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PostPosted: 07 Nov 2017, 18:26 
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Location: Australia
This is the second of the is style of headphone amp I have built. The first got sold, should never have done it. Here just SS rectification and three stage filtering. A small 5H Hammond choke starts the smoothing process. A Hammond power tranni which should be in spec but runs awful hot. I built this for someone else but because of the tranni issue I'm keeping it and building the client another with a more robust power tranni and tube rectification.

The 6.3V winding is rated 2.5A but also the 12AU7 filament is being drawn from here adding an extra 300mA. Could this be the heat problem? The tranni is rated 65 or 75mA depending whether you believe the Hammond web site or the sticker on the tranni. Total draw is 60mA. As best as I could measure the tranni gets to 40C or 100F. Too hot? I can hold my hand on it for about 5 seconds.

Power tube is a RCA 6AS7 which I tested is 105% emissions and the 12AU7 is an Ozzi Miniwatt testing 95%. Good combo of tubes. Sockets are ceramic and most of the wiring is solid core. Alps Blue Velvet pot with solid brass knob. All WBT silver soldered (45).

With a 12AX7 there is a lot more gain but you can just detect a very faint hum. With the 12AU7, less gain (not a problem) and no hum. The amp drives both my 32ohm and 250ohm HPs without any problem. Lots of bass. The schematic is similar to the standard Bottlehead design. I'm using only 100uf 100V NP output caps. On the schematic they are recommending 680uf. I can't see how that is required I'm finding bass plentiful. All power caps are snubbed and the output cap is snubbed with a 220nf poly. I like to snub any large audio signal cap. I think is makes them quieter and I believe it allows better flow of HFs.

Base once more by my amp buildimg buddy Don and is Australian Spotted Gum. Top plate is thick ABS. The sound is absolutely fabulous.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 08 Nov 2017, 00:32 
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Absolutely gorgeous! :thumbsup:

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PostPosted: 08 Nov 2017, 01:21 
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Thanks mate.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 08 Nov 2017, 15:01 
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Location: Arizona, USA
Hi Mark, looks nice. What schematic?

Good listening
Bruce

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PostPosted: 08 Nov 2017, 18:44 
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gofar99 wrote:
Hi Mark, looks nice. What schematic?

Good listening
Bruce

Thanks Bruce. I've assembled three Bottlehead HP kits for others and this schematic is the same as a basic Bottlehead. I built one of these some years back with the massive Sophia Princess rec tube on external PS. Foolishly sold it. Im about to build another again for another guy. With this one the tranni gets too hot. It maybe the fact that the power tranni works straight into 100uf cap direct from the diodes. I'm going to add two 220ohm resistors before the first cap to see if that lightens the tranni load. The power tranni is CTed. So from the two high voltage leads I'll add the resistors.
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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 08 Nov 2017, 21:38 
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Hi Mark. The build looks great.

The heat problem may be two fold. Drawing 2.8A on 2.5A rated transformer will definitely cause it to heat up. My experience with the new Hammond transformers is that they run pretty hot as you reach full rating so going over doesn't help. The heat from the nearby 6AS7 may also add to the heat.

Are you using strictly 6AS7. The 6080 works well here too and are inexpensive. The downside is they look awful compared to the shapely 6AS7.

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PostPosted: 08 Nov 2017, 22:35 
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Yes the RCA 6AS7 being a little close doesn't help. But I'm not using it thinking I'm going to burn out the tranni and lose $117 worth. I have bigger tarnnies on the way. I'm enjoying it so I want to keep using it. Ambient here is about 20C, 70F.

Thinking the duty factor and load on the tranni was high with me having 100uf of cap directly after the diodes I added two 100ohm restistors on each tranni HT lead. I then dropped the last limiting resistor to ensure 150VHT to the amp cct. This doesn't seem to of helped a great deal though it maybe slightly better.

Also I rewired the driver tube heater to take a E88CC of which I have Genelex Gold Lions. The original cct. uses E88CCs. Not sure now what to do keep running it hoping the tranni doesn't pop or use some other HP amp. Wife's home sick so I'm on the phones.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 08 Nov 2017, 23:08 
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Well I just happen to have a NOS GE US of A 6080. I kind of like thier shape and used a similar tube in my first build. I'll give this a go and see if the tranni handles this tube better though I can see no reason why. I'll let you know. At this stage not as smooth as the big old RCA 6AS7 buts spankers (new).

Which would you use Gio? Cant load an image it is too big. Soon.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 08 Nov 2017, 23:16 
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With 6080.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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PostPosted: 09 Nov 2017, 06:16 
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For some strange reason on some tracks there is a crackling sound. It is appears to be distortion and appears to follow passages of heavy bass. The tranni is slightly under powered and I have replacement tranni on the way. Not sure this is the problem though. The HT does not sag on these tracks and ii'm not convinced it is all the trannis fault.

But I think Ill scrap the whole amp innards and build Suncals 12A*7/6AS7 HP amp instead in the same chassis with the bigger tranni. It will be a week or so until the tranni arrives so time to do something else.

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Projects: "Sanctum" - 12AU7 and 6AS7 direct coupled headphone amp | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "Mimic Carbon" - carbon resistors and PIO caps. MM phono preamp
Website: retro-thermionic


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