Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Skip Hagey on June 08, 2015, 03:05:09 pm
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I am putting together a parts list for a Hammond organ ao39 to Marshall 18 watt lite and there is a cap value that I don't understand. It reads 50-500uf in parallel with a 150/5w resistor in the output section, between the two cathodes of the el84's. Could someone enlighten me as to what that is? Thanks...
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between the two cathodes of the el84's
OR
connected to the cathodes of both tubes and to ground on the other side ?
K
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Yep. this is where I found this Sch...... Hammond AO-39 conversion to MARSHALL 18 WATT Lite
Drawn By Steve Luckey 02/27/2011
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I'm betting it's a 50 volt, 500 uf cap. But that might be a typo, 500 uf is really high. (I think)
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Without looking at the actual schematic, I'm betting it's 50uf to 500uf and it doesn't specify voltage.
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Without looking at the actual schematic, I'm betting it's 50uf to 500uf and it doesn't specify voltage.
BINGO!
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But it could mean equally '50V 500uF'. Lots of schematics don't put the 'V' in the cap value. And seeing as how its a cathode bypass cap at the output stage, a 50V rating is fairly typical. (Even tho' a 25V rating would work safely for EL84s as well)
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But it could mean equally '50V 500uF'.
I would never list the voltage rating of a cap before the value.
50-500µF means exactly what it says. :wink:
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But it could mean equally '50V 500uF'.
I would never list the voltage rating of a cap before the value.
50-500µF means exactly what it says. :wink:
:m4
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Ok, so it means 50-500 uF, does that mean just choose a value that "sounds" good or am I looking for a variable cap? I think I hear someone laughing..... :l2:
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My name is Steve and I have poor communication skills. :icon_biggrin:
That 50-500µF value was a direct copy from the Pedal Monkey schematic that was floating around over at 18watt.com. A lot of guys were experimenting with that value based on "something that sounds good". I just used a 50µF @ 50V because I had one in stock. Never thought any more about it.
BTW, my AO-39 conversion later morphed into an AC-15. Then this past January I just used the iron and tubes and a new chassis to build a Dual Lite, Marshall 18W Lite IIB + AC-15 Lite. I much prefer the Vox sound. If you're still in the planning stage you may want to take a look.
http://home.comcast.net/~seluckey/amps/dual_lite/dual_lite.htm (http://home.comcast.net/~seluckey/amps/dual_lite/dual_lite.htm)
If you plan to use the original AO-39 chassis it would be a real challenge to put both preamps in that box.
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Thanks Steve, I think I'll stay w the project at hand. Yes I am using the 39 chassis and yes it would b a challenge! This is my first build and I'm struggling w the basics so I think I'll keep this one simple, but maybe in the future...
Thanks to all....Skip
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We'd like to follow your project. Stick around, share your progress, ask questions if needed, but above all, post pics. We really like that! You're in a good place for this kind of stuff.
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> 50-500 uF, does that mean just choose a value that "sounds" good or am I looking for a variable cap?
If you *never* over-drive the final stage, you do not need the cap. The push-pull equal-but-opposite action means the cathode voltage never tries to change, as long as you never over-drive the amp.
What am I saying? Where am I? Yes, a guitarist WILL over-drive the amp, perhaps rarely, perhaps all night long.
There is no "perfect value" here. Values of 500uFd and more *might* "improve sound" in Hi-Fi applications. While once costly, today a 1,000uFd 25V is very cheap.
But guitar tonal accent is NOT "Hi-Fi". We want distortion, changing dynamically.
The interaction between the playing style, dynamics, over-load, cathode-cap action, the grid-cap time constant, and the *music* is too complex to figure out ivory-tower style.
Many thousands of fine amps just threw about 50uFd in there. I suggest you do likewise. No-cap is also a good plan.
If, after serious beating (remember this cap has no effect below over-load), you are bored and restless and just gotta solder something, try other caps. I don't expect major differences. Nothing that would be worth drilling for a switch.
BTW, there are/were "variable capacitors" but the biggest were a million times less uFd than you need here.
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Thanks again, PRR, I knew they did make/use variable caps but mors for radio/TV (tuning) but I couldn't see how one might work or even look like in this position. Great answer! I will stick around and probably wear out my welcome. :icon_biggrin: Pics will be along, anon....
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If you don't see a capacitor simbol crossed by an arrow or something that looks like a "T"
don't worry
that isn't a variable capacitor or a preset capacitor (this last may be also a variable capacitor trimmer) :icon_biggrin:
(http://www.learnabout-electronics.org/ac_theory/images/variable-cap-symbols.gif)
Oh, a variable capacitor looks like this, so is near impossible you don't see it looking to the circuit :wink:
(http://upload.wikimedia.org/wikipedia/commons/6/6c/Variable_Capacitor.jpg)
The small round thing on this image is one of the possibles capacitor trimmer
the big one is a variable capacitor with a pair of capacitor trimmer incorporated
(https://tronixstuff.files.wordpress.com/2010/06/variablesmall.jpg)
K
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Yeah, I remember seeing them in radios and TVs I believe in the tuner section. I did pay a little attn. to what my dad was doing just not nearly enough. Ahhh, youth is wasted on the young :violent1:
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> variable caps ...but I couldn't see how one might .... look like in this position.
Like kagliostro's image or your memory, except over a million times more capacitance.
Can we set the plates a million times closer? Not likely.
Can we make them a million times bigger in area? 1,000X each way? From 2 inches to 2,000 inches? 166 feet (50 meters)? I guess we could, though mechanical floppiness would be a problem suggesting wider spacing and thus even more area. And an amp with a 266 foot cap is hard to fit in a 7 foot door.
Interesting to imagine.
Similar heroic-scale parts exist in artificial lightning research, though probably not this exact part.
And we do not need the 100 discrete channels needed in an AM or FM tuning system. We don't do narrow-band. And a single simple R-C has such a soft roll-off that you can hardly assign it "a frequency". The -3.01dB point is convenient in the math and suggests the observed response, but we can often move an R-C filter a half-octave or even a whole octave and nobody notices. (2-pole and 3-pole filters are sharper knives and need finer control.) Also the cathode cap on a push-pull stage is not a frequency response but an overload time-constant. When it works, the signal is already pretty mangled and exact numbers don't tell us much. If it has "any" effect on your amp for your music, I'd expect a 3 or 6 way switch and a flock of 20-cent caps would cover all the choices. In 2 inches, not 2,000 inches.
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Skip, here's a pic of my AO-39 to 18W Lite conversion...
http://home.comcast.net/~seluckey/amps/hammond/ao39-18w.jpg (http://home.comcast.net/~seluckey/amps/hammond/ao39-18w.jpg)
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Steve, it looks like in the photo there is a third preamp tube(PI?) and the schm. only shows two, but the tubes are labeled V1 and V3 with no V2. am I mistaken? And not just Steve, but Anyone?....Bueller.....Bueller....
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Steve, it looks like in the photo there is a third preamp tube(PI?) and the schm. only shows two, but the tubes are labeled V1 and V3 with no V2. am I mistaken?
There is a third socket. But only the filaments are wired. I originally planned on doing an 18W TMB Lite, hence the extra unused socket. You don't need that socket for an 18W Lite or the AC-15 Lite.
Notice that I removed the hum balance pot from the chassis, enlarged that hole and installed a 9 pin socket. That's the PI and it is close enough to the power tubes to make point to point wiring easier. The preamp tube is in the original socket located in the corner of the chassis near the input jack and knobs. This also made wiring easier.
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Im glad to hear that, I don't need to many other confusions at this point, but I'm having fun....Thanks again