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 Post subject: Re: 1930 171-A SE amp
PostPosted: 15 Apr 2013, 17:31 
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Location: Australia
I have had to use my jFet based BoZSE preamp to drive Intermezzo due to the fact that the input resistance is 400ohms and the BoZSE is my only preamp which will drive large voltage swings into the amp. I like the sound of this amp but didn't feel it was up to the level of some of my other amps. So I tried putting my most recent Universal single stage preamp in front of the BoZSE. Now I have a tube front end with a Fet backend driver. The sound improved dramatically with far more body and better overall musical weight. The sound went from a little dry to rich and meaty.

Today I hope to build a small BoZ and mount it in the 4SUT preamp which is the 4S Universal single stage tube preamp Suncalc designed. This will allow me to remove one alps pot from the audio path and have a single stage single tube preamp with a drive\buffer jFet stage all in the one box.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 15 Apr 2013, 21:07 
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Here is the BoZ module which will be installed in the 4SUTea preamp.
Attachment:
BoZFor4SUTea.jpg


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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 15 Apr 2013, 22:50 
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Here is the 4SUTea preamp with the BoZ back-end driver stage. The pot is between the output of the tube stage and BoZ. BoZ drives the output direct. This sounds a hell of a lot better than when BoZ was used as the preamp to Intermezzo. Now I have it all in one box (literally - its a tea box). This is a neat solution and combines tube front-end with a low impedance back-end.
Attachment:
P4162280.JPG


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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 16 Apr 2013, 06:09 
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Forgot to mention I received another 71-A today from eBay. I now have three tubes which look the same and have similar output levels. One other tube which has low output. I got this last tube for $34 plus shipping much cheaper than I paid for the others .

Also the BoZ driver stage is all component-2-component wiring. There is no hookup wire in the signal path. The big brown caps are the 10uf output caps. They are polypropylene and take about 10 hours to break in. I have used these lots in the past and for a cheap cap they sound good. Also I increase the source resistor to a reduce the gain slightly (I didn't need it here as much as I needed low impedance output) and provide a little more local feedback.

I have had a chance to listen for a few more hours. Sounding much better.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 16 Apr 2013, 09:33 
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Joined: 05 Jun 2012, 13:10
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Location: Darboy, WI, USA
Can you find a 80 tube with the same outline shape as the '71s? I think I've seen them that shape before...

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 16 Apr 2013, 16:23 
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RossD wrote:
Can you find a 80 tube with the same outline shape as the '71s? I think I've seen them that shape before...

Yes there is a balloon on eBay now. I have just purchased two more 80s with the same same shape as in the image. The one on eBay looks very frail and may not make it to Australia. It's a long swim.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 18 Apr 2013, 03:55 
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Some photos of the the filaments of Intermezzo:
Attachment:
71-AFilamaments.jpg
Attachment:
71-AFilamamentsII.jpg


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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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 Post subject: Re: 1930 171-A SE amp
PostPosted: 27 Apr 2013, 04:12 
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Intermezzo had always had a bit of a buzz and hum. With the Fostex efficiency that made the buzz audible from my seating position when the room was quiet and the music had stopped. But only just. At 1m from the speaker the buzz was quiet audible. With nothing earthed on the amp and everything floating, earthing various points and parts appeared the way two go to try to reduce the buzz.

I had tried connecting the mains earth to the frame of power trannies and the filament tranni, that did nothing but made the amp safer. Today I pulled the amp apart and with the help of a digital CRO went buzz hunting. I had 30mV pp odd of spikey buzz at 50hz with inputs shorted and connected to a speaker. No one earthing made a big difference but all the earths I tried, combined, fixed the buzz issue.

Firstly I attracted the mains earth to the HT negative this dropped the buzz by 5mV. This then became my new earth to which I would connected other sections of the amp. Then I earthed the frame of the input transformers (Hammond nickle core inter-stage trannies) this dropped the buzz by about a further 5mV. Connecting the ground of the RCAs to the amps now common ground dropped the buzz another 10mV.

How's your arithmetic? The buzz is now only 10mV pp, a long way down from the original 30. I tried earthing the metal case the OPTs are in and the OPTs but this made no difference. I tried earthing the common side of the OPTs secondary, this made no differnce. I like to let the OPTs secondary float so I was pleased when I didn't have to earth them.

Now not only is the amp safer it is very quiet. Only by pressing my ear against the Fostex driver (96db efficient) can I hear the faintest buzz and a little hum. There is only 10mV peak-2-peak of buzz. My 300B is quieter but it cost $4,000 to make in today's money so it should be. At one meter I can hear nothing at all. Only with my ear hard against the speaker would you know the amp is on.

It took me a few hours to get the amp to this point but it was worth it and the use of my digital CRO made seeing the result of what I was trying a lot easier. I'm a happy man. Can't wait to spend a few hours with my much improved amp.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 27 Apr 2013, 22:35 
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Silly me. I was hoping for a noise level of 2mV. But when thinking about it I really have a noise level of about 3.5mV. The 2mV I was hoping for was RMS my 10mV is peak-2-peak. So in way I have achieved very close to what I was after. 10mV pp is 3.5mV RMS. No wonder it is quiet.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: 1930 171-A SE amp
PostPosted: 20 Jun 2013, 07:37 
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A SQR wave through the amp is a scary sight. The secondary of the inter-stage tranni and small capacitance of the tube makes the perfect ringing (tank) cct. The rising edge overshoots the plateau then rings (dampened) all the way to the falling edge. By putting a 100K resistance across the secondary I stopped the oscillation after two rings. There is still some over-shoot but far more controlled than without the resistor. Using a 47K resistor stops most of the over-shoot and all of the oscillations. I went with the 100K.

Without the resistor the amp had a 3db sharp peak above a flat response at 10K. The 100K resistor tamed this resonance considerably.

Here is an image of the amp re-configured in my system with the Fostex horns. Look here for more details. http://www.diyaudioprojects.com/Forum/viewtopic.php?f=6&t=2494&p=36361#p36361
Attachment:
InSystemIntermezzo.jpg


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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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