Hi!
Ah, I didn't check carefully. I thought it is the B+ supply. For the filament supply it makes sense of course. I like to use SB850 Schottky diodes in the filament supply of 211/845
Make sure the diodes you are using can handle the peak currents (check the data sheet).
It might be worthwhile to try choke input (no cap after the rectifier) but the resulting voltage will be lower.
Best regards
Thomas
Ah, I didn't check carefully. I thought it is the B+ supply. For the filament supply it makes sense of course. I like to use SB850 Schottky diodes in the filament supply of 211/845
Make sure the diodes you are using can handle the peak currents (check the data sheet).
It might be worthwhile to try choke input (no cap after the rectifier) but the resulting voltage will be lower.
Best regards
Thomas
Finally got some time to assemble and test my amp.
the results are good, sounds great!
the only thing that needs to be reviewed is the lack of gain in the drive stage.
The bias of the 845 are 70V/60mA the OT Hashimoto H-20-7-U connected at 5K.
The drive stage is one D3a at 18mA with IT LL1692a ALT S.
I am considering the possibility of going from two stage amp to a three stage Amp
but would like to keep the same IT(budget issue) any suggestions, maybe D3a as input than a 2A3, IT LL1692a
is this a good idea?
Once again thank you all for your precious help!
the results are good, sounds great!
the only thing that needs to be reviewed is the lack of gain in the drive stage.
The bias of the 845 are 70V/60mA the OT Hashimoto H-20-7-U connected at 5K.
The drive stage is one D3a at 18mA with IT LL1692a ALT S.
I am considering the possibility of going from two stage amp to a three stage Amp
but would like to keep the same IT(budget issue) any suggestions, maybe D3a as input than a 2A3, IT LL1692a
is this a good idea?
Once again thank you all for your precious help!
Hi!
Inorder to make a good choice, it would be helpful if you determine how much more gain you want. 6dB? 12dB?
A 2A3 has rather low gain. If you move the D3a from the transformer coupled position to a resist loaded input stage, it will have a bit less gain, which needs to be taken into account.
A good choice might be the 6AH4. You could use it with the LL1692A. It has an amplification factor of 8. With the loss you will get in the gain of D3a (assuming you will use it resistor loaded) you will probably end up with a boost in gain of 6x. If that turns out to be tot much you can use the 6CK4 instead or wire the IT for further step down
Best regards
Thomas
Inorder to make a good choice, it would be helpful if you determine how much more gain you want. 6dB? 12dB?
A 2A3 has rather low gain. If you move the D3a from the transformer coupled position to a resist loaded input stage, it will have a bit less gain, which needs to be taken into account.
A good choice might be the 6AH4. You could use it with the LL1692A. It has an amplification factor of 8. With the loss you will get in the gain of D3a (assuming you will use it resistor loaded) you will probably end up with a boost in gain of 6x. If that turns out to be tot much you can use the 6CK4 instead or wire the IT for further step down
Best regards
Thomas
Hi MFF.
Looks like a great amplifier. I always wanted to build 845 monoblocs but I'm a bit 'nervous' with voltages higher than 400v..
According to the Designer II soft, in your PSU the mains 100hz hum would be of the order of 16mV RMS at +B. That's a bit high.
Depending on the sensibility of your speakers maybe it won't be a problem, but it's fairly easy and cheap to add an additional filtering stage. For instance, a 100R/2w resistor and a 50uF capacitor ( 2x100uF/400V in series). The voltage dropout through that resistor is fairly small, (7.8V at 78mA) and the hum would go down to 4mV RMS. i.e.12 db less than now.
Looks like a great amplifier. I always wanted to build 845 monoblocs but I'm a bit 'nervous' with voltages higher than 400v..
According to the Designer II soft, in your PSU the mains 100hz hum would be of the order of 16mV RMS at +B. That's a bit high.
Depending on the sensibility of your speakers maybe it won't be a problem, but it's fairly easy and cheap to add an additional filtering stage. For instance, a 100R/2w resistor and a 50uF capacitor ( 2x100uF/400V in series). The voltage dropout through that resistor is fairly small, (7.8V at 78mA) and the hum would go down to 4mV RMS. i.e.12 db less than now.
I did a simulation on PSU Designer II with the values of your PSU -> 460V/0,2A trafo, 51uF Cap as first filter and 10H choke + 51uF Cap. as second filter stage, for a total load of 78 mAmp.
