Mayhem on AmRad 40 m. band

[BusyBee]

Some interesting historical perspective there....

If/as/when I get some rod pentodes to play with I think I'll be using my method. It's comparatively complicated but I reckon it should satisfy the needs I have for circuitry with cathode biasing, grounded grid, cascode amps and cathode followers etc.

What I'm thinking of is a master filament supply of something like + & - 12V (another separate isolated set of supplies floated up at say 40V or whatever for high cathode potential uses such as cathode follower or cascode as necessary) very well smoothed and regulated, which power current sources and sinks. The valve filament connects between them and has a pair of 1N4148 or a 1.25V bandgap reference between filament + & - to maintain 1.25V actual filament voltage (hence actual filament see's voltage drive) whilst a DC servo op amp adjusts one of the current sources to centre the cathode voltage to where it is required. Op amp and current source forming a sort of DC transconductance servo loop. This last bit would also allow bias to be accurately adjusted and maintained as desired to any reasonable point between around + & - 8V ish, hence simultaneously providing adjustable fixed bias whilst the cathode floats at many Meg Ohm away from ground and rails from DC to at least a few 100 KHz (filament chokes or common mode choke could be used to extend it to MHz etc if needed. Obviously the scheme is mainly intended for audio work). Et voila, instant "indirectly heated" valves   There could be a gotcha I haven't thought of here of course!

Probably talking 6 transistors and an op amp per rod pentode to do this. Worth it? For potential high end hi fi use's then yes.

That's taking me a bit to follow. Although I do occasionally deviate into digital control I must admit my experience often tells me to go for "kiss" methodology.

Tracy

[Mike Watterson]

High End HiFi could use less than that and no valves

But why not if you find it fun.

It's too complicated for a dubious value for me. Grounded grid (for low RF input impedance like 50 or 75 Ohms) and cascode amps are only of much value in niche RF applications. I suppose a ribbon mic might usefully use a "grounded" grid amp.

[Mark Hennessy]

High end hi-fi would be more accurately described as High-Price Hi-Fi.

Conventional circuitry that works flawlessly is too boring, too hard to market. But stumbling across an old cache of nuvisters worked out pretty nicely for a certain Mr Michaelson back in the day. That's the thing - do something unusual, create a bit of excitement and a "story", sell stuff. Who cares how it measures and actually sounds... (not passing judgement on that particular example, but some of what you see today is really quite sketchy)

No harm in experimentation, of course. Should be interesting and fun. But there won't be gold at the end of the rainbow. We already found that with the bipolar transistor

[Murphyv310]

Hi
The worst amps I've come across were the world amp series, terrible design and under rated components.
The other one was around £3000 with single ended triodes, 6 watts per channel, the mains transformer was 220v, HT and filaments way too high, regular GZ32 failure, amongst other things including poor audio and distortion.
The dealer who I did work for was gobsmacked when I played the 3-3 stereo amp in comparison, wanted me to build them on a regular basis, I declined.

[Jez]

High end hi-fi would be more accurately described as High-Price Hi-Fi.

Conventional circuitry that works flawlessly is too boring, too hard to market. But stumbling across an old cache of nuvisters worked out pretty nicely for a certain Mr Michaelson back in the day. That's the thing - do something unusual, create a bit of excitement and a "story", sell stuff. Who cares how it measures and actually sounds... (not passing judgement on that particular example, but some of what you see today is really quite sketchy)

No harm in experimentation, of course. Should be interesting and fun. But there won't be gold at the end of the rainbow. We already found that with the bipolar transistor

You've basically got it yeah but I can't entirely agree with all of it.

As many will know I waged a one man war against audiophoolery for several years (and got myself banned from all the hi fi sites over it!). Magic fuses, enchanted speaker cables, snake oil coax and "grounding boxes" (literally a wooden box full of "earth" ie soil from the garden, with binding posts on it to which you "ground" your system. £500 - £4000. No I'm not making this up!), amongst other things were all given "both barrels" by me on a daily basis... which made a lot of audiophools very angry

However.... on mainly engineering led sites there is a tendency for people to take things way too far in the opposite direction! On sites such as this people think that all decent measuring amplifiers sound the same. They do not! (and that Quad 33/303, 44/405 are really good.... nope! Especially the pre amps which are poor).

Two units both measuring <0.01% THD and flat from 10Hz - 40KHz, good square wave performance, good damping factor, low noise etc can sound very different to one another with one sounding great and the other one grating on the ear. No one knows why. Which seems to get the backs up of people used to being able to correlate measurements with observed performance in all other fields of electronics. It simply doesn't work like this with hi fi. This is one of the main reasons it has kept my interest and fuelled my frustration since I was a young 'un and why I made it my career.

Bipolar transistors can sound great yes. As can valves. As can FETs and mosfets. But it's horses for courses and they can all sound different depending on how they are used. Square law V's exponential law of devices is just one facet of this (well covered by Baxandall many years ago... but only the technical side. He was virtually booed off stage at an AES meeting late in his life for his "everything can be measured and all decent kit sounds the same" rant which he delivered!).

Valves can often sound particularly good and with fairly simple circuitry. Yes of course there is audiophoolery surrounding this as well and many will (wrongly) say that valves always wipe the floor with transistors for sound quality and even that a single solid state device in the signal path can ruin the sound! (valve rolling I find particularly weird!). Strange then that all audio is not unlistenably awful with all those NE5532's in the mixing desks that most recordings have passed through!

