I am here to seek assistance regarding the construction of Pultec EQP

[winner]

Hello everyone, I am using this schematic to build an EQP, but I found that when ECC82's 6 is connected to the transformer winding, it will produce noise similar to that of a tractor, which is very regular and very loud

When I disconnected this winding, it could work normally, but the frequency response of its amplification circuit was very abnormal, and it began to decrease significantly after 2K

From the test results, it seems to be caused by a change in phase

I have uploaded the relevant pictures. Thank you for your help

 

[Brian Roth]

Perhaps the negative feedback resistors R11 and R12 are "swapped" and not connected to the correct transformer windings?

Bri

 

[winner]

也许负反馈电阻R11和R12被“交换”，没有连接到正确的Transformer绕组？

BRI

Thank you for your reply, dear BRI

I have tried what you said. After swapping, it no longer works and the continuous noise. When I switched it back and disconnected the 6 pins of ECC82, it returned to the way I described it

 

[ruffrecords]

First you should try removing R11 and R12, then connect ECC82 pin 6 and see if it works. It may have too much gain but it should work. If it does not then there is something else wrong

Cheers

Ian

 

[ruffrecords]

Unfortunately that did not get translated into English. But the first part of your reply says:

"I feel like I'm one step closer to the truth. As you said, when I disconnect R11 R12 and connect pin 6, the whole device is muted, it no longer makes any sound. ECC 83 pin 1 and 7 nominal voltage 140v becomes 256v"

OK, this is because t=we disconnected the cathode connection to the ECC83. The next step is:

Disconnect R11 and R12 from the transformer.
Disconnect R13 (1K) from the transformer
Connect the free ends of R11, R12 and R13 together.

You should now get some signal through the amplifier

Cheers

Ian

 

[winner]

Unfortunately that did not get translated into English. But the first part of your reply says:

"I feel like I'm one step closer to the truth. As you said, when I disconnect R11 R12 and connect pin 6, the whole device is muted, it no longer makes any sound. ECC 83 pin 1 and 7 nominal voltage 140v becomes 256v"

OK, this is because t=we disconnected the cathode connection to the ECC83. The next step is:

Disconnect R11 and R12 from the transformer.
Disconnect R13 (1K) from the transformer
Connect the free ends of R11, R12 and R13 together.

You should now get some signal through the amplifier

Cheers

Ian

I feel like I'm one step closer to the truth. As you said, when I disconnected R11 R12 and connected pin 6, the entire device was silent and it no longer made any sound. The nominal voltage of pins 1 and 7 of ECC 83 has changed from 140v to 256v

I'm not sure if this is related to the power supply part, as I don't have a high-voltage power Transformer, so I used a rectifier bridge instead of a 6X4 in the high-voltage part. The voltage I received is almost normal. To be honest, I don't quite understand the purpose of R5 and C7 in the picture, and I'm not sure if it's because of them. The attachment is a schematic diagram of the power supply section

 

[winner]

Unfortunately that did not get translated into English. But the first part of your reply says:

"I feel like I'm one step closer to the truth. As you said, when I disconnect R11 R12 and connect pin 6, the whole device is muted, it no longer makes any sound. ECC 83 pin 1 and 7 nominal voltage 140v becomes 256v"

OK, this is because t=we disconnected the cathode connection to the ECC83. The next step is:

Disconnect R11 and R12 from the transformer.
Disconnect R13 (1K) from the transformer
Connect the free ends of R11, R12 and R13 together.

You should now get some signal through the amplifier

Cheers

Ian

I'm sorry, I have edited them again

I followed your instructions (without using the feedback winding of the transformer, connect R11 and R12 directly to pins 3 and 7 of ECC83, and then connect the other end of the two resistors to GND), and it emits a huge gain sound. When I turned down the signal source to -40dB, it sounds better

I tested the current frequency response using REW, and it slowly dropped from 5k to 20k, resulting in a loss of 2dB. I believe this is a good phenomenon compared to before

At present, I think it may be due to the inappropriate feedback resistance value in the feedback winding of the transformer, but I will not calculate it. Can you continue to help me? Thank you very much

 

[ruffrecords]

The fact you now have huge gain is exactly what is expected. This confirms the circuit is basically OK but the negative feedback needs to be corrected.

As someone pointed out at the beginning, the most common cause of problems with this circuit it the polarity of the feedback. If the polarity is wrong it will oscillate as it did at the beginning. So now you need to reinstate the feedback from the transformer. First you need to check the output transformer wiring. From the 9530 data sheet it looks like you should connect the violet and red wires together to the 1K resistor and then to ground. R11 and R12 should be connected to the yellow and orange wires respectively. If this oscillates then swap the yellow and orange wires.

