U47 with EF14 tube Build

[Spencerleehorton]

kathode = 1.2v
Anode = 27.8v
2M/3M = 56.9v
H+ = 5.27v
B+ = 104.1v
Relay = 4.95v

i have noticed ive put in the 60M and the 100M round the wrong way so i'll change that but i thought the anode was meant to be 40v?

 

[Spencerleehorton]

i have corrected the mistake and swapped the resistors round, i now get:

kathode = 1.3v
Anode = 28.8v
2M/3M = 56.5v
H+ = 5.27v
B+ = 105v
Relay = 5.05v
10nf/100M = 5.65v

still just top end, no bottom end?

going to try lowering the 100k plate resistor down to 10k and see if bass comes back

 

[Spencerleehorton]

tried lowering the 100k resistor to 50k and 100R burns up!!!????

changed 100R to 2W and now have 31v on anode, so going the right way!!

 

[MagnetoSound]

How do the voltages sit with R4 = 50k?

 

[Spencerleehorton]

kathode = 1.2v
Anode = 31v
2M/3M = 75v
H+ = 5.11v
B+ = 103v
Relay = 5v

100k to 50k

Sounds so much better now!!

Keep going until I get 40v on anode?

 

[Khron]

How did the capsule bias divider voltage jump up to 75v though?

 

[MagnetoSound]

No. It appears there is too much bias current. You’d be looking for around 0.5mA to get 40v at the anode through the stock resistor values. Ohm’s Law says you’re pulling about 0.7mA. It doesn’t sound like much of a difference but at these values that’s a lot. Perhaps try to tinker the cathode resistor values until you get ~1.6v, and see how the plate voltage sits then.

 

[MagnetoSound]

How did the capsule bias divider voltage jump up to 75v though?

Yeah, that’s weird. Are the 30k and 100k resistors ok?

 

[kingkorg]

well first impression isnt good, all works, seems to lack bottom end? lovely clear top end, much clearer than my dany U47EF which im comparing it too as that seems to be my stand out U47 so far.
will change capsule and see if its the capsule
also will leave it on for a few hours for the EF14 to burn in and seem if it improves

Are you sure that relay is appropriate for such HiZ application? Maybe you have some leakage there? Try disconnecting the relay. Also is it working properly, you are not in omni? With omni you would experience low end loss at close proximity.

 

[Spencerleehorton]

I will check the voltages again but the low end has definitely come back with changing 100k to 50k.
Relay is fine and I don’t think it’s the problem.
So change the 29R? Try a 47R or go lower?

 

[MagnetoSound]

I’d probably try a 33 if I had it. Or lower the 100R to 82. Or if you have a trimmer somewhere near to either of those values you could probably dial in your 40v plate voltage without too much trouble.

 

[Spencerleehorton]

I don’t have a 2w 82R only 0.5W so I’ll try the 33R first, pretty sure it’s a 30R in there at the moment.
Might be easier using some trimmers and then make up the resistors.

 

[Spencerleehorton]

Just tested voltages again

Kathode - 1.2v
Anode - 32.62v
2M/3M - 56v
H+ - 5.2v
B+ - 104.2v
10nf/100M - 5.6v

Will try lowering 50k to 22k and try 33R on kathode

 

[Spencerleehorton]

Cathode - 1.26v
Anode - 35.3v
2M/3M - 57.2v
H+ - 5.04v
B+ - 105.9v
10nf/100M - 5.8v
22k/30k - 65.5v

31R cathode
22k for 100k

recorded a bit of drums and still no bottom end compared with U47 with EF800 danys version?

 

[MagnetoSound]

You shouldn’t need to lower the plate resistor so much!

Get a higher voltage on the cathode, you should find the plate voltage comes up due to reduced idle current. Aim for at least 1.5v, maybe closer to 2.0v at the cathode. If this doesn’t fix it, I think the tube may be bad.

 

[Spencerleehorton]

ive changed out the M7 capsule and txf from aliexpress and now put into the dany U47EF800 i get a thin sounding mic, must be the transformer, so i'll change txf back to other one to verify.

 

[MagnetoSound]

It still doesn’t address the low plate voltage. There’s gotta be something up with either the tube or the circuit for that to be off.

Is the output cap leaky? If you’ve magnetised the core with DC it would explain both the poor bass response and the low plate voltage.

 

[Spencerleehorton]

the transformer is the problem, ive tested both the M7 capsule and the transformer, the BV08 copy transformer tests as 6ohm per secondary winding, 14ohm in series and 300ohm per pri winding 600ohm in series.
doesnt seem to marry up with the BV08 winding sheet, so i'll take it apart and wind it again!!!

 

[Spencerleehorton]

well ive got not heater now? i think the tube is faulty?
I changed the capacitor as it may well have been leaking?
Voltage on the heater sits at 16v and doesn’t go down?

 

[MagnetoSound]

Definitely seems like that tube was on it’s way out.

Cap can be measured in Megohms range …?

Are you using an unregulated heater supply?