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Output Transformer Question and Draining Caps

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yyz2112

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Regarding my previous questions about my Peavey amp and odd hiss...
The filter caps are not axial as per the norm, but radial. Axials would be much easier to drain, as the legs are easily accessible. Any advice on draining radial caps?
Also, if it is necessary to replace the OT, would "any" replacement be suitable, as long as it is a 2 6L6 compatible unit?
 
yyz2112 said:
Also, if it is necessary to replace the OT, would "any" replacement be suitable, as long as it is a 2 6L6 compatible unit?
Click to expand...

Yes and no. Next to the tubes used and circuit design, the OT has a big effect on the tone and feel of the amp. Don't go on the cheap in this area. Check with Mercury, Hoyboyer, Hammond, etc. Also if there is anything you would like to change with the tone and feel, these companies can makes some great recommendations regarding OT design differences.
 
Normal warning.  Tube amps can kill if you don't have some sense about going inside them.  I am assuming you have a volt meter.  Put the ground (black) probe in a screw hole on the chassis and look for the rectifier diodes (I am assuming that the amp has these as well)  Check for voltage here.  One (or two) should be greater than the rest, and that should be B+.  It sort of depends on the rectifier circuit.  You should be able to clip a wire here as well to drain the voltage to ground with the proper tool.  Verify that the voltage is drained with the voltmeter.
Patrick

 
Patrick from Davis said:
Normal warning.  Tube amps can kill if you don't have some sense about going inside them.  I am assuming you have a volt meter.  Put the ground (black) probe in a screw hole on the chassis and look for the rectifier diodes (I am assuming that the amp has these as well)  Check for voltage here.  One (or two) should be greater than the rest, and that should be B+.  It sort of depends on the rectifier circuit.  You should be able to clip a wire here as well to drain the voltage to ground with the proper tool.  Verify that the voltage is drained with the voltmeter.
Patrick
Click to expand...

So I'm sure I understand...clip to the diode, as I otherwise would if the cap was more accessible?
 
In most of the common modern diode rectified amps there are four diodes.  I cannot stress enough to use the voltmeter to check this.  On the schematic they generally look like a diamond with all of the arrows (diode symbols) pointing towards one corner of the diamond.  The corner where the arrows meet is the high voltage one, B+.  The two corners connected to this are where the transformers lines come in, and the corner that the arrows point away from is generally ground.  This is commonly done on a circuit board as four diodes in a line, in adjacent parking spots, ( IIII )  The side of the diode with the line printed on it is the direction of the arrow, so if you check from those sides of the diodes with a voltmeter (with the black lead on ground), you should see the highest voltage side.  A setup with a resistor to ground should discharge it with out creating sparks or hurting things.  CB has a post that describes the bleed resistor setup, I'd look it up to figure out what you need.  I cannot stress enough to use the voltmeter to check this.  Set it at 600V and then check the voltages.  The bleed resistor set up will discharge the high potential across the amp's caps, completing the high voltage to ground circuit.  The resistor will let it drain slowly as to not fry you or other items in the amp. 

Remember that you are connecting high voltage to ground to drain it.  Only use one hand (you do not want the voltage going through you to ground) to connect the drain resistor to the high voltage.  Don't rest your hands on the chassis.  And I will say it a third time,

I cannot stress enough to use the voltmeter to check for the high voltage.
Patrick

 
yyz2112 said:
Regarding my previous questions about my Peavey amp and odd hiss...
The filter caps are not axial as per the norm, but radial. Axials would be much easier to drain, as the legs are easily accessible. Any advice on draining radial caps?
Also, if it is necessary to replace the OT, would "any" replacement be suitable, as long as it is a 2 6L6 compatible unit?
Click to expand...

Last first - you'll need to match the impedance of the output to the impedance of the speaker load.  Thats a given.  But... how big of a transformer for your amp is also a question.  That is, you can get the windings right, and the wattage right. but maybe the wattage is wrong for the tone you want.  If the transformer can handle the electrical load, the amount of iron determines its saturation point - or ability to "transform".  Cleaner - go over wattage and big iron.  More grind?  Go a bit short on wattage, and smaller iron.

As for draining caps - it would be a sad day if Peavey didn't put in self draining circuitry... but in case they didn't... just get a test probe and fit an alligator clip to the end of its lead.  Get a 2w 2megohm resistor (or 5megs works too), and fit that to the lead wire inside the test lead, or fit it within two layers of shrink wrap along the lead wire.  That must be a smooth splice - no spur wires sticking out to poke thru the wrap and zap ya, please.

Clip onto the chassis and touch the plus side of each cap... hold for a second.  If the circuit is what I imagine, touching one electrolyic hot side will drain them all, but test with a meter.

Electrolytics grow back.... so then ya need a double alligator lead wire to go from chassis to the hot leg of one cap to "keep" the voltage from climbing back up.
 
