First question on my build

[eschertron]

I took a peek at your facebook pics. Amazing work! Thanks for sharing

[pdf64]

Adding a dc blocking cap in series with one of the probes should set things straight. 0.1uF should suffice as the meter input resistance should be high.

[Chuck H]

Adding a dc blocking cap in series with one of the probes should set things straight. 0.1uF should suffice as the meter input resistance should be high.

That's the first thing that occurred to me. I was just debating whether to open up my cheapie and try to fit that cap into the circuit when switched to AC... I think not.

[Dave H]

That's the first thing that occurred to me. I was just debating whether to open up my cheapie and try to fit that cap into the circuit when switched to AC... I think not.

Try it first. I don't think it will work (because of the half wave rectification).

[Dave 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).

I'm surprised it isn't oscillating with getting on for 100% NFB. If the 820R is also disconnected there's no path for the PI tail current so it won't work like that. The gain with the 100R connected is 9 times the gain without it. That's nearly 20dB more gain with the 100R, no wonder the volume comes up quickly.

OP, I've just noticed that the 220k mixing resistor from the Normal channel to the PI input is missing on your schematic. If you add 220k from PI input to ground the gain will be reduced by 6dB. If that's not enough reduce its value. 27k would give it about the same gain as removing the 100R.

[J Luth]

I suspected the meters may be a problem.

Where the amp stands today

Moved the red yellow wire from the PT directly to the ground of the first filter cap, moved the lead from the 820R to the 4 ohm tap of the OT, installed the 100R and connected it to the ground of the speaker jack. The amp has minimal hum but that could be where I am doing the work, I tend to hear some hum on all amps when in the work area. Weird how the 100R is OK connected to the speaker jack ground but when connected to the 2 - 25/25 ground or to the star ground the amp hums loud and volume is real loud. I'll see what happens a bit later, have bring it upstairs and see what happens. The volume control may be working a bit different, appears as though it has to be turned up a bit more to get to the same volume level as before I stated doing the work. That may not necessarily be a bad thing.

Not sure if this is the correct fix, please let me know.

[J Luth]

Thanks eschertron for the comment about the bass page

[J Luth]

Put the amp back in the cabinet and brought it upstairs. Very minimal hum and volume appears to be OK.

[J Luth]

Dave H

Are you referring to the connection where there are normally 2 - 220K resistors connected to the 500PF capacitor which connects to pin 2 of the PI? If so, I do have a single 220K there. This is a single channel amp and I only put 1 resistor there for that reason. If you are referring to something else or if I should have an additional 220K from the existing 220K connection to ground please let me know.

[Dave H]

Dave H

Are you referring to the connection where there are normally 2 - 220K resistors connected to the 500PF capacitor which connects to pin 2 of the PI? If so, I do have a single 220K there. This is a single channel amp and I only put 1 resistor there for that reason. If you are referring to something else or if I should have an additional 220K from the existing 220K connection to ground please let me know.

If you want your single bass channel amp to have the same gain as the two channel amp's bass channel then you need the 220k to ground otherwise it will have twice the gain of the original two channel circuit.

[J Luth]

Dave H

I will give it a try to see what it does. The amp doesn't appear to have a lot of gain without it but I will try it.

[J Luth]

I may have spoken too soon about the amp appearing to be OK. I just rechecked and I had connected the lead from the 100R to the wrong terminal of the speaker jack. I had it connected to the the tip terminal where the output from the selector switch is connected rather than the ground. That's what I get for doing it before I was completely awake. DOH! I moved it to the ground terminal and am back to square one, loud volume and loud hum.

[J Luth]

Dave H

If I understood you correctly, I added a 220K to the where the 220K connects to the 500PF cap like it is on the Fender layout. Connected the other end to ground. When the amp is turned on it squeals, when turned off it sounds like it is connected to a tone sweep, it goes from a low squeal to a high squeal as it powers down. Maybe I misunderstood what you were stating.

[Dave H]

I may have spoken too soon about the amp appearing to be OK. I just rechecked and I had connected the lead from the 100R to the wrong terminal of the speaker jack. I had it connected to the the tip terminal

I suspected that when you said it doesn't appear to have a lot of gain with the 100R connected at the speaker jack. Connecting it to the tip is like not having the 100R fitted. It has three gain stages before the PI. That's a lot of (too much?) gain for a bass amp. It is going to be sensitive to hum and noise. As a test try changing the added 220k to ground to 27k.

In your later post are you saying the turn on/off instability is worse with the extra 220k to ground? That shouldn't be the case.

[J Luth]

Dave
It goes from bad to worse.

I tried the 220K connected to where the 220K is connected to the 500PF cap and the other end to ground and as I stated the amp screeches and when turned off the noise goes from a low squeal to a high pitched squeal. Now with the that second 220K removed and the 100R not connected the amp squeals a bit and there is a lot of distortion at low volume all the way up. It appears the distortion does not start until completely warned up. With the 100R connected to ground lug of the speaker jack it is loud and distorted, it was only loud before and no distortion. With the 100R connected to the other side of the speaker jack which would be the tip it is not loud and does not appear to be distorted but I need to check further. I ran out of time for a bit. Also while it was distorting I change the selector between 4, 8 and 16 ohms, it only appeared to distort it at 8 and 16. I went through and checked all resistors and caps quickly and they appear to be OK. Hopefully the 220K din't mess up the OT. I will check further later.