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Old 10th July 2011, 07:53 PM   #111
mogliaa is offline mogliaa  United Kingdom
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Hi Rod,
Thanks, hard work but is slowly coming to life.
I'm using a circular 8-pole connector from Rapid to connect the two chassis and it seems it has a false contact that I lost the connection with the bias supply when I tested my HT supply. My HT voltage regulator obviously couldn't cope with the current load and I'm lucky that this one died and nothing happened to my 45 valves. Now I added a fuse and a HT switch, but still need to review what is going on with these connectors.

Thanks for the testing tips, will change my test layout then!

RE the ripple, have a look at the attached diagram. I have a couple of 15mF capacitors between the 0.47R resistor....

Cheers,
Ale
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File Type: png LT Supply 45SET.png (14.0 KB, 367 views)
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Old 11th July 2011, 09:29 AM   #112
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Ale, raw supply looks perfect.

Was the measurement taken while the HT was energised, or with only the filament ON?

With the values you have there, the ripple should be 150mV or so in the raw dc, and <<1mV across the filament - ie nothing should be visible if a scope is connected, and the most sensistive mV range selected.
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Old 11th July 2011, 09:41 AM   #113
mogliaa is offline mogliaa  United Kingdom
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Measured without HT supply connected. What difference should be seen?
Will have to retake the measures as my measure on the filaments was mainly noise....
Thanks
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Old 11th July 2011, 09:59 AM   #114
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Without HT is best, since HT noise couples into the scope.

It's sometimes hard to avoid measurement errors with a scope, especially if the probe's ground clip has long wire. Taking some bare, solid tinned copper wire [about 0.8mm dia or so] and wind around the probe ground band [clip removed]. Slide it off the probe and solder to the filament terminal [usually F+] that is connected to HT GND. Slide the probe back in, and solder a short wire to the other Filament terminal.

Long wires can pick up HF/UHF noise. This can be a problem if you live near big radio stations, Underground trains, trams, industrial works etc. If the noise is still evident with short probe wires, investigating some ferrite chokes and/or common-mode chokes can be very worthwhile for the sound quality. Use them in the HT supply and the filament supply.
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Old 31st July 2011, 02:40 PM   #115
mogliaa is offline mogliaa  United Kingdom
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Slowly more progress on the amp. Have tweaked the driver stage as per attached.
Mosfet CCS. Va=260V and HT=350V. Got bias at around -1.8V with a yellow LED. Haven't measured Ia, but from anode curves and ltspice should be around 3.5mA.
When measured distortion got 0.12% @ 2.8Vpp and 0.04%@1Vpp.

I know that Thomas would say that is far from being and optimal driver, but it doesn't perform bad at all. Final judge would be the listening test and see how will respond to dynamics, which it may be the downside of running the valves cooler at 3.5mA.

Options I think I have is to aim for Vg=-2V (Red LED) Ia=5mA so will need to raise HT to 400V or look for an alternative driver i.e. 6N7 as suggested earlier and add a pre-amp stage (which I'm keen to build soon).

I will complete the build as is, so can listen to the results before changing this.
Any recommendations on the driver options or further tweaking the current one?

Many thanks
Ale
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Old 31st July 2011, 07:28 PM   #116
mogliaa is offline mogliaa  United Kingdom
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Well, managed to do a quick end to end test and it sounds brilliant!
Output valves are running at 33mA and 300V as suggested by Kevin Maier :-)
No hum at all, just used my Grado headphones and played a bit of Charlie Mingus, nice!
Good bass response, clear tone and warm sound...

Those Rod Coleman's regulators are supreme. No AC hum at all could be noticed, so quiet. Will measure the output levels when I get the chance...

More than 6 months on this... happy to see some results coming up finally!
Cheers,
Ale
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Old 31st July 2011, 08:26 PM   #117
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Nice progress!

(But hey, don't ever call Mingus "Charlie". He hated that)
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Old 31st July 2011, 08:51 PM   #118
mogliaa is offline mogliaa  United Kingdom
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You are right, but his music is brilliant
Still need to finish the amp to enjoy it with my full range speakers!
Didn't used the beer cans as Rod suggested, just put the amp upside down at the edge of the table, at least the valves are not horizontal though...

Click the image to open in full size.
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Old 1st August 2011, 03:27 AM   #119
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Mogliaa, no hum with Grados is quite an accomplishment with a DHT amp, folks pay thousands and thousands of dollars for commercial DHT headamps without hum.

I was all set to use my set of coleman boards but the recommendation that they have to be in the same chassis as the tubes and fed from a separate umbilical cord than B+ had me delay the project. Who wants 2 umbilical cords ?

Can you explain how you accomplished the 2 chassis scheme using the colemen regs, i.e. board placement, tranny, cable used, etc.

thanks
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Old 13th August 2011, 10:51 AM   #120
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by regal View Post
Mogliaa, no hum with Grados is quite an accomplishment with a DHT amp, folks pay thousands and thousands of dollars for commercial DHT headamps without hum.

I was all set to use my set of coleman boards but the recommendation that they have to be in the same chassis as the tubes and fed from a separate umbilical cord than B+ had me delay the project. Who wants 2 umbilical cords ?

Can you explain how you accomplished the 2 chassis scheme using the colemen regs, i.e. board placement, tranny, cable used, etc.

thanks
I ended up with three chassis as I ran out of space on the HT chassis :-)
HT chassis has also the bias supply and 8 core cable to the main chassis.
LT supply chassis is housing two e-core trannies for the 45s and one toroidal trannie I had at hand for the driver filament supply. On this chassis I have three raw supply boards and another multipole connector with 4 core cable from this chassis to the main one.

Now need to do the final assembly before I can test this amp with my Fostex 167 full range speakers!

Cheers,
Ale
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