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 Post subject: Need Help to Add Negative Feedback to Vox Tube Guitar Amp
PostPosted: Feb Tue 13, 2018 4:17 am 
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Location: South Jersey East of Philly
I have a 1965 Vox Cambridge Reverb model V-3. Usually no guitar player wants to add negative feedback...but I would like a little more headroom on this amp. Not much output on this probably 10-12 watts..but I'd like to see what it sounds like with NF. It's a long tailed pair PI (i believe). I know you run it from the speaker positive, thru a resistor, to the input grid on V-3b on the schematic...I'm not sure what I have to add in that part of the circuit to adapt it.
If anyone can help with that I appreciate it. Here's the schematic...Just click on the thumbnail schematic in the first posting on the page...
http://music-electronics-forum.com/t37195/


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Tue 13, 2018 3:32 pm 
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Location: Fayette County, Pa
Adding negative feedback is sort of a vague term. What do you want to accomplish? If simply a matter of gaining headroom you can do that by eliminating the bypass cap on the output which effectively lowers the gain and provides a more gentle cutoff at saturation. You can also add a resistor from speaker to grid as you say, the value is a mtter of how much feedback you want to incorporate. You might even experiment, put a pot in (1-2 meg) and adjust it to your liking then insert that value resistor. If you want the feedback to be non-linear over the spectrum, use a resistor and capacitor which will provide differing amunts of feedback at different frequencies. Again values are a matter of preference.

This is something you can experiment with; as long as you don't miswire something you really can't do any harm. (Just be careful you don't inadvertently create some high frequency feedback which could cause problems. A scope on the output will resolve that possibility.)


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Tue 13, 2018 3:50 pm 
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If I understand "headroom" correctly, adding feedback will have no effect. I think headroom is the difference between normal listening levels and the maximum peak without clipping. This is a property of the output stage, and will not be affected by feedback.

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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Tue 13, 2018 4:43 pm 
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Location: Cortez, Colorado
https://www.ampbooks.com/mobile/amplifi ... /feedback/

Rk = 1200
Rg = 470K
Rt = 47K
Rp I would put at maybe a 10K pot
Rfb Maybe another 10K pot. Adjustable is good.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Tue 13, 2018 6:30 pm 
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If I understand "headroom" correctly, adding feedback will have no effect. I think headroom is the difference between normal listening levels and the maximum peak without clipping. This is a property of the output stage, and will not be affected by feedback.

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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Wed 14, 2018 1:52 am 
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OK, I figured out how to add the NF circuit by copying a Fender one that's similar, and changing some values. Thanks.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Wed 14, 2018 2:00 am 
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Yea, but how did that work out for you?


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Wed 14, 2018 3:40 pm 
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Too much gain loss for a guitar amp with 12 watts.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Fri 16, 2018 4:06 am 
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I can't be sure from the schemo, but output is about 12 Watts, and you have EL84 power tubes, right? They are the European 6BQ5's.

To increase headroom, rewire it for EL34 tubes, 6CA7's. You will need to add octal tube sockets. The result will be almost twice the output power, so louder without distortion, which seems to be what you want. However, the plate voltage will be limited to what your transformer can supply. So you may not get up to 20+ Watts.

The guy that builds Neil young's amps converts the Tweed Deluxe, which came out with 6V6's, to 6L6 output, which does what you are thinking, increases the output without clipping. But then they go the next step further and drive the bigger tubes into distortion and create chaos, Crazy Horse style. 8)

Example: Powder Finger.

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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Fri 16, 2018 5:15 am 
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Stepping back a bit: If you don't have enough gain after adding feedback, then just add a gain stage. If there's no room in the chassis, then maybe use a transistor.

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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Fri 16, 2018 5:42 am 
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That amp has British mojo. 8)
I personally feel that a vintage 1965 Vox amp should be left as is. Build your own custom amp if you want to experiment. JMO.

Attachment:
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Wait a bit, Petula Clark? Must be her backup band used Vox.

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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Fri 16, 2018 6:24 am 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Quote:
I personally feel that a vintage 1965 Vox amp should be left as is. Build your own custom amp if you want to experiment. JMO.
If it was a Brit made '65 Vox I could see that..No British mojo here...It is a 12 watt Thomas Vox American made amp, and had very little in common with the Brit AC Vox amps. Would be stupid to build a custom amp just to try a NF circuit. I only did a couple reversible changes to try the circuit. Nothing sacred about this amp.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Fri 16, 2018 5:43 pm 
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OK, it is yours to do with as you wish.
But when I searched for the description you gave I found an attractive vintage amp, so even a Thomas Vox seems to be worth saving as is, JMO.

As far as building a custom amp, it is not hard or stupid, and you can change it any way you wish, just attaching sockets to a bread board. Or a bread pan, even. Anyway, good luck and have fun.

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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Fri 16, 2018 11:44 pm 
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Tbirdkid wrote:
Too much gain loss for a guitar amp with 12 watts.

Doesn't matter is it's 12 Watt or 100 Watt. When you apply negative feedback to an amp that previously had none the volume will decrease by the amount of feedback applied and, if it's to be effective, perceived loudness will decrease by at least half. I.e. 10 dB of feedback reduces volume by 10 dB or, conversely, it will take 10 dB more signal drive to reach the same power output level. The same applies if the feedback is 12 dB, or 16 dB, or any amount of dB. It's a direct tradeoff of gain for negative feedback.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Sat 17, 2018 4:38 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4025
Location: Sunnyvale CA
Tbirdkid wrote:
I have a 1965 Vox Cambridge Reverb model V-3. Usually no guitar player wants to add negative feedback...but I would like a little more headroom on this amp. Not much output on this probably 10-12 watts..but I'd like to see what it sounds like with NF. It's a long tailed pair PI (i believe). I know you run it from the speaker positive, thru a resistor, to the input grid on V-3b on the schematic...I'm not sure what I have to add in that part of the circuit to adapt it.
If anyone can help with that I appreciate it. Here's the schematic...Just click on the thumbnail schematic in the first posting on the page...
http://music-electronics-forum.com/t37195/


I generally agree with the other posters, it's unlikely to give you anything you want, but, seeing as how it's a guitar amp, only you can make an assessment if it worked or not. Only one way to find out for sure. It would be a different story with a hi-fi amp, there is an objective definable goal.

I would only caution you that you need to be very careful and monitor your amp on a scope as you experiment with it, because it is a very real possibility that adding feedback incorrectly will make it go unstable. If so, it could be wildly oscillating at 50 MHz and burn everything up, and you will never hear it.

Brett


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Sat 17, 2018 3:37 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
The amp is back to original now...experiment is over.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Sun 18, 2018 2:10 am 
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Wha hoppin?


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Mon 19, 2018 3:52 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Already noted result in a previous post above.


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 Post subject: Re: Need Help to Add Negative Feedback to Vox Tube Guitar Am
PostPosted: Feb Thu 22, 2018 9:34 am 
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Location: Gainesville, Florida
change V2 to a 12AU7


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