[Hammarlund] SP-200 Audio Bypass

[Roy Morgan]

At 07:27 PM 7/18/2006, you wrote:
>     I just picked up a very nice Hammarlund SP-200 that plays like a dream.

Good for you. A fine radio.

>      In looking at the 2nd AF stage, I see some issues:
>
>     1.   There is supposed to be a 40 mfd/150V cathode bypass cap. Was there
>any production change that eliminated this cap?

I have no idea. Maybe it was just not installed!

>Any reason to eliminate it? If
>not, I plan to install a 47 mfd/150V cap. Any problem with this?

One reason to eliminate it *might* be to increase negative feedback in the 
output stage, thereby increasing audio bandwidth and linearity ("cathode 
self-bias feedback").  Re-installing a good cathode bypass cap is likely a 
good idea, however:

The rated audio output of the radio is more than a couple watts.  (Do I 
remember something like 4 to 6 watts for a pair of 6F6's in push 
pull?)  almost NO one, needs 4 watts of audio from such a radio.  So 
reducing the max output by leaving off the cathode bypass cap does little harm.

>     2.  The cathode-bypass resistor is supposed to be 750 ohm/10W. The 
> one in there measures about 800 ohms but is only a 6-watt Koolohm unit. Is that
>wattage too light? Should I replace it with a 750-ohm/10W unit?

Consider these ideas:
  - we do not need the full power the radio was designed to deliver to the 
speaker
  - tube life is GREATLY extended if we run them at reduced plate dissipation
  - Running the 6F6's at 70 percent of design plate current will reduce the 
plate dissipation by about half. You will still get lots of audio from the 
set.  Lots.

Here is what I normally do with such audio output stages:
  - increase the cathode resistor until the standing plate current is less 
than 80 percent of normal design current. (This takes into consideration 
modern increased line voltages.)  Compute design current from the manual 
resistor value and cathode voltage.
  - replace the bypass cap with a  modern one, possibly larger in 
capacitance than the original. Cathode voltages can be expected in the 12 
to 20 volt range, and are not subject to power-on surges as power supply 
voltages are.  So a 150 volt cap is not needed.
  - compute the power dissipated in the resistor with ohms law: 
P=E*2/R  Use a resistor that is rated for at least twice the expected power.

Then: I put in good tubes and expect them to last a VERY long time, and 
deliver plenty of audio.

An interesting exercise is to measure the power being delivered to your 
speaker at fairly loud listening levels.  You have to assume the speaker's 
impedance because it is for sure not what it is marked at any but one or 
two frequencies*. Approximate is good enough.  (An audio power output meter 
is very handy for this, the General Radio more modern one being most 
desirable.)  Do  not be surprised to find that you are listening to half a 
watt or LESS at loudness levels that disturb the whole household and nearly 
drive you from the radio shack.

* Another very interesting exercise is to measure a speaker's impedance or 
just the resistance across the audio bandwidth.  For an 8 ohm speaker, for 
instance, you may well discover variations from 3 ohms to 50 ohms.  Only a 
resistive dummy load will be anywhere near what you expect it to be: real 
speakers vary all over the lot.

Sooo many projects, soo little time.

Roy
who got his SP-200 in 1965


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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