The official G9 help thread

[andow]

First post, so hello everybody!  8)
Really great forum with lots of valuable info!

I'm about to source parts for my first G9 build and I can't get C4 which is 1,5µF. Is it ok to use 1µF or 2,2µ here?

C4 is a signal coupling cap, so making it smaller will effect lower frequencies.  So 2.2uF would be fine but 1uF I am not sure if the effect would be audible ( as I don't know the impedance of the circuit at that point). 

General rule, don't make signal coupling caps smaller. 

As C4 is part of a feedback circuit I am not sure if a smaller cap would increase or decrease low frequencies, but using a larger cap won't effect them.

Thank you very much for the fast reply!
That's pretty much what I thought, but I wasn't sure if C4 has any other purpose than DC filtering...

Speaking of filtering, another question comes to my mind 
The way I see it the variable high pass filter is formed by C8 (10nF) or C9 (22nF) and 47kOhm from the output pot. However, using these values I get cut-off frequencies at 338Hz and 153Hz respectively, while they are supposed to be 80Hz and 160Hz.
Can someone please explain this?

 

[Alexandru marian]

1.5uF is made by many of the usual brands.  A cheap one would be the claritycap ESA 250V. 

I think the resistance of the filter comes from before the caps, not after (pot).  And that part is complicated, no idea

 

[andow]

1.5uF is made by many of the usual brands.  A cheap one would be the claritycap ESA 250V. 

I think the resistance of the filter comes from before the caps, not after (pot).  And that part is complicated, no idea

The problem is I don't want to pay shipping costs for ordering 2 caps 
The two shops I order at (Tube Town and Reichelt) don't carry them....

Edit:
Resistance before the caps wouldn't make sense for a highpass.

 

[Deepdark]

Hi guys

Quick one. I see some folks using one PT rated 15V+15V and 1 PT rated 12V + 12V. Also, one the schematic it's stated 2x 15V+15V but one the shop list (from the gyraf page) it's stated 1x 15V+15V and 1x 12V+12V.  I would like to know if we're better with one or the other. Also, I saw some folks hadn't have much success with 30VA PT. Is there any conclusion such as what power rating we should take.

I plan going with Avel. So there is already the 12+12 and 15+15 at 30VA available.

Avel Y236102 30VA 12V+12V Toroidal Transformer
Avel Y236103 30VA 15V+15V Toroidal Transformer

Thanks.

 

[Alexandru marian]

You need one 2x12 and one 2x15 to be on the safe side in terms of B+ voltage.  The downside is sometimes the second trafo (12V) might not like being fed 15V and it will make a mechanical noise.  Also the first trafo can get hot.  It happened to me using 80VA Nuvotem trafos so avoid those. 
Each secondary should be 30VA to be on the safe side, so buy 60VA trafos, maybe 40 if they are good quality.  The biggest draw comes from the heaters.  0.6A, say 1 to be safe, x15 you get 15VA.  High and 48 don't draw that much but it is always better not to run the trafos close to the limit. 

If you are rich, think about a custom made step-up trafo.  275V 100mA, 12V 2A, 15V 0.5A secondaries, electrostatic shield, magnetic shield.  It can go up to 200 euro but worth it imo.  Less hum, and it will never heat.  Also feeding less voltas in the heater regulator will generate less heat.  I can feel a clear difference on the heatsink just from going from 15 to 14VAC.  12 should be even better.   

 

[Deepdark]

feeding 15v into 12v?? I would think 12v through 15v. Why we can't get 2x15V+15V?

 

[Alexandru marian]

The original design used 15 - 15 resulting in 220VAC input to the B+ supply.  Some people complained of not getting enough voltage at the output. In theory it shoudl be enough: you multiply by 1.4 to get DC:  308V. Sounds like plenty for the regulator to step down to aprox 255 and then 245 after the last resistor but in real life it might not work as well under full load.
So Jakob changed the second trafo to 12V.  So you now get 15:12x220=275VAC,  385DC after diodes. 

 

[Deepdark]

Ok, I just found 2 Multicomp from Farnell rated 50VA.

