I have built a simple EL84 amp using parts from an old tube amp and the schematics from diy paradise. Now that it's built I am getting very little output. I think one of the problems is that my b+ after rectification is about 400V and it needs to be 300V. Can I fix this and if so how? I am not willing to unwind the PT.
If this is impossible could somone tell me what I could do to salvage what I have built into something useful? I have a total of 6 9 pin sockets.
If this is impossible could somone tell me what I could do to salvage what I have built into something useful? I have a total of 6 9 pin sockets.
Hi djnubz,
You say you used old parts, is the power trannie rated according to the schematic?
If so i would suspect you are not drawing current somewhere. Hence i would start by checking you're wiring. please check the pinouts of all tubes and then double check them.
Do some measuring to, check the grid and cathode voltages on the various tubes. Given the high supply voltage values can be a bit off but shouldn't be off to much. please report back.
If it really would be due to the psu voltage, try this for starters. replace the first cap in the psu with a 0.1 uf cap. Next add a 100R resistor in series with the second cap and place the 100uf cap you took out in the first stage of the psuafter the resistor. This should at least lower you're psu voltage.
Oh and just try connecting a 500k to 1m resistor from the input tubes grid to ground.
hope this helps
Joris
You say you used old parts, is the power trannie rated according to the schematic?
If so i would suspect you are not drawing current somewhere. Hence i would start by checking you're wiring. please check the pinouts of all tubes and then double check them.
Do some measuring to, check the grid and cathode voltages on the various tubes. Given the high supply voltage values can be a bit off but shouldn't be off to much. please report back.
If it really would be due to the psu voltage, try this for starters. replace the first cap in the psu with a 0.1 uf cap. Next add a 100R resistor in series with the second cap and place the 100uf cap you took out in the first stage of the psuafter the resistor. This should at least lower you're psu voltage.
Oh and just try connecting a 500k to 1m resistor from the input tubes grid to ground.
hope this helps
Joris
Hi DJNUBZ,
You need what the original power supply calls for. That means a choke. You can not use dropping resistors, the current variation is too high. If you don't want to use a choke, you need another power transformer with lower output. Your hum level may be too high if you go that route.
An electronic regulator for your B+ may work. It may possibly cost more than the choke.
You are also using the wrong tube type for the voltage amplifier. Try to stick as close as you can to the original diagram.
-Chris
You need what the original power supply calls for. That means a choke. You can not use dropping resistors, the current variation is too high. If you don't want to use a choke, you need another power transformer with lower output. Your hum level may be too high if you go that route.
An electronic regulator for your B+ may work. It may possibly cost more than the choke.
You are also using the wrong tube type for the voltage amplifier. Try to stick as close as you can to the original diagram.
-Chris
Here is the thread of how I built the PS.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=68702
http://www.diyaudio.com/forums/showthread.php?s=&threadid=68702
So reading the previous thread, it seems you went for a CRC smoothing filter.
Now the value of +B wil depend greatly on the value of R. The current consumption should be about 80 mA in total, so you can use this as a basis to work out a value for R.
Measure the voltage out of the rectifier and subtract 300 (the desited voltage) from it. Divide 80 into it (the expected current in milliamps) and you will get the value for R in KOhms.[ (V1-V2)/I ]. R is likely to dissapate lots of heat: Dropping 100V for example, would dissapate 8W, so probably a 10W (minimum) resistor would be needed. [ V*I ].
This is the first iteration in finding the value of R. In practice, the value might be a little different due to loading effects, but you must start somewhere.
Mount the resistor so that it can be cooled by a convection air flow, not close to other components, especially electrolytics.
I don't believe that current demand will vary. The OP stage is class A, and the Push-Pull should cancel out variation due to 2H distortion as well, so this solution will work to lower the voltage.
I'm still not clear about what you've used for an input stage. Pehaps you can explain. As a previous posters have remarked, the high +B is probably not the reason for your poor sensitivity/low output symptom, but probably something wrongly wired, such as the output transformer, or a wrongly designed input stage.
Now the value of +B wil depend greatly on the value of R. The current consumption should be about 80 mA in total, so you can use this as a basis to work out a value for R.
Measure the voltage out of the rectifier and subtract 300 (the desited voltage) from it. Divide 80 into it (the expected current in milliamps) and you will get the value for R in KOhms.[ (V1-V2)/I ]. R is likely to dissapate lots of heat: Dropping 100V for example, would dissapate 8W, so probably a 10W (minimum) resistor would be needed. [ V*I ].
This is the first iteration in finding the value of R. In practice, the value might be a little different due to loading effects, but you must start somewhere.
Mount the resistor so that it can be cooled by a convection air flow, not close to other components, especially electrolytics.
I don't believe that current demand will vary. The OP stage is class A, and the Push-Pull should cancel out variation due to 2H distortion as well, so this solution will work to lower the voltage.
I'm still not clear about what you've used for an input stage. Pehaps you can explain. As a previous posters have remarked, the high +B is probably not the reason for your poor sensitivity/low output symptom, but probably something wrongly wired, such as the output transformer, or a wrongly designed input stage.
Here are rest of the pages.
http://diyparadise.com/buildel84b.html
http://diyparadise.com/buildel84c.html
http://diyparadise.com/variation.html
http://diyparadise.com/buildel84b.html
http://diyparadise.com/buildel84c.html
http://diyparadise.com/variation.html
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.