[Heath] D-104 amplified or microphone for SB-101 Heathkit Rig

[Kenneth G. Gordon]

On 9 Dec 2012 at 21:15, Jerry Rego wrote:

> Looking for the appropriate microphone for a SB-101 Heathkit.

The most appropriate microphone for the Heathkit rigs is the one that 
Heathkit sold separately (there was never a mic included with those rigs), the 
Electrovoice 638. This was sold by Heathkit as their HDP-21A.

Although the input impedance to those Heathkit rigs is high impedance, not 
all high-impedance mics will work properly with them.

An UN-amplified D-104 will work fine with them also, unless there is a 
problem in the circuitry of V-1.

An UN-amplified mic works best with the Heathkit rigs: turning up the 
amplification on an amplified mic always causes severe distortion in the mic 
amp stage.

BTW, low mic gain is a common complaint about the Heathkit rigs, but that is 
usually caused by either the incorrect mic being used, or problems with the 
circuitry around V-1, and NOT the design. The design is correct.

>  I have
> tried a D-10 without any success.

What exactly do you mean by "...without any success..."?

>  If anyone has an sb-101 tell me
> what microphone you are using

I use a Turner Plus-3 microphone with both my HW-101 and with my 
SB-102, both of which have the absolutely identical mic circuits as your 
SB-101.

I keep the gain control on the mic turned very low with my Turner Plus-3, not 
above about 2. You can buy these on eBay for very little money, and they 
sound superb.

Other users have used the Astatic DN-HZ (Hi impedance) mic.

If you are having low mic gain problems with your SB-101, there are several 
things you might wish to check: first of all check, very carefully, ALL the 
resistors around V-1, the 6EA8.  Also, make certain you have a good 6EA8.

That stage operates properly with what may seem like very low voltages to 
you, if you should measure them, but those voltages are correct. 

There is a resistor in that circuit, I think it is 100K, that can go very high, or 
even open, on you and thereby cut the mic gain very low. I have had several 
HW-101s or SB-101/102s that had problems with that resistor.

Secondly, I have had very good luck replacing the 6EA8 with a 6GH8A (NOT 
the non-A versions, which are almost all bad, even when new).

The 6GH8A has identical pin-outs as the 6EA8, and has an identical triode 
section, but the pentode section has more gain than that section in the 
6EA8. This can help if your mic is not quite correct or has low gain.

If everything is correct, and with a good 6GH8A in place, your mic gain 
control should not have to be turned much above about the 2:00 o'clock 
position. If you have to turn the mic gain control much above the 10:00 
o'clock position, then something is not right.

When the correct mic is used, and when the mic input stage (V-1) is working 
properly (and everything else is correct), the Heathkit rigs have excellent 
audio.

vy 73,

Ken W7EKB