corbato,
NO - don't loose the grid stoppers - EL84s are like any output tube, they have some tendency toward parasitic oscillation. The grid stoppers will prevent that (If they don't then increase their value).
In addition, with Ultralinear Amps screen grid stoppers are usually a good idea and paradoxically the better the output transformer the more they are required, so the correct place for those resistors shown in the screen grid are right up against the tube pin socket just as you do for the control grid stoppers.
The bias scheme is "auto-bias" - we are just using a current source in place of the usual resistor. I found a definite improvement when I changed from the resistors (470R for EL84s) to the current sources, particularly in the bass, probably due to better matching of the idle currents.
Cheers,
Ian
NO - don't loose the grid stoppers - EL84s are like any output tube, they have some tendency toward parasitic oscillation. The grid stoppers will prevent that (If they don't then increase their value).
In addition, with Ultralinear Amps screen grid stoppers are usually a good idea and paradoxically the better the output transformer the more they are required, so the correct place for those resistors shown in the screen grid are right up against the tube pin socket just as you do for the control grid stoppers.
The bias scheme is "auto-bias" - we are just using a current source in place of the usual resistor. I found a definite improvement when I changed from the resistors (470R for EL84s) to the current sources, particularly in the bass, probably due to better matching of the idle currents.
Cheers,
Ian
Thanks Ian
Usually I have noticed that loosing Grid Stopper result in a 'tighter' sound. Only with 6L6 family I have faced oscillation issues. I usually start with the GS's and remove them if things dont oscillate. I was just wondering if somebody has already tried this with Baby Hueys.
AUTO-BIAS
I meant the "old" type auto-bias
Did you try the LM317 trick here?
Perhaps somebody can also get lucky with Rg1 and try Fixed Bias.
Thanks again .
Usually I have noticed that loosing Grid Stopper result in a 'tighter' sound. Only with 6L6 family I have faced oscillation issues. I usually start with the GS's and remove them if things dont oscillate. I was just wondering if somebody has already tried this with Baby Hueys.
AUTO-BIAS
I meant the "old" type auto-bias
Did you try the LM317 trick here?Perhaps somebody can also get lucky with Rg1 and try Fixed Bias.
Thanks again .
Hello Gingertube,
I've tried to apply the shunt feedback a la "Baby Huey" in the amp I'm designing.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=79555
(last page)
Could you evaluate it, and explain me how the feedback ratio is calculated? I would like to know how much gain I will "burn" this way, since I have this amp with a bit too much gain.
Thank you very much!
I've tried to apply the shunt feedback a la "Baby Huey" in the amp I'm designing.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=79555
(last page)
Could you evaluate it, and explain me how the feedback ratio is calculated? I would like to know how much gain I will "burn" this way, since I have this amp with a bit too much gain.
Thank you very much!
LM334 Current Source for diff pair
Corbato,
Had a quick squiz. at the data sheet and application notes.
I think it will be OK BUT I don't think it will be as good as the cascode transistor BECAUSE:
Zout for 1 mA is quoted at 1MOhms at 10kHz. Graph doesn't go far enough but by extrapolation this maybe down to around 100K Ohm by 100kHz. This may mean that its a little harsh on the top end. This is due entirely to the 15pF shunt capacitance specification (For 15pF Zc=106 KOhms at 100kHz).
I can only suggest you try it and see. The Current set resistor you need with LM334 for 1mA current source is 68 Ohms.
ALL: Shows the need for keeping wiring on the current sources "short and sweet" - keep that shunt capacitance low for best results. I actually shifted some cathode to current source wires on my unit the other night and was sure I heard an improvement but talked myself out of it. That calc. above says that maybe I was right the first time and I did hear a difference.
I havent looked at using LM317 for EL84 cathodes BUT its been done before. Better check its shunt capacitance on the data sheet too.
I'm running the shunt feedback set resistor on mine up at 27K now. Its definitely not as "live" (or should that be "colored") as using 16K but its a lot more accurate and the top end is tighter and more detailed.
