Poor Man's Tube Gain Make Up Stage

[Joechris]

Ian, have you tried a 8cg7 tube in this. Apart from a 8,4V heater they look similar. So just by rectifying the 6,3V I should be in the ballpark..?
I can get this tubes at a local supplier dirt cheap. Will the HT voltage tap off stil work ok?

 

[ruffrecords]

Ian, have you tried a 8cg7 tube in this. Apart from a 8,4V heater they look similar. So just by rectifying the 6,3V I should be in the ballpark..?
I can get this tubes at a local supplier dirt cheap. Will the HT voltage tap off stil work ok?

Yes, I am pretty sure the 8CG7 is just a 6CG7 with a different heater voltage. I am not sure if rectifying 6.3V will work though. Were you planning on smoothing it as well?

The HT voltage will be the same.

Cheers

Ian

 

[Joechris]

Rectifying a 6,3V ac will give me 8,8 V, so a little on the high side. Mabye a smal resistor and a large cap, and Im done??
By HT tap I was thinking of the tap that lifts the heater potensial.

 

[Kamel]

don't forget, at low voltage, the diode voltage loss (2 x Vf) so rectified is more near 7,5v (avoiding transformer loss)

 

[ruffrecords]

Rectifying a 6,3V ac will give me 8,8 V, so a little on the high side. Mabye a smal resistor and a large cap, and Im done??

Whether this happens in practice depends on how close to fully loaded the 6.3VAC secondary is. If you apply its full load current and have sufficient smoothing to reduce ripple to a small fraction of a volt, you tend to end up with close to 6.3V dc. Lower loads lead to a correspondingly higher dc votlage.

By HT tap I was thinking of the tap that lifts the heater potensial.

OK, understood. This tap remains the same. The small change in heater voltage will not matter.

Cheers

Ian

 

[strangeandbouncy]

Hi Ian,


  just a couple of questions if I may.

      Firstly, if you build this with only one channel, ie just one 6cg7, do the 1k resistors in the power supply need changing to another value . . .

  Also, in the spirit of ~ Poorman (ie me . . . .) . . . I have a number of 47uF 450v caps, and not enough to quad them up to make four @ 220uF. Will there be a great drop off in hum rejection if I build with 47uF or even 100uF instead of 220uF . . . can i increase hum rejection some by quad or doubling just one, and which one . .

  One more . . . I have 6sn7 as well as 6cg7. Are they the same apart from base and pin connection, or any other changes to circuit necessary with 6sn7 . . .


  many thanks in advance,


      ANdyP

 

[ruffrecords]

Hi Ian,


   just a couple of questions if I may.

      Firstly, if you build this with only one channel, ie just one 6cg7, do the 1k resistors in the power supply need changing to another value . . .

Yes, they should be changed to 2K.

   Also, in the spirit of ~ Poorman (ie me . . . .) . . . I have a number of 47uF 450v caps, and not enough to quad them up to make four @ 220uF. Will there be a great drop off in hum rejection if I build with 47uF or even 100uF instead of 220uF . . . can i increase hum rejection some by quad or doubling just one, and which one . .

You will probably be fine with 47uF with just the one tube and the increase of the series resistors to 2K will help too. The important thing is to use all four caps

   One more . . . I have 6sn7 as well as 6cg7. Are they the same apart from base and pin connection, or any other changes to circuit necessary with 6sn7 . . .

Apart from the base and pin connections they are identical so no circuit changes are necessary.

Cheers

Ian

 

[strangeandbouncy]

Hi Ian,


  many thanks,


    ANdyP

 

[ej_whyte]

Anyone know a good source for the 4.7uF 400v WIMAs? I tried mouser but didnt fanyc the 19 week wait :/

Cheers

 

[culteousness1]

Anyone know a good source for the 4.7uF 400v WIMAs?

I just checked conrad.de and they will stock those with a 27.5 mm pitch in a few days.
So you may check www.conrad-uk.com.

Hope this helps,
Carsten

 

[ruffrecords]

Anyone know a good source for the 4.7uF 400v WIMAs? I tried mouser but didnt fanyc the 19 week wait :/

Cheers

Thinking about it, I don't think we really need an output coupling capacitor as big as 4.7uF. That gives a 3dB down point into 10K of about 3Hz, which means the response would be 1dB down at 6Hz and 0.5dB down at 9Hz.

If we changed it to 2.2uF then it would be 3dB down at about 7Hz, 1dB down at 14Hz and 0.5dB down at 21Hz which should be perfectly acceptable for most uses. I have never been a particular fan of the huge WIMAs anyway because, as you have discovered, then can be very hard to get hold of. So what I suggest is I layout the PCB to accommodate a pair of 2.2uF caps in parallel. That way you can fit one or two and the sourcing problems should be eased.

Edit: I just checked Farnell and they stock both a Vishay and WIMA 2.2uF 400V cap on the identical 27.5mm lead spacing. Dubillier also do one that fits in the same spacing.

