Build Thread:PQD2

[Gustav]

Meanwhile I'll ask this again if you don't mind-

Sorry, I dont have any real time frequency response measurements available.

I set my tone generator to square, and read the frequency on my scope.

Gustav

 

[band_master]

Is this with no tubes installed, as your picture would suggest?

If so, consider for your findings, that (I) with no tubes installed is significantly lower than (I) with tubes installed.

Gustav

Correct. I should have searched the thread, but thanks for answering this question again.

I installed the tubes and am getting 234V after the drop resistor now.

 

[antisleep]

For the attentuation not working - try different frequencies. Theres a cap setting the value of each, and if all but one/most/even just one is working, the problem is those caps not making connections.

Maybe the switch is stopped on the wrong side for the frequency select, if its all of them. To check, take out the stop-pole, turn the switch all the way counter clockwise, and insert the stop.

Got this sorted - bypass toggle again. When I desoldered the switch the first time, I must have lifted one of the pads on the pcb. So the switch is working but the trace doesn't. Jumped it and all is well. I'm not proud.

The only thing I see left on these test sweeps is what looks like some noise on the right channel. I've got the power transformers moved as far away as possible now but it remains (along with that tiny bump at 60hz). Curious if I'll be able to hear it.

 

[band_master]

Finally getting around to some serious bench testing...I had an issue where I mis-wired the IO to my patch bay when first trying to pass audio. After sorting that out I was able to get the left channel working. I opened it up and found that if I moved the tube around in the right channel I could get it to go in/out. The top/bottom assembly is a more loose in that socket than the left channel so not an ideal scenario. I switched the drop resistor to 20K so that the high voltage wouldn't exceed 250v in the event that the right channel tube socket craps out but with both channels active these are the power calculations.



The right channel's output is quite a bit louder than the left channel even in bypass mode so I took DC voltages at the tube socket pins to ground.



These all look consistent except for the heater voltages at 4 and 5 but when I measure DC across the pins I am getting 5.7v so that seems expected.

I switched out all cabling across the channels, even the internal wiring from the board to the XLRs and it all points to something on the main board.

Could it be the output transformers aren't matched? Or could the left channel transformers have been damaged while the IO wiring was incorrect?

I swapped the tubes and re-flowed all of the solder joints on the main board before swapping out drop resistors.

Should I take voltages at the output transformer pins next?

BD

 

[Gustav]

Finally getting around to some serious bench testing...I had an issue where I mis-wired the IO to my patch bay when first trying to pass audio. After sorting that out I was able to get the left channel working. I opened it up and found that if I moved the tube around in the right channel I could get it to go in/out. The top/bottom assembly is a more loose in that socket than the left channel so not an ideal scenario. I switched the drop resistor to 20K so that the high voltage wouldn't exceed 250v in the event that the right channel tube socket craps out but with both channels active these are the power calculations.

View attachment 86276

The right channel's output is quite a bit louder than the left channel even in bypass mode so I took DC voltages at the tube socket pins to ground.

View attachment 86277

These all look consistent except for the heater voltages at 4 and 5 but when I measure DC across the pins I am getting 5.7v so that seems expected.

I switched out all cabling across the channels, even the internal wiring from the board to the XLRs and it all points to something on the main board.

Could it be the output transformers aren't matched? Or could the left channel transformers have been damaged while the IO wiring was incorrect?

I swapped the tubes and re-flowed all of the solder joints on the main board before swapping out drop resistors.

Should I take voltages at the output transformer pins next?

BD

1. How much is "quite a bit louder". This could be 0.3dB or 10dB, depending on the person speaking, so its easier to help, if you are specific.

2. Did you try to swap the tubes to see if the discrepancy follows? The raw-tube gain will differ.

3. If its in the 0-2 dB range, I would tell you to replace the last resistor step on the high cut attenuator with a trimmer and adjust.


Gustav

 

[band_master]

1. How much is "quite a bit louder". This could be 0.3dB or 10dB, depending on the person speaking, so its easier to help, if you are specific.

2. Did you try to swap the tubes to see if the discrepancy follows? The raw-tube gain will differ.

3. If its in the 0-2 dB range, I would tell you to replace the last resistor step on the high cut attenuator with a trimmer and adjust.


