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Old 16th April 2018, 01:11 AM   #41
gabdx is offline gabdx  Canada
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When driven over 8 watts in class B, the bias point shift for the 300B, regulated supplies please! One 300B can play 15 watts no problem if properly implemented. All schematics I see are mostly 8 watts designs limited to class A.

At no feedback it produce 1%thd at 1 watt and continue to climb steady. So this design is good. but the power supply not.

Personally I would use a regulated cathode bias and negative bias for the pre-driver stages. All regulated.

Otherwise why build a S.e.t. using the most expensive tube/transfo for no power?
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Old 16th April 2018, 01:15 AM   #42
tomchr is offline tomchr  Canada
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6A3:

Yeah. I wouldn't design a "2A3 amplifier" to dissipate 40 W in the "2A3" either. That's just asking for disaster. Someone will eventually plug in a 15 W rated 2A3 and have a meltdown.

I agree with your observations re. OPTs. Good ones are worth paying for.

Tom
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Old 16th April 2018, 01:18 AM   #43
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by gabdx View Post
When driven over 8 watts in class B, the bias point shift for the 300B, regulated supplies please!
I get 10 W (3 % THD) out of my DG300B. Class A with regulated supplies all around.

Using a regulated B+ is also one of the best way to reduce hum on the amp output.

Tom
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Old 16th April 2018, 03:04 AM   #44
gabdx is offline gabdx  Canada
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Quote:
Originally Posted by tomchr View Post
I get 10 W (3 % THD) out of my DG300B. Class A with regulated supplies all around.

Using a regulated B+ is also one of the best way to reduce hum on the amp output.

Tom
It is possible to get at 15.5 W at 3% thd, with a wide bandwidth.

I know we are talking low power, which is silly, with 10W 3%, you must get grid current distortion, only avoidable with a cathode follower which can cope with grid current.

To get to the next level you must regulate the cathode bias of the 300B, as well as the grid stabilized with a negative potential. This is more complicated than usual

I didn't built it, I got those figures from a designer.
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Old 16th April 2018, 04:24 AM   #45
tomchr is offline tomchr  Canada
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10 W @ 3 % is actually with a cathode follower. You can get a bit higher output power with a source follower, but I liked the sound of the cathode follower better. You're absolutely right about the grid current. That plus the ~200 Vpp swing needed to drive a 300B well is the challenge of a 300B design.

Also, that's with a 5 kΩ primary on the OPT.

Tom
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Old 16th April 2018, 04:40 AM   #46
6A3sUMMER is offline 6A3sUMMER  United States
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gabdx,

Perhaps I misunderstood what you said in post # 41 . . .
I thought you were talking about one 300B tube, not two 300B tubes.
The only practical and good sounding 300B amplifiers in class B that I know of, are push pull, not single ended amplifiers.
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Old 16th April 2018, 04:59 AM   #47
6A3sUMMER is offline 6A3sUMMER  United States
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tomchr,

There are good reasons to design an amplifier that uses the JJ 40 Watt plate dissipation 2A3.
With a 2.5V filament, there is less field difference across the filament as it affects the multi-electron paths past the uni-potential grid on their way to the plate, than for the same construction that uses a 5V filament.
That JJ 2A3 has a plate structure that is at least as big as a real 300B, and it also has a glass envelope that is bigger than a real 300B. More plate and more glass = 40W dissipation.
If people want to tube-roll on such an amplifier that is designed for 40W on the plate, they should know that they do so at their own risk, when using tubes that "just plug in", because all real 15 Watt plate 2A3 tubes do "fit".
Especially with an amplifier like that, right?

But then I have to ask the question about persons who start with a 6L6 amplifier, and plug in the not-so "compatible" KT88, KT90, KT120 that has the same wiring and same socket.
Or, how about the person who starts with a KT88 amp, and plugs in 6L6 or 5881 tubes; the connect and fit so nicely.
How well might that work?

I use non regulated B+ for 300B and other tube amplifiers, and always get less than 500uV hum. All without global feedback too.
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Old 18th April 2018, 09:25 AM   #48
xcicc is offline xcicc  Italy
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Thank you all for the strong interest in this project of mine. But at the moment I'm more confused than before, summarizing, is it better a fixed or automatic bias? In my opinion with a transformer that delivers 320v x 1,414 = 452v-50v of voltage drop we have about 402v and with a fixed bias a - 75v we have a dissipation of about 30w right ?. With two 10hom resistors connected to ground on the cathode of the 300b
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Old 18th April 2018, 04:39 PM   #49
waltube is online now waltube  Italy
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Which type of calculation you done with voltage?
With 400 vdc the bias of -75 v is too high.
Last, you must have two resistors of 47 ohm and one of 1 ohm 1 w to ground for current monitor


Walter
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Old 18th April 2018, 05:32 PM   #50
xcicc is offline xcicc  Italy
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Quote:
Originally Posted by waltube View Post
Which type of calculation you done with voltage?
With 400 vdc the bias of -75 v is too high.
Last, you must have two resistors of 47 ohm and one of 1 ohm 1 w to ground for current monitor


Walter
Why? 47k 10k and 220 ohm is not enough???
Also 10k to ground......

Anyway I can reduce the grid bias from the trasformer from 250v to 220v

Last edited by xcicc; 18th April 2018 at 05:51 PM.
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