Tightening bass in Fender blackface

[SixStringBender]

We checked the SSS schematic. It is a 100K slope resistor, but I noticed there is no 120pF bright cap. How does the SSS achieve it's bright tones? I removed the 47pF bright cap from my DRRI and I did not like it. So I put the 120pF in and I like it. I did not try it with my lipstick tube pups on the neck pickup though. That may have been a good combination. I was playing in position 4 with my Fat 50's and I missed the bright cap.

My Blues Junior, with BillM mods, has no bright cap, but it is brighter than my modded DRRI when the bright cap is removed. Even with the bright cap in the DRRI I can make the BJ brighter with the sparkle control and presence control.

Here is an example of my modded BJ. This is a more scooped tone than the tone I'm in search of. The BJ is the same as the modded DRRI, not tight enough in the bass to get the Tin Pan Alley tone from 1985 at Montreux. This is still great tone though.

http://youtu.be/hADsYNsgOzs

[Roe]

I recommend a 3n3 cap after the second gainstage (or a .68uf cathode cap ala Diaz). and try 500-1000pf as the PI input cap. Try the SF PI with 68k bias splitters and 47k plate resistors. perhaps increase mains filtering to 47uf

[SixStringBender]

Roe, I already have the .68uF cathode bypass cap on the second stage, V2B.

The main filters in the 64 Vibroverb Custom are two 220uF at 285V in series with balancing resistors. I removed the balancing resistors and put two 50uF 600V caps in parallel.

The Tonequest article mentions the Vibroverb having the silverface PI 330K grid resistors instead of the 1M resistors.

Where do you mean a 3N3 cap? C17 is .0033uF.

Do you mean 1000pF for C25? It is already 1000pF. .001uF on the schematic.

What change in tone would the PI changes you recommend make? Honestly I think any PI changes will be my last step. As stated, I want to keep the bandwidth through the PI and output stage.

I'll probably change C18 first and go from there since it is, IMO, a problem that needs to be corrected since I no longer have signal going to ground through the intensity pot. I'll do .022uF there.

As for the bypass caps. I don't have any 5uF electrolytics on hand. I do have a single 1uF at 63V, I think.

[SixStringBender]

We replaced C18 with a .022uF 225P cap. The difference was minor. Still requires a lot less bass than treble. I also was wrong about how I play the EQ on this amp earlier in this thread. I play my BJ with low treble settings (4-5). I play with the DRRI treble set higher. I don't have the knobs on, so I don't know how high. Probably 7-8, or so. With C18 changed the bass was a little tighter.

The bypass caps are next.

[Tone Lover]

The 1uf right there definitely remove some bass alot actually you could try it and see if you like. But without doing the math your cross over frequency with a 1.5k resister will be in the 160 to 200 hz range low e is around 82hz . So might be way to far.
Bill

[SixStringBender]

Tone Lover, are you talking about V2A's bypass cap (C6)?

Thanks for the info, I'll search info on resistors. I'll probably go with the 5uF caps and the same resistor values as the Dumble SSS. That is the sound I'm in search of.

I'm in the process of removing the footswitch jack and installing a dwell control with a 1M audio pot. Gonna replace the 100K linear reverb pot with a 100K audio pot too. I gotta tame the reverb on this thang. I like long decay, but the presence of the reverb is overkill. The touchiness of the reverb pot is annoying too. One tweak and not enough reverb. Tweak it again and too much.

[dimitris]

This trick was posted by user phatt on freestompboxes.org.Some guys tried it and really liked it.

[pdf64]

BF guitar amps intended for use with closed back cabs tended to have a 500pF coupling cap to the power amp long tail pair input, eg http://bmamps.com/Schematics/fender/tre ... _schem.pdf

I suggest that the value of the grid feed bias resistors for the power tubes (typically 220K) is reduced, eg to 100k.
That one change (well, two components) can often be enough to get the amp under better control, as it has a four-fold benefit-
The gain and output voltage swing of the LTP is reduced a little, increasing the margin of stability and reducing bias excursion under overdrive.
The time constant at the power tube control grids (formed by the coupling cap and grid leak resistance) is lowered, further reducing bias excursion under overdrive, see http://www.ampbooks.com/mobile/amplifie ... excursion/
Lower grid leak resistance results in a more stable amp, more able to accommodate higher levels of control grid positive current; note that the limiting value for control grid resistance of 6V6 and 6L6 in fixed bias is 100k.

[SixStringBender]

dimitris, can you give some detail on the diagram you posted so I'll know where to begin to look? I'm blind and my wife has to read schematics to me.

pdf64, 500pF for C25 noted. The cab is open back though, however, an open back 4x12 is still a totally different animal than a 1x12 combo.

What impedance is the circuit? I went to the calculator and input the following, but I don't know the impedance. I do understand less time of excursion is better, but what will the effect of the 100K resistors do to tone. I guess improve it since there is less crossover distortion?

