Bleeder resistor - why not?

[pickupcentral]

On another Forum someone pointed out that the Vavle Jr. v2 has a bleeder resistor (R11) to drain the caps whenever you shut off the amp.

Why would this be a bad idea to install in my other amps?

I have an old Bassman 10 that don't work for sh*t, and I will be doing multiple repair sessions on it. If I install the bleeder resistor on it, then it will be that much safer to work on, no?

I plan on building some kits - why not install on them as well?

Thanks!
-Phil

[John Phillips]

There is no reason why any amp should not have a bleeder resistor - if the value is high enough, it won't affect the normal operation of the circuit at all, and it will definitely make draining the caps much easier, since you then don't have to actually do anything other than shut the amp off and wait a minute or two.

[pickupcentral]

Hi John!

What would be a good typical value? The on on the VJ is 220k 1 watt.

-Phil

[VaughnC]

Bleeder resistors are typically used to equalize the voltage across filter caps that are connected in series. However, from a manufacturers point of view, if there are no series caps in an amps power supply, adding that one bleeder resistor represents money out the door to them. Yeah, 220K will be fine.

[John Phillips]

220K is fine, but you may really need the 2W rating. (Assuming a maximum B+ voltage of 450V, which is the most you are likely to have in an amp without stacked filter caps - power = voltage-squared divided by resistance, that gives 0.92W, so a 1W resistor is much too close for safety.)

470K or 1M would work equally well, and not need such large resistors, but will increase the drain time - not enough to be a problem though really.

To be absolutely safe on the power rating (bearing in mind that the resistor will be inside a hot enclosed space, which reduces the effective rating) I would probably go with 470K 2W or 1M 1W for any B+ over about 320V.

[JimmyR]

Where would you put it in the circuit?

[John Phillips]

You need to go on a case-by-case basis, there is no one right answer.

Usually, directly across the first filter cap; or if the amp has a stacked first filter stage followed by a standby switch (like most of the higher-powered Fenders - this potentially allows the second and lower stages to remain charged, if the standby switch is turned off), across the second filter cap.

There are many amps that don't need it at all - 100W Marshalls are a good example, because the standby switch is upstream of all the filters and there are divider resistors on the second stage, so there is no time when all the caps won't discharge via the second stage.

[Rich M]

John, I respectfully disagree. In most marshalls, if the HT fuse blows, the first cap bank becomes isolated from the bleeders. I found this out the hard way changing a fuse on a Major that had been unplugged for 7 hours. At least the caps are good!

I've used much higher values (~2 Meg) so I can use lower wattage parts and minimally impact the circuit. It just takes longer to bleed off the voltage. An important consideration not pointed out is resistors have a voltage rating as well as a power rating. The resistor I ordered (from Mouser) were high voltage types and somewhat more expensive than your generic ones.

[John Phillips]

John, I respectfully disagree. In most marshalls, if the HT fuse blows, the first cap bank becomes isolated from the bleeders. I found this out the hard way changing a fuse on a Major that had been unplugged for 7 hours. At least the caps are good!

I've used much higher values (~2 Meg) so I can use lower wattage parts and minimally impact the circuit. It just takes longer to bleed off the voltage. An important consideration not pointed out is resistors have a voltage rating as well as a power rating. The resistor I ordered (from Mouser) were high voltage types and somewhat more expensive than your generic ones.
All true. Yes, I should have pointed out that the Marshall case is only if the HT fuse isn't blown (I did think about it while posting, too :(.) Sorry...

Good to find someone else that's been 'Majored' and lived to tell about it, though :). I can't remember exactly how mine bit me... I don't have a clear recollection of the seconds before it did :eek:. It was one of the worst two shocks I've ever had, anyway.

[Rich M]

Yeah, it reached out from the fuse holder - felt it all up my arm. :dude

[Ronsonic]

I'm not aware of a full size Fender amp that doesn't already have bleeder resistors. Hang on a mo.... http://www.ampwares.com/ffg/ .....
Yeah, the Bassman Ten's got 'em.

