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Correct negative feedback for a plexy marshall

9/28/2003 3:19 PM
Karl Correct negative feedback for a plexy marshall
Whats the correct way to route the negative feedback for a 100w marshall plexy cirquit?  
4/8/or 16 ohm tap?47k or 100k resistor?  
I use an OE Output tranny,thanks Karl.
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9/28/2003 6:58 PM
Hoffman shows 100k to the 8-ohm tap in his layout. Marshall's own 1959 diagram shows 47k(100k) (their parens).. indicating that both were used. They are using the 8-ohm tap assuming that the user will be plaing through a pair of 16-ohm cabinets in parallel.  
Keep in mind that the higher imp taps have move voltage. 100k on the 16-ohm tap is a lot more -fb than 100k on the 8 ohm tap. You generally want to take the -fb from the same tap you are using to drive the speaker, but you don't *have* to. Bigger R = less -fb, lower tap = less -fb. More -fb will have a flatter freq response clean, and will transition more quickly to OD. Less -fb will have more of a mid hump clean and will transition more gradually to OD. It's all a matter of taste. There really isn't a "right" answer. Try several setups and see what works for you. have fun!
9/29/2003 6:08 AM
Thanks SpeedRacer,I understand that if you plan to use the amp with only one cabinet you HAVE to experiment..
9/29/2003 6:53 PM
Bryan James

speed, or anyone else,  
how much of a difference is there between 47k to 100k on the negitive feedback loop. and does it make an audio/tonial difference especially with what cabs you use?  
what about puting a 47k and a 50k pot and series and you could adjust the amount of negitive feed back, or is the defferance not worth it? (if you think it worth a try i have a homebrew amp i would try it on. but is it worth the time?)  
9/29/2003 11:56 PM

Hmmm well between the 47k and 100k,  
With the 47k you'll seem to have more clean volume.  
Or easier way to put it..with the 100k as you turn up your volume control, it will distort alot sooner.  
And the 47k will give you a wee bit more clean before distortion. Maybe a number or 2 on the control. In some of the older amp books they would say use the EL34s feedback on the 8 ohm tap.When switching from amps that used 6550s  
And i believe some have on here have used a pot in the circut for fine tuning,may be a bit scratchy when adjusting. I think people on here have tried about everything  
9/30/2003 5:31 AM
Bryan James

might be an idea if they make a 50k locking pot? you could mount it on top of the chassis and then tune it to your likes then lock it in place. of course if you mount it ontop on the top of the chassis a locking may not be need. or even one of those flat head screw driver type pot?
10/2/2003 5:29 PM
SpeedRacer Re: understanding voltage feedback in common MI amps
Your idea with the pot is a sound one, I know bc I have done this for years in customer' Fender amps! :) It makes a nice control (in the aux spkr jack hole) to "loosen" the amp up. Very popular with the Blues guys.  
The Q about feedback: While I am not the math guy, figuring out the -fb loop in these amps seems basic enough that I will explain how I look at it. Then perhaps folks that know what they're doing can add/correct/etc to this. It's a nice thing to be able to understand. (assuming *I* understand it! ;))  
Ok, so you have an amp that puts out 30W into a 16-ohm load. Great! Watts are volts squared divided by the resistance (we're going to assume 16-ohms is just 16-ohms ok?) so you wind up with about 22 volts. Where does this come in to the feedback discussino? Well, you have the feedback resistor (Rfb), say 100k. That comes from the "hot" side of the output transformer secondary and forms a voltage divider with the presence control (we'll talk Marshall for the example). So this becomes a basic algebra ratio problem. You start with 22 volts, and X gets dropped over the 100k and n-x get dropped over the 5k pot to ground. The signal voltage at the top of the pot is your feedback voltage. You can see that by varying the rfb, you change the ratio and thus change the feedback voltage. This voltage is added to the signal voltage at the phase inverter, so if you have a 10V signal and you add 2V of -fb to it you can now calculate how many dB of -fb you're using (and how much gain you're losing in the output section).  
Since it is a straight forward process to calculate, you can also see how much -fb the various amps are using. You will see from experimenting that the original JTM45 used by far the most -fb of any popular guitar amp basically bc they borrowed the 5F6-A design right down to the Rfb, which was used on the *2* ohm tap of the Bassman.. and slapped it on the *16* ohm tap of the JTM45. This resulted in a lot more feedback. 2-ohms at 30W is about 7.7V, vs 22V. Nearly 3 times as much (on a voltage basis)  
Hope this is clear as mud! :)  
ps: yes, it is totally worth the time to add a pot into your -fb loop to learn the effects of it. Easily reversed, less than $2 to do (if you already have a knob!)

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