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5703 replacing AC701k



 
 
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  #1  
Old May 17th 04, 02:18 PM
randomexpress
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Default 5703 replacing AC701k

I've seen some tube mics(Korby?) with 5703 tube- a small triode.
Some even suggested that this tube could replace AC 701 in KM 54,56 other
neumanns.
Is this possible?
If so--how is it done?

jens


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  #2  
Old May 17th 04, 03:20 PM
Scott Dorsey
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Default

randomexpress > wrote:
>I've seen some tube mics(Korby?) with 5703 tube- a small triode.
>Some even suggested that this tube could replace AC 701 in KM 54,56 other
>neumanns.


It's a nice tube, the 5703. It was in production in the Raytheon plant
in Massachusetts until the late 1980s, and there are a lot of them out
there because they were used in a lot of military equipmnent.

>Is this possible?
>If so--how is it done?


Well, the plate voltage is in the right ballpark. I think that the filament
voltage may be different so you may need to replace the zener in the filament
supply. You will almost certainly need to change the cathode resistor so that
the tube is biased properly (and I don't recall what grid-cathode voltage it
wants but the Raytheon Tube Handbook will tell you).

It will sound different than the AC701. I think this is a good thing
personally.

The 5899 pentode is another possibility. Even strapped into triode mode
it will have much better top end than the AC701k. However, it is very
microphonic because of the tight element spacing required to fit a pentode
into that size package.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #3  
Old May 17th 04, 03:20 PM
Scott Dorsey
external usenet poster
 
Posts: n/a
Default

randomexpress > wrote:
>I've seen some tube mics(Korby?) with 5703 tube- a small triode.
>Some even suggested that this tube could replace AC 701 in KM 54,56 other
>neumanns.


It's a nice tube, the 5703. It was in production in the Raytheon plant
in Massachusetts until the late 1980s, and there are a lot of them out
there because they were used in a lot of military equipmnent.

>Is this possible?
>If so--how is it done?


Well, the plate voltage is in the right ballpark. I think that the filament
voltage may be different so you may need to replace the zener in the filament
supply. You will almost certainly need to change the cathode resistor so that
the tube is biased properly (and I don't recall what grid-cathode voltage it
wants but the Raytheon Tube Handbook will tell you).

It will sound different than the AC701. I think this is a good thing
personally.

The 5899 pentode is another possibility. Even strapped into triode mode
it will have much better top end than the AC701k. However, it is very
microphonic because of the tight element spacing required to fit a pentode
into that size package.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #4  
Old May 17th 04, 08:25 PM
randomexpress
external usenet poster
 
Posts: n/a
Default


"Scott Dorsey" > ...
> randomexpress > wrote:
> >I've seen some tube mics(Korby?) with 5703 tube- a small triode.
> >Some even suggested that this tube could replace AC 701 in KM 54,56 other
> >neumanns.

>
> It's a nice tube, the 5703. It was in production in the Raytheon plant
> in Massachusetts until the late 1980s, and there are a lot of them out
> there because they were used in a lot of military equipmnent.
>
> >Is this possible?
> >If so--how is it done?

>
> Well, the plate voltage is in the right ballpark. I think that the

filament
> voltage may be different so you may need to replace the zener in the

filament
> supply. You will almost certainly need to change the cathode resistor so

that
> the tube is biased properly (and I don't recall what grid-cathode voltage

it
> wants but the Raytheon Tube Handbook will tell you).
>
> It will sound different than the AC701. I think this is a good thing
> personally.



Any schematics floating around..?


  #5  
Old May 17th 04, 08:25 PM
randomexpress
external usenet poster
 
Posts: n/a
Default


"Scott Dorsey" > ...
> randomexpress > wrote:
> >I've seen some tube mics(Korby?) with 5703 tube- a small triode.
> >Some even suggested that this tube could replace AC 701 in KM 54,56 other
> >neumanns.

>
> It's a nice tube, the 5703. It was in production in the Raytheon plant
> in Massachusetts until the late 1980s, and there are a lot of them out
> there because they were used in a lot of military equipmnent.
>
> >Is this possible?
> >If so--how is it done?

