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Author Topic: peavey classic B coupling caps  (Read 13540 times)

ilyaa

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Re: peavey classic B coupling caps
« Reply #15 on: July 04, 2014, 12:17:21 PM »
they are passing like 37 mA with about -50V on them - that seems normal, right?

g1

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Re: peavey classic B coupling caps
« Reply #16 on: July 04, 2014, 12:18:38 PM »
Quote
Where are you getting those "should be" voltages from?  Are you using a different schematic than the one from your first post?

its a classic A, not a classic B, i discovered

the new schematic is attached above (in my post that starts with NOTE:)
Ok, got the correct drawing now, sorry about that!
One thing very critical for power is screen supply.  Have you checked the voltages at pin 4 of both power tubes?

ilyaa

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Re: peavey classic B coupling caps
« Reply #17 on: July 04, 2014, 01:32:06 PM »
yeah screen voltage is more or less equal to plate voltage on both power tubes

nashvillebill

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Re: peavey classic B coupling caps
« Reply #18 on: July 04, 2014, 05:05:09 PM »
I certainly don't claim to be an expert, but I've got a few questions:

1) When you checked the amp's output power, were you still on the DBT?
2) The DBT isn't indicating any shorts, so why would we still want to have it on the DBT?  Seems to me that we no longer need any current limiting on the amp's power supply primaries if we're trying to get the amp to full power!
3) The amp apparently still has the original filter caps.  Question a): Is the amp still holding close to its designed 470 and 460 volts when it's pushed to full volume--what do these DC readings fall to?  Question b): How much AC ripple do we see on the 470 volt supply, both quiescent (idling) and at full power? (merely switch the meter to AC)
4) On a similar note with the bias supply, what conditions are the two caps?  Similarly, what are the DC and AC readings on the bias voltage both at idle and at full power?

Roly

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Re: peavey classic B coupling caps
« Reply #19 on: July 05, 2014, 04:59:39 AM »
Quote from: nashvillebill
(merely switch the meter to AC)

Nope - we've been around this one before.  Most common VOM's and DMM's will see DC as AC and give wildly inaccurate readings.  Apparently some of the more expensive DMM's have a true AC input but most don't, they just peak rectify and assume no DC.  To measure ripple with one of these a cap, say around 0.1-1uF, of suitable voltage rating has to be used in series with the probe to block the DC.  On older moving coil multimeters this was sometimes inbuilt as a socket marked "Output" or just "Out". {the one in the multimeter to hand is 0.047uF at only 400V working}


@ilyaa please try this;

If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: peavey classic B coupling caps
« Reply #20 on: July 05, 2014, 07:31:05 AM »
they are passing like 37 mA with about -50V on them - that seems normal, right?
"They" means "each one passes 37mA" or "the pair passes 37 mA" ?
Anyway, if both pass similar currents , they  look fine.
Unfortunately we are eliminating other possibilities so the shorted OT possibility remains.  :(
Please do the test Roly suggests.

nashvillebill

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Re: peavey classic B coupling caps
« Reply #21 on: July 05, 2014, 05:45:41 PM »
Quote from: nashvillebill
(merely switch the meter to AC)

Nope - we've been around this one before.  Most common VOM's and DMM's will see DC as AC and give wildly inaccurate readings.  Apparently some of the more expensive DMM's have a true AC input but most don't, they just peak rectify and assume no DC.  To measure ripple with one of these a cap, say around 0.1-1uF, of suitable voltage rating has to be used in series with the probe to block the DC.  On older moving coil multimeters this was sometimes inbuilt as a socket marked "Output" or just "Out". {the one in the multimeter to hand is 0.047uF at only 400V working}


Sorry, I forgot about the averaging meters that can measure a DC battery as AC; I'm using a true-RMS Fluke. 

My old Simpson 260 analog meter did indeed have the Output connection (cap coupled) to measure AC on DC....

ilyaa

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Re: peavey classic B coupling caps
« Reply #22 on: July 05, 2014, 07:53:37 PM »
i dont have a neon light handy -

could i juse a 9V, an LED, and a resistor in the same configuration? is there something special about the neon light (brightness, sensitivity, or ???) that makes it ideal for this kind of test?

all im testing is if there is a shorted path around the light, so the light passes no current through it, right? if all is good, the LED will light brightly. if there is a short in the winding, it wont light at all or quite dimly. correct?

