Blew up my parallel 5881 champ

[garyvalve]

Welp, after 47 years of playing and 8 amp builds under the belt, I have finally fried an amp.  Not paying close enough attention, I plugged my amp backwards with a Swart Night Light attenuator, and had turned it on cranked!  There was a sputter, and I immediately turned it off.  It smelled something fierce, and the chassis was hotter than hell. 

I put it on the bench a couple weeks later, and did some troubleshooting with my lightbulb limiter, checking things out.  Removed the lightbulb limiter, and checked all voltages and bias, and everything seemed normal.  I live in an apartment, so could not turn it up. But played through it, and it seemed fine.

Well, when I took the amp to rehearsal, I cranked it up, and blamo!  There was white smoke coming out of the jacks.  At least it looked cool.  Turns out the bias bypass cap blew completely.  Pretty much disintegrated.
So, I have ordered a new SE output transformer from mojotone and a bypass cap.  Also for sure will get a new set of 5881’s.
What other things should I maybe replace?  Maybe a new Bias resistor and SSR to be safe?  Thanks!  (note:  The below photo of the amp is pre- explosion…)

[Willabe]

and had turned it on cranked!

You turn your amps on with the volume at full blast? 

Removed the lightbulb limiter, and checked all voltages and bias, .... Turns out the bias bypass cap blew completely. .... Maybe a new Bias resistor....

This is a || (parallel) SE Champ amp? So should be cathode (K) bias.

So why are you saying you checked the bias, bias bypass cap, bias R?

Do you mean the K R and K bypass cap.

[garyvalve]

No, usually do not do that with volume up full.  With the attenuator, it probably wasn’t cranked at the start, but then had turned it up full on.

Yup, it is indeed cathode biased.  The bypass cap in parallel with the 10 watt 270 ohm cathode resistor blew up.

[kagliostro]

I don't understand exactly what you mean when you say

I plugged my amp backwards with a Swart nite light attenuator

Also don't know exactly what is a Swart nite light attenuator

   

Can you explain ?

Franco

[Latole]

I don't understand exactly what you mean when you say

I plugged my amp backwards with a Swart nite light attenuator

Also don't know exactly what is a Swart nite light attenuator

   

Can you explain ?

 
Franco

Google Swart nite light attenuator

 https://www.humbuckermusic.com/products/swart-night-light-attenuator

[ShoemanGB]

I'd love a gut shot of that gizmo.  There sure is plenty of hype in the product description. At least they did not use the word "infamous" 

[HotBluePlates]

... Turns out the bias bypass cap blew completely.  Pretty much disintegrated.
So, I have ordered a new SE output transformer from mojotone and a bypass cap.  Also for sure will get a new set of 5881’s.
What other things should I maybe replace?  Maybe a new Bias resistor and SSR to be safe? ...

I would check what's in the amp before replacing a bunch of stuff.  Sure, the bypass cap blew, but that doesn't tell me anything about the health of the tubes, cathode resistor or output transformer.

Cut out the old bypass cap.  What's the resistance of the 5881 cathode resistor?

Measure the resistance of the OT primary.  What is that?

If the 2 items above look "normal" then try powering up the amp with the 5881s in their sockets & no bypass cap (but with a regular speaker connected).  DC Voltages at the output tube sockets?

Google Swart nite light attenuator

https://www.humbuckermusic.com/products/swart-night-light-attenuator

Scroll to "AO-44" in the Hammond schematics.

There is a resistive bridge at the bottom annotated "Volume Compressor."  It is a limiter that was placed between the Hammond organ's Dry speaker & the Reverb Tank, to reduce the speaker-signal to a level suitable for the tank's input.  The little #12 bulbs in the bridge increase their resistance when they get hot (from the applied signal), so they dynamically reduce the voltage-output of the bridge.  This imparts a heavy compression effect that can squash all the dynamics out of a sound (but that didn't matter for sustained organ tones, and helped reverb ring longer).

I've never been since a Swart Night Light, but I'd bet they just lifted the AO44 Volume Compressor circuit, maybe with a couple minor tweaks.

[jeff]

Scroll to "AO-44" in the Hammond schematics.

There is a resistive bridge at the bottom annotated "Volume Compressor."  It is a limiter that was placed between the Hammond organ's Dry speaker & the Reverb Tank, to reduce the speaker-signal to a level suitable for the tank's input.  The little #12 bulbs in the bridge increase their resistance when they get hot (from the applied signal), so they dynamically reduce the voltage-output of the bridge.  This imparts a heavy compression effect that can squash all the dynamics out of a sound (but that didn't matter for sustained organ tones, and helped reverb ring longer).

I've never been since a Swart Night Light, but I'd bet they just lifted the AO44 Volume Compressor circuit, maybe with a couple minor tweaks.

