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Post subject: 1968 Fender Showman Reverb Vibrato Speed Intensity problem
Posted: Sat Apr 28, 2012 3:06 am
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Hi. I have a 1968 Fender AA768 Dual Showman Reverb TFL5000D amp head.

All four inputs work great. Both Bright switches do their job and work great.
The reverb is outstanding. The footswitch is brand new and works great.
The tone controls all work good. There is no hissing or humming
or anything or any noise at idle or play.

The only problem is the Vibrato/Speed/Intensity controls when switched on.
They produce a low-level helicopter wop wop wop wop sound.
Not too loud, low key. If there's any vibrato, it is faint.

I had this issue on another amp, but the helicopter sound was expremely loud.
I believe there may have been pre-amp tube issue involved, maybe the two
preamp tubes around the footpedal RCA inputs or around the reverb transformer.

Anyway, I am not technical, so if it is more than swaping tubes, I will have
to take it to an amp repair tech. I do not solder, nor read schematics, etc.

Any suggestions for Vibrato/Speed/Intensity issues? Thanks. Toppscore


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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Sat Apr 28, 2012 3:16 am
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V5 is the oscillator for the tremolo circuit but I'm doubtful that it's causing the symptoms you've described. You might try swapping in a known good 12AX7 but failing that, it will need diagnostic troubleshooting and repair service by a qualified-and-competent amp tech.

Arjay

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Sat Apr 28, 2012 3:18 am
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V5= counting left to right?

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Sat Apr 28, 2012 3:22 am
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Right to left, both on the chassis and the tube chart.

Arjay

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Sat Apr 28, 2012 5:10 am
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Toppscore, the "whop-whop-whop" sound is what old-timers call the "Huey Effect." Sorry, the dang Devil made me write that. :mrgreen: What you prolly have is a motorboating issue (my WAG).

Have heard this when the 20mfd/500VDC caps in the PS rail stages start to dry out and inadequately isolates the stages tapping off the PS rail (like the power tubes, input tubes, PI, vibrato, reverb section). Interacting stages usually results in poor stage gain (thus poor vibrato, if the dry cap is part of that circuit) and lots of interaction (positive feedback) with other stages---> whop-whop-whop.

When was the last time the entire power supply filter caps were replaced? And the rail's dropping resistors checked? These things are usually a relatively simple fix (I hope that I didn't curse you!).


Thanks!


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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Sat Apr 28, 2012 6:30 am
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BMW2002Ti wrote:
Toppscore, the "whop-whop-whop" sound is what old-timers call the "Huey Effect." Sorry, the dang Devil made me write that. :mrgreen: What you prolly have is a motorboating issue (my WAG). Have heard this when the 20mfd/500VDC caps in the PS rail stages start to dry out and inadequately isolates the stages tapping off the PS rail (like the power tubes, input tubes, PI, vibrato, reverb section). Interacting stages usually results in poor stage gain (thus poor vibrato, if the dry cap is part of that circuit) and lots of interaction (positive feedback) with other stages---> whop-whop-whop. When was the last time the entire power supply filter caps were replaced? And the rail's dropping resistors checked? These things are usually a relatively simple fix (I hope that I didn't curse you!). Thanks!


Wow!!! Thank you, BMW.
Your information is incredibly helpful. Although not good news, but it is something
within the filter capacitors or chassis electronics. This means it is over my head
and I must see an amp tech. Your advice will save me time pulling tubes, etc.

Thank you. Toppscore 8)

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Mon Apr 30, 2012 5:30 pm
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Does the rate of the wops change with the speed control?

Is the noise always there or after the amp warms up?

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 12:29 am
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mhowell wrote:
Does the rate of the wops change with the speed control? Is the noise always there or after the amp warms up?
Hi! No change in speeds. And it's low bedroom level volume/agressive.
I had another amp that was quite loud and kind of took over all the
controls, so I have obviously seen worse.

EVERYTHING else works great. Tone is fabulous.
All switches are good. All tones make a difference.
The reverb tank, control (next to the speed & intensity) and sounds are outstanding.

