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Discussion in 'Amp Central Station' started by Krackle, Mar 20, 2021.
I have 4 silverface amps...never considered blackfacing them..
What is it that you gain from it?
I would not call that circuit a transitional thing. Ommv. These circuits that were introduced in the summer of 1968 were actually the first CBS circuits, and all of the 6L6 amps got this mixed Biasing treatment. I have never left those cathode resistors in one of them. Fender did away with this mixed Biasing in the fall of 1969......as I understand it the amps were not well received. This amp still has the bias balancing circuit in it, as well.
A full tech evaluation is advisable. Without knowing the voltages and particularly the biasing, it is difficult to make much of an assessment of the breakup characteristics. I personally like those 10” Oxfords. If I wanted to replace them, I would have to consider Emi ALK1028s....Kapton formers and 35 watts of capability. The Allesandro SC10s are built on the same platform, but the paper formers limit their capability...20 watts??? I would want more than 40 watts of speaker capability in that amp...personal thing.
I love a good VIbrolux Reverb. Congrats, Krackle.
And..,.as always.....the AC needs to be rewired in best and safest manner.
Everyone has pretty much covered the waterfront on your amp. I'd just add that were it mine (which it ain't, so take me with a grain of salt) I'd convert it to BF spec, which is a piece 'o cake to do, and at the same time I'd replace those electrolytic caps with F&Ts or Spragues. The "brown blob" caps in those developed a bad rap due to early failures, but if they've made it this long they may be just fine. The orange drops in the phase inverter are replacements, and it seems odd to me that the tech would replace the A/C shunt (.1uv 200v) and just one of the grid caps. But hey, the tech was in charge. There's something of a school of though developing that recommends replacing the original .1uf 400v PI grid capacitors with modern 600v caps, thinking being that those caps get slammed with more than 400v every time the amp is fired up and if they fail the power tubes and OT could pay the price. I've never seen that happen, but.... Were it mine I'd probably replace all three of those PI caps with a better quality brand polyester cap (Mallory 150, SDE PS6 or 418P, perhaps the Sozo blue caps).
Your headroom concern is frankly endemic to the Vibrolux Reverb. VRs run a rather low plate voltage for a Fender 6l6GC amp. You minght get some more headroom by changing the rectifier tube to a GZ34, which is what was OEM in the blackface VRs. One of the reasons CBS Fender adopted the ill fated 868 circuit you have was to clean up the amp, i.e., increase headroom. They changed the bias adjustment from bias adjust to bias balance (that's what all those resistors are by the PI circuit), added the .02 grid capacitors and big cathode resistors on the power tubes, and changed some of the phase inverter circuit resistor values. The circuit isn't as wretched as legend tells us, but when I was in retail sales in those days I had guys who'd had the blackface versions of the same amplifiers complain that the amps sounded dead and sterile. Your ears are the best judge for you.
all interesting and thanks Wally and Sok..
I wouldn't know where to begin..I could handle the basic electrolytic recap including filter caps..anything else? Too much for me..
im no electronics concept guy but, with good directions and the right parts, im a good solderer.
I usually seek out and triple check info on what to desolder and replace..
All of this sounds expensive..I use a very respected local tech but i wouldn't know where to begin with directions on how to proceed..i immediately thought of printing out the last two posts from you guys (Wally and Sok) and handing them to him and then having a discussion..
If I told him in simple terms 'let's blackface this circuit and do a recap' would that get me where i need to be?
Im doing this groundwork here because the current Covid situation tends to stunt the curbside drop off discussions many folks are insisting on
I would never describe this amp as sounding sterile..it's got a very sweet bloomy quality that i was immediately addicted to--but that's probably due to age and degradation
Im sure it would change drastically with major change..not a prob really, i've gone through this before..'thought' i loved everything about my Deluxe until the tech i use recapped and tidied up a few abberations in it..it sounded ten times better after that..so
This guy always asks me.."do you like the sound of the amp right now?"..lol, sometimes it's a mistake to say 'yes'..
Anyhow..what's the best way to offer direction in a simple bundle?? ..like mentioned..Blackface it and recap it"..that will get me this amp sounding great and functioning at safe spec?
Not a lot on normal Fender amps, but you do get an adjustable bias by going from SF to BF.
But with the odd '68 circuit with the hybrid bias etc (short lived) the amp sounds dirtier to me and less like most BF/SF Fenders.
Now see..i do notice this and in it's current condition..i like the sound of it..but that's just an alignment of random factors..age, oddball changes and other whatevers..
I think it could be a good thing for lower volume playing etc in some ways. It does have a certain character to it... Kind of a hint of Cathode bias sound... Albeit I haven't played one a lot, not gigged one.
Some of the lower headroom is probably about the circuit. Some of it is definitely the original Oxford speakers. I myself like speakers like those, but, as they say, your mileage may vary. I just finished building a BF Vibrolux clone, and mine has some old CTS ceramic speakers in it. They break up early but also have a nice chime. The SF Vibroluxes are going to run at higher voltages generally, which, as others have noted, will change the sonics a bit from a BF one. The differences between the BF AA964 and SF AB568 (or AA270) are minimal, components-wise. See the bottom of this web page under "Blackfacing," where it describes the differences.
