Poor Man's Tube Gain Make Up Stage

[ruffrecords]

Hi again, I'd just like to ask if the PSU can handle anymore than 2x 6cg7 with the right transformer?

Yes, you just need to adjust the series resistors to maintain about 10V drop across each. If you add many more 6CG7 stages you may need to increase the capacitor values too. In my mixer, which has 16x 6CG7 and 2x ECC99, I use 100 ohm resistors and 470uF caps. You do have to watch the inrush current into the first resistor when the supply first turns on. In my mixer supply I use a 5W part for this.

Also, will a 10k/600ohm output transformer enable this to drive 600ohm loads without the adjustments mentioned a few posts earlier?

Yes, but the headroom will not be that great. The 6CG7 stage will do about 20V rms into 10K which via a10K:600 transformer is only 5V rms  or +16dBu. If you really want to drive a 10K:600 transformer I would recommend using an ECC99 as an SRPP at 15mA (you can do this on the same PCB) and this will do +22dBu into 600 ohms. In both cases you do lose 12dB of gain in the transformer of course.

one more...with the EQ input impedance being 10k could I take the signal from the output of one gain stage straight to the EQ input?

Yes, that is exactly what I intend to do in mixers using the poor man's EQP1A!

I suppose what I'm aiming at is a one-bottle type mic preamp with PM Pultec/gain makeup.
cheers
Matt

Sound like a cracking project to me.

Cheers

Ian

 

[mylesgm]

So what about using the makeup pcb for summing amp?  Starting with passive summing.  So transformer balanced inputs hardwired to a buss bar with mix resistors into the makeup stage.  want to be able to alter the bias, the load and possibly the supply voltage to alter the tone of the amp.  Any thoughts?

 

[ruffrecords]

So what about using the makeup pcb for summing amp?  Starting with passive summing.  So transformer balanced inputs hardwired to a buss bar with mix resistors into the makeup stage.  want to be able to alter the bias, the load and possibly the supply voltage to alter the tone of the amp.  Any thoughts?

I use this same circuit for gain make up of passive mix buses in my mixers - it is quite a versatile circuit. Its limitations are its gain (fixed at about 25 to 26dB) which limits you to about 20 sources and its noise floor which on a good day will be about -80dBu at the output.

Mu followers are renowned for their relatively low distortion combined with good output drive capability which is why I use them all over the place in my mixer designs so it is not easy to make them distort. The key to the low distortion is the 10K resistor between the triodes which effectively gets multiplied by the top tube mu and appears as the bottom tubes plate load - in this case it looks like about 20 x 10K = 200K. Not that huge you might think but with 5mA standing current you would need an HT supply of 1000V to achieve the same effect in a common cathode design!

The smaller the resistor the lower the plate load and the higher the resulting distortion. Make the resistor zero and you effectively have an SRPP and SRPPs definitely have a lot more distortion than mu followers. So, here's an idea, how about replacing the 10K resistor between the tubes with a 10K pot and you can smoothly move from a very good mu follower to a SRPP in one control. The pot will need to be able to take the 5mA and I don't know if it will sound scratchy as you turn it with that dc flowing through it but it is worth a try. The tubes should be fine since only about 50V is dropped across the 10K resistor so the tubes plate voltage only change by 25V each. As designed the circuit is symmetrical so each tube should have the same plate volts.

To avoid the scratchiness of a pot with dc you could retain the 10K and place a pot (or a switched resistor) with a series cap across it to alter the ac conditions of the circuit. Sounds like quite a fun experiment to me.

The mu follower is remarkably tolerant of dc operating point and the unloaded distortion is not very dependent on the plate current - the output drive capability is, of course, directly proportional to dc current.

Cheers

Ian

 

[mylesgm]

The srpp/mu follower variation is an interesting idea and sounds incredibly easy to implement.  What about the alteration of the voltage supply?  I realise this will alter the gain of the circuit but it would also reduce the headroom, increase the distortion characteristic etc right?

What mix resistors should be used to interface correctly to the input of the valve for a 12-16 channel mixer (so 6-8 channels per summing amp) with no faders or pans?

 

[Joechris]

The summing is straightforward passive unbalanced. The Left and Right switches ground the summing resistors in the deselected position and connect them to the mu output when selected (mu follower output R is about 800 ohms). I used 47K bus resistors and the bus is loaded by the 100K log master pot which gives a bus loss with 6 channels of close to 16dB. With 26dB in the mu follower gain make up that leaves 10dB in hand on the master fader.

