Costly AC701k's will not be used for new builds

zandru

"Hard At Play"
Fellow Ak'ers,

If you please; I would appreciate some feed back pertaining to the use of the
AC7651, EC70/6778 & EC71/5718. These three tube are basically dead ringers'
with minor exceptions to the famously expensive AC701K tube; at a fraction
of the cost.

It had been suggested that some of these tubes are superior to the AC701k,
and that the AC701K is nothing more than myth and just pure snob appeal!.
(certainly not taking any audio brilliance away from the AC701k which is
undoubtedly "Primo extraordinaire")

I am in the process of building a group of microphones based on the AKG C-60
& Telefunken ELA M260 small diaphragm microphone (all orignally built by
AkG). Which I just so happen to own several dozen original mic capsules, and as
an added bonus, they just so happen to operate using the same basic type of
easy to construct power supply which will include a filament pot adjustment to
accomodate each tubes optimum performance within their individual amp circuit.

Any knowledge regarding the AC651, EC70/6778 and the EC71/5718 tubes, along
with any possibly comparison or obsevation to the AC701K would be most helpful.
Much Thanks
 
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The AC701 can be substituted with modifications required both in the mike and the power supply. The modifications depend on the mike, but most AC701k substitutions involve an increase of the heater voltage from 4 volts to 6.3 volts. BE CAREFUL working on the mic and/or the power supply since the HT (120 volts) will deliver a good shock and may cause injury and/or collateral damage.

Note that modifying a mic may decrease its value. However, this must be weighed against having a working microphone versus a decoration. New build microphones should use an affordable tube that is commonly available.

First, make sure the AC701 isn't bad. The tube is often blamed when a noisy resistor or leaky cap is the problem. If the mike is old, the cable insulation may be bad, especially at strain relief locations in the connectors (especially at the mike end). Last but not least, one can run the mike off batteries as a test. Nine volt batteries are cheap...11 of them in series is good enough for HT and 3 D cells in series (through a 5 ohm resistor in series on the plus end) will run the 4 volt heater. If excessive noise is still present, proceed.

Since a AKG C 60 mic is referenced, the bias voltage for the mic is derived from the IR drop across the 82 k resistor in the AC701 cathode circuit. Since the normal anode current of a AC701k is 0.5 to 0.7 mA, the capsule bias voltage will range from 41 to 57 volts, which is within the nominal 60 volt operating voltage of the capsule. The anode of the AC701 goes directly to B plus in the supply.

The modification to a 7586 nuvistor requires an increase of the heater voltage to 6.3 volts. This modification must be done first. Use a 63 ohm 2 watt resistor as a substitute load for the tube for initial settings. Next, deal with the mic mods. Typically, a series 4.7 k resistor (0.5 w) is required between B plus and the plate for a 7586. The operating point of the 7586 is set for the tube to draw approximately 1 ma of anode current. The 82 k resistor in the cathode divider (for the AC701) is excessive since one does not want to put 82 volts across the capsule and risk collapsing it. Change the 82 k resistor to 56 k. Use low noise metal film resistors. Do not change any other components now. With the mic open, apply power and immediately check the heater and anode supply voltages. The heater voltage (at the tube) must range between 5.9 and 6.4 volts (NO MORE!). The anode supply must be 120 volts. Measure the voltage drop across the 56 k ohm cathode resistor; it should not exceed 60 volts. The grid bias can be measured between the junction of the 1500 ohm and 56 k ohm resistors, and the tube cathode. This should measure between 1 and 1.5 volts.

If the initial tests are good, close up the mic and connect the audio output to a suitable amplifier and listen carefully for residual hum, noise, or other artifacts. If a calibrating input is present (or remove the mic capsule and inject a test voltage through a high value resistor) and check the frequency response, etc. Last, make a recording and listen carefully.

The Capps CM2250 mic used a 5703 tube. The old Altec M30 used a 5840 triode connected. I haven't tried the 5718 or the 6111. The Neumann U64 (export model of the KM64) used a 7586 nuvistor. The Sony C37 used a 6AU6. Other models used the 6AB4 and their European equivalents. The venerable Neumann U47 used a VF14 pentode triode connected with a underheated filament. The AR47 kit supplied for some Neumann U47's was a "quick fix" for the discontinued VF14. Connecting both halves of a 407A in parallel and using the 40 volt heater is a better substitute by providing a lower impedance load to the output transformer.

Good luck...
 
The AC701 can be substituted with modifications required both in the mike and the power supply. The modifications depend on the mike, but most AC701k substitutions involve an increase of the heater voltage from 4 volts to 6.3 volts. BE CAREFUL working on the mic and/or the power supply since the HT (120 volts) will deliver a good shock and may cause injury and/or collateral damage.

