Silverface Super Reverb bias question

[Randall]

I have what I believe to be a 1968 AB763 Super Reverb in for a re-tube and re-cap. I have never seen the bias circuit in it. The left 0.1 cap sees a fixed -48v from a pair of 10K ohm and a 100K resistor and goes via the yellow wire at the bottom of the board to pin one of the left output. The right 0.1 cap gets it's bias supply from the middle wiper of the pot, but instead of going to the junction of two 220K ohm resistors it goes to that eyelet, but only a 100K resistor is there and feeds pin one of the right tube via a red wire.

So, the right tube is adjustable, but the left tube is stuck at 23mA at 477v. That seems awfully cold to me. What's up with this? One of CBS early improvements?



http://i668.photobucket.com/albums/v...issimo/001.jpg

[g1]

Like AB568 ?
http://www.davidsonamp.com/sf/images...everbab568.gif

[Jazz P Bass]

The Tremelo must be off when adjusting the 'bias' voltage.

If your amp is similar to the AB568, it is cathode biased.
So the 'bias' adjust is more for the Tremelo.

Edit: a much clearer schematic is attached.

[Randall]

Wow, I am confused! It has an AB763 tube chart in it, but the bias circuit is indeed wired like the AB568? How does it work, and what is this about the tremolo? I am not understanding.

[Gingertube]

It is not a bias pot, it is an output tube idle current balance pot.

Turn OFF the Tremlo and set volume control to zero. You want no signal into the power amp stage, either audio signal or the tremlo modulation signal.
Then stick your ear against the speaker and adjust the pot for minimum hum which will occur at the point where output tube idle currents are exactly equal.

Once that is done you can turn up the volume control and adjust the hum pot if it has one, again just adjust for minimum hum.

Cheers,
Ian

[Randall]

HA! I am not going to stick my ear against the speaker! But I can measure and match output tube idle currents.

So, if I understand this... it's an idle current balance adjustment. And it's cathode biased, which explains the cement 150 ohm cathode resistors, which I've never seen on a Super. Interesting, I've never heard of this version. I owned a '69 that looks almost exactly like this one, but had the "normal" bias circuit, so was it something that was tried, and then ditched?

Can someone explain how this circuit works? I understand cathode bias, but not with this adjustment pot.

[Gingertube]

Its a "Hybrid" bias - part cathode bias and part fixed bias. The 150 Ohm cathode resistors on each tube would need to be more like 560 or 620 Ohms or so to develop enough bias voltage to be fully cathode biased.
Cheers,
Ian

[mozwell]

If the bias balance pot wiper went open circuit, we lose the negative bias to the top valve on the sch.
If we added a 220k from the wiper of the bias balance pot, to the end of the pot connected to the diode & capacitor (-56V end) if the wiper ever had a bad connection, we would get maximum negative bias & this would protect the tube. (please let me know if this is wrong).
this mod might save a tube failure if the bias balance pot wiper ever failed

[Enzo]

You do this with no signal applied, turn all the volumes to zero. Nothing much comes out the speaker except background hum, there is nothing to assault your ears.

The point is that it is a hum balance control. You can balance the currents, but that may or may not null the hum. That is why it is done by ear rather than current measurements. The best setting may not be where currents are equal. other factors enter the equation, such as differences in OT winding resistance.

[Chuck H]

FWIW...

I was just reading in copy of Fender Amps (The First Fifty Years) by Teagle and Sprung. It's a good book from a historic perspective. Carefully fact checked and edited so far as I've been able to tell.

The book pretty much mirrors what has been said here. In 1968 Fender switched to the SF cosmetics and soon after changed to the AB568 circuit, enlarged the cabinet and scooched the speakers askew rather than in the corners. Then in 1969 they undid what is considered their least good bias circuit for the AA1069 model and a year after that they made the cabinet smaller again and dropped the silver trim from the SF cosmetics. Interesting to me is that this implies there are bona fide AB763 amps from early 68 with BF circuitry and SF cosmetics.

Not mentioned in the book is how Fender commonly used left over parts, including tube charts, when changing model numbers and designs. It seems dumb to me that something as cheap as a piece of paper, one with the potential to cause considerable misinformation, was reused out of context rather than thrown out. Hey Leo/CBS, Can you guys spare a penny so I can know what amp I own, please?!?

[Leo_Gnardo]

Can someone explain how this circuit works? I understand cathode bias, but not with this adjustment pot.

The hybrid bias works like crap. I snip out those 150R's and accompanying cathode-to-cathode cap, wire cathodes to ground, good time to put in 1 ohm 1% 3W WW bias current sense resistors. On the balance pot, instead of just a resistor to ground, add a 10K or 20K trim pot in series with a resistor 10K or so, and there's your bias adjustment. Now you have balance, bias, and current sense resistors, and a good sounding reliable output section.

Once I built a couple of amps with hybrid bias. Sound was awful 'til I went back to normal fixed bias. I can't imagine why Fender went there, maybe the engineers' coffeepot was spiked with extra-strength brew.

[Randall]

I must say, I don't like this circuit. And Fender must not have liked it either, because I had '69 AB763 Super that had the standard fixed bias circuit, so that suggests they went back to it. I think I will adjust it as described and let the customer play it for a while. And if he wants to change it out to fixed bias later we can do that.

[pdf64]

Mixed cathode / fixed bias works pretty well in 60s Vox AC30 (though the early tube rectified ones were fixed bias only).
Compared to the SR AB568, they are more 'fixed' than 'cathode', with 47 ohm cathode resistors (unbypassed) per EL34.