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Old 26th October 2006, 06:24 AM   #1
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Default transmitter tube amp

I am new to the audio hobby. Anyone have experience with type 813 tubes in audio application?

I constructed the following breadboard and it sounds good so far.

813 output tubes in push pull
ultralinear taps at 40%
custom output transformer, 3" stack EI-1 3/4, interleaved windings 4section primary, 5 section secondary (one week to design and wind.) .010 Nomex between sections. .005 Nomex between layers. Soleniod wound using an engine lathe with wire tensioning control.
815 phase inverter and driver tubes
Goal: 100 watts rms class A, 200w class AB

So far, I am running 600v on the plate, (final design is for 1200 or more. ) I use a SMPS for the filaments and it works very well in addition to being light weight. I am designing a SMPS for the plate supply also, so that the finished mono block will be movable without a fork lift.

I use no cathode bypass caps. I have tried a little negative feedback, but no improvement over already good sound.

Providing I don't electrocute myself, I will report future developments. Anyone here ever work with this tube?
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Old 26th October 2006, 07:06 AM   #2
Gluca is offline Gluca  Italy
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me ... se parafeed 900V@100mA, DC (current regulated) heated and I use forklift to move it.

I am going to upgrade it one day (100TH ccs loaded as driver + step down intestage tranny).

I luv the light and sound.

Ciao
Gianluca
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Old 26th October 2006, 02:13 PM   #3
7N7 is offline 7N7  United Kingdom
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When I was building, I spent a lot of time with 813.

Since I do not understand feedback (and now, never will!) I opted to strap the 813 as a triode. I have posted curves here I think some years ago. It makes a lovely triode. Ra 1.8K, 100W Pa, better than a 845 at a fraction of the price for a genuine one - and much easier to drive.

My last 813 amp was of course, push-pull. I ran the 813s at about 850V and 90mA, from memory bias was about -90V. Output tranformer was 7k a-a and I ran feedback from the secondaries to the 813 "cathodes" i.e. to the wiper on the hum balance pots.

The driver stage was balanced. I used 6SN7 DC coupled to 6BX7 with a EL822 as a current sink for the 6BX7.

I worked well, was reliable and sounded fine. I miss it.

7N7
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Old 26th October 2006, 04:31 PM   #4
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Hey Sparky, this may be of interest to you:

http://www.pmillett.com/813_se_triode_amps.htm

Victor
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Old 27th October 2006, 04:22 AM   #5
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Thanks for your replies. Gluca, I am not sure about the 100th. It would be overkill, but it lights up nice. Thanks HollowState for the link to the SE amp. I am amazed by the resources on this DIY board and thankful for them. 7N7, what happened to your 813 amp, anyway?

The 813 is very inexpensive, easy to drive. I purchased tube sockets and plate caps from an eBay dealer at a very low cost, also. I have a supply of 813's from surplus ham radio gear, which is one reason I chose this tube to start experimenting with audio.

My OPT is 10kohm P-P and I will increase the plate voltage to 1200v as soon as I have vacuum varnish dipped the xfmr and dielectric tested it to 4000vac. I have never built a SMPS for plate voltage at this power level (200w +) so I am not sure if it will be a flyback or a feed forward design. In either case, the capacitor energy storage will be small compared with a conventional supply. I plan several levels of safety interlocks.

Sparky
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Old 27th October 2006, 06:49 AM   #6
7N7 is offline 7N7  United Kingdom
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Hello Sparky.

I am the Paul Leclercq referred to in the Millett article.

I started building only in 1997 after a long absence from the hobby and the 813 amplifier was the first of a number that I built, usuually using obscure or strange valves. I actually wrote a small paper about triode strapping since it is/was a favourite topic.

