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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Wed May 02, 2012 4:26 am
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Arc-n-spark wrote:
Not much and since this will be blackfaced and thus not restored to original specs I guess I could put on anything I like however I think you told me that the original grill cloth is not available am I right?

Ken

It looks like this only not so much turquoise.

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I took another look and this seems like its what I have. I'm glad MoJo has it.

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Wed May 02, 2012 6:30 pm
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Tonight I decided to look at the reverb tank. It's a 64063 and I think part of why I don't think it sounds so great is because one pair of springs doesn't meet in the middle. Is this a cause for concern? Also I measure 2 ohms at one jack and 142 ohms at the other. Please don't tell me I need one of these too.

Ken

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Wed May 02, 2012 6:35 pm
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Post a pic so we can ascertain the damage.

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu May 03, 2012 1:01 am
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Ken, those readings aren't bad. The springs just transfer mechanical energy between the transducers, at either end. As long as the springs aren't loose and the transducer coils are tight, all is well. You can clean all the connections, inside and out of the tank. The 12AT7 and 12AX7 tubes can really make a difference in the signal strength. JMO Art....Stratele, Arjay or Beemer might have better advise!

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu May 03, 2012 12:50 pm
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I have a 1968 Showman Reverb Silverface. One of the first ever made.
Personally, regarding tolex and refurbish, I would make sure the amp
ends up working great. Once you have a great working amp,
then you decide if, along with the original cabinet, grill, drip-edge, tolex
and exterior chrome, is the amp all original?

Or, did you modify the the chassis, pots, trannys, choke, etc.

So, do you have an all original amp, or is it modified?

If original, then say to yourself, is it a keeper or should I sell it?
If you will sell it, then it looks bad.
If you keep it, and do not refurbish it, then it is your ugly baby.
If you decide to refurbish it (grill, tolex, chrome) then it is not an
original Fender Showman, it is a Fender Showman that was once original.

Personally, I love reconditioned amps by those who can do a good job.
I buy them cheap, pretend $500 (you can get a clean Silverface for $800).
If I use it for two years, did not I get my $500 out of it from usage?

Then I can sell it again as a good working, good looking refurbished Showman.

But, if you make the amp work great and is all original, but is disgusting
to smell and look at, then it is your baby that you love, cause nobody else
will love it like you do.

Personally, the amp looks bad. Retolex it, add good looking metal/chrome,
nice handle, new faceplate & knobs, new grill and reshine the drip edge trim.

Make it an original internal amp and a great looking external amp.
You will have fun with it, and when you sell, it will move faster than
an ugly amp sitting an a shed for 24 years :lol:


Check out my purchase of a 1968 Fender Concert II Rivera "Redhead" 1x12 Combo amp:

http://www.ebay.com/itm/300648476303?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

***Make sure to look at the 11th of 12 photos to see what the amp
looked like originally to the previously owner = piece of $hit!

Now, it is beautiful, and as you can read, highly super modified.
It is the most modified amp I own, and the 1983-1986 Fender Rivera amps
with push/pull electronics are some of the greatest amp modification
posibilities offered to work on.

So, say to yourself, "Would I rather have that rat's nest of an amp pictured
in the 11th picture? Or, would I rather have the beautiful red head with
the great boob job, pearly white teeth and freshly showered & scented."

No, you cannot date my amp, but you can see the difference.
Look at your "FREE" amp and see what you can do with it.

Remember, once you make a change, it is not a FENDER anymore.
It will be a FENDER with changes, like a 1966 Ford Mustang Shelby.
Those have new tires, paint, batteries, spark plugs, seat covers,
wiring, muffler, breaks, exhaust, filters, oil, bearings, etc. Right???
And they still sell for millions. Right?

Hope my thoughts help provide more choice towards your decisions.

***Please. do tell me true what you think about my:
1983 Fender Concert II Rivera Redhead
Thanks, Toppscore 8)

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu May 03, 2012 1:51 pm
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aclempoppi wrote:
Ken, those readings aren't bad.


Correct. The internally-grounded Fender tanks typically measure one or two ohms (sometimes even less) at the input and between 80 and 230 ohms at the output, assuming good transducers at both ends, and both spring sets installed. The tank on my early '68 Deluxe Reverb measured 0.82 ohms at the input and 81.5 ohms at the output.

