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Old 1st April 2007, 01:36 PM   #1
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Default never say never - 300B amps...

OK OK OK now I know I always said that indirectly heated tubes were the way to go

BUT...

I seem to have got me a couple of Shuguang 300B-98's and some 3.5K OPTS - oops! - I already have a range of HT transformers.

This amp is supposedly something simple, whilst I mull over the Gargantuan P-P amp that will eventually take over the world... Heck, I'm even considering cathode bias.

So my first question (& this is a SE amp). The common (Standard)bias point seems to be around -80V with 400v B+ (ok maybe not exactly there but in that ballpark)

WHY? What am I missing?

looking at this from the point of view of a total noob to DHT and certainly the 300B, whats 'wrong' with at most 300v on the plate at 90-100Ma, OK that's not max Pout but the tube is then operating in a linear area...

At the moment I'm looking at say 260 - 270v on the plate, Just -50v (fixed) bias, and 90-100ma But that seems really unconventional.

So what gives.

As always any guidance / pointers gratefully received - & I did search honest!

Andy
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Old 1st April 2007, 02:08 PM   #2
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Just curious: how well does your output transformer like 100mA?
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Old 1st April 2007, 03:09 PM   #3
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Quote:
Originally posted by analog_sa
Just curious: how well does your output transformer like 100mA?
a fair point, I have a selection of 2 types at the moment, and I have little idea about either still the favourites weigh in at >3 kg a piece. so I'm hopeful.

I understand a little of what you mean though, having just dumped KT88's at 100MA with some piddly little transformers in favour of EL34's because the reduction in standing current paid big dividends. However, in that case the tube still has plenty of beans. Biasing an 300b at -80v when my chart runs out of curves at -120 seems optimistic.

I'm on thin ice here and I know it, unknown OPT's and an unknown tube type. heh heh, well I guess this'll be what they call "a learnin' experience"

Andy
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Old 1st April 2007, 03:54 PM   #4
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Andy,

A chart of 300B operating points gives: 350 V. on the plate, -76 V. on the grid, and Ib = 50 mA. into a 3.6 KOhm load as yielding 7.8 W. That should get you going.

You don't see many "fixed" bias 300B amps due to the possibility of runaway. There are a number of ways to prevent runaway. Combination bias is the method I favor. You'll have to bypass the smallish cathode resistor with your O/P trafos. Driving the 300B with a voltage follower allows for a small grid leak resistor and little chance for runaway. A 3rd technique is the use of a grid choke, instead of a grid leak resistor. Low DCR is combined with a high AC impedance.
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Old 1st April 2007, 06:53 PM   #5
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Default Re: never say never - 300B amps...

Quote:
Originally posted by andrew_whitham
whats 'wrong' with at most 300v on the plate at 90-100Ma, OK that's not max Pout but the tube is then operating in a linear area...

At the moment I'm looking at say 260 - 270v on the plate, Just -50v (fixed) bias, and 90-100ma But that seems really unconventional.
I have not tried the 300B, so I can't comment from experience, but I suspect that *some* users are trying to squeeze as much power out of them as possible. 8W might not seem like a lot, but it's more than the 3 to 4W you'll get from a 2A3 or the 1.5 to 2W that you might get from a 45. I think those are the popular contenders to the 300B. Some people love the 300B on its own merits, but some just want a DHT with more power....

Anyway, I think that running at a lower operating point offers the very real advantage that you could build a two stage amp. Avoiding a third stage is worthwhile IMHO. A 5687 or its cousins can drive a 300B grid with 50V peak directly from a CDP output.

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Old 2nd April 2007, 03:02 AM   #6
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Default Re: never say never - 300B amps...

Quote:
Originally posted by andrew_whitham
I seem to have got me a couple of Shuguang 300B-98's

My experience with this particular tube is that is can sound hard, but running the heaters low (mine are at about 4.3V) improves things considerably.
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Old 2nd April 2007, 05:34 AM   #7
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Default Re: Re: never say never - 300B amps...

Quote:
Originally posted by dsavitsk
My experience with this particular tube is that is can sound hard, but running the heaters low (mine are at about 4.3V) improves things considerably.
Now thats worth knowing. The power supply for the plates will be based on an LM317K (since I've seen it done and have a copy of the schematic) So adjustable is fine. Whilst we're at it what does reducing the heater power do for tube life? It must be good right? up to a point anyway.

When [and if] the thing works sucessfully then different tubes can be tried. I didnt want to shell out on the expensive ones only to find out that running closer to the 100Ma limit would effectively deal them hot electric death...

Andy
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Old 2nd April 2007, 05:59 AM   #8
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Default Re: Re: Re: never say never - 300B amps...

Quote:
Originally posted by andrew_whitham
Whilst we're at it what does reducing the heater power do for tube life? It must be good right? up to a point anyway.
Mine have been fine for a few years. I guess too low could lead to stripping, but so far so good for me. There is an article somewhere where someone claims that lowering heater voltage on DHT's (but not IDHT's) lowers distortion. I have done no measurements myself, so I have no idea, but I do know that these particular tubes sounded better.

Oh, you also might want a higher current VReg instead of the LM317 -- the heaters draw 1.2A (1.22A or so for mine IIRC) which is pretty close to the 1.5A limit o nthe part.
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Old 2nd April 2007, 07:23 AM   #9
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I've made a couple of 300B amps, from 450V, 400V, 350V plate voltage and I finally settled on 350V. I didn't care for all the power I could get since my speakers can be driven to party level by even a type 10. To me the 350V operating point posted by Eli has the better SQ based on my equipment. Never used DC heaters and has always used AC on my DHT amps.

Having said that, I was able to convince my friend who's suffering from a type 26 hum issues and for the longest time does not want to use DC as it will "affect" the sound. So after much encouragement, he finally went DC with his 26 but never liked the sound. He tinkered around and said, if fed 80% of the filament voltage, he's getting "close to AC" performance. He's using current regulation (did I say that right?)
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Old 2nd April 2007, 08:04 AM   #10
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Quote:
Originally posted by Eli Duttman
Andy,

A chart of 300B operating points gives: 350 V. on the plate, -76 V. on the grid, and Ib = 50 mA. into a 3.6 KOhm load as yielding 7.8 W. That should get you going.
Eli, yep I've seen that chart, but looking at the curves I was using it looked like one half of the wave form was going to spend its time in the scrunched up bits... basically I dont follow WHY thats the recommended setting. I wondered if there was a simple explanation.

Actually I wonder If I'm actually confused over the ratings, since my data sheet is a JJ one, or the STC one (all 3 pages) that you pointed out. Its clearly saying that 100Ma is max for quiescent.

Is there a decent datasheet anywhere I could be pointed at?

I clearly need to do more maths...

For the sake of discussion I've attached the curves I was using, from the electro harmonix datasheet, (I'm allowed to do this? apologies if not, but it WAS in the public domain.)

Andy
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