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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 5:15 am
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Something is wrong for sure. My TR is louder than any amp I have right now. My DR doesn't even come close. I can't approach "10" with the Twin Reverb. I can't hear what I'm playing. But, the neighbors on the next block sure can. :lol:

The DR is loud, but can but cranked to "10" in the middle of the day --- in short bursts --- without a visit from the Federales.


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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 5:39 am
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BMW2002Ti wrote:
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


Having trouble reading those schematics, but pretty sure its the ac568 because of the 100uf-70vdc bias cap.
Not looking at the chassis now, but seem to recall that at least one of the diodes had a different center color than the others.
Betcha its a diode problem meaning I'll have to check them out of circuit. Can those be unsoldered right from the top of the board?


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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 5:49 am
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I'm betting that there was a voltage problem to begin with when I purchased the amp.
Could tell right away that the volume wasn't there..... but decided to buy it anyway and look for a cap job to remedy the problem.
BTW....all the replacement caps are very close or exact value. Went through multiple sources to accomplish this.


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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 6:47 am
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yomammaaho wrote:
Having trouble reading those schematics, but pretty sure its the ac568 because of the 100uf-70vdc bias cap.

Not looking at the chassis now, but seem to recall that at least one of the diodes had a different center color than the others.

Betcha its a diode problem meaning I'll have to check them out of circuit. Can those be unsoldered right from the top of the board?


The AC568 circuit has prominent 150-ohm, 7-watt wire wound "sandbox" resistors tied from the cathode-to-ground of each 6L6GC. And 2000 pico-farad ceramic disc caps tied from the signal grid of each 6L6GC to ground. These resistors and caps are unique to the AC568, in the TR. And are part of the hybrid bias setup (fixed AND cathode biasing), plus the anti-high frequency, parasitic oscillation circuit. Whew! :lol:

The AC568 and the AA769 circuit have a 50mfd/70VDC bias supply filter cap.

Photos of your amp's circuit would help a lot. You need to use the diode test option on your DVM or do a forward-and-backward continuity test on each diode. If there is obvious physical damage --- replace all diodes. I like UF5408 for the six main PSU bridge. And a UF4007 for the bias supply. WATCH polarity!!!

You should definitely check the two 220k-ohm, 2 watt resistors that parallel the 70mfd/350VDC OEM caps in the main PSU supply. If these are off by more than 10%, replace them. They must be in good shape --- so that these first two caps of the first filter stage (just before the standby switch) function properly. If they do not operate correctly, wrt an even voltage load across both caps --- you will never get the proper voltage readings off the power supply taps.


Rectifier bridge and bias diode board:

Image

-----

Two 220k-ohm resistors paralleling first two main caps (left) under the "doghouse."


Image
-----

Now, once you've made sure the caps and resistors are ok, under the doghouse. And the diodes of the rectifier bridge and the bias supply are ok --- measure the voltages off the main caps and compare them to the schematic. Tubes removed. Nothing plug into inputs. Zero volume. Speakers connected. The values must be within 15-20VDC of the values on the schematic. All values. I'd use the AC568 schematic as a voltage guide for the power rail in your amp.

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

See the schematic: VDC at the standby switch. After the choke (TR2). At the plates of the 6L6GC (pin 5 of the socket). And at each point off each 20mfd/500VDC cap in the power rail (B, C, D on the schematic). And the reading at the junction of the bias supply and the two 220k-ohm, 1/2 watt grid-to-ground resistors (these are on the main tagboard, in characteristic v-position).

If you look at the photo of the caps under the doghouse, you should be able to figure out where B+, before the standby switch, as well as points B,C, & D are. Leo made reading these important values a very easy task. Do you see where they are in the photo? Remember, the standby switch must be in the "amp-on" position to measure B,C, & D.

All voltages must be good and follow a reasonable relative relationship to each other (on the main power rail) --- in order to have a proper working amp. Study the schemo and you'll see how each tap off the main power supply rail drops wrt to the last tap point. This is important.

BEWARE!!! you will being dealing with a live amp that has ~500VDC coursing through its veins. Something, that needs the upmost respect and attention. A good tech maybe in order.


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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 12:42 pm
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Wait - red, red, black, orange, gold?/silver?

Red, red, black is 22 OHMS, what's up with these? Even a resistor calculator doesn't show this combination.

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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 1:13 pm
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For 5 band resistors, read the first three colors, in this case red-red-black is 2-2-0, 220, then the forth band is the multiplier, in this case orange means x 1000, so you have a 220K ohm. Gold is still 5% tolerance and silver is still 10% tolerance. For the multiplier band, the color indicates the number of zeros, brown is one zero (x 10), red is two zeros (x 100), orange is three zeros (x 1000), etc.