But yes if the rod pentodes have the sort of virtually non existent microphony, very low noise and fine linearity hinted at then they could possibly perform very well in audio gear... and yes the punters are likely to go daft for something "new and different"

(no I can't get the quote thing to work with my browser and computer. There is no "end on quote" to start beneath on my outdated browser and I tried putting several empty lines between the quote and my reply as you can see but still no difference. I'll just keep adding quotation marks)

[Mike Watterson]

No-one has made rod pentodes since 1991. Valves are still made. Transistors & ICs more so. No-one will ever make rod pentodes again. The USSR continued with them out of a mix of inertia and inability for a long time to make silicon transistors. I remember repairing mad "newish" USSR TVs and radios in late 1970s. All Germanium.

Also you are very limited in power. The sorts of things rod pentodes can do at audio can be done better by ICs. The rod pentodes main advantage is outperforming B7G battery valves and US subminiature wire ended valves in battery applications. Using them in mains gear is even more a curiosity than battery applications. Fine for a bit of fun.

They are quite microphonic, also work to over 100 MHz. USSR actually mounted them on alumina thickfilm substrates with SMD parts. Only earliest applications used point to point wiring.

Pros:
High to very high efficiency on filament except "radar/fuze" O/P application devices.
Can work at low HT (The 1j42a ONLY works at very low HT, 6V to 12V, 9V is good).
Very tolerant of filament volts, because designed for NiCd and silver rechargeable cells.
Sharp cut off.
Pentode like characteristic
Lower noise than regular pentodes.
Works well as Pentode mixer with direct drive / bias to g2 (20V (off) to 45V (full on) approx )
Lower electrode capacitance than any common battery valve.
Ideal for replacing 2V or 1.4V battery valves (I've done KK32, KLL32, DK91, DK92, DK96, DF97, 1T4, DAF96, DL9x. planing a "no re-tune" IL6 replacement)

Cons:
The g2 needs an HT rail or potential divider. The current changes too much to set voltage with a series resistor.
For RF / IF AGC you need to control gain via g2, not g1.
Very sensitive to magnetic fields
Sensitive to vibration
Gain very sensitive to voltage on g2
Filament needs small ballast resistor for NiMH and larger ballast resistor for Alkaline. Better suited to NiMH than Alkaline.
Needs electrostatic shield. Almost no built in shield.
Can be triodised, but unlike real pentodes the g3 should be at about f- or f+ voltage, never tied to anode.
No tetrode simulation.
RF devices to over 100 MHz, so care on audio.
As awkward on mains equipment as traditional 1.4V B7G valves.

[Mark Hennessy]

(no I can't get the quote thing to work with my browser and computer. There is no "end on quote" to start beneath on my outdated browser and I tried putting several empty lines between the quote and my reply as you can see but still no difference. I'll just keep adding quotation marks)

Quote fixed.

You're not clicking in the right place before you start to type your reply. Please see the screenshots in the "sticky" I've referred to a couple of times.

Also, you're likely not using "View source" mode, which lets you see the closing Quote tag, which is ending up after your reply, rather than in front of it. To fix the quote, I'm just moving that tag up from the bottom of your message to the bottom of the quote. That's all you have to do.

Because you've posted on lots of forums over the years, you surely must be familiar with BBCode? If nothing else, that's what UKVRRR uses. If so, you'll understand the business of tags, which are enclosed in square brackets. Such as B for bold, I for italic. At the end of the text you're tagging, you "close" the tag with a forward slash:

Code:

Here is some text. This [B]word[/B] is in bold.

Here's how a quoted reply should look:

Code:

[quote]
This is the original message.
[/quote]

And this is your reply.

But here is what you're doing:

Code:

[quote]
This is the original message.

And this is your reply.
[/quote]

As you can see, the closing quote tag is in the wrong place.

If you follow the instructions in the "sticky", you'll be absolutely fine. Pay close attention to the second image, which I've annotated in red to show people exactly where you need to type.

Here is the link to the stick again (3rd time lucky?): https://golbornevintageradio.co.uk/forum...p?tid=4638

So, there are 2 options. Either use the WYSIWYG editor by default, but pay close attention to where you click before you start typing your reply. Or use it in "raw" mode, where you have full control of the BBCode, which will be the same as many other forums (and if you prefer the raw mode, you can set that to be your default mode of operation). It's all in the sticky, and I have explained it several times recently.

If you can't figure it out, I'll break into your house and steal all your analogue oscilloscopes. And leave you a Rigol DS1054Z in their place 



PS: outdated browsers are an absolutely massive security risk. Please, please try to do something about that. Start a new thread for advice if you wish.

[Mike Watterson]

(no I can't get the quote thing to work with my browser and computer. There is no "end on quote" to start beneath on my outdated browser and I tried putting several empty lines between the quote and my reply as you can see but still no difference. I'll just keep adding quotation marks)

Click  preview or  type Ctrl shift S (same as toggle View Source)
Make sure javascript isn't blocked.

You using NCSA Mosaic, Lynx, Netscape, IE4 or something on windows 3.1?

[Jez]

My PC is from 2006 and 32 bit hence as browsers etc have been continually updated many no longer support 32 bit running windows 7.

[Mike Watterson]

This looks like a current 32 bit Windows version:
https://www.mozilla.org/en-US/firefox/windows/

More private than Chrome and less mad than IE.

But in any case you can toggle source mode (Ctrl Shift S).