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posting in the sewatt, too..


had a 40W bulb in..


haven't done this for a long time. if memory serves, the bulb was supposed to glow.. then dim very much.

this seems to glow.. not full brightness-I don't think..then stays at that level.


Swapped to 100W bulb.. about the same effect, just a tick brighter.


tested with bulb in R10 12 and 13


42.3, 10.3 and 8.5


I have no real clue what that tells me.. voltage drops the way it should ratio seems a bit off..


hints anyone?


should I test with bulb out?



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What is it that you're trying to accomplish? I've heard of using a light bulb in the AC line as a "fuse," to protect against catastrophic meltdown on first power up of a new build, but I've never heard of using one in place of resistors on the rail.

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Yes just using it for the first power up.

but it wont dim.


used it on the octal, building two at once, and it dimmed right down.. but the only sound I get is with the volume up all the way and then just very small signal.

that's an active guitar, and new, new/used, but I doubt it's the battery in the guitar..

can't figure it out yet. had it once a long time ago.. but can't recall.


on this one, the bulb wont dim.. and it should. I tested voltages just for the heck of it.. that's why the readings are low..


and when I fired it up sans bulb to test.. which I see now I shouldn't have done since the bulb didn't dim, I got smoke off the

power resistor .. the one running from the power cord to the switch.


Sooooo. I'm gonna replace that. But I'm sure it wont cure the no dimming problem.

I guess I'll just recheck everything again.. though I'm really so sure it's right.. famous last words, eh?

and see what's up.


feeling stupid about this one, but good about being so close on the octal.



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the light bulb is between the amp and the wall outlet.

It's just the usual light bulb tester.. when you power it up, the bulb glows then dims, as it's supposed to.

Then, I take it out and fire up the amp.

Nothing has been replaced.


It was just my poor expression.. what I meant to say was, I tested the amp voltages with the bulb connected..

and got readings at r10 etc.

I didn't mean I replaced r10 with a light bulb.


working on two amps today so my language got hurried and sloppy. beg pardon.


The only problem right now is the signal from the guitar.. and I checked the guitar in another amp, it was fine.. but in te octal the volume is barely audible with the amp

all the way up.

somehow I'm not getting any volume.


I'm going to go check voltages, as I've had the amp on enough without the bulb tester in long enough to know it's not gonna fry.

And maybe that'll give me a clue as to where the power is dropping out and reducing my volume.


on the other amp, the bulb wont dim. a sign that somethings not right somewhere. I'll just have to track it down.


I can't find any faq on voltages for the octal tube in the vj..

I remember it's around 350V for .. was it r10.. ? but I don't know the rest.


Does every octal build remove c3 and c4 as shown at turretboards?




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Ahh yes, the old light bulb current limiter trick. I use that on every build. Hey, stuff happens, and light bulbs save parts.


The old pop test with a meter probe may help you localize your preamp problem, but don't forget the good old chopstick test.



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Never heard of this, but I haven't built an amp in years. TWANG, you say that you have the light bulb between the amp and the ac socket? The resistance of the bulb will cut the voltage approximately in half. If you take off the plate on a wall switch and measure across the terminals of the switch with the switch turned off, you will read about 60 volts, since you are reading through the resistance of the tungsten filament of the bulb. So, when you power up with the bulb in series with the amp, I would guess that you are applying about that voltage to the amp...


Maybe that's the intended use of the bulb, to apply less voltage to smoke test the amp?


BTW, the current in a series circuit will be same at all points. All electrons flow through all components. The voltage drop will vary depending on the resistance and thus, the wattage across a component will vary according to P=EI. So, the bulb will not limit current, but will limit the voltage according to its resistance. An inductive load would change the phase of the AC voltage but not change the total current flow. A resistive load would not affect the phase but would drop some voltage.


Maybe too much info, but I'm trying to understand the intended purpose of the bulb in the circuit.

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Light bulbs actually present a variable resistance to current flow. You could buy a fancy current limiter bulb box from Kendrick or Weber, but why waste the kind of money they're asking for 'em when you can build one out of scrap? Here's all the info on it from the SEwatt "In Depth Info" FAQ.







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That's an interesting test tool. It took me a little while to see why you would want to limit the voltage to the amp, but apparently it works to prevent amp failures if there is a problem.


The theory that I was taught years ago involved the fact that current is analogous to the water volume flowing through a hose. All the water flows from one end to another, even if you pinch the hose. A pinch is equivalent to a resistance. The resistance causes a backup and the water flows more slowly, but it all gets through. Voltage was equal to water pressure, in this case EMF.


This link




is the way the current flow was explained to me. I guess the thing that I missed is that although the current flow is the same at every point in the circuit, a resistance will limit the current flow to a lower rate because of the voltage drop across the resistor, just like a pinch in a hose will limit the amount of flow of water past the pinch, but the pressure rises at a lower rate of flow. Past a resistance, the current flow will be limited, but all parts of the circuit will have the same current flow at any measured point.


I learned something new here.

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On the el84.. thanks to the light bulb I didn't hurt anything. That's way cool for a geezer like me still trying to learn.

I got smoke off the power resistor.. which I always put in until I do the first voltage tests, at least.

Another 'try not to wreck anything' idea.

I need 'em!

The problem I found was a doofus thing, so I aint tellin' ya..

but I just went over the amp stem to stern, hooked in the bulb, fired it up on standby.

glow, dim.

hit the standby, glow dim again.

So, I'm taking a break right now and then I'll go take the bulb out and test.


This one's looking good. And I have it much like my everyday VJ so I think it will be fun to test the sound of the Weber OT against

the MPS 08 OT.


The 6v6 I haven't gotten to yet.

It gives me guitar only if the vol. is all the way up.

and then only very lightly.

I think once I really run this through my head in stead of letting that giant explanation point float over me, I'll figure that out.


On that I'll be testing voltages and comparing against what I got from Gil and couple others. And luckily they all had some mention of key component values they used, so I figure I'll be deviating from the Turretboards choices which I have in.


Then.. I'm really thinking of the Baby Will. Two el84, two ax7.. how cool it would be to build that and have it work!


I found a drawback in the way I was soldering. Man, redoing anything was just horrendous. Solid is nice but I was complicating it.

No amp should be that hard to repair.


So, except for the solder up my nose, it's been a good day.

and thanks to all for everything along the way..


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well the good day is over with.

testing from r10





288 at the sgr and 285 at pin 9.... I thought!


continuing to the pre







uh oh.


the power






suddenly pin 9 isn't givine me that 285 anymore.


No smoke. No smell. Nothing unusual about the way the tube glow.


Shut down, recheck.

.1 at the heaters? that's a new one on me.



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seems like.. pt or tube.


does that led/battery tester work for PTs? I haven't used it in ages. I remember how but not if it works on Pts.

I'd like to test the PT before pulling it, and I'd like to know it's ok before I risk another tube.



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