Bass tube amp head - no voltage readings on filter caps?

[banjerpickin]

Hey guys and gals, I know this forum is for pedals but I had a quick question about amp repair.  I'm sure many of you will be knowledgeable due to the fact you probably dabble in both!

So I've repaired a few amps in my day, safely draining the filter caps before prodding around inside the chassis.  Last night I took apart my old Fender Bassman 20 for the first time so I could replace the input jack, which was having connection issues.  It's a somewhat obscure PTP amp made only in 1982 (about 2-3000 made) with only 4 control knobs and a 15" speaker.  Supposedly it's the last PTP amp that Fender made before moving completely to PCB.  It's impressive that all of the OEM components lasted 30+ years, I've never even had to swap a tube out!  So, except for the jack, it's in tip top working shape. 18 watts and runs at about 350 volts according to spec sheet.

So I let it sit overnight after being played at an hour long rehearsal, and then I pulled the chassis out and took a measurement of the 2 70uF filter caps, attaching my ground lead to the ground terminal.  I got a reading of 5 mV from both caps.  Thinking my DMM had gone bad I carefully used my "not quite as safe" pocket DMM and got the same readings.  Thinking they both had gone bad I measured a pedal I had open on my bench and got appropriate readings.  Then I really started scratching my head!  I couldn't find any info on whether or not it has a self draining feature; many other amps by Fender from that era do.  However, in other Fender amps with the self draining feature, I've seen it only drop to about 12-16V unless you drain it the rest of the way.  I should have been measuring around 16V or 300+V, not 5 mV.

Confused, I decided to tempt the Tube Amp Gods and broke rule number by plugging in the chassis and turning it on with my DMM still measuring, curious if I'd see a spike in volts (speaker was plugged in, too).  Still 5 mV.  I turned the amp off and carefully attached the ground clip directly to the chassis, just in case.  Now I've got 5.2 mV (whoop-dee-doo). 

Seeing that the issue with the jack was only that it had become bent, I carefully used my shock-safe pliers to bend it back into place.  And, that problem was solved.

Anyone care to help me understand what was going on with the voltage readings?

[thermionix]

Well a tube amp won't amplify with only 5mV.  So if it's working, making a sound, you have the voltage in there somewhere, just not reading it right for some reason.  Maybe oxidization on the solder joint or component leg you're probing with your meter(s).

Do you have a schematic?  If you post it we can see if the filter caps are self-draining.  In many Fender amps you have to flip the standby switch on to drain them all.  When in doubt, bleed each cap with a jumper wire before commiting your body parts.

[banjerpickin]

In many Fender amps you have to flip the standby switch on to drain them all.  When in doubt, bleed each cap with a jumper wire before commiting your body parts.

Here's the schematic: https://www.elektrotanya.com/PREVIEWS/63463243/23432455/fender/fender_bassman-20_sch.pdf_1.png
You can see the filter capacitor assembly in the bottom right of the page.  I'm not quite sure what I'm looking for in a bleeder resistor, my understanding is that it would be wired in parallel to the caps and go straight to ground (just like my draining stick).  I see that 15K going to ground parallel to the 70uF's, could that be it?

It doesn't have a standby switch, only an on/off switch.  It was very clean on the inside, I didn't see any oxidation, but that certainly doesn't mean it wasn't present!  Even though I wasn't getting a reading I should have noted that I did attempt to drain the caps using my draining stick (10K 10 Watt resistor) for about 15 minutes each before using the pliers, should have been more than enough time as long as there was an actual connection going on. 

Thanks for the help!

[banjerpickin]

Good lord every time I edit one of my posts I end up creating an entire second message. 

[PRR]

> every time I edit one of my posts I end up creating an entire second message.

Depending on your forum settings, clicking EDIT opens a new edit-box but the browser sometimes drops to the Quick-Reply box. Which looks nearly the same.



> I see that 15K going to ground

Follow that around the twisted drawing. That is Bias supply.

I don't see any for-sure drain. Everything leaks, so it will go to Zero some day. Often if you turn-off the residual cathode-glow will suck it down to 50%-25% in a few seconds.

