Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: ilyaa on June 11, 2014, 03:57:37 AM

Title: peavey classic B coupling caps
Post by: ilyaa on June 11, 2014, 03:57:37 AM
fixing this one up for a friend -

incidentally i at one point had 3 of these identical amps in my garage -  a popular one in my milieu, i guess....

anyway, this one was doing something weird and i traced the problem to the phase inverter (solid state differential amp), which was giving me wonky voltages. replaced the transistors and all voltages are back to normal.

amp sounded fine - nice and loud - but had an unpleasant kind of congestion and loss of volume still when cranked up.

noticed the bias voltage for the PI (which should have been 330V) was only at 260 or so. saw that the last resistor in the PS filter network on the main cap was not, in fact, 68K but 87K, for some reason. change that out and voltage is closer (now at about 300V) - the first resistor in the HT filtering is 1K (instead of 400), so that might be it, but ill leave that as is for now.

the amp is making power. the slightly low HT voltages seem an unlikely suspect. i have noticed, looking through the amp, that many of the smaller coupling/DC blocking caps have a white residue on them as if they have suffered a tiny explosion. is this a sure sign of age/wear? should any caps with this kind of sign of stress be replaced? just wondering if that's a clear indicator or one of those 'maybe you can change it but it's probably okay' kind of things.

thanks!
Title: Re: peavey classic B coupling caps
Post by: g1 on June 11, 2014, 12:34:45 PM
  I have had new caps like that with some kind of white powder on them, it's not leakage.  Generally you only see issues like that with electrolytics.
Title: Re: peavey classic B coupling caps
Post by: nashvillebill on June 11, 2014, 04:07:52 PM
There are several electrolytics being used as coupling caps, have you replaced all the electrolytics?

If any caps look suspect--even film caps-- I'd replace them.  And getting that voltage back up a few more volts might help too.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on June 30, 2014, 11:16:30 PM
NOTE: in the subject i classified this as a classic B - its actually a Classic A! who knew?! (schematic attached)

i swapped out the electrolytics in the preamp (some of the other ones looked newer...) and i think this amp is as good as its going to sound -

im fixing another one (identical amp), though, and am a bit puzzled -

it had the same phase inverted issue - i changed those transistors and that seemed to do it, except the PI signal swing was low
the JFET driving the phase inverter was giving me weird voltages so i changed that out and im getting about ~150V p-p when the bias is turned all the way down to -75V - so the PI is working good, i think!

BUT, the amp is only making like 12 watts RMS (measured at 4 ohm dummy load)

everything up to the power tubes is looking good, but they cant seem to deliver power. they are pretty much new, ive only used them a little bit in some other amps, so im really hesitant to blame the problem on them, but they are NOT brand new - could anything else be responsible? there is nothing between them and the load but the OPT and thats working so - any thoughts?

i was thinking maybe the big filter cap, although giving me good static voltages, maybe cant handle the voltages when the amp is driven? that cap is definitely old - could it be the culprit? i took some voltage measures when slamming a 1kHZ wave into the amp, and all the voltages are lower than their static values - is that to be expected? obviously im measuring DC when there is some activity (not quiescent), so maybe those values are useless.
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 01, 2014, 11:19:27 AM
BUT, the amp is only making like 12 watts RMS (measured at 4 ohm dummy load)

everything up to the power tubes is looking good, but they cant seem to deliver power. they are pretty much new, ive only used them a little bit in some other amps, so im really hesitant to blame the problem on them, but they are NOT brand new - could anything else be responsible? there is nothing between them and the load but the OPT and thats working so - any thoughts?
you need some 50V RMS to drive those tubes, do you have them?
To be more precise, signal peak at the grid *must* be able to beat bias voltage and drive that grid a few mV  positive , that´s when the tube delivers maximum possible current.

Quote
i was thinking maybe the big filter cap, although giving me good static voltages, maybe cant handle the voltages when the amp is driven? that cap is definitely old - could it be the culprit? i took some voltage measures when slamming a 1kHZ wave into the amp, and all the voltages are lower than their static values - is that to be expected? obviously im measuring DC when there is some activity (not quiescent), so maybe those values are useless.
No, it´s normal to drop somewhat, say 10/15% under load.