Cheers

Ian

from http://www.groupdiy.com/index.php?topic=45388.0

 

[ruffrecords]

The srpp/mu follower variation is an interesting idea and sounds incredibly easy to implement.  What about the alteration of the voltage supply?  I realise this will alter the gain of the circuit but it would also reduce the headroom, increase the distortion characteristic etc right?

As I said, the mu follower is very tolerant of dc conditions. At dc each tube looks pretty much like a 30K resistor (mu x 1K plus ra~10K). So you have a 30K tube then a 10K resistor and another 30K pot. Feed in the HT voltage and you can work out the the operating points. As you lower the HT the tube current drops as does the headroom. I know there is very little difference in performance between 4mA and 8mA operating current but below 4mA (i.e lower HT voltage) I have not tried but the lower you go the lower the headroom and I would expect distortion to rise too.

What mix resistors should be used to interface correctly to the input of the valve for a 12-16 channel mixer (so 6-8 channels per summing amp) with no faders or pans?

Picking mix bus resistor is always a compromise. The bigger the value the better the crosstalk but the higher the noise. The lower the value the lower the noise but the higher the interaction between controls. The input of the valve does not really care so long as the total resistance between grid and ground is below about 1Meg. Generally you want as low a value as you can get away with.

That said, you have made things somewhat simpler by avoiding faders and pans so interaction and crosstalk are not issues. You only really have to think about noise so your best bet is to choose the lowest value the source can drive without distortion. 10K would probably be a good value to start with. With 8 channels per summing amp the bus resistance will be 10K/8 = 1250 ohms which will produce a noise level of slightly better than
-121 dBu.  As expected, this means the noise of the tube gain make up stage will dominate.

Cheers

Ian

 

[MatthisD]

Ahh! part of my plan was to use these mu follower preamps after a balanced passive mixer to bring the levels back up, but now this option sounds great too.
Again, I presume you are talking 10k/10k transformers unbalancing?

Talking about noise, the gain makeup required for a balanced mix buss would be twice as much as unbalanced, so in theory the noise is doubled as well and with worse crosstalk?

At this stage I'm thinking I'll make one external PSU to use with either a dual EQ, dual preamp or possibly this summing mixer.

I suppose what I'm aiming at is a one-bottle type mic preamp with PM Pultec/gain makeup.
cheers
Matt

Sound like a cracking project to me.

Cheers

Ian

 

[ruffrecords]

Ahh! part of my plan was to use these mu follower preamps after a balanced passive mixer to bring the levels back up, but now this option sounds great too.
Again, I presume you are talking 10k/10k transformers unbalancing?

I was answering a question by mylesgm who said "So what about using the makeup pcb for summing amp?  Starting with passive summing.  So transformer balanced inputs hardwired to a buss bar with mix resistors into the makeup stage."

So, in short, yes.

Talking about noise, the gain makeup required for a balanced mix buss would be twice as much as unbalanced, so in theory the noise is doubled as well and with worse crosstalk?

With a balanced bus I would recommend you unbalance just prior to the gain make up tube using a suitable transformer. If this is 1:1 then the signal reaching the tube is exactly the same as for the unbalanced case so noise would not be affected.

Cheers

Ian

 

[MatthisD]

Yes I'm with you, thats my intention which should result in lower noise than my previous, more cost effective plan:

balanced inputs/passive summing buss/balanced outputs - to - Mic Pre (40+/- dB gain makeup)

thanks for your suggestions, I'll go in on some more pcbs in the blackmarket thread.
cheers
Matt

I was answering a question by mylesgm who said "So what about using the makeup pcb for summing amp?  Starting with passive summing.  So transformer balanced inputs hardwired to a buss bar with mix resistors into the makeup stage."

So, in short, yes.

 

[ruffrecords]

I have just lashed together the poor man's EQP1A together with the tube gain make up and run some tests. I attach a pic of the test rig.

Results graphs can be seen here:

http://www.ianbell.ukfsn.org/pics/alteq/

The pic title tells you what the test is so hiboostcut6 is max simultaneous hi boost and cut with the switch at position 6 i.e fully clockwise (highest frequency).

You can quite clearly see the Pultec bump at low frequencies and we have a modest HF bump too with simultaneous max hi boost and cut.

Cheers

Ian

 

[buildafriend]

Around how much does it cost without the chassis/front panel/all other stuff?

