Silver face vs black face

[m-fine]

I'll play around with that on my next SF project. My TR has a Vari-Watt now so if I want power tube distortion I just dial back the B+ to the 6L6's and I can drive them hard without needing to put on hearing protection. Lift the ground from the mid pot to overdrive the preamp too and it sounds NOTHING like a Fender anymore, but that is beyond the scope of this thread.

[Euthymia]

The way I frame it is that the SF circuits were themselves modifications of the earlier BF circuits. What I do when I mod SF amps for clients is undo some of the circuit mods that the CBS engineering staff introduced.

"Matters of taste" and all, but I will say that I have yet for a client to take me up on my BF mod guarantee, which is that I will reverse the mods and refund that part of their repair fee if they are not happy with the sound afterward. Usually it's more like their jaw hits the floor when they play their first chord through the BF'd amp.

The standard package of BF mods I do is: global bias, 220K bias feed resistors, 100K/82K plate resistors & 1M grid leak resistors on the PI.

[JoeM]

... Usually it's more like their jaw hits the floor when they play their first chord through the BF'd amp..

People will hear what they expect to hear. None of the changes you make would cause a great difference. Speakers would be an area where definite improvements are possible.

[m-fine]

I disagree, just getting the power tubes biased right for the first time can make a very noticeable difference. It just dpends on how bad the thing was to start.

[Euthymia]

People will hear what they expect to hear. None of the changes you make would cause a great difference.

Not so. Both theory (in a nutshell, we're both changing the operating point of the PI tube and throwing away less signal) and empirical observation tell me that they do make an audible difference (if they didn't, why did the CBS engineers change them?). I've actually gone through this procedure several times with friends observing, as I make each individual mod. I install new power tubes, then make the global bias mod, then we play the amp, then proceed to the next mod. Each one definitely affects the sound and response of the amp. You might not think the differences "great," but they are audible, and their results may even be seen on the voltmeter and oscilloscope.

When I first started fixing amps full time, the only BF mods I routinely did were the 220K resistors and global bias, and then I decided to try the rest of the popular BF mods. I did them one at a time, and some of them actually did make the amp sound different (more pleasing to my ear). Some of them did not, and I do not do those.

I'm not in the business of selling snake oil; for instance when I do filter cap jobs, I use whatever off-the-shelf generic electrolytics I get from my distributor. I tell people that if they want to source Sprague or whatever, I can install them, but I don't bother to keep them in stock. Same with most other parts (not tubes, of course). When people ask me what kind of caps I use when I replace coupling caps, I just say "film" and leave it at that. If people want to bring me fancy components to install, I am happy to do so, but can't be bothered to keep them in stock for repairs.

The "BF" mods I make are audible, and the difference they make is not what I consider subtle.

[JoeM]

The "BF" mods I make are audible, and the difference they make is not what I consider subtle.

Sorry, but I just cant buy that the mods are an 'improvement'. But if your customers do that's great.

[m-fine]

Sorry, but I just cant buy that. But if your customers do that's great.

Joe, have you never adjusted the bias on an amp or turned a tone knob? The changes are easily audible and the reason they are is well know, well understood science. You can easily model how these things impact frequency response before you heat the iron up. This is not snake oil $100 coupling caps, we are talking about different resistor and cap values that significantly change voltages, bias points gain and frequency response in highly preditable and measurable ways.

To suggest that this stuff is not audible is quite a stretch.

[JoeM]

Joe, have you never adjusted the bias on an amp or turned a tone knob? The changes are easily audible and the reason they are is well know, well understood science. You can easily model how these things impact frequency response before you heat the iron up. This is not snake oil $100 coupling caps, we are talking about different resistor and cap values that significantly change voltages, bias points gain and frequency response in highly preditable and measurable ways.

To suggest that this stuff is not audible is quite a stretch.

I'm not suggesting the changes arent audible. But I've played through many, many SF and BF amps. If you had 2 Twin Reverbs for example, both stock, one BF and one SF, but with new filter caps, good sounding speakers (the most important IMHO), and good tubes (properly biased), both amps are going to sound good. Just changing some resistors isnt going to change your dull, boring sounding SF amp into some kind of fire breathing dragon. I've never felt the changes to the SF amps were detrimental to the sound as all the BF hype indicates. For example, changing the 220k bias feeds from the 100k isnt going in the right direction if you're trying to reduce blocking distortion - it's going to make it worse.

[m-fine]

If you take two twin reverbs, use the same speakers (hook them both to the same cab) and use the same exact tubes, you can have a pretty noticeable difference. Assuming you use a matched set of power tubes, baring luck, the simple fact that you CAN "properly" bias with one and not the other can be a pretty noticeable difference on it's own. All of the other changes are additive so I would not critisize Erik or his customers for hearing the difference and feeling it was more than subtle.

We can argue whether some of the changes make it sound better or worse, but they definitely sound different.

[JoeM]

Older is always better to some, and just keeps the whole vintage mojo BS perpetuated ad nauseum.

[Rhodesplyr]

One aspect of the BF design I don't see discussed that much is the fact that, according to the original 6L6GC datasheets, the maximum control grid resistance (i.e., the grid/bias feed resistors) for a 6L6GC is 100k when using fixed bias. With 220k resistors, you're over twice the published limit. I may be prejudiced because the first Fender amp I did major work on was a SF Super Reverb that had had some BF mods, and it was having problems with the output tubes going into thermal runaway. Before someone starts saying, "Well, did you check...." Yes. I checked everything you could possibly mention, and the output tubes were Philips 7581As.

Or, let me put it this way: has anyone checked, when going from 100k to 220k, for an increase in grid emission, which you could measure by monitoring the voltage drop across the 1.5k grid stop resistors?

I'm perfectly willing to accept that 6L6GCs can tolerate 220k grid-leak resistors if that's the general consensus. I put some 1960s GE 6L6GCs in the amp mentioned above, and, according to its owner, it has not red-plated in the year and half since I worked on it (with 220k resistors installed by a previous tech).

[JoeM]

Definitely an area where the SF amps got it right. I think this was posted above, but here's something from Aiken amps on blocking distortion:

Fender amps are particularly susceptible to this because of the large values of coupling capacitors on the grids of the power tubes (0.1uF). Blackfacing your Super Reverb can actually make the problem worse, because you change the grid bias feed resistors from 100K to 220K, which increases the time constant of the AC coupling to the output tube grids. You will note that most Marshalls use 0.022uF coupling capacitors and 100K resistors, which gives a much faster time constant. In addition, the preamp stages have a much more rolled off low frequency response. This is why they sound tighter when played wide open.

I've used 150K with .047, which helps too.