I KILLED IT... (neg feedback)

[Buddha's Guitar Tech]

First time this has happened...

putting in the NEG FB from the secondary.

as far as i knew, the worst that could happen was a squeal.
No squeal, just 1/20th of the volume. Dead.. Well, almost dead.

So bummed, don't even know where to start looking.

Champ: 6V6, 12AX7, 5R4

Any thoughts?

EDIT: it's not the tubes, not the OT (phew)
it's nothing in cathode bypass world, i've even bypassed the tone controls...
good voltages everywhere as far as i can tell.

GAH!!

[benoit]

Could your NFB resistor value be way off? Way too small --> Lotsa feedback/cancellation --> lower output?

[Buddha's Guitar Tech]

NFB isn't in at the moment. i started with it out, it sounded great, i connected it, things got crippled...

i swapped the OT into my (working) champion 600 and it's fine.

i have checked and swapped almost every component in and out so far and i am nowhere nearer to finding out what the problem is.

worse still, at one point i could hear a COMPONENT oscillating. didn't track it down.

it's to Champ AA764 schematic spec.

[benoit]

NFB isn't in at the moment. i started with it out, it sounded great, i connected it, things got crippled...

i swapped the OT into my (working) champion 600 and it's fine.

i have checked and swapped almost every component in and out so far and i am nowhere nearer to finding out what the problem is.

worse still, at one point i could hear a COMPONENT oscillating. didn't track it down.

it's to Champ AA764 schematic spec.

When you say you've checked each component do you mean read the value or actually metered them? Please don't be insulted by the basic nature of my suggestions, you may well have already covered these bases, but in my own experience when something is wrong in a situation like this it's usually something silly and obvious. If it sounds great with it out it's very likely something to do with the way it's wired. You're not connecting it to the wrong side of the 12ax7 are you? Or taking the tap from the primary instead of the secondary? Hope you get this sorted!!

Ben

[Buddha's Guitar Tech]

Ha ha, no offence inferred. i know all too well about the dumb and obvious, seen it a million times!

Here's how it happened...

had the amp upside down, guts exposed, guitar to my left, gently hitting a few strings, watching the two scope channels.

Secondary of the OT was same phase as the cathode input - thought to myself, i'll prepare for this horrid squeal... (couldn't remember if the phase coming in was supposed to be opposite to the cathode or its accompanying plate... inversion and all that (probably too much thinking))

anyway, got the crocodile clip out, TOUCHED the 2K7 resistor which is supposed to follow to OT sec connection and the sound just died.

haven't been able to rescue it yet. So frustrating.
swapped the OT into another amp, it's fine, all tubes seem to be working in other amp... what the hell is left?

200-odd VDC on both 12AX7 plates, 2 volts on cathodes.
25 volts on 6V6 cathode, bypass cap checked
decent AC on all grids and plates (i think)

PS by checking components, i mean taking them out of the equation one by one and inserting a new one... if problem persists, put old one back in... NO change.

[briane]

Not sure as to your issue....

But that should be DC on the plates, not AC.

[Structo]

You've got three threads now on this same amp.

[Buddha's Guitar Tech]

The AC is signal.

DC looks fine, 360 @ plate. 20 at @ cathode.
Screen is higher than plate, but it was like that when it sounded good (for all of five minutes )

Sorry 'bout the multipost, but i am tearing my hair out! (think it's two to be honest; a possible third thread is about a Champ 600 - this is a champ clone)

Just trying to avoid replacing every resistor and cap to shotgun the problem. i like to fix AND learn.

[j-po]

I like to connect the nfb first then switch on the amp without using the standby. This way the squeal comes in slowly as the tubes warm, just flick the switch back off as you hear it coming.

[Buddha's Guitar Tech]

Shall save as note to self, thanks.

For me, there was no squeal, just an instant drop in volume. It's like it blew something very quickly before getting the chance to squeal.

i *think* i've localised the problem to the output tube stage, but not sure where. There's not much to it, just the power supply, OT primary, resistor etc. If the AC signal on the grid looks healthy, and the DC voltage on the plate looks healthy, what fundamental principle am i missing?

