Maxie's amp is back for service

[Phil_S]

You might remember this one, or maybe not: http://home.comcast.net/~psymonds/Maxie-tone.htm
This is an amp I built for a friend's daughter. She's gone on the road for a couple of weeks. She flew to join her band, so she couldn't take it. It's got a serious static-like crackle to it. It happens when playing, volume about half way up. It happens when I slap the top of the cab. There's also a smell...my nose wanted me to turn it off, but I played it long enough to produce the problem, a minute or so. I figured she's been using it and it wouldn't hurt it worse.

I haven't had time to crack it open yet.

Volume output is good, not weak at all. This amp is louder than I remember ;-}

I'd be grateful for a suggestion of what I'm looking for.

[Stevem]

There's a ton of things this could be, but the smell of something burning somewhat narrows it down to a arcing output tube socket, burning output tube screen resistor, bad power supply dropping resistor or the power supply connections, or even just a simple bad standby switch.

Trouble shooting 101, pull out the preamp tubes and smack the amp , does it still crackle and pop?
If not replace the tubes one by one starting from the output end until the issue picks up again.
Of course just subbing in known good tubes is a start and you could always wire up a output lead off of the output side of any coupling cap and feed that into another amp to test out which stage the issue may be in.

[Phil_S]

It turns out the smell is dust burning off. There was a layer of dust on the power tubes. I suspect Maxie hasn't powered it up in a while. I think she may have been afraid of hurting my feelings by telling me she sidelined it, which was not going to be the case at all. Actually, I was glad to see the dust as it told me she had the good sense to stop when something wasn't right.

After looking inside last night and seeing nothing obvious, I concluded that tube swapping is the thing to do, too. Thanks for confirming on this.

The recording doesn't do justice to the noise. I am thinking it is the power tubes. I'm sorry to see those old GE 6L6CG go, but if their time is up, then it is what it is. Nothing lasts forever!

I'm hoping it isn't the 7247 (12DDW7) because I'll have to swap a 12AU7 or 12AT7 in it's place as I don't have another one of those. The AU side is the cathodyne inverter. I suppose I could rework the inverter resistors to make it work with a 12AX7, but I'd rather not.

[Leo_Gnardo]

I'm hoping it isn't the 7247 (12DDW7) because I'll have to swap a 12AU7 or 12AT7 in it's place as I don't have another one of those. The AU side is the cathodyne inverter. I suppose I could rework the inverter resistors to make it work with a 12AX7, but I'd rather not.

No fear, JJ has their ECC832 = 12DW7. I've used about 6 or 7, they work fine. Get 'em from the usual suspects, not awfully expensive either.

[Stevem]

Don't give so easy on those outputs, as over the years I have had 6 to 7 tubes that only needed to have one of the pins reflowed to end a bad connection!
Take a close look at all the pins as most times it's easy to see one that is iffy.

[67plexi]

I was trained to go to the power transformer first read no load voltages.
And if they read low find the cause. Shorted power tube most of the time
and will cause the exact symptoms that you state. runaway bias see how hot the PT is under load. I had some real challenging amps run across my
bench last week. Voltage chart the complete amp. Starting with the power tubes then the phase inverter to the preamp tubes in that order.
A 65 Twin Reissue It had weird artifacts in the sound and crackle it was a PT going away ran real hot the cause a shorted out 6L6GC.
Next was a Mesa amp very low output all preamp and PI and reverb tubes
tested 100% the 5Y3 and the EL84 tubes were shot.
So I thought easy replaced the faulty tubes the power section same problem then I voltage charted the amp bad PI replaced the 12AX7 PI
all good. Now the reason I'm hammering on voltage chart is someone had
gone through the Mesa amp changed all the transformers and a few relays
I did reflow some cold solder joints but if I was going off of my tube tester
readings I would have been stumped.

[Phil_S]

Thanks everyone. This turned out to be an easy one. Stevem was right when he said put it on the bench and slap the amp. I took the handle side of the screw driver to tap each tube. One of the 6L6's was identified as the culprit.

Aside from a bit of dust and needing every single nut tightened (including the fuse holder) everything else was fine. I put in some cheap Russian 6P3S that I had, biased them and closed it up. I had to reduce the power a bit for these crappy tubes. I think it will be fine, and Maxie doesn't have to pay anything for this, so she'll be glad. The amp played fine afterwards.

One thing that is bothering me is that I swear it left my house with a pair of GE 6L6GC. What I found was an RCA 6L6GB and a Sylvania 6L6GC. I suppose my memory could be faulty, but I think not because those GE's are nowhere to be found in my stash. I'm not a suspicious person, but I think there was a tube swap in the last 5 years since I built it and not an appropriate one, if you ask me. Things happen...

[Pierre]

I just had similar symptoms on a home built amp. I noticed the pilot light flickering as well. It turn out to be a loose cap on the fuse holder. I put a wrap of electrical tape on it to prevent that from happening again. Another lesson learned!

[Phil_S]

If I had that sort of problem with a fuse holder, I'd replace it. Tape is not reliable.

[Stevem]

Your running that 6l6gb right at its limit for class AB1 at 360 VDC!
Also tape does not make for a road worthy fix!

[Phil_S]

Dang, you're right about the voltage Stevem. I'll have to get her some proper 6L6GC. She won't be back for a couple of weeks.

[Stevem]

Even if that where not the case the two different tubes would never have the same frequency responce output per signal input, if you really want a amp that sings, then get a set of Groovetubes as they are the only ones that are matched for frequency responce and not just current and as such worth every penny to me!
The best of this type that Groovetubes has to offer is the 6L6S model.

[tictac]

On that ajustable feedback control, seems like you'd want a resistor in series with that 50k pot so as to not have s direct short from the speaker to cathode...

just a thougt....

TT

[Phil_S]

Hi Tic Tac. I had trouble with that NFB and couldn't get it it right. After reversing the phasing, still no luck, it was removed. Yes, you are quite right about using a fixed R with the pot. Thanks for noticing and taking the time to mention it.