dumble type presence on a fender

[Heisenberg]

has anyone tried the dumble feedback network on a fender?

[M Fowler]

Like the LNFB on V1? Yes

Mark

[boldaslove6789]

Like the LNFB on V1? Yes

Mark

I think he's talking bout the Global NFB Presence circuit. No?

Its possible, pending on what Fender amp/circuit, you'd have to reverse the leads on the OT, and add some parts, but its most definitely doable.

Also, the LNFB circuit (like whats used on 124, 102 ect.) can be be done on a fender, and can even be put on a switch to turn on/off....

Also, another great mod for the Fender amps circuit & Verb is to change the Cathodes from shared to split Cathodes and mess around with different combo's.You can even put it on a Muti Position Rotary for all sorts of combinations from Vox, to Dumble, to Marshall, whatever.... As well as adding the 500p/220k Grids on the 2nd V4B (12ax7).

So many mods.... not enough amps

[Heisenberg]

yes, global nfb for a 763 fender deluxe. do I need to make any other changes aside from simply switching the output transformer leads? grid stoppers look to be bigger on the dumbel for example. 1k5 vs. 3k3/5k1.

for boldaslove: the grids on the first stage of the reverb are shared so I am not sure what you meant by 27k and 500p/220k.

[ic-racer]

I implemented a Dumble style presence on my Fender Super Twin Reverb conversion as shown in this schematic.
http://ampgarage.com/forum/files/final_ ... ic_594.pdf

[boldaslove6789]

yes, global nfb for a 763 fender deluxe. do I need to make any other changes aside from simply switching the output transformer leads? grid stoppers look to be bigger on the dumbel for example. 1k5 vs. 3k3/5k1.

for boldaslove: the grids on the first stage of the reverb are shared so I am not sure what you meant by 27k and 500p/220k.

Woops, I apologize, late night

I meant add a 500pF/220k grid to V4B (12ax7) in the Reverb, and change that tubes cathodes from Shared to Split cathodes. Try 2k2/22uF for the added Cathode resistor/cap on V4A.

[Heisenberg]

I implemented a Dumble style presence on my Fender Super Twin Reverb conversion as shown in this schematic.
http://ampgarage.com/forum/files/final_ ... ic_594.pdf

did you end up using the 820 ohm or did you beef up to the 4k7?

[ic-racer]

I implemented a Dumble style presence on my Fender Super Twin Reverb conversion as shown in this schematic.
http://ampgarage.com/forum/files/final_ ... ic_594.pdf

did you end up using the 820 ohm or did you beef up to the 4k7?

I believe I left the original 820 in there.

[glasman]

If you inplement the dumble skyline presence in a Fender you should change the 820 to a 4700 (or 3200) if you are using the 390 tail resistor otherwise you will seriously limit the gain of the output stage. It is all about the ratio of the two resistors. Try to maintain the 8.2 ratio of the original Fender circuit.

Dumble ratio 4k7 / 390 = 12

Fender 820/100 = 8.2

Scaling the presence

8.2 *390 = 3198 FB resistor (3200)

Gary

[rogb]

Gary, what would you suggest for a 50w implementation of the 102 NFB 390/4K7? Reduce the 4k7 and keep the ratio? Leave it as is?
I notice one kit builder leaves it the same for the 50w.
I'd be interested to hear your thoughts.
Thanks
Rog

[Heisenberg]

If you inplement the dumble skyline presence in a Fender you should change the 820 to a 4700 (or 3200) if you are using the 390 tail resistor otherwise you will seriously limit the gain of the output stage. It is all about the ratio of the two resistors. Try to maintain the 8.2 ratio of the original Fender circuit.

Dumble ratio 4k7 / 390 = 12

Fender 820/100 = 8.2

Scaling the presence

8.2 *390 = 3198 FB resistor (3200)

Gary

thanks gary. what about increasing the tail resistor? say 4k7/560?

[glasman]

Raisng the tail resistor would give the desired gain results, but change the presence break frequency as well, at guess it would drop the presence range more into the upper mids / lower highs, which might be a really nice effect.

Gary

[ic-racer]

Gary thanks for your comments on the NFB resistor ratio. I have scanned my notes and schematics and I believe leaving the original 820 ohm resistor in there was an omission or error.

Since it is a big deal to take that chassis out of the cabinet, I was scanning my pictures of the finished amp to see if I had actually put a 4k7 in there and forgot to mention it anywhere. Either way, I'll be taking the chassis out to check it out. The amp is not very loud for a 180W amp, so I suspect the NFB loop is the issue.

[Roe]

If you inplement the dumble skyline presence in a Fender you should change the 820 to a 4700 (or 3200) if you are using the 390 tail resistor otherwise you will seriously limit the gain of the output stage. It is all about the ratio of the two resistors. Try to maintain the 8.2 ratio of the original Fender circuit.

Dumble ratio 4k7 / 390 = 12

Fender 820/100 = 8.2

Scaling the presence

8.2 *390 = 3198 FB resistor (3200)

Gary

yes and the ratio of the secondary. 100k@4ohms=141k@8ohms=200k@16ohms

[aflynt]

I was messing around with a single channel, non-trem Blackface Vibroverb build I did a long time ago, trying to get it sounding decent last weekend. I replaced the preamp with the clean channel of #102 with no bright, mid switch hard-wired off, and rock hard-wired on. This amp had previously sounded really tight, bright and painful with way too much low end. The #102 preamp helped a whole lot. I also wound up changing the reverb mix resistor network a bit (killed the 10pF bleeder, lowered the reverb mix ratio, and brought the overall signal level down by reducing the 220k ground resistor), added a 220k/500pF grid network to the mix stage, lowered the pre-pi .1uF coupler to .047uF, messed with the droppers to get V1 down to 200-210VDC on the plates, and tweaked the negative feedback slightly. For the negative feedback I kept the 820 ohm but lowered the tail resistor to 76 ohms to give it the same ratio as the Dumble. I'm not sure how much of a difference the NFB change made in the overall scheme of things, but the amp has a really nice warm clean sound now with a character very similar to my #102 clone.

-Aaron