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In Reply to: RE: Ahhhh, you want the magic of tube hype posted by Best audiotransformers on December 18, 2023 at 08:17:06
>>A 300B can make easely a swing of 450Vpp with a 1:1 interstage.
How to? max plate dissipation of 300B is 40W
putting typical 300B plate to cathode 450V is dangerous, even risk in 450V plate voltage you need 80mA+ to stay in linearity straight line, It eat up the plate max dissipation, the tube can only last less than a month
Even if risk 450V plate to cathode 450V, the 300B could only produce max 370V p p swing at high distortion, or 350V p to p for safe play.
If that is so easy for 300B swing the sufficient voltage then Lundahl and Sowter do not need to produce 1:2 especially for 845 coupling
Follow Ups:
How? To do the calculations right to start with.Look at the datasheets of WE and you see that at Vdc= 400. Ia =60mA load =3.5K the power is 10.5 Watts.
That means in 8 Ohm you have 9.2 V and 1.125 A (=10.5 W) That also means with a transformer with a ratio of 21:1 (3500Ω to 8Ω) the primary Voltage is 192Vrms. I hope you can do the calculations Vrms to Vpp yourself.
Edits: 12/18/23 12/18/23 12/18/23
I know the datasheet but I did several experiments.
Have you looked at the secondary waveform at 1:1 at plate 400V 60mA when reaching 192V RMS
With 420V 65mA at 1:1 1kHz before the secondary reaching 140V rms the waveform is obviously distorted. half cycle start having a notch. Afterwards I got an Lundahl 1677, with 350V 60mA the secondary goes all the way to 160V rms without deformed shape
I once bought the Cary 805B it's 845 was 860V plate and 110mA idle current at 8K OPT. The bias was negative 105V. The Cary use 420V 65mA on 300B and any grid voltage after 75V RMS will cause it to reach A2. So 1:1 is OK for this case
My case is 1200V 70mA bais -190V with 11.5K OPT , so this need more clean swing
Hi,
The rear one is the stock EI 1:1 interstage with permalloy sheet, as said, output approaching 135V RMS the waveform becoming apparent distorted, I then changed to Lundahl 1677 1:2, no deformed waveform up to 160V rms. So what would you choose
Transformer manufacturers produce 1:1.5(Tango NC21)and 1:2(Lundahl 1677, Sowter 8423)，there must be reason they did that
You need to buy a better (much bigger) 1:1 interstage. Your measurements only show that the transformer you used in the past was not designed to allow the swing you need.
Lundahl says max is 145Vrms @30Hz for the ll1677 and i know this is a correct value. At low frequencies the interstage has more limitations then at 1kHz. Lundahl at 20Hz will be even worse then 145Vrms.
Edits: 12/25/23
The weight of the previous 1:1 EI interstage is about the same as Lundahl 1677, both weigh around 0.8kg, but the Lundahl 1:2 could see more swing at less distortion
Another advantage of using step up interstage is using lower 300B plate voltage, say 335V at 80mA is lying on quite straight line in transfer curve. but with 1:1.5 it is virtually 500V plate V allowing more linear AC swing within 150V rms(which is the max I need)，335V 80mA is also not stressing the 300B talking about 26.8W idle plate dissipation.
However with 430V plate for the less output swing, to obtain the similar straight line it might need run to 90mA which already eat up the 300B
My latest interstage is 1.5kg (photo attached), twice as big as Lundahl 1677, it is 1:1.5, 27Henry primary, so do you think it is still good to go back 1:1, for the moment I cannot find any reason
"Another advantage of using step up interstage is using lower 300B plate voltage, say 335V at 80mA is lying on quite **straight line in transfer curve**." Can you show me this?Thanks
Here is a poor example of a voltage gain tube. The graph on the right is the plate curves (with the operating point and load line plotted) and the graph on the left (derived from the plate curves with the operating point and load line plotted) is the transfer curve. The input wave is on the left and the output wave form is on the right. Can you see how truncated the one half of the output waveform is? Who ever did this picked a poor operating point and load impedance for that tube. The output wave form is very distorted.
Is this what you meant by "transfer curve"?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/20/24
The same weight between EI core and C-core means a lot of differances.
Roughly the EI has 50% loss in performance because of the core (if the bobbin and windings are the same).
Weight alone is not enough to know if the interstage is capable to do the job. You have to know inductance value, Idc and Vrms max of the core at a give frequency.
Also, since I use 1200V bias -190V, to get 845 beyond that and started to enter A2 it only take 1 or 2 mA to flow into grid
With 1:1.5 the equivalent 300B tube resistance at secondary will be like 1500 ohm, the estimation is that by 210V grid it drive 845 to put out 42 watt rms which is the current target
this
Hi,
Having looked at the article, are you implying the operating point causing asymmetry output? It may be in some scenarios but may not be my case, when we talk about A1 region where 845 grid remains relatively high input impedance
With coil load it can swing plenty voltage, however when it approaches to the boundary then the output may not be as linear. Putting a 1:1.5 ITC may help on utilizing more on linear portion of the output
When I email to Lundahl I got the above reply, and the experiments I did confirm that
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