Feeler: EZ Tube Lunch Box

[dipfrik]

Hey Ian,

a picture with or without the actual wiring?
I will make two later at home, because the wires are actually blocking the view of the board.

Thanks, Sven

 

[dipfrik]

Hey Ian,

so here a very detailed shot (JPG) plus my schematic and board layout as PDF.

A few notes:

1) i added a dotted line to visually separate the amp area from the rest of the board
2) to eliminate anything that additional on the board, all test were performend without them. Which means the problems are not coming from the 48V/L-M/Pad/Pol etc. Also not from the 12V Utility Rails, the BO Hansen DI add ons or the send-return mimic thats also on the board.
3) "the yellow wire": to absolutely make shure, that this design is exactly like your amp card, i recreated the gain setup "before" the last change i did on the pcb. (gain switching zero reference)
4) also "new" is the additional 100nF accross the HT and N4007 for protection
5) i didn't mention one thing that Holger found out: the 2nd gain stage gain trimmer acts strange. Fuully CCW and with an EQ between the gain stages, it's always to much gain. Around 2-4 dB. Which means, that i can't trim to unity gain. On your amp card, i definitely need to trim with a few turns CW to catch up.
6) Holger did a remarkable job to make this board "silent" but we couldn't find a way to solve the "flooding" problem

So based on all the research and help from Holger over the last weeks with this protoboard, this is my personal view:

1) component placement is the key and my test-board s**ks
2) there is a small but important mistake in my schematic or pcb layout
3) wiring and shielding - this is the part that makes me crazy on top of it all!
Really, i did a terrible (don't show your kids stuff *g) wiring job on my 1st amp card with all the eq's and the psu in one rack and this creepy setup works like a charm!

Bottom line: I should try a new iteration (going 3-5 steps back) and mimic your inital component placement, which seems to be THE great deal if schematic is definitely right.

Thanks, Sven

Here the files:

https://dl.dropboxusercontent.com/u/3150146/groupdiy/trouble-ez/TUBE_AMP_BIGBOARD_V01_pcb.pdf
https://dl.dropboxusercontent.com/u/3150146/groupdiy/trouble-ez/TUBE_AMP_BIGBOARD_V01_schematic.pdf

 

[ruffrecords]

Hi Sven,

Now I begin to understand. I thought you had two of my boards - one that worked and one that did not. But you have one of my boards that works and your open PCB layout which does not work. Now things are becoming clearer.

I have spent a little time looking at you PCB layout. There are several things I can see which may contribute to the problems you have:

1. You have the 12AX7 in between the two 6922 tubes. This means the ground current of the first stage 6922 flows in the ground lead of its 12AX7. This is a classic cause of instability. Note how in my layout both the ground from the 12AX7 reaches the HT input via the grounds of the output tubes. This means the output tube current does NOT flow in the 12AX7 input. In addition. you have the gain switch ground connected to the 6922 ground which is then connected to the 12AX7 ground. You have therefore split the gain switch ground from the 12AX7 ground and taken it via the 6922.

2. The two 150K anode  loads of the 12AX7 are a long way from the tube. As a rule you should keep components connected to tube pins as close to the pin as possible to avoid unnecessary coupling between circuits which is another cause of instability.

3. You have a fat 0V(ground)  but you do not have a ground plane covering the entire preamp. A proper ground plane serves several functions. First it provides a very low resistance 0V connection to all parts of the circuit. Second it acts as a screen to reduce the effects of external interference.. Third , it adds small stray capacitances to all the signal leads. This gently rolls off the amp's response and reduces coupling between signals, both of which reduce the possibility of instability.

4. You have long traces plus wires from both inputs to the 12AX7. These are very high impedance inputs and likely to pick up interference. The most important one is from the mic transformer secondary to the first amp 12AX7 triode. This must be kept as should as possible.

5. You have the gain switch a long way from the 12AX7 components it connects to. Another chance for instability

6. You have the first amp output close to its input. The two signal are in phase so they should be kept well apart.

Any one of the above factors could cause the oscillation problems you have but in combination it is almost inevitable that they will cause instability.

Sorry to be the bearer of such bad news.

Cheers

Ian

 

[dipfrik]

Hey Ian,

thanks again for the priceless feedback!

Now I begin to understand. I thought you had two of my boards - one that worked and one that did not. But you have one of my boards that works and your open PCB layout which does not work. Now things are becoming clearer.

Sorry my mistake. Your boards are rock solid!

Any one of the above factors could cause the oscillation problems you have but in combination it is almost inevitable that they will cause instability.