>
> Well, the plate voltage is in the right ballpark. I think that the

filament
> voltage may be different so you may need to replace the zener in the

filament
> supply. You will almost certainly need to change the cathode resistor so

that
> the tube is biased properly (and I don't recall what grid-cathode voltage

it
> wants but the Raytheon Tube Handbook will tell you).
>
> It will sound different than the AC701. I think this is a good thing
> personally.



Any schematics floating around..?


  #6  
Old May 17th 04, 08:30 PM
Scott Dorsey
external usenet poster
 
Posts: n/a
Default

randomexpress > wrote:
>"Scott Dorsey" > ...
>> randomexpress > wrote:
>> >I've seen some tube mics(Korby?) with 5703 tube- a small triode.
>> >Some even suggested that this tube could replace AC 701 in KM 54,56 other
>> >neumanns.

>>
>> It's a nice tube, the 5703. It was in production in the Raytheon plant
>> in Massachusetts until the late 1980s, and there are a lot of them out
>> there because they were used in a lot of military equipmnent.
>>
>> >Is this possible?
>> >If so--how is it done?

>>
>> Well, the plate voltage is in the right ballpark. I think that the

>filament
>> voltage may be different so you may need to replace the zener in the

>filament
>> supply. You will almost certainly need to change the cathode resistor so

>that
>> the tube is biased properly (and I don't recall what grid-cathode voltage

>it
>> wants but the Raytheon Tube Handbook will tell you).
>>
>> It will sound different than the AC701. I think this is a good thing
>> personally.

>
>Any schematics floating around..?



Same schematic, same topology. You aren't changing the circuit at all,
just the tube bias point and (maybe) filament voltage.

Be aware that this will devalue a Neumann mike, but just about any change
will.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #7  
Old May 17th 04, 08:30 PM
Scott Dorsey
external usenet poster
 
Posts: n/a
Default

randomexpress > wrote:
>"Scott Dorsey" > ...
>> randomexpress > wrote:
>> >I've seen some tube mics(Korby?) with 5703 tube- a small triode.
>> >Some even suggested that this tube could replace AC 701 in KM 54,56 other
>> >neumanns.

>>
>> It's a nice tube, the 5703. It was in production in the Raytheon plant
>> in Massachusetts until the late 1980s, and there are a lot of them out
>> there because they were used in a lot of military equipmnent.
>>
>> >Is this possible?
>> >If so--how is it done?

>>
>> Well, the plate voltage is in the right ballpark. I think that the

>filament
>> voltage may be different so you may need to replace the zener in the

>filament
>> supply. You will almost certainly need to change the cathode resistor so

>that
>> the tube is biased properly (and I don't recall what grid-cathode voltage

>it
>> wants but the Raytheon Tube Handbook will tell you).
>>
>> It will sound different than the AC701. I think this is a good thing
>> personally.

>
>Any schematics floating around..?



Same schematic, same topology. You aren't changing the circuit at all,
just the tube bias point and (maybe) filament voltage.

Be aware that this will devalue a Neumann mike, but just about any change
will.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #8  
Old October 14th 13, 05:48 PM
rmburrow rmburrow is offline
Junior Member
 
First recorded activity by AudioBanter: Oct 2013
Posts: 2
Default

Quote:
Originally Posted by Scott Dorsey View Post
randomexpress > wrote:
>"Scott Dorsey" > ...
>> randomexpress > wrote:
>> >I've seen some tube mics(Korby?) with 5703 tube- a small triode.
>> >Some even suggested that this tube could replace AC 701 in KM 54,56 other
>> >neumanns.

>>
>> It's a nice tube, the 5703. It was in production in the Raytheon plant
>> in Massachusetts until the late 1980s, and there are a lot of them out
>> there because they were used in a lot of military equipmnent.
>>
>> >Is this possible?
>> >If so--how is it done?

>>
>> Well, the plate voltage is in the right ballpark. I think that the

>filament
>> voltage may be different so you may need to replace the zener in the

>filament
>> supply. You will almost certainly need to change the cathode resistor so

>that
>> the tube is biased properly (and I don't recall what grid-cathode voltage

>it
>> wants but the Raytheon Tube Handbook will tell you).
>>
>> It will sound different than the AC701. I think this is a good thing
>> personally.