I remember talking about this years n' years ago. Isn't it really, really bad if one of the bulbs blows? Not THAT bad if used for reverb(like hammond) but MAJOR, MAJOR problem for power section if a bulb blows.
I might be remembering that wrong.


Were you using that device when the amp blew?
Possible that plugging it in backwards damage the unit, therefore using it damaged, damaged the amp?
Any way to test bulbs/unit?


Just some thoughts

[Merlin]

Since the cathode cap blew we can assume your cathode voltage went high. That might be because the coupling cap has gone leaky, causing the output tubes to pass too much current. Or it might be that the bypass cap simply didn't have enough voltage rating for the job. As a rule, the output tube bypass cap should be rated for at least 4x idle bias voltage, in order to cope with start-up transients.
Or maybe it's just too close to that bias resistor and has been slowly cooking to death. What's the bias voltage, normally?

Isn't it really, really bad if one of the bulbs blows?

Presumably the bulb only forms a part of a parallel load, so there is still a relatively safe load attached even if it blows.

[Merlin]

I'd love a gut shot of that gizmo.

Google is free, you know!
http://swartamps.com/tech/nightlight_mod/

[HotBluePlates]

Isn't it really, really bad if one of the bulbs blows? Not THAT bad if used for reverb(like hammond) but MAJOR, MAJOR problem for power section if a bulb blows.

As Merlin says, in Hammond's implementation the Dry speaker is always connected and is a load for the organ's power section.

The Volume Compressor circuit taps signal from the Dry speaker (which is still as-loud as it always was), and knocks down signal before it gets to the reverb tank.  The whole rest of the AO44 is just an amplifier to boost the return from the Reverb tank large enough to drive its own Reverb speaker.

I'd love a gut shot of that gizmo.

Google is free, you know!
http://swartamps.com/tech/nightlight_mod/

At a glance, it looked like Swart made an L-pad of some sort, and used a single #47 bulb to offer the compression effect.  There were also some LEDs that appeared to let the user know the circuit is active if one is not using the compression.

[Willabe]

It's a passive attenuator for low wattage amps that has the option of a light bulb compressor, up to 22w with the light bulb compressor, up to 30w with it off. 

Swart Night Light Attenuator & Stereo Drive

FAQ section;

Swart Night Light Attenuator & Stereo Drive

[Merlin]

I have my doubts about whether it can really dissipate >20 watts with that aluminium enclosure.

[Willabe]

I have my doubts about whether it can really dissipate >20 watts with that aluminium enclosure.

The attenuation resistor is an aluminum finned heat sink type that is bolted to that aluminum box, so it's a heat sink.

Guys are using that with his amps, a few he rates as 20w, 1 he rates as 22w.   

[garyvalve]

Thanks so much everyone!  Highly appreciate your guidance as usual.

I should mention that I had been gigging and playing regularly with this amp with zero issues or problems for almost 1 1/2 years.  until the attenuator “plugged in backwards” mishap.

I just had assumed at this point that it had no load on it when that happened, and I had fully cranked it up.  It smelled like electronics burning, and the chassis was super hot.

The 50uf 50v cathode cap did not blow then, but blew right away at a later rehearsal when I had first turned it on, and the volume turn up.  The attenuator was being not used when this happened.

[garyvalve]

Getting:

268.6 ohms on the 10watt 270 ohm cathode resistor

79.5 ohms on OT primary.

hmm…

[garyvalve]

Oops, I should have mentioned that it is the Night Light Jr. that I have.  The little 15 watt version.  Sorry everyone for the misinformation.

[garyvalve]

Checked the pot and the 30 watt resistor in the Night Light Jr, and everything looks okay there.

I just plugged in my 8 ohm speaker to the amp connection of the Night Light Jr, and measured the speaker side (which the amp had incorrectly been plugged into).  With the pot turned down low, I get a reading of around 400 ohms.
So there it is.  There was around a 400 ish ohm load hooked up to the dimed amp.
Would this cause flyback into the OT?

[Merlin]

So there it is.  There was around a 400 ish ohm load hooked up to the dimed amp.
Would this cause flyback into the OT?

Not ideal but I would expect the OT to survive that. 400 ohms is pretty good at damping high dv/dt flyback.

[Merlin]

and the chassis was super hot.

Do you mean the amp chassis? I'm guessing the tubes are suspended underneath?

[garyvalve]

Yes, deluxe chassis set up in a tweed deluxe cabinet.  It was very hot, and this all happened with a minute or two.  And the smell of burning hot electronics….

[HotBluePlates]

Getting:

268.6 ohms on the 10watt 270 ohm cathode resistor

79.5 ohms on OT primary.

hmm…

Neither of those sound like they need replacing.

I would consider installing tubes (could even be the ones you already have in there) and measuring DC Volts on each pin of the output tube sockets.  And if the voltages look good, play-testing with a normal speaker-load.