One doubt: when you measure those 12W, do you scope the output and reach visible clipping?
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 01, 2014, 11:36:21 AM
Quote
you need some 50V RMS to drive those tubes, do you have them?

yeah ive got about 50V RMS at the grid - maybe a bit lower. and 12W at the output with visible clipping. if the PI was giving me a bit less than 50V RMS at the grid, would that really make the difference of 30+W at output, though?

Title: Re: peavey classic B coupling caps
Post by: Roly on July 01, 2014, 11:48:46 AM
What I would do.

Hook it up to my dummy load (and wattmeter  ;) ).

Crank it up until I see clipping on the load, then see where that was coming from, i.e. is the PI clipping, or the output stage?

I hate to say it, but low output with everything else good (I assume the voltages and currents around the OP stage look reasonable?*) could be a shorted turn in the OPT.

So I'd set up some OPT tests, perhaps try to measure its turns ratio using heater power.

There's also the kickback test with the battery and neon.

(* are they?)
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 01, 2014, 07:53:54 PM
Quote
if the PI was giving me a bit less than 50V RMS at the grid, would that really make the difference of 30+W at output, though?
Wrong question.
The sequence is this:
1) Class AB1 biased tubes reach maximum current, hence maximum power (whatever it is) with grids at 0V.
So peak driving voltage must be *at least* bias voltage.
2) Peak power fed into the OT will be peak current squared into plate impedance.
So if we do not get expected current, we won´t get expected power ... simple as that.
And since it´s a squared function, getting half the current means getting one fourth the power.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 02, 2014, 11:33:38 AM
hmmm okay heres what im getting

with the bias voltage at about -50V, one side of the PI is giving me about 45V RMS and the other side only about 35V RMS - could this be the problem?

ON SECOND MEASURE: the figures above are when i measure at the collectors of the PI transistors - if i measure at the grids of the power tubes (after the grid stoppers) i get about 35V RMS on each grid.

thats reflected in the ouput, where one side clips way harder and sooner than the other...and there is pretty bad crossover distortion, too, when the PI isnt even clipping yet...

is a little imbalance a good thing or is that too much?

(the output clips way before the PI, also)

ALSO: the DC voltages around the PI dont look perfect. oddly enough, on the power supply end of the 82K 1W resistor, the voltage looks high (should be 460 and its at about 490), but on the PI end, its low (should be 300 and its only 260). the collectors of the PI transistors are a bit low at 130-140V, instead of 150. could the PI bias be off, causing the low output? (these measurements are with the power tubes installed - without the power tubes in, all the voltages look just like their schematic values)
Title: Re: peavey classic B coupling caps
Post by: Jack1962 on July 03, 2014, 06:25:30 AM
if your using a light bulb limiter (I hope you are) the voltages will be lower than those on the schematic , from what I have read here check your PI tube or try a known good tube in that socket .
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 03, 2014, 11:42:22 AM
Pretty bad crossover distortion translates to horrible biasing plus hard to drive (hence low power) tubes.
And you might simply have worn/bad/poor quality tubes.
Dont trust just the bias *voltage*  but its effect on the tubes.
Add a 1 ohm cathode resistor to each tube, measure voltage across it, 1mV means 1 mA current, and bias so they are around, say, 30 mA each.
Post results because I guess both tubes are not matched and then we continue.
Title: Re: peavey classic B coupling caps
Post by: g1 on July 03, 2014, 12:31:07 PM
on the power supply end of the 82K 1W resistor, the voltage looks high (should be 460 and its at about 490), but on the PI end, its low (should be 300 and its only 260). the collectors of the PI transistors are a bit low at 130-140V, instead of 150.
Where are you getting those "should be" voltages from?  Are you using a different schematic than the one from your first post?
Title: Re: peavey classic B coupling caps
Post by: Roly on July 03, 2014, 01:08:51 PM
I'm still thinking shorted turn in the OPT (if the OP bottles aren't sick "they are pretty much new").

