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Messages - nodz

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1
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: July 05, 2013, 08:46:03 AM »
That's good if they have their own fuses. Makes the wiring up much easier.  Just an on off switch and the standby switch to worry about.

2
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: July 05, 2013, 04:13:45 AM »
Hahaha yes Phil.  What I meant was, is all I need a fuse of a small value on the ht side of the transformer because I am using an a.c. wall wart multiplied up

3
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: July 04, 2013, 11:12:09 PM »
Just wondered now, that I have built the PSU by taking the AC output from a wall wart and then multiplied it back to 240V from a back-to-back transformer, whether I need some kind of over-current failsafe to prevent damage to the Wall wart, PSU and preamp.

4
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 16, 2013, 06:52:01 PM »
Quote from: Roly
I rat caps out of CRT TV's at the local dump...

I get them from CRT too.  Whenever a mate gets rid of one, I get them to drop it over to my place.  I then take various bits and pieces out of them and then dump the rest.

I will be doing that with the old computer PSUs now, so that I can gather together enough hi-voltage caps for voltage multipliers.

Quote from: Roly
"Where do you keep your transformers?"
Blank look from young sales dweeb.
"We don't stock toys" he sneered.

Oh you had this problem too. :lmao:  Towards the end of the component stocking days, I'd walk in there and they wouldn't have a clue.  I wanted an electrolytic cap of a specific value, bloke looked on the computer and said they had stock and then tried to sell me a ceramic cap for $20.  What I wanted was a hi capacity one for use in a 13.8V, 10A PSU for amateur radio supply.

Hopefully, I won't need too many bits to get moving on the preamp.  I'll be reviewing over the next couple of days

5
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 15, 2013, 03:32:05 PM »
Quote from: Roly
A lot of builders make the mistake of miniaturising their board build, then putting it into a large case   - just making life difficult for themselves.

Since I am building modular and probably going to build into 19" racks for ease of use and storage around the home, size is not really an issue, except height that is.

The 2 x 100uF/400V caps that are in the HT supply line, were snap-ins.  I just straightened the pins slightly with a pair of needle nose pliers and soldered them to the board.  I'm a fan because of their half height, although they have a larger footprint.  For some reason this type of capacitor is cheaper new than many of the others.  I bought a packet of 6 for $9.  It means that they fit in a 1U rack easily.  If i need to buy, rather than scavenge, I buy at www.rockby.com.au and they quite often have specials like that.

Years ago when i was regularly building amateur radio projects and when Dick Smith's was not just a home entertainment store, i used to buy in bulk.  I have 200 x 1N4004 diodes, that I think were $5 for the bag, so as I said for the sake of a 2 cent diode to protect a $2 regulator, cheap insurance.

Over the next few days, will be sorting through the junk box to determine what is needed for the preamp.  I think that I have most of the components, just a few pots and resistors needed, I think.

6
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 14, 2013, 06:06:16 PM »
Woot, woot.  Preamp PSU completed.  R2 - 2 x 1K2 in series, R1 - 270R for a calculated 12.36V.  Actual measured voltage 12.5V with no load.
Changed out regulator, just in case.
Replaced the second 4,700/50V cap with a 100uF/25V, no problems  :tu:
Placed a diode across I/P & O/P, may not be required now that large cap has been replaced but for the sake of a 2 cent diode.
Simplified circuit diagram attached.

Board is larger than probably necessary, but I deliberately spaced everything out for ease of trouble shooting and repair, should it ever be required.

Tested supply on heaters of an ECC83 valve.  Nice glow.  :cheesy:

Thanks for the input, definitely some head scratching there for a while.

Now to start rounding up all the parts for the preamp.

7
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 11, 2013, 03:26:47 PM »
Rightho changing heaters to 12.6v. I did have a diode across the lm317t but I thought that this may have been the cause of the problem so I removed it.  Will change the capacitance to something like 10uF, will see what I have.  Will remove the elevation link then when put the circuit board into the case ensure that both circuits are earthed properly.

8
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 11, 2013, 08:46:58 AM »
Most of the ideas are taken from audiophile circuits.  Pretty much a standard circuit for hi-fi preamp.  Perhaps may be overkill for a guitar amp but was potentially going to use the PSU for a hi-fi preamp as well.  Might try losing the voltage elevation link for the moment and just working on the heater by itself. I can change out the 270 resistor and replace with a 220 and I can just have 2 x 1k resistors in series for R2, voila 12.61volts.   I''ll see what else I have in my junk box by way of a smaller cap with approx 25wv to replace the 4700uF.

9
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 10, 2013, 08:19:20 PM »
Rightho,
Will have to check over, especially around that area
PSU is on veroboard.
Attached diagram shows how it is wired
Anything obvious from that?
Cheers,
Chris

10
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 09, 2013, 11:45:55 PM »
Appears to be a problem with R2 supposed to be 1K and 100R in series.
When I measure R2a its 100R, when I measure R2b it's also 100R even though it's marked 1K.  When I measure the resistance across both R2a and R2b expecting 1K1 I get 0 ohms.  Something very fishy.