I've modded mine. I had the older 50W version (if you've got the 70 this won't all apply). Mine had an extra winding on the power tranny for a tube rectifier heater, so out came the diodes and in went a 5AR4. I rewired the front end to resemble a Super Reverb, without the reverb and trem. My speakers were bug eaten (pretty common in old unloved amps here in Florida) so I replaced them with 8 Ohm drivers wired series-parallel. The originals are 32 Ohm parallel. I also plugged the vents in the speaker cabinet. This is a huge improvement for guitar.

Anyway, bleed resistors are typically found in parallel with the first bank of filter caps. They also tend to equalize the work load between the caps when they are stacked like they are in most Fenders.

And yes, do be careful in there. At the least these things can punch your reset button and dump everything you had unsaved in RAM.

Ron

[John Phillips]

I'm not aware of a full size Fender amp that doesn't already have bleeder resistors.
Do you count a Deluxe Reverb as 'full size'?

At the least these things can punch your reset button and dump everything you had unsaved in RAM.
That's exactly what happened to me when I got shocked by the Major. To this day I don't know what I did wrong. I found myself sitting on the floor several feet away from my bench with a terrible cramp pain in my arm, and slight twitching. I'm probably lucky. The amp was actually on at the time though, it wasn't a filter cap holding its charge that did it.

[Old Tele man]

...imagine how a "tech" with an emplanted heart pacemaker "worries" about discharged capacitors!!!

[500VDC]

All true. Yes, I should have pointed out that the Marshall case is only if the HT fuse isn't blown (I did think about it while posting, too :(.) Sorry...

Good to find someone else that's been 'Majored' and lived to tell about it, though :). I can't remember exactly how mine bit me... I don't have a clear recollection of the seconds before it did :eek:. It was one of the worst two shocks I've ever had, anyway.

What was the other?

[Rich M]

Hey we should start a show on National Geographic channel: "When good amps go bad" :eek:

MY other bad shock was in 8th grade when I made a strobelight out of an old TV transformer and was too lazy and stupid to insulate the leads. Well, I was showing it to my mom when my arm leaned into the 500V lead off the transformer and send my arm hurtling involuntarily into the wall behind me. I remember there was a little hole burned into my elbow, but I still don't understand that - the current couldn't have been that high :messedup

[samwheat]

i put the bleeder resistor on the 1st preamp RC filter ...... it take more time to drain the B+ than if you put it across the 1st cap

[tremolux]

My "NDE" with a Major was around 96-97 on a cold winter nite up trying to locate an intermittant downstream and getting very impatient. I shut it off (I think), reached for the switch on the metal bench light mounted to the bench directly behind the chassis and the next thing I knew I was waking up in the dark (tripped the breaker) after a few seconds, laying flat on my back still in my chair with the back of my head throbbing and right arm feeling numb. A faint smell of ozone (or was that me :crazy ) in the air. Was very happy to finally figure out I was ok.....but it took a while.

My bench lights now hang from the ceiling and are switched by the entrance doorway. They have pulleys to raise and lower them for close up and the reflective bottom surfaces are made from glass painted with aluminum paint. No mo' metal near the bench.....tho Im not sure the light reflector was part of the problem.. I reach for stuff to my right or left now, never over an open chassis.

My other memory of that 1967 model amp was that it appeared to be factory stock with 6550s vs. KT88s....also had what looked to be stock Tungsram ecc83s....

[pickupcentral]

I'm not aware of a full size Fender amp that doesn't already have bleeder resistors. Hang on a mo.... http://www.ampwares.com/ffg/ .....
Yeah, the Bassman Ten's got 'em.
Ron
Would that be the one labeled 2K2, 7W? I'll have to double-check that it is there - whoever got into the amp before me did a bunch of "upgrades".
And yes, do be careful in there. At the least these things can punch your reset button and dump everything you had unsaved in RAM.
Ron
Measure twice, cut once. Or just measure twice. Or something like that. Point taken. I'm WILLING to learn about fixing amps, but I'm not DYING to learn.

Thanks!
-Phil

[John Phillips]

What was the other?
A tube TV. A lesson in not fiddling with things you don't know anything about. (I was only about 19 at the time, and thought I knew enough about electronics to just 'investigate'.)

Nice forum name, BTW :).