> I decided to tempt the Tube Amp Gods and broke rule number by plugging in the chassis and turning it on with my DMM still measuring

No, this should be Standard Practice. Watching the B+ while the amp works. BUT you treat it like a basket of poisonous serpents. Who are invisible. And tricky. Pre-connect the meter. Stand back. Keep your hands out and off the chassis OR meter. Use a switched power-strip, or use a short plastic tube to work the power toggle.

Why you are not getting 400V DC, I do not know. Oxidation DOES happen, almost instantly, so a meter probe has to be jiggled. (Which means toothed clips, NOT poking a slippery probe at live snakes!) Some meters don't read over 300V. DC/AC switch confusion is popular (discounted cuz you say you got OK numbers on a pedal.)

[thermionix]

No they aren't self-draining in your amp.  Cool that you have a draining stick.  If you just have a jumper wire handy you can connect a preamp tube plate (pin 1 or 6 on a 12AX7) to ground.  And with no standby switch, that's it, all your caps will drain.  Probably good to go in 30 seconds, but always double check, you know.

[banjerpickin]

Depending on your forum settings, clicking EDIT opens a new edit-box but the browser sometimes drops to the Quick-Reply box. Which looks nearly the same.

THAT is good to know.   

Follow that around the twisted drawing. That is Bias supply.

Oop yes I see it now. 

Pre-connect the meter. Stand back. Keep your hands out and off the chassis OR meter. Use a switched power-strip, or use a short plastic tube to work the power toggle...

...Why you are not getting 400V DC, I do not know. Oxidation DOES happen, almost instantly, so a meter probe has to be jiggled. (Which means toothed clips, NOT poking a slippery probe at live snakes!) Some meters don't read over 300V. DC/AC switch confusion is popular (discounted cuz you say you got OK numbers on a pedal.)

I do have some nice clips that plug straight into my DMM which I've always used to measure volts in the other amp chassis.  I wiggled them around a bit (from a safe distance) to no avail.  Here's the DMM I have, supposedly rated up to 1000V DC:  https://www.amazon.com/gp/product/B00EYYJRC0/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

I kind of regret getting the auto ranging model, there is something that feels way more accurate about turning the dial to the correct max value.  My little pocket DMM from RS isn't auto ranging but rated to 500V DC, so it should have been just as capable of reading the measurement.

No they aren't self-draining in your amp.  Cool that you have a draining stick.  If you just have a jumper wire handy you can connect a preamp tube plate (pin 1 or 6 on a 12AX7) to ground.  And with no standby switch, that's it, all your caps will drain.  Probably good to go in 30 seconds, but always double check, you know.

Draining stick was easy to make, just bought some of those cement resistors on Amazon and a pair of quality small electronics gator clips for like $10.  Soldered the resistor to one of the clips, and then used electrical tape to put a few wine corks together (so far my wife's only contribution to this hobby) to make a safe little handle.  Has worked great so far, and I usually leave the DMM plugged up so I can watch the volts drop, because, ya know, science is cool.

Well, the good news is it worked great at a show last night with the jack fixed, so I'll probably just leave it alone as it seems to be working just fine.  Pretty cool little bass amp, perfect for stage monitoring or recording due to the line out in the back.  Plus it drives all of my elitist musician friends crazy I have a PTP wired amp

[GibsonGM]

No they aren't self-draining in your amp.  Cool that you have a draining stick.  If you just have a jumper wire handy you can connect a preamp tube plate (pin 1 or 6 on a 12AX7) to ground.  And with no standby switch, that's it, all your caps will drain.  Probably good to go in 30 seconds, but always double check, you know.

What would happen if you

1) touched the plate with your finger
2) touched the other end of the jumper to connect it to ground
and
3) how would that be different than just touching a charged cap with your finger?

I've done this too, but I grab the alligator clip with INSULATED pliers - not my hand! - to put it on the plate, or something connected to it... 

Using the shorting stick is recommended procedure - with a nice fat resistor or 2 on it, even better, as it doesn't actually SHORT but drains the caps...

[vigilante397]

I've done this too, but I grab the alligator clip with INSULATED pliers - not my hand! - to put it on the plate, or something connected to it... 