HTH

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: '69 Dual Showman Reverb questions
Posted: Thu May 03, 2012 6:40 pm
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OK.. cool. Now for the bias. I have removed the 56k resistor on the one leg of the pot to ground and replaced it with a 20 k pot just to see what kind of change I would get. I have installed a 1 ohm resistor on the left most 6L6 and also one on the right most 6L6 pin 8. I'm a little confused on this part though, when I measure the voltage on pin 8 to ground through these resistors am I measuring the current through one tube or through the pair? I'm trying to use the Weber tube bias calculator thingy here http://www.webervst.com/tubes1/calcbias.htm The tubes do seem to get pretty hot now as compared to before when I had -54V going into the bias pot. I now have -37V at the bias pot, 405V between plate and cathode, 51mA current measured at the 1ohm resistors to ground. According to the weber calculator the amp should now be balanced at about 70% which they say is hot. Before it was well below cold. So now I'm thinking I need to increase this bias pot resistor to the 27 ohms that is called for. does this sound right?

Also... I decided to poke around a bit with a fiberglass tweeker and the PI and Vibrato tubes seem to amplify the scraping sound i made by rubbing the tweeker on the metal tube cover. Does this mean they are microphonic?

As always... thanks for the input.

Ken

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu May 03, 2012 8:18 pm
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Arc, the idea of installing a precise (1% tolerance is best, 1/2 to 1 watt resistor is fine)---is you measure the VOLTAGE drop (in DC mV) across each resistor (cathode to ground). Unit set to no volume, with load (speakers hooked up), and no input. The mV read = mA of idle current passing across each resistor. As in Ohm's Law:

E (voltage) = I (current) X R (resistance)

Or if the resistance = 1 ohm. Then, voltage = current.

If your (plate voltage - cathode voltage) is around 405VDC, then you measure across the 1-ohm resistor and its measurement = 45mV DC, that would equal 45mA at idle.

(45mA) = 0.045 amps.

and (watts of dissipation) = (Current, in amps) times (plate volts-cathode volts)

or (0.045) x (405) = 18.225 watts idle dissipation per 6L6GC.

Which is a nice conservative point to start with for a Class AB1, push-pull amp (which your Fender is design to run).

You do NOT need the bias supply voltage (-37VDC) for calculating the idle dissipation. The negative voltage is to "set" the idle bias current (equal to adjusting car's idle). But, does not tell you what the actual current passing through the tube. This is tube and circuit dependent. It's why to adjust bias with every change of output tubes. Each one idles at its own unique point.

Now, you only have one bias pot. And two readings (one for each 6L6GC). This is a great way to match output tubes. Just measure a bunch until you get about the same mV reading across the two one-ohm resistors. Doesn't matter if they are the made by same company or vintage. Though, most NOS tubes are built to tighter tolerances than newly made tubes. But, putting a GE and an RCA together is no problem---esp if those two come closest in measurement to each other.

You can adjust the idle bias point to within tube manufacturer's recommended range. Depends on the tubes, too. NOS RCA black plates can take a lot higher idle bias settings, than a run-of-mill Sovtek. I would keep the dissipation around 18-20 watts per tube.

Check tubes for red plating. A sign of either too high current passing through tube (too hot) or bad tube. Too cold biasing will give amp a sterile, solid-state sound. Lacking rich harmonics and texture.

HTH!


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Fri May 04, 2012 4:09 am
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Joined: Sat Mar 31, 2012 6:38 am
Posts: 593
Toppscore wrote:
I have a 1968 Showman Reverb Silverface. One of the first ever made.
Personally, regarding tolex and refurbish, I would make sure the amp
ends up working great. Once you have a great working amp,
then you decide if, along with the original cabinet, grill, drip-edge, tolex
and exterior chrome, is the amp all original?

Or, did you modify the the chassis, pots, trannys, choke, etc.

So, do you have an all original amp, or is it modified?

If original, then say to yourself, is it a keeper or should I sell it?
If you will sell it, then it looks bad.
If you keep it, and do not refurbish it, then it is your ugly baby.
If you decide to refurbish it (grill, tolex, chrome) then it is not an
original Fender Showman, it is a Fender Showman that was once original.