My "NDE" with a Major was around 96-97 on a cold winter nite up trying to locate an intermittant downstream and getting very impatient. I shut it off (I think), reached for the switch on the metal bench light mounted to the bench directly behind the chassis and the next thing I knew I was waking up in the dark (tripped the breaker) after a few seconds, laying flat on my back still in my chair with the back of my head throbbing and right arm feeling numb. A faint smell of ozone (or was that me :crazy ) in the air. Was very happy to finally figure out I was ok.....but it took a while.Hmmm... this seems like quite a common occurrence with these amps, doesn't it! :eek:

My other memory of that 1967 model amp was that it appeared to be factory stock with 6550s vs. KT88s....also had what looked to be stock Tungsram ecc83s...The Tungsrams might have been (with an ECC82 in the driver stage), but I'd be very surprised if the 6550s were. It was definitely designed around KT88s, and the US distributor didn't start fitting 6550s in the EL34 models until around 1970/71, so there would be no reason for there to be any stock of them.

Would that be the one labeled 2K2, 7W?No, that's the "choke" resistor, that all the 70/135W Ultra-Linear models have instead of an actual choke. The divider resistors are the 100Ks on the tag strip at the end of the amp.

[Ronsonic]

Would that be the one labeled 2K2, 7W? I'll have to double-check that it is there - whoever got into the amp before me did a bunch of "upgrades".

Thanks!
-Phil


No, the bleed resistors are the ones that parallel the main filter caps. On the schematic I linked they are at the lower right parallel with the caps just before the standby switch.


Ron

[unklmickey]

No, the bleed resistors are the ones that parallel the main filter caps. On the schematic I linked they are at the lower right parallel with the caps just before the standby switch.


Ron

hi Ron,

those resistors function as bleeders, but that's not why they're there!

those are balancing resistors, necessary to equalize the voltage across the stacked caps.

if Leo didn't need to stack the caps, those resistors wouldn't be there. he wouldn't have spent a dime more than necessary.

just look at any and all the Fender amps where the voltage is low enough that stacked caps aren't necessary. i doubt you'll find a single instance where a bleeder resistor is used.

[tremolux]

"Hmmm... this seems like quite a common occurrence with these amps, doesn't it!":eek: (originally posted by John Phillips)


AND........it was the only time for me to get hit that hard (knock knock) and then within a few days start re-redesigning the bench area out of concern, which was a good thing in the long run. It made a BIG impression on me.


My other memory of that 1967 model amp was that it appeared to be factory stock with 6550s vs. KT88s....also had what looked to be stock Tungsram ecc83s...

The Tungsrams might have been (with an ECC82 in the driver stage), but I'd be very surprised if the 6550s were. It was definitely designed around KT88s, and the US distributor didn't start fitting 6550s in the EL34 models until around 1970/71, so there would be no reason for there to be any stock of them. (originally posted by John Phillips)

That particular amp may have had an east coast music store sticker on the back, tho I dont recall it being Mannys, maybe a chain store. The KT88s might have been replaced by a US tech with 6550s when it arrived just because he had'nt dealt with KT-88s before and wanted something in there he was familiar with. Or like a lot of older Marshalls Ive run across, it was brought over from GB and never made the trip back after a tour, being sold and then getting serviced, re-sold etc. The amp was stock otherwise, never serviced before as far as circuit components go.

The 6550s looked like they'ed been there a long time, as did the Tungsrams.....youre probably right about them not being stock. Ive wondered........ After a cap job and tubes it sounded lincredible. Great amp, made me want to play and keep it for that one outdoor gig I do every 5-6 years

[minesaguinness]

220kOhm is fine with 400V ish B+ would recommend not less than 1 Watt rating.
THat's what's across the two 100uF capacitors (either side of the choke) in my RV50H (anode voltage = 400V). Roughly gives a time constant (ignoring the choke) of about 45 seconds and it will take about 3 time constants for HT to drain (add a little for the choke) so about 3 minutes drain time- i.e. have a cup of tea and eveything'll be fine.

Different amps have different arrangements - on mine the HT secondary winding is fused on both sides immediately out of the secondary winding, followed by bridge followed by standby switch and everything else is downstream - which I would consider good practice.

I have a habit of earthing the chassis, and probing all exposed parts prior to touching anything and mimise risks - I'm still here after 49 years and lots of cups of tea.
Cheers
Brian