U47 clone plate voltage of various tubes

[Banzai]

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.

From 130K to 160K is a small change. Halving or doubling plate resistance will give better results.

Can also increase B+ to 120V. Just change your divider to 2x 1M from the original 2M+3M to keep 60V at the capsule. If your capsule can take it, try 1M+2M for 75-80V and see how it sounds. Experiment with all of it.

Not familiar with what you're building though, so all this is assuming you're using a modern PSU (separate heater and B+ supplies)

 

[Purplenoise]

Changing chatode bias will change your anode current. I already explained what you have to do

Yes, I just try to make sense of it all. Basically Banzai offers a way to change plate current by varying the 100k resistor and you by varying the 100ohm bias resistor...unless of course you guys talk about two different things. Of course since my knowledge is limited I can’t evaluate...just keep experimenting and reading until it starts to make sense.
Thanks man, I am getting a lot of great info

 

[Banzai]

If you're not sure how to go about it, what you'll be measuring to set bias is the plate voltage.

Step by step:
1) Choose a plate resistance (50K, 100K, etc)
2) Set B+ (105, 120V, etc)
3) Choose a target plate current (0.6mA, 1mA etc)
4) Calculate plate voltage needed to achieve target (50, 60, 70V etc)
5) Adjust bias until you measure plate voltage calculated in step 4

Example:
1) 100K plate resistance
2) 120V B+
3) 0.6mA plate current
4) 0.6mA x 100K = 60V. 120V - 60V = 60V plate voltage
5) Easiest way to adjust bias is with a trimpot on the cathode. Play around with trimmer whilst measuring plate voltage, mic powered on.

0.6mA is set once you measure 60V plate voltage

 

[Moby]

Yes, I just try to make sense of it all. Basically Banzai offers a way to change plate current by varying the 100k resistor and you by varying the 100ohm bias resistor...unless of course you guys talk about two different things. Of course since my knowledge is limited I can’t evaluate...just keep experimenting and reading until it starts to make sense.
Thanks man, I am getting a lot of great info

Just put the trimpot instead 100ohm resistor (go with multiturn one, say 470ohm-1k) and carefully find the cathode bias by measuring voltage at anode. With the present anode resistor, when you achieve 34-36v at anode, you are at the desired tube bias. Listen how it sounds, change the tube and repat the bias. Then, when you choose the tube, measure the resistance of the cathode trimpot and replace with the same value fixed resistor.

 

[Purplenoise]

If you're not sure how to go about it, what you'll be measuring to set bias is the plate voltage.

Step by step:
1) Choose a plate resistance (50K, 100K, etc)
2) Set B+ (105, 120V, etc)
3) Choose a target plate current (0.6mA, 1mA etc)
4) Calculate plate voltage needed to achieve target (50, 60, 70V etc)
5) Adjust bias until you measure plate voltage calculated in step 4

Example:
1) 100K plate resistance
2) 120V B+
3) 0.6mA plate current
4) 0.6mA x 100K = 60V. 120V - 60V = 60V plate voltage
5) Easiest way to adjust bias is with a trimpot on the cathode. Play around with trimmer whilst measuring plate voltage, mic powered on.

0.6mA is set once you measure 60V plate voltage

Thank you so much Banzai. I gave it a try and I changed the 100k to a 51k for a total of 81k plate resistance. That only raised the plate voltage by roughly 5v so now the plate current is 40v/81k=0.5mA. This sounds really good and after raising the B+ to 110v it brought the sound even closer to the original u47 that I had here for a short time.
Thank you so much for taking the time to share this info, really appreciate it.

 

[Purplenoise]

Just put the trimpot instead 100ohm resistor (go with multiturn one, say 470ohm-1k) and carefully find the cathode bias by measuring voltage at anode. With the present anode resistor, when you achieve 34-36v at anode, you are at the desired tube bias. Listen how it sounds, change the tube and repat the bias. Then, when you choose the tube, measure the resistance of the cathode trimpot and replace with the same value fixed resistor.

Thank you so much Moby, i really and always appreciate your help. I will give it a try, for now I tried banzai’s suggestion because I had the parts available.
I got another newbie question. Why stick with 34/35v plate voltage? I thought that was specific for the vf14. I see on the Neumann schematic for the u47 with nuvistor the plate voltage is specified to 70v. Am I misunderstanding what you suggested?

 

[Moby]

Thank you so much Moby, i really and always appreciate your help. I will give it a try, for now I tried banzai’s suggestion because I had the parts available.
I got another newbie question. Why stick with 34/35v plate voltage? I thought that was specific for the vf14. I see on the Neumann schematic for the u47 with nuvistor the plate voltage is specified to 70v. Am I misunderstanding what you suggested?

I thought that you want to find best replacement tube within U47 schematics with the ninimal values changes. That's why I sugested bias trim. Of course, Banzai's advice is OK if you know what you are doing. U47N schemo you mention has nuvistor in impedance mismatch operating point with Bv.08 transformer. To figure out what I talk about you should look at nuvistor vade-mecum.

 

[Banzai]

Thank you so much Banzai. I gave it a try and I changed the 100k to a 51k for a total of 81k plate resistance. That only raised the plate voltage by roughly 5v so now the plate current is 40v/81k=0.5mA. This sounds really good and after raising the B+ to 110v it brought the sound even closer to the original u47 that I had here for a short time.
Thank you so much for taking the time to share this info, really appreciate it.

Sounds like you nailed it. Job well done 8)

 

[Purplenoise]

I thought that you want to find best replacement tube within U47 schematics with the ninimal values changes. That's why I sugested bias trim. Of course, Banzai's advice is OK if you know what you are doing. U47N schemo you mention has nuvistor in impedance mismatch operating point with Bv.08 transformer. To figure out what I talk about you should look at nuvistor vade-mecum.

Oh I see, sorry for the misunderstanding. I am just trying to find the best operating point and bias of currently popular tubes used in those builds ( ef800, 802, 184 etc). Your method though the way I see it is totally useful in this case too. I am sure a combination of the bias resistor, plate resistor and b+ can be achieved for maximum results. Thank you sir, I really appreciate it.

 

[Purplenoise]

Sounds like you nailed it. Job well done 8)

Haha...with a “little” help from my friends. Thank you so so much banzai, I really appreciate it taking the time and sharing the knowledge.

 

[Purplenoise]

With the 51k plate resistor ef802 also sounds great. Plate voltage is at 55v and plate current at 0.64mA with B+ at 108/110v and heater at 5.05/5.2v.
Comparing the two tubes ef184 sounds a little fuller and more “vintage”...maybe a little dryer too. Ef802 a little more defined with a touch more upper mids and a little smoother and 3 dimensional. Ef184 has a little more of that 1k color which can make some vocals sound a little nasal at times. Both sounds are totally in the u47 “family “