Cheers

ian

 

[kante1603]

So what I suggest is I layout the PCB to accommodate a pair of 2.2uF caps in parallel. That way you can fit one or two and the sourcing problems should be eased.

+1!

Udo.

 

[ej_whyte]

Anyone know a good source for the 4.7uF 400v WIMAs? I tried mouser but didnt fanyc the 19 week wait :/

Cheers

Thinking about it, I don't think we really need an output coupling capacitor as big as 4.7uF. That gives a 3dB down point into 10K of about 3Hz, which means the response would be 1dB down at 6Hz and 0.5dB down at 9Hz.

If we changed it to 2.2uF then it would be 3dB down at about 7Hz, 1dB down at 14Hz and 0.5dB down at 21Hz which should be perfectly acceptable for most uses. I have never been a particular fan of the huge WIMAs anyway because, as you have discovered, then can be very hard to get hold of. So what I suggest is I layout the PCB to accommodate a pair of 2.2uF caps in parallel. That way you can fit one or two and the sourcing problems should be eased.

Edit: I just checked Farnell and they stock both a Vishay and WIMA 2.2uF 400V cap on the identical 27.5mm lead spacing. Dubillier also do one that fits in the same spacing.

Cheers

ian

Aah ok cool, sounds like a good plan to me, a bit of flexibility is always good 

Cheers

 

[ej_whyte]

So yep i will be down for a set please when u get all the EQ boards done and have time to sort this out

Cheers

 

[ruffrecords]

So yep i will be down for a set please when u get all the EQ boards done and have time to sort this out

Cheers

LOL, I got the last EQ boards in the post at 1pm Friday, then at 1.30pm my four granddaughters turned up for a sleepover while their parents went to an awards night. It is now Saturday, just gone 3.30pm and they have gone home and the missus and I have mopped, hoovered and cleared up all their mess. Doing a PCB layout seems like a bit of light relief.

Cheers

Ian

 

[gemini86]

Doing a PCB layout seems like a bit of light relief.

Cheers

Ian

Same here. For me it's like doing a crossword puzzle, just nice relaxing brain work.

 

[ruffrecords]

Here's a first draft of a single amp with two paralleled output caps. I have a couple of quaestions regarding the PCB size and the fixing brackets.

The PCB is at present 1.5 inches high and I am not sure this will fit in a 1U enclosure (there's only 0.25 inches less the casework thickness clearance). I guess it depends somewhat on the case but does anyone know the maximum PCB height that will fit into a 1U case?

Regarding the fixings, it has been suggested that I should make the holes for a specific bracket and supply the brackets with the PCB. Having just posted 30 odd packets of PCBs I know that anything weighing over 60gms costs over 2 gbp to send internationally. Even if the brackets weigh only 50gm each it will add 1.50 gbp to the postage costs and the brackets only cost 1 gbp each so this is a very expensive way of suppling brackets. An simpler and cheaper alternative would be to let people sort out their own brackets and just leave a blank area at each end of the PCB for people to drill their own holes. Comments?

Cheers

Ian

 

[gemini86]

People who can't drill a simple bracket hole really don't want to DIY, do they? Food for thought.

 

[chrispbass]

Here's a first draft of a single amp with two paralleled output caps. I have a couple of quaestions regarding the PCB size and the fixing brackets.

The PCB is at present 1.5 inches high and I am not sure this will fit in a 1U enclosure (there's only 0.25 inches less the casework thickness clearance). I guess it depends somewhat on the case but does anyone know the maximum PCB height that will fit into a 1U case?

Regarding the fixings, it has been suggested that I should make the holes for a specific bracket and supply the brackets with the PCB. Having just posted 30 odd packets of PCBs I know that anything weighing over 60gms costs over 2 gbp to send internationally. Even if the brackets weigh only 50gm each it will add 1.50 gbp to the postage costs and the brackets only cost 1 gbp each so this is a very expensive way of suppling brackets. An simpler and cheaper alternative would be to let people sort out their own brackets and just leave a blank area at each end of the PCB for people to drill their own holes. Comments?

Cheers

Ian

I'm having some cases made based upon these front panels Ian. http://www.holtbs.co.uk/blank_panels.htm
I've just measured the 1U extruded front I have infront of me with a digital caliper. To the outer edge of the returned flanges is 40.25 mm and to the inners, is approx 36.75 mm. Taking these as potentially realistic extremes averages to 77/2 = 38.5 which is about 0.4 mm greater than 1.5". I don't think you're far off with your current dimensions as being a reasonable average. I suppose the main thing, specially given tube voltages, is that traces and wiring pads are well inside the edge if a little bit of filing of the pcb was required to get them to fit in, if anybody did have a particularly 'stingy case' in the internal height department.

hope this helps, if a tad un-scientific?  ;D

 

[ej_whyte]

So yep i will be down for a set please when u get all the EQ boards done and have time to sort this out

Cheers

Sorry for all the messing around, but i think i'm gonna have to go with the solid state make-up after all. Been pricing up all the components and the transformers are killing me, im just too much of a poorman at the moment.

Cheers