11
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 09, 2013, 09:03:49 PM »
RIghtho, Just wired up the PSU for the preamp as shown in the diagram.  All voltages as measured as shown also.
All seems to check out except for the heaters.  I was expecting a calculated 6.34V from the LM317T as per spec sheet, that is,  1.25 x (R2/R1 + 1).
R2 is 1100 ohms (a 1K and 100R in series), R1 = 270 ohms.
No matter what I do, all I get is 1.247V, which to me, doesn't seem correct, effectively R2/R1 appears to be 0???

Pin 1 is the adjust
Pin 2 is the output
Pin 3 is the input

My understanding this was from left to right when the device was viewed from the front, i.e the plastic part closest to you and the metal tab at the back furthest away from you.

Can anyone offer any help please??

Cheers
Chris

12
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 02, 2013, 10:24:46 PM »
Ah right that explains it.  From the circuit diagram of my amp.  Wishing to feed the 150k plate resistors with 250v rms or approx 350v peak.  The amp was designed to drop about 100v across the plate resistor so the tube was fed at about 150v.  Therefore plate current is about 100/150k or 0.7mA.  This checks out on the valve curves using grid voltage of -1.25v and a cathode resistor of 1k8, gives voltage of 145v and a current of 0.7mA.

Now on to the spice load resistors.  Require 250v at 0.7mA which means they should be 357k.  Selecting nearest standard value of 330k to load each stage, I can change the values of the filter resistors R2 and R5 to get 350v peak on the traces.

13
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 01, 2013, 06:45:50 PM »
Hi Roly,

Also, if you have time, would it be possible for you to do an LTSpice on the PSU, I have modified the PSU to have only two points, for the two stages of the preamp
I have done one but I am not sure of the resistors I have in place.  What I'm actually struggling with is how to load each stage in LTSpice, so that I get an indicative voltage.  I've put a 1K resistor to GND at each filter stage.  Is this correct?

Cheers
Chris

14
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: June 01, 2013, 05:08:07 PM »
Hi Roly,
Thanks for the input.

Re: PSU

The PSU shown is for the preamp only, due to the fact that it has powered by a 16VAC @ 1.25A wall wart.
I was going to build modular, so the other units will each have their own PSU.  May be a little OTT for each stage to have its own PSU but as you said:
  • Avoids the in phase/decoupling issues
  • I can take my time to build
  • I can spread the cost of the build over a longer period of time
  • Makes debugging easier
  • Makes refinements/alterations to circuits easier
  • Makes repair easier, should anything go wrong
I was also going to simplify the PSU.  Originally when it was first designed, it used back-to-back transformers and it did have filtering points A, B, C and D.  Now that I am using a wall wart for the preamp only, I can dispense with the other filtering points.  I just haven't got around to modifying/simplifying the PSU circuit yet.

Also the fact that we are discussing AVA106, the 450V hextupler off-line for the quad of 6L6GC, negates the use of the PSU shown for the PI/PA.


Re: Input Impedance

I'll probably just go with a 1M resistor rather than the input drive control. As you mentioned previously only really duplicating the effects of the volume control on the guitar and as you also said, still probably not as good as the 5M/FET combination that you have been developing.
However, having said that, with the input control and the unit being built as stand alone, there is the option to be able to use it with other devices such as a cd player.  Originally I was considering duplicating the preamp for stereo channels for hi-fi listening. That's why the heaters of the 12AX7 are running 6.3V and elevated, as I am sure you are aware, common practice in the audiophile world.  However, I haven't decided whether I am going to do this yet.  In all likelihood I won't and therefore will change the input drive as suggested.
 

Re: Overdrive

I can't for the life of me find the circuit diagram that I used for this part of the circuit.  I know that it came from the EL34World/Hoffman amp site and was labelled as something like a Blues Amp.  I am trying to find the circuit to determine if I've transposed it incorrectly.  Ah, I've just found it Tweed Overdrive Special or Carolina Overdrive and yes you are quite right. I have got it wrong.


Re: Tremelo

Oops my mistake on the labelling.  When I moved the diagram around, I reattached the labels incorrectly.

The circuit was taken from the article:
http://www.preservationsound.com/wp-content/uploads/2012/07/Vibrato.pdf

I know that is says that it can be capacitively coupled to the first audio stage, but I inserted another stage prior to the effects loop and PI because it and the oscillator will be a stand alone unit.
I take your point about the possibility of obtaining sufficient depth and that the LFO/LDR is commonplace these days but I did find the uniqueness of this circuit appealing.  However, uniqueness doesn't necessarily means that it works...hahaha.


Re: OPT
You've mentioned this mod previously and I thought I had included it.  I must have missed it in the subsequent diagram revisions.

Updated the diagram and attached

Cheers,
Chris

15
The Newcomer's Forum / Re: Newcomer, Melbourne, Australia
« on: May 31, 2013, 05:20:45 AM »
Well now we probably are going over the top  :loco


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