U47 clone plate voltage of various tubes
« on: March 10, 2021, 12:01:27 AM »
Hi, I would like to hear your experiences with plate voltages in u47 builds using different tubes. Particularly with ef184 and ef802 but experiences with other tubes are welcome too.
I have just installed an ef184 in my d-ef47 ( Oliver/Dany) and the plate voltage is on the high side ( 60v) but the bias is spot on (1.1v) at 5.2v heater and the mic sounds great. Is there any reason to try and lower the plate voltage and what would be the best way to do it in this build?
Thanks in advance.


abbey road d enfer

Re: U47 clone plate voltage of various tubes
« Reply #1 on: March 10, 2021, 01:08:58 AM »
Plate voltage is not a very important parameter. It needs to be low enough to prevent excessive grid current. How low is low enough? It depends on the tube's construction and it's generally a non-documented parameter. One tube can be as high as 70V, when one would need to be at 40V.
Bias current is generally the dominant parameter.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Gus

Re: U47 clone plate voltage of various tubes
« Reply #2 on: March 10, 2021, 05:19:55 AM »
Who says that 1.1VDC at the cathode is good for that tube?

You might be stuck looking at the wrong thing

Think more about plate out circuits

Re: U47 clone plate voltage of various tubes
« Reply #3 on: March 11, 2021, 12:48:26 PM »
Who says that 1.1VDC at the cathode is good for that tube?

You might be stuck looking at the wrong thing

Think more about plate out circuits
Good point, I am new to this so yes I am going with the general consensus. I see that in with vf14, ef14, ef800 this is the cathode voltage they go for so I never questioned it. Is there a practical way to find the optimal voltage? Thanks.

Moby

Re: U47 clone plate voltage of various tubes
« Reply #4 on: March 11, 2021, 01:51:36 PM »
Hi, I would like to hear your experiences with plate voltages in u47 builds using different tubes. Particularly with ef184 and ef802 but experiences with other tubes are welcome too.
I have just installed an ef184 in my d-ef47 ( Oliver/Dany) and the plate voltage is on the high side ( 60v) but the bias is spot on (1.1v) at 5.2v heater and the mic sounds great. Is there any reason to try and lower the plate voltage and what would be the best way to do it in this build?
Thanks in advance.
Every tube has his own characteristics and different "emission". So, to put the tube in the right operating point you should trim the bias point and in this case to make the current similar as Vf14. So, change the bias, it can be even 0.5 or 2v depending on tube, and make it at anode approx 34v. I don't say that every tube you put in that operating point can do the job, but most of them can. Happy experimenting :)
For microphone transformers,  BV.8,  Bv.11,  Bv.12, etc.. contact me at mobyelectronics at gmail dot com

Re: U47 clone plate voltage of various tubes
« Reply #5 on: March 11, 2021, 02:21:43 PM »
Every tube has his own characteristics and different "emission". So, to put the tube in the right operating point you should trim the bias point and in this case to make the current similar as Vf14. So, change the bias, it can be even 0.5 or 2v depending on tube, and make it at anode approx 34v. I don't say that every tube you put in that operating point can do the job, but most of them can. Happy experimenting :)
Hey Moby, thanks. So I guess in the d-ef47 the 30ohm is regulating the bias? Maybe I should have a trim resistor there and experiment haha...
As far as the plate voltage I was a little confused. I thought that by changing the 100k resistor would affect the voltage but when I did it didn’t change. It stayed at 60v

Re: U47 clone plate voltage of various tubes
« Reply #6 on: March 11, 2021, 03:48:20 PM »
Plate voltage is not a very important parameter. It needs to be low enough to prevent excessive grid current. How low is low enough? It depends on the tube's construction and it's generally a non-documented parameter. One tube can be as high as 70V, when one would need to be at 40V.
Bias current is generally the dominant parameter.
Thank you so much

Moby

Re: U47 clone plate voltage of various tubes
« Reply #7 on: March 11, 2021, 05:06:25 PM »
Hey Moby, thanks. So I guess in the d-ef47 the 30ohm is regulating the bias? Maybe I should have a trim resistor there and experiment haha...
As far as the plate voltage I was a little confused. I thought that by changing the 100k resistor would affect the voltage but when I did it didn’t change. It stayed at 60v
hey, I'm not familiar with the d-ef47 circuit but as I mentioned before,  you should find correct operating point by biasing tube. :)
For microphone transformers,  BV.8,  Bv.11,  Bv.12, etc.. contact me at mobyelectronics at gmail dot com

Re: U47 clone plate voltage of various tubes
« Reply #8 on: March 11, 2021, 06:04:44 PM »
hey, I'm not familiar with the d-ef47 circuit but as I mentioned before,  you should find correct operating point by biasing tube. :)
Yes of course...but how? Haha...any good reads/books on tube mic and amp circuitry? 

Moby

Re: U47 clone plate voltage of various tubes
« Reply #9 on: March 11, 2021, 06:30:49 PM »
Yes of course...but how? Haha...any good reads/books on tube mic and amp circuitry?
If I look at the correct schematics, there is a voltage divider 100ohm/29ohm. At their junction you measure 1.1v. right? If yes, then you can vary 100ohm to achieve the correct bias (operating point). Ok?
For microphone transformers,  BV.8,  Bv.11,  Bv.12, etc.. contact me at mobyelectronics at gmail dot com


Re: U47 clone plate voltage of various tubes
« Reply #10 on: March 11, 2021, 06:43:07 PM »
If I look at the correct schematics, there is a voltage divider 100ohm/29ohm. At their junction you measure 1.1v. right? If yes, then you can vary 100ohm to achieve the correct bias (operating point). Ok?
Correct, yes I believe both those resistors are affecting bias. I think in the 100ohm position I could use a trim resistor. That could also give me room to play around with B+ for capsule polarization. Go up a little...maybe.

