Fender Dual Showman Reverb

zoork_1

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I have a Fender Dual Showman Reverb (w/ push/pull master vol).

This amp should be loud as he** but it's like a mouse. I think it's weaker than my Deluxe reverb...

I changed all caps, didn't get any better. Any ideas what can cause this...?
 

D'tar

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4x6l6 should be kinda loud IMO. LOL. Is it weak on both channels? try swapping preamp tubes around, If both channels are weak I'd likely start at V6 phase inverter. Are all 4x6l6s working. A complete voltage chart may help identify a problem area.
 

Pappy58

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Have you tested all the power tubes and checked the bias? Beyond that, partial short on OT? Bad PI? Is it the same with different speaker configurations,,,and even cables? Few idea's. Good Luck!

ps; One final thought is the MV pot itself. Maybe try and few shots of deox on it.
 

Greg's Guitars

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Possibly the transformer? I have one stuffed into an old twin cab ( as opposed to a head) and have to take two power tubes out and drop it to 50 watts so I can even play mine....LOUD. Mine is pre push/pull though...



olwhitey.jpg
 

zoork_1

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Thanks all for input, I really appreciate it.

I have tried different speaker configurations and the cables, no difference whatsoever…

I will check if it’s weak on both channels, check all tubes and try to get some help to test the transformer.
 

Thumpalumpacus

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Yes, if your caps are good check tubes and transformers. Four 6L6s should be able to deafen you fairly quickly -- they sure could in my 71 Bassman.
 

zoork_1

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Ok, it’s weak on both channels. I have tested all tubes (replaced two power tubes) and it still lacks balls... :(
So I'm now leaning toward the transformer.
 

Soul Tramp

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A question for you: 1) If you dime the amp (vol & MV), is the background hum/hiss loud (what you'd expect if the amp were working properly)?

If not, the problem is in the power amp. Check the speaker jacks & OT leads (primary & secondary) soldering. Verify the speaker jacks aren't pulling the output signal to ground.

If the hum/hiss is loud, the problem is in the preamp/PI.
 

Leña_Costoso

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There are two models of the Dual Showman Reverb with push pull MV.

One is the more or less conventional derivative of the AB-763 circuit, using basically everything the same as a blackface or silverface Twin Reverb. The Dual Showman Reverb was just a Twin Reverb in a head form (maybe a little tweak on the phase inverter coupling capacitor). These are the 100 watt Dual Showman Reverbs.

The other is the 135 watt variation, which has what's called "UltraLinear" screen grid taps on the huge output transformer. There is a lot of iron there, and Fender was trying to keep things as clean as possible, while also upping the power level quite a bit.

You can try this: Try running the two outboard output tubes only. That should give you 50w or so. Then do the same by running the two inboard output tubes.

Adjustments - those amps have a bias "balance" control, not a bias level control. There is also a "hum balance" control. The latter first, the hum balance varies the ground reference for the filament supply, so that if you have a higher current filament draw on one side of the split supply, you can "offset" it a little bit with balance control to minimize hum. With really good tubes, you can have basically no hum at all, just a faint hiss, with the amp dimed. The bias balance evens out the negative bias voltage going to each set of output tubes - it does nothing for the level.

And therein I suspect is your problem. You said it sounds weaker than your Deluxe Reverb. There should be approximately 6dB difference between a DR and a DSR. That 6 dB could easily get eaten up by really cold bias.

If you're comfy with working in an amp (which I suspect) then I'd check the bias current, and check the voltage on the plates of the output tubes. That will tell you if you have a bad power transformer, which I doubt. It would also be a stretch, but not unheard of, to "blow" an output transformer, but usually they get really distorted and crappy sounding if they "partially" blow (aka, short unto themselves inside). Twins and DSRs have huge overkill in the OT iron... I don't suspect an issue there as a first item of failure.

Check the screen grid resistors. They DO blow. Best replacement is 470ohm 5 watt "sandblock" ceramic body ones. They last really well.

I don't suspect the Master Volume control. It would get scratchy as hell, be loud n soft and goofy.

And.... I'll toss out two things. First, is... you ARE using speaker cord, not guitar cord for the speakers, yes? And second is, you are plugging into the correct speaker output, yes? If you plug into the EXT output with nothing in the main output, you'll get some sound, but it will be low even with the amp dimed, and it will be distorted, since the amp is dimed. Reason being is the shorting safety switch in the main output jack.

Lastly.... and its a longshot... if the amp is the Export model, some of which were sold in the USA to touring pros, it may be wired for other than 120v input. Those had 100v, 120v, 130v, 200v, 240v and 260v taps on the primary. They were sold in the USA as 240v stage models, where stages were wired with 240vAC which reduced wire sizing for heavy loads on stage, allowed lighting loads, etc etc.

I'm interested to see what you come up with.
 

zoork_1

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A question for you: 1) If you dime the amp (vol & MV), is the background hum/hiss loud (what you'd expect if the amp were working properly)?

If not, the problem is in the power amp. Check the speaker jacks & OT leads (primary & secondary) soldering. Verify the speaker jacks aren't pulling the output signal to ground.

If the hum/hiss is loud, the problem is in the preamp/PI.
Thanks Soul Tramp, the hum/hiss is as loud as I expect from a properly working amp. I'll do some reading and see what I can find.
 

zoork_1

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Tanks Leña_Costoso. My amp is the conventional one, not the 135W UL.

When it's fixed, I'll test it with two output tubes thats for sure.


I will check the bias, the voltage on the plates of the output tubes, and screen grid resistors. I have the "sandblock" ceramic body in my Deluxe Reverb, but not in the Showman (looks like the original ones).

I'm using speaker cord and the correct speaker output. Not sure about the transformer but I will take a look at the top/taps.
 

zoork_1

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I checked the (fixed) bias and it's -45 V.
If understand this correctly it should be close to -52 V...?
Not sure about the implications of this, but I'm thinking of swapping a 1,2k resistor in the circuit for a 470 ohm and measure the bias again.
 

Leña_Costoso

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Another words if you say it's -45 volts that's on one side the other half might be - 35 or - 55. You can refit that potentiometer so that it raises the entire bias up and down instead of raising one side as it lowers the other
 

Leña_Costoso

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One last thing. It's not what the bias voltage is but what the bias voltage does that's important. Does as in what Idle current the tubes are conducting.

Bias of -55 v would be considerably cooler than -45 volts.
 

Leña_Costoso

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There are a number of references on how to do this, online.

Fender changed from bias level to bias balance... sometime during transition from blackface to silverface. The very first silverface amps were still just blackface circuits, but as "improvements" crept in, things changed. The pot is a funky looking affair, with an odd stub of a connection right out the back of the pot. That connection is a "halfway" tap, and can basically be ignored. Some guys just cut it off. Then there's some minor rewiring and voila, you can set the bias correctly. And... use tubes, please, that are fairly matched at idle and for transconductance.

How'd the screen grid resistors check out?

Excuse my posting... I'm in bed w/flu... voice typing.
 

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