The official G9 help thread

[gyraf]

- Toroids: minimum 30VA each. If you have 50VA, that's probably good.

- Capacitors: As you say, don't go for snake-oil. The sonic contributions of the G9 does NOT come from capacitative fancyness. You can always experiment later, but you imo won't gain 1% extra quality from putting in €1000 worth of capacitors.

- HT 245V: Don't spend money on fancyness - rather spend it on upgrading to two 330 or 470uF/400V electrolytics (not larger) - this may help quiten down the supply a bit.

And last, but not least - do the recommended trace-cut and direct-wireing mod for the front-panel Hi-Z jacks. Can save you a lot of troubleshooting later.

Take care when working on high voltages (!)

Jakob E.

 

[Alexandru marian]

Thanks!

I've found a guy in this forum who got by with 25VA.  If I don't find better I'll try it too. 

Upgrading to 330uF, you mean for C14 and C15 right?  I had that on my list.  Maybe motor caps. 

jacks - I removed these completely, I'm the same guy with the simplified schematic on previous page. 

Speaking on previous page, I'll repost APL's questions just so they don't get buried because of me:

-----


Posted by: APL
« on: November 09, 2014, 05:10:39 AM »

    Insert Quote


I'm Just finishing the HT supply and  thinking about bleeder resistors.

1. I read that people have perminent  resistors directly across terminal s of capacitors (C14  and C15). Recomended values range 220k to 1 M.  Would 100k 2 W be OK?

2. Instead of having bleed resistor across terminals. Could I change mains switch to 4 pole and wire  2 bleed resistor/LED circuits  via the switch so C14 & c15 discharge in the "off" position.  Hope that makes sense.

 

[collinTHEbrewer]

Just finished my build. I'm having issues with the heater voltage. It's a little high from the transformer at around 18V. It's putting a lot of stress on regulator I think. It is getting pretty hot even with a heat sink. Eventually it craps out and the heater voltage does out. Any ideas?

 

[gyraf]

Just finished my build. I'm having issues with the heater voltage. It's a little high from the transformer at around 18V. It's putting a lot of stress on regulator I think. It is getting pretty hot even with a heat sink. Eventually it craps out and the heater voltage does out. Any ideas?

You just need plenty of heatsink. Make sure to isolate electrically.

Jakob E.

 

[APL]

Sorry if this has been asked/answered before but I’m working on the PSU unit and concerned about the voltages I am measuring.

The first transformer is  230:2x15 v .I measured input as 248 VAC (I’m in UK). I got 16.2 VAC on the secondary’s , which I assume is OK to feed the heater and Phantom rectification circuits.

Second transformer is 230: 2 x 12 v. So when I connect to the 16.2V, I get 294VAC at the output.

With the 294v input to the HT rectifier circuit I got an increasing output voltage I turned it off when it reached 375V dc. I am measuring without a load (I haven’t built the rest) so is this expected would the voltage regulate with tubes connected or do I have a problem?

 

[gyraf]

Maybe you should reverse order when in the UK - 12V transformer first, then the 15V transformer for the secondary?

Or get a 15V transformer for the second one?

Jakob E.

 

[collinTHEbrewer]

You just need plenty of heatsink. Make sure to isolate electrically.

Jakob E.

Do you recommend a particular heatsink?

 

[gyraf]

No particular. But must have a c/w (centigrade per watt) rating low enough to get rid of whatever heat you need to dissipate. Alternatively, mount (again, electrically isolated) to chassis.

Jakob E.

 

[collinTHEbrewer]

No particular. But must have a c/w (centigrade per watt) rating low enough to get rid of whatever heat you need to dissipate. Alternatively, mount (again, electrically isolated) to chassis.

What's a good C/W rating for this application?

 

[APL]

Just for information, as Jacob recommended a few posts back, I have reversed the power transformers.

1st transformer 230: 2x 12v 50VA - The 2 “12v” outputs were 14.1 vac

2nd transformer 230: 2x 15 v 50VA (reversed)  -  With the 14.1v as input the output  was 194V.

