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Old February 15th, 2013, 10:02 AM   #1 (permalink)
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Does removing master volume on SF Pro Reverb increase clean headroom?

My amp (silverface 40W Pro Reverb, no mid control, with push/pull master volume) is going in to have its lack of clean headroom looked at.

I have faith in the engineer's ability to find faults as he already made improvements to the amp by replacing HT capacitors to bring the power up from 20W to 30W, at which point we agreed I would try it for a while as it might have been fine for my needs, but in fact I do still want more clean power/headroom. So it's going back in tomorrow and he will do some more testing.

However in addition, and as I get all my overdrive from a pedal and therefore have no need for gain from the amp, is there any benefit in having the master volume removed altogether? As it is, I usually have the 'master volume' quite high and as little 'volume' as possible to maximise the clean sound, but it's still breaking up before I want it to, especially on chords.

I don't care about anything other than squeezing every bit of clean headroom out of the amp but don't want to have mods done just for the sake of it.

Thanks in advance for any shared experience on this.

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Old February 15th, 2013, 10:56 AM   #2 (permalink)
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Why don't you set ur master to 10??

How about more efficient speakers? No mod whatsoever, just louder...

sry, no comments about removing the master...
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Old February 15th, 2013, 11:23 AM   #3 (permalink)
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I doubt you'd be able to tell any difference between having it set on 10, or having it removed.
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Old February 15th, 2013, 11:31 AM   #4 (permalink)
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I doubt you'd be able to tell any difference between having it set on 10, or having it removed.
That's what i thought...
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Old February 15th, 2013, 11:43 AM   #5 (permalink)
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you may - your tech will know if you have the available voltages- be able to change tubes to get more clean headroom - kt66 or 6l6 for output tubes - and/ or change the preamp tube(s) to something that has less gain -
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Old February 15th, 2013, 12:21 PM   #6 (permalink)
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Thanks for the input chaps.

I doubt tube changes will help - this amp used to be much louder/cleaner with stock tubes (and it now has new ones) and speakers so changing them are a last resort.
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Old February 15th, 2013, 12:57 PM   #7 (permalink)
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Gigsby wrote: "I have faith in the engineer's ability to find faults as he already made improvements to the amp by replacing HT capacitors to bring the power up from 20W to 30W"

I am not sure I understand the meaning of this statement. IF the amp is supposed to be making 40 watts....and a healthy 40 watt Pro Reverb will....before breaking up, what does the 'bring the power up from 20W to 30W' mean? Did your tech tell you that the amp was producing on ly 20 watts before the recap and only 30 watts after the recap? IF the amp is only making 20 or 30 watts before breaking up, something is wrong with the amp. An electrical engineer would understand this...if this tech is an engineer. IF the tubes are good, if the power supply is good, if the voltages are good and if the components in the signal path upto and including the power tubes are good but the amp only makes 20 or 30 watts, then one starts to suspect the output transformer.
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Old February 15th, 2013, 01:15 PM   #8 (permalink)
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Originally Posted by Mr Ridesglide View Post
you may - your tech will know if you have the available voltages- be able to change tubes to get more clean headroom - kt66 or 6l6 for output tubes - and/ or change the preamp tube(s) to something that has less gain -
It's not about voltage. Swapping 6L6 to KT66s looks dandy on paper until you reailze KT66s draw twice as much filament current. Real ones do, anyway.

Lotta tube swapping goes on without anyone lookin' at the spec sheet. You don't accomplish anything if your transformers aren't up to the task. Actually you may accomplish something. You might melt the goo outta your transformers.
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Old February 15th, 2013, 01:27 PM   #9 (permalink)
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It's not about voltage. Swapping 6L6 to KT66s looks dandy on paper until you reailze KT66s draw twice as much filament current. Real ones do, anyway.

Lotta tube swapping goes on without anyone lookin' at the spec sheet. You don't accomplish anything if your transformers aren't up to the task. Actually you may accomplish something. You might melt the goo outta your transformers.
well then - current is our key! Thank you!
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Old February 15th, 2013, 01:45 PM   #10 (permalink)
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The tech is a very experienced engineer. I may not have the terminology right, but he measured 20W before the cap job and 30W afterwards. We both agreed that I should gig the amp before deciding whether to get him to more test and investigations. I will mention the output transformer to him though.

As I said, this amp had plenty of clean headroom once, with the same types of tubes as it now has, so I think there is a fault somewhere that should be found before we experiment with different types of tubes.
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Old February 15th, 2013, 01:48 PM   #11 (permalink)
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I'm with Wally on the cap issue, I don't get it either.

Assuming the amp is healthy, here's a list of headroom inducing wizardry:
  • Efficient speakers.
  • SS rectifier.
  • Bias set around 60%.
  • SF Bassman OT.

