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Old 26th April 2011, 01:50 AM   #101
kevinkr is offline kevinkr  United States
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A couple of words.. 40mA/250V...
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Old 26th April 2011, 08:40 PM   #102
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by kevinkr View Post
A couple of words.. 40mA/250V...
I was aiming at 34mA, why you recommend 40mA. Is it not beyond the recommended power dissipation limits?
Cheers, ale
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Old 27th April 2011, 12:21 AM   #103
kmaier is offline kmaier  United States
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A later specification for ST-style 45 shows an increase in maximum plate voltage at 300V and maximum plate dissipation at 10 watts.

With a 5K load, I prefer ~34ma but also prefer higher plate voltage and running the plate up at 10-watts. My 45 SETs are rated at 2-watts RMS and clip ~2.25-watts. Grid drive needs to be around 120V pk-pk however.

On your SE amp CAD sim.... your output power is shown in peak, not RMS watts, which means you're only getting 1-watt of RMS power. Is this your goal?

Regards, KM
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Old 29th April 2011, 09:45 AM   #104
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by kmaier View Post
A later specification for ST-style 45 shows an increase in maximum plate voltage at 300V and maximum plate dissipation at 10 watts.

With a 5K load, I prefer ~34ma but also prefer higher plate voltage and running the plate up at 10-watts. My 45 SETs are rated at 2-watts RMS and clip ~2.25-watts. Grid drive needs to be around 120V pk-pk however.

On your SE amp CAD sim.... your output power is shown in peak, not RMS watts, which means you're only getting 1-watt of RMS power. Is this your goal?

Regards, KM
Hi KM,
Never seen that spec, can you send me the datasheet if you have it at hand?
Thanks for pointing out the peak power oversight. I will need to change my power supply then, but first will build as is and test results. I can then modify my HT PS and bias points to increase power.

I have etched two PCBs for the PNP and MOSFETs CCS and will do a listening test to see the difference in sound which I'm keen to listen to...

This weekend I will work on Rod Coleman's DHT supply boards and do a bit of wiring on the amp.

Will post results shortly...
Cheers,
Ale
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Old 2nd May 2011, 07:30 PM   #105
mogliaa is offline mogliaa  United Kingdom
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Default 45SET build...

Here I'm back from an intensive weekend building the chassis and getting the different circuit blocks to work. Attached are some pictures of the current build. The 6sL7 driver has the PNP CCS board and the 45s have the Rod Coleman's DHT circuits.

Had some problems with my LT bench supply so ended up wiring an alternative supply that could cope with the heaters current demand...

Now have the pair of 45s regulating at 2.5V and 1.5A. Bias voltage at -45V but have some issues measuring the cathode current across an 1 ohm resistor which is connected from one filament to the HT ground bus. Without HT applied have some 10mV sensed across the resistor so I guess I have some leakage current across? I have emailed Rod to seek for some assistance...

Either way, I did apply some input to the 6SL7 and got 160Vpp across the 45 anode with an 1.4vpp input. See my graphs attached, looks quite good and this should be about 2.5W across the 8ohm resistor?

Will keep working on it...
cheers,
Ale
Attached Images
File Type: jpg 6SL7 driver.jpg (89.6 KB, 529 views)
File Type: jpg 45 anode voltage.jpg (77.9 KB, 520 views)
File Type: jpg 45SET build.jpg (127.6 KB, 513 views)
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Old 21st May 2011, 06:46 PM   #106
mogliaa is offline mogliaa  United Kingdom
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Default Modifying the driver circuit

Hi everyone,
Here I come again with an update. Have converted my valve rectifier to a hybrid one, so now my HT power supply can deliver 450V. This will definitely help me in fixing my driver circuit :-)

Have modified the 6SL7 driver circuit based on SY's CCS using the DN2540.
My constrain still is two octal sockets, so may change the valves if needed (e.g. 6SN7, 6BL7, etc), but these need to be a double triode in a single valve envelope

I have removed my paralleled 6SL7 and now have a 6SL7 grounded cathode biased at -2V (green LED) and 1.8mA and direct coupled to a cathode follower using the other half of the 6SL7 biased at 1.8mA as well. I suppose I can modify the bias point of the cathode follower, any recommedations?
With the current circuit the output impedance is around 660 ohms, the gain is 36.7dB (or 68.4), sensitivity is 5.4vpp delivering up to 374Vpp to the 45 output valve (I need 160vpp to deliver full SET power). Bandwidth is around 7Hz to 312kHz.

will you suggest changing the cathode follower to 6SN7 or 6BL7 to reduce further the output impedance? (these are the only double triode valves I have in octal envelope)

Many thanks for the help....
Cheers,
Ale
Attached Images
File Type: png Picture 2.png (29.4 KB, 469 views)
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Old 21st May 2011, 09:26 PM   #107
mogliaa is offline mogliaa  United Kingdom
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Forgot the protection diode for the second valve and also need elevating the heater? Cathode is around 250v, what is the Vhk max for the 6sl7? can't find it in my brimar data sheet.
Thanks,
Ale
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Old 26th June 2011, 04:11 PM   #108
mogliaa is offline mogliaa  United Kingdom
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Slowly making some progress on the chassis. Have added a wood front cover to the amp.

Now need to complete the heater raw supply to feed Rod Coleman's boards and do some testings!

Cheers,
Ale
Attached Images
File Type: jpg IMG_0665.jpg (107.0 KB, 407 views)
File Type: jpg IMG_0670.jpg (106.4 KB, 162 views)
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Old 9th July 2011, 12:14 PM   #109
mogliaa is offline mogliaa  United Kingdom
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Rod Coleman's board are working great. Input ripple is reduced from 760mVpp to less than 25mVpp. Input voltage is 9V and regulator is adjusted to 2.5V

Thanks Rod for the help to tune the LT supply and thanks Andy Evans for giving me a piece of that nice multicore cable!

Cheers
Ale

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Old 10th July 2011, 11:40 AM   #110
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Hi Ale, Glad to see it's coming along!

Please take care with measurements and position of the 45s - many DHTs will behave strangely if operated at any angle away from the vertical. This is especially a risk in old DHTs, where the tension of the filaments may have slackened a little. In this case the filament-to-grid separation distance changes when running horizontally, and the anode current changes in a corresponding way! In the extreme, a short might occur.

I prefer to prop the amp up on four cans of beer to take measurements. When all the adjustments have been made, the four beers seem to improve the sound even further.

Do you have small values of capacitor on the filament raw dc? 760mV of incoming ripple seems high. With a CRC-connected supply of 10 000uF x2 and about 1R, it should be near to 250mV. This level should allow unmeasurably small ripple on the filament - much less than 1mV.

I bet it sounds good though....
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