More mic design ??? Under heat a tube or not ? biasing schemes which one ??

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gary o

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What are all the reasons for under heating (impedance?)...... Im messing with 5840 tube in M49/Royer/AKG circuits in some schematics im using its under heated 5,7v others down to 4V .....

Also biasing I see the different schemes that exist but not shure how they effect the mic sound which is most important to me I read that the U47s sheme contributes to the U47 souind.....how & why...I have one of Maxes fantastic kits the Mk47 its sound great BUT to me not better than my messy thrown together point to point circuits with their different biasings ..... also my MK47 has more back ground hiss dont get me wrong its not bad highm impedance board is all clean & I have tried several tubes so I dont think there is anything wrong & did I read that the U47 circuit is noisier than most tube mics...... & can this be because of the biasing scheme ..... im thinking of disconnecting the cathode & using a 1.V AA battery, will that work.....

Thanks for reading
 
gary o said:
What are all the reasons for under heating (impedance?)...... Im messing with 5840 tube in M49/Royer/AKG circuits in some schematics im using its under heated 5,7v others down to 4V .....

The reason you underheat is to extend filament life. Wire burns cooler, lasts a bit longer (sometimes a lot). Important when tubes are scarce types, and may require hand selection in order to perform as intended.

Underheating has to be carefully implemented. Too much will lead to sensitivity loss, as transconductance can fall off steeply in some tubes as cathode temparature is lowered.

Do the circuits you refer to actually specify using the 5840 at those voltages, or are you just following the schematic to the letter and seeing if the 5840 works?

Also biasing I see the different schemes that exist but not shure how they effect the mic sound which is most important to me I read that the U47s sheme contributes to the U47 souind.....how & why...

In a nutshell ... self-biased tubes adjust their own idle current, this sets an operating point, and this is affected to some extent by fluctuations in signal level. As transients are produced, the operating point shifts around, imparting a subtle dynamic tonal variation to the signal.

In a fixed bias circuit such as the U47, the tube is held to a fixed operating point and the signal cannot impart much influence on the timbre of the amplifier, producing a more 'dynamically uniform' character to the sound.

It should be emphasised that the difference is quite subtle to the untrained ear! Adjectives are tired and hackneyed, the only true way to decide which biasing scheme you prefer is to record test clips and compare as carefully as possible while listening closely for variation in timbre.

.. also my MK47 has more back ground hiss ...

... & can this be because of the biasing scheme ...

... im thinking of disconnecting the cathode & using a 1.V AA battery, will that work.....

Background hiss is down to the tubes. It may be necessary to try quite a few before you find the right ones. The biasing scheme is susceptible to excessive hum from the B+ line, but hiss would be filtered out by the B+ caps.

You can try a battery, but you will probably forget to take it out and then wonder why the mic doesn't work next time you go to use it.  :)

 
gary o said:
I have tried several tubes so I dont think there is anything wrong & did I read that the U47 circuit is noisier than most tube mics.

Not true (like a lot of things that you read on the web). In its time the U47 was about the lowest noise mic; the shortbody version had a noise spec of 15 dB-A - despite the "lowish" 60M grid resistor. That's lower noise than most tube mics even today, and in fact some FET mics. Subjectively (I own one) it appears even lower noise as its inbuilt 30 Hz hi pass suppresses a lot of rumble.

Not all tubes take underheating well; there were some threads about that before. The VF14 can run seriously underheated for decades, other tubes may become noisy within a few weeks. So, depending on the tube type, underheating can either prolong or shorten tube life.
 
Hi Dan & Rossi you two must be fed up with my questions ...... So big thanks for your replies great info as ever ....great I get the biasing now Im bit slow but now I see .... the tube in question is a 5840 & circuits like Olivers TLM149 conversion M49 type circuit on schematic heater is a 4V also the Royer mod 5-6V heater.... plus what I read here ..... I have a Oliver M49 circuit running at the moment trying different heater voltages right now

I ask the qestions as Im trying to decide which circuit to concertrate on & to give best capsule & parts too Im liking the Oliver M49 ( with orig T14) as it is but wondering weather to borrow schemes from other circuits same for my paint by numbers MK47 thought that may cure the not too bad hiss .....but if thats tubes I guess having two tubes in one mic makes it harder to find quiet tube tubes.....I must got lucky I only have one 5840 & it seems quieter than my 6028 combos & on a par with my 6072s.....Im now wondering if I can utilise my BV08 in a 5840/6072 mic circuit...

Rossi sadly not got access to classic mics so I have to read from the web did seem strange such classic would be thought of as noisey, I guess that grew from a report of noise from a ill U47....

Thanks once again
 
Thanks Odbfs of coarse I had ruled AC701 out of mind I didnt even register it on the schematic ..... anyway I had already tried 6.3v 5.7 & 4V...... yes 4v seemed to slightly effect the output I had decided on 5.7v anyway so that nice to know.... Im off to tweek cathode voltage now..... the circuit sounds great !
 

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