Note that the driver diff pair are within the NFB loop and may only be reflecting a downstream problem - such as a shorted turn in the OPT.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 03, 2014, 03:19:45 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:)
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 04, 2014, 06:51:19 AM
Up to 20% difference in DC voltages is not usually a problem, so don't worry about 460 vs 490 or even 260 vs 300 .
I'd worry today about *real* voltage needed for each tube passing, say, 30 mA which is an indirect  indication of how worn they are.
As in: a tube with no emission left will need a very low bias voltage just to pass idle current.
And if they are confirmed good and reasonably matched and drive is there, yes, the dark possibility of a shorted OT raises its head.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 04, 2014, 12:17:21 PM
they are passing like 37 mA with about -50V on them - that seems normal, right?
Title: Re: peavey classic B coupling caps
Post by: g1 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?
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 04, 2014, 01:32:06 PM
yeah screen voltage is more or less equal to plate voltage on both power tubes
Title: Re: peavey classic B coupling caps
Post by: nashvillebill 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?
Title: Re: peavey classic B coupling caps
Post by: Roly 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;
(http://www.ozvalveamps.org/repairs/xform_test.gif)
Title: Re: peavey classic B coupling caps
Post by: J M Fahey 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.
Title: Re: peavey classic B coupling caps
Post by: nashvillebill 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....
Title: Re: peavey classic B coupling caps
Post by: ilyaa 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?
Title: Re: peavey classic B coupling caps
Post by: J M Fahey 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.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 06, 2014, 12:39:40 AM
wait - it turns on with 90V?

roly's diagram has it hooked up to a 6V...
Title: Re: peavey classic B coupling caps
Post by: J M Fahey 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
Title: Re: peavey classic B coupling caps
Post by: Roly 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 (http://www.allaboutcircuits.com/vol_1/chpt_15/2.html)
and;
http://www.electronics-tutorials.ws/inductor/inductor.html (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).
Title: Re: peavey classic B coupling caps
Post by: ilyaa 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).
Title: Re: peavey classic B coupling caps
Post by: Roly 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

Title: Re: peavey classic B coupling caps
Post by: J M Fahey 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.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 07, 2014, 11:54:07 AM
when i test the center tapped primary, does it matter which two leads of it i test across?

also, where i live (in the san francisco bay area) there are a few too many amp and music 'enthusiasts' for any amp to survive in a junkbin long enough for me to find it - im fixing this for a friend and hes okay to buy a new OT -

any suggestions where to get one online? ebay seems like a bit of a crapshoot for something like this as far as getting one with appropriate specs that does in fact work (no more shorted turns, please!) - is triode electronics reliable?
Title: Re: peavey classic B coupling caps
Post by: DrGonz78 on July 07, 2014, 06:34:16 PM
I was really impressed with Classic Tone on how fast it arrived at my door (phoenix) and the quality of their transformers. I ordered the transformer on Monday morning and it was at my door Wednesday.

http://www.classictone.net/

Edit: Note that I am pretty sure I ordered from Amp Parts Direct and I think you were already referring to Triode USA. Both sell those brands.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 07, 2014, 10:39:10 PM
this one is fine, right:

http://www.classictone.net/40-18000.html

?

Title: Re: peavey classic B coupling caps
Post by: DrGonz78 on July 08, 2014, 06:45:41 AM
That one looks good to me. Also curious if that OT is mounted by rivets? The Peavey Classic that I have has rivets mounting the OT in place. Pain in the butt that you have to drill them out or something. Anyone know good ways to get rivets out the best?
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 08, 2014, 11:37:22 AM
yeah definitely rivets.....
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 08, 2014, 04:24:49 PM
Same nominal diameter drill so in theory at least it drills out just the aluminum shaft without enlarging the mounting hole.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 12, 2014, 09:35:32 PM
alright got the rivets out

got the new OT - tested it with my tester and it looks good

plugged it in, wired it up, etc.

all voltages look good

BUT when i turned the amp on and tried to bias the tubes up, something bad happened - as soon as i turned the grid bias positive enough to let a tiny bit of current pass, seems like the floodgates opend and the tubes started passing like 140mA right away! and started to glow blue....i tried a couple different sets of tubes and they all did it - whats happening?? seems like there is some kind of path for current that there shouldnt be but i cant imagine what it is - i didnt change anything in the amp - just took the old OT out and out the new one in.....

