Poor Man's Tube Gain Make Up Stage

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ruffrecords

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Edit: 10th June 2011. Below is the buyers list with the amount to pay for each person. I'll update this as payments are received and when I ship the PCBs.

Please pay by paypal using my email which is ianbellATukfsnDOTorg with the AT and ORG replaced by their respective characters. The price shown includes shipping and payapl.

Please ensure you send me, either in a message with your payment or separately, your real name, shipping address and group name. Often paypal does not give a shipping address and trying to tie real names from paypal to group names can be quite time consuming.

Code:
Name                PCB Qty            Price (GBP)
mylesgm              8                      shipped
Albrebreton          6                      shipped
detonator            2                      shipped
kante1603            4                      shipped
Grooveteer           4                      shipped
fazer                4                      shipped
leadbreath           1                      shipped
Hank Dussen          2                      shipped
baadc0de             4                      shipped
danijel              2                      shipped
Joechris             4                      shipped
culteousness1        2                      shipped
Matthew Jacobs       8                      shipped
Script               2                      shipped
MatthisD             2                      shipped
Winetree             4                      shipped
strangeandbouncy     2                      shipped
buschfsu             2                      shipped
Citrus Hill          10                     shipped
noulou               2                      shipped
MrClunk              2                      shipped
dirtyhanfri          2                      shipped


Here's my first shot at a poor man's tube gain make up stage to complement the poor man's Pultec EQP1A. To keep cost down it uses just one tube per stage in a mu follower configuration, dispenses with interstage transformers, uses ac heaters and a very simple CRCRCRC HT filter. Since there is just one tube there is not a lot of gain to play with (about 26dB of which we need 21dB for gain make up) so we have to use a 1:1 output transformer. I have discussed this with Brian Sowter who recommended using the ST5069 transformer. Although this is a 600:600 transformer this circuit is not designed to drive into a 600 ohm load. It will however quite happily drive over +26dBu into a 10K bridging load which should be fine for most people. I built a couple of these a few months back and they do work. This mu follower has the advantage that it produces low levels of distortion. This circuit will achieve well below 1% THD at 20V rms (+28dBu) into a 10K load and at normal operating levels (+4dBu) it is below 0.1% and almost entirely nice tasty second harmonic.

The pots at the input are for use with the EQ and a bypass switch. The 470K LIN pot sets the output level with the EQ switched in and all controls set flat. Normally you would set this for an overall unity gain. I used a LIN pot because we have 26dB of gain and 21dB or so of loss so the pot will need to add about 5dB of loss for overall unity gain. The mid point of a LIN pot give about 6dB loss at the centre position. The other preset and associated resistors is a pad for when the EQ is in bypass mode. This pad is designed to present about 10K load to the input transformer secondary and have an attenuation range from about 18 to 27dB with about 21dB at its centre point.

If there is enough interest I'll have a play with a stereo PCB.

Cheers

Ian

PMTGMsm.jpeg
 

michal_k

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Do you have to lift heaters? Datasheet states 200V max between cathode and heater, but I have no real experience with mu-follower circut and alikes.
 

ruffrecords

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culteousness1 said:
Do you think it's possible to fit the make up stage in a 1RU case? Maybe an angled pcb will do the job.

Cheers,
Carsten

I remember several people on the poor man's EQP1A thread talking about building it into a 1U rack so I had it in mind that this tube stage should also fit into a 1U rack. The Sowter transformer are 34mm diameter so they will fit OK and you would probably need to find a suitable toroid mains transformer. The B9A socket is about 25mm diameter so that is OK but if I would need to make the PCB about 38mm high to get tracks around the socket.

So, overall, the answer is yes and that is what I was aiming at.

Cheers

Ian
 

ruffrecords

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michal_k said:
Do you have to lift heaters? Datasheet states 200V max between cathode and heater, but I have no real experience with mu-follower circut and alikes.

Usually you do. The 200V is dc PLUS signal. The top cathode sits at about 180V dc (the HT is about 310V). The signal on that cathode can reach as much as 40V rms (+34dBu) or 56V peak which takes the cathode up to 236V. With 75V of heater elevation the difference is kept down to a safer 161 volts.

Cheers

Ian

 

chrispbass

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culteousness1 said:
ruffrecords said:
If there is enough interest I'll have a play with a stereo PCB.

Great Ian! I am interested.

Do you think it's possible to fit the make up stage in a 1RU case? Maybe an angled pcb will do the job.

Cheers,
Carsten

That's exactly what I was thinking. Strange, as there was a thread recently about right angle tube sockets. Obviously meant to be!  :)
C


 

Matthew Jacobs

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Same here. I'd be interested in 4 stereo sets to complement your PCB EQ sets and the 2 extra circuits I plan to build P2P using Russian capacitors...
 

ruffrecords

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OK, looks like there is some serious interest in this so I'll look at a possible PCB layout over the next few days. Assuming many people are thinking of fitting this into a 1U rack are there any preferences for how this PCB would be mounted in such an enclosure?

Cheers

Ian
 

tmuikku

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ruffrecords said:
OK, looks like there is some serious interest in this so I'll look at a possible PCB layout over the next few days. Assuming many people are thinking of fitting this into a 1U rack are there any preferences for how this PCB would be mounted in such an enclosure?

Cheers

Ian

Do a vertical PCB so one can use regular pcb mount tube sockets and then L-bracket to attach the PCB into case bottom ;) This could be then used as a gain stage in  other projects too ( for example console channels trip, no?). Put 8 EQs side by side in one 4u case etc. Separate psu pcb.
 

ruffrecords

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tmuikku said:
Do a vertical PCB so one can use regular pcb mount tube sockets and then L-bracket to attach the PCB into case bottom ;) This could be then used as a gain stage in  other projects too ( for example console channels trip, no?). Put 8 EQs side by side in one 4u case etc. Separate psu pcb.

Ok, so that would mean this PCB could be no wider than 4U less a bit and no higher than 1U less a bit right? Say 6.5 inches by 1.5 inches?

Edit: So transformers would definitely have to be off board.

Cheers

Ian
 

chrispbass

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Vertical, will prob be easiest as long as the height factor makes it efficient enough for component placement and we can find some decent 'L brackets'. Probably the neatest. This will also work well for people who want to mount horizontally in a 2U+ case using normal pcb spacers.

Having a horizontal pcb means that we may either need an extra pcb for the skt and an 'L bracket' to fix it or some form of angle bracket/chassis mount socket and hard wiring. Potentially messier/more metalwork depending upon peoples construction skills.

Is there away of satisfying both? A slim pcb with a circular valve base footprint(to mount vertically 1U/horizontally 2U+) and also a neat way of wiring off to a chassis socket/angle bracket, for people who prefer that method?

Chris
 

0dbfs

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How about mounting the tube in a standard socket/L-bracket combo (or chassis) off of the PCB and wiring it into the PCB as required?

Ian, I was also wondering what you think of using this circuit to drive a 600 ohm load.

Thanks!

Cheers,
jonathan

 

Ptownkid

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I'm reasonably interested as well...to ask a dumb question though...does this thing remain "poor man's" when all these building blocks that are coming together come together?
 
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