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Post subject: Odd Issue last night
Posted: Sat Oct 08, 2011 11:27 am
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Was playing my recently redone Showman ('65, redone to aB763 as you've seen in the other thread). Amp currently only has the middle 2 6L6 as one of the outer ones was defective. Running it into my 2x12 Bandmaster Cabinet with the Warehouse G12C speakers, wired for 8 ohms.

Last night while playing at a show, after about 1 1/2 hours all of a sudden the amp stopped making sound. after confirming it wasn't my tuner mute, I shut off the amp, and waited 2 minutes. Flicked power back on, and amp came back up. It did the same thing 20 minutes later and shut if off for good.

Luckily I had my bandmaster combo for a guest; their songs were over so I moved it across the stage, plugged the bandmaster head into the cab and finished the show.

Would only having 2 of the power tubes running do this? I've got a full set of 4 Groove Tubes 6L6's but don't want to put them in until I know what the issue is.

Amps (3 guitar amps, bass amp, 6 JBL EONs) all plugged into the same circuit, but this was the only amp that seemed to have an issue.

thank goodness for backups; changing the battery in my Smokey amp (ultimate backup amp) as we speak.


thanks for any help/suggestions/recommendations.

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Post subject: Re: Odd Issue last night
Posted: Sat Oct 08, 2011 1:35 pm
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When the amp went silent were the tube filaments and pilot lamp still lit?

No blown fuse?

Regards,
Roger

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Post subject: Re: Odd Issue last night
Posted: Sat Oct 08, 2011 1:52 pm
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pilot lamp went out, fuse stayed on since the amp powered back on.

didn't check filaments.

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Post subject: Re: Odd Issue last night
Posted: Sat Oct 08, 2011 2:01 pm
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In that instance, I think the issue is power supply related -- possibly the heater tap on the PT. That's 6.3VAC (ie: un-rectified) and it also powers the pilot lamp. I doubt it's a short as that would've likely popped the fuse. If there's an intermittent open somewhere within the 6.3V winding or the green wires from the tranny that run to the pilot lamp socket that would've shut everything down without blowing the fuse.

HTH

Arjay

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Post subject: Re: Odd Issue last night
Posted: Sat Oct 08, 2011 5:29 pm
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So just got back from retubing the amp; pulled the 2 tubes and replaced with 4 matched groove tubes (the GE versions).

Let amp warm up on standby for 4 min and then played on 5 on the vibrato channel for 30 min - no problem. Played through my humbucker tele and a strat. Then played for 30 min on 8 (with headphones) no problem.

Then plugged into the normal channel and played bass through it for 30 min with no problem.

Only differences:
Playing in recording studio vs. A stage where there may have been more vibration from drums, bass player jumping around b

Used thicker speaker cable vs the flat F cap zip wire Fender speaker cable.

Matched quartet of tubes vs. just inner 2.

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Post subject: Re: Odd Issue last night
Posted: Sat Oct 08, 2011 8:13 pm
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When you say "inner two"---do you mean the two 6L6GC on the inner portion of the four sockets? These are NOT push-pull pairs. But, rather 1/2 of the push-pull pairs of the four tubes (See layout). In essence, you were playing a two 6L6GC parallel single-ended amp. And most push-pull trannies won't liked that sort of impedance load. Plus, the 6L6GC will be pushed harder, sometimes into near Class "A" mode to make music.

FWIW--- the first two and last two 6L6GC are the push-pull paired tubes.


http://www.prowessamplifiers.com/schema ... ayout.html


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Post subject: Re: Odd Issue last night
Posted: Sat Oct 08, 2011 11:15 pm
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If your pilot light goes OFF , this must be a power transformer problem . Not the transformer itself but solder on their fillements wires and like Retroverbial said.

Chek also solder on the primary ; on fuse holder ,AC power chord.

If the amp was " rebuilt" chance are bad soder or forget to solder.

It is not a big trouble to fix


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Post subject: Re: Odd Issue last night
Posted: Sun Oct 09, 2011 12:26 am
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stratele52 wrote:
Chek also solder on the primary ; on fuse holder ,AC power chord.


+1, Stratele

I didn't even consider that. It could be something in the input power circuit -- literally anything between the wall plug and the on/off switch.

Good call, mon amí!

Arjay

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Post subject: Re: Odd Issue last night
Posted: Sun Oct 09, 2011 10:11 am
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Ok, I agree (kinda) with Stratele & Arjay... except for the 20 miuntes everything is ok... then the amp goes off. And the fuse never blew. Yes, you may have an issue with the heater wirign to the lamp and heaters of the tubes.

But, running the two inner 6L6GC is not what you should be doing. Run the two 6L6GC on the left or right sides of the socket layout. The impedance will be wrong for the OPT primary winding. But, at least you are running in push-pull mode and not single ended-parallel mode. With SE-mode, the amp maybe drawing more current, instead of the benefit of one tube being close to off with push-pull topology.

Anyhow, best to have the amp checked out by a good tech.


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Post subject: Re: Odd Issue last night
Posted: Sun Oct 09, 2011 10:23 am
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Thanks all - this being my first quartet-fueled Fender, I had mistakenly heard somewhere that to run in 2 tubes, you pulled either the outside pair or the inside pair.

If I have to run it with only 2 tubes going forward, will run either left pair or right pair.

thanks!

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Post subject: Re: Odd Issue last night
Posted: Mon Oct 10, 2011 12:55 am
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Your problem could be a bad fuse , yes it could be intermittent, replace it. It could be also a bad fuse older.

One thing is sure with your report ; The problem is between the AC plug and secondary wiring of your power transformer.

nedorama, if you want to run with only 2 tubes remove the 2 outer OR the 2 inner tubes, but you have to run at higher impedance speakers


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Post subject: Re: Odd Issue last night
Posted: Mon Oct 10, 2011 8:19 am
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stratele52 wrote:
Your problem could be a bad fuse , yes it could be intermittent, replace it. It could be also a bad fuse older.

One thing is sure with your report ; The problem is between the AC plug and secondary wiring of your power transformer.

nedorama, if you want to run with only 2 tubes remove the 2 outer OR the 2 inner tubes, but you have to run at higher impedance speakers


Stratele - this is what I had always heard, but BMW posted this:

Quote:
When you say "inner two"---do you mean the two 6L6GC on the inner portion of the four sockets? These are NOT push-pull pairs. But, rather 1/2 of the push-pull pairs of the four tubes (See layout). In essence, you were playing a two 6L6GC parallel single-ended amp. And most push-pull trannies won't liked that sort of impedance load. Plus, the 6L6GC will be pushed harder, sometimes into near Class "A" mode to make music.

FWIW--- the first two and last two 6L6GC are the push-pull paired tubes.


Guys - can someone clarify which it is for me for future reference? thanks.

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Post subject: Re: Odd Issue last night
Posted: Mon Oct 10, 2011 8:35 am
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If you examine the Showman schematic, you'll note which two 6L6's among V7 through V10 are paired with one another to produce symmetrical conductance through the OT. Stratele is correct.

Arjay

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Post subject: Re: Odd Issue last night
Posted: Mon Oct 10, 2011 12:27 pm
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nedorama, you can remove any two tubes ( pair ) except those combinations ; the two first one or the two last one.

This mean you can remove V7 and V9 or V7 and V10 or V8 and V9

You can't remove V7 and V8 or V9 and V10


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Post subject: Re: Odd Issue last night
Posted: Mon Oct 10, 2011 12:33 pm
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And an 8-ohm or even a 16-ohm output load would be recommended for this configuration.

HTH

Arjay

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