First question on my build

[Enzo]

OK, I'll believe you, all I saw was continuity to ground from some 25uf caps.

[J Luth]

I'll try to get some shots in a day or so.

[J Luth]

No wire no 100r. Without it the amp is dead quite withe the 100r it hums.

[Chuck H]

No wire no 100r. Without it the amp is dead quite withe the 100r it hums.

But that doesn't tell the whole story. Also, and correct me if I have this wrong, the amp is louder with the 100R in place. And if you lift the 820R resistor in the feedback circuit WITHOUT the 100R in place the amp goes almost dead. These two things support what Enzo just posted. And Enzo's observation is the same as mine in post #24 (page one) and you proved it out with testing in post #30.

I think the amp is simply higher gain than you suspect AND there is a hum issue that is more apparent when the amp is operating at it's full gain. The thing that is confusing you, I think, is that you perceive the amp to be operating more correctly with the circuit intentionally miswired. If you want the amp to work "correctly" you will need to wire it correctly and move forward with trouble shooting from there. Taking out or omitting the 100R resistor fixes nothing. It doesn't solve the hum problem, that is elsewhere. If the amp has too much gain for you that can also be changed. But you have been happy with the amp so far and this is keeping you from understanding how incorrect the amps circuit and behavior actually is.

Gut shots please. A few good, clear ones. But only if you can stand to change the amp. It won't sound or behave the same once it's fixed.

[Enzo]

Yes, lacking either a wire or a 100R resistor to ground, the phase inverter tail has no ground connection. Imagine leaving out the 1.5k cathode resistor of the input stage. Add the 100R or a wire, I don't doubt it gets louder, it has a complete circuit then. I surmise it was dead quiet without the resistor because the gain through the stage was so low. The branch path to ground via the feedback network is not enough.

[J Luth]

But that doesn't tell the whole story. Also, and correct me if I have this wrong, the amp is louder with the 100R in place. And if you lift the 820R resistor in the feedback circuit WITHOUT the 100R in place the amp goes almost dead. These two things support what Enzo just posted. And Enzo's observation is the same as mine in post #24 (page one) and you proved it out with testing in post #30.


Gut shots please. A few good, clear ones. But only if you can stand to change the amp. It won't sound or behave the same once it's fixed.

With the 100R it is louder, with the 100R not connected and the 820R lifted the amp is just about dead. If I turn the signal generator or the amp way up you can then hear the tone.

I have audio files of what I am talking about but not sure if I can upload them without them being video.

That's why I am here. I know that something is wrong and would like to track it down. Like I sated before, I'm a rookie at this and trying to learn.

I have a lot of gut shots that I am going to try and upload.

[J Luth]

I created an album of amp pictures, link below. There are a lot of pictures of the amp figured an album would be easier than having them in the thread.
Hopefully there are enough and detailed enough to figure this out.

This is my first build, it was a bit cleaner before I started trying different components. I need to get in there and clean it up a bit, it's a bit sloppy right now. And yes all the solder spatter has been cleaned up. I still need to fix a few wires I hit with the iron.

They are a bit large, I left them that way figuring it would be easier to see everything.

In the pictures you will see the 100R connected to the board, the other end is not connected. it is in there to show where I have it connected. The yellow wire that runs under the main board is connected to the volume control. The shielded wire running under the board is connected to the input jack. Also in the pictures it shows an 82K going from the 1uf caps to the first PF cap, that is not in there, it is back to a 100K.

http://music-electronics-forum.com/u...587-albums246/

[J Luth]

Photo album cerated, see link in my post.

[J Luth]

Chuck,

I don't mind changing things. It's all about learning.

[Chuck H]

Aight... Let's start.

I haven't spent a lot of time on it, but here's what I found so far. The 100R resistor thing looks to be just what I expected. Let's fix a miswire I spotted first, test it, and then try the 100R back in the circuit after. That is, I'd like for you to correct the miswire and try the amp as it is without reconnecting the 100R BEFORE reconnecting the 100R. In the red circle below there is a red line/dash. That dash is a jumper connecting those two eyelets.