So with your list of possible factors causing this problem, i should definitely re-layout everything.
I can imagine, that the new layout will look like yours at the end

One question regarding the gain switch of the 1st amp gain stage:

You have the gain switch a long way from the 12AX7 components it connects to. Another chance for instability

In a 6U pcb design, i would like to uncouple the stepped gain switcher from the main board.
So if i'am designing a small external board with only the grayhill and the resistors, i should put the "in-points" as near as possible to the 12AX7 or do you mean all resistors should be on the main board and only the grayhill on a separate board connected via e.g. an IDC cable/connector?

And what are the rules for input and output transformer?
Far away as possible from each other or?

Thanks, Sven

 

[ruffrecords]

Hey Ian,

thanks again for the priceless feedback!

So with your list of possible factors causing this problem, i should definitely re-layout everything.
I can imagine, that the new layout will look like yours at the end

When I first looked at the layout and saw the 0V problem I thought you might be able to fix it by cutting parts of the ground plane and adding some wire links. However, as I looked further and saw the other issues I changed my mind, so I think you are right, a new layout is the only cure.

One question regarding the gain switch of the 1st amp gain stage:

You have the gain switch a long way from the 12AX7 components it connects to. Another chance for instability

In a 6U pcb design, i would like to uncouple the stepped gain switcher from the main board.
So if i'am designing a small external board with only the grayhill and the resistors, i should put the "in-points" as near as possible to the 12AX7 or do you mean all resistors should be on the main board and only the grayhill on a separate board connected via e.g. an IDC cable/connector?

You can put the switch and resistors together on a separate PCB and take a couple of flying leads to the 12AX7 circuit. This is the same as I plan to do with the 47K REV LOG pot as a gain control. The sequence of connections in the original design is:

0V --> 100uF cap --> gain resistor/switch --> 12AX7

If you keep the 100uF close to the 12AX7 then the 'in points' will be close together. If you are changing the layout you could also change this sequence to:

0V --> gain resistor/switch -->100uF cap --> 12AX7

This means the gain resistor is now ground referenced. This means if you do not use the gain resistor you can use the connection to the 100uF as a virtual earth either for mixing or for attaching the EZ EQ circuit.

And what are the rules for input and output transformer?
Far away as possible from each other or?

Thanks, Sven

Make sure the connections from the secondary of the transformer via the input capacitor to the 12AX7 grid are as short as you can  make them. Make sure the amp output is not lose to the amp input. Keep the transformers far apart and keep any leads connected to the mic input away from the leads to and from the output transformer.

Good Luck.

Cheers

Ian

 

[dipfrik]

Hey Ian,

one short question regarding the 47K REV LOG for substituting the gain switch.
I understand your math but why "REV"?

Thanks, Sven

 

[G-Sun]

A little back and forth, looking at my workflow and options,
I believe a EZ channelstrip is the thing for me, Preamp + PMEQP1 w/mid + Ezcomp (DOACbased, right?)
in a 3U case.

One question is the EQ-section and case: There is no PM EQ option for the EZTubeLunshbox? At least not with the mid-addon, right?
Or is that the Universal EQ? Does it fit into the Eurocard-system for the EZTubeLunshbox?
One card, two?

 

[ruffrecords]

Hey Ian,

one short question regarding the 47K REV LOG for substituting the gain switch.
I understand your math but why "REV"?

Thanks, Sven

The gain pot needs to be configured as a variable resistor rather than a potential divider. The basic requirement if for gain to increase as the pot rotates clockwise. What we need is:

1.  Zero resistance with the pot fully clockwise, (we add a 430R resistor in series to set the maximum gain)
2.. 4K7 with the pot half way.
3. 47K with the pot fully anti-clockwise

This is a REV log law. A LOG pot would give:

1. 47K when fully clockwise
2. 4K7 at the middle
3. Zero when fully anti-clockwise.

Cheers

Ian

 

[ruffrecords]

A little back and forth, looking at my workflow and options,
I believe a EZ channelstrip is the thing for me, Preamp + PMEQP1 w/mid + Ezcomp (DOACbased, right?)
in a 3U case.

Not sure about the Ezcomp; what do you mean by DOAC based?

One question is the EQ-section and case: There is no PM EQ option for the EZTubeLunshbox? At least not with the mid-addon, right?
Or is that the Universal EQ? Does it fit into the Eurocard-system for the EZTubeLunshbox?
One card, two?

There is no lunch box option for a Pultec EQ of any sort. The problem is the  Pultec EQs have so many controls they just won't fit on the front panel. You might squeeze a PMEQP1A without the mid in there but a 3 band Pultec does not fit. You need to find room on the front panel for 3 x rotary switches, 5 x pots and a toggle switch. You have about 110mm of panel height and about 60mm of panel width to fit these in. If you want to have a go at a front panel designer layout for it I would be very interested to see it. I have a PCB layout ready to go for a 3U 3 band Pultec which has the three switches mounted on the 3U PCB and all the other controls connected by flying leads. If you can come up with a workable front panel layout I'll happily have the PCBs made and give you one from the first batch for free.

