BF Showman choke

[deci belle]

I'm going through the filter section on my amp and I measured the resistance of mine today. It's 93.7Ω.

Does that sound about right?

I don't have any reason to believe anything is wrong with it. I've never measured another choke before.

[Tom Phillips]

Sounds about right to me for DC resistance.

[Chuck H]

Sounds about right to me for DC resistance.

Why? I wanted to say the same thing based on a general knowledge of things in general... But I held out because I've never actually measured one.

[deci belle]

Chuck said:

I wanted to say the same thing based on a general knowledge of things in general... But I held out because I've never actually measured one.

Oh Chuck, you really must!! haha!!

[Enzo]

I never check chokes unless I have a specific reason to do so. WHen they fail, they can melt down, but miore likely the insulation punches through or a couple turns short. Your ohm meter would never detect a couple shorted turns. Nor would it detect a high voltage arc or otherwise damaged insulation short of a short to frame. If the amp works and the choke doesn;t get boiling hot, then chances are real good it is just fine. In the past 25 years I may have replaced two chokes tops.

[Chuck H]

Ok, I'll Showman mine if you'll Showman yours

Seriously though... After some Google searching I'm finding reports of 105-115 ohms for the 125c1a/22699 choke that went into the majority of Fender amps. Including all Showmans as far as I can tell. I can't say if your 94 ohm reading is normal or not, but it's lower than the reports I'm reading.

[deci belle]

Ya, I'd asked Patrick at Mercury a while back and he said chokes almost never fail. Nothing is getting hot. I'm having issues with a long standing problem and now I believe it is in the power section. I've already upped the values of all but the preamp filter cap to 100 and 30mfd Now I'm considering 47mfd for the screens and PI filter caps. (I'm getting ringing oscillation in both channels since I 5f6'd one of them). But I've started other threads about it already.

That's why I wanted to know about the choke.

Thanks you guys!!

[jmaf]

Ya, I'd asked Patrick at Mercury a while back and he said chokes almost never fail. Nothing is getting hot. I'm having issues with a long standing problem and now I believe it is in the power section. I've already upped the values of all but the preamp filter cap to 100 and 30mfd Now I'm considering 47mfd for the screens and PI filter caps. (I'm getting ringing oscillation in both channels since I 5f6'd one of them). But I've started other threads about it already.

That's why I wanted to know about the choke.

What does 5F6'ng an amp mean? You converted your blackface showman into a two channel tweed bassman?

Some ideas: It's probably not in the power supply, have you tried moving leads around using a chopstick while it oscillates? Does moving leads change anything? It may give you clues, that's how I normally find a cold solder joint too.

Do your power tubes have grid stoppers(1K5 to 5K6 common values)? Do you have that pesky little 47pF cap between the phase inverter plates? If you bypassed the cathode of V1, try removing that cap, bassmans used 12AY7 and the 12AX7 will sometimes make it wild. Hope this helps.

[Tom Phillips]

Sounds about right to me for DC resistance.

Why? I wanted to say the same thing based on a general knowledge of things in general... But I held out because I've never actually measured one.

I have actual measurements recorded somewhere. However, since I didn’t have access to my notes I referenced published specs for chokes in the 4H / 90ma range to get the “sounds about right.”

Even if I found my measurements I didn’t do a statistical study and I’d think that there could be a good amount of variation between different Fender samples. Maybe ±20%. Anyway…if I was troubleshooting the amp for “ringing oscillation in both channels” a 94Ω choke coil DC resistance wouldn’t be a concern to me.

Regards,
Tom

[deci belle]

Yes, Tom! I'm sure the choke is fine too. I hesitate to hijack my own thread, but… (I knew it'd happen)

jmaf said:

Some ideas: It's probably not in the power supply, have you tried moving leads around using a chopstick while it oscillates? Does moving leads change anything? It may give you clues, that's how I normally find a cold solder joint too.