EDIT: Looking at the AA864 layout it show a connection between these eyelets under the board. If you've done this then, obviously, adding a jumper is redundant.

EDIT: Those are your 1R cathode sense resistors. Nevermind

Hear from you soon.

[J Luth]

Chuck, my apologies.

There is a connection between the resistors and the 2 25/25 caps. It's under the board. Sorry about not putting in the thread.

Probably shouldn't say this because it will probably change after I do but here goes.

I may be on to something. I grounded the 820R, I also installed the second 220K in the phase inverter circuit that Dave H suggested in post #50. I only had one there due to it being a single channel amp. Having the second 220K to ground appears to change the gain of the amp slightly but not in a bad way. The amp was humming but just for the heck of it I turned the bias pot slightly to see if it changed the hum much. A very slight turn and just about all the hum disappears. So I tried turning it a little bit at a time in both directions and the hum comes and goes but it is not smooth, it just abruptly stops turn a little more and it comes back. But it is one area of the pot, if I turn it past that area in one direction or the other there is little to no hum all the way down to the end. When I turn the other way very slight hum increase but you really have to listen. When I get to the spot in the pot the hum jumps out, turn slightly and it just about disappears, continue turning and the hum comes back and slowly increases but I assume that is normal because of the bias voltage change.
Once past the spot where the hum jumps out I can vary the amount smoothly in either direction. Just kind of weird that the hum would increase like it did with the 100R installed and now it appears not to with the pot moved. I will be checking the 100R connected and disconnected for hum again a bit latter.

Also just to try, I mounted a second 47uf 160V capacitor in parallel in the bias circuit mounted on the rectifier board.

I reckon I'll replace the pot when I get a chance hopefully that is the problem but not sure yet..

I will keep looking and trying things if you have any suggestions.

Where I have the pot now there is little to no hum, as the amp warm up there is what I would call a normal amount of hum and virtually gone once the tubes are warmed up.

The current voltage readings are:
Pin 3 419
Pin 5 -49
At the 1R resistor -31.8

[Chuck H]

That's biased pretty cold. Not a problem at all if you don't hear a problem. I'll take a look at the bias circuit to see what I can see and check for anything that might cause the output tubes to imbalance at higher bias levels. How much use have the tubes had?

On the issue of the voltage divider to the PI. Dave is correct that there's a voltage division in the signal via the plate loads at the stage feeding the PI. But with your arrangement if you just tack a resistor to ground you are connecting HV to ground because the division happens before the coupling cap (which blocks DC). It's a high enough resistance that it won't damage anything right now, but it's not ideal so we should change it. You can't simply move the voltage divider to the other side of the coupling cap because the PI bias must remain isolated from 0V via the coupling cap. The first solution would be then to sandwich the voltage divider between two coupling caps. But we don't need to do that. Dave recognized that there would be a voltage division because the HV supply has a very low AC impedance. So... You don't need to take the second 220k resistor to ground. You can take it back to the HV node for the same signal division and avoid shunting HV to ground here. Details are in the schematics below. The top one would be the circuit you have now. The red line shows the HV path to ground. The bottom one is an alternative that doesn't shunt HV to ground and doesn't require an extra coupling cap. Just move the second 220k resistor from terminating at ground, instead to the HV rail side of the plate load resistor for the stage feeding the PI. This is actually exactly what's happening in the Bassman.

[J Luth]

OK,I'll take a look at the schematic you posted and see if I can understand what you are saying.

The power tubes are a year to a year and a half old. Not used all that much for about the first 3 month's or so. But between to 2 - 4 gigs a month after that up till now.

[J Luth]

I see that you stated that the amp is biased pretty cold. Do you have a suggestion for voltages that I can try to see what happens?

[J Luth]

Not sure if I understand you on the 220K resistor.
Are you saying to connect it to the eyelet on the board that is marked A in the picture I uploaded. It's where the red wire coming over from the preamp side is connected.