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Old 27th February 2014, 04:37 PM   #21
45 is offline 45  Italy
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I would consider even a bit less, about 650V, under load (also considering some volts dropped into the OT). I would still run it at 60 mA. Increasing the current to 80 mA I guess will result in lower "usable" Pout as that driver will be not able to drive the 845 into positive grid field with low distortion.
To get it right with you actual 845's I would start with 1K plus +10R precision reistor to measure the current + a 1K variable resistor, all in series. Once you know what value you need you change them with the nearest convenient value.
I agree with MelB, that 20 uF cathode bypass is a bit small. I would use 100 uF. It's cheap if you use electrolytics....However initially you don't need to change it if you run measurements at 1KHz only.

Last edited by 45; 27th February 2014 at 04:39 PM.
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Old 27th February 2014, 04:41 PM   #22
kevinkr is offline kevinkr  United States
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Check the current ratings of the 5AR4 at these voltages, you might want to consider a pair in parallel or a 5U4 instead particularly with modern examples which generally aren't too reliable at these voltages. You can improve the ruggedness of the 5AR4 considerably by installing a pair of UF4007 or similar diodes in series with the plates.
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Old 1st March 2014, 03:57 PM   #23
MelB is offline MelB  Canada
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I'm missing something about the lower operating point you suggest. If you run at 80mA vs. 60mA you get into a positive grid voltage when your anode voltage is higher and therefore your grid current is lower requiring less drive current? Or do I have that backwards?
It seems counter intuitive that a lower operating point would yield more power out?
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Old 1st March 2014, 09:39 PM   #24
45 is offline 45  Italy
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I was simply saying that one is not going to get a real benefit from the increased dissipation as the output tube will first run into grid current than cut-off and the clipping will be harder. The difference in anode voltage will only be about 10V between the two operating points I guess, without considering various drops along the supply and OT. Assuming 650V total, for 575V at - 75V bias you would get about 60 mA and for 75V swing it will be still several milliamps, on average almost 10 mA, away from the cut-off with 5K. It could even run at lower primary load without problem, for example if the speaker load drops to 6R the max Pout would not change significantly. However he can try both 60 and 80 mA and decide what he likes. The Pout will be a bit less, equal or a bit more mostly depending on the max THD you want to accept otherwise if you consider the absolute max 10% THD the lower current might just give a bit more power for any secondary load equal to or greater than 6R!

Last edited by 45; 1st March 2014 at 10:03 PM.
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Old 12th March 2014, 04:34 PM   #25
MFF is offline MFF  Portugal
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Thank you all for the help!
I'm still waiting for some parts to arrive.
meanwhile I'm trying to figure out what the best design for the filaments power supply.
Please see the design.
Tried to simulate, but the soft does not have the differential model for the choke.
One of the questions is, should I apply the differential choke immediately after the first cap as in the schema or should I apply it at the output?

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Old 12th March 2014, 06:19 PM   #26
wicked1 is offline wicked1  United States
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Can someone verify if MFF has the common mode choke wired properly?
I thought the dots should both be at the same end.
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Old 17th March 2014, 03:58 PM   #27
MFF is offline MFF  Portugal
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Default Help on IT drive stage

Originally Posted by 45 View Post
I think you will get about 7W max with 610 total supply voltage at 60 mA and 5K.
If you go for the LL1692a/18mA you can experiment the connection ALT T (36 mA and 35H) with the 6C45 or the connection ALT S 4:3.5 ratio with the D3a.
The connection ALT S will allow 18 mA that are enough for the D3a with 125H primary inductance.
With the 6C45 and LL1692a ALT T you should need less than 0.7 V RMS for full power. With the D3a and LL1692a ALT S you will need about 1 V RMS. For 1W output you will need about 0.25 V RMS in the first case and 0.37 V RMS in the second. This could be enough if you use a modern CD player and 90 dB efficient speakers in most cases without adding a step-up input transformer. I would first try this two stage amp with normal volume control at the input and then if you feel the gain is not enough you can think about increasing the gain further.
I'm trying to draw the IT drive stage but I'm not sure if my calculations are correct this IT for Alt S connection have a 18mA rated.
should I set the bias to 18mA or to be safe, go for a lower set point like 15mA or 10mA. I'm trying not to burn the IT.
I real appreciate your help!!
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Last edited by MFF; 17th March 2014 at 04:05 PM.
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Old 17th March 2014, 06:24 PM   #28
Vinylsavor is offline Vinylsavor  Germany
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You can safely run the IT at the rated current of 18mA. In fact these transformers have some margin. Up to 20mA would still be ok.

There is no danger to burn such transformers at these currents. What happens when you apply too much DC is core saturation. But that kicks in gradually.

Best regards

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Old 18th March 2014, 03:39 PM   #29
MFF is offline MFF  Portugal
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Hi Thomas,
Thanks for the clarification about transformers.

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Old 18th March 2014, 04:22 PM   #30
rdf is offline rdf  Canada
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From memory the max rated voltage for that James is 500 VDC. I once asked about using it with an 828.
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