1979 Fender Silverface Super Reverb bias too cold...

['66Cuda]

I am trying to bias a Silverface Super Reverb, by turning the "Matching Output Tubes" pot on the back. Utilizing the 1 ohm resistor to ground measurement method on the output tubes, they balance at .018mv with 503 volts. That is only about 9 watts of dissipation. The amp sounds like an ice pick. This is how the amp came to me. When I turn the pot, only one of the output tube's current changes. I suspect this is a balance pot like the Blackface Bassmans, with the other tube being fixed. I have been able to get warmer bias in the Bassmans by changing the resistor that grounds to the back of the adjustment pot, to raise the bias on the fixed tube. In this Silverface, it is a 33k resistor. Can I change the value of this resistor to control the bias in this amp, like you can in the Blackface Bassmans?

Thanks

[Leo_Gnardo]

In this Silverface, it is a 33k resistor. Can I change the value of this resistor to control the bias in this amp, like you can in the Blackface Bassmans?

Exactly. What I do is leave that 33K in place, and parallel another resistor while monitoring bias current. I'm lucky to have a resistor sub box, it comes in handy right about now. Couple of clip leads and some resistors say 470K, 220K, 150K, 100K, you'll find one that puts you in the right ball park in minutes, then solder it onto the 33K once you've settled. I like to see at least 30 mA, generally 35 is plenty for a Super Reverb. Note you'll see the hi voltage drop into the high 400's as you draw more bias current. FWIW I don't take "70%" as a target value, typically there's plenty of bias current before you get there.

[Enzo]

And do not assume that the entire tone of the amp is due to the power tube bias.

[g1]

Yes, I'm interested to hear whether warming the bias has a major or minor effect on the ice-pick tone.

[Justin Thomas]

I just added a bias adjust pot to my Bassman w. Balance. Nice having both. I do it to all my Silverface amps.

Justin

[potatofarmer]

1979, 500v, sounds like a 70W. You can add an additional bias pot, but you need the balance pot to cancel out the ripple on the screens.

If you want to warm it up, clip out the NFB resistor.

[Justin Thomas]

Re: ice pick: Speakers? Lead dress issues leading to cancellation? Preamplifier voltages too high? Has it been modded in any way?

Justin

['66Cuda]

Re: ice pick: Speakers? Lead dress issues leading to cancellation? Preamplifier voltages too high? Has it been modded in any way?

Justin

This amp has been "Blackfaced" by someone, according to the owner-whatever that means. They also added reverb and vibrato to the normal channel, and there is some sort of mod coming off of the "Hum Balance" pot. It consists of a 220mf/50v capacitor in parallel with a 2.7k resistor going to pin #7 of V1. V1 pin #1 voltage is 295 volts, where schematic calls for 250 volts. I can't remember what the others were. The normal channel output was low, and I found a 100k resistor that had failed. I fixed that and then had to leave.

In reference to the biasing, have you ever seen an amp biased that cold from the factory? I guess I am wondering why the plate dissipation is only 28%? That seems really low from the info I have read. It seems .017ma is about half the current it needs, isn't it? Should I be looking for another problem somewhere else? Also, this amp has a "Hum Balance" pot. When turned, it doesn't seem to make any difference on the hum. Any idea what the Hum balance mod might be for? When I initially started to turn the tube balance up, I could hear the hum increase. Turning the hum balance made no difference. I set everything back where it was when it came in.

I appreciate the input...

[Tom Phillips]

...have you ever seen an amp biased that cold from the factory? I guess I am wondering why the plate dissipation is only 28%? That seems really low from the info I have read. It seems .017ma is about half the current it needs, isn't it? Should I be looking for another problem somewhere else?...

Yes. That is very common for stock Fender and Mesa Boogie amps as they are shipped from the factory. Maybe other brands too.

[Enzo]

Monster gain amps 5150/6505 Peavey heads are typically running 11ma-15ma on the power tubes.

By .017ma, do you mean 17ma or .017A?

[Mick Bailey]

And Blackstar runs their amps cool, too.

[pdf64]

...If you want to warm it up, clip out the NFB resistor.

Yes, I too tend to prefer to reduce / eliminate the global NFB, as that will give a smoother transition into overdrive.
But it will also tend to make the amp brighter, as the power amp gain will then tend to track the speaker impedance (which due to voice coil inductance, above the low mids at which nominal impedance is assessed, rises with frequency).
So improvement may tend to be counter-acted by that.

My guess is that cold bias/crossover distortion isn't the problem here, the issue may be due to the particular speaker type used in the amp, combined with the nature of an array of 4 speakers acting in the same plane; the on-axis response will tend to a create an evil super beam of ice pick

If so, ways forward may be a different speaker type / mitchell donuts http://www.thegearpage.net/board/ind...tivity.470956/

[Justin Thomas]

My experience with removing negative feedback in amps that were designed with it was that the noise went up a lot, and it broke up on @ instead of 5... not really a problem if you play crazy-ass loud guitar solos all the time, but not much good for aught else. That said, none of the four amps I built ended up getting it, and the one I tried it in was less than spectacular.

I do think warming the bias may help add some lows and mids girth; it may not just he an excess of highs, but an absence of lows. If you turn the bass over 4 at a reasonable volume, is it too much? I would expect any vintage unmodified Fender to be pumping out way toomuch lows at any usable volume with the bass pot turned to about half up...

I guess, let's explore every FREE possibility before springing for new part$$$, right?

Justin

[pdf64]

The late 70's Fenders tended to use a 30% taper bass pot, rather than the 10% taper that tended to be used in the BF and earlier SF era.
Combined with the 10nF-330k (bootstrapped) HFP at the LTP input (500pF or 1nF -1M in the BF era), that tends to result in bass control settings above ~3 being excessive / fart inducing at higher volume settings.