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Old 26th August 2011, 08:14 PM   #1151
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Hi Andy,

ah, I forgot, filament bias, so you adjust your current through B+. Still I would not feel comfortable if that change would have a significant sonic impact. As the circuit and tubes age things move and would then change the sound too.

How did you get these current values? Did you measure them or just read them from the datasheet? Keep in mind that actual tube samples can differ quite significantly from the nominal values. Even if they are NOS. You can have a spread from 60 to 110% ! Again a reason why the circuit should be tolerant to such variations.

It is a common misconception that transformer coupled circuits have no cap in the signal path. The signal travels in loops. Each stage has a input and output signal loop. The output loop is in case of transformer coupling: tube plate to transformer primary to B+ from there through the last B+ cap to ground from ground through the cathode resistor anf bypass cap (if any) to the cathode and through the tube to the plate.

This is where ultrapath comes into play. The ultrapath cap 'shortens' the signal path from B+ directly to the cathode. There is a similar idea behind filament bias, the cathode resistor is of smaller value which has less sonic impact (IME) and which does not need a bypass cap.

With LC coupling the plate choke efectivly decouples the B+ from the signal path since it is a high AC impedance. The signal travels from the plate through the coupling cap to the grid of the following stage. From here through the grid resistor to ground and back to the cathode through the cathode resistor and bypass cap.

This is an idealized way of looking at the signal path. In reality there will be a multitude of parallel paths due to stray capacitances, leakage inductances and what not. Still looking at it this way helps to understand which parts are important. In your case the sound will be mainly influenced by the tube, transformer and that 47uF cap. This cap should be of best quality. An electrolytic here can ruin the whole thing. Also note that a cap which works well as coupling cap, might not sound as good as last B+ cap.

This is why I remind people to be cautious to jump to conclusions how a certian part sounds. It is always all those parts in the signal path which act together.

Don't get too hung up about the current through the transformer. You do not use the full signal swing it can deliver, which gives more current headroom.

Still I would encourage you to start to do some basic measurements to make sure everything is where it should be. Sure the ear should be the final judge, but the basic technical parameters need to be in order to ensure the listening test is meaningful.

Best regards

Thomas
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Old 26th August 2011, 08:21 PM   #1152
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Hi again,

one more remark: keep in mind that depending if you have the filament resistor connected to the + or - side of the filament, half of the filament voltage will be either added or subtracted from the bias voltage.

With filament bias or fixed bias, plate current can be easily measured by getting the voltage drop through the transformer primary winding. Then calculate the current by ohms law from that voltage and the DCR of the winding. Also if you use RC decoupling in the B+ you can do the same by measuring the voltage across the dropping resistor.

Thomas
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Old 26th August 2011, 09:36 PM   #1153
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Hi Thomas,

Don't worry, I measure extensively to determine operating points, current, voltages, resistances etc. I have a huge set of equations I've put into Excel spreadsheets. I calculate everything even though I don't use a scope.

I had in fact read the operating point off the curves in the first instance. The true current going through the 26 is between 3 and 3.5mA depending which valve. That's actually fine and does not need changing.

Next to look at which way round I have the LL1660 - this turns out to be 4:4.5. Now, what I don't know is how many Henries I would have on the "secondary" of 4.5 (1.25K static resistance) as opposed to the "primary" of 4 (1.1K). If I get as many Henries (or more) on the 4.5 side I can also try this in 4.5:2 (alt T).

Do you have any idea of these impedances in Henries?

Andy
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Old 26th August 2011, 10:04 PM   #1154
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Hi Andy,

the inductance scales by the square of the windings. So the factor is (4.5/4)*(4.5/4) which is about 1.26

Current handling scales about linearily with the windings (more windings/inductance = less current capability)

Best regards

Thomas
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Old 26th August 2011, 11:02 PM   #1155
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Quote:
Originally Posted by Vinylsavor View Post
Hi Andy,

the inductance scales by the square of the windings. So the factor is (4.5/4)*(4.5/4) which is about 1.26
So the other way around is .79. That's more of a difference than I thought. Must try it in 4.5:4.