What is the bass measurement? I noticed it increases when I input smaller coupling cap values.

.047uF vs .1uF
220K vs 100K
1.5K control grid stoppers
68K impedance

edit, if I understand correctly the impedance is from the PI plates. I guess about 34K?

I input the following to my guess with the MOSFET source followers.

0.68uF
100K
1.5K control grid stoppers
1.5K impedance

On another note. The dwell control we installed on this amp is such an improvement. I haven't had a chance to play cranked, but it really got the reverb under control. I don't think the 100K linear return/mix pot is an issue now. I can turn it a bit higher to get lush reverb now with the dwell backed off to prevent doinggy verb and forever long decay. And I like long decay. Just not like I'm in Mamoth Cave.

[Shamadan]

Hi SixStringBender
I have found some info on dimitris post at http://www.guitargear.net.au/discussion ... c=27602.45

[pdf64]

Yes, it's generally beneficial to cut down the signal level being fed to the reverb driver V3.
I thought that your 4x12 had some holes drilled in the back panel. Depending on how extensive these are, they may act as a port rather than a true open back. I suggest that at least 30% of the back panel area is removed, focused on the areas directly behind the speaker cones.

Regarding the low end, there are lots of useful calculators at that site, eg http://www.ampbooks.com/mobile/amplifie ... -response/

http://www.ampbooks.com/mobile/amplifie ... alculator/

A useful old time guideline RC coupling was for resistance x capacitance to =10 for full bass extension (total R value in k ohms x C value in uF).
Bring that down to 5 to de-flub for guitar use.

The LTP output impedance will depend mainly on the tube type and plate load resistor value; 12AX7 and 100k tends to result in around 35k-40k.

[dimitris]

Hi SixstringBender,The diagram is just a small capacitor(1nf to 3nf suggested values) between the guitar's output signal and the low input of your amp.
This capacitor in combination with low input's resistors network creates a high pass filter that rips off all the excess bass and increases the clean headroom by reducing the preamp gain.

[pdf64]

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

[SixStringBender]

Shamadan, I read that thread. I got a laugh at some of those guys. Always folks who are jealous of SRV's abilities.

The only guy who ever impressed me when playing along side SRV was the late great Jeff Healey.

To each is their own, I guess, but SRV will always be my favorite. Levi Platero is one of my favorites and he is younger than me by about 12-13 years. No point in me being jealous. I'd like to play along side him. He is great and not a straight out copy cat. It does get old listening to guys who can't play like SRV who try to, but hey so many wanna copy him because he and his tone was so great. I can't dog them like many people do. I used to be one of them who didn't really sound like SRV. Now I can, but now I want to take his piece of the tone puzzle and do something original with it. Thanks for the link and thanks to dimitris
too for suggesting it. I think another guy suggested something similar with a bass cut cap in the guitar on page 1 of this thread. I have a .0012uF Orange Drop PS cap. I'll try it. The guy in the thread had a point. In the mid and late 80s SRV did sound the same with tight low end through any rig. A little too clean and tight at times in the late 80s.

pdf64, I found the following to achieve what I think you are suggesting in the calculator.

.030uF-.033uF
100K
1.5K grid stoppers
1.5K impedance with MOSFET source followers

I noticed lowering the grid stopper resistors reduced excursion a little too, but I don't know what is optimal for the MOSFETs.

The holes are directly behind the magnets. They are 2 inch round holes. I wanted a 4 inch hole saw, but 2 inches is all I had.

A question, why does the blackface have a 82K and a 100K on the PI plates? Why not balanced? The later Silverface was balanced at 47K, I think???

I'm gonna give this cap trick a whirl to see how it works. If it makes me happy I may not try to reduce bass by as much in the circuit. I guess I'll taylor it to how it sounds cranked from there if it is something that amazes me.

Is there a reason this won't work into input 1 or an amp with a single input jack? Is input 2 suggested simply to reduce preamp gain by putting more resistance between the guitar and the preamp?

[pdf64]

If you have a buffer after the LTP, then its low impedance output will negate any bias excursion issues.

Regarding the LTP outputs balance issue, see http://www.valvewizard.co.uk/acltp.html

Is there a reason this won't work into input 1 or an amp with a single input jack? Is input 2 suggested simply to reduce preamp gain by putting more resistance between the guitar and the preamp?

Input 1 generally has an input impedance of 1M and no attenuation, whereas input #2 (when used on its own) has an input impedance of 136k and 6dB attenuation.
Assuming a direct connection between guitar and amp (eg no buffered pedals, tuners), then the 136k will act to load the pickups, thereby reducing the frequency, magnitude and Q of its resonant peak.
And it will act with the series cap to form a high pass filter.
If input #1 is used, then the high pass filter frequency will be ~9x lower (as the R term is 1M compared to 136k).