I presume it is there. I start the amp with standby, then power, the test
the Normal channels, then the Vibration channels then the reverb effect.
I want to make sure nothing else has trouble. Then, I try the speed/intensity.
And it appears and I do not get much of an effect playing my guitar.

I can retest it for you, but what do you mean by warming up?
Do you mean ~ does the issue get progressively worse?
Toppscore

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 12:49 am
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As mentioned that's Motorboating. Mains Filter Caps are bad. When the Volume/Power is turned up the caps can't handle it. Thus the oscilation. At low volume the old caps can cope.

Are you Toppscore who has the Rare Tweed Bassman 5D-6 and wrote extensively on it ?
Would have thought you would have knowledge of such tech issues ? You deal gear on ebay with over 4000 Feedbacks. I heard you got Banned from Plexipalace Forum and recently the TDPRI Forum ? :shock:

Anyhow a New Start here at the Fender Lounge ! :D


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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 1:38 am
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:mrgreen:

Arjay

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 4:32 am
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Arjay,

Love it! Now, couldn't you see Mom & Dad pop in for a visit, with you wearing that get-up? "Rosin Rot" for sure, For sure! :lol:


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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 12:12 pm
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spankthatplank1 wrote:
As mentioned that's Motorboating. Mains Filter Caps are bad. When the Volume/Power is turned up the caps can't handle it. Thus the oscilation. At low volume the old caps can cope.
Thanks for the confirmation. I thought that motorboating
would mean the same as helicopter sounds. But, motorboating
is the word to use. I must take the amp to the pro repair man.
Sad, but good to know what to do. Yes, I wrote quite a bit
about the amps from 5B6-5E6, their history, parts, transitions,
components, model numbers, serial numbers, transition of
one amp into another, etc. Took a year and some special
interviews, but got it done in February. As for the rest,
I have things in the works. Another story. Take care. Toppscore :?

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 12:16 pm
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mhowell wrote:
Does the rate of the wops change with the speed control?
Is the noise always there or after the amp warms up?



Hi. I checked it out again. Yes, the rate of the motorboating
does fluctuate with the controls.
Guess what, I forgot to plug in the the footswitch.
The intensity & speed channels/controls sounded great.
They were just like the normal channels. No problem at all.
Then, after realizing no pedal hooked up,
well, the motorboating came back.
That'll probably help isolate the problem.
Thanks again. Let me know what you think. Toppscore :)

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Tue May 01, 2012 9:12 pm
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Toppscore wrote:
mhowell wrote:
Does the rate of the wops change with the speed control?
Is the noise always there or after the amp warms up?



Hi. I checked it out again. Yes, the rate of the motorboating
does fluctuate with the controls.
Guess what, I forgot to plug in the the footswitch.
The intensity & speed channels/controls sounded great.
They were just like the normal channels. No problem at all.
Then, after realizing no pedal hooked up,
well, the motorboating came back.
That'll probably help isolate the problem.
Thanks again. Let me know what you think. Toppscore :)

I'm not a tech. I asked because I had a similar issue with a Princeton reverb and a tube replacement fixed it. Not sure it the Showman has a similar type vibrato circuit so I wouldn't know if that would fix your issue or not.

I stand to be corrected but I don't think the rate of the motorboating would change with the vibrato speed control.

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Post subject: Re: 1968 Fender Showman Reverb Vibrato Speed Intensity probl
Posted: Wed May 02, 2012 5:39 am
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Quote:
I'm not a tech. I asked because I had a similar issue with a Princeton reverb and a tube replacement fixed it. Not sure it the Showman has a similar type vibrato circuit so I wouldn't know if that would fix your issue or not. I stand to be corrected but I don't think the rate of the motorboating would change with the vibrato speed control.
Great. That gives me hope. Before I take it to a tech, I will double check
the tubes, expecially with amps with working vibrato/speed/intensity.

Honestly, it is hard to say if/when the rate of motorboarding increases at all
and or which controls. Have to study with more intensity, myself. Toppscore 8)

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