Personally, I love reconditioned amps by those who can do a good job.
I buy them cheap, pretend $500 (you can get a clean Silverface for $800).
If I use it for two years, did not I get my $500 out of it from usage?

Then I can sell it again as a good working, good looking refurbished Showman.

But, if you make the amp work great and is all original, but is disgusting
to smell and look at, then it is your baby that you love, cause nobody else
will love it like you do.

Personally, the amp looks bad. Retolex it, add good looking metal/chrome,
nice handle, new faceplate & knobs, new grill and reshine the drip edge trim.

Make it an original internal amp and a great looking external amp.
You will have fun with it, and when you sell, it will move faster than
an ugly amp sitting an a shed for 24 years :lol:


Check out my purchase of a 1968 Fender Concert II Rivera "Redhead" 1x12 Combo amp:

http://www.ebay.com/itm/300648476303?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

***Make sure to look at the 11th of 12 photos to see what the amp
looked like originally to the previously owner = piece of $hit!

Now, it is beautiful, and as you can read, highly super modified.
It is the most modified amp I own, and the 1983-1986 Fender Rivera amps
with push/pull electronics are some of the greatest amp modification
posibilities offered to work on.

So, say to yourself, "Would I rather have that rat's nest of an amp pictured
in the 11th picture? Or, would I rather have the beautiful red head with
the great boob job, pearly white teeth and freshly showered & scented."

No, you cannot date my amp, but you can see the difference.
Look at your "FREE" amp and see what you can do with it.

Remember, once you make a change, it is not a FENDER anymore.
It will be a FENDER with changes, like a 1966 Ford Mustang Shelby.
Those have new tires, paint, batteries, spark plugs, seat covers,
wiring, muffler, breaks, exhaust, filters, oil, bearings, etc. Right???
And they still sell for millions. Right?

Hope my thoughts help provide more choice towards your decisions.

***Please. do tell me true what you think about my:
1983 Fender Concert II Rivera Redhead
Thanks, Toppscore 8)


TC: this is gonna be my ugly baby until I get some time and money together to either cover it and make like new or until I turn it into something completely different. I keep see sawing on this but until then I'm just trying to make it into the best amp I can.

Arjay Thanks for the confirmation my tank meters out OK.

_________________
...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Profile
Post subject: Re: '69 Dual Showman Reverb questions
Posted: Fri May 04, 2012 4:34 am
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Joined: Sat Mar 31, 2012 6:38 am
Posts: 593
BMW2002Ti wrote:
Arc, the idea of installing a precise (1% tolerance is best, 1/2 to 1 watt resistor is fine)---is you measure the VOLTAGE drop (in DC mV) across each resistor (cathode to ground). Unit set to no volume, with load (speakers hooked up), and no input. The mV read = mA of idle current passing across each resistor. As in Ohm's Law:

E (voltage) = I (current) X R (resistance)

Or if the resistance = 1 ohm. Then, voltage = current.

If your (plate voltage - cathode voltage) is around 405VDC, then you measure across the 1-ohm resistor and its measurement = 45mV DC, that would equal 45mA at idle.

(45mA) = 0.045 amps.

and (watts of dissipation) = (Current, in amps) times (plate volts-cathode volts)

or (0.045) x (405) = 18.225 watts idle dissipation per 6L6GC.

Which is a nice conservative point to start with for a Class AB1, push-pull amp (which your Fender is design to run).

You do NOT need the bias supply voltage (-37VDC) for calculating the idle dissipation. The negative voltage is to "set" the idle bias current (equal to adjusting car's idle). But, does not tell you what the actual current passing through the tube. This is tube and circuit dependent. It's why to adjust bias with every change of output tubes. Each one idles at its own unique point.

Now, you only have one bias pot. And two readings (one for each 6L6GC). This is a great way to match output tubes. Just measure a bunch until you get about the same mV reading across the two one-ohm resistors. Doesn't matter if they are the made by same company or vintage. Though, most NOS tubes are built to tighter tolerances than newly made tubes. But, putting a GE and an RCA together is no problem---esp if those two come closest in measurement to each other.

You can adjust the idle bias point to within tube manufacturer's recommended range. Depends on the tubes, too. NOS RCA black plates can take a lot higher idle bias settings, than a run-of-mill Sovtek. I would keep the dissipation around 18-20 watts per tube.

