Crochambeau Bogen

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Misomusist
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Crochambeau Bogen

Post by Misomusist » Thu Mar 16, 2017 1:04 pm

Hey Crochambeau,

Did you ever do anything more to that Bogen? I got one a couple of years back and found the schematic on your blog, just curious.
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Post by crochambeau » Thu Mar 16, 2017 4:00 pm

Which one? I did recap the K-15 somewhat recently, and it worked well for a while...

but after an extended use everything went into runaway and I haven't had the time to look at it since.

The MX-60 and the green shell of a 30 watter remain untouched..
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Post by Misomusist » Thu Mar 16, 2017 7:00 pm

M-60.

Mine has the original preamp tubes, but someone had dropped in a pair of EL-34's. they worked but were seriously underbiased and ran very hot. I bumped up the bias voltage, but need to drop a pot in there to adjust it down, way too cold now., sigh :mrgreen:
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Post by crochambeau » Thu Mar 16, 2017 9:19 pm

8417 are great tubes, it's a shame they're NLA.

I'll chew on it a bit. I want to say I have a couple of the fuckers (amps) and not enough of the proper tubes to feed them.

That said, I'm in the middle of a move and moving sucks - so my mind is focused on other things.

KEEP CRACKING THE WHIP AND MAYBE WE CAN BE BOGEN TWINS!

hahaha
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Post by Misomusist » Fri Mar 17, 2017 6:18 am

I have a multi-turn pot to drop in the bias circuit, I just can't figure out where to put it in all this tagboard.

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If I mount it on the tagboard, I have to pull the panel off the amp, and turn it upside-down to get at it, and I won't be able to fit the tubes in unless I make some kind of stand to hold it up off the bench upside down, which would make setting bias a pain, and this area is nowhere near the rear panel, so I'd have to run a loop of cable to get the adjuster somewhere near the panel and I don't know what I'd mount it on.

Couple that with the fact that in all this disorganized chaos (my workspace not the amp) I can't actually find my bias checking widget for my meter :mad:

Hmmm, maybe I should mount it facing upwards, and drill an access hole on the bottom plate? Though I still can't leave the amp in it's case, if I have a bias widget under the tube, damn. :mad:
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Post by MKULTRA » Fri Mar 17, 2017 6:50 am

What type EL-34's?

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Post by crochambeau » Fri Mar 17, 2017 7:16 am

Unless you're fiddling with changing out tubes all the time there's really no harm in making the process involve pulling the chassis out of the outer shell. This will result in an invisible modification with regards to normal use.

Those recessed sockets might make using your widget annoying even in the best of circumstances (chassis out) so save yourself the trouble. I would punch a hole somewhere in the chassis to affix the adjustment though, that way you needn't touch the amp while mounting & dismounting the bias probe/tubes or adjusting the voltage.

I look forward to seeing the results!
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Post by Misomusist » Fri Mar 17, 2017 8:10 am

You mean the power tube sockets being recessed?

Yeah, even with the recess, the probe fits, and the wires don't foul, but yeah, it needs to stay out the chassis. The bottom of it is a separate panel though.

Now you've got me thinking, I have to take the top part off, but there's no need to take the bottom part off, if I make the bias adjustment visible from the top, so rather than drilling the bottom panel, I should face it the other way up, and dril the chassis itself.

That way I can keep it oriented upright, with the probe on, and the bottom panel still on, and make my adjustments. I should be able to drill a hole by that tagboard that holds the bias supply, and just mount the pot upside down to how I look at it, so the little brass screw pops out, or at least is visible topside.

Just have to check what is around that area, and of course find the bloody probe. :?
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Post by Misomusist » Fri Mar 17, 2017 8:11 am

MKULTRA wrote:What type EL-34's?
Standard Octal ones.
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Post by Misomusist » Fri Mar 17, 2017 8:18 am

crochambeau wrote:8417 are great tubes, it's a shame they're NLA.

I'll chew on it a bit. I want to say I have a couple of the fuckers (amps) and not enough of the proper tubes to feed them.
I've seen people use 6550's in their place, but again I think you really need to fix up the bias supply.

It's simple enough, sub the 22K 5W resistor (or whatever it was rated at) for something lower, I went 5K, which bumped the voltage way up. You also have to replace the smoothing cap as it's a really low working voltage like 25V or 50V or something real small, and would pop otherwise. I'd like to get a bigger one, all I had was a 10uF with a 450V range. Replacing a 500uF :shock:

I'm planning to put this 10K pot inline with the diode which is in parallel with the 3.3K so that it and the other 10K make a divider that allows me to adjust the bias voltage. Hopefully there's enough swing, it should also allow me to properly bias if for 6550's if I care to fit them.
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Post by crochambeau » Fri Mar 17, 2017 9:21 am

Why reduce R47 when you can increase R46?

Knocking R47 to ~22% of its original value looks like an unnecessary load on the 50 year old transformer secondary to me.
Misomusist wrote:I'm planning to put this 10K pot inline with the diode which is in parallel with the 3.3K so that it and the other 10K make a divider that allows me to adjust the bias voltage. Hopefully there's enough swing, it should also allow me to properly bias if for 6550's if I care to fit them.
Check that schematic I linked on my blog and confirm we're talking about the same amp.

What I'm looking at is the voltage divider being 22K R47 (load) & 3K3 R46 (reference), pre-diode. If you slip your 10K pot in series with R46 it will allow you to swing the tap point of the voltage divider above where it is at. Then the diode rectifies and the 10K (R45) supplies a loaded DC reference point.

I agree 25 volts is puny, 50 or 63 are standards to grab - but I would not sub 10u for 500u.

If your R46 is in parallel with the diode, I think we've got problems..
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Post by crochambeau » Fri Mar 17, 2017 9:32 am

For brevity:

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Post by crochambeau » Fri Mar 17, 2017 9:35 am

I'd also confirm that Jaguar is not leaking DC like a motherfucker, that would play hell with your bias.

I mean, I'm a preservationist and I've seen failures in that cap class.
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Post by Misomusist » Sat Mar 18, 2017 5:26 am

Yeah, I typed that badly. That is the schematic, no the diode is not in parallell with R46, it's coming off in a parallel branch with the 10K.

My thinking was the reference resistor, as you call it, had to be of a lower value than the branch with the 10K and diode, so that the load of that side did not start affecting the reference voltage, but that is silly now I think about it, there's not usually anything much in the way of current draw on the grid circuit.

I need to measure the actual voltage at the end of that cap. I was thrown by fact they used a 5W resistor, but I think it's probably overkill, but if I throw a 10K in there, it's still going to be only a third of the full voltage.

Too early for that kind of thinking, measure, then calculate, instead of trying to reverse engineer the value.
I think trying to quantify music is one of the biggest wastes of time in the world, like discussing your favourite colour or deity or pizza topping. People should realise that and get on with their life.

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