2x 15V 1.67A
2x 12V 2.08A

Is it ok? I would like to know if they are good quality transformer or a total dump? Thanks

 

[Alexandru marian]

Too high is not bad!  Again, Jakob's safest recommendation is 60VA total.  I think the confusion (been there) arises from the website/schematic asking ratings per each winding while sellers state the total power rating. 

If you are OK with ordering overseas, the cheapest stepup  trafo I found is here:  http://www.ampmaker.com/store/20W-toroid-power-transformer-PT06.html      I don't know how good it is but it has the perfect voltage. 
You will use just the 275 - 0 taps to power the B+.    Then also order a let's say 50VA 2x15.  One winding powers the heater, the other the phantom.  You do need separate windings for these two otherwise the 12V bridge will blow up. 
Trafos are separated and you won't have any heating or saturation issues. 

 

[Deepdark]

When you say 60VA total, is it 30VA per transformer or 60VA per transformer?

 

[Alexandru marian]

per transformer.    When a seller says 50VA, it means 25VA per secondary assuming they are identical.  For example 15V x 1.6A

 

[Deepdark]

ahhh ok so when Jakob said 30VA it was refering 30VA per winding? So 60VA per transformer?

 

[Alexandru marian]

Right.  It is noticeably over-spec but better than to run them at the limit and risk them burning.  Also think twice about using a real stepup trafo instead of a back to back.  If the cheap one does not attract you, give Plitron (in Canada) a call and they will make you a killer single trafo.  Again:  275V 100mA,  12V 2-3A (or 13-14 to be sure)  and 15V 0.5A for phantom.

 

[Deepdark]

ok. thanks. What I want to know is if Jakob meant to use min 30VA total or was he referencing to 30VA per winding? Because some dude use the Avel (rated 30VA) and don't seem to had trouble

 

[Alexandru marian]

My exact exchange with Jakob a few pages back was: 
--toroidals: is 30VA required for each secondary or in total.
J:  - Toroids: minimum 30VA each.

Not a clear as day answer, one it can read it "30VA each [transformer in total]", but I did not want to stress J even more than I already did.  I choose to read it 30VA each winding as having thicker copper is always a safer bet.    Bigger trafos do not cost a  fortune more and you shoudl have plenty of space for them.  If something "works" for a certain dude it is not a 100% guarantee. My trafos were cold after 10 or 20 minutes of use. After 2 hours they were HOT and who knows, left powered overnight might have burned?  This being said I've read good things about Avel.  If you can find bigger  than 30VA, really do it. It is just a few $. 

 

[Deepdark]

I just found the thread. I think He means 30VA PER transformer instead of 30VA per coil. But, if you got more and it works, it's cool then

 

[Deepdark]

Hi

I just found a 50VA triad which is rated 4.16A. Is it too much overkill???

 

[andow]

Thank you very much for the fast reply!
That's pretty much what I thought, but I wasn't sure if C4 has any other purpose than DC filtering...

Speaking of filtering, another question comes to my mind 
The way I see it the variable high pass filter is formed by C8 (10nF) or C9 (22nF) and 47kOhm from the output pot. However, using these values I get cut-off frequencies at 338Hz and 153Hz respectively, while they are supposed to be 80Hz and 160Hz.
Can someone please explain this?

Anyone else? I'm planning to modify the filter...

And regarding the 1,5µF cap: I found this one rated as "interference suppression capacitor" but I can't find any difference to a conventional film cap...
https://www.reichelt.de/Supressor-Capacitors/PAN-X2-1500N/3/index.html?&ACTION=3&LA=2&ARTICLE=129280&GROUPID=3157&artnr=PAN-X2-1500N

 

[gyraf]

Try building the unit as recommended - then only later change/upgrade parts if it does not work for what you need.

Modifying BEFORE actually having an unit built will most often only result in problems and frustrations while finding errors.

Jakob E.

 

[andow]

Try building the unit as recommended - then only later change/upgrade parts if it does not work for what you need.

Modifying BEFORE actually having an unit built will most often only result in problems and frustrations while finding errors.

Jakob E.

Of course that's what I will do! But I want to order all the parts I will need later at once to save shipping costs. Besides, it won't hurt to understand the circuit...

Any info on the interference suppression cap as C4? Can I use it for an audio circuit or should I substitute with a conventional 2,2µF film cap?