Thermionic Valves

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daveejhitchins
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Re: Thermionic Valves

Post by daveejhitchins » Sat Mar 03, 2018 4:32 pm

martinw wrote:
daveejhitchins wrote:
martinw wrote:Two shelves in this cabinet are now looking very f’ing cool
So . . . When are you open for visitors :roll:

Dave H :D
Now :)

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On -- my -- way . . . :lol:

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Re: Thermionic Valves

Post by martinw » Sat Mar 03, 2018 4:40 pm

daveejhitchins wrote:
martinw wrote:
daveejhitchins wrote: So . . . When are you open for visitors :roll:

Dave H :D
Now :)

Martin
On -- my -- way . . . :lol:

Dave H :D
Excellent ... see you soon Dave, I’ve got a few bottles of malt waiting too :)

Martin

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Re: Thermionic Valves

Post by daveejhitchins » Sat Mar 03, 2018 4:50 pm

martinw wrote:[Excellent ... see you soon Dave, I’ve got a few bottles of malt waiting too :)
I see a summer charabanc trip being arranged . . . :mrgreen:

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Re: Thermionic Valves

Post by martinw » Sat Mar 03, 2018 4:58 pm

daveejhitchins wrote:
martinw wrote:[Excellent ... see you soon Dave, I’ve got a few bottles of malt waiting too :)
I see a summer charabanc trip being arranged . . . :mrgreen:

Dave H :D
Anytime Dave :)

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Re: Thermionic Valves

Post by Diminished » Sun Mar 04, 2018 5:44 am


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Re: Thermionic Valves

Post by martinw » Sun Mar 04, 2018 7:17 am

Diminished wrote:Found this re: Mullard:

https://www.youtube.com/watch?v=-GgWIlvyEL8
Cool 8)

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Re: Thermionic Valves

Post by flaxcottage » Sun Mar 04, 2018 11:46 am

Seen this before. It is fascinating.

What I'd like to know is who made the machines that made the machines that .... :wink:
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Re: Thermionic Valves

Post by Diminished » Sun Mar 04, 2018 12:16 pm

flaxcottage wrote:What I'd like to know is who made the machines that made the machines that .... :wink:
Yes, the machines are an interesting question about the whole process. I mentioned earlier that the company making the valves I bought recently was set up in the 90s. The only reason they were able to begin production at all is that they managed to secure the equipment from the old (ex-Soviet?) TESLA company.

I imagine this applies in a few areas. Vinyl pressing is another one I sometimes think about, where it's still possible to do it because old pressing equipment is still around, but I would imagine that trying to bootstrap a brand new vinyl production facility from scratch would likely be extremely difficult. You still have people who understand how the pressing process works, and they can teach those skills to other people, but what about the engineers who designed the pressing equipment in the first place? Does that expertise still exist?

It makes me think of Asimov's Foundation series during the fall of the Empire, in which old Imperial-era atomic equipment is still kicking around, but it slowly falls into disrepair because nobody knows how to fix it.

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Re: Thermionic Valves

Post by BigEd » Sun Mar 04, 2018 3:28 pm

Diminished wrote:Found this re: Mullard:

https://www.youtube.com/watch?v=-GgWIlvyEL8
By coincidence just came across this story of the EF50, a wartime essential. It seems Philips, the parent company, were now regarded as the enemy, or in enemy territory, and Mullard was at risk of compromise...

With the number of Radar systems growing also the demand for EF50 tubes increased.
Up to that moment nobody seemed to have worried about the supply of EF50s. C.O. had a large stock of them in a warehouse at Newmarket. The tubes were naturally supplied by Mullard, and everybody must have assumed that Mullard was well capable of producing this new tube. Bowen writes: “we were told that the EF50 was manufactured by Mullard, and we were assured that it was in good supply.” In reality, large-scale production of the new glass envelope presented major difficulties so that most of the EF50 tubes that Mullard supplied to Pye came directly from Eindhoven.
The technicians and tool shops at the Emmasingel work day and night to comply with Tromp’s request, and finally in the evening of the 9th of May a truck was loaded with the machines which had been hurriedly put together, and no less than 25,000 complete EF50s and 250,000 tube bases. It was for everybody clear that the Germans could invade Holland any moment, but nobody loading that truck that evening could have guessed that hours later, around five o’clock the next morning, the Germans would actually cross the border with Holland.
From being a neutral country, it became a company many of whose senior staff had close relatives in enemy-occupied territory, and as such it could no longer be considered totally secure. Thus when the main production of EF50s was transferred to Blackburn, in the latter part of 1940, the Mullard and Philips parts of the complex were kept strickly separate, and commissionaires were posted outside the Mullard factory with instructions to keep all Philips personal out, though after a year or so relations became more relaxed. At Mitcham, Philips’ Plant director was for a time under house arrest, being driven to and from work each day. Mullard also had many Dutch staff, from Eriks downwards, but were excluded from the more secret projects, and a young graduate who joined Mullard at Mitcham in 1941 found that they had not yet gone over entirely to war work.
The suspicion was there on the other side too:
The Germans clearly did not entrust to Philips any secret project or even any element of a project large enough to be discernible as part of a secret. Thus they made substantially no use at all of the exceptionally fine laboratory organisation before mid-1942, and failed to get two percent of its potential usefulness after that date.
It turns out there's a connection to early computing too:
Williams and Kilburn who built the Manchester Baby (SSEM) had worked on Radar during the Second World War.
The Store, being composed of CRT tubes uses many circuits cribbed from Radar. The machine contains 156 EF50s