Cheers

Ian

 

[winner]

The fact you now have huge gain is exactly what is expected. This confirms the circuit is basically OK but the negative feedback needs to be corrected.

As someone pointed out at the beginning, the most common cause of problems with this circuit it the polarity of the feedback. If the polarity is wrong it will oscillate as it did at the beginning. So now you need to reinstate the feedback from the transformer. First you need to check the output transformer wiring. From the 9530 data sheet it looks like you should connect the violet and red wires together to the 1K resistor and then to ground. R11 and R12 should be connected to the yellow and orange wires respectively. If this oscillates then swap the yellow and orange wires.

Cheers

Ian

Thank you for your patient reply
Perhaps the actual situation is much more complex than people imagine, because all the transformers are wound by myself

Based on your guidance, I used a potentiometer to adjust and determine the appropriate value for R11 and R12. Currently, it can work normally, but the high-frequency drop is still significant. As shown in the first frequency test chart, I suspect it is caused by my DIY's "9330", as its inductance is now very large, with 240H for the primary and 890H for the secondary. I don't quite understand the role of a resistor with R2 of 820R, and I don't know if it will affect the entire circuit

In addition, the entire circuit also has a slight 50Hz AC sound, and I am not sure why. Although these are still unresolved issues, they have taught me a lot through them

 

[ruffrecords]

High frequency loss in inductors usually has two causes. The first is leakage inductance and the seconding is winding capacitance. Both depend on the physical construction of the transformer. The primary inductance (240H in your case) mainly determines the limit of the low frequency response. Your transformer should still have an input impedance of more than 30K at 20Hz which I would think is good for this EQ design.

Cheers

Ian

 

[winner]

High frequency loss in inductors usually has two causes. The first is leakage inductance and the seconding is winding capacitance. Both depend on the physical construction of the transformer. The primary inductance (240H in your case) mainly determines the limit of the low frequency response. Your transformer should still have an input impedance of more than 30K at 20Hz which I would think is good for this EQ design.

Cheers

Ian

I have been trying to change the resistance of R11 and R12 to test it

By the way, I have removed R13 directly and grounded the taps of the two windings directly; Because only in this way can the entire gain be the value I truly want,

I just don't understand why the frequency response of the circuit is like this now. By adjusting R11 and R12 to control the gain, if the gain is relatively large, it seems like it has been low-pass filtered, but when the gain is smaller, it is attenuated in a certain frequency band. The following is the graph formed by adjusting R11, R12, or R2 that I don't understand. I think they have a certain relationship, but I can't find the pattern in it.

I guess it's because the inductance or capacitance value of the feedback winding itself and the feedback resistance R11 R12 form an LC or RC filter, but I haven't found a way to calculate it

 

[ruffrecords]

What values are you now using for R11 and R12?

With small values of R11 and R12 you will have high gain. This means there will be very little negative feedback so your frequency response will vary a lot. As you increase the values you get more negative feedback and the response becomes flatter.

Cheers

Ian

 

[winner]

What values are you now using for R11 and R12?

With small values of R11 and R12 you will have high gain. This means there will be very little negative feedback so your frequency response will vary a lot. As you increase the values you get more negative feedback and the response becomes flatter.

Cheers

Ian

Can you see the image I uploaded? The different curves were tested using different R11 and R12, all within the range of 500R-1K, because if these values are exceeded, the gain will be very large (but the high frequency will be cut off and turned into a low-pass filter) or will not work

 

[ruffrecords]

Can you see the image I uploaded? The different curves were tested using different R11 and R12, all within the range of 500R-1K, because if these values are exceeded, the gain will be very large (but the high frequency will be cut off and turned into a low-pass filter) or will not work

Yes I saw the curves but there is no information about the value of R11 and R12 for each one.

Cheers

Ian

 

[winner]

Yes I saw the curves but there is no information about the value of R11 and R12 for each one.

Cheers

Ian

Thank you for your patient response. This is my latest measurement result. I have marked the value of R11 and R12, removed the resistance of R11 in the output transformer, and directly grounded its tap. Only in this way can the waveform look better. However, no matter how I replace R11 and R12, they always attenuate at high frequencies. I feel like I don't know what to do anymore

 

[ruffrecords]

You need to go back to basics. We do not have enough information to work out what is going on. First we need to make sure the EQ circuit is not at fault. So can you measure the frequency response from the input via the EQ to the secondary of T2. You should do this with the power OFF.

Cheers

Ian