I take your point about the current. Some experiments coming!!

Andy
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Old 27th August 2011, 01:32 PM   #1156
mogliaa is offline mogliaa  United Kingdom
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Hi guys,
I have been playing around with a test bed for a 26 pre-amplifier. What I also did was to measure various valve distortion on a simple setup. Mine is a CCS loaded 26 valve with LED bias for simplicity, and also what I had at hand.

My laptop/soundcard is having some issues in measuring distortion, however I had applied same method across the board, so what I aimed at in my tests was to identify which valve/brand had the lowest distortion figure.

I run the 26 at 8mA and anode voltage was around 145V to 160V depending on the valve. In summary what I found was that lowest distortion was provided by the Sylvania ST 26s. See below my table:

The LED array provided Vg=-8.2V @ 8mA

Valve THD
Sylvania 26 ST 0.022%
National Union ST 0.0305%
CX326 globle 0.046%
Super Silvertone ST 0.046%
Sylvania 26 globe 0.024%

I did get slightly better results with the 4P1L! This valves is really linear. Well, Andy said it's very microphonic. So will need to do a listening test.

I will build a modular pre-amp so I can play around and exchange different valve types, loads like choke, CCS, etc. and different bias

Hope you find this simple testing a bit interesting as I did!
Cheers,
Ale
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File Type: jpg 26 TEST.jpg (117.3 KB, 591 views)
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Old 27th August 2011, 01:54 PM   #1157
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Check my calculations here to see if they are correct (I've simplified a bit):

Voltage into 26 = 2v
Mu = 8
Ra = 8
Voltage out of 26, into LL1660 = 16v
Voltages out of LL1660:
4:4.5 = 18
4.5:4 = 14
4.5:2 = 7
4.5:1 = 3.6

If this is correct, I personally would need 4:4.5 or 4.5:4 for my 3 amplification stages (followed by 46 and 300b). I can try 4.5:2 but it looks a bit short.

Andy
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Old 27th August 2011, 02:12 PM   #1158
regal is offline regal  United States
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Quote:
Originally Posted by mogliaa View Post
Hi guys,
I have been playing around with a test bed for a 26 pre-amplifier. What I also did was to measure various valve distortion on a simple setup. Mine is a CCS loaded 26 valve with LED bias for simplicity, and also what I had at hand.

My laptop/soundcard is having some issues in measuring distortion, however I had applied same method across the board, so what I aimed at in my tests was to identify which valve/brand had the lowest distortion figure.

I run the 26 at 8mA and anode voltage was around 145V to 160V depending on the valve. In summary what I found was that lowest distortion was provided by the Sylvania ST 26s. See below my table:

The LED array provided Vg=-8.2V @ 8mA

Valve THD
Sylvania 26 ST 0.022%
National Union ST 0.0305%
CX326 globle 0.046%
Super Silvertone ST 0.046%
Sylvania 26 globe 0.024%

I did get slightly better results with the 4P1L! This valves is really linear. Well, Andy said it's very microphonic. So will need to do a listening test.

I will build a modular pre-amp so I can play around and exchange different valve types, loads like choke, CCS, etc. and different bias

Hope you find this simple testing a bit interesting as I did!
Cheers,
Ale
Those #'s are very good, this was on the mu out? What was the load? Most soundcard have a terrible load for testing preamps (~1k).
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Old 27th August 2011, 02:49 PM   #1159
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Hi Andy,

Quote:
Originally Posted by andyjevans View Post
Check my calculations here to see if they are correct (I've simplified a bit):
Yes they are correct except that the mu of the 26 is 8.3 but that doesn't make a big difference. Also the numbers are only correct if the transformer secondary is unloaded

Thomas
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Old 27th August 2011, 02:58 PM   #1160
jamesdb is offline jamesdb  United States
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Andy, vt4c.com has purpose built plate chokes that are 200H 30ma DCR 2800 ohms at comparable prices. I wonder if those would be better than Hammond general purpose chokes. Plus two of those in series would increase your inductance to 400H total.
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