First question on my build

[J Luth]

I did, see above

[J Luth]

Everything you ask I will gladly try.

[J Luth]

eschertron

You mentioned the red yellow wire should be as close as possible to the first filter cap. Currently that wire is connected to the star ground which is about an inch away from the trans but is connected to the chassis. There is a ground wire from teh start to the first filter cap. Would you suggest that I move the red yellow wire directly to the first filter cap ground end?

[eschertron]

eschertron

You mentioned the red yellow wire should be as close as possible to the first filter cap. Currently that wire is connected to the star ground which is about an inch away from the trans but is connected to the chassis. There is a ground wire from teh start to the first filter cap. Would you suggest that I move the red yellow wire directly to the first filter cap ground end?

That's one thing I've learned here on MEF, quite a bit of theory and conventional wisdom behind ground paths. Getting the PT as close to the reservoir cap as possible (physically and electrically) is really high on the list. Try it and see. It may not be the source of your hum, but it's not likely to make it any worse.

[Devil's Advocate] If you like your amp as it was, then why change it? Making it closer to stock may require a substantial investment of time and troubleshooting. Consider the pros and cons. The PI's ground via the OT secondary (as Chuck reasoned above) is not as noisy as the ground through the 100R resistor. Maybe leave the 100R resistor off? [/Devil's Advocate]

Which makes me ask: Where does the 100R reach ground? Maybe by moving it's ground reference around the circuit you can find a 'quiet spot' to land it. How far away is the PI from the OT secondary? If the output jack's ground connection is dead quiet, can you land the 100R resistor there and get the same effect?

edit: On the layout, the 100R simply has an arrow -> GND but no indication of where it lands, is this correct?

[Chuck H]

Which makes me ask: Where does the 100R reach ground?

Indeed! I was thinking the same thing since installing it injects a "S" ton of hum and seems to put AC on the circuit!?! But I digress... And I agree that the amp can be used as it is. Unusual, maybe.?. But it's not hurting anyone.

Clear photos that show the wiring might reveal what's hinky about the 100R situation. But even then it's possible that the amp working in a fully stock configuration could be perceived as too powerful? So there ya go. I'd fix it on principal, but that has nothing to do with tone or music.

EDIT: I did notice that the filament circuit looks a bit low though. At 2.8 per side that's only 5.6V on the filaments, which is out of spec. and probably indicative of an over current condition. Maybe that added fan circuit? Those fans don't pull much current. As long as it doesn't overheat I suppose.

[J Luth]

escherton

I may try moving the wire and see what happens.

[Devils Advocate] I have thought about that but I don't mind spending the time and trouble shooting. Plus would like to find out why it's doing it It's a good learning process. Hell at my age I don't have much else to do. I like proving you can teach an old dog new tricks, and I am old but still going and going. Hell even started playing in bands again which I thought I would never do. [/Devils Advocate]

With regards to the layout drawing it is drawn as if you were looking in the underside of the amp and is to scale. The 100R when grounded would be connected to the same place as the2 - 25/25 caps are grounded. I also tried connecting it direct to it's own chassis ground and to the star ground, no change.

The OT is mounted on the top side of the amp above the main filter cap board, behind the preamp tubes and close to centered on the second 12AX7. The location on the layout where it shows the OT wires coming back in under the filter cap board and going to the selector is wrong. They actually come out to the left of the filter cap board and closer to the back of the amp then to the selector switch. That side of the OT is about 2 or 3 inches or more away from the PI. It is also turned so that the bells face the PT.

[J Luth]

Chuck
I rechecked the filament and got 3 V, I don't trust my meters to be 100% accurate. On the layout the 100R would be connected to the same location as the 2 - 25/25 caps. I put that in the layout after I realized I forgot it and was not sure where to connect it. I can try and get some photo's of the wiring if it might help.

As you state it works the way it was and isn't hurting but as you stated it the principal that it's not correct.

[Chuck H]

As you state it works the way it was and isn't hurting but as you stated it the principal that it's not correct.

Well we're here however you want to handle it. I just wouldn't want you to end up unhappy for your efforts. So, onward then...

Test the junction of the two 25/25 caps where you were connecting the 100R for continuity to the chassis. Obviously it should be zero ohms.

[J Luth]

Chuck,

No problem on my end, I would not end up unhappy, I love a challenge. At my age it would take a lot more than this to bother me plus it is a learning experience. I have always strive to learn as much as I can with things I am interested in and I generally don't give up. I just don't want to become a pain in the butt with this. I'll keep trying things until everyone else gives up.

0 ohm's at the 25/25 connection to chassis.

[Chuck H]

Okey dokey then. It's pretty clear to me by now that the 100R resistor is supposed to be there. Yes the amp starts to break up on 2 with a modern bass. Maybe that's normal, maybe not. Maybe some extra gain is related to the hum? I'm still inclined to think that regardless of the inconsistency between meters that it can't be ignored that they are all measuring some AC on pin 7 of the PI. How about measuring pin 7 pin the PI without the 100R installed.?. Any AC?

I guess I should ask what sort of meters you're using. Are these old Radio Shack needle meters or DMM types? I happen to have an amp on the bench right now with a long tailed pair PI. Maybe I'll break out one of my cheapies and see if it reads any AC there. My Fluke doesn't. I know that because I checked that earlier to see if there was anything strange that I wasn't aware of.

[J Luth]

I will check again tomorrow. My day wont end for another 3 hr's, 3AM my time but the wife has gone to bed and I don't want to wake her, I work right below the bedroom.

As far as the bass I use it is new but not what would be considered modern. I make all my own instruments not from parts, I start with raw lumber. I wind the pickups make the bridges and whatever else I can. I mostly use P Bass and J Bass styles due to the style of music I am playing lately and at the request of the band leader. My pickups are a bit hotter than stock Fender but not as much as an active set would be. Here's a link to some of what I make if you want to take look. https://www.facebook.com/Bass-Guitar...1862644892324/ it's public so you don't need a facebook account. There is also a shot of the front of the amp.

The multi meters are digital and are Extech MN35 and Velleman DVM850BL. At some point I will pick up a Fluke but it wont be for while, I need to wait for the MIT Flea Market to start up in the spring.

[J Luth]

Also forgot to list that I have a very old Variac, no meter on it just a dial for voltage.

[Dave H]

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.

It will be the meter. Cheap meters are half wave rectified and don't have an isolating capacitor so they can read twice the DC voltage on the AC range. They only work on true AC (e.g. mains voltage) with no DC component. A good way to check is to measure a 9V battery on the AC range. If the meter is half wave rectified without a capacitor it will measure well over 9V in one direction and zero with the meter leads reversed. Also there is no compensation for the diode voltage drop so low AC voltages read low.

[J M Fahey]

Even worse, it will read some 2.25X of the actual value.
Meaning a 9V battery will read over "20VAC" and those "141VAC" you measured at the cathodes probably mean some real 50 or 60 VDC .
Even easier: the ubiquitous $10 meter with only 2 VAC scales, typically 200VAC and a usually 750VAC:

belongs definitely to the "wrong AC" class.
AC scales can only be trusted to measure wall voltage and straight transformer taps (such as secondary and filament voltage) but nothing else.
Ripple in a supply cap? ... forget it.

[Chuck H]

Thanks gents. I learned something today. And it explains a few times I've been stuck with a cheap meter (I keep a cheapie in my work truck) and thought "What's wrong with this thing?"