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Old 22nd Jul 2019, 6:36 pm   #1
its ur aerial
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Default Linear Amp using an 807

Does any one know of a circuit diagram for a small linear Amplifier using a single 807.
And would 550 - 600 volts of HT be enough?

Thanks
Ken, G6HZG
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Old 22nd Jul 2019, 6:52 pm   #2
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Default Re: Linear Amp using a 807

Higher HT generally gives greater linearity. I recall somewhere seeing a 1950s US design for a triode-strapped grounded-grid 807 linear, the big thing about GG operation is that it does away with the need for separate screen-supplies and neutralisation.

Some of the old US TV "Sweep Tubes" were essentially 807-derivatives, and these were popular for use in linear-amps.
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Old 22nd Jul 2019, 6:56 pm   #3
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Default Re: Linear Amp using a 807

How much drive do you have?

David
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Old 23rd Jul 2019, 9:07 am   #4
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Default Re: Linear Amp using a 807

Quote:
Originally Posted by G6Tanuki View Post
Higher HT generally gives greater linearity. I recall somewhere seeing a 1950s US design for a triode-strapped grounded-grid 807 linear, the big thing about GG operation is that it does away with the need for separate screen-supplies and neutralisation.

Some of the old US TV "Sweep Tubes" were essentially 807-derivatives, and these were popular for use in linear-amps.
There appear to be a number of examples on the net of this sort of thing. A 6L6 is electrically similar to the 807 except for the top cap. Other great valves would be the 6BG6 sweep tube as you say. See attached for 6L6 2MHz circuit, does require a moderately high drive voltage/power as I think David was alluding to.
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Old 23rd Jul 2019, 11:41 am   #5
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Default Re: Linear Amp using an 807

GG mode needs a fair bit more drive than grounded-cathode.

But as said, it gives good linearity and circumvents the need for neutralisation.

I'm guessing the wanted mode is SSB because of 'Linear' in the title.

6L6 and 807 are common in the UK. American sweep toobs are fairly rare. The PL509 and PL519 was the common line output family here and there are HF amp designs floating around using them ( EG: 'Frinear')

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Old 23rd Jul 2019, 4:55 pm   #6
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Default Re: Linear Amp using an 807

Thanks for the replies, it`s intended use is AM, on the VMARS net, driven by a 10 watt valve home brew TX, very similar to a Codar AT5.
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Old 23rd Jul 2019, 5:04 pm   #7
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Default Re: Linear Amp using an 807

I did find a circuit using the PL519/ 509, but I have not got a PL519, and the I recall the heater voltage being a bit high ? At the moment I have a Home brew PSU with 550 v DC HT and a 6.3 v LT.
I would also imagine PL519`s are getting a bit thin on the ground.
It` must be 30 years plus since I changed one in a Decca Bradford !

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Old 23rd Jul 2019, 5:33 pm   #8
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Default Re: Linear Amp using an 807

Sounds like the 807 is the best bet.

Linear mode would be rather inefficient. Have you considered adding a modulator to the anode feed of the 807 and keeping it in class-C? I think you'll get a lot more bang for the buck.

David GM4ZNX
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Old 23rd Jul 2019, 7:55 pm   #9
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Question Re: Linear Amp using an 807

I see that alternative valves to the 807 have been mentioned. How about a pair of TT21? Are they still available? And typical prices, new & used?

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Old 23rd Jul 2019, 10:32 pm   #10
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Default Re: Linear Amp using an 807

I'd go for a 6BG6 any day. These were one of RCA's first dedicated line scan valves, Brimar got the dies and made them too, so they are in the UK. AWV in Australia made them too. I see there is one on UK ebay now, they have a larger cathode surface area than an 807:

https://www.ebay.co.uk/itm/VINTAGE-B...gAAOSwEfVcTa~b
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Old 23rd Jul 2019, 11:13 pm   #11
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Default Re: Linear Amp using an 807

TT21 are silly money being KT88 with top cap anode. You can get about ten NOS 807 for the price of one TT21.
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Old 23rd Jul 2019, 11:27 pm   #12
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Default Re: Linear Amp using an 807

Quote:
Originally Posted by its ur aerial View Post
Thanks for the replies, it`s intended use is AM, on the VMARS net, driven by a 10 watt valve home brew TX, very similar to a Codar AT5.
Assuming that's 10W DC input like an AT5, you will be getting about 30W PEP in AM mode.

That would need a lot of 807's to get a useful increase in power (say 10dB, 300W PEP, 75W carrier). A couple of 813's would be more like it!

You'd be a lot better off just building a bigger AM Tx. How about the classic 2x 807 Class C finals, with 2x 807 in the modulator as Class B triodes, all running of 600 or 700V HT ? About 80 W carrier output (320W PEP fully modulated).
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Old 24th Jul 2019, 8:04 am   #13
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Default Re: Linear Amp using an 807

Another solid option could be the PL38, if you don't mind a small transformer suited to the 30V heater. They have an anode dissipation similar to the 807 at 25W I think. The PL38 is certainly a very attractive valve.
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Old 24th Jul 2019, 1:07 pm   #14
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Default Re: Linear Amp using an 807

As far as '807' linear-amp designs are concerned, it's worth remembering that the STC/Brimar 5B254M was essentially an 807 electrode-assembly fitted into a more-modern 'bottle' and base.

http://www.r-type.org/exhib/aab0149.htm

There are plenty of designs for 1950s/1960s linear-amp designs using the 5B254M in SWM, RadCom, the various RSGB handbooks etc - it could be worth seeking out a few of these for circuit-inspirations!

[Don't copy them slavishly though: the 5B254M and similar had shorter lead-out wires so significantly lower inter-electrode capacities. You may get away without neutralisation with one of these whereas the longer grid/cathode/screen-leadouts of an 807 could be sufficient to make it hoot impressively on the upper HF bands!]
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Old 24th Jul 2019, 5:59 pm   #15
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Default Re: Linear Amp using an 807

The FT101 series original used a TV LOPT valve, 6JS6A.
The later ones used 6146B, which while an Octal base, it's not pinch construction, but "really" button base.

I might have used an 807 in 1960s, but not today.
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Old 26th Jul 2019, 7:44 pm   #16
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Default Re: Linear Amp using an 807

This circuit is also viable using a single 807 ;

https://www.qsl.net/vu2awc/ham_radio.../atn_qro-2.jpg
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Old 29th Jul 2019, 10:16 am   #17
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Default Re: Linear Amp using an 807

Re using a 6BG6 in this circuit : There is a meatier version, the 6CD6, in circulation, and I think they may be more common than the 6BG6. Tony.
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