Tightening bass in Fender blackface

[martin manning]

increases the clean headroom by reducing the preamp gain

How are you defining clean headroom?
I can't see how a frequency selective attenuation of the input signal could affect it http://en.wikipedia.org/wiki/Headroom_% ... cessing%29

It would if those frequencies that are being attenuated are the ones that are distorting due to insufficient bandwidth, no?

[pdf64]

I was hoping to raise the issue that headroom is a meaningless, or at least undefined, concept in the context of guitar amps, as non of that audio engineering terminology seems applicable, due to guitar amps generally being used such that the signal is harmonically enriched by them.
ie they are not intended to have a linear frequency or dynamic response.
The point being that it may be helpful if an alternative word could be used.

[roberto]

I use the term "headroom" too.

Not in terms of harmonically free signal, but in what is perceived as a clean sound. Nothing absolute, but an easy term to be understood by everyone.

I call it frequency shape of the sound to increase the headroom of the amp.

[TUBEDUDE]

I see headroom as the quality of having additional dynamic power, regardless of distortion level. The ability to have a transient sound louder than the average signal. So a dynamic accent isn't the same level, like most over compressed metal.

[pdf64]

So which has more headroom; a 5 watt low gain practice amp that's still clean with controls cranked, or a 100 watt Super Lead Hi Treble channel, in which the 4n7F bright cap makes a clean setting almost impossible to achieve?

[TUBEDUDE]

Depends on how hard you drive it. The headroom is the difference between the max available output and the average level you are driving the amp, regardless of wattage.

[pdf64]

The typical use of a tube guitar amp is to be driven to at least some degree of power amp overdrive; I don't perceive how the concept of headroom can be valid in such a scenario?

[roberto]

So which has more headroom; a 5 watt low gain practice amp that's still clean with controls cranked, or a 100 watt Super Lead Hi Treble channel, in which the 4n7F bright cap makes a clean setting almost impossible to achieve?

Potentially the latter, in reality probably the former.
That's exactly what I'm referring to when I say "frequency shaping to increase the headroom". You exaclty wrote a wrong frequency shape that erase a potentially good headroom.

[SixStringBender]

Okay, so I was going about ideas and I was going to build a box to put a pot and a switch with multiple caps to dial this in. I even thought about putting it in the guitar. Then I recalled reading about a bass cut tone control before. Well right there it was. My idea already done years ago in a G&L called the PTB. Without the switching caps though.

Since I use pedals I believe this can only be done in my guitar or in a box before my first effect. I'd just plug into input 1 on the amp.

So one question I'm not understanding clearly. Does more resistance allow more bass through or less?

pdf64 wrote, " If input #1 is used, then the high pass filter frequency will be ~9x lower (as the R term is 1M compared to 136k)."

Based on this I think it means more resistance to ground means less bass is cut and more bass gets through.

So I wire this in my Strat like this?

Edit: Changed plan because the last one will not work as described.

Following orientation is with the pot shaft facing down shell facing up with lugs facing you.

Disconnect the signal wire to the output jack from the volume pot's wiper and connect it to the left lug of the new bass cut pot. Don't solder yet.

Connect the volume pot's wiper to the bass cut pot's wiper. Solder the wire at the volume pot wiper, but don't solder the bass cut wiper.

Put a short lead or wire from the bass cut's wiper to the right lug. Solder the bass cut's wiper lug now. It has two wires in it. One from the wiper of the volume pot and the short lead just installed.

Put a .001uF (1nF-1000pF) cap in the two outer lugs of the bass cut pot. Left lug has the signal wire to the output jack with one cap lead. Solder it. Right lug has the jumper from the wiper with the other cap lead. Solder it.

Remember I'm nearly blind, so I have to ask in text to make sure I'll have my gal wire it up right.

[pdf64]

With RC filters, the bigger the R term, the lower the break frequency.
Your idea with the bass cut control wired into your guitar should work.
How about having a switched pot for the control, so that the bass cut cap can be taken out of circuit (shorted across), thereby putting the guitar back to normal?