J M Fahey

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Re: peavey classic B coupling caps
« Reply #23 on: July 05, 2014, 08:21:04 PM »
Sorry but no.
A Neon lamp is needed there.
a) it turns on with some 90V applied.
2) it is visible with very low currents, a few microamperes.

What you are testing is whether the transformer still has  a very high inductance (good) or it lost most of it (shorted turn)

But you may get a neon lamp from a broken/dead electrical appliance such as a toaster, clothes iron, etc.
Ask the local retired electrician/repairman or search for suff people junks.
Or get a neon pilot light from a local electrical shop.
Or buy a $1 neon voltage indicator screwdriver.

ilyaa

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Re: peavey classic B coupling caps
« Reply #24 on: July 06, 2014, 12:39:40 AM »
wait - it turns on with 90V?

roly's diagram has it hooked up to a 6V...

J M Fahey

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Re: peavey classic B coupling caps
« Reply #25 on: July 06, 2014, 12:51:17 AM »
EXACTLY  :o  .... and that´s the trick  :loco   :duh

The 6V are NOT able to light the neon bulb.
But the **momentary** switch (you briefly pulse and release it) first drives some current into the transformer winding and then stops it .

I should say "tries"  to stop it because the transformer winding, being a high value inductor will violently try to oppose that by creating a high voltage, as high as it can, to keep that current flowing. :trouble

THAT VOLTAGE PULSE CAN REACH well BEYOND 100 VOLTS !!!!!!  :duh
That is, IF it is a healthy transformer.
One with shorted turns will not.

Amazing? .... your car ignition system uses the exact same principle to generate *thousands*  of volts for the spark plugs out of meager 12V  :o

Roly

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Re: peavey classic B coupling caps
« Reply #26 on: July 06, 2014, 12:12:01 PM »
When you close the switch the battery drives a current through the transformer winding and this current causes a magnetic field to build up in the transformer (or any inductor actually).

When the current is interrupted the magnetic field collapses very quickly and this collapse induces a "back EMF" (or voltage) in the windings.  The value of this voltage is directly related to how fast the field collapses and it can be very fast and therefore very high.

v = -L * di/dt

"induced voltage equals (minus) the inductance times the rate of change (amps per second) of current through the inductor".  If the rate of change, di/dt is large then the (induced Back EMF) voltage will be also.

The Kettering car ignition system typically generates 30-40kV for each spark.

Detail;
http://www.allaboutcircuits.com/vol_1/chpt_15/2.html
and;
http://www.electronics-tutorials.ws/inductor/inductor.html

When chasing problems with a new industrial controller I was getting spikes from relays being released that were confusing the logic, even 'tho all relays had "catch diodes" across the coils intended to prevent this from happening.  Now diodes don't turn on instantly, they take a few microseconds, and with a high speed CRO I was able to see a spike only a few microseconds wide but what I estimated to be 4kV high.  With a perfect inductor this collapse takes zero time and the induced voltage is therefore infinite - and it's bad enough with real-world inductors.

In the days before DMM's when you were checking inductor/transformer continuity you had to remember to keep your fingers clear of the wires or you would get bitten as you disconnected the probes (DMM's use much lower currents than moving-coil multimeters to measure resistance).  So you could just try connecting a battery across the primary and see if it gives you a shock when you disconnect it again after a couple of seconds.   :o   {on second thoughts...get a neon}

You should be able to buy an NE2 neon for under a dollar (or if you collect some used disposable cameras they come free).
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: peavey classic B coupling caps
« Reply #27 on: July 06, 2014, 04:59:13 PM »
sooo

i popped a neon out of some thing i had laying around and built the transformer tester -

i used a 9V and some diodes to get the voltage down to 6ish

the results were not encouraging but also a little confusing (trying to understand the dynamics of the transformer tester itself):