Cheers,
Ian
Corbato,
Had a quick squiz. at the data sheet and application notes.
I think it will be OK BUT I don't think it will be as good as the cascode transistor BECAUSE:
Zout for 1 mA is quoted at 1MOhms at 10kHz. Graph doesn't go far enough but by extrapolation this maybe down to around 100K Ohm by 100kHz. This may mean that its a little harsh on the top end. This is due entirely to the 15pF shunt capacitance specification (For 15pF Zc=106 KOhms at 100kHz).
I can only suggest you try it and see. The Current set resistor you need with LM334 for 1mA current source is 68 Ohms.
ALL: Shows the need for keeping wiring on the current sources "short and sweet" - keep that shunt capacitance low for best results. I actually shifted some cathode to current source wires on my unit the other night and was sure I heard an improvement but talked myself out of it. That calc. above says that maybe I was right the first time and I did hear a difference.
I havent looked at using LM317 for EL84 cathodes BUT its been done before. Better check its shunt capacitance on the data sheet too.
I'm running the shunt feedback set resistor on mine up at 27K now. Its definitely not as "live" (or should that be "colored") as using 16K but its a lot more accurate and the top end is tighter and more detailed.
Cheers,
Ian
Re: LM334 Current Source for diff pair
Thanks Ian
One of the reasons I am looking at LM334 is that it can be mounted right on the socket. Right now I am drawing out the wiring layout. The discreet CCS will require a small vero-board construction and its accompanying capacitance issues. Dunno what would be a better choice. Will report back after trying both.
Thanks again.
Added later:
OK. Further reading of the LM334 date sheet suggests possibles issues with thermal effects, and slew rate. The first is high, second is low!. Best it to build and see.
Thanks Ian
Exactly!gingertube said:
One of the reasons I am looking at LM334 is that it can be mounted right on the socket. Right now I am drawing out the wiring layout. The discreet CCS will require a small vero-board construction and its accompanying capacitance issues. Dunno what would be a better choice. Will report back after trying both.
Thanks again.
Added later:
OK. Further reading of the LM334 date sheet suggests possibles issues with thermal effects, and slew rate. The first is high, second is low!. Best it to build and see.
gemby,
Have a look at post #21 on this thread
http://www.diyaudio.com/forums/showthread.php?s=&threadid=78712&perpage=10&pagenumber=3
- links to original circuit and my modified version of the Music Angel EL34. Will try to get some photos
corbato,
There are not a lot of voltages to check - these are the voltages on my amp:
Main HT Voltage: +280V
Diffamp Anodes : +170V
EL84 Cathodes: +10 to +12V
Hint: The current sources adjust the EL84 bias for matched bias currents. You can then simply select pairs of EL84s which end up with the same bias voltage to match tubes.
Example: if you had cathode voltages of 10.2 and 11.1 say on one channel and 10.4 and 10.9 on the other then rearrange the tubes to put the 10.9 and 11.1 in one channel and the 10.2 and 10.4 in the other channel.
Cheers,
Ian
Have a look at post #21 on this thread
http://www.diyaudio.com/forums/showthread.php?s=&threadid=78712&perpage=10&pagenumber=3
- links to original circuit and my modified version of the Music Angel EL34. Will try to get some photos
corbato,
There are not a lot of voltages to check - these are the voltages on my amp:
Main HT Voltage: +280V
Diffamp Anodes : +170V
EL84 Cathodes: +10 to +12V
Hint: The current sources adjust the EL84 bias for matched bias currents. You can then simply select pairs of EL84s which end up with the same bias voltage to match tubes.
Example: if you had cathode voltages of 10.2 and 11.1 say on one channel and 10.4 and 10.9 on the other then rearrange the tubes to put the 10.9 and 11.1 in one channel and the 10.2 and 10.4 in the other channel.
Cheers,
Ian
Hello Ian,
sorry for the partial offtopic but I haven't been able to get in contact with you in private.
