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Old 1st April 2014, 02:08 PM   #41
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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
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Old 1st April 2014, 02:58 PM   #42
MelB is offline MelB  Canada
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MFF your secondary resistance of 5 ohms in your PSU simulation seems quite high for a > 3.5 amp transformer??
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Old 25th June 2014, 09:54 AM   #43
MFF is offline MFF  Portugal
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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!
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Old 25th June 2014, 12:50 PM   #44
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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
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Old 25th June 2014, 02:09 PM   #45
Artur5 is offline Artur5  Andorra
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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.
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Old 26th June 2014, 04:13 PM   #46
MFF is offline MFF  Portugal
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Hi Artur5,

Thanks for your reply,
I intend to review the power supply, you're right 16mv is a bit high.
Can you tell me how you have come to these numbers,only for my education.

Cheers
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Old 26th June 2014, 04:59 PM   #47
Artur5 is offline Artur5  Andorra
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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.
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Old 26th June 2014, 05:42 PM   #48
MFF is offline MFF  Portugal
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Default two drive stage

This is a draft for a two drive stage,




I tried to incorporate the Thomas suggestion but
I don't know if the design is correct, any comments are welcome.
thanks.
Attached Images
File Type: jpg two stage.jpg (194.8 KB, 127 views)
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Old 26th June 2014, 05:45 PM   #49
MFF is offline MFF  Portugal
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Artur5,
Thanks I appreciate the help
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Old 26th June 2014, 06:03 PM   #50
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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
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