http://computerconservationsociety.org/ ... etypes.htm

the store amplifiers which detect if there is a "dot" or a "dash" on the face of the Kilburn-Williams tube use 4 x EF50 and 1 x EF55

http://computerconservationsociety.org/ ... t-amp.html
(Dave Wade via cctalk)

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Re: Thermionic Valves

Post by martinw » Mon Mar 05, 2018 11:13 am

Very interesting :)

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Re: Thermionic Valves

Post by Diminished » Wed Mar 07, 2018 6:35 pm

I might as well ask this here and see if anyone has any ideas. Not Acorn or computing related, so apologies for that.

So I have a guitar amplifier -- a Laney VC30 -- which according to some commentators has a design flaw afflicting some models. Essentially, some seem to think that the speaker is mounted too close to the valves, and the magnetic field disrupts the electron flow within the valves. This page has an overview of the supposed problem.

Mine was one of the very first models of this amplifier, and contains a speaker that didn't feature in later revisions, so I can't find any opinions on whether my model is affected or not. There is about a centimetre of clearance (or possibly a tiny bit more) between the glass of the EL84s and the back of the loudspeaker.

In use, it definitely seems like the valve most directly adjacent to the magnet is fluorescing differently to the others ...
vc30.jpg
I had hoped to solve this problem by just replacing the speaker with one that was slightly shallower in depth. The replacement (a Celestion) arrived this morning, but preliminary measurements indicate that this will only get me an extra half-centimetre or so of clearance, so I'm wondering if slightly more drastic measures like modifying the cabinet might be in order (some other owners of this amp have done exactly that).

It seems there are at least a couple of people in this thread who really know this subject well, so I wondered if anyone had any experience with this sort of thing, or any thoughts on the matter?

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Re: Thermionic Valves

Post by flaxcottage » Wed Mar 07, 2018 8:47 pm

A magnetic field certainly affects electron flow in thermionic devices. Just look at the CRT TVs that were about. Both the horizontal and vertical scanning was done by electromagnets effectively. In one of my old sets BITD the focus control was actually a moveable magnet.

Both the klystron and cavity magnetron rely on strong magnetic fields to cause their high frequency oscillation.

It is not surprising therefore to see some effect in the amplifier due to the speaker magnet. It is looking like the magnetic field is diverting the electrons away from the anode so that they hit the screen grid hence the high current drawn by it and the lowered power output from the amp.

It is looking like the speaker is much too close to the valves and even a mu-metal screen may not reduce the magnetic field sufficiently.

BITD I worked for Orange building high power amplifiers for pop groups. All our amplifiers had separate loudspeakers so that this effect would not happen. I should imagine a pair of KT88s delivering over 300W RMS audio output would put on an interesting display if strong magnets were close by.
:lol:
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Re: Thermionic Valves

Post by daveejhitchins » Wed Mar 07, 2018 9:10 pm

My last job was for a company (Filtronic Group) that designed and built GHz transceivers plus various GHz filters. Some of the filters used magnets in their design. I have a stack of them - very difficult to separate, once they attach themselves together . . .

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Re: Thermionic Valves

Post by martinw » Thu Mar 08, 2018 10:50 am

Diminished wrote:I might as well ask this here and see if anyone has any ideas. Not Acorn or computing related, so apologies for that.

So I have a guitar amplifier -- a Laney VC30 -- which according to some commentators has a design flaw afflicting some models. Essentially, some seem to think that the speaker is mounted too close to the valves, and the magnetic field disrupts the electron flow within the valves. This page has an overview of the supposed problem.

Mine was one of the very first models of this amplifier, and contains a speaker that didn't feature in later revisions, so I can't find any opinions on whether my model is affected or not. There is about a centimetre of clearance (or possibly a tiny bit more) between the glass of the EL84s and the back of the loudspeaker.

In use, it definitely seems like the valve most directly adjacent to the magnet is fluorescing differently to the others ...

vc30.jpg

I had hoped to solve this problem by just replacing the speaker with one that was slightly shallower in depth. The replacement (a Celestion) arrived this morning, but preliminary measurements indicate that this will only get me an extra half-centimetre or so of clearance, so I'm wondering if slightly more drastic measures like modifying the cabinet might be in order (some other owners of this amp have done exactly that).

It seems there are at least a couple of people in this thread who really know this subject well, so I wondered if anyone had any experience with this sort of thing, or any thoughts on the matter?
Maybe a sheet of mu-metal between the two might help?

https://mumetal.co.uk/

http://custommagneticshielding.magnetic ... t-and-foil

Martin

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Re: Thermionic Valves

Post by Diminished » Thu Mar 08, 2018 5:05 pm

Thanks a lot, guys.

