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PostPosted: 27 Nov 2012, 21:00 
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Joined: 26 Feb 2012, 13:13
Posts: 23
Location: Virginia
EXILE wrote:
It heats up to about 60 Celsius in summer but it has been giving me reliable power for about 6 months now :)

That's extremely HOT :hot: In old money that's 140f

I thought mine was hot at about 25c. I can put the back of my hand anywhere on the chassis or transformer without any fear of it being unbearable or burn inducing, as long as I avoid the glass things :)
EXILE wrote:
I use 32-60ohm phones and paralleled 6as7 gives me a better sound mostly on the low ranges.

Any tips on using mine with low impedance phones? My amp hates anything down the 32 ohms level. Early distortion at listening volume. My HD650s are 300 ohm and sound as you'd expect from these reference phones.

Is it as simple as adding a switched resistor in series with the phones socket to allow 'cheap' low impedance phones to be used? Or is the use of the parallel tube the way you achieved the lower impedance handling?


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PostPosted: 27 Nov 2012, 22:32 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
EXILE wrote:
It heats up to about 60 Celsius in summer but it has been giving me reliable power for about 6 months now :)


About the same as a ST-70's stock power transformer.

I used a Hammond on a pre and even at 2/3 ratings, hit 74C. on the I.R. thermometer :hot:

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PostPosted: 21 Oct 2013, 07:21 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
I was thinking about building this HP amp. Two things concern me: firstly I don't have any E88CCs tubes but there are some nice NOS tubes on eBay for about $40 for a a pair. My big concerns is the output cap is 680uf at 200V. I don't have one (or even ones I can gang together to make the value). Not sure where I would get it.

Does the output cap have to be this big? My phones are 33 ohms.

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PostPosted: 21 Oct 2013, 10:26 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
Forty bucks!!!!!!????? That's pure robbery!!
Check for equivalents like ECC88, 6922, 6DJ8 or the Russian 6N23P (or the mechanically more tougher 6N23P-EV). You should be able to find tubes from 5- 10 USD.

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PostPosted: 21 Oct 2013, 16:48 
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Soundbrigade wrote:
Forty bucks!!!!!!????? That's pure robbery!!
Check for equivalents like ECC88, 6922, 6DJ8 or the Russian 6N23P (or the mechanically more tougher 6N23P-EV). You should be able to find tubes from 5- 10 USD.

These where Phillips tubes NOS. There were dearer ones.

Soundbrigade, have you seen a schematic for a OTL 6AS7 headphone amp which uses a different driver tube e.g. 12AX7? Could the ECC88 be substituted for a 12AX7 in this cct?

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PostPosted: 21 Oct 2013, 16:56 
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I found an article about the HP amp and originally only 220uf was used as output caps. This I can manage. I would snub them with 2.2uf polies. I would like to drive this amp with another tubes???

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PostPosted: 11 Nov 2013, 06:47 
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Soundbrigade: I'm really going to upset you now I just paid $66 for a pair of Tesla E88CCs shipped. Others were more expensive and this I could buy now. I was hoping to build some weirdo HP amp with vintage tubes and trannies but had no schematic to go on. I have a a bunch of 6AS7s and now some driver tubes and the thought of no OPT makes me smile.

One guy built one and said it appears to drive his low impedance phones OK. Mine are 40ohm so it may test the amp. The PS is the next worry and I hoping to use a Sophia 274B mesh plate rev tube I bought. Current draw should be right.

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PostPosted: 11 Dec 2013, 22:03 
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My Tesla E88CCs came today. I has pleased to see the gold pins. I didn't know. Now for the build using a Sophia Electric Princess 274B. Two chassis on for PS and the other for the amp.

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PostPosted: 27 Dec 2013, 16:49 
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Joined: 09 Oct 2012, 19:43
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Location: Vancouver Canada
I wanted to know if the 6as7 tubes can be paralleled directly. It would seem to be the answer to driving lower impedance phones such as 32 ohm.


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PostPosted: 27 Dec 2013, 19:29 
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Location: US Pacific Northwest
laurie54 wrote:
I wanted to know if the 6as7 tubes can be paralleled directly. It would seem to be the answer to driving lower impedance phones such as 32 ohm.
The 6AS7/6080 can have current sharing issues if directly paralleled. If you choose this configuration, then each tube should have at least 150Ω of series resistance added on each plate. They should also get individual grid stoppers.

In an October-December, 1947 "Ham Tips" publication was this little ditty concerning using the 6AS7 in audio amplifiers...
Attachment:
6AS7 considerations.jpg
The very first item is "The two units should always be used in push-pull, never parallel.". For individual designs where they can be watched, you can get away with parallel the sections as I've described above, but in general purpose designs, it not a good thing to do with this tube.


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