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Post subject: 69 Twin Bias
Posted: Tue May 13, 2014 5:47 pm
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Z.K.....Here's what I did to a stock 69.
New electrolytes (power supply, bias supply, and all cathode bypass caps).
I also converted to a grounded plug.

Using a Bias Rite cathode current probe AND an Amp Head Plate voltage probe on the arbitrary same tube.....after 10 min. Warmup at zero volume....here's my results:
Cathode current=54ma (increased from 46ma@ start up)
Plate voltage= 402v (fell from 406 @ start up)
That's about 72% idle.

I didn't mod the bias circuit to blackface specs........nor did I turn it up to sample the sound.
I'm concerned about the hot bias.


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Post subject: Re: 69 Twin Bias
Posted: Tue May 13, 2014 6:23 pm
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To check whether the other set of 6L6's were also running hot, I put the Bias Rite and Amp Head into one of the other tube sockets.....here's the data:
Cathode current =49ma, and Plate V = 395
About 65% idle


Maybe I just need to burn in the new tubes, but I'm not overly thrilled with the sound so far just doesn't seem as loud and sustaining as I'd hoped/expected.
Wondering if I should've ever changed out the caps now.


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Post subject: Re: 69 Twin Bias
Posted: Tue May 13, 2014 8:36 pm
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Did you try swapping the 2 parallel sets of 6L6's and re-measuring?


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Post subject: Re: 69 Twin Bias
Posted: Tue May 13, 2014 10:36 pm
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yomammaaho wrote:
To check whether the other set of 6L6's were also running hot, I put the Bias Rite and Amp Head into one of the other tube sockets.....here's the data:
Cathode current =49ma, and Plate V = 395
About 65% idle


Maybe I just need to burn in the new tubes, but I'm not overly thrilled with the sound so far just doesn't seem as loud and sustaining as I'd hoped/expected.
Wondering if I should've ever changed out the caps now.


There is something wrong with the plate voltage. 395VDC is way to low, for this amp. For the AC568 circuit the plate voltages should be in the 475-480VDC range, with a healthy PT, filter caps, and diodes in the PSU. For the AA769 circuit, the values should be in the 415-430VDC range. The voltage readings are with today's wall outlet voltages.

Though, I am not sure why the AA769 schematic shows 70VDC lower B+ voltages. Both the AC568 and the AA769 use the same PT and OPT. Same PSU cap and resistors values. I have not worked on a AA769 TR to confirm these values.

Maybe, check your amp's power rail caps and resistors values. Esp the two 220k-ohm load leveling resistors, in the first filter cap stage. Are the diodes in the main rectifier original?


Which TR circuit does the amp have? Here is the schemo with nominal values:

The AC568 & the AA769:



http://bmamps.com/Schematics/fender/twi ... _schem.pdf


http://bmamps.com/Schematics/fender/twi ... 0aa769.gif


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Post subject: Re: 69 Twin Bias
Posted: Tue May 13, 2014 10:54 pm
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BMW2002Ti wrote:
There is something wrong with the plate voltage. 395VDC is way to low, for this amp. For the AC568 circuit the plate voltages should be in the 475-480VDC range, with a healthy PT, filter caps, and diodes in the PSU. For the AA769 circuit, the values should be in the 415-430VDC range. These readings are with today's wall outlet voltages.

Maybe, check the power rail caps and resistors values. Esp the two 220k-ohm load leveling resistors, in the first filter cap stage. Are the diodes in the main rectifier original?

Which TR circuit does the amp have? Here is the schemo with nominal values:

The AC568 & the AA769:



http://bmamps.com/Schematics/fender/twi ... _schem.pdf


http://bmamps.com/Schematics/fender/twi ... 0aa769.gif


+1!

And double dittos on checking those 220kΩ resistors on the power rail. The reservoir caps are crucial for the power supply to develop sufficient B+ voltage.

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 Twin Bias
Posted: Tue May 13, 2014 11:34 pm
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BMW2002Ti wrote:
...There is something wrong with the plate voltage. 395VDC is way to low, for this amp...Which TR circuit does the amp have?...


According to the OP, it is a Twin, not a TR. :wink:

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Post subject: Re: 69 Twin Bias
Posted: Tue May 13, 2014 11:41 pm
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Mike,

Yeah. Could be the difference. For some reason, I can't find the schematic for the 1969 Twin. Going to let Google be my friend:

http://lmgtfy.com/?q=1969+Fender+Twin+schematic

Hmm.... :?: :mrgreen: :?:


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Post subject: Re: 69 Twin Bias
Posted: Tue May 13, 2014 11:50 pm
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There is no 1969 "Twin" amp.

It's a little early in the week to be starting a new circle-jerk.

:roll:

Arjay

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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 12:28 am
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Arjay, it's never too early. :lol:

OK. All kidding aside... do you know why the AA769 & AA270 TR schematics shows plate voltages a good 60VDC+ below the AC568 or AB763 schemo? Seems that all three models use the same iron (PT, OPT, and choke), same resistor and cap layout in the PSU. There seems to be a 40VDC nominal drop at the standby switch (whether on-or-off). I usually get a 15-20VDC drop when the standby switch is set to allow voltage to the OPT.

Have you ever worked on a AA769 or AA270 TR? I can't remember when I worked on one of these. But, I don't remember this big of a discrepancy in plate voltages between the AC568 or AB763 and the later TR circuits. I usually measure about 475 VDC on the plates.

Thanks!


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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 12:44 am
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There certainly were Twins in '69, and from what I hear, they were quite biased.

http://www.baseball-reference.com/teams/MIN/1969.shtml

Then there were the '69 Twin Turbo Corvettes.

Image

Yeah, I'm working on a Deluxe at the moment. Care to guess what amp I have on my bench?

It isn't that difficult to refer to an amp by its actual name. :wink:

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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 12:47 am
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I owned an AA270 TR back in 71/72 but I never had occasion to work on it since it never failed while in my possession. That 40-volt deficit in the B+ spec does seem to be quite a discrepancy from the AB763 and AC568 revisions. Frankly I'm wondering if it might actually be a typo on the schematics. At 405 VDC to the plates, I doubt that the amp would produce much more that 65 watts or so. And I can attest that my AA270 was damn loud -- easily the equal of a Dual Showman.

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 Twin Bias
Posted: Wed May 14, 2014 12:55 am
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Mike,

Billy Martin was their manager? Now, I know where the missing 40VDC went. :lol:


Arjay, the Dual Showman Reverb line shows the same discrepancy between early SF model (AA 768) versus later circuits. Odd, I never read or heard about this before.


http://bmamps.com/Schematics/fender/sho ... _schem.pdf

http://bmamps.com/Schematics/fender/dua ... _schem.pdf


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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 1:05 am
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I'll mention this to my Fender amp tech buddy up in Tucson. He's got nearly every schematic and TSB that Fender ever issued in his archives -- mebbe he can shed some light on this apparent anomaly.

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 Twin Bias
Posted: Wed May 14, 2014 1:07 am
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Retroverbial wrote:
There is no 1969 "Twin" amp...


There's the answer right there. :lol:

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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 3:38 am
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Pardon..........it IS a Twin Reverb.
Yes-the diodes are original.
Will do some checking of pwr. rail resistors over the week.
And yes.......I would guess output @around 60 watts dimed. My drri seems close in volume.
I did move the bias and plate voltage probes between the two sets of tubes, and on one got a cathode current of 85 ma WAY OFF with appox. 395v plate.
Definately something wrong.


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