On these results I suppose if anything I should replace 2nd transformer with another 12 v to increase 194 v. But I will go ahead and build rest and see what I end up with a load connected.

 

[milosh]

Dear forum members,

I am in the middle of stuffing the main board for my G9.
I have some questions about some electrolytic caps.

C-16 C17 C18 (blue on the photo) - on the schematic they are1000u 25 V (thats what I bought) but i noticed that on the PCB they are 35V
When i put them on the PCB they look small..
Should i buy bigger 35V ones?

C25 (10u/250V - yellow on the photo) - I bought not expensive cap but I see some people go with some extraordinary caps here.
On gyraf audio site it is stated C25/R31 are filtering the input stage. Should I buy some extraordinary caps here or what I have is OK?

 

[Deepdark]

You're ok with your "cheap" cap. It's up to you to put the most expensive caps on earth or put some average, standard quality caps there. For the other which didn't fit the footprint, If you respect the value and the voltage rating, it's ok. Some manufacturer developped smaller and compact version of their caps, such as Nichicon.

 

[milosh]

Thanks Deepdark!

 

[Alexandru marian]

Mundorf makes an interesting 300+300uF 500VDC 3 pin cap. 

It has a common negative lead.  I was wondering if it would work for C14 and C15.  But in between the negatives of 14 and 15 there is a resistor connecting to ground.  Does this changes things or can I use the 3 pin cap?  The schematic will change to this:

Also,  is the power rating enough, or too close?

 

[Alexandru marian]

And another matter

I've never used a preamp with separate gain and volume control. What does the volume pot do exactly? 

Assuming all I want from the pre is 35 to 45 db of clean gain, can I replace the pot with a certain fixed resistor and use just the switch?

To answer myself, I imagine the pot allows to get some tube distortion at an acceptable output level.  So you turn the gain switch up high, then reduce the volume via pot as much as you need?    So for a clean, simple operation sound, the pot should be replaced with a full open value: a 47k - 0k voltage divider. 

 

[gyraf]

..switch is first-stage gain, pot is drive to the output stage. Both are nice to have, as sonic imprint is different when input is high and output is low than when opposite...

But yes, you can wire in two resistors for the pot if you really want fixed output drive.

Jakob E.

 

[Alexandru marian]

Thanks!  I'll keep the pots in after all, found some nice TKDs  

Now that i have all parts and figuring out layout and details: 

-both valves need to have the heaters powered, right?  PCB photo seems to confirms this.  Just that schematic omits "ff" for V1. 
-heater polarity doesn't matter. I see one valve has +12 to pin 4, the other to pin 5.  This is likely to ease up the PCB layout. 
-pins 9 are unused (they are used just for 6V heating)

Lundahl wiring:  hope I have this OK.  trying to make sure I won't put them in reverse  

Edit:  my scheme ignores the phase switch, which I want to exclude.  I tried to verify it using the PCB file but from there it seems I am having it all wrong. :|

 

[Alexandru marian]

Let's just look at 1528 first.  From the PCB files I can look at the traces and pin numbers then apply them to the Lundahl schematic.    It looks nothing like the Gyraf schematic.  Probably the pre schematic shows the wiring for OEP?

Scrolling down the LL pdf, they show the wiring for 1:5 and seems to be the same as my drawing so i guess this one is solved. 

 

[Alexandru marian]

5402:  why "normal" phase has no 10k resistor (besides R44) but reversed phase has both R32 and R44?  (trying to  figure out wiring by excluding the circuit with R32. I assume this is the reversed phase which I want to exclude)

 

[APL]

Sorry Alexandru I can’t help with that hope you don’t mind if I ask a question meanwhile.
I’ve just bought some old gear off ebay for the Lundahl  LL5402s. There were also 2 transformers which I believe are Marinair T1442. I done a search and these are 2:1 transformers. I assume they could be used instead of the LL5402s on the output? or as a line input.
I assume they would not be suitable replacement for the input LL 1528. However there is a comparison by JLM audio which appears to compare the T1442 with LL1538 which is described as a mic transformer in Lundahl data sheet. Would T1442 work as an input on G9?