The final simulated +B is 620Volt and the difference between the max. and min. values at C2 is 45.2mVolt. Being this a peak-to-peak value, that's aprox. 16mV RMS.
If you add a third filter with a 100R resistor and another 50uF cap, +B drops to 612Volt. and the hum goes down to 12.2 mV peak-to-peak = 4.3mV RMS.
Of course, I don't know the resistive value of your choke ( I assumed 100 Ohm) and the exact regulation of the trafo, thus the real +B might differ some volts from the simulation, but the hum should be quite accurate, for 78mAmp. of standing current ( 60mA the power tube and 18mAmp the driver)
Cheers.
The final simulated +B is 620Volt and the difference between the max. and min. values at C2 is 45.2mVolt. Being this a peak-to-peak value, that's aprox. 16mV RMS.
If you add a third filter with a 100R resistor and another 50uF cap, +B drops to 612Volt. and the hum goes down to 12.2 mV peak-to-peak = 4.3mV RMS.
Of course, I don't know the resistive value of your choke ( I assumed 100 Ohm) and the exact regulation of the trafo, thus the real +B might differ some volts from the simulation, but the hum should be quite accurate, for 78mAmp. of standing current ( 60mA the power tube and 18mAmp the driver)
Cheers.
Hi!
In order to provide good advice, it would be helpful if you answer the question about the gain increase you want to reach. It would also be helpful if you explain a bit about the specific choices you made in this schematic to understand your train of thought.
It would also help if you give some details about the output stage, grid bias of output tube to understand the drive voltage needed.
Some initial findings:
- The D3a should have a grid stopper or ferrite bead at the grid pin.
- The grid to ground resistor of the 6AH4 is unnecessarily low
- Why only 14ma through the 6AH4 if the IT can handle 18?
- Use as high a B+ as possible on the driver to get more headroom
best regards
Thomas
In order to provide good advice, it would be helpful if you answer the question about the gain increase you want to reach. It would also be helpful if you explain a bit about the specific choices you made in this schematic to understand your train of thought.
It would also help if you give some details about the output stage, grid bias of output tube to understand the drive voltage needed.
Some initial findings:
- The D3a should have a grid stopper or ferrite bead at the grid pin.
- The grid to ground resistor of the 6AH4 is unnecessarily low
- Why only 14ma through the 6AH4 if the IT can handle 18?
- Use as high a B+ as possible on the driver to get more headroom
best regards
Thomas
Hi,
About the gain I'd like to be able to connect direct to any CD.
The output stage is 845 low voltage the OTP is 5K.
with cathode bias set at 70V/60mA. but I want to try to go far as 80mA.
To be able to driving the 845 with low distortion, might be difficult with a single stage,that is why I am considering moving for two stage.
Cheers
Hi!
Are you sure you need more gain?
Typical CD output level is 2V RMS.
The D3a amplifies that to 140V RMS
You wired the interstage 4:3.5 which reduces that to 120v RMS
That is about 340V peak to peak
Your output tube is biased at -70V at the grid which means it needs 140V peak to peak for full output power.
You have about 8dB gain reserve which should be enough.
Did you measure the voltage levels you have at the output of your CD player and the grid of the output tube?
Best regards
Thomas
Are you sure you need more gain?
Typical CD output level is 2V RMS.
The D3a amplifies that to 140V RMS
You wired the interstage 4:3.5 which reduces that to 120v RMS
That is about 340V peak to peak
Your output tube is biased at -70V at the grid which means it needs 140V peak to peak for full output power.
You have about 8dB gain reserve which should be enough.
Did you measure the voltage levels you have at the output of your CD player and the grid of the output tube?
Best regards
Thomas
a thought stacking caps increases total capacitance but their power rating remains..
Stacking is simply connecting caps in series, for example take three 400V/100uF caps and stack them - you end up with a 33uF 1200V capacitor. Note that you must install voltage equalizing resistors across each capacitor, and each one must be rated for operation at voltages greater than they will ever encounter in operation.
I posted a response yesterday but it didn't seem to have come on the board...
Yes, what Kevin said. If you look at big power 845 and 211 amp PSU schematics for example, you'll see what I mean. You'll even see these 'stacks' paralleled to get the capacitance up to requirement. So in the example above, two of those in parallel would equate to 66uf 1200v.
Yes, what Kevin said. If you look at big power 845 and 211 amp PSU schematics for example, you'll see what I mean. You'll even see these 'stacks' paralleled to get the capacitance up to requirement. So in the example above, two of those in parallel would equate to 66uf 1200v.
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