[Collins] 75S-3B audio output tube

Tue Feb 3 20:14:42 EST 2009

Jerry,

Thanks for bringing all that information together in one post. After  
looking at a wide variety of options, my solution to the 6BF5 heat  
challenge in 75S-3/3B receivers turned out to be replacement of V10  
with one of Bill Noonan's "T-tubes". 2nd harmonic distortion  
(measured, by the way) is higher than the 6BF5, but at normal  
listening levels the audio is quite satisfactory and the AF Gain  
control settings for normal listening (at least to my ears)  are the  
same I use when the 6BF5 is in place. After hours of continuous  
running, the T-tube and the V10 socket are just warm to the touch.  
Unfortunately, Bill is not currently able to produce these due to a  
critical part availability problem. I purchased the last of the V10  
Mohicans from him just a few weeks ago.

Bob, KF6BC

On Feb 1, 2009, at 2:10 PM, Dr. Gerald N. Johnson wrote:

> On Mon, 2008-12-15 at 18:43 -0600, Larry WA9VRH wrote:
>> Hi Jerry,
>>
>> Please let me know what you find on the 51S-1. I have one that I  
>> would like
>> to do this to.
>>
>> Glad I could find it.
>> 73 Larry WA9VRH
>>
> Simply said, the K5UGM mod is to reduce the grid bias by putting a  
> 330k
> from grid to ground. Then add some cathode bias back by ungrounding  
> the
> cathode with a 47 ohm resistor bypassed with a 25 uf capacitor.
>
> It will work. But all the receivers are not the same grid resistance,
> cathode resistance, plate voltage or screen voltage.
>
> I analyzed the operations based on the receiver manuals, including  
> their
> parts list, normal voltages chart, and schematic, plus tube data  
> sheets
> from General Electric and others.
>
> Actually, simply plugging a 6AQ5 in the 6BF5 socket will work in the  
> 75S
> receivers so long as they are running on AC or external 6 volts. Where
> the heaters are run in series parallel as in the 75S receivers and the
> 51S-1() the few tubes in parallel with the 6BF5 socket (now holding a
> 6AQ5) will be seeing a higher heater voltage that will shorten their
> lives, probably seriously. And the rest of the receiver's tubes will  
> be
> running low voltage making them seem to work less well with time.
>
> The simple change is not optimum. First off, the 6AQ5 doesn't need as
> much bias voltage as the 6BF5 so it will be less linear showing
> considerable second harmonic distortion. Secondly the gain will be  
> lower
> because the transconductance of the 6AQ5 is half that of the 6BF5.  
> Third
> the power output will be lower. How much the power output will be  
> lower
> depends on the model and MCN version of the receiver. My books show
> sometimes different values on the schematic, the voltages chart, and  
> the
> parts list and for different editions of the books.
>
> The 6AQ5 would prefer a higher plate voltage and in any case a 5K load
> rather than the 2.5K load in the 75S receivers. The 6AQ5 is rated at a
> significantly higher plate dissipation value, 12 watts vs 5.5 watts  
> for
> the 6BF5.
>
> I compute from the tube curve and supply voltage that in the 'S-1
> through S-3A, with 16 volts peak to peak audio on the grid, the 6BF5
> produced 1.47 watts. The 6AQ5 produces 0.287 watt under the same
> conditions with 11 volts of grid bias (circuit not changed). With the
> 6AQ5 bias reduced to 8 volts (as in the K5UGM circuit) the power out
> with 16 volts peak to peak drive is 0.43 watt, down 5.3 dB from the
> maximum power out of the 6BF5 and at 8 volts of bias the 6AQ5  
> distortion
> is less.
>
> Were I to use a 6AQ5 in the 'S-1 through S-3A, I would probably add a
> shunt resistor grid to ground to achieve -8 volts on the grid, which
> computes to be a shunt resistor of 620K. Though I don't object to the
> cathode resistor, it makes changing tubes less critical, and if not
> bypassed reduces the gain further while significantly improving the
> linearity from the inverse feedback. Changing the total grid bias from
> -11 volts to -8 also significantly improves the linearity of the  
> 6AQ5 at
> the cost of heat by raising the plate current to something around 32
> milliamps.
>
> In the 'S3B/C, there may be a 27 ohm unbypassed cathode resistor and  
> the
> grid resistor might be 220K or 330K. Plate and screen voltages are
> higher. I compute that the 6BF5 will put out 1.54 watts of audio while
> dissipating 4.47 watts in the plate. The 6AQ5 with no circuit mod will
> put out 0.33 watt (both with 16 volts peak to peak grid drive), down  
> 6.7
> dB. With the 6AQ5 grid bias reduced to -8 volts (grid to cathode), the
> plate current will be 35 milliamps and the plate dissipation 6.48  
> watts
> and the power output will be 0.48 watt, down 5 dB.
>
> If the 3B/C grid resistor is 220K, and the cathode resistor 27 ohms,
> 330K grid to ground is an appropriate and simple mod. The 6AQ5 is
> dissipating 2 more watts in the plate than the 6BF5 but well within  
> its
> ratings while saving 4.7 watts of heater power but producing 1/3 the
> audio power for the same drive.
>
> The power output of the 6AQ5 can be helped by using an 8 ohm speaker
> instead of a four ohm speaker at the cost of output transformer
> bandwidth. The capacitor shunting the plate will have a lower  
> frequency
> roll off, possibly making the audio sound muffled. That capacitor has
> been unhooked in my receiver for probably 40 years because it rolled  
> off
> 2975 RTTY high tone too much. Cutting that capacitor in half would  
> help
> this situation. The transformer will also raise the LF roll off from
> insufficient primary inductance with the 8 ohm load.
>
> In summary: The 6AQ5 mod is practical at a cost of at least 5 dB lower
> gain and power output. Its best when the grid to cathode voltage is
> adjusted to -8 volts (grid negative) with any combination of  
> resistor to
> ground at the grid and a 27 or 47 ohm cathode resistor. It shouldn't  
> be
> used when the heaters are in a series parallel string, whether 75S
> receiver operating from external 12 or 24 volts or a 51S-1(). Despite
> its heat, the original 6BF5 will produce more audio with less drive.
>
> Its important to be sure the bias on the 6BF5 is still a total of -11
> volts, and there is a small electrolytic in the bias divider that if  
> it
> shorts will reduce or eliminate that grid bias frying the 6BF5. That  
> is
> C29, positive side grounded.
>
> -- 
> 73, Jerry, K0CQ, Technical Advisor to the CRA
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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