Do your power tubes have grid stoppers(1K5 to 5K6 common values)? Do you have that pesky little 47pF cap between the phase inverter plates? If you bypassed the cathode of V1, try removing that cap, bassmans used 12AY7 and the 12AX7 will sometimes make it wild. Hope this helps.

I've been dealing with this for a year. Ringing oscillation only when I pick certain notes on 1st string. I simply can't refine lead dress any more than it is. I'm guessing the oscillation is shared between both channels where they join at the PI input.

Nothing I can do about that except install a switching relay to isolate them (which seems a little beyond me at the moment). So I'm refining everything else I can think of for now. It rings even when the hot channel pots are @ 1 and I'm on the clean channel.

I was studying the Dual Rectifier Mesa and noticed the higher filter cap values, so that's the only reason I decided to check the choke! —since it's part of the system. It does have a waxy residue— but that's probably from issues before I got the amp (nothing runs hot anymore).

I initially wanted to see the values of their grid stoppers, jmaf— knowing the Mesa is a high-gain Fender but found it only has regulation 1k5s at the sockets just like every other 6L6 amp. I'm going to try 4k7 just to see, but I was hoping someone else (Mesa) had found it a necessity to up those values.

When I scoped my amp and went through the system, things were definitely hopping at the plates— noticeably more than anywhere else (my maiden voyage with a scope), so upping the grid stopper values is my next planned experiment.

Ya, I have that li'l 47p at the PI plates! But it doesn't sound worse when it's not there. Even tried the 100p bypass caps at the PI plate Rs like the Mesa~ no dif. I have a 5k6 at the cathode of the extra gain stage (no cap); a 1k8/2.2uf at the CF's gain stage cathode (it's grid has a 300Ω at the socket) and a 1k for the CF's pin 3 cathode (no cap).

The oscillation is the same when I use AX7 or AY7 in the gain stage; tone sounds better with AX7. Both CF and 763 tubes are the best of a succession of 5751s and the driver has tried numerous AT7s~ each more expensive than the other. 6L6str are new tubes. Everything is shielded— and/or twisted and have noticed symptomatic improvements, but nothing affecting the root issue.

I'll try the higher value grid stoppers and start a new thread if I find any improvement.

[jmaf]

Geez, deci belle, you've been fighting that battle for much longer than I would have guessed, and surely you've tried tons of more trivial stuff. Part of the trivial tests, have you disconnected one of the channels entirely? Like unsoldering where the two join and mix together?

[deci belle]

OK, I'll do that as soon as I patch up the filter section, jmaf (waiting for a 2k2 glaze resistor to put between the screen and driver caps (my line voltage has gone up a bit lately and has been hovering between 129 and 136 volts)!!!!!

It's the same elsewhere in the neighborhood, I just haven't been able to contact the power company yet to ask about it, but I'm getting over 510v at the plates, dang! Glad I got 6L6str instead of JJ6V6s.

My guess is that the high gain circuit is the culprit and the traditional Fender layout is not helping matters, but taming the all frequency oscillations has been the battle from day one— and I have learned so much in this process. That little gremlin is my buddy!

[Tom Phillips]

Can you post detail gut shots of your build?

[g1]

(my line voltage has gone up a bit lately and has been hovering between 129 and 136 volts)!!!!!

Don't mean to derail your post but that's mighty hot line voltage. For legal reasons I would verify the voltmeter and start documenting line voltage at various times and dates. Sure the power company has some leeway with the +/- on the line voltage but at some point they must be liable. Maybe find out what they guarantee tolerance wise. 136VAC and things could start to blow, potential fire issues etc. Insurance companies would not like paying out for things like that caused by the power company.
I thought our 125VAC was hot, it's the upper limit of the tolerance here, not sure where the liability kicks in.
Make sure you get in touch with the power company and document when they were informed of the problem.

[deci belle]

Yes g-one… The power company just left and their side is good. It was my meter, low battery