Fender Silverface Super Six Recap Job

[Wittgenstein]

Finally got around to putting new caps in a buddies super six. I swapped out all the electrolytics and added all new tubes, but I have yet to mess with the bias. I was reading that it is not a true bias pot on this amp and that is a balance pot? Any suggestions on how to dial in the bias of the new 4 x matched pair of Fender Groove Tubes 6L6's. Is it worth adding 1ohm resistors to ground off the cathode?

I played it a bit tonight through a 2 x 12 cab that i have with 50W speakers wired to 4 ohms. It sounds really good and the tremolo and reverb circuit sound great. The bass pot on the first channel is a bit scratchy, but some pot cleaner should take care of that.


High Res Flickr Photos

[Mick Bailey]

You can often be lucky with the biasing on 'balance pot' Fenders and no further work is required. I always check to see what they're drawing just to be sure. Set the hum balance pot for minimum hum and ensure that the tubes aren't red-plating. Then check the dissipation. Some modern tubes can pull a lot more current than the Silverface bias circuits were designed for.

I don't install resistors - measure the DC resistance of each half of the primary (with the amp safely off - tubes can be in place. Each side will have a different resistance - that's normal) then switch on the amp and measure the DC voltage drop across each half to arrive at the total current draw for each side. Multiply this by the plate voltage and you'll get the idle dissipation for each side. If you divide it by two you get the dissipation for each tube on that side.

This method only holds good for a new set of matched tubes, as there's the assumption that each one of the pair of tubes on either side will be drawing the same current. It wouldn't show if one tube was drawing too much and the other too little - something that can happen if the tubes are mismatched or have drifted with age.

[soundguruman]

I will recommend F&T filter capacitors.
Noticeably better sound quality, to my ears.

[Chuck H]

Thank you helpy helperton! He already replaced the filter caps. Are you suggesting he do it again?

[Chuck H]

The "balance" pot in the Super Six isn't a "humdinger" circuit on the filament wind 0V reference. Looking at the schematic I can't quite tell WTF it is. Some sort of bias/drive adjustment represented by a pot like schematic symbol I've never seen before. Maybe if I looked into the actual amp that symbol would make more sense. Maybe not. If it were "my" amp I'd take one look at the schem and get straight to changing it to a normal bias adjust circuit. I can't say as this circuit ever balanced mismatched tubes well anyway (it seems to alter drive on one side and bias on the other???) and getting decently similar tubes isn't a problem when you buy them that way.

BTW... There are either a lot more of these very cool and supposedly rare amps around than I'm led to believe or those that come across them are making it a point to post about it!?! A Fender combo amp with six tens and four power tubes!!! How cool IS that?

[potatofarmer]

The "balance" pot in the Super Six isn't a "humdinger" circuit on the filament wind 0V reference. Looking at the schematic I can't quite tell WTF it is. Some sort of bias/drive adjustment represented by a pot like schematic symbol I've never seen before. Maybe if I looked into the actual amp that symbol would make more sense. Maybe not. If it were "my" amp I'd take one look at the schem and get straight to changing it to a normal bias adjust circuit. I can't say as this circuit ever balanced mismatched tubes well anyway (it seems to alter drive on one side and bias on the other???) and getting decently similar tubes isn't a problem when you buy them that way.

It's a bias balance circuit using a tapped pot. Can't remember who said it in what thread, but dial the master to zero and adjust the 'output tube matching' pot for minimum hum and you've balanced the power tube current. Still a good idea to check actual dissipation, but a handy thing to have nonetheless.

Some people convert the tapped pot just to a 'bias adjust', other people replace the 15k to ground (or the 3.3k after the bias diode) with a fixed resistor in series with a trim pot to get a balance pot and an adjustment pot.

[Chuck H]

So a typical bias adjustment (after balancing the sides via hum?!?) is made via adjustment to the 15k resistor value indicated in the schematic as being on the balance pot tap. Nothing to it! I can see it pretty plain actually, but the explanation seems confounded. Provided this info is clear enough to understand I might at least replace the 15k resistor with a 10k and add a 22k trim pot in series as a bias adjustment pot.

[g1]

The "balance" pot in the Super Six isn't a "humdinger" circuit on the filament wind 0V reference. Looking at the schematic I can't quite tell WTF it is. Some sort of bias/drive adjustment represented by a pot like schematic symbol I've never seen before.