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Old February 15th, 2013, 01:48 PM   #12 (permalink)
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The recap may have come late and the OT has been compromised. An OT can 'function' while being much compromised...yielding poor sonics and low output.
Or...there may be another problem....
Good luck with it.
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Old February 16th, 2013, 07:40 AM   #13 (permalink)
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Well, well, well. Gigged it last night and swapped the (new) 5U4GB rectifier for a Groove Tubes solid state thingy and the amp seemed born again - louder, cleaner...just wonderful. I also found myself rolling off lots of bass to further lose the distortion on chords.

Funny , but I tried the SS years ago and didn't like the tone, but now it seems to have done the trick.

Thanks for reminding me about this funny little orange thing I've had in a box for 20 years!

I'm still taking the amp to my guy to take a look and do some measurements...a report to follow next week.
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Old February 16th, 2013, 01:41 PM   #14 (permalink)
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Well, well, well. Gigged it last night and swapped the (new) 5U4GB rectifier for a Groove Tubes solid state thingy and the amp seemed born again - louder, cleaner...just wonderful. I also found myself rolling off lots of bass to further lose the distortion on chords.
You're so right about the "rolling off bass" part: I've got mine set to 3. If i add bass, it starts breaking up a lot sooner indeed!

If you want more bass without distortion, try different speakers. I had A Jensen Blackbird in there for a while, and had to turn down the bass to 0,5!

Treble stays at around 5, with "bright switch" off, with a strat mind you...

I found that the Pro has a very scooped sound. It took me a while to find good settings on the amp. It's all about finding the right amount of mids, since there is no mid knob. I think adding a middle pot might be a good mod on a Pro... Anyone has experience with such a mod?
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Old February 16th, 2013, 08:53 PM   #15 (permalink)
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The Pro has a scooped EQ, but they all (Fender blackface/Silverface) do. It's what a Fender tone stack does. It's the Fender sound.

As far as a mod, the larger the value for the mid resistor (resistor or pot), the less effect the Bass and Treble have.

Try replacing the 6.8K resistor at the bottom of the stack with a 10K or 22K when there is no mid knob. Models with a mid knob use a 10K pot there.
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Old February 16th, 2013, 09:26 PM   #16 (permalink)
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Some of the silverface era Pro Reverbs came with really crappy and inefficient Oxford speakers.

first thing I'd look into is an electrical checkup to make sure everything is healthy and happy. Once the guts gets a clean bill of health, if the headroom is still too low, then replacement speakers would be in order of you have the Oxfords in there now.

I see you are in UK, so you can probably get Celestion V30s at a good price. Else i would suggest Eminence Swamp Thangs.
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Old February 17th, 2013, 07:33 AM   #17 (permalink)
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Ellis, yes that was a huge difference between the tube rectifier and the solid state one: with the tube one I generally has bass on about 8 and treble on about 2 - with the solid state one I need to have bass on about 2 and treble on about 8 or the amp was muffled.

FenderLover thanks but the actual tone is fine and the EQ actually isn't as scooped as the setting that I usually have on amps with a mid pot (I usually take a lot of the mids out to get that Fendery sound from other amps).

Bigger John this amp has Utahs, which I know some people hate but I have always found the amp to sound wonderful until these issues with the clean headroom. I did wonder whether it was because the speakers were getting tired, but I played the amp through another cabs with a very efficient speaker a while back and heard no significant difference - that speaker also cab seemed to break up at the same volume, so I doubt that there is a problem with the speakers.

Like I said, the switch to a solid state rectifier seems to have fixed things, but my amp guy is taking a look to see if there are any problems that have gone undetected so far. It could be that the old rectifier tube was part of the problem, and that the new one that the previous amp guy (who was worse than useless) installed was also faulty, so we didn't factor it as a possible cause.
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Old February 19th, 2013, 10:06 AM   #18 (permalink)
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Quick update in case anyone is interested in clean headroom!

The engineer tested the amp with the tube and solid state rectifiers.

At full power with the tube he measured HT at 400v. With the solid state he measured HT at 470v.

With the tube he measured 28w. With the solid state he measured 46w.

So I guess even relatively small differences in voltage have a big impact on power.

We decided to stick with the SS rather than try other tube rectifiers and he is making some modifications that are necessary as a result: a couple of capacitors need replacing to ones more suitable for the SS, and then a rebias.

Forgive me if my terminology is not spot on but I hope this makes sense to those in the know.

The upshoot is that my amp is finally back to its former glory.
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Old February 19th, 2013, 11:07 AM   #19 (permalink)
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Quick update in case anyone is interested in clean headroom!



The upshoot is that my amp is finally back to its former glory.
Sweet! Good news...
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Old February 19th, 2013, 12:43 PM   #20 (permalink)
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I would guess thatt that tube rectifier is underperforming because that is very low B+ if he was taking measuremnts with the suggested 5U4 in there. Teh schematic calls for somewhere around 440VDC for the B+. A good GZ34 would bump that up some, and a SS would be somewhere above the GZ34. OF course, wall voltage is another variable, right?

I have alos found that output power is not the on ly consieration. IF the voltages are high in the preamp, a Fender amp will lose some of its 'musicality'.....it gets sterile, ime, if the preamp voltages are commensurately high with thta B+ of 470V.
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