(all of these tests were done with all transformer windings open circuit - disconnected from anything)
a) the peavey transformer secondary did not light the bulb at all
b) the peavey transformer primary did not either (i wasnt sure how to test it (because its center tapped), but no configuration worked to light the neon
c) to make sure my tester was working, i tried it on the output transformer of a little champ i built last year - the secondary of this transformer also did not light the bulb! totally possible that output transformer has some issues because this was the first amp i had ever worked on and i probably turned it on open load who knows! the PRIMARY of this transformer did light the bulb however
d) for a final  test of my tester i tried it on an unused brand new transformer i bought for a mixing desk power supply im building - the primary windings are 125V windings and they DID light the bulb - the secondary windings are only 25V windings and they did NOT.

seems to indicate that the peavey secondary (and my poor little champ!) has a shorted winding. i guess a follow up question, though: should a healthy output transformer in a 50W amp be high enough inductance to give me 90V+ to light the bulb? isnt the amp only putting out about 40V p-p through the transformer secondary?

and: does it seem likely that the peavey PRIMARY also has a shorted winding? wouldnt this be fudging the voltages on the amp side of the output transformer?

and finally: its hard to tell because it flashes so quickly, but seems like only one of the little strips in the neon lights up when it does (if i just have the neon across line voltage from the wall - with a resister in-line, of course, both strips light up brightly).

Roly

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Re: peavey classic B coupling caps
« Reply #28 on: July 07, 2014, 08:21:31 AM »
Quote from: ilyaa
i used a 9V and some diodes to get the voltage down to 6ish

The driving voltage is not at all critical, we just want to get a few tens of milliamps flowing in the winding.

Sorry, I guess it wasn't clear that this test applies to the primary (which has lots of turns).  I wouldn't expect much result from a secondary.  The fact that you got a flash from the Champ tells us that it's okay, no shorted turns there, and that your tester is working as intended.

Quote from: ilyaa
does it seem likely that the peavey PRIMARY also has a shorted winding?

Sadly, it does.  Incidentally this test will show up a shorted turn on any winding because they are magnetically coupled and any shorted turn anywhere will suck all the collapsing field - no voltage, no flash; but you will only see a flash (if there is going to be one) on the side with lots of turns.  It is for this same reason that a shorted turn will almost always be in the primary, generally due to an internal flashover caused by driving the amp without a load, so don't never do hit.

You can try a little experiment with your power tranny (or whatever) and confirm that the primary give you a flash until you short the secondary and lose the flash, open the secondary and you get the flash back.  This simulates a shorted turn.

Quote from: ilyaa
seems like only one of the little strips in the neon lights up when it does (if i just have the neon across line voltage from the wall - with a resister in-line, of course, both strips light up brightly)

The voltage from the mains is bi-polar at 50/60Hz, so each half cycle one of the electrodes is negative and glows.  The Back-EMF from an inductor is mono-polar (like the driving battery), so your observation is correct  :dbtu:,  only one electrode should flash.

{You can have more fun than is legal with a single neon; you can make them oscillate, build a crude voltmeter, even detect radiation, and as you have seen they are a pretty sensitive voltage detector.  Just for reference, normally the current through an NE2 should be limited to around 1mA, a series resistor of 1k per volt.}

If you want to do a confirmation test on the suspect OPT you can try feeding the 6.3VAC heater voltage (or anything similar to hand) into the speaker side.  It should have a turns ratio of somewhere between 20 and 30 to 1, so 6.3VAC should appear as 6.3 * 20 = 126VAC to 6.3 * 30 = 189VAC on the plate side (careful!).  If it has a shorted turn it should draw a lot of current from the 6.3V (so be quick with your measurements), pulling it down to maybe 4 or 5 volts, and give a lot less than 100V on the plate side.

HTH

If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: peavey classic B coupling caps
« Reply #29 on: July 07, 2014, 09:27:43 AM »
Agree on 2 counts.
1) the pulse will be visible on the high turms (high inductance) windings; secondaries or low voltage windings don´t have enough.

2) unfortunately it looks like you have a shorted turns transformer :(
Start shopping for one.

FWIW maybe you can get for peanuts a dead amp deemed junkbin stuff and reclaim a working OT from it.

Any 2 x 6L6/EL34 one will do, no matter brand or model.