It's a tapped pot. The drawing is funny, wiper goes to a 15K which is grounded (doesn't look like it!). By running the bias to the carbon track via the tap, you still have bias if the wiper contact fails. So it's just an odd bias balance pot. But it helps a lot with hum in the amps it is used in. Adding an extra bias level pot is preferable to converting the bias balance into bias level.
The pot and the hum were also discussed here: http://music-electronics-forum.com/t35216/

[Chuck H]

See my post made four minutes prior to yours... SIMULPOST BIACHE!!!

[Tom Phillips]

It's a bias balance circuit using a tapped pot...other people replace the 15k to ground ... with a fixed resistor in series with a trim pot to get a balance pot and an adjustment pot.

IMO that's the most versatile bang for the buck and it won't hurt the "tone" one bit. If you don't want to add a pot you can just test select the resistor value to give the correct bias amplitude after the bias balance pot is set. Added 1 Ohm cathode resistors to provide a means to monitor the actual current work out best for most people.

[Chuck H]

I'm fine with the bias balance if anyone thinks there's any real advantage to it WRT guitar amps. Tone can be partly confidence in the gear. I even pointed out a way to keep the circuit intact and add an over all bias adjustment. But...

A slight mismatch is the most any player will need to deal with in these times. The balance pot isn't really doing anything useful. Since a slight mismatch has even been demonstrated to be a good thing isn't the reduction of a small amount of hum a small condolence to the loss of an asymmetry that may actually be a tonal improvement? Just playing devils advocate now so don't start flaming. It just doesn't seem like an appropriate circuit for any amp used today. I've had plenty of trouble with preamp tubes that hum right out of the box, but I've never had a problem with unacceptable hum levels due to mismatched power tubes. In fact, when power tubes are perfectly matched in guitar amps I've noticed:

Objectionable noise due to crossover distortion in bias vary tremolo's.

"Swirl" as it is known can become a fizzy, buzzy sound instead of a pleasant phase shift.

Second order harmonics seem to dampen and fade.

The lack of asymmetry in the clipped wave form just sounds and feels less organic and warm. More like an SS circuit.

I'm not saying it's wrong for everyone. And a gross mismatch is bad, and even potentially dangerous for amplifier performance. Today power tubes are bought in sets and quads that are "close enough" for rock n' roll as it were. And I think that's better than perfectly matched. In this regard a balance circuit could be doing more harm than good! Well... Unless you plan to use it as a PA or something. The fact that you need to balance it to be sure it's not imbalanced ensures that an amp would be at least partly robbed of the attributes I listed above. It's really just an inappropriate idea implemented in the wrong venue by frustrated engineers.

I relinquish the floor.

[potatofarmer]

"Swirl" as it is known can become a fizzy, buzzy sound instead of a pleasant phase shift.

Second order harmonics seem to dampen and fade.

The lack of asymmetry in the clipped wave form just sounds and feels less organic and warm. More like an SS circuit.

To play double devil's advocate (?) what you're describing is a "dial-a-mojo" pot.

In the typical amp, they're useful when you don't have a matched quad - say you have one tube that goes bad, you can just replace that one tube, adjust the pot for no hum, and as long as you don't get any redplating you can get through the gig.

Though with a quad of tubes the whole "...but now you don't have to have them matched!" argument doesn't work quite as well.

In the "ultralinear" amps that don't have any ripple filtering on the screens, the imbalance in plate current is much more heinous. On the Sunn Model T they used a CLC filter before the connection to the OT CT and a regular bias adjust. I imagine a tapped pot is significantly cheaper than a choke rated to filter plate + screen current.

I have two amps with this bias circuit, the Twin I'm talking about in another thread and a PA 100. The PA 100 I converted the tapped pot to just a bias adjust.

[Wittgenstein]

It has a new matched quad set of Groove Tube 6L6's. Without having to modify the circuit at all, what would be the best way to bias these tubes? I guess I am still not clear on the differences between the bias balance pot like this has? versus a typical bias pot. Having all cathode fixed bias amps, I have never had to adjust the bias before, especially not on 4 tubes. I am confident in my ability, but any suggestions would be appreciated. Does anyone have a link to a clear walkthrough on the process?

[Chuck H]

Follow the balance pot adjustment procedure outlined above. Then check the bias by whatever means you like. If it requires adjustment find the 15k resistor on the balance pots tap. Adjust the value of that resistor (as you would normally adjust a bias pot) to adjust the overall bias.

[potatofarmer]

I use the same method as Mick Bailey mentioned in post #2, but you can also install 1 ohm resistors between cathode and ground. Without modifying the circuit at all, the bias isn't adjustable per se.

More than you could want to know about biasing, courtesey of Randall Aiken: https://web.archive.org/web/20130301...m/Biasing.html