Cheers

In

 

[dipfrik]

Hey Ian,

that confuses me, i thought "rev" only means that the curve is log but "on top" of the linear curve?

I found this:

In this case full CCW or CW is always the same?

Thanks, Sven

 

[G-Sun]

A little back and forth, looking at my workflow and options,
I believe a EZ channelstrip is the thing for me, Preamp + PMEQP1 w/mid + Ezcomp (DOACbased, right?)
in a 3U case.

Not sure about the Ezcomp; what do you mean by DOAC based?

One question is the EQ-section and case: There is no PM EQ option for the EZTubeLunshbox? At least not with the mid-addon, right?
Or is that the Universal EQ? Does it fit into the Eurocard-system for the EZTubeLunshbox?
One card, two?

There is no lunch box option for a Pultec EQ of any sort. The problem is the  Pultec EQs have so many controls they just won't fit on the front panel. You might squeeze a PMEQP1A without the mid in there but a 3 band Pultec does not fit. You need to find room on the front panel for 3 x rotary switches, 5 x pots and a toggle switch. You have about 110mm of panel height and about 60mm of panel width to fit these in. If you want to have a go at a front panel designer layout for it I would be very interested to see it. I have a PCB layout ready to go for a 3U 3 band Pultec which has the three switches mounted on the 3U PCB and all the other controls connected by flying leads. If you can come up with a workable front panel layout I'll happily have the PCBs made and give you one from the first batch for free.

Cheers

In

Thanks a lot Ian!

Ezcomp: Ok (misspelled DAOC.
But anyway I'm maybe not uptodate on the Ezcomp-module for the EZLunshBox, or if such a thing excists
(saw some scribbling on a photo at least )

Your 3U 3 band pulteq sounds very interesting. Is it for mounting in the Eurorack of the EZTubemixer system?
A custom front-panel, one piece is what I have in mind. But of course there are benefits with the modular approach as well.
My possible build will have a little uncertain start date. I'm doing a little other stuff first.

BTW: Those 5pots with switch could be mounted on both sides of a second pcb, I guess? With a ribbon-cable to the first one. Would maybe fit into the system then?

 

[ruffrecords]

It is easy to get confused and I think there is an error in my list which only made it worse - apologies.

That graph shows the resistance between the wiper and the ground terminal versus pot rotation and in both case it will be zero at the fully anti-clockwise position and 47K at the fully clockwise position. The log pot has 10% resistance at the half way point and the rev log is 90% resistance at the mid point. Here's a simplified table showing the two

Type	anti-clockwise	middle	clockwise
LOG	zero	4K7	47K
REV LOG	zero	42K3	47K

           

Since we need zero resistance at the fully clockwise position we connect between the wiper and the TOP of the pot. If you work this out from the graphs you will see that both give zero when fully clockwise and 47K when fully anti-clockwise (that was where I made a mistake in my last post). However, the log gives us 90% at the half way point but the reverse log gives use the 10% (4K7) that we want.

Hope that is a little clearer.

Cheers

Ian

 

[ruffrecords]

Thanks a lot Ian!

Ezcomp: Ok (misspelled DAOC.
But anyway I'm maybe not uptodate on the Ezcomp-module for the EZLunshBox, or if such a thing exists
(saw some scribbling on a photo at least )

OK, now I understand. Yes, I did build a simple prototype that will fit in the lunch box but there was little interest so I took it no further.

Your 3U 3 band pulteq sounds very interesting. Is it for mounting in the Eurorack of the EZTubemixer system?

Yes, it would be compatible with the EZTubeMixer and the lunch box.

A custom front-panel, one piece is what I have in mind. But of course there are benefits with the modular approach as well.
My possible build will have a little uncertain start date. I'm doing a little other stuff first.

OK. On a regular 19 inch wide 3U front panel you can use the regular 3 band Pultec PCB. It is too tall to fit vertically in a 3U case but it fits horizontally with a good space between the switches.

BTW: Those 5pots with switch could be mounted on both sides of a second pcb, I guess? With a ribbon-cable to the first one. Would maybe fit into the system then?

Fitting them onto a PCB is not a problem. It is finding enough front panel space for them such that you can still get your fingers between them!!

Cheers

Ian

 

[G-Sun]

Yes, I did build a simple prototype that will fit in the lunch box but there was little interest so I took it no further.
..

Fitting them onto a PCB is not a problem. It is finding enough front panel space for them such that you can still get your fingers between them!!

Thanks!

