Blues Deluxe standby buzz after cathode bias mod

[thelonious]

I converted a friend's 90's era Blues Deluxe to cathode bias. I grounded the 220k grid leak resistors and added a 500R 10W shared cathode resistor. The conversion went fine, it sounds good, and the 6L6 cathodes check out at 37v. BUT... now it hums when it's in standby. When the amp is fully on, the heater hum is only slightly higher than it was with fixed bias, quite tolerable even at bedroom volumes. But when I flip the amp back to standby, it's dead quiet for about 10 seconds and then an obnoxious hum fades in. It's definitely coming from the speaker. Any ideas about what would cause this and how to fix it?

(BTW, we converted it to cathode bias in preparation for installing simple VVR scaling on it---he can't afford the nearly $150 he would spend on a LP/KOC SV1 kit with bias-tracking + raw bias aux kit + shipping, and while I know enough to etch and build a simple VVR, I don't know enough to build him one with bias tracking from scratch. Other solutions are welcome!)

[dude]

I don't know what the hum could be but you can install a VVR on a fixed biased amp.

Wouldn't cost much more then a VVR on a cathode biased amp, if you interested I could post a schematic.

al

[thelonious]

if you interested I could post a schematic.

Would definitely like to see that! After reading a bunch of threads on the topic, I got the general impression that some of the simple bias tracking solutions out there (the ones I could understand and build) were considered sort of sketchy. If there's a reliable low-parts-count solution, please share!

[Willabe]

After reading a bunch of threads on the topic, I got the general impression that some of the simple bias tracking solutions out there (the ones I could understand and build) were considered sort of sketchy.

That's the problem with fixed bias VVR, the bias tracking. Kevin's got that worked out where others are sketchy at best.

Plus his new(er) Power Scaling kits do not have the high dcv on the PS pot so you can use any type/size of pot you want.



                      Brad       

[thelonious]

That's the problem with fixed bias VVR, the bias tracking. Kevin's got that worked out where others are sketchy at best.

Thanks Brad. That's what I was afraid of. Given his tight finances, it looks like we're stuck with the cheapie option of switching it to cathode bias and etching a VVR for him.
...which brings us back to the original question. Why does this infernal buzz happen only in standby, why does it take 10 seconds to fade in after I flip it from "on" to standby, and how do I get rid of it? I tried different grounding points for the grid leak resistors, and while they mildly affect how much heater hum is present when the amp is on, they don't change the standby buzz.

Is there any chance it has to do with where I re-grounded the Fender flyback protection diodes that go from plates to ground? Shouldn't one ground be as good as any other for those? I've attached the schematic for reference.

Thanks guys.

[dude]

I can't find the schematic for fixed bias VVR I had but there has been a lot of discussion here, Check this link http://www.el34world.com/Forum/index.php?topic=10918.0

There is a schematic posted above in the link and a search on Google (VVR schematic for fixed bias) comes up with several posts on the subject right here

al

[sluckey]

Kinda grasping for straws, but, since you no longer need the fixed bias supply, remove (or lift one end) of CR19. Any help?

[PRR]

It "can't" hum in standby.

How is the switch rigged?

Check DC voltages when in standby.

How about when 6L6es are pulled, does it hum?

[thelonious]

It "can't" hum in standby.

UNLESS an idiot like me accidentally re-"grounds" one of the flyback protection diodes to a heater wire instead of ground and it starts half-wave rectifying the heater voltage, putting DC on the plates and screens when the amp is in standby. When the amp was fully on, the diode was reversed biased and didn't do anything.

I had found a place to ground it by touching pads near it with my meter until it beeped continuity to ground. Which was super dumb and lazy. If I had looked at my meter I would have seen that it said 25 ohms and was beeping continuity to ground through the PT's heater winding.

I am super lucky I didn't fry the amp. Or fry myself. These are the moments I think I shouldn't be doing this...

Thanks for the help!

[PRR]

> -"grounds" one of the flyback protection diodes to a heater wire

You can't be much of an idiot if you *found* that.

> my meter until it beeped continuity .. I would have seen that it said 25 ohms

That's one well-learned lesson I harp on. "Continuity" is for trailer lights, where you either have <1 ohm (good circuit) or >10K (busted wire, rusted socket).

The beep is a GREAT tool when laying in the mud under a trailer hitch. Even then you may need to re-check (<1 ohm good wire or ~~2 ohms lamp?), but no-go where you should have go is bad.