EQP 1-A tube program (Analogvibes project) Help needed

[Takroc]

Hi everyone, I would like some help. I'm working on my EQP1-A, and I'm getting feedback. If you check my earlier post, this is the original issue I had a while ago. I have checked all the solder joints, confirmed the components and matched the position of everything compared to the other unit, which works just fine. Would you happen to have any ideas on where to look for the issue? I am going to check the ground in the chassis. You can hear the oscillation if you watch the attached video with sound.

any help is much appreciated,

Paul

 

[thomasdf]

Interesting

Sounds like a capacitance problem and or power supply oscillating. Did you check your grounds (clean connections and no loop) and shielding of the transformers ?

Do you have an oscilloscope ?

 

[Takroc]

Interesting

Sounds like a capacitance problem and or power supply oscillating. Did you check your grounds (clean connections and no loop) and shielding of the transformers ?

Do you have an oscilloscope ?

I don't have access to an oscilloscope, but I did check the grounding. I found the full chassis was not grounded; it is now, and I still have the same problem.

 

[Takroc]

Interesting

Sounds like a capacitance problem and or power supply oscillating. Did you check your grounds (clean connections and no loop) and shielding of the transformers ?

Do you have an oscilloscope ?

How do you check for "no loop?" The grounds are the same as the unit that is working, and I checked the solder joints and connections.

 

[thomasdf]

Be sure you have no ground loop and the correct impedance at the input and output of the unit.
I don’t know how one can measure ground loop. I just sit and watch / analyse the grounding to avoid these.

Try using tidier wiring and shielded cables for signal (pay attention to ground loops here also, with the shields)
Are the transformers and inductors in shielded and grounded enclosures ?

Are the voltages correct across the circuit ?

I suppose Analog Vibes can also provide customer support ?

Noisy Pultecs builds have been discussed on the forum in the past, a search should come up with answers

 

[Takroc]

Be sure you have no ground loop and the correct impedance at the input and output of the unit.
I don’t know how one can measure ground loop. I just sit and watch / analyse the grounding to avoid these.

Try using tidier wiring and shielded cables for signal (pay attention to ground loops here also, with the shields)
Are the transformers and inductors in shielded and grounded enclosures ?

Are the voltages correct across the circuit ?

I suppose Analog Vibes can also provide customer support ?

Noisy Pultecs builds have been discussed on the forum in the past, a search should come up with answers

Hey thanks for the information. The wiring is the same as the other unit so for now I’m assuming it’s ok. I don’t think Analog vibes much customer service so I will have to continue with the forums. Voltages we’re good but I’m going to re check them.

 

[ruffrecords]

Can you post the schematic of the gain make up stage? This is definitely a very high frequency oscillation. Could be due to poor layout or incorrectly applied negative feedback.

Cheers

Ian

 

[Hee Jung Park]

It is expected that there will be more NFBs connected with 12AX7 passing a 1kohm resistor across the output transformer NFB coil neutral point. Therefore, first try cutting the connecting wire with NFB. If so, once the oscillation stops, let's take a look at the circuitry involved

 

[Takroc]

Can you post the schematic of the gain make up stage? This is definitely a very high frequency oscillation. Could be due to poor layout or incorrectly applied negative feedback.

Cheers

Ian

Thanks for taking a look Ian

 

[Takroc]

It is expected that there will be more NFBs connected with 12AX7 passing a 1kohm resistor across the output transformer NFB coil neutral point. Therefore, first try cutting the connecting wire with NFB. If so, once the oscillation stops, let's take a look at the circuitry involved

Hi, I'm not sure what you are suggesting to cut. The 1K resistor that connects the tertiary coil to the ground connection? The violet and red wires are together that connect to the 1K resistor and then ground. See attached pic of the output transformer.

 

[Takroc]

Hi everyone,
Here is an update on my issue. As recommended by Hee Junk Park, I removed the 1K resistor from the ground connection from the output transformer, and the oscillation stopped. I played some music and a test tone but could hardly hear it, even with a gain boost. This connection is the tertiary center tap. Not sure what to do next. Any advice would be greatly appreciated.

 

[ruffrecords]

Attached is the original EQP1A amplifier section. The output transformer is on the right and you can see the 1K resistor. The other two connections on that winding are the negative feedback. Disconnecting the 1K means the is no connection from the cathodes of the 12AX7 input tube to ground so there is no gain which is why you hear. What I suggest is you reconnect the 1K but also reverse the connections of the negative feedback winding nothing which is the yellow and orange wires connected to the tag strip near the 12AX7.

Cheers

Ian

 

[Takroc]

Attached is the original EQP1A amplifier section. The output transformer is on the right and you can see the 1K resistor. The other two connections on that winding are the negative feedback. Disconnecting the 1K means the is no connection from the cathodes of the 12AX7 input tube to ground so there is no gain which is why you hear. What I suggest is you reconnect the 1K but also reverse the connections of the negative feedback winding nothing which is the yellow and orange wires connected to the tag strip near the 12AX7.

