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Post subject: Dual Showman Reverb Export model
Posted: Fri Apr 13, 2018 2:32 pm
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Good evening.
I'm new to this forum, signed in because I'm working on this amp that has some strange modifications that I can not yet figure out. There is a kind of universal powertransformer in it (marked "old twin") that supplies 345VAC for B+ and 50VAC for the bias circuit. The filter caps toward the centre of the schematic that filter the platevoltage for V1-V4 are bridged by a 330K resistor and there are two parallel resistors (22K+10K=>6875Ω) placed between the filter choke and the rest of the world, but past the branch to the 470Ω resistors to the screens.
I somehow fear to remove all modifications because I don't really trust the powertransformer.
BTW, the amp plays quite OK, be it with a loud 100Hz hum, probably due to bad filter caps.
Can anybody comment on this? Would I take a big risk undoing the modifications? Should I order a new powertransformer?
Tanks for your replies.
Grtz,
Eric


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Post subject: Re: Dual Showman Reverb Export model
Posted: Sun Apr 15, 2018 8:37 pm
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Which Showman Reverb schematic are you referring to, and what year showman reverb are you working on? Does it have a master volume control?

From your post, it kinda sounds like someone put a power transformer for a 100 watt model in a 80 watt model and made some mods to reduce voltage to V1 through V6.

Just a guess without more info or pictures.

Any numbers on the power transformer? Numbers on the other transformers and choke?

Almost forgot, welcome to the forum! In the UK, I presume?


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 1:26 am
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Hi, Vinyl.
First of all, thanks for your reply. I am in The Netherlands, you were very close.

The amp has the master volume control; the output transformer, the filter choke and the reverb driver transformer are originals as far as I can see.

What you state about reducing the voltage is what I thought at first, too.
But the amp seems to be the 100W model, so there should be no need to reduce voltages, or is there?

BTW all pre-tubes are replaced by their ECC "equivalents".

Some pictures of the amp are here:
https://www.dropbox.com/sh/a6luu0a50ndg ... R0vYa?dl=0

The schematic is there too; it matches this amp quite well as far as I can see, apart from the things mentioned above.

Grtz,
Thanks,
Eric


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 9:16 am
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Looking at that 330K-ohm resistor addition... looks like whomever thought that these two 20mfd filter caps were in series (like the first two 100mfd ones). And put, what he/she thought are 330k-ohm equalizing resistors into the circuit. As it is, they are acting like bleeding resistors.

https://photos-4.dropbox.com/t/2/AADAaa ... ize_mode=3


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 10:41 am
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Agree with beemer as to one possible explanation. Another is the caps in this picture
https://photos-1.dropbox.com/t/2/AAC0XDPNcLiu7ku6GfpbjFZkW6pT85z5GAB4lVgAYFNPMA/12/102111891/jpeg/32x32/3/1523916000/0/2/IMG_9212.JPG/EO3zjU8Y7usHIAcoBw/IWBpeWsvTPzmO8Ey5bJBPTwZLrOZnT31ASTx7dee95w?dl=0&size=800x600&size_mode=3
appear to only be rated at 350 volts, and would probably puke their guts out (slang translation: vent the electrolyte out) if the voltage isn't reduced, as the schematic shows 450 and 349V across those caps.

On a different subject, if the voltage on the primary side of the power transformer is approximately double (230V versus 117V) would a 2 amp slow blow fuse be more appropriate than a 3 amp, allowing for transformer not being 100% efficient?


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 11:37 am
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Vinyl,

Actually, this is an intended use of lower voltage rated caps. By putting two similar value caps in series, the resultant capacitance is half original (50mfd, in this case), but rated at double the voltage (700VDC).

Manufacturers did this during the 1940-60s, when single, higher voltage rated caps were unavailable or too expensive.

Image

BTW... does your nom-de-plume mean you are into old analog? Me too! :)

Image


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 12:04 pm
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With only a paltry 345 VAC to the plates, I'd be surprised if the amp was producing anything more than 50 watts. And that low a voltage at the output stage is likely stifling much of the traditional Fender tone that the Dual Showman Reverb was known for. Were the amp mine I'd find the correct export PT for it, gut the mods, and return the wiring to stock configuration.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 12:34 pm
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Hey, Guys.
Thanks for your involvement.
Reading all your comments points me into the direction of replacing the PT by a suitable one and removing the modifications. Apart from the stuff I already mentioned the only other thing I could find were the plate resistors of the phase inverter. They are 68K now.
Classictone has the export PT; unfortunately my predecessor has destroyed the selector switch as well.
Grtz, again,
Eric


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 1:27 pm
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Do you need an export version of this amp? If not, maybe a regular Dual Showman PT will suffice.

https://www.angela.com/hammondpowertran ... 290fx.aspx


The PI plates are loaded with 47k-ohm, in your original schemo. You may want to "match" the two closest 47k-ohm resistors (using a good DVM). For these big amps --- could make a difference in the "clean" at super loud.


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 1:40 pm
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BMW2002Ti wrote:
BTW... does your nom-de-plume mean you are into old analog? Me too!

Steve,
Yes, I'm looking forward to Record Store Day on the 21st. I'm the same vinyl that hung out here for a while a few years back. Aging parent, work, personal battle with cancer, etc, have kept me away for a while.

I'm also the same vinyl that still owes Arjay $1.00 :oops:


ericxke wrote:
Hey, Guys.
Thanks for your involvement.
Reading all your comments points me into the direction of replacing the PT by a suitable one and removing the modifications. Apart from the stuff I already mentioned the only other thing I could find were the plate resistors of the phase inverter. They are 68K now.
Classictone has the export PT; unfortunately my predecessor has destroyed the selector switch as well.
Grtz, again,
Eric


Eric,

Here is a switch:
https://www.ebay.com/itm/NOS-Voltage-Selector-Switch-6-Position-Single-Pole-for-Fender-Amps-w-black-knob/272548616265?hash=item3f7529d849:g:ocIAAOSw44BYmWlr

Dump the mods and replace all of those electrolytic caps.


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Post subject: Re: Dual Showman Reverb Export model
Posted: Mon Apr 16, 2018 2:56 pm
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Well, the PT should be able to take 230V primary; that is what we have here.....
There are PT's on the market with solely the 230V primary, I'm waiting for specs, because the secondary voltage / output is different from the ClassicTone or the Hammond version or from what's in the schematic.


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Post subject: Re: Dual Showman Reverb Export model
Posted: Tue Apr 17, 2018 6:03 am
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vinyl wrote:
Steve,
Yes, I'm looking forward to Record Store Day on the 21st. I'm the same vinyl that hung out here for a while a few years back. Aging parent, work, personal battle with cancer, etc, have kept me away for a while.

I'm also the same vinyl that still owes Arjay $1.00 :oops:



I've just fixed my turntable and got the old audio system running, again. A lot of new re-issued vinyl, these days. :)
Gonna send you a pst. I lost your email address, on my latest computer upgrade.


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Post subject: Re: Dual Showman Reverb Export model
Posted: Sat Apr 21, 2018 7:18 am
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Sure wish you were closer!
I have a UL Twin with an export PT that I hope to replace, (it is huge!)


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Post subject: Re: Dual Showman Reverb Export model
Posted: Sat Apr 21, 2018 11:54 pm
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Hi, Bill. Maybe would have been a solution.
In the meantime I ordered a replacement from Ampparts Direct. Tink that one is huge too, had to pay over $70 for shipping..


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