Since the 6j1 pre is discussed in various threads, I will send a PM to you or create a separate thread to discuss 6j1 preamp.
I would like this thread dedicated to this PL504 based simple tube amp. It's a great little project which even novices like me can build. Your schematic was very helpful
I would like this thread dedicated to this PL504 based simple tube amp. It's a great little project which even novices like me can build. Your schematic was very helpful
Got the amp working. Did some measurements.
HT = 65.5 V obtained by doubling 24 VAC
HT after 2.2K resistor 64.8 V
Anode and G2 of PL504 64.7 Volt
As output transformer serves a 220 to 18V transformer. No air gap. EI75 - 27
On the cathode 6.9 Volt e.g. 27 mA (the PL504's are used and have some experience)
On the anode of the 6J1 is 19.2 Volt
G2 is 43.6 V
Cathode is at 1.34 Volt
Anode current approx 0,21 mA.
I fed the amp with a CD player signal 200 - 300 mV
after the pot I only find approx 10mV.
At low volume no distortion, but as soon as the volume goes up distortion kicks in.
The 6J1 seems to be set for a very low sensitivity.
May need some changes.
Continue tomorrow.
HT = 65.5 V obtained by doubling 24 VAC
HT after 2.2K resistor 64.8 V
Anode and G2 of PL504 64.7 Volt
As output transformer serves a 220 to 18V transformer. No air gap. EI75 - 27
On the cathode 6.9 Volt e.g. 27 mA (the PL504's are used and have some experience)
On the anode of the 6J1 is 19.2 Volt
G2 is 43.6 V
Cathode is at 1.34 Volt
Anode current approx 0,21 mA.
I fed the amp with a CD player signal 200 - 300 mV
after the pot I only find approx 10mV.
At low volume no distortion, but as soon as the volume goes up distortion kicks in.
The 6J1 seems to be set for a very low sensitivity.
May need some changes.
Continue tomorrow.
Had a look again at the curves of the 6J1 connected as triode (G2 connected to anode). From a post on the 6J1 pre amp I could derive that the internal resistance of the 6J1, as triode, increases when the current through the tube decreases. With 59 volt on the anode and a current of 0.95 mA the internal resistance of the tube is 59 / 0.00095 = 62K. With 38 volt on the anode a current of 2.6 mA was measured -> 38 / 0.0026 = 13.6 KOhm for Ri.
So it makes sense to keep the current high, and thus the internal resistance of the triode low, in order to have enough current swing over the anode resistor. Example. HT 65 Volt . Current 0,95 mA. 65 / 0.00095 = 68.4KOhm. When the internal resistance of the tube is already 62K you only have 5 K left for the anode resistor, which will not generate enough output voltage swing over Ra (only 5 V in this case).
So suggest you copy the settings of the 6J1 pre-amp schematic in which the 6J1 is connected as a triode; Ra -> 4.7K; Rk -> 200 Ohm (with feedback eg. 150 +47). Rg1 of the PL504 goes from 470K to 100K. The 6J1 should then run at approx 3.5 mA with an amplification of around 20-22. You will now see a larger voltage drop over Ra of apoprox 15 Volt.
Will try these changes tomorrow evening. Too busy now.
So it makes sense to keep the current high, and thus the internal resistance of the triode low, in order to have enough current swing over the anode resistor. Example. HT 65 Volt . Current 0,95 mA. 65 / 0.00095 = 68.4KOhm. When the internal resistance of the tube is already 62K you only have 5 K left for the anode resistor, which will not generate enough output voltage swing over Ra (only 5 V in this case).
So suggest you copy the settings of the 6J1 pre-amp schematic in which the 6J1 is connected as a triode; Ra -> 4.7K; Rk -> 200 Ohm (with feedback eg. 150 +47). Rg1 of the PL504 goes from 470K to 100K. The 6J1 should then run at approx 3.5 mA with an amplification of around 20-22. You will now see a larger voltage drop over Ra of apoprox 15 Volt.
Will try these changes tomorrow evening. Too busy now.
Agreed. Will try to wire up tomorrow the new values. Parafeef was just a random idea when I was thinking about the problems of getting a economical OPT in my place,which is very difficult right now. Thanks .will update
Works Right! The distortion is now completely gone. It's only barely noticeable only at heavy bass songs are played. A volume control will certainly fix this issue. The voltages measured are as you predicted. There's 13.5 volts across 4.7 anode resistor. I used a 220 ohm cathode resistor . In series with 100 ohm to ground. My only limitation now is the OPT. My line trafi is great above 100 Hz or so. Highs are excellent.If I use a power trafo as OPT, Bass improves quite a bit,but highs are lost. I plan to open up the line trafo and see if I can modify the EI core and create a air gap. A very good design, Indeed. Extremely silent,no hiss/ hum.
Btw, what's the power output in this amp? At roughly 70 to 80 volt?
Also, the cathode resistor in PL504, IS NOW 220 OHM, 1 watt. I notice voltage of around 9 across it, and it gets fairly hot to touch after 15 minutes. When it gets very hot, distortion kicks in. What shall I do?
Btw, what's the power output in this amp? At roughly 70 to 80 volt?