Re: U47 clone plate voltage of various tubes
« Reply #11 on: March 13, 2021, 10:05:13 AM »
Hi, I would like to hear your experiences with plate voltages in u47 builds using different tubes. Particularly with ef184 and ef802 but experiences with other tubes are welcome too.
I have just installed an ef184 in my d-ef47 ( Oliver/Dany) and the plate voltage is on the high side ( 60v) but the bias is spot on (1.1v) at 5.2v heater and the mic sounds great. Is there any reason to try and lower the plate voltage and what would be the best way to do it in this build?
Thanks in advance.
What's your B+ and plate resistor?

(stock U47 = 105V / 130K)

Gus

Re: U47 clone plate voltage of various tubes
« Reply #12 on: March 13, 2021, 10:45:12 AM »
Posts are missing what you want

I don't want to post what you want however, you should be able to figure it out if you read and think about it

Re: U47 clone plate voltage of various tubes
« Reply #13 on: March 13, 2021, 11:23:11 AM »
What's your B+ and plate resistor?

(stock U47 = 105V / 130K)
B+ is 105v and the resistor is 100k. Actually I raised it to 130k but the plate voltage remained the same, no drop. I am a little confused by the outcome haha

Re: U47 clone plate voltage of various tubes
« Reply #14 on: March 13, 2021, 11:29:45 AM »
Posts are missing what you want

I don't want to post what you want however, you should be able to figure it out if you read and think about it
Hey Gus thanks, I am mostly looking for the right way to do it. I mean specific details would be great too but being new to this sometimes I don’t even know where to start or what to search for haha. So far all my builds have been successful and compare very favorably to the originals but I lack the knowledge and deeper understanding of how things actually work

Re: U47 clone plate voltage of various tubes
« Reply #15 on: March 14, 2021, 08:44:52 AM »
B+ is 105v and the resistor is 100k. Actually I raised it to 130k but the plate voltage remained the same, no drop. I am a little confused by the outcome haha
U47 has 2 plate resistors (100K + 30K). Check to make sure you know the exact value in your build as you need it to calculate plate current.

3 measurements you need are B+, plate voltage, and plate resistance.

Using values from original U47 schematic:
105V (B+) - 34V (plate voltage) = 71V (voltage drop across plate resistor)
71V / 130K (plate resistance) = 0.55mA plate current

That's the number you want to focus on.

Using your values:
105V - 60V = 45V
45V / 100K = 0.45mA

So your bias is running a bit cold. But if it sounds good to you at 0.45mA, then it is good – I like what 1mA does to the lowend in U47 builds, you might prefer 0.45mA to 0.6mA. You'll have to try them all out and see.

You can adjust bias with trimpot on cathode, smaller/bigger plate resistor, lower/higher B+ etc.

Re: U47 clone plate voltage of various tubes
« Reply #16 on: March 14, 2021, 10:39:35 AM »
U47 has 2 plate resistors (100K + 30K). Check to make sure you know the exact value in your build as you need it to calculate plate current.

3 measurements you need are B+, plate voltage, and plate resistance.

Using values from original U47 schematic:
105V (B+) - 34V (plate voltage) = 71V (voltage drop across plate resistor)
71V / 130K (plate resistance) = 0.55mA plate current

That's the number you want to focus on.

Using your values:
105V - 60V = 45V
45V / 100K = 0.45mA

So your bias is running a bit cold. But if it sounds good to you at 0.45mA, then it is good – I like what 1mA does to the lowend in U47 builds, you might prefer 0.45mA to 0.6mA. You'll have to try them all out and see.

You can adjust bias with trimpot on cathode, smaller/bigger plate resistor, lower/higher B+ etc.
Thank you so so much. Now it makes sense to me.
In my build I have both 100k and 30k resistors so my plate current is around 0.35mA...
So if I replace the 100k to a 50k ( total plate resistance 80k) I should be getting 0.56mA plate current.
105-60=45v
45/(50+30k)=0.56mA

Re: U47 clone plate voltage of various tubes
« Reply #17 on: March 14, 2021, 10:57:45 AM »
Changing plate resistance will also change your plate voltage. So you'll have to measure again with the new plate resistance and redo the calculation  :)

Re: U47 clone plate voltage of various tubes
« Reply #18 on: March 14, 2021, 11:13:36 AM »
Changing plate resistance will also change your plate voltage. So you'll have to measure again with the new plate resistance and redo the calculation  :)
Yes I meant to ask you about that...I guess it’s going to be a balancing game haha. When I raised the 100k to 130k though thinking that this will lower the plate voltage there was minimal drop if any.

Moby

Re: U47 clone plate voltage of various tubes
« Reply #19 on: March 14, 2021, 12:19:09 PM »
Yes I meant to ask you about that...I guess it’s going to be a balancing game haha. When I raised the 100k to 130k though thinking that this will lower the plate voltage there was minimal drop if any.
Changing chatode bias will change your anode current. I already explained what you have to do
For microphone transformers,  BV.8,  Bv.11,  Bv.12, etc.. contact me at mobyelectronics at gmail dot com


 

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