There was a small amp using 6528 in p-p driven by a couple of 6SN7s one a cathode follower DC coupled. Then there was the 13E1 amplifier with +ve and -ve supplies (thanks to EC8010 for this idea), then finally the "Instock" and "Dashboard" amplifiers; these last were written up on this forum I think. There was also a pre-amp using 955 "acorns" with 6GK8 triode pentodes as cathode followers with built in pentode sinks; balanced output . This was very good although I never ran the finished pre-amp. The prototype was -3dB at 180kHz with very low distortion.

The last and maddest project, which was never finished sadly although all the elements had been tested in prototype form, was a stereo mid-range amplifier. This used triode strapped YL1071 (there is a US number but I have forgotten it) driven by 7788 pentode cathode followers with active G2 supply, with 7N7s at the front, all balanced of course. It was especially a pity as the driver stage had incredibly low distortion.

At the beginning of last year my employer went bust; after a couple of months of looking for a job,I ran away to France. All my equipment and amps I sold to fellow enthusiasts in London as I am living a somewhat basic life here...

7N7
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Old 27th October 2006, 01:09 PM   #7
AD5GB is offline AD5GB  United States
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Hello Sparky813,

I too have thought of fooling with high-powered 813s for audio since I don't use them for radio anymore (moved on to 4-400s and various ceramics like the 4CX1000A etc) and I have nearly a dozen laying around here. I'm curious about your output iron though. Did you wind it yourself or have it done? I was a bit reluctant to go that route for high voltage ( I'm talking in the 2KV range) as I was a bit concerned that the sheer amount of wire required to create the right load would result in so much C that it would hurt the high frequency response. Please let me know how this goes for you. It would surely be simpler than this 8x807 project I'm considering.

Thanks!
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Old 27th October 2006, 01:36 PM   #8
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by AD5GB
Hello Sparky813,

...I'm curious about your output iron though. Did you wind it yourself or have it done? I was a bit reluctant to go that route for high voltage ( I'm talking in the 2KV range) as I was a bit concerned that the sheer amount of wire required to create the right load would result in so much C that it would hurt the high frequency response...Thanks!
I have never understood how a true "hi-fi" amplifier can be made with very high-voltage devices for this reason exactly.

I always took the view that above say 9k one would start to run into trouble; I know that there are some very clever transformer designers around, but for me 8k a-a (only push-pull at "Differential Towers"!) was the limit. This is OK as far as I recall with 813s at about 900V or so, although I did play a bit at 1.2kV.

Of course I am talking about triode mode here. With beam tetrode operation of course everything is different and feedback is necessary. I should think that a very mighty amplifier indeed could be built with a pair of 813s in BT mode running at about 1kV.

Paul
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Old 27th October 2006, 02:27 PM   #9
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For the record, I am a beginner, and this is the first audio transformer I have ever wound. In order to get high frequency response, I am using interleaved primary and secondary. There are four primary sections, and five secondary sections. The secondaries are for 4, 8, and 16 ohms, so there are a total of 19 windings. Since this is a high voltage application, the insulation type and thickness is important. Between sections, I am using .010 Nomex. Between wincing layers, I am using .050 Nomex. VPI varnish is a must. Iron is 14 mil grain oriented silicon steel which is stacked with one on one interleave for minimum theoretical air gap. Windings are carefully soleniod wound, not dump wound, to get closest coupling possible. I have run the amp at listening levels all day long for several weeks and the transformer does not get even warm.

The high frequency response by calculation is 100khz, but this remains to be carefully tested. Right now, I have more roll off in my cap coupled driver stage than I see at the OPT, so I am far from done. I hope I don't have to wind this one twice. It was dog gone hard to do the first time. I am using a hands-on audio transformer design manual that I obtained from Ram Sales (Los Angeles) years ago. I will post the authors name and Ram's phone number for anyone interested. I will also post more details of the OPT if it is successful. Sparky
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Old 27th October 2006, 03:45 PM   #10
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by sparky813
I am using a hands-on audio transformer design manual that I obtained from Ram Sales (Los Angeles) years ago. I will post the authors name and Ram's phone number for anyone interested.
I should think everyone here would be interested to see that manual - I certainly would.
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