Cheers

Ian

Thanks Ian I will try that tonight when I get home from work. Another suggestion that someone gave me was to reverse the connections on pin 1 and 6 of the 12AU7 tube. Should I try that after your suggestion if the issue isn’t resolved?

 

[ruffrecords]

Thanks Ian I will try that tonight when I get home from work. Another suggestion that someone gave me was to reverse the connections on pin 1 and 6 of the 12AU7 tube. Should I try that after your suggestion if the issue isn’t resolved?

It is effectively the same thing so you could do either but not both!

Cheers

Ian

 

[Takroc]

It is effectively the same thing so you could do either but not both!

Cheers

Ian

Hi Ian,
So I reversed the connections like you suggested and now I have a very different oscillation noise. See the attached video with the sound to listen. Hopefully, you can offer more guidance. I'm also attaching my voltage check pic. There is a difference between pin #1 and Pin# 6 on the 12AX7. Not sure if that is significant or not.

 

[beatnik]

Since you have a working unit, have you tried swapping the tubes ?

Have you ensured the terminal strips that are used as a ground point have a good connection to chassis ?
Before painting the chassis manufacturer should mask the holes that are used as a ground connection, if that has not been done you should consider using tooth washers between the grounding lug and the chassis, or even better removing paint around the hole.
Maybe it isn't the cause of this particular issue, but it is very important practice in builds that have multiple ground connections to the chassis and in my experience the vast majority of diy chassis manufacturers don't bother with masking the holes.

Another advice, it's good practice to twist the wires from transformers and other signal wiring as you would do with AC tube filaments, it helps to reject noises.

 

[Takroc]

Since you have a working unit, have you tried swapping the tubes ?

Have you ensured the terminal strips that are used as a ground point have a good connection to chassis ?
Before painting the chassis manufacturer should mask the holes that are used as a ground connection, if that has not been done you should consider using tooth washers between the grounding lug and the chassis, or even better removing paint around the hole.
Maybe it isn't the cause of this particular issue, but it is very important practice in builds that have multiple ground connections to the chassis and in my experience the vast majority of diy chassis manufacturers don't bother with masking the holes.

Another advice, it's good practice to twist the wires from transformers and other signal wiring as you would do with AC tube filaments, it helps to reject noises.

Yes I have exchanged the tubes and the feedback does not resolve so I am assuming the tubes are ok. If you look at my latest post I swapped the two wires Ian suggested and I get a very different feedback sound. Plus the voltages do not match at pin 1&6 on the 12AX7 tube.

 

[ruffrecords]

OK, I listened to your second video/sound cllip. As you say, the high pitched oscillation has gone. What you have now sound to me like "motor-boating". This occurs when the negative feedback stability breaks down at low frequencies. It is often caused by too little or poorly implemented HT decoupling. So you need to make sure all the electrolytics are the right values, fitted the right way round and securely connect to 0V.

The other thing that concerns me is that swapping the leads changes the voltages on the two anodes of the 12AX7 are not the same. Can you post exactly what these voltages are and also post the voltages on pins 3 and 8. It is possible the motorboating is affecting the dc readings so it would be an idea to measure the 12AX7 voltages with the 12AU7 tube removed.

Cheers

Ian

 

[Takroc]

OK, I listened to your second video/sound cllip. As you say, the high pitched oscillation has gone. What you have now sound to me like "motor-boating". This occurs when the negative feedback stability breaks down at low frequencies. It is often caused by too little or poorly implemented HT decoupling. So you need to make sure all the electrolytics are the right values, fitted the right way round and securely connect to 0V.

The other thing that concerns me is that swapping the leads changes the voltages on the two anodes of the 12AX7 are not the same. Can you post exactly what these voltages are and also post the voltages on pins 3 and 8. It is possible the motorboating is affecting the dc readings so it would be an idea to measure the 12AX7 voltages with the 12AU7 tube removed.

Cheers

Ian

Hi Ian,

Success!! I was checking the capacitor between pins 1 and 6 on the 12AX7 tube to see if it was shorted or burnt out, and I noticed that the label was facing away. I checked on the unit that works, and the label was facing me. I assume this cap is unpolarized, but I reversed it anyway (grasping at straws at this point), and the feedback is gone, and it works perfectly. I don't understand why, but I'm good with it. Can you offer some insight? Just curious now...

But thank you for all of your advice; very much appreciated.

 

[ruffrecords]

I am pleased you got it going. That capacitor is not a polarised type so it should not matter which way round it is fitted. Perhaps one of the joints was not good. That capacitor is part of the circuit that tames the high frequency response so it will make a difference to stability. On the other hand, even some unpoloarised capacitors do have a 'preferred' orientation which is a function of the way they are constructed.

I should point out that your current construction is not ideal from the point of view of ensuring stability. Particularly the cables from the output transformer - you should aim for it to look more like the picture yo posted with leads cut to length and dressed appropriately. I made exactly the same mistake on the first tube audio amplifier I built back in the 60s. It squealed like a pig.

Cheers

Ian