Purpose of resistors across HT

[Timbo]

Can someone explain the function of this resistor? It’s actually two in parallel, measured value 150k. They’re directly across the HT supply immediately after the rectifiers.

[mozz]

To drain the filter caps when the unit is turned off?

[Jon Snell]

To drain the filter caps when the unit is turned off?

Correct.

[Timbo]

Thanks. Why isn’t that done more often? I don’t remember seeing it in any other amps.

[trobbins]

Penny pinching for commercial amps. The resistor needs to be rated for the B+ level, which often requires a 2W resistor. DIY should imho always include a bleed. Some designs have bleeds in place, for example with balancing resistors across series capacitors, and with heater elevation resistors.

[mozz]

To help global warming. I myself, never use them, as i see them as creating unnecessary heat and a few more milliamps current draw from the power transformer. But i also use a #47 lamp where possible or LED. From a safety standpoint and to help others from getting shocked, use them.

[Justin Thomas]

Would it be effective to put them at the END of the filter cap chain? I believe it may take longer to drain the caps but I don't immediately prod my amp 2 seconds after shutting it off, either.

Jusrin

[Helmholtz]

To help global warming. I myself, never use them, as i see them as creating unnecessary heat and a few more milliamps current draw from the power transformer.

Wow, I assume you also bias your tubes very cold?
I mean the bleeder resistors might increase idle power consumption by something like 3%.

[mozz]

No, i only use NOS tubes and bias them beyond specs. I'm just joking, as seeing many times power resistors used to give off heat. Where is that schematic where they use a 30w resistor to ground to drop voltage? If people weren't so stupid i would say never use them, but these days you have to protect the idiots. I would actually have the caps hold a charge, it's easier on the power transformer next startup.

[g1]

Where is that schematic where they use a 30w resistor to ground to drop voltage?

Fender "The Twin" supply node? That one is only 10W though.

[Justin Thomas]

These? That's a "Concert II" & a Fender 75... I think it was Matchless who used a 30W. I just asked about this a few months ago, go figure.

[eschertron]

Fender Super 60 has a 10W resistor there. Dissipates less than 6W at rated voltage. Post #2 of this thread (below) shows it. I was thinking it was a 30W resistor, but I remembered incorrectly. it's 30 kilOhms and 10 Watts.

[Chuck H]

Would it be effective to put them at the END of the filter cap chain? I believe it may take longer to drain the caps but I don't immediately prod my amp 2 seconds after shutting it off, either.

Jusrin

Well now HERE is a good point of discussion. The bleeder resistors are usually placed on the first filter and behind the standby switch. If they were placed in the preamp and the standby switch were left open then the main filter wouldn't discharge. So...

Same scenario but what discharges everything but the main filter cap/s if the standby switch is left open? Would best practice mean putting a bleed on both sides of the standby switch? Thin as it applies to standby arrangements that break the HV rail.

[Justin Thomas]

I don't build with Standby switches so it never crosses my mind. I sure do love me some tube rectumfriers, cuz I've got about 100 of them. Not being able to switch off any "background noise" also encourages me to build as quiet as possible & learn how stuff works.

When I'm working on my Fenders, I leave the tubes in & Standby "off" (in "Play" mode) out of habit, learned from some gurus who advised that to discharge the caps. I read it in some books somewhere ... But then when I'm working in the amp I always use a jumper or occasionally tack in a resistor to ground if I'll be working on the amp over a few days.

More seriously... Maybe I can mess about in my next build, whenever that is. Whatever best practice may be, though, REAL best practice is to be proactive - discharge the lil zappers manually & making sure they stay that way instead of trusting your life to a $0.15 part or two. [Small Wisdom Nugget taken from Juan]

Edit: if at the end of the filter chain at the preamp side, wouldn't I NOT need a 20W block?

Jusrin

[pdf64]

I have a hypothesis that it may be better for the cap life, especially with gear that might not get used often, to allow them to retain some level of residual charge, rather than forcing a drain down to 0. As it may help to keep them 'formed'.