Death by Uberschall said:
...... the OT has a big effect on the tone and feel of the amp. Don't go on the cheap in this area.
Click to expand...

Aren't we glad there wasn't a Hammond or Mercuiry in the amp Clapton used to record Layla?

Sometimes cheap transformers are the BEST.  Hence "Radio Spares" in Marshall amps... cheap, but great. 

But... going with a proven tone OT is always good.  BTW, be sure to throw Hammonds very conservative transformer specs out the flippin' window, or you'll end up with a Hi-Fi instead of a guitar amp.
 
If your amp has been " OFF" for a while, I'd be surprised if theres any charge on any Cap, but that doesn't mean don't take precautions, if you get zapped, it's probably just gonna surprise you and piss you off.

I doubt it will kill you, I'm sure someone will chime in and say otherwise.  You'le probably get hurt more when you yank your hand out of there at the speef of WTF, and pull a muscle in your arm.

if you check points with a meter, that will likely drain anything down low enough to be safe.
 
Thanks for the info guys.
Last but not least. In re-reading some of the cap replacement info, I see a variac is mentioned to bring the amp up to power after the job is done. How important is this?
 
It is a safety measure I believe.  I dunno I have never seen am amp with one (other than EVH's, but that is something different)  I am guessing that it keeps the voltage from creeping too high too quick.  If an electrolytic cap is installed backwards (polarity wise) they boil on the inside and blow up.  Not pretty at all.  I would guess the variac would keep the voltage down so that you could prevent that, figure it out, and fix it without killing something.  But, the amp didn't have one to begin with so...  I am sure CB or one of the other guys that knows more could tell you more.  I have replaced caps in amps before and never used one.  I think that this is the first I have heard of it used that way.  Anyone else want to chime in?
Patrick

 
Death by Uberschall said:
Supposedly the variac is a safety measure and conditions the caps on the way up to full voltage.
Click to expand...

This is what I understood. Is it something that is always utilized?
 
The variac thing was started out of some half-a-brain information read in the old RCA manual, and appropriate for the capacitors made in the 50's.  Today's materials are not subject to the same stresses.  Also, Gerald Weber (cough cough) thought that this was one of the secrets to "bluz-tone" and advocated using a variac over something like a day and a half to "properly form" the capacitors.  Total BS, like a lot of what he spewed.

If you're sure they're in right, they're ok.  But, having said that.... its not a bad idea to use a variac AND a solid state rectifier plug-in (if your amp is tube rectified).  This is - a safety - in case you have a wire whisker with B+ shorting to ground, which will create a bit of a show when it does.  The reason you want a solid state rectifier, is because it will rectify at low voltage, whereas a tube rectifier needs the heater hot... which means you're already at 1/2 to 2/3 the operational voltage when the B+ comes... too high for safety.

I use a variac to run things up slow on the amps I BUILD, but not on the amps I service, since I've got the experience in the latter to know the job is ok.  First timers might want to use a variac, or have a buddy ready to unplug the power cord in case of the 4th of July being celebrated in your chassis.
 
Ya know, I offered advice regarding your situation here in another thread.

But considering you chose to be an @$$ in my clips thread, you can "forgeddaboudd" any more.  :icon_thumright:
 
All previous posts in this thread, excluding SL's, are the best part about this forum!
Good luck with repair, be safe.

good advice =CB=!
 
On some amps, closing the standby switch with the power off will drain the filter caps.
 
I'm definitely not going to trust that they've drained on their own, even if there is a drain circuit in the amp.
I build pedals, so I basically know how to measure voltage and understand cap polarity.Of course at far lower voltages, a mistake can't kill you.
I'll measure, have a drain jumper at the ready, and have a go of it.
 
Death by Uberschall said:
dbw said:
On some amps, closing the standby switch with the power off will drain the filter caps.
Click to expand...

That can work, but don't trust it completely. You can build a drain down jumper for next to nothing.

http://tubenewbie.com/discharge_capacitors.html
Click to expand...

Some Fenders have a path to ground on the rectifier side of the standby switch.  This works as a drain resistor for the filter caps on the other side of the choke when you close the switch.

http://www.schematicheaven.com/fenderamps/bandmaster_reverb_aa1069_schem.pdf
 
dbw said:
Death by Uberschall said:
dbw said:
On some amps, closing the standby switch with the power off will drain the filter caps.
Click to expand...

That can work, but don't trust it completely. You can build a drain down jumper for next to nothing.

http://tubenewbie.com/discharge_capacitors.html
Click to expand...

Some Fenders have a path to ground on the rectifier side of the standby switch.  This works as a drain resistor for the filter caps on the other side of the choke when you close the switch.

http://www.schematicheaven.com/fenderamps/bandmaster_reverb_aa1069_schem.pdf
Click to expand...

Yeah, they definitely exist, but again, what gets me is the warning about caps recharging without current being applied. I know something like that would happen to me. You don't know my luck.
 
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