Also, the cathode resistor in PL504, IS NOW 220 OHM, 1 watt. I notice voltage of around 9 across it, and it gets fairly hot to touch after 15 minutes. When it gets very hot, distortion kicks in. What shall I do?
If you want to improve performances of this circuit, increasing output power and decreasing distortion at Hi-Fi level, this is the modified schematic.
The plate voltage of the PL504 has been increased at about 110Vdc (Caution, dangerous voltage!) and there is a global negative feedback to linearize bandwidth and decrease total distortion.
T.b.d resistor should be verified sperimentally to obtain about 90 Vdc on the 47 uF capacitor.
The two output transformers are Hammond 119DA
Hammond Mfg. - "Classic" 600 Ohm - Speaker Matching Transformer - Hammond P/N 119DA
Audio Line Transformers are not meant to be DC biased by plate current. Low frequencies will be diminished.
Availability of OPTs in India is a surprise to me anyway. Over many years in the hitech electronics buz I've seen quite a bit of electronic components on display at various trade shews, much of it coming from India.
One of the EE I worked with came from India & eventually went back to work in hitech. He was a funny guy, he teased the secretaries with stories of elephant jams on the highways,
One of the EE I worked with came from India & eventually went back to work in hitech. He was a funny guy, he teased the secretaries with stories of elephant jams on the highways,
With 9 Volt on the cathode of the PL504, with a resistor of 220 Ohm, you have biased the tube at 9 / 220 = 41mA. That may be a lot for a transformer with no air gap. The cathode resistor has to dissipate 9 x 0.041 = 0.37 W. So a 2 watt resistor should do.
Increasing the HT to 110 or 165V will seriously improve the performance of the amplifier. However in this project a voltage of approx 65 V was chosen, which allows the use of a cheap 24 V transformer with voltage doubling for the HT suply. And 65V is relatively safe for newbies to tubes.
Increasing the HT to 110 or 165V will seriously improve the performance of the amplifier. However in this project a voltage of approx 65 V was chosen, which allows the use of a cheap 24 V transformer with voltage doubling for the HT suply. And 65V is relatively safe for newbies to tubes.
Availability of OPTs in India is a surprise to me anyway. Over many years in the hitech electronics buz I've seen quite a bit of electronic components on display at various trade shews, much of it coming from India.
One of the EE I worked with came from India & eventually went back to work in hitech. He was a funny guy, he teased the secretaries with stories of elephant jams on the highways,
There are very few tube audio manufacturers in India. They have custom winders. As far as general diy audience is concerned, there's no Hammond or edcor in India.Although, there are Good quality power transformer manufacturers are widely available here.
Many thanks for the schematic.! I just asked a possibility of using 6j1 and PL504, and you came up with a complete schematic. In addition to that,you have detailed explanation,which made me understand things better.My next plan is to open a trafo and see if I can modify .
Is it high voltage that saturated the trafo or is it high current? Supposing we operate plate voltage at 165 ,and still maintaining 30 mA bias, will it saturate?
Check these site with google translate:
Netztrafos als Ausgangs?bertrager?
air gap: output transformer air gap adjustment
From this you can see that the more DC current in an transformer, the lower the induction will get. A low primary induction will get you less lower frequency audio performance. So assuming you have a tube rated at a max power rating of 10 Watt (VA) then this implies at 250 Volt you can run it at 40 mA. At 60 volt you can run it at 416 mA to reach the same VA rating. Assuming the tube could handle such cathode current.
The implications for the output transformer are clear. In the 60 Volt case you would hardly have any induction left to work with. If the transformer has nbo air gap this will only become worse.
Just to be clear the tube power rating is NOT the output power of the amplifier. That is much less, as only a fraction makes it to the speaker in Single Ended.
If you read through the articles below you will understand that DC saturation is not directly a problem in Push Pull output transformers. Reason- The DC current of the 2 output tubes eliminates each other's magnetic field. That's why it is easier to build a PP amp with a power transformer as an output transformer. E.g. 110 -110 primary and 6+6 V seconday.
Especially toroid transformers are suitable for this. You will find enough propositions of PP schematics with alternative toroid transfformers as output transformer on internet.
Netztrafos als Ausgangs?bertrager?
air gap: output transformer air gap adjustment
From this you can see that the more DC current in an transformer, the lower the induction will get. A low primary induction will get you less lower frequency audio performance. So assuming you have a tube rated at a max power rating of 10 Watt (VA) then this implies at 250 Volt you can run it at 40 mA. At 60 volt you can run it at 416 mA to reach the same VA rating. Assuming the tube could handle such cathode current.
The implications for the output transformer are clear. In the 60 Volt case you would hardly have any induction left to work with. If the transformer has nbo air gap this will only become worse.
Just to be clear the tube power rating is NOT the output power of the amplifier. That is much less, as only a fraction makes it to the speaker in Single Ended.
If you read through the articles below you will understand that DC saturation is not directly a problem in Push Pull output transformers. Reason- The DC current of the 2 output tubes eliminates each other's magnetic field. That's why it is easier to build a PP amp with a power transformer as an output transformer. E.g. 110 -110 primary and 6+6 V seconday.
Especially toroid transformers are suitable for this. You will find enough propositions of PP schematics with alternative toroid transfformers as output transformer on internet.