I use pretty intensely insulated alligator clips I haven't been zapped in quite a while, but I've had my fair share of 400V to the hand (HAND, SINGULAR, ONLY ONE HAND IN THE AMP AT A TIME) to know that it is ALWAYS better safe than sorry.

[thermionix]

No they aren't self-draining in your amp.  Cool that you have a draining stick.  If you just have a jumper wire handy you can connect a preamp tube plate (pin 1 or 6 on a 12AX7) to ground.  And with no standby switch, that's it, all your caps will drain.  Probably good to go in 30 seconds, but always double check, you know.

What would happen if you

1) touched the plate with your finger
2) touched the other end of the jumper to connect it to ground
and
3) how would that be different than just touching a charged cap with your finger?

I've done this too, but I grab the alligator clip with INSULATED pliers - not my hand! - to put it on the plate, or something connected to it... 

Using the shorting stick is recommended procedure - with a nice fat resistor or 2 on it, even better, as it doesn't actually SHORT but drains the caps...

I guess I should have been more specific.  I use an insulated wire (duh) with insulated alligator clips.  Clip to ground first, then to a preamp tube plate, generally one that has a 100k or 220k plate load resistor.  It's usually easier to clip to the resistor lead as the 9-pin socket can be pretty crowded.  Then the caps drain out through the resistors in the power supply plus that plate load resistor.  I've never had a problem doing it this way.  After the 30 seconds or so of draining, I leave the ground clip connected, and touch the other end to the + of each filter cap just to make sure I got the last few volts.

[GibsonGM]

Nice!  That's not a bad way, altho I do prefer a stick.    One time I did the clip trick, did my work....turned the amp on, and watched some nice blue smoke waft up from the cap I left connected...ha ha.....lucky I didn't kill the PT! 

I left it connected because of noob fear of that "mystery returning voltage" that caps sometimes experience. Later learned it was not any issue...

[banjerpickin]

I left it connected because of noob fear

There is certainly a much high barrier of entry in terms of safety to getting into building tube amps, I still haven't gotten over my noob fear.  After I shocked myself real good assembly a tube preamp for my record player, I started taking it a bit more seriously.

[GibsonGM]

I left it connected because of noob fear

There is certainly a much high barrier of entry in terms of safety to getting into building tube amps, I still haven't gotten over my noob fear.  After I shocked myself real good assembly a tube preamp for my record player, I started taking it a bit more seriously.

Oh, for sure, Banjer. Mine was a long time ago, but the lessons stay fresh! It pays to take some extra precautions.

My big shock was trying to clean out a TON of cig butts from the motor assembly of a trash compactor.  It was unplugged.    I was using a finger to ream them out of the motor...and woke up across the room, and to this day don't actually remember the shock.  BFC in there.   My parents thought I was h*gh, LOL!   

[PRR]

> regret getting the auto ranging model

Get the magnifying glass, unfold the origami, there's instructions in an English-like language but very tiny.


http://vctec.co.kr/web/product/BAE/img/108752-5.pdf

I assume that press-press-press cycles through 600V, 60V, 6V, 600mV. And presssssss throws it back to auto.

I often do this on the Fluke. Set to 200V range, it does not have to cycle through all the lo-V ranges to tell me 123V. While my wall-voltage is pretty sure 108V-125V, pre-set 200V range also makes it harder to mis-read 100+mV leakage as normal 100+V. And when I did more tube amps I wanted to KNOW RIGHT NOW (faster than auto-range) if the cathode voltage was going to a happy 24V, or skyrocketing to way-excess. (There was the 400V supply I mis-wired for 800V, and noticed because a manual meter on 500V range was going up way too fast.)

[GibsonGM]

My cheap Extech does something like this, when measuring current. 

If it's on A rather than mA, it will show me having a draw of 2.3A or something when I'm measuring current into an opamp distortion or such thing.  If the reading looks like BS, it probably is...

[banjerpickin]

Cool I will definitely try that out next time I'm using it!  Yes that is also a gripe that I have, it takes way longer to auto range than if I just selected it myself.