Blackfacing a Silverface reverb amp

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1,071
There are a million threads on this but I have one thing to say. The silverface reverb is much more lush and full than the blackface reverb. Lately I have been keeping the reverb standard silverface. 560pf cap from cathode to blue transformer lead. 470ohm unbypassed cathode resistor. 2000pf cap from 220k resistor to ground. That seems to be the magic combination. Just my 2pesos worth.
 

SoK66

Member
Messages
243
That circuit is very hard on reverb transformers, which will tend to burn and short internally, causing a raspy static/distortion on lower notes. Frankly when I see one I change it back to the BF spec for reliability.
 
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pdf64

Member
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9,299
470ohm unbypassed cathode resistor.

With the anode voltage up around 450V, that arrangement also puts its 12AT7 under stress, breaching the limiting values of both anode dissipation and voltage .

I came up with this to at least make the dissipation a little friendlier for it, yet retain much of the tonal benefit mentioned. With regard to @SoK66 's point, it should keep the transformer happier too :)

Reverb-Driver.jpg
 

gldtp99

Member
Messages
4,613
I think about the only thing a typical SF Fender amp really "needs" (and that's debatable)

Is a conversion of the Bias circuit to Fully Adjustable (as in most BF amps) instead of the usual SF Bias Balance (which only adjusts one tube ---- in a 2x6L6GC amp)

Other than that I think most SF amps sound great just the way they are

I spent lots of bench time and effort "improving" a Mid '70's SF Twin Reverb w/Pull Boost (eliminated) amp and ended up with a great sounding Twin Reverb that had a combination of BF and Brownface (PI) specs and a PPIMV (in place of the orig Pull Boost MV)

I didn't know when to stop on that BF'ing project ----- looking back I could have left much of the SF circuit alone and just eliminated the ridiculous "Pull Boost/MV" feature and ended up pretty much in the same sonic general area as I eventually did

I made many, many changes and it's difficult now to tell if all that time/effort/money spent was worth it

I ended up with a great sounding Twin Reverb but I don't know if it was all that much better than a properly serviced SF Twin Reverb
 
Messages
6,965
Thanks you guys. I probably shouldn't have been playing one in my SFDR last night then. Woops!
There are a million threads on this but I have one thing to say. The silverface reverb is much more lush and full than the blackface reverb. Lately I have been keeping the reverb standard silverface. 560pf cap from cathode to blue transformer lead. 470ohm unbypassed cathode resistor. 2000pf cap from 220k resistor to ground. That seems to be the magic combination. Just my 2pesos worth.

So, by ‘standard Silverface’ you are referring to the changes made to the reverb circuit with the master volume amps…and the volume boost DR in 1978…. rather than the first 6 years of the SF era in which things in the reverb were pretty much the same as the BF era??? Thanks for bringing the subject up, though.
 

Kabukiman

Member
Messages
362
I think about the only thing a typical SF Fender amp really "needs" (and that's debatable)

Is a conversion of the Bias circuit to Fully Adjustable (as in most BF amps) instead of the usual SF Bias Balance (which only adjusts one tube ---- in a 2x6L6GC amp)

Other than that I think most SF amps sound great just the way they are

I spent lots of bench time and effort "improving" a Mid '70's SF Twin Reverb w/Pull Boost (eliminated) amp and ended up with a great sounding Twin Reverb that had a combination of BF and Brownface (PI) specs and a PPIMV (in place of the orig Pull Boost MV)

I didn't know when to stop on that BF'ing project ----- looking back I could have left much of the SF circuit alone and just eliminated the ridiculous "Pull Boost/MV" feature and ended up pretty much in the same sonic general area as I eventually did

I made many, many changes and it's difficult now to tell if all that time/effort/money spent was worth it

I ended up with a great sounding Twin Reverb but I don't know if it was all that much better than a properly serviced SF Twin Reverb

I would say you are spot on, the proper bias adjust is probably worthwhile* and swapping the master volume to a post phase is something I have done with great success, not really needing any further modifications.

*On some of them like the bassman 100 they have a really low primary impedance, from memory, which is why I think they were biased so cold in the factory form. The one I worked on was <50% at idle but didn't have the typical cold bias farty sound so I guess you need to also respect the original design goals too
 
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6,965
I just brought a 1972 Vibrolux Reverb back to life yesterday. The cathode bypass cap on The reverb driver cathode bypass cap was severely ‘bloated’. Side note: that 12AT7 coincidentally had lost vacuum. Anyway, I measured the 2,2k cathode resistor and found it was showing 3.5K. So, I replaced that in the process. So, this SFVR from 1972 still had the BF reverb circuit.
I had already replaced the dropping resistors in the power rail the 2.2k there had drifted upwards. Since that is one change from the
BFVR circuit, I went back with the 10K that the BF carried there. I used two new 10K CCs for those two dropping resistors.
 

Ampegged

Silver Supporting Member
Messages
1,663
I just brought a 1972 Vibrolux Reverb back to life yesterday. The cathode bypass cap on The reverb driver cathode bypass cap was severely ‘bloated’. Side note: that 12AT7 coincidentally had lost vacuum. Anyway, I measured the 2,2k cathode resistor and found it was showing 3.5K. So, I replaced that in the process. So, this SFVR from 1972 still had the BF reverb circuit.
I had already replaced the dropping resistors in the power rail the 2.2k there had drifted upwards. Since that is one change from the
BFVR circuit, I went back with the 10K that the BF carried there. I used two new 10K CCs for those two dropping resistors.