....(only thing that looks weird in the amp is that one of the death caps (this amp has a 3 prong cord but still has the old death caps in it) looks it exploded inside - it looks all bloated!)
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 12, 2014, 10:37:25 PM
1) pull that death cap off ... NOW.

2) bias voltage is *never*  positive.
Set it, without the tubes, to what the schematic asks for or , say, to -52V just to go to a classic value as a starter.

3) 140mA idle (by the way, how do you know that?) .... strongly blue tubes ... all points to loss of bias.
Check that (bad trimmer, cracked connection, etc.)
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 13, 2014, 04:04:30 AM
woo!

1) pulled the death cap

2) i know bias is never positive - i meant as i made it more positive/less negative

3) im measuring across a homemade bias probe that interjects a 1ohm resistor between cathode to ground - but i think i figured it out!!

there did not seem to be any issues with the trimpot or any of that stuff. i scoped the power tube grids and turned on the amp and waited for the tubes to start pulling crazy current and lo and behold i got a huge AC signal on the grids with no input or anything! i opened the bias filter cap - no difference. i opened the caps coming from the PI and voila! once those were open the tubes biased up normally and current stayed where it should. i guess those were leaky! the high voltage on the PI collectors must have been leaking through those coupling caps and appearing as a large signal on the grids, pulling current through as if i was driving them!

so ill just some new caps for that spot pop them in and we'll see how that goes -

im curious why the tubes biased up properly with the old/bad OPT - any ideas? if those caps were leaky they must have been leaky before, too, but this problem arose only once i put the new OPT in....
Title: Re: peavey classic B coupling caps
Post by: Roly on July 13, 2014, 09:32:15 AM
"AC signal" ain't leakage!

The OPT phasing is incorrect so NFB is actually PFB and the OP stage is jumping into full power oscillation when the bias allows.  It stopped because you broke the signal loop. (strong blue glow means healthy emission BTW)

Reverse anode connections (or secondary tho the anodes are normally easier) and try again.

A shorted turn on the OPT is an AC condition and bias is a DC condition so the shorted turn can have no effect of the DC idle conditions.
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 13, 2014, 01:57:27 PM
oh, man!

doh!

you're right, of course - put the caps back, flipped the anode connections and we are all good! 49 watts into 4 ohms. and sounds good!

i learned something! i knew about transformer phasing but for whatever reason figured that because this was a new one and was made for exactly this kind of amp why should it not be in phase? but i guess its a crapshoot! should have tried that FIRST not last!

thanks, guys!

now on to the tremolo that isnt tremoloing......
Title: Re: peavey classic B coupling caps
Post by: Roly on July 14, 2014, 07:03:09 AM
I'm 65, built my first radio when I was about 12, and I'm still learning.

Technique tip: when replacing an OPT initially leave the NFB connection off (if it has one, not all guitar amps use NFB).  Pass a signal at low level through the amp, then touch the NFB connection onto where it should go on the secondary.  If the phasing is correct the level will drop, if incorrect the level will rise or even oscillate.  This is more reliable than just "no oscillation" because many valve guitar amps don't have enough feedback to cause oscillation if incorrect (and yes, I have been caught by this  :-[ ).

Lead colours on a specific OPT should be dependable, but as you have just discovered, not always.

Quote from: ilyaa
now on to the tremolo that isnt tremoloing......