[Merlinb]

Si *think* i've localised the problem to the output tube stage, but not sure where. There's not much to it, just the power supply, OT primary, resistor etc. If the AC signal on the grid looks healthy, and the DC voltage on the plate looks healthy, what fundamental principle am i missing?

How do you know the tubes are OK? It's possible that you could have toasted the screen grids if the amp tried to instantaneously oscillate when you touched the clip to the cathode. (Screen resistors measure ok too?)

[Buddha's Guitar Tech]

i have two champs - this dying one and one that works. swapped tubes out, they're fine in the healthy champ. Same with the OT.

[marcoloco961]

This is a pretty lame suggestion, but you might try cleaning the tube sockets on the non-working amp. I have seen flux work it's way around the pins and end up on the contacts causing a bad contact between the tube pins and the socket.

Or maybe it's just a bad socket. I would remove and measure my grid and screen resistors on the power tubes just for shits and grins. I had a bad resistor in a dropping string yesterday, it was supposed to be a 9.1K, was a new resistor and it measured over 1.5 Meg. I either burnt it up soldering, or it was just bad to begin with.

Also had over 315 VDC on the plate of one side of the PI. I swapped out resistors and capacitors in line with the plate voltage and it was still at 315. Then I cleaned the socket and changed the tube and it was right back around 200 VDC, where it should be. I don't know if it was a bad tube or a dirty socket, but I am leaning towards the second. It sounds like your power tubes are not seeing your grid voltage.. (just a wild guess). Maybe you created an arch inside the socket and dirtied the connection. Might try cleaning the inside of the socket where the tube pins make contact. I would carefully inspect and test all solder joints on the tube sockets, make sure I have the proper B+ and a good connection on the center tap of the OT primary. Sounds like something simple, it was up and running and all the expensive parts are still working properly.

P.S. I would chopstick around the volume and tone pots with a signal coming in. I had a pot that measured correct yesterday, but when I hit the wiper contact with a chopstick it would go from normal volume to almost nothing, kept oscillating and making a sizzling sound until I replaced it. You might want to check there and around the actual guitar input, a bad contact in these areas would cause similar symptoms.

[Buddha's Guitar Tech]

Perhaps one would never guess, but i tracked it down... the filter caps.

i knocked together a spare network, but i had to use around 50uF + 50uF... all was well until i cranked the volume. died a tiny death again.

So i PRESUMED the replenishing current killed THOSE caps and maybe damaged the rectifier, but not sure.

Dug out and knocked up another network (this time 20uF + 22uF) and no change, but i'm leaning to the idea that the plate voltage is too hot..

there's a 5R4 in there and i think it's delivering more than a 5Y3 would, so maybe that's a big problem

this build is CURSED!! Last two 50W builds went off with barely a hitch, this little 5watt b*sta*rd is ruining my life!

i guess there's an art in these little guys.

[marcoloco961]

Perhaps one would never guess, but i tracked it down... the filter caps.

i knocked together a spare network, but i had to use around 50uF + 50uF... all was well until i cranked the volume. died a tiny death again.

So i PRESUMED the replenishing current killed THOSE caps and maybe damaged the rectifier, but not sure.

Dug out and knocked up another network (this time 20uF + 22uF) and no change, but i'm leaning to the idea that the plate voltage is too hot..

there's a 5R4 in there and i think it's delivering more than a 5Y3 would, so maybe that's a big problem

this build is CURSED!! Last two 50W builds went off with barely a hitch, this little 5watt b*sta*rd is ruining my life!

i guess there's an art in these little guys.

I would be more worried about the screen voltage than the plate voltage. A 6V6 shows a max. plate voltage of 350, so yes, I would lower that some, but the max. (G2) or screen voltage is 315. So if that is higher than your plate voltage as you stated earlier in this thread, then screen voltage is way over the max. and would probably explain your failing filter caps.

I have read that some amp builders keep screen voltages a little lower than needed as a "saftey" feature for the component life. Are you sure your screen voltage tap comes from the right spot?? If so, maybe your screen resistors are not high enough wattage or have gone bad. I believe they should be a wire wound type and at least (5 watt)?? Not sure on a champ. Does the champ have a choke? Or does it use a big high wattage resistor like the Wreck. Maybe one of those has gone south causing a higher screen voltage.