Amp volume on zero still produces quite a bit of volume

[Ambugaton]

All of a sudden my amp retains quite a bit of volume on zero. Didn't do this before and nothing else has changed. It drops off when I began to turn it then comes back up normally. Is this a problem? I thought I read something about it being some type of ac leak

[plexi50]

Could be a bad coupling cap but are you sure you have the volume pot grounded at your far left terminal facing the pot from the back?
Check you input jacks and be sure the metal ground strip actually has enough tension on it making a good shunt to ground on any jacks not being used. What amp is this your speaking of?

[John]

The wiper on your volume pot might not be making good contact with the trace anymore?

[Ambugaton]

It is the Stout TMB. I will look into the wiper on the volume pot. All inputs are tightened and making good contact.

I've also just realized that how I thought NFB worked was entirely wrong. In my head the more resistance equaled the more NFB that was looped back. Boy was I wrong. I don't even know how I came to that conclusion. I increased the resistance to 147k and gave up when I felt like I wasn't getting the desired outcome (more linear, less gain)

I might try adding a pot on the NFB loop to mess around with it. I am sure there are some schematics I can find on here.

[Ed_Chambley]

I might try adding a pot on the NFB loop to mess around with it. I am sure there are some schematics I can find on here.

Just hook it up as a voltage divider.  I have a NFB pot on most all my amps.  If you find you get oscillation when completely off, no resistance wire a resistor from lug 3 to ground.  It is a simple mod, but I really like them because when playing at lower volumes I can get a greasy tone by reducing and if I need to play louder I can increase and the amp will tighten up.

I use a 100K linear, which can be a lot.  On one of my amps it will actually clean it up so much that I sometimes use it to play vinyl.

[Ambugaton]

Can anyone explain what differences are made by connecting the NFB loop to the different taps on the impedance switch? I am using an 8 ohm load cabinet but also use a 16 ohm cabinet from time to time.

I am still having difficulty understanding how the impedance selector is tied in. I've attached a picture of how I installed it but there is a mislabeled OT secondary. The orange tap is 16 ohm. I've looked at the layout and schematic over and over and it appears correct but I feel like something is still wrong.

[plexi50]

I had this exact issue today on a Hot Rod DeVille.
V1 preamp tube bad. New tubes don't mean they are any good. The tube was a JJECC803S.

Fact is stranger than fiction/

[PRR]

> NFB loop to the different taps

Same as changing the long NFB resistor by a factor of 1.414.

[Ambugaton]

I had this exact issue today on a Hot Rod DeVille.
V1 preamp tube bad. New tubes don't mean they are any good. The tube was a JJECC803S.

Fact is stranger than fiction/

I've switched out V1 a couple times now and didn't notice a difference but I was also switching for another purpose so I didn't pay too much attention. If someone could give a quick look at my attachment and let me know I was correct that would be helpful. Thanks for the help thus far!

[HotBluePlates]

If someone could give a quick look at my attachment and let me know I was correct that would be helpful.

No, not correct.

Lug furthest to the left look like your Hot output from the Impedance Select Switch. That connects only to the Speaker Jack Hot terminal. There should be no wire from this lug to the 4Ω lug, which is the next lug to the right.

4Ω, 8Ω and 16Ω taps connect to the lugs you've drawn (acknowledging that you meant the Orange wire to be 16Ω).

Which tap/Lug to connect the NFB depends on what amp build you made/copied (not stated here so far?).

[Ambugaton]

Thanks! On the layout/schematic it appears to make two connections to the rotary switch. That is why I am getting confused.

This is for a Stout TMB

[Sowndman]

I had a friend who grew up in infancy of tube amps. He explained to me why the NFB was implemented. He said in early used of tube amps in theaters using microphones, the speakers would actually act like a microphone making the sound produced garble fro the speaker absorbing the wave coming back that had bounced from the back wall. The NFB loop was implemented to cancel the inverted wave coming back.

[HotBluePlates]

Thanks! On the layout/schematic it appears to make two connections to the rotary switch. That is why I am getting confused.

This is for a Stout TMB

The Stout TMB layout/schematic shows 2 connections on the wiper/output of the impedance switch: each goes to a tip of the two speaker jacks, one wire per jack. The layout version shows an electrically-equivalent way of wiring it that uses a single wire and rotates the jacks so the tip lugs are close/touching.

[jjasilli]

Can anyone explain what differences are made by connecting the NFB loop to the different taps on the impedance switch? I am using an 8 ohm load cabinet but also use a 16 ohm cabinet from time to time.

I am still having difficulty understanding how the impedance selector is tied in. I've attached a picture of how I installed it but there is a mislabeled OT secondary. The orange tap is 16 ohm. I've looked at the layout and schematic over and over and it appears correct but I feel like something is still wrong.

The different OT secondary taps put out different voltages.  Power (Watts) = V² / I.  E.g, 100W into 8Ω is 28V.  100W into 4Ω is 20V.  So, the higher Ω tap puts out more volts.  But note, doubling the Ω's does NOT double the voltage.  Voltage changes only by 1.414 as PRR states above.

Many manufacturers source NFB from the highest voltage tap, and attenuate & refine it from there in the NFB loop circuit.  Some DIYers source the NFB from a speaker Ω selection SW.  That way the amount of NFB automatically changes to "match" the speaker load.

[PRR]

> source the NFB from a speaker Ω selection SW.  That way the amount of NFB automatically changes to "match" the speaker load.

Except the "optimum amount of NFB" does not change with speaker impedance.

It may "work out" for a small selection of speakers such as most players have at hand. It "works out" because we never really have an "optimum NFB", the happy-zone is wide.

In guitar work, it is often useful to have several amounts of NFB handy (a pot is cheapest), to suit the amount of NFB (tightness, damping) to the specific speaker (loose, sealed, extended bass) and the musical genre of the gig.