First step, check that the LFO is actually oscillating, anode and cathode voltages.  These normally require the gain of a 12AX7 and a common problem is that somebody has replaced it with a 12AU7 or 12AT7 (because they look the same).
Title: Re: peavey classic B coupling caps
Post by: J M Fahey on July 14, 2014, 08:01:34 AM
This particular amp has an SS tremolo, but standard troubleshooting still applies.

1) as suggested, check that you have (low frequency) oscillation .
Or start by checking DC voltages there, as shown on the schematic.
*If* you use a needle multimeter you´ll see it wiggle up/down.
The footswitch grounds oscillator bias to kill it, check that too.

2) that oscillation must reach the FET

3) the FET rhytmically grounds preamp signal through the depth pot

4) the bias pot adjusts the best tremolo effect

So you see that you have a lot of stuff to check. ???
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 15, 2014, 05:33:48 PM
k its all good -

i guess i just had to "tune" it using the tremolo bias pot -

i turned the pot with my scope on the preamp output -
at some settings there was no tremolo at all, until i hit a sweet spot where it looked great, after which if i kept turning it disappeared again - i was not able to find a setting that was 'perfect' - no matter where i set it the depth knob seemed to not really give me a full range of tremolo, only activating the effect once i hit 5 or 6 - but perhaps thats just the nature of this effect!

anyway, amps working great now, thanks, guys!!
Title: Re: peavey classic B coupling caps
Post by: ilyaa on July 21, 2014, 01:37:38 PM
one quick follow-up question which applies to this and many other amps:

whats the right way to interpret the two speaker jacks on the output?

the transformer needs 4 ohms for maximum power transfer - currently, there are two 8ohm speakers in parallel connected to one of the jacks. if i wanted to connect another cab, would it appear in parallel or in series with the first jack? looks like parallel.

so the only way to properly load the amp using both jacks would be two separate 8 ohm loads, is that right? or is there something im missing?
Title: Re: peavey classic B coupling caps
Post by: Enzo on July 21, 2014, 05:12:46 PM
Look at the schematic.

On the V Classic, there are two jacks, marked MAIN and AUX.  See the tip contacts are connected together?   That is a parallel wiring.  The only difference between the jacks is that the MAIN jack has a shorting contact.   That protects your output transformer.   The bottom line of that is simple:  You have to plug into the MAIN jack first.  The AUX is only used for a cord to a second cab.

Matching is not so much about "maximum power transfer" as it is about sounding right.   Remember, these are just guitar amps, not precision laboratory systems.  It won;t hurt the amp if you are off by a step on impedance.   If you plug one 8 ohm load into the amp, it will still be happy.

If you have a pair of parallel 8 ohms in there now, that is a 4 ohm load.  If you then add another cab, it will parallel those, and your total will be lower than 4 ohms.  Wonb't hurt the amp, but you may or may not like the results.
Title: Re: peavey classic B coupling caps
Post by: Roly on July 22, 2014, 01:05:21 PM
Quote from: ilyaa
whats the right way to interpret the two speaker jacks on the output?

What's the right way to wire an XLR?  There are two ways you can do it (keeping ground on the same pin), and both are used, PLUS people like me use XLR's off broadway as speaker connectors &c.  How many ways can you stuff a three-pin standard?

So no, there isn't a standard for this either, and if there was, somebody would break it, so the only real way to know what you gear does for sure is go inside and have a look.  They may be just connected in parallel as in Fenders and hang the mismatch, or they may have switching contacts on the Aux to select a lower tapping - could be anything depending on how many the designer had for lunch.


Generally speaking valve amps are more tolerant of load variations than transistor amps and are not particularly bothered by a factor of 2:1 high or low on nominal (e.g. 16 or 4 on "8", but the maximum available power may be reduced).  Guitar amps tend to the low side plate-to-plate impedance of the Hi-Fi "optimum" for minimum distortion, trading distortion for power.
Title: Re: peavey classic B coupling caps
Post by: Enzo on July 22, 2014, 01:25:02 PM
Except in this case, there is a schematic telling us how it is wired, or was intended to be wired if someone has changed it.

Peavey Classic B series.