However, I don't understand the rationale of the input filter idea; compared to a Fender tone stack, what's its advantage over turning the bass and mid down?
Regular Marshalls etc don't have the same facility to roll of bass and mids.

[SixStringBender]

HA I'm indeed using a DPDT 250K push/push pot to bypass the bass cut pot.

I haven't finished it yet, but we'll see how effective it is. I have my doubts, but I just gotta try it. If I do indeed like it I think I'll make the treble cut none adjustable with a fixed resistor and cap so I can have my blender back. I almost never roll the tone pot down to cut treble. I just don't like that sound. The only way I like treble rolled off is with a .010-.015uF cap to get some Slash Guns'N'Roses tone.

Or I can install a push/pull or a push/push for the treble cut tone and just make the bridge pup switchable to get all three pups.

I like all three pups in parallel. I like that quacky sound.

Back to the phase inverter coupling caps to the output stage. 0.030uF puts them at your recommendation, correct? I do tune down half a step. Sometimes a full step. Although, not likely a full step down with this amp.

[SixStringBender]

I found some .001uF (1000pF=1nF) 100V caps I had bought last year for another project so I tried the bass cut control in my Strat. I can't tell much of a difference in it. I made the pot and the cap fully bypassable using the DPDT switch on the push/push pot so I could go back and forth comparing it with and without the bass cut on the fly. It is coming back out and the blender is going back in. This was with alnico 6 bar lipstick tubes, so maybe I didn't notice enough difference due to that. There was a very minor difference, but I can guarantee this was not SRV's reason for his tight mid and late 80's tone. I did what the guy said in the thread with the bass rolled up on the amp, treble maxed too, and it was way too much bass. It was not a good sound. Far from SRV's tone. Even with all the bass dialed out on the amp it still wasn't as tight as SRV's SSS tone.

I also noticed something else I didn't like. The resistance when the pot was rolled up reduces signal/volume to the amp. Even switching the cap in and out with the pot rolled down to no resistance seemed to make a slight difference in volume. I guess due to the bass frequency cut with the cap.

Repeat: Back to the phase inverter coupling caps to the output stage. 0.030uF puts them at your recommendation, correct?

I found another article recommending the buffer after the PI. I think it was from the Valve Wizard. He also recommended the 100K bias resistors and smaller coupling caps to the output tubes as you, pdf64. He did mention the 100K bias resistors do cause some attenuation and I had noticed that when calculating them in on another calculator. The bass gets a slight bit more attenuation than the treble.

Back to the cathode bypass caps, PI coupling caps and MOSFET source followers I go. I believe it is the only way to shape the bass tighter. It certainly isn't in the guitar.

I read the 220K mix resistor to the PI for the normal channel drains signal from the reverb channel. Do you have any recommendations for this other than just disconnecting the resistor (any other changes need to be compensated for)? I read it increases gain, but I'm not so sure I want to increase gain at this point. However, curiosity will make me try it.

I have to see if I can determine witch resistor it is on the DRRI when my wife gets a chance to go over the schematic with me. Or someone can just let me know what R# to clip. HAHA edit: We just checked and I think I need to clip R12. It seems R12 and R35 form a voltage divider. R12 from V1B and R35 from V4B.

I wonder if the 220K resistor from the normal channel was disconnected on SRV's blackface amps? I know V1 was pulled.

[Roe]

just try it and see if you like the 220k

[pdf64]

Back to the phase inverter coupling caps to the output stage. 0.030uF puts them at your recommendation

I generally default to CR=10; 0.1uF with 100k grid leaks usually seems fine.
Reduce CR to ~5 if it seems a bit blocky, or ~2 or 1 to roll off some bass.

Yes, R12 feeds the normal channel to the phase splitter input.
If it's lifted at the C5 end then it can be put in paralle with R35.
That gives the option of a 220k, 110k or 0 (R35 linked out) series input resistor to the phase splitter. These may gives slightly differing performance as the power amp transitions into overdrive.

[SixStringBender]

pdf64, so you are saying I can try R12 clipped and R35 at 220K alone, 220K in parallel with R35 to get 110K and jumper R35 to compare how all three configurations sound?

I'll try a couple of those combinations later.