>
>Any schematics floating around..?



Same schematic, same topology. You aren't changing the circuit at all,
just the tube bias point and (maybe) filament voltage.

Be aware that this will devalue a Neumann mike, but just about any change
will.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

************************

Replacing AC701k with 5703, 7586, etc.

BE CAREFUL, you are working around 120 volts, carelessness may result in injury or damage components that are expensive or near impossible to replace!

First, eliminate the AC701 as the problem. A noisy anode resistor (typically 200k) will generate noise. Leaky or intermittent capacitors in the mic can generate noise. Resistors are the big culprit. Check the cable for leakage. Old cable insulation (essentially at strain relief connector fittings) can break down and cause noise. If the power supply is a old NKM type with the "stabilyte" heater voltage regulator, disconnect those and connect three AA rechargeable batteries (pay attention to polarity!) in their place. Fire the supply up (with mic connected) and IMMEDIATELY check the heater voltage. It MUST NOT EXCEED 4.00 volts! Acceptable range is 3.8 to 4.0 volts...If the heater voltage is high, immediately shut down the supply. Disconnect the mic, load the B plus with a 120k resistor and the DC heater with a 40 ohm 2 watt resistor. Troubleshoot the supply to get the correct voltages for the AC701 using the load resistors. Then disconnect the load resistors, reconnect the mic and recheck.

If the problem is DEFINITELY with the AC701 and replacing it with a 5703 or 7586 is the option, note that this may devalue a Neumann or AKG mic..but if you want a working mic and don't plan to sell it, any work done inside the mic or supply MUST be done neatly and in a reversible manner in the event AC701's become available. (Email or call Eurotubes.com and email JJelectronics.com and encourage JJelectronics to manufacture the AC701!)

The 5703 or 7586 requires 6.3 volts on the heater. The supply must be modified carefully to increase the heater voltage. MARK THE SUPPLY so it doesn't accidentally get connected to working AC701 powered mics! For a old NKM supply with the stabilytes, disconnect them and replace with 4 AA type rechargeable batteries in series. You may need to make other changes to get the correct heater voltage. Load the heater supply with a 63 ohm resistor, and the B plus with 120k resistor for testing. Get the heater voltage correct first. The tube must be oriented so the grid faces the capsule connection using a short direct connection! Other leads must be bundled to stay away from the grid lead!

The bias will vary depending on the mic. Some AKG mics get the capsule bias off a voltage divider from the cathode. This means you have to change the divider string so the capsule bias DOES NOT EXCEED 60 volts. This means you have to remove the mic cover and measure the cathode voltage with the mic connected. I recall the C60 used a 82 k resistor to ground (common) and a 1500 ohm resistor (in series) for bias for the AC701. If the replacement 5703 or 7586 draws 1 ma of current, the voltage across the 82k resistor will be 82 volts and this is excessive for the capsule. A 56 k resistor to ground and a 1 k resistor (in series) may provide the correct voltages but this needs to be measured since the drop across the tube is affected by this.

Some of the Neumann mics derive the capsule bias off a resistive voltage divider off the B plus rail and the capsule is isolated from the tube grid by a coupling capacitor. This is easier to handle since the operating point of the tube can be set easier. You may find that a 120 k resistor in the plate, and 2.7 k in the cathode should work with a 7586; you may need to change these values slightly for a 5703.

Some of the mics specify a small (typically 3 pf or so) capacitor for feedback. This can be derived by twisting two pieces of small gauge (i.e. #20) hookup wire together (insulate the open end!) using a "western union" type twist; two turns should do. Check the schematic.

Last but not least, use the calibrating input (if the mic supply has one) and measure the response and distortion...and then connect the mic up to a preamp and use your ears critically...

Remember, LABEL modified mics and power supplies so they don't accidentally get interchanged with AC701 supplies and mics...a mistake here will ruin a AC701! I have referenced the 7586 because Neumann and AKG used this tube in export version (i.e. U64) mics and successfully modified a AC701 mic to a 7586...

Good luck...
 




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