Check tubes for red plating. A sign of either too high current passing through tube (too hot) or bad tube. Too cold biasing will give amp a sterile, solid-state sound. Lacking rich harmonics and texture.

HTH!


Thanks for the reply. I think I understand how the measurement is made and current calculated better now but must i connect only one tube through a 1 ohm resistor or can/should I connect both pin 8s on each pair through a single 1 ohm resistor. In other words to be sure... is it OK to have one resistor for each pair so that 2 tubes pin 8s are connected through one 1 ohm resistor. (I only have 2 1 ohm resistors and have them on only one tube of each pair right now).

And now to understand this a little more correctly. The -37V i get from the bias adjust pot can and will vary as I adjust the pot. The -37V on the schemo is just a reference voltage. I adjust for the right bias current and whatever the voltage is from the bias pot just is what it is? The Bias adjust is a voltage divider and the resistor from one end of the pot to ground is to set the range of adjustment that the pot has no?



Ken

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Fri May 04, 2012 8:59 am
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Arc, remove each ground strap from the 6L6GC cathode ---pin 8--to the chassis. Replace each with a precise (1% is best), 1/2 watt -1 watt, one-ohm resistor.

You will measure mV (DC) across each one-ohm resistor to calculate the idle bias of each 6L6GC. This is the best way. Then, you can match up the closest mV measured 6L6GC's with each other (after measuring a batch of tubes). Do not use a shared resistor between the 6L6GC, as you want to measure each 6L6GC independently.

Now, the bias supply pot and resistors are separate from the measured mV (and corresponding current flow) through each 6L6GC. The negative voltage varies depending on that line-to-ground resistor and the bias pot. The negative volts produced whether the output tubes are in the socket or not. What this circuit does is put a negative voltage onto the control grid of the 6L6GC's. Negative voltage from the bias supply onto the control grid helps adjust the flow of electrons through the tube. Which pass from the chassis to the cathode, up to the anode. Then, out to the OPT.

So, you have TWO sources of electrons. The main B+ which supplies the current flow through the 6L6GC to allow the tube to amplify. And the Bias Power supply which helps set the idle of the the 6L6GC's.


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 08, 2012 4:26 am
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How's it gettin' along, man?

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat May 19, 2012 10:30 am
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So, What is the STORY, MORNING GLORY?
PICTURES please and maybe some You Tube Video/Audio, PLEASE!!!!



Arc-n-spark wrote:
Hello all,
I have a few opening questions about my new acquisition but first a short story. I overheard a guy at work say that he never throws anything away. Knowing that this guy has been playing since '74 and gigging every weekend for the past 30 or so years I asked him if he had any old tube amps laying around that he might want to get rid of. He told me he has a Fender Dual Showman Reverb that was repaired 24 years ago and has been laying around ever since. It moved from garage to shed to the shed and he said he needed to clean it out before giving it to me. I guess some small mammals made a home in it along with numerous insectoids and arachnids. It could also pass for a elementary school project as a mold experiment. Needless to say I have my hands full. I gave it a good cleaning without going overboard and when hooked up to the speakers in my Peavy Renown 400 I was amazed at how quiet this amp sounded. It all works OK which is a nice place to start. Now for the questions. I want to make this amp safe and revitalized. I will change out all the tubes to start with since all the 6L6 tubes are different. What electrical parts should also be replaced? How should a grounded cord be installed? I realize this next question may be more of a mater of opinion but should I keep the amp as much "as is" or should I try to restore this with new tolex, hardware, knobs, etc? If I decide to restore it would enhancing it with a master volume control or other mods be cool or would it detract from it's value as a vintage amp? I don't plan on selling it but plans can change.

Any and all replies are much appreciated. Thanks for reading

Image
Image




So, What is the STORY, MORNING GLORY?
PICTURES please and maybe some You Tube Video/Audio, PLEASE!!!!
Toppscore 8)

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun May 20, 2012 8:23 am
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I hope to get to some work on it today but who knows. So much to do and so little time.

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun May 20, 2012 8:28 am
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Do me a favor, Ken.

Measure your valence panel (H & W) and let me know those dimensions. I *may* have some grill cloth for you, contingent upon what the size needs to be.

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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