[Collins] 75S-3B audio output tube

[Jim Brannigan]

A small low speed muffin fan works just great here.
There is one each sitting on top of the 75S and 32S.

Jim


> 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