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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 1:42 pm
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Ok, whew. I thought I was going crazy for a minute when the resistor calculator I used (I'm not familiar with 5-band resistor codes) didn't show it. A different one does. Thanks!

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Post subject: Re: 69 Twin Bias
Posted: Wed May 14, 2014 5:51 pm
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Jim,


Actually, this is a flame-proof, 1% tolerance, 2-watt resistor. The final band is a light brown, not gold. Mike is correct about the multiplier. You need this band with resistors of 2% or better tolerance. As you can have 100, 101, 102-ohm 1% resistors.

-----

Image


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Post subject: Re: 69 Twin Bias
Posted: Thu May 15, 2014 10:43 am
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This 69n Twin Reverb may be the AA769 circuit, which I am researching has a plate voltage of 405 volts.
http://books.google.com/books?id=kxtcys ... te&f=false

It might be I am unlucky enough to have one of the less toneful Twins.


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Post subject: Re: 69 Twin Bias
Posted: Thu May 15, 2014 11:23 am
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yomammaaho wrote:
It might be I am unlucky enough to have one of the less toneful Twins.


Mischievous tinkering by CBS's nefarious battalion of electronic dilettantes turned virtually all of the more powerful Fender platforms into sonic turds. Mercifully though, most of the original hardware remained and some modest re-wiring is all that's required to restore them to their former tonal glory. The AA769 was one of the worst offenders but reversion back to AB763 specs is possible. Raising that B+ to 460 VDC is a good first step.

Arjay

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Post subject: Re: 69 Twin Bias
Posted: Thu May 15, 2014 11:52 am
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If I knew for sure my AA769 405 vdc plate voltage could be increased w/out having to start with another pwr. transformer, I'd consider it.
For now, I'm gonna replace the oxfart speakers to much more efficient ones and see if any saving grace.
Otherwise, this might be a chassis to have a builder hack.


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Post subject: Re: 69 Twin Bias
Posted: Thu May 15, 2014 12:27 pm
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The power tranny for the AA769 carries the same P/N as that of the AB763 -- 125P34A. Thus, the potential for 460 VDC from your power supply exists so long as the tranny itself is not tired or damaged.

Arjay

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Post subject: Re: 69 Twin Bias
Posted: Thu May 15, 2014 1:58 pm
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yomammaaho wrote:
If I knew for sure my AA769 405 vdc plate voltage could be increased w/out having to start with another pwr. transformer, I'd consider it.
For now, I'm gonna replace the oxfart speakers to much more efficient ones and see if any saving grace.
Otherwise, this might be a chassis to have a builder hack.


Like Arjay said, with that PT and PSU circuitry on that AA769, I can't figure out how Fender dropped the B+ from 470 to 410VDC. One way would be to use inferior power supply caps. Like ones rated at nominal value, but much greater tolerance (like +/- 50%). Or Fender started using 6L6GC tubes that draw a LOT of extra current, dropping B+ a whole 60VDC. Also doubtful.

Your PT is made by Schumacher. I doubt the PT is the problem. And the OPT is also Schumacher. Same part numbers, between BF, early SF and your model. Same part number choke.

The big problem is if the plate voltage is truly 405VDC, you'd need to run those 6L6GC's at 50mA each, to get 20 watts of idle dissipation per tube. And when you do this, it seems that red plating is an issue. This just doesn't make sense. Fender runs their 6V6GT amps at higher plate voltages.

OK... try this. Pull all 6L6GC tubes. Now, measure the B+ at the (+) end of the second filter cap (the cap, whose lead goes to the standby switch, output tube side). Right were the standby switch meets the choke (TR2).

Look at the photo above, of the caps under the doghouse. Read at the (+) end of the third cap from the left. This is the (+) end of the first 22mfd/500VDC cap.

Measure with the standby switch on and off. Record those values.

Now, put the 6L6GC's back into the amp and re-measure B+ at the same points. Standby switch on-&-off. Lets see what you get & go from there.

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

Good luck!


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Post subject: Re: 69 Twin Bias
Posted: Thu May 15, 2014 2:07 pm
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BMW2002Ti wrote:
OK... try this. Pull all 6L6GC tubes. Now, measure the B+ at the (+) end of the second filter cap (the cap, whose lead goes to the standby switch, output tube side). Right were the standby switch meets the choke (TR2).

Look at the photo above, of the caps under the doghouse. Read at the (+) end of the third cap from the left. This is the (+) end of the first 22mfd/500VDC cap.

Measure with the standby switch on and off. Record those values.

Now, put the 6L6GC back into the amp and re-measure B+ at the same points. Standby switch on-&-off. Lets see what you get & go from there.


+1!

Said check should reveal all.

Arjay

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