Gustav

There is a difference of about 12.5dB attached is a pic of a square wave going into each channel on bypass and recorded back into pro tools. I did try swapping the tubes.

 

[Gustav]

There is a difference of about 12.5dB attached is a pic of a square wave going into each channel on bypass and recorded back into pro tools. I did try swapping the tubes.

I would start by bypassing the filters - the 3 pin connector spots going from the main board to the filters - if you disconnect those, and place a jumper from 1 to 3, the signal path will be isolated to the input, gainstage and output.

If the 12dB difference remains, it will be limited to a very small area.

And, try plugging XLR in to XLR out directly on the cables to make sure this is related to the unit, and not something else int he set-up. I dont know how common it is for you to use outboard, but many errors have been traced down to these types of routing problems, a fader set differently in an interface mixer, etc, so best to check

Gustav

 

[band_master]

I would start by bypassing the filters - the 3 pin connector spots going from the main board to the filters - if you disconnect those, and place a jumper from 1 to 3, the signal path will be isolated to the input, gainstage and output.

If the 12dB difference remains, it will be limited to a very small area.

And, try plugging XLR in to XLR out directly on the cables to make sure this is related to the unit, and not something else int he set-up. I dont know how common it is for you to use outboard, but many errors have been traced down to these types of routing problems, a fader set differently in an interface mixer, etc, so best to check

Gustav

I tried plugging the XLRs into each other and I got consistent levels across the channels at -12.43dB.

Then I unplugged the filter section and put a jumper on the outside pins of the connector from the main board to the filter board. I was able to get signal on the left channel at 1.57dB but nothing on the right channel. Even if I wiggle the tube I can't get the right channel to pass signal anymore. I wonder if I damaged the output transformers when I first wired it to the patchbay I think had pins 2 and 3 reversed on the input/output.

These are the voltage readings on the pins when the right channel is out.





When I first took the reading on pin 8 the voltage was higher but dropping fast finally settled around 14VDC.

I ordered some new tube sockets as well.

BD

 

[gyraf]

look for bad solder joints and unstable cabling

output transformers are not easily damaged - and if they are they react by not passing anything

 

[Gustav]

Set your meter to AC, and send that square in - measure VAC left/right after the input transformers, after the gainstage, and after the output transformers.

That will narrow it down some more

Gustav

 

[band_master]

Well, I removed the board and re-mounted it. I went to check those VAC voltages and somehow I've lost my heater voltage completely.

I checked the DC voltage on the input leg of the 317T and it's reading 0.5v the output leg reads 0v DC. The terminal block for the secondaries reads 12VAC so does that mean the rectifier failed? Below is a link to the ones I am using. The rectifier for the high voltage is still working.

https://www.mouser.com/ProductDetail/Vishay/2W10G-E4-51?qs=sGAEpiMZZMuGN2ig0xo6G04AwmIzburs
For measuring the VAC at those locations, should I be measuring across the resistors below?

After the input transfomers: R1 (10K)
After the gain stage: R113 (1K)
After the output transformers: R118 (2K2)

BD

 

[gyraf]

take care - the high voltage gets really high and stays for a long time when there's no heater to make current flow

 

[band_master]

take care - the high voltage gets really high and stays for a long time when there's no heater to make current flow

Thank you. Yes, I did notice that. I left the unit alone for a couple hours, removed the tubes, and turned it back on. The heater voltage is back so I put the tubes back in and now I am able to pass signal on the right channel. The level -1.68dB is consistent with the left channel when the filter board is shorted.

I then put the filter board back in the circuit and in bypass mode I am getting -15.24db in the left channel and -2.62dB in the right channel.

So I guess that points to the filter board for the inconsistent output volume?

I'm getting 13V at the input leg of the 317T now...does that rectifier look up to spec?

I think I am going to replace the tube sockets just to eliminate them as a potential point of intermittent failure. Speaking of...I am using Sylvania 6DJ8 NOS tubes which I read were direct replacements for ECC88s. Do they seem up to spec for this circuit?

https://datasheetspdf.com/datasheet/6DJ8.html
[LATE NIGHT UPDATE]

I swapped out the tube sockets for peace of mind but still had a problem with the right channel cutting out. I started probing for VAC across those resistors points and discovered I could get the right channel to pass audio when probing across R113 so I think I have a cold solder joint there to repair in the morning.