Did you change change the bias balance circuit at all or leave as-is? I rehabed' a 70' Vibrolux Reverb a couple weeks ago & just added a mini bias control to the existing bias balance pot. Worked out great.
 
Messages
6,965
Did you change change the bias balance circuit at all or leave as-is? I rehabed' a 70' Vibrolux Reverb a couple weeks ago & just added a mini bias control to the existing bias balance pot. Worked out great.

I left it stock. This one was bought by a customer to sell. He doesn’t even play guitar these days. I checked the bias, and it is fine as is.
Had it been my amp or a customer wanted it done, I would have added the trim pot to adjust the voltage. He is getting a bargain on the labor as it is. If the bias had been odd, I would have changed it. All voltages are dead on correct to the 1964 BFVR schematic..including the bias. Note: I do not hold that schematic bias voltage as the defining aspect of biasing, but this one is operating fine as it is.
 

zenas

Member
Messages
8,871
I'm kinda over the whole "blackface" thing on silverface amps. Used to be that was the first thing you did and I really don't know if it did much.
Now I just change out the electrolytic caps, service them and add a bias voltage trim pot to the stock balance set up. That added pot only takes a minute or two to do and no changes to the the phase inverter.
All of that needs to be done anyway so you're not out anything if you decide to do further changes but so far I've just let them be.
The pull boost master I just push in and turn to 10 and forget it's there.
Early on I tried different MVs with the things and soon figured out I don't like MVs of any sort.
 

Laurent Brondel

Silver Supporting Member
Messages
4,492
I stopped fooling around with SF amps when I bought BF amps and sold the SF’s…

Don’t get me wrong, most SF‘s are great amps, especially the small ones like PR’s, DR’s and VR’s as the circuit didn’t change much besides the change of rectifier type.

But I got to compare the same multiple BF and SF amps many times, and they just don’t sound the same. Better ot worse is purely subjective, and besides modifying the balance to adjustable bias for the bigger amps for practicality, they’re probably best as is.

Transformers? Components? It’s really hard to pinpoint to one thing only.

The cabs are different, baffle attachment and materials. Dadoed particle board baffle vs. fastened, solid pine vs. ply pine, finger joints vs. rabbeted joints etc.

The BF’s always sound clearer to me, with a little more high end and tighter low end.
The best values are probably the drip-edge era which are BF amps in SF clothing.
 

Tone_Terrific

Silver Supporting Member
Messages
38,615
The best mods solve a problem.
I ended up with a great sounding Twin Reverb but I don't know if it was all that much better than a properly serviced SF Twin Reverb
Modding as a form of looking for a problem to solve is also popular.;)
 

zenas

Member
Messages
8,871
The best values are probably the drip-edge era which are BF amps in SF clothing.

They can be. However drip edges that actually have blackface guts are rare, compared to drip edges with alert over blackface tube charts. That gets a lot of people if they don't look inside. We've all seen the guy that paid blackface money for a drip edge Super Reverb. Look inside and find the dog turd caps and that goofy fixed/cathode bias set up.
The prices on drip edges got to the point a few years ago where I just don't even bother with them.
Honestly I'd rather skip ahead a few years in the silverface era to where the cabs "got bad". The early ones often run lower plate voltages and never quite sound right to me and the prices are so close to a real BF there's no point in messing with them.
 

blackba

Silver Supporting Member
Messages
12,404
I am working on some 1970 Twin reverbs, 3 to be exact. One thing I noticed comparing the AA270 schematic with the AB763 is that the dropping resistors in the power supply off the 20uF caps are different, thus leading to Lower voltages on the AA270.

This surprised me as I had read that the silverfaces they raised voltages, but on these pretty early ones, not the case.

So I was considering changing the dropping resistors from the 2.2k to 1k, and the 10k to 4.7k like the AB763 BF circuit. Any thoughts on this change?

 
Messages
6,965
I am working on some 1970 Twin reverbs, 3 to be exact. One thing I noticed comparing the AA270 schematic with the AB763 is that the dropping resistors in the power supply off the 20uF caps are different, thus leading to Lower voltages on the AA270.

This surprised me as I had read that the silverfaces they raised voltages, but on these pretty early ones, not the case.

So I was considering changing the dropping resistors from the 2.2k to 1k, and the 10k to 4.7k like the AB763 BF circuit. Any thoughts on this change?


The plate voltage change rebiases those preamp tubes, and ime you will hear a difference. Do the experiment to see if you prefer one situation over the other.
 
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blackba

Silver Supporting Member
Messages
12,404
So I wanted to report back on the resistor change in the dog house I mentioned a few posts back. Tried the BF values and really liked the change. Amp broke up a bit earlier and was warmer sounding. Did test this through a power station.
 
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jay42

Member
Messages
7,473
Amp broke up a bit earlier and was warmer sounding. Did test this through a power station.
The changes put higher voltages on the preamp circuits, resulting in slightly higher gain, but mostly a higher clean swing into the power amp. Earlier means at a lower Volume pot setting, right?
 




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