I mean if I mounted this thing in a cardboard box with some old XLR jacks i have laying around and only needed the tubes, tube sockets (if needed), resistors, pots, output xfrmr, caps, power xfrmr, and diodes..

This is of course, to build a pair of them.

Also, can anyone fill me in on what they sound good on? What instruments do you think they work great for?

Thanks!

 

[gemini86]

Caps should be on that shopping list. Resistors are pennies, caps- not so much.

 

[buildafriend]

Caps are on the shopping list

 

[gemini86]

Around how much does it cost without the chassis/front panel/all other stuff?

I mean if I mounted this thing in a cardboard box with some old XLR jacks i have laying around and only needed the tubes, tube sockets (if needed), resistors, pots, output xfrmr, caps, power xfrmr, and diodes..

This is of course, to build a pair of them.

Also, can anyone fill me in on what they sound good on? What instruments do you think they work great for?

Thanks!

DOH!

 

[baadc0de]

I have just lashed together the poor man's EQP1A together with the tube gain make up and run some tests. I attach a pic of the test rig.

Results graphs can be seen here:

http://www.ianbell.ukfsn.org/pics/alteq/

The pic title tells you what the test is so hiboostcut6 is max simultaneous hi boost and cut with the switch at position 6 i.e fully clockwise (highest frequency).

You can quite clearly see the Pultec bump at low frequencies and we have a modest HF bump too with simultaneous max hi boost and cut.

Cheers

Ian

Looking good! Did you run this output transformerless?

 

[ruffrecords]

Looking good! Did you run this output transformerless?

Yes. There is an input transformer mounted on the other side of the aluminium sheet so its leads poke through. You can just see its nut just below the larger pot at the top.

Cheers

Ian

 

[ruffrecords]

Around how much does it cost without the chassis/front panel/all other stuff?

I mean if I mounted this thing in a cardboard box with some old XLR jacks i have laying around and only needed the tubes, tube sockets (if needed), resistors, pots, output xfrmr, caps, power xfrmr, and diodes..

This is of course, to build a pair of them.

Are you intending to use the PCBs or build it point to point?

Also, can anyone fill me in on what they sound good on? What instruments do you think they work great for?

Well, at the low end, the Pultec bump has been emulated many many times. I have a dbx mic channel with built in 'enhancer' and the bass part of this in frequency response terms is pretty much a straight copy of the Pultec bump. It is very good for enhancing kick drums and toms for example.

At the top end, the poor man's EQP1A does not achieve the sharp peaking response of the original Pultec (that'so ne way we saved money) but is can produce a gentle bell for enhancing the mif range and as a stright shelving boost for adding sparkle to almost anything.

Cheers

Ian

 

[buildafriend]

Around how much does it cost without the chassis/front panel/all other stuff?

I mean if I mounted this thing in a cardboard box with some old XLR jacks i have laying around and only needed the tubes, tube sockets (if needed), resistors, pots, output xfrmr, caps, power xfrmr, and diodes..

This is of course, to build a pair of them.

Also, can anyone fill me in on what they sound good on? What instruments do you think they work great for?

Thanks!

DOH!

hahaha its cool buddy

 

[buildafriend]

Around how much does it cost without the chassis/front panel/all other stuff?

I mean if I mounted this thing in a cardboard box with some old XLR jacks i have laying around and only needed the tubes, tube sockets (if needed), resistors, pots, output xfrmr, caps, power xfrmr, and diodes..

This is of course, to build a pair of them.

Are you intending to use the PCBs or build it point to point?

Also, can anyone fill me in on what they sound good on? What instruments do you think they work great for?

Well, at the low end, the Pultec bump has been emulated many many times. I have a dbx mic channel with built in 'enhancer' and the bass part of this in frequency response terms is pretty much a straight copy of the Pultec bump. It is very good for enhancing kick drums and toms for example.

At the top end, the poor man's EQP1A does not achieve the sharp peaking response of the original Pultec (that'so ne way we saved money) but is can produce a gentle bell for enhancing the mif range and as a stright shelving boost for adding sparkle to almost anything.

Cheers

Ian

Oh I'm not ready for point to point yet, I am a newb.

It would be with the PCB's

 

[ruffrecords]

Oh I'm not ready for point to point yet, I am a newb.

It would be with the PCB's

OK, so a power supply PCB plus two gain make up PCBs plus four EQ PCBs will cost about £40. Mains transformer will set you back around £50 and a couple of 6CG7s will cost about £30. Allow another £20 for the pots, caps and resistors and your total looks to be in the region of £140.

Cheers

Ian