Here's a couple pictures of the amp. I do not like how close the rectifier is to the power transformer, but was the only place I had room for the socket. The chassis was originally home to a Voice of Music single ended pentode stereo 6BQ5 amplifier.
The current power cord is going away and will be replaced by an IEC jack.
I tried a 5Y3 and here's the numbers.
Plate voltage 281.5Vdc
Screen voltage 282.8Vdc
Cathode current 49.1mA
Wanted to try a 5AR4 as I've got one somewhere, but while looking for it I found a 5R4 and tried it instead.
Here's the numbers with the 5R4.
Plate 293.4Vdc
Screen 295.3Vdc
Cathode current 51.1mA
I'm satisfied with those numbers.
Will add banana jacks later tonight so that I can measure the cathode current without having to open the amp up as there's a bottom plate that is normally on the amp.
Also realized the 6.3V winding has a center tap so I ditched the 220 ohm resistors and just connected the tap to the bias voltage.
The current power cord is going away and will be replaced by an IEC jack.
I tried a 5Y3 and here's the numbers.
Plate voltage 281.5Vdc
Screen voltage 282.8Vdc
Cathode current 49.1mA
Wanted to try a 5AR4 as I've got one somewhere, but while looking for it I found a 5R4 and tried it instead.
Here's the numbers with the 5R4.
Plate 293.4Vdc
Screen 295.3Vdc
Cathode current 51.1mA
I'm satisfied with those numbers.
Will add banana jacks later tonight so that I can measure the cathode current without having to open the amp up as there's a bottom plate that is normally on the amp.
Also realized the 6.3V winding has a center tap so I ditched the 220 ohm resistors and just connected the tap to the bias voltage.
Not sure why, but there's a slight hum in the speaker which sounds like a 60Hz hum. Now instead of the bias voltage going to the heater center tap, should I do it like some manufacturers such as Scott did and use a pot across the heaters with the wiper connecting to the bias voltage and adjusting the pot until I get the least hum?
That said should I move the grounds to the chassis instead of the ground buss I used?
That said should I move the grounds to the chassis instead of the ground buss I used?
I am just curious .. what is the voltage at the grid of 6BQ5 ?
What is the voltage across the 196 resistor ?
Thanks in advance.
What is the voltage across the 196 resistor ?
Thanks in advance.
I would suggest suing a LM317 as CCS for the common cathode for the 6BQ5 instead of fixed bias.
Johnny
Johnny
The heaters are connected to the negative bias voltage, right? That makes the heater-to-cathode voltage even bigger, and may be a source of hum.
What's the benefit of that?
When the heaters weren't connected to the negative bias I had a slight buzz in the speaker.
Was the 6.3V centre tap connected to the ground before or hanging in the air?
Using CCS at the common cathode would reduce the distortion. Also, you don't have to worry about the power tubes taking excessive current.
There's no adjustment required when replacing the power tubes.
There's no adjustment required when replacing the power tubes.
If the 6.3V heater is center tap to -11VDC, it looks to me like
a heater is elevated to a negative voltage instead of a positive elevation voltage (30 to 60V) according to
https://www.valvewizard.co.uk/heater.html
a heater is elevated to a negative voltage instead of a positive elevation voltage (30 to 60V) according to
https://www.valvewizard.co.uk/heater.html
Hanging in the air. Didn't think about grounding it.
So with a CCS I wouldn't need a - bias voltage either, right?
Also does an electrolytic capacitor get put across the CCS so that the AC signal bypasses the CCS?
Didn't figure it mattered what voltage was on the heaters.
Hi Tube Radio,
Using the CCS for push pull circuit doesn't require bypass capacitor at all. The two power tubes will share the current through the CCS, i.e., one takes more and the other takes less. I done this in two of my PP amps (KT66, and 2A3) with good result. No extra bias voltage requires. Just to make sure the CCS provides the amount of current for Class A operation.
On the other hand, the two power tubes need to be matched. Both tube should take same of DC current. Most of the PP output transformer would allow small amount of DC current going through. A 100 ohm potentiometer at the cathode may be used for the DC balance.
Using the CCS for push pull circuit doesn't require bypass capacitor at all. The two power tubes will share the current through the CCS, i.e., one takes more and the other takes less. I done this in two of my PP amps (KT66, and 2A3) with good result. No extra bias voltage requires. Just to make sure the CCS provides the amount of current for Class A operation.
On the other hand, the two power tubes need to be matched. Both tube should take same of DC current. Most of the PP output transformer would allow small amount of DC current going through. A 100 ohm potentiometer at the cathode may be used for the DC balance.
Hi Tube Radio,
Oh, sorry CCS doesn't really good for the 6BQ5 because it will reduce the output power significantly. With 340V B+, the 6BQ5 is operating if class AB mode.
Oh, sorry CCS doesn't really good for the 6BQ5 because it will reduce the output power significantly. With 340V B+, the 6BQ5 is operating if class AB mode.
As I mentioned in the previous post, using CCS would reduce the distortion at the power stage.
Running the amp in class A mode with CCS would need to reduce the B+ to 280Vdc. CCS current 40mA. The 6BQ5 strapped in Triode mode.
The max output power : 5 watts rms. Estimated THD : 1.23%
The max output power : 5 watts rms. Estimated THD : 1.23%
to lower the B+ voltage, you can use a small capacitor of say 0.5uF instead of 22uF after 5R4 rectifier tube. Try different size of capacitor to get the right B+.
Going fixed bias makes a lot of sense if you are using Russian tubes, which are equivalent to 7189 rather than 6BQ5. With fixed bias you get considerably higher power and lower distortion (if power supply is adequate, of course).
Also according to the data I need to swap to an outout transformer with an 8K primary in order to get about 17 watts of audio.
I'm happy with it as is so it might be a change I make down the road if I find I need close to 17 watts of audio unless using a transformer with a higher primary impedance will make the tubes last longer.
I'm happy with it as is so it might be a change I make down the road if I find I need close to 17 watts of audio unless using a transformer with a higher primary impedance will make the tubes last longer.
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