I'm in the process of prototyping an EL36 PP triode amp: the input stage now is a CCS loaded ECC83 LTP, then dc coupled to a 5687 cathode follower, then ac coupling to the EL36 in fixed bias.
Now the question is: is it worth it to implement shunt feedback and partial feedback since I'm using ECC83 frontend? From this point of view, if I do this could I remove the cathode follower driver or it will degrade performance (since EL36 are not so easy like EL84). Please note that I need AT LEAST 35Vrms drive to the EL36 grids with 2Vrms input.
Thank you very much, and if you feel that this is too OT you can answer me at my email addres in my profile.
sorry for the partial offtopic but I haven't been able to get in contact with you in private.
I'm in the process of prototyping an EL36 PP triode amp: the input stage now is a CCS loaded ECC83 LTP, then dc coupled to a 5687 cathode follower, then ac coupling to the EL36 in fixed bias.
Now the question is: is it worth it to implement shunt feedback and partial feedback since I'm using ECC83 frontend? From this point of view, if I do this could I remove the cathode follower driver or it will degrade performance (since EL36 are not so easy like EL84). Please note that I need AT LEAST 35Vrms drive to the EL36 grids with 2Vrms input.
Thank you very much, and if you feel that this is too OT you can answer me at my email addres in my profile.
Giaime,
Check out post #54 on this thread. Its the schematic for the 6V6 fixed biased variant. What you would need to do is the same thing except your tube cathode followers replace the MOSFET source followers shown (and the zener protections are then not required). You probably would need to increase the anode resistors on the 12AX7 up to say 150K or 180K and drop the current source a little to balance the DC voltages. Also you would not want to put too much shunt feedback - considering the voltage you need to swing into the output tube grids BUT perfectly do-able and will give you a little improvement in damping plus the self balancing action.
Start Point if you decide to do it:
I'd try 150K on the 12AX7 anodes to 39K from output tube anodes networks with about 10K as the cross connect (shunt feedback set) for a start and drop the CCS current to 1.5 mA (that is 750uA each side of the diff amp). Fire it up and see what happens - you want about +150V to +160V on the 12AX7 anodes
If you have plenty of volts then you might like to go 180K on the 12AX7 anodes - if you don't have enough volts then you need to drop the CCS current a little.
Cheers,
Ian
Check out post #54 on this thread. Its the schematic for the 6V6 fixed biased variant. What you would need to do is the same thing except your tube cathode followers replace the MOSFET source followers shown (and the zener protections are then not required). You probably would need to increase the anode resistors on the 12AX7 up to say 150K or 180K and drop the current source a little to balance the DC voltages. Also you would not want to put too much shunt feedback - considering the voltage you need to swing into the output tube grids BUT perfectly do-able and will give you a little improvement in damping plus the self balancing action.
Start Point if you decide to do it:
I'd try 150K on the 12AX7 anodes to 39K from output tube anodes networks with about 10K as the cross connect (shunt feedback set) for a start and drop the CCS current to 1.5 mA (that is 750uA each side of the diff amp). Fire it up and see what happens - you want about +150V to +160V on the 12AX7 anodes
If you have plenty of volts then you might like to go 180K on the 12AX7 anodes - if you don't have enough volts then you need to drop the CCS current a little.
Cheers,
Ian
panos29,
PL84 could be made to work BUT there are a few issues:
1) Screen voltage limitation - you would not want to run HT of more than say +250V
2) Output Transformer Raa needs to be 4K Ohm compared with 8K Ohm for EL84
3) distortion will be a little higher
Output Power will be much the same.
Plus the 15V @ 300mA for PL84 Heaters
Cheers,
Ian
PL84 could be made to work BUT there are a few issues:
1) Screen voltage limitation - you would not want to run HT of more than say +250V
2) Output Transformer Raa needs to be 4K Ohm compared with 8K Ohm for EL84
3) distortion will be a little higher
Output Power will be much the same.
Plus the 15V @ 300mA for PL84 Heaters
Cheers,
Ian