have some vertical movement, pinch them with a pliers and wiggle until they break off.

I've also had the experience where they get loose and the hole is big enough to accept a countersink (like for a nail). Put the counter sink in the hole and strike with a hammer. Often, that will finish breaking the rivet and it will drop out.

Good luck with the rivets. It's relatively easy to get them out because they are typically made with soft metal.

[xtian]

The best part is he ended up selling it for $1500 to some idiot.

THAT's the amp I want to build.

[Phil_S]

The best part is he ended up selling it for $1500 to some idiot.

THAT's the amp I want to build.

We need to document that amp so it can be cloned. Then we need to clone the buyer

[xtian]

First steps lead to fail.

I replaced the frayed, 2-conductor power cord, and removed the death cap. Hooked up an 8ohm speaker. Used bulb limiter to start up with no tubes, then with all tubes installed. Was drawing a little current, but not enough to light up the heaters in the tubes. So far, so good.

Took the bulb limiter out, and tried another startup. I noticed the 5Y3GT go red plate quickly, so turned it off.

Pulled out all the tubes and switched on. 6.3vac heather wiring is working fine. Power light goes on, measurements OK.

BUT, the 5vac pair which is connected to pins 4 and 6 of the 5Y3 recto are not reading right. They cause my meter to go right off the scale and then beep like it's yelping for mercy. Remember, I have no 5Y3 installed, and nothing is connected to the 5vac wires from the PT. They don't read as shorted...185 ohms.

Is the PT shot? What could cause an AC voltage that my meter can't read?

[jjman]

Unloaded will be significantly higher. How high does the meter read? Is the Roman candle cap still in there?

[xtian]

Tried once more with bulb limiter in place, no tubes installed. AC voltage on 5v pair still goes out of range, though amp is not drawing enough current to light the 40watt bulb in my limiter.

[xtian]

Unloaded will be significantly higher. How high does the meter read? Is the Roman candle cap still in there?

I took the green Tropicap out. I'm using a DMM. Manual says it measures up to 750vac.

[xtian]

Now that I'm looking at a 5f1 schem, I see that I thought incorrectly that 5vac was connected to recto pins 4 and 6. In fact it is the PT's HT that goes there. So, the combined AC of both legs could actually be out of range of my 750vac DMM.

Which leaves the question: Why is the amp red plating my rectifier tube?

[Cliff Schecht]

You need to make sure you know the pinout of the power transformer. I don't have my buds notes or I'd send you those. IIRC the pinout didn't make a lot of sense though.. With nothing attached you shouldn't see anything more than 5.7 or so Volts, probably less than that. The winding resistance seems high though, shouldn't secondary DC resistance for the LV stuff be much lower than the primary resistance (usually a few Ohms)? I suspect you might actually have a bad PT or an improperly placed wire somewhere.

Also you need to remove all of the paper electrolytics, while they might work they are known to pop "at any time" and can literally ruin everything they spray electrolyte gel on. These could already be bad enough to draw excess current when you turn on the amp and cause the rectifier to redplate.. Also it looks like the value of that first cap is 40uF which is too large for a 5Y3, this should be reduced to 20uF-ish to keep the rectifier from arcing.

TBH I wouldn't expect much from this amp sound-wise though, the B+ is much too high for 6V6's (the JJ's handle it alright though) and the design isn't really conducive to great guitar audio (doesn't it have an RIAA tonestack for phono use?). It has MASSIVE B+ droppers (47k and 100k in the preamp) which IME makes for an amp that is easily distorted and overly sensitive to picking dynamics. I prefer more tweed Fenderish values for getting the best tradeoff of B+ voltage and power supply induced distortion (from the B+ lowering), something along the line of 10k-22k after the power amp. Of course this is also dependent on what sound you are going for..

[Phil_S]

+1 on what Cliff says. I think maybe someone munged up the PT wiring to the rectifier. If you can't find the SAMS Photofact for the amp (try your local public library for a free one), disconnect all wires except the primaries. Use your DMM set to Ohms to determine what terminals are paired. Then power it up and see what the voltage is on the pairs. Remember, on the high voltage pair, unloaded, it is possible that you will blow your meter, so you may want to only measure from each outer leg to the center tap. You'll know the high voltage one because it is likely to be the only center tapped winding.

Then you'll see, someone miswired the rectifier socket.

Good luck.