First question on my build

[Chuck H]

Do you have a impedance selector switch on your build? Is the position of that switch matching the speaker load?

Just as a tangent, I see this schem has the NFB tapped at the speaker. What do others think about putting that FB point on a set tap?

Good catch. And the NFB lead absolutely should be from the 4 ohm tap and not the speaker jack. But that's not what is causing the hum.

I can't even wrap my head around how 114VAC could even get on pin 7 of the PI. I'm hoping that ones an error in the measurement. If the filament readings come back 3.3ish VAC from each side and there is, indeed, 114VAC on PI pin 7 I'm really going to be scratching my head.

[Chuck H]

My first concern is that everything may be normal, and the amp is simply way louder than you'd expect when up close and personal.

I expect this too. But objectionable hum loud enough that the amps volume control doesn't contribute until halfway up seems excessive. IMHE an amp like this should only exhibit a hint of hum by the time it's up loud enough to be hissing like a snake anyway. Perhaps if we can solve for the hum issue Jim can learn to love the amp as loud as it really is

[J Luth]

eschertron

Yes there is a impedance selector switch and it is set to match the current speaker load which is 8 ohms

The reason I asked about the wire swap is that I tried that before and the amp sequels real bad.

The does not change much if any with both the 100 and 820 in circuit but I will check again.

[eschertron]

eschertron

Yes there is a impedance selector switch and it is set to match the current speaker load which is 8 ohms

The reason I asked about the wire swap is that I tried that before and the amp sequels real bad.

The does not change much if any with both the 100 and 820 in circuit but I will check again.

Okay. Not squealing is correct (!) and proves the NFB is working. On the the 8R winding the effect is almost 50% greater than if the tap were on the 4R winding. Not a big deal, but may become important as you you get used to the amp.

[J Luth]

Chuck and eschertron,

I appreciate all the suggestions and help and will try all. Just a bit more on the amp. See if this helps any, not sure if makes any difference. I have used the amp many times over the past year playing out with full bands and in almost all cases, I have never been more than 3 or 4 on the volume knob and it was plenty loud enough and some of the bands were a bit loud. But this was before I discovered I didn't have the resistor installed. Maybe it is supposed to be a lot louder than it currently is.

Chuck I will recheck the ACV reading and check the filaments. The wire from the 820 is connected to the speaker jack. I was following the Fender layout. Also, with the multi tap output transformer, would it correct to connect the feedback to the 4 ohm tap or someplace else?

[Chuck H]

It should be on the 4 ohm OT secondary. The feedback loop is a voltage dependent circuit. Since it feeds voltage back to the PI, which operates at a consistent voltage regardless of speaker tap, you need to connect the NFB to a consistent voltage source at the transformer end too.

And I don't doubt that the amp is loud. Maybe since it works acceptably WRT hum without the 100R in place (or with a much higher value in place) the 100R isn't being installed correctly? Because, really, nothing about installing that resistor should make the amp louder. Without it the NBF goes straight to ground so the amp should be louder without the 100R and adding it should make the amp quieter. Replacing the 100R with 4.7k would probably cause weird distortions at useful volume. What's more is that there's not much about that resistor's function that should cause any hum.

Can you explain how you have been adding and removing the 100R resistor? It is supposed to be in series with the PI tail going to ground. So if you just remove it (or forget it?) the PI tail resistor isn't grounded and very little (if any) current will flow through the PI.

[J Luth]

Chuck, escherton

With the 820 lifted and the 100 connected the hum may be a little louder.

I rechecked the ACV on pin 7 and was still getting 114 VAC. I tried a different DVM and got 43 VAC and 32 VDC.

On the filaments I am getting 2.8 VAC on the heater pins on all tubes, and that is with both meters.

Maybe there is something wrong on the AC side of the meter with the higher reading.

[J Luth]

Chuck

I will try moving the wire to the 4 ohm tap.

With regards to the 100R if you look at the attached layout, the 100R is shown as not installed. When installed it is connected as shown with the other end connected to where the double 25/25 caps are grounded.

The image is what it looks like looking down into the bottom of the chassis.

[Chuck H]

It occurs to me that without the 100R in place, that is, simply removed, that the PI would find a DC path through the the 820R and speaker. I would expect some hum operating like this though. And the circuit would be operating with 100% NFB (well, maybe not really because of impedance anomalies, but pretty close).

We may need gut shots that clearly show the wiring before we're done.

[J Luth]

Back again.

I moved the wire to the 4 ohm tap. The volume is not as touchy as it was with it connected to the speaker jack. It is louder than with it connected to the sparker jack but not as much as the other way. The hum is still there did not change much if any.

[Chuck H]

Can you do something for me? Go ahead and set the amp up as it was working before. That is, with the 820R FB resistor in place and the 100R removed (don't install the 4.7k). Does the amp perform as you know it to? Now, lift the 820R. What happens to the sound?

[J Luth]

Chuck

I posted the ACV readings and the filament readings above.

I also posted a layout drawing of the inside of the amp. See the two post by me above yours.

With regards to the 100R, with it not in there the amp is dead quite. I have used the amp at a lot of gig's for over a year without the resistor and have never had any hum which seams weird. I would think just as you stated that the 100R it should be in there.

I'm still a rookie and still learning.

[J Luth]

Chuch

Just saw your post. I never ran the amp with the 4.7K I just tried it to see if it made a difference then took it out. Over the past year of running the amp I never had the 100R installed.

I think I already tried the amp wired as before with the 820R lifted and had no sound but I will double check.

[Chuck H]

Please indulge me and follow the instructions in post #26

[J Luth]

Chuck,

In case you didn't see it I have never used the 4.7K just tried it to see what happens. The amp has been run with no 100R and the 820 connected to the speaker jack for over a year. It always sounded real good and not to loud.


As you asked.

Wired as before no 100R and 820R to speaker jack amp works as it always did.
No 100R, 820R lifted but the other end connected to either the speaker jack or the 4 ohm tap, no output until the volume is close to 3/4 up and then there is very little output and very distorted. Also little to no hum

I appreciate all the help and hopefully can track this down.