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Old 12th May 2008, 12:42 AM   #1
GK is offline GK  Australia
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Default 811a Opt

Gday

Im about to start ordering parts for a ~100W class AB push-pull amp using either a pair of 807s or 811As per channel.

The 807 would probably be the more sensible option, but a bit boring. The only thing stopping me from buying a set of 811As right away is trouble sourcing a suitable output transformer.

Has anyone here built such an amp with 811As using an off-the shelf transformer from one of the typical manufacturers? If so, which transformer was it?

Cheers,
Glen
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Old 12th May 2008, 01:21 AM   #2
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I would talk to Jack at Electra Print for the 811 tubes. Depending upon what topology you intend Dave Slagle at Intact Audio for SE Amorphous core or my company for PP for the 807. Don't neglect Lundahl either, though you will have to look a bit to make sure the voltages involved with the 811 are suitable.

For best performance from the OPT in push pull you really want to use the 807 tubes. The dielectric losses, distributed capacitance and leakage inductance for an OPT designed to handle the 811 will turn your sound hard and the plate electron bounce due to rejection, due to those losses, will drive up your distortion.



Bud
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Old 12th May 2008, 02:05 AM   #3
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Hi,
Gotham Audio made a push pull class A2 cutting head amp using 811A tubes with a B+ of 550 - 560 volts, I think Triode Electronics has a schematic for it in their "dusty files" section of their website.
Anyway, I built a pair of these amps using the Hammond 1650 series of transformers and was blown away by their sound. I used the 60 watt Hammonds rated at 4300 ohms primary but you can also use the 1650R (I think) that's rated at 100 watts with a 5K primary.
I don't think that they will do 100 watts but you can get 50 watts/channel from them. Since they are class A2 they will sound much more powerful than 50 watts.
They are a little out of the ordinary to set up but they don't use crazy voltages and can be made pretty compact. I am using 6.3vac on the filaments with absolutely no hum at the output and the power supply is pretty straight forward to build. I used a differential amp (a la Curcio) driving the 6BX7 cathode followers which in turn directly drives the 811A's. The original front end of this Gotham amp I feel is overly complicated and better results can be had with a simpler driver.
I feel this amp will put any 807 amp to shame and won't be that much harder to build. Keep the 807's for a guitar amp.
Bottom line... when using the 811A at lowish voltages, you don't need a very high primary impedance output transformer, so anything commonly available should work fine. Of course, if you want to spend money on the good stuff, all the better the results will be.
To get an idea where I am coming from, I really like triode amps for stereo use, all the better if all voltages are regulated (b+, bias, etc). I like permalloy and other exotic core materials for transformers, you know... the good stuff.
But the Gotham Audio 811A amp scheme is excellent without having to resort to exotic tricks and high dollar parts.
Hope this helps...
Good luck,
Daniel
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Old 12th May 2008, 02:10 AM   #4
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Here's a link to the Gotham amp schematic:

http://www.triodeel.com/gothamp.gif

One more thing... The cathode follower driver is actually a 6BL7 not a 6BX7.
Daniel
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Old 12th May 2008, 03:59 AM   #5
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Quote:
Originally posted by danFrank
Here's a link to the Gotham amp schematic:

http://www.triodeel.com/gothamp.gif

One more thing... The cathode follower driver is actually a 6BL7 not a 6BX7.
Daniel

Thanks for the link.
A pair of 807's in AB2 with 600V and 300V on the plate and screen respectively will give me 80W. Close enough to 100W I guess.

I agree with BudP with respect to the OPT performance with the 811A's, but I guess that I could run four 811A's in parallel push-pull to get my desired 100W with a reasonable plate impedance and plate supply voltage.

Hmmmm.................

Cheers,
Glen
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Old 12th May 2008, 04:20 AM   #6
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Hmmmmm......... On further inspection of the datasheets, at lower plate voltages the case for the 811A at the output power I desire is actually better that the 807.

According to the 807 STC application report, a pair of 807's with 600Vp 300Vs require a p-p load impedance of 6400 ohms to deliver 80W into the load.

RCA's datasheet for the 811A specifies a plate-plate load of only 5100 ohms with a 750V plate supply in class B to deliver a whopping 178W into the load.

I only want 100W, so I guess a lower plate supply combined with a higher bias current for class AB is the way to go.

Cheers,
Glen
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Old 12th May 2008, 04:35 AM   #7
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Hi Glenn:

Here's a schematic you might enjoy looking at--- for the Altec 1570B amp--- 170 watts in class B.

http://www.triodeel.com/al1570b.gif

note that the Peerless 16492 output trans has a 6400 ohm CT primary and if you used it at say half the power--- you would surely have a fair amount of magnetic headroom.

MSL
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Old 12th May 2008, 07:20 AM   #8
GK is offline GK  Australia
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Quote:
Originally posted by MQracing
Hi Glenn:

Here's a schematic you might enjoy looking at--- for the Altec 1570B amp--- 170 watts in class B.

http://www.triodeel.com/al1570b.gif

note that the Peerless 16492 output trans has a 6400 ohm CT primary and if you used it at say half the power--- you would surely have a fair amount of magnetic headroom.

MSL

Thats a nice looking circuit. After some more datasheet inspection and procrastination Ive decided that a pair of 811As is overkill for a 100W amp.
I will order the 811As and reserve these for a higher power amp.

For my 100 watts Ive decide to go parallel-push-pull with four 807 per channel in class A.

The operating conditions for a pair of 807s in class A push-pull are as follows:

Vplate
600V

Vscreen
300V

Vgrid
-29.5V

Load p-p
10,000 ohms

Distortion
2.2%

Pout
47.5W

Ill use a pair of 1650R output transformers from Hammond that are rated at 100W with a 5k primary impedance, which will be ideal for two pairs of 807s.

Cheers,
Glen
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Old 12th May 2008, 03:52 PM   #9
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Hi again,
You will not get 100 watts class A from 4 807's. Class AB1, yes, but not class A.
What are you going to use these amps for?
For audiophile use, you aren't going to beat class A triodes for sound.
If you want a PA or utility amp, then go for the 807's as they are much less fragile than any directly heated triode.
Basically it will come down to what you like. I would suggest getting parts that could be used with both types of amp topologies and try both of them to see what you like best.
Good luck,
Daniel
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Old 12th May 2008, 06:21 PM   #10
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If it's a choice between Class AB2 807s and Class AB2 811s, then definitely go with the "boring" 807s. The grid load of the 807s is a good deal easier, and require just the small added complication of a regulated screen supply.

Since the 811 is a "zero bias" RF final (large u= 160) it doesn't have the advantages of an audio triode: the greatly reduced r(p) and favouring mainly h2. The 811 is really quite pentode-like in that it has a large r(p) and produces lots of h3 and higher order harmonics. In either case, you'll probably be needing local NFB to supplement gNFB both for distortion reduction, and to get the effective r(p) down for good damping.

Unless you are determined to use the 811 for some other reason, the Class AB2 807s are the best choice on technical merits.
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