BD

 

[band_master]

Well reheating R113 fixed the right channel cutting out. I spent some time probing around the filter board comparing resistor values across channels and doing traces from the i/o blocks and bypass switches. Eventually I reheated all of the solder joints on the right channel and finally I am getting consistent output across both channels. When I bus the square wave inside pro tools and record it I am getting -12.77 dB while running the tone through either channel of the PQD2 I am getting around -18.4 dB. The "main thread" for Pultec projects was helpful in terms of knowing what to expect from the circuit.

https://groupdiy.com/threads/g-pultec-problems-please-help.37837/page-32
Time to box it back up and move on to testing the EQ settings. BTW, if anyone has a board and needs parts DM me I have an extra set of Lorlin switches and electrolytic caps...when I ordered them the supplier sent two shipments by mistake.

cheers,
BD

 

[antisleep]

After a long break, I got my EQ over to the studio to audition it. Sounds very good! A few questions:

1. Pro-Q screenshot is a 1k tone being sent from Pro Tools, with the EQ in bypass. Are the overtones expected? They're in both channels.

2. The LF attenuate is much more aggressive than other Pultecs I've used. It's noticeable right away, and the boost/cut trick seems happier with the cut at 1 or 2 at the most. Do other folks experience this?

3. I misplaced a red knob cap. Anyone have a part number that I can hopefully get in the US? (btw I quite like these knobs)

 

[andow]

After a long break, I got my EQ over to the studio to audition it. Sounds very good! A few questions:

1. Pro-Q screenshot is a 1k tone being sent from Pro Tools, with the EQ in bypass. Are the overtones expected? They're in both channels.

2. The LF attenuate is much more aggressive than other Pultecs I've used. It's noticeable right away, and the boost/cut trick seems happier with the cut at 1 or 2 at the most. Do other folks experience this?

3. I misplaced a red knob cap. Anyone have a part number that I can hopefully get in the US? (btw I quite like these knobs)

Yes overtones are to be expected and in most cases also desired. If you use a Pultec style EQ you wouldn't expect it to be clean...

I also noticed this behaviour on the LF attenuate control when I first tested mine. It's just a matter of how the log curve of the pot looks and these can of course differ. I did mine with stepped rotary switches so I changed the values of the resistors, but still hardly ever use it above 2 or 3.

 

[antisleep]

Yes overtones are to be expected and in most cases also desired. If you use a Pultec style EQ you wouldn't expect it to be clean...

Sure, but I was surprised by how loud they are. I'll test on a friend's Pultecs soon.

I also noticed this behaviour on the LF attenuate control when I first tested mine. It's just a matter of how the log curve of the pot looks and these can of course differ. I did mine with stepped rotary switches so I changed the values of the resistors, but still hardly ever use it above 2 or 3.

Mine is the stepped switches & resistor values that came in the full kit from Gustav. Glad it's not just me at least.

 

[gyraf]

..you don't tell us about the absolute level of your input signal? In analogue hardware the amount of residuals depends entirely on the operating level

The gyraf-version of the Pultec (I think the PDQ2 is the same) is optimized for working on music with levels around 0dBfs=+14dBu

That said, I wouldn't consider residuals at -75dB surprisingly loud for a tube eq

/Jakob E.

 

[antisleep]

Yeah, sorry for just throwing that up without much info. Long day.

That's through an Avid HD I/O (calibrated to the default, -18 I think), and with Signal Generator set to its default value (-20dB). I'll bring a meter to the studio to double-check that but I think that's 0dBu?

I asked a friend to do the same test with his Pultecs (tube and solid state) and Mercury EQ-H1. No overtones on the real Pultec!

 

[Gustav]

Sure, but I was surprised by how loud they are. I'll test on a friend's Pultecs soon.




Mine is the stepped switches & resistor values that came in the full kit from Gustav. Glad it's not just me at least.

The great thing about DIY is, you can change the resistor ladder to your liking. For the low end, we are not dependent on the whole range of the switch, so just put larger values in per step

Try replacing the inductors with shielded types.

Gustav