Ok, looks like preamp vertical and 3 band pulteq horisontal would be the way to go.

Now I see better:
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/3BandPultec/P3bandDoc.pdf

Sorry to hear no interest for the comp.
A pure la2a addon, is that possible? Or just stupid?

Btw: I'm looking for the psu specs (rails/voltage) but don't find it.

 

[ruffrecords]

Thanks!

Ok, looks like preamp vertical and 3 band pulteq horisontal would be the way to go.

Now I see better:
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/3BandPultec/P3bandDoc.pdf

Sorry to hear no interest for the comp.
A pure la2a addon, is that possible? Or just stupid?

Btw: I'm looking for the psu specs (rails/voltage) but don't find it.

Pure LA2A add-on should be possible and you can lose the input transformer if you don't want to make the comp patchable. The only issue is squeezing it into your box.

My designs are relatively insensitive to HT voltage; as long as it is between 275V and 325V you should be OK . The Eurochannel mic pre PCB consumes about 10mA from the HT as does the twin line amp. This document shows how to work out HT cap and R values for a CRCRCRC typr HT supply like the HT250:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/power/HTdesign.pdf

It is aimed at EZTubeMixer builders but the principles are equally applicable to a rack mounted project.

You might also want to read this:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf

Cheers

Ian

 

[dipfrik]

Hey Ian,

thanks, now i got it!

Cheers, Sven

 

[G-Sun]

Thanks!

Pure LA2A add-on should be possible and you can lose the input transformer if you don't want to make the comp patchable. The only issue is squeezing it into your box.

Well, it was just an idea from one who doesn't know very much.
Was thinking they could share psu, twinamp and yes, the in/out transformers.

My designs are relatively insensitive to HT voltage; as long as it is between 275V and 325V you should be OK . The Eurochannel mic pre PCB consumes about 10mA from the HT as does the twin line amp. This document shows how to work out HT cap and R values for a CRCRCRC typr HT supply like the HT250:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/power/HTdesign.pdf

It is aimed at EZTubeMixer builders but the principles are equally applicable to a rack mounted project.

You might also want to read this:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf

Thanks!

Is this a typical transformer to use for in/out line level?
http://www.sowter.co.uk/acatalog/SOWTER_TRANSFORMERS_ISOLATOR_BALANCERS_2.html#a4

 

[ruffrecords]

Is this a typical transformer to use for in/out line level?
http://www.sowter.co.uk/acatalog/SOWTER_TRANSFORMERS_ISOLATOR_BALANCERS_2.html#a4

Yes, that would be more than good enough for a balanced line input. Which one to use for the output depends a lot on the specific output circuit.

Cheers

Ian

 

[ruffrecords]

Your 3U 3 band pulteq sounds very interesting. Is it for mounting in the Eurorack of the EZTubemixer system?
A custom front-panel, one piece is what I have in mind. But of course there are benefits with the modular approach as well.
My possible build will have a little uncertain start date. I'm doing a little other stuff first.

BTW: Those 5pots with switch could be mounted on both sides of a second pcb, I guess? With a ribbon-cable to the first one. Would maybe fit into the system then?

I like a challenge so I had a go at squeezing 5 pots and two switches onto a PCB to sit vertically to the right of a 3U Pultec board. Like the EQ board, the pots board cannot be more than 100mm tall and you need to leave 2.5mm unpopulated at top and bottom which gives you 95mm to play with. I decided to use ALPS RK09L pots which are 12.1mm wide. The three frequency selelction swotches are 25.4mm apart (1 inch) and for the hi and lo switches there is both a boost and a cut pot. So I placed one pot level with the switch and the other above it, in the case of the hi band, or below it in the case of the lo band. Then, between the lower pot on the hi band and the higher pot on the lo band, there is just enough room to squeeze two ALPS push button switches, one for the mid boost/cut switch and the other for the EQ in out. I laid this out in front panel designer and the resultant .fpd file is here:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/lunchbox/PMP/3Upassivepultec.fpd

I printed it out full size and laid the smallest knobs I could find on it. They are 13mm in diameter. They fit fine for the three frequency select switches. They are a bit tight but workable for the five pots. The only thing I am not 1005 happy about is the two switches which seem a little too close to the pot knobs. A reasonable compromise might be to have just the boost/cut switch for the mid section of the PCB and have the EQ in/out as a toggle on flying leads. It could probably go next to the lo cut pot. A picture of the layout with the knobs in place is attached:

Cheers

Ian

 

[dipfrik]

I like a challenge so I had a go at squeezing 5 pots and two switches onto a PCB to sit vertically to the right of a 3U Pultec board.

Squeezing is always the fun part.
Awesome outcome!

Cheers, Sven