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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 10:07 am
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Location: Coastal Bend, Tx.
When I removed the tubes I noticed the 5y3 glass was loose on the base, may still work but iffy for sure. No sign of vacuum leak inside the tubes glass.
MDF finger joined cabinet w/ particle board baffle. No label Oxford 465-7019
Doesn't appear to be any mods
20/20/20/20 Cap Can w/ 2x20's in parallel
Here is the chassis pic:
Image


Last edited by sfceric64 on Sat Jul 08, 2017 6:49 am, edited 2 times in total.

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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 10:11 am
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sfceric64 wrote:
MDF finger joined cabinet w/ particle board baffle.


Are you sure? The cab should be finger-joined pine planks.

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: 1970 Bronco
Posted: Wed Jun 28, 2017 10:54 am
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Well I thought I was until you said something, thanks :lol: . Took a little 0000 to the bottom and yes its PINE and particle board baffle. Its covered in a grayish/black coating, what is that stuff?


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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 11:18 am
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Well, the circuit does look original. Fender used a 20/20/20/20 cap can, from the Princeton Reverb line --- and tied the first two sections together (parallel). To get final capacitance = 40mfd, in the first stage. All other stages are 20mfd.

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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 1:03 pm
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Well I went ahead and ordered all the electrolytic from AES(had to get the 15% off sale).
ordered the CE 40/20/20x525 (the original can was dtd 69 33d week)
Went w/ my gut on a couple for the SR, are these suitable replacements?
ordered 2x Sprague 80x450 to replace the 2x 70x350
ordered 1x F&T 100x100 to replace the 50x70

also started thinking about the bottom tolex(vinyl) piece, re-looking at the 1st pic it appears the amp was probably in a basement and possibly sat in an inch or so of water long enough to dissolve the glue and permanently stain the baffle board while deteriorating the tolex material somewhat to appear like vinyl.

Mom update; while still in the hospital she is doing remarkably well and is likely to be released soon.


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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 8:36 pm
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One question regarding the 2nd Cap from the right, V1B cathode to intensity pot of the Vibrato channel.
The SC AA764 schematic says it is a 2ufx25vdc and the ampwares bronco AB764 layout says it is a 10ufx25vdc but the amp has a 10ufx35vdc. I ordered a 10ufx50vdc to cover the specs of the actual cap that's in the amp should I have followed either of the schematic specs and what are the effects between the three?

I'm guessing it regulates or supplies a sturdier juice to the intensity of the amps vibrato as the #uf is increased while the #vdc is just the max amount expected to be seen and the V1B cathode voltage increases similarly.


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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 10:21 pm
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Higher voltage rating on those 'lytics is fine. Up to a point. IME, 50-100VDC for 25VDC is fine.

:)


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Post subject: Re: 1970 Bronco
Posted: Wed Jun 28, 2017 10:23 pm
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sfceric64 wrote:
One question regarding the 2nd Cap from the right, V1B cathode to intensity pot of the Vibrato channel.
The SC AA764 schematic says it is a 2ufx25vdc and the ampwares bronco AB764 layout says it is a 10ufx25vdc but the amp has a 10ufx35vdc. I ordered a 10ufx50vdc to cover the specs of the actual cap that's in the amp should I have followed either of the schematic specs and what are the effects between the three?

I'm guessing it regulates or supplies a sturdier juice to the intensity of the amps vibrato as the #uf is increased while the #vdc is just the max amount expected to be seen and the V1B cathode voltage increases similarly.


Correct. That cap is only passing the biasing voltage -- a 50 VDC rating is more than adequate. Since that current is basically pulsed DC fed via the tremolo oscillator, the increased value should in theory provide a more uniformly-shaped pulse to extend the life of the 6V6GTA.

There are many anomalies that to this day remain unexplained but it's well-documented that Leo often ordered circuit deviations from the official schematics -- this instance seems to be one of those cases.

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: 1970 Bronco
Posted: Wed Jun 28, 2017 11:38 pm
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Thanks, was hoping I was in the ballpark. :D


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Post subject: Re: 1970 Bronco
Posted: Thu Jun 29, 2017 3:15 am
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Indeed, you're good to go.

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: 1970 Bronco
Posted: Thu Jun 29, 2017 9:37 pm
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sfceric64 wrote:
One question regarding the 2nd Cap from the right, V1B cathode to intensity pot of the Vibrato channel.

The SC AA764 schematic says it is a 2ufx25vdc and the ampwares bronco AB764 layout says it is a 10ufx25vdc but the amp has a 10ufx35vdc. I ordered a 10ufx50vdc to cover the specs of the actual cap that's in the amp should I have followed either of the schematic specs and what are the effects between the three?

I'm guessing it regulates or supplies a sturdier juice to the intensity of the amps vibrato as the #uf is increased while the #vdc is just the max amount expected to be seen and the V1B cathode voltage increases similarly.



These are all bypass caps. Changing the mfd will affect the tone. That in mind, I've used 10-25 mfd/50VDC caps to replace those oddball low mfd caps with no problems.

As for main PSU caps, I like 500VDC rated ones. And the two 70mds/350VDC caps, in the SR first filter stage... I've been using F&T 80mfd/450VDC ones. Can't go much above 80mfd (two in series = 40 mfd) --- when using the caps in a PSU, that has tube rectification.


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Post subject: Re: 1970 Bronco
Posted: Fri Jun 30, 2017 8:30 am
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I was a little click happy when I placed my order. I ordered a mix of Sprague & F/T in the SR for the PSU, the 80x450 Sprague is 2x the price of the F&T. The drop down box for Sprague is an "easy button" and I pushed it.
So the doghouse will be black and blue on the inside 2x80x450 and 3x22x500. The 5x25 that I missed on the original order wont be here until after the 4th, shipping cost more than the cap. :(

$100 bucks for all the electrolytics(18) in both amps is still a pretty reasonable amount and the piece of mind is priceless, imo.


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Post subject: Re: 1970 Bronco
Posted: Fri Jun 30, 2017 7:17 pm
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Checking all 20 of the resistors today. Making a #reference sheet.
Edited to add the results below:
5 out of spec
The 10k dropper is open, will change the 1k & 10k to MOx3watt
The 470-ohm V3 Cathode resistor needs changed
Both resistors 47-ohm/2.7k to the spkr jack didn't measure properly, gonna order them JIC.
Pulled the death cap
Pulled the two prong cord
Pulled the electrolytics


Last edited by sfceric64 on Sat Jul 01, 2017 1:36 am, edited 1 time in total.

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Post subject: Re: 1970 Bronco
Posted: Sat Jul 01, 2017 1:20 am
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For all voltage droppers, metal oxide replacements are superior for their resistance to thermal fatigue. However, for any resistors that pass a signal I prefer to stick with original carbon-comps.

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: 1970 Bronco
Posted: Sat Jul 01, 2017 1:56 am
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Thanks, Arjay
3watt MO it is for power and 1wattMF/WW on the output tube Cathode


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