Note that modifying a mic may decrease its value. However, this must be weighed against having a working microphone versus a decoration. New build microphones should use an affordable tube that is commonly available.

First, make sure the AC701 isn't bad. The tube is often blamed when a noisy resistor or leaky cap is the problem. If the mike is old, the cable insulation may be bad, especially at strain relief locations in the connectors (especially at the mike end). Last but not least, one can run the mike off batteries as a test. Nine volt batteries are cheap...11 of them in series is good enough for HT and 3 D cells in series (through a 5 ohm resistor in series on the plus end) will run the 4 volt heater. If excessive noise is still present, proceed.

Since a AKG C 60 mic is referenced, the bias voltage for the mic is derived from the IR drop across the 82 k resistor in the AC701 cathode circuit. Since the normal anode current of a AC701k is 0.5 to 0.7 mA, the capsule bias voltage will range from 41 to 57 volts, which is within the nominal 60 volt operating voltage of the capsule. The anode of the AC701 goes directly to B plus in the supply.

The modification to a 7586 nuvistor requires an increase of the heater voltage to 6.3 volts. This modification must be done first. Use a 63 ohm 2 watt resistor as a substitute load for the tube for initial settings. Next, deal with the mic mods. Typically, a series 4.7 k resistor (0.5 w) is required between B plus and the plate for a 7586. The operating point of the 7586 is set for the tube to draw approximately 1 ma of anode current. The 82 k resistor in the cathode divider (for the AC701) is excessive since one does not want to put 82 volts across the capsule and risk collapsing it. Change the 82 k resistor to 56 k. Use low noise metal film resistors. Do not change any other components now. With the mic open, apply power and immediately check the heater and anode supply voltages. The heater voltage (at the tube) must range between 5.9 and 6.4 volts (NO MORE!). The anode supply must be 120 volts. Measure the voltage drop across the 56 k ohm cathode resistor; it should not exceed 60 volts. The grid bias can be measured between the junction of the 1500 ohm and 56 k ohm resistors, and the tube cathode. This should measure between 1 and 1.5 volts.

If the initial tests are good, close up the mic and connect the audio output to a suitable amplifier and listen carefully for residual hum, noise, or other artifacts. If a calibrating input is present (or remove the mic capsule and inject a test voltage through a high value resistor) and check the frequency response, etc. Last, make a recording and listen carefully.

The Capps CM2250 mic used a 5703 tube. The old Altec M30 used a 5840 triode connected. I haven't tried the 5718 or the 6111. The Neumann U64 (export model of the KM64) used a 7586 nuvistor. The Sony C37 used a 6AU6. Other models used the 6AB4 and their European equivalents. The venerable Neumann U47 used a VF14 pentode triode connected with a underheated filament. The AR47 kit supplied for some Neumann U47's was a "quick fix" for the discontinued VF14. Connecting both halves of a 407A in parallel and using the 40 volt heater is a better substitute by providing a lower impedance load to the output transformer.

Good luck...

Welcome to AK rmburrow,

It appears that your the first post to this thread, which I thought was dead.

It's nice to know that there are a few Vintage Microphone Heads around,
or perhaps your just "in-the-know" and surfed around the web to be
able to reply to my query. Thanks …. and I commend your valued input.

As it turned out (as I already knew), the nuvistor & other such subminiature
tubes just don't sound very good (to suite my taste & needs) for the most part.

I was looking for quality substitute for the Telefunken AC701K that could
be used in my hand built versions of the AKG C-60 type small diaphragm
microphone.

I got very lucky when a friend of mine told me about this contact he had
from Italy who was an old Ham & egger like himself. This gentleman
Frequently sent him lists of items that he had access to; or was selling.

On the list there was a reference to replacement AC701's for Siemens
mobile transmitters, So we called the man to inquire about the AC701's.
He told me he had around 20 or so left in a box; and that he used to be
a policeman that used those old Siemens "Walkie-Talkies", and that
when he retired he took a few of them with him and the box of tubes.

I think he knew that they had value but did not know what price they were
selling for these days. I made him an offer However, He said that he had
a better Idea; if I would go along with it.

In Italy there was a ban on large screen HD LED-LCD TV's, and If I could
purchase a Samsung 46" 1080p LED HDTV, unpack & repack it in an ATA
case to be shipped to his home; That this transaction would be a fair trade.

We made it So. "You do the math"

(26) Telefunken AC701K's arrived at my door safe & sound.

My mic build went smoothly with out a hitch, consuming only (14) AC701K's, with
(6) designated for a current project and the few leftovers have been added to my
personal back-up collection.

The new old stuff is just about evaporated out of existence, So! if we are going
to keep doing what we do with quality components, modern tube manufactures
need to get on the move to improve and move tube technology forward, and be
willing to reproduce a lot more tubes that are no longer in production.
 
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