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Just how hard can you drive a 300B ?

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ArcheusLedoux

Member
Joined 2015
#1
Studying an interesting amplifier.

https://i.pinimg.com/originals/8b/1e/00/8b1e00728ef12eb4be4d8b8aa7b075d2.jpg

Here's the guy that designed it.

https://i.pinimg.com/originals/de/7a/8e/de7a8e45524e36203d58d458fee2401f.jpg

MV Kiebert. Looks like a pretty serious joe to me.

So this amp was designed for the BBC. 100 watts from a quad of 300Bs.
Now my first reaction to this claim was he's probably talking about Peak power not continuous.

Most 300B quad amps I've seen use an OPT with 2500 ohm Primary.
Here Mr Kiebert is using a Freed 18777 OPT with a Primary rating of 1650 Ohms and an idle current of 300Ma and output up to 700Ma with a plate of 450 volts.

And he mentions 100 watts normal output with instantaneous peaks of up to 200 watts are capable.

Full article is here
https://worldradiohistory.com/Archive-Electronics/50s/Electronics-1955-04.pdf
Page 167.

Now I've always understood a PP 300B amp to be around 20 watts and a quad at 40 to 45 watts.
But Kiebert has lowered the plate resistance a considerable amount, and driving them ridiculously hard.
Or so I thought.

Is this thing a UNICORN?

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#2
Max. plate power dissipation is 36W. This leads to 20W pp with two tubes or 40W with a quad.

WE recommends a max. of 450V and 80mA for a single tube, which will give 18W of output power with a 2K load.
 
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#3
He's got them running in AB2. Each tube is conducting less than a full cycle, and its running in grid current. I can believe that kind of power output.

Most "modern" 300B amplifiers are working in Class A for about 20W per pair or AB1 for about 30W per pair whereas this amplifier is working in Class AB2 and getting 50W a pair.
 
#6
My more primitive simulation shows 50 W a pair with the following parameters:

+ Ub = 450 V, Ug1 = -96 Vdc, Ia = 75 mA/tube, g1 (drive) = 224 Vpp, Pout = 50 W, THD = 0.5 %. OPT primary resistance = 40 ohms/halve, secondary = 0.2 ohms.
Pd (at full power) = 31.8 W/tube.
 

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#8
then 50W should be pretty easy
Click to expand...

For a pair of 300Bs ? It is not as easy as with pentodes. The required drive is > 220 Vpp with moderate amount of grid current,
so the driving stage must be low impedance type and DC-coupled the grids of 300Bs.

As a reference, a pair of EL34 can be driven to same (50 W) power with a 12AX7 cathodyne with some 1 mA idle current.
 
#10
OK, well back to the circuit in question.
The Freed 18777 clocks in at 1650 Ohms.
This was the main question I had about how hard you can drive a quad????
Most OPT's run in the 2400 to 2500 Ohm range.
This just seemed a pretty low value to me. Or am I missing something here?
 
#12
Good luck getting that kind of power out of a quartet of modern production 300B, the JJ and a couple of EML types will likely do it, not so much anything else.

Technically the 300B is not supposed to be run with grid current, but there are a number of designs that did including a Japanese 300B SE rated (legit) at 17Wrms output. (You can find it somewhere in the collection of MJ schematics circulating here on the forum and on the net.)

My first 300B PP design (in VTV) achieved between 25 - 40W depending on the output transformer chosen. The ones I built for the article could do 30W @ 2% and 25W @ 1% THD into 3.8K with 400V supply and 70 - 80mA per tube idle current. Fixed bias...

Wayback Machine

Article starts on page 25. It was a somewhat popular design with a number of people building them. Note in the 21 years since I designed that amp and wrote that article my opinions have shifted pretty drastically. It's not a good design to tinker with, since doing so generally broke it..
 
#13
ArcheusLedoux said:
OK, well back to the circuit in question.
The Freed 18777 clocks in at 1650 Ohms.
This was the main question I had about how hard you can drive a quad????
Most OPT's run in the 2400 to 2500 Ohm range.
This just seemed a pretty low value to me. Or am I missing something here?
Click to expand...
Nobody knows which loading "most OPT" are working at. Because "most" people are after cheap prices rather than quality. Cheap prices and quality never matched. Not in the presence, nor in the past. And because designers know the aim of their customers for the most bang for a buck, they design with low impedance transformers.
"Most" people go for a cheap asian 300B SET or simple PP design amp, just because they want to add that tube- myth to their audio gear and hear whats all the rumors are about. Thats the real goal of "most" people- and they fail. Because low eff. speakers doesn't match or the whole system doesn't suit this tube. And speaking of 300B tube, were speaking of replicas.

The 300B datasheet states clearly, that, with a lower output trans impedance, output power can be maximized.
Some people say, that Tango X 2.7 is the best 300B SET output transformer.
It has 2.7K Ohm impedance.
But nowadays, more people come to the conclusion, that maximizing of power output may not result in the best sound, so they aim for higher impedance matching, say 4K or 5K Ohm for a SET 300B tube.
To go for highest power output, this tube is already a looser against the PP- pentodes amps. It was, maybe, worthwile, to discuss in those yesterdays when 300B were as cheap as pentodes. Today, 300B is synonemous for quality in sound- not for high powered amps.
 
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#15
Schmitz77 said:
Nobody knows which loading "most OPT" are working at. Because "most" people are after cheap prices rather than quality. Cheap prices and quality never matched. Not in the presence, nor in the past. And because designers know the aim of their customers for the most bang for a buck, they design with low impedance transformers.
Click to expand...

Let me qualify my statement.
When I said "most" I meant most of the Push Pull circuit specs and or transformer specs. In particular when you look at Triad and UTC they all show 2500 Ohms matching up with a quad of 300Bs, again in Push Pull.

So when I see the Kiebert 100 watt circuit using a quad and a Freed OPT with 1650 Ohms primary, that's considerably lower than 2500 ohms.
 
#17
The primary load of the output transformer is least interesting part of that article. It's the driver with kicker, good power supply and the feedback layout that allow that power not the primary impedance of the output transformer. I mean that without a good driver, power supply and feedback one will never be able to get that power at such low distortion regardless of the output transformer. In the end if the output transformer has 2.5K plate-to-plate instead of 1.65K it will require some (limited) tuning of the best quiescent conditions and power will be less. But I challenge anyone to tell the difference between 80W and 100W when listening to some music....it's just 1 dB difference at hearing damage levels!

I have made myself a few 300B amps for friends with different primary loads and all worked fine with the right speakers.
Regarding output power I am more familiar with 300B SE amps. The most powerful was working at 420V/70mA with 5K load and it was possible to get more than 13W in class A2. 45% efficiency!!!....Pentode-like! 🙂...with quiescent plate dissipation of less than 30W. Chinese tubes worked fine too. 🙂)
This at least will guarantee good tube life in normal use with appropriate speakers (such that the amp is not driven close to clipping too often). However after years of squeezing as much power as possible I am not doing this anymore. It might be worth with low power output tubes to make amp more usable but who really needs more than 50W for home HiFi? It's more academic than else, IMHO.
 
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