Even if one changes these components to the BF values, one will face the issue of the higher voltages offered by the SF power transformer, as opposed to the BF one. Those voltage differences definitely affect the sound and feel, though both specimens can be great. Depending on what you're looking for, you may well prefer the SF Vibrolux or the BF Vibrolux. Let your ears be the judge, if you're able to compare at some point. Beyond the sonics of "blackfacing," there is a sort of mystique that goes along with an attempt by some to "return CBS-era Fender amps to their ideal state" by matching the pre-CBS Fender circuit.
If it were mine, it would get all new filter capacitors, and I'd check the cathode bypass capacitors and the bias capacitor. But there's no need to go changing anything unless it's out of spec or just poorly done.
I think it would communicate clearly simply to tell your tech to "blackface" the amp and re-do anything that looked janky. Then you can see if you like the results better or not. It's not a huge deal to put everything back, if you don't.
Voltages....I know that the SF schematics show higher B+ than does the BFVR schematic....465VDc in the SF versus 410vdc in the BF. However, my real life experience is different. The near mint 1976 VR I owned was running 411VDC after the recapping and service. It was biased at about 62% of max plate dissipation when that voltage was measured. It was a glorious sounding amp, and the signal did not fall apart at three or four on the dial...with the OEM Oxfords in it. I use 5s for the volume and tone settings to judge BF/SF amps....I know how they should act there. With a proper pick attack, any BF/SF will be clean and lively with those settings. One can use a lighter pick attack for pianissimo....and a harder pick track will bring on strong signal....forte. It would take beating——improper and excessive pick attack imho—-on the strings to cause any flatulence on the low end. I have watched people play such an amp at 2.5 on the volume without being able to get a truly clean signal...and I choose not to listen to those people ‘play’ guitar.
A player who has good pick attack and guitar output control can set these amps at 5s and NOT have excessive volume....just a lively, responsive amp....ime.
Trying to understand..
RE blackfacing this circuit..
What blackface circuit version do you end up with? ( of course assuming there were revisions through the Bface era..I recall ‘yes’)
The blackpanel Vibrolux Reverb is AA964. I think that's the only one.
They'll be fine. That amp might put out 40 watts cranked with a big boost in front. Emi's rating is pretty conservative.
This is a clip of Chris Scruggs that I poached from another thread on here. Don’t know if the amp has been altered in any way but he says it’s a ‘73 and is playing a telecaster through it if you want a ballpark sound to reference
The VR is the only 6L6 Fender reverb amp that did not get the major revisi9ns that came later on....MV and UL OT. This amp does have that mixed biasing scheme that did not last long in the Fender lineup anyway. I remove that and also the 2000pfd snubbers. ommv.
The Biasing circuit should either be modded back to a true bias voltage adjustment circuit, or one should install a trim pot to adjust that voltage while leaving the balancing act in place. The plate load resistances on the phase inverter are different from the ‘64 circuit as is the first dropping resistor in the power rail. Change one of those two aspects and you might want to change the other.
Regarding that 1973 VR in that video, the stock circuit is the same as the 1968 circuit AFTER one removes those cathode resistors and the snubber caps from the 1968 circuit. That would be the AA270 schematic.After that, the only change that came to the VR was the pull boost mod.
Interesting video there with Scruggs. Thanks.
Guys...thanks so much, y'all are great and i do appreciate your time..
Have to say that Chris Scruggs 73 Vibro in that video took the Mustangs to task..thing sounds incredible...of course so different..but what a sweet organic feel and tone when he first plays
Im thinking of doing some work myself on this amp before i drop it at the tech..He's out of town for a couple of weeks so i can proceed slowly..
But I'm entering the part i hate..which is where i realize that i don't know what I'm doing except when it comes to the mechanics of it..snipping and soldering..I get stressed on identifying and ordering proper parts and then making sure im putting them in the right spot
So, i was thinking that I'd like to replace the filter caps with some known and good quality caps..this is the kind of task that's perfect for me..things in a row, easy to follow.
Is there a resource out there that would give me the accurate values for this amp? Have Filter Caps ever changed in value through the various Vibrolux revisions?
I'm Also thinking about doing the dumping of the mixed bias setup in this amp..but i'd need some guidance and visuals..
Here are both the AA964 and the AB568 schematics. Both schematics call for 16uF filter caps, rated for 450V. If it were mine, I'd at least up the two reservoir caps to 22uF, and I'd use F&T caps in all positions. The 22uF F&T's are rated for 500V, and the 16uF are rated at 475V. The 22uF's are actually cheaper than the 16uF's, and they'd work great in all positions -- that is, 5x22uF F&T caps. That's what many people do, anyway. A bit more filtering and higher voltage rating.
Wally is right about that bias circuit and getting some voltage measurements. You have a 52 year old carbon comp resistor dictating your bias voltage. As he said, you’re going to want to either have that converted to a Blackface style bias circuit, or replace that resistor that goes from the back of the pot to the middle (right?) pot lug with a resistor/trim pot in series to adjust overall bias voltage. If you want to to that yourself, folks here can walk you through it, otherwise it’s tech time.
Headroom and bias voltage are related, and I can see that part of the circuit being more of an issue than those Oxfords, which are pretty good IIRC.
professor, thank you..
will upping those values stiffen the tone?
I’d like to stay in the realm of, i guess, the ‘historic’ sound on this amp..