The question I was trying to ask here was "how close is too close?", which I now realise is about as sensible as asking how long a piece of string is, what with magnetic fields being infinite and all. I think I was hoping that someone would pat me on the head and tell me everything would be just fine, which was rather naive.

Anyway, I decided flaxcottage was right, and so I plumped for the most drastic option that would gain me as much clearance as possible. Unfortunately this meant spending the day mucking about with a chisel and a hacksaw to change the speaker's mounting position:
clearance.jpg
I now have nearly 2 inches between the valves and the magnet, which is probably as good as it's going to get without building a whole new cabinet for the thing. I did have the presence of mind to set up a microphone at a known distance away from the cabinet and make some recordings before and after the swap, and it definitely appears to be louder and less harsh now (although you could equally attribute that to the new speaker, of course). Happily this amp has a facility for driving two speakers at once, so a carpentry project for another day will involve building another cabinet for the original driver. Not that I really need any more volume, but ...
martinw wrote:Maybe a sheet of mu-metal between the two might help?
I was wondering where you could get this stuff from, so thanks for the sources. I may consider adding some, just to finish the job.

One other thing before I shut up: There is a piece of folklore amongst guitarists which states that "a 30W valve amplifier is much louder than a 30W solid-state one". This assertion has always baffled me. Surely 30W RMS is 30W RMS? If anything, I would have expected the valve amplifier to be quieter, due to losses in the output transformer (which isn't needed for a solid state device). The only thing I can think of that seems plausible to me is that valves introduce a degree of harmonic distortion, which compresses the sound, making it louder.

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Re: Thermionic Valves

Post by Coeus » Fri Mar 09, 2018 12:03 pm

Diminished wrote:One other thing before I shut up: There is a piece of folklore amongst guitarists which states that "a 30W valve amplifier is much louder than a 30W solid-state one". This assertion has always baffled me. Surely 30W RMS is 30W RMS? If anything, I would have expected the valve amplifier to be quieter, due to losses in the output transformer (which isn't needed for a solid state device). The only thing I can think of that seems plausible to me is that valves introduce a degree of harmonic distortion, which compresses the sound, making it louder.
I think this is about the distortion characteristics of each type of amplifier. For class A amplifiers, or once you are well out of the crossover zone, transistors are pretty linear until the required output voltage exceeds the power supply whereupon they clip very suddenly. That distortion is very harsh and not well tolerated by the ear so one would normally avoid driving a transistor amp that hard.

I believe valves become a bit non-linear long before hard clipping, i.e. a "soft clipping". The distortion from this is easier on the ear and is the classic distorted guitar sound so people are quite happy driving their valved-based amps that hard.

The RMS measurement will almost certainly be made with neither amp clipping and when driven just shy of any kind of clipping both amps will be delivering the same power, but in actual use people will drive the valve amps harder and therefore get a bit more power out of them.

Interestingly NAD used extra circuitry to add a "soft clipping" to some of their HiFi transistor amps to make them behave a little more like valve amps.

Apart from being technically interesting I am not sure what practical use this is, though. BITD people may have been interested in building bigger and more powerful guitar amps but these days, if you suddenly found yourself playing the Pyramid stage at Glastonbury, they'd happily put a mic in front your 30W valve amp and feed it through the same few KW of (transistor-based) PA that isn't been driven to clipping as everything else. As the PA is not be driven to clipping it should be transparent and people should hear your guitar amp sound just like they were stood in front of it.

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Re: Thermionic Valves

Post by Coeus » Fri Mar 09, 2018 12:26 pm

BigEd wrote:By coincidence just came across this story of the EF50, a wartime essential. It seems Philips, the parent company, were now regarded as the enemy, or in enemy territory, and Mullard was at risk of compromise...
What a find. That is both a fascinating story about the technology itself and am interestingly reminder that history is generally told from a particular perspective. Reminds me of another story involving the English and the Dutch, I think, at a battle , possibly against the French, and though I don't remember which way round it was it was something like the English version being that the English has just about won by the time the Dutch turned up such that the Dutch need not have bothered and the Dutch version was that the English were losing until the Dutch turned up and the Dutch saved the day.

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Re: Thermionic Valves

Post by crj » Fri Mar 09, 2018 5:38 pm

Coeus wrote:The RMS measurement will almost certainly be made with neither amp clipping and when driven just shy of any kind of clipping both amps will be delivering the same power, but in actual use people will drive the valve amps harder and therefore get a bit more power out of them.
Things are a bit more analogue than that.

No amplifier is ever truly linear at any output level. What you have to quote is a pair of figures: total harmonic distortion and the maximum output it can achieve within that THD.

A transistor has a wide band which is very linear, rapidly descending into very harsh distortion outside that range, followed almost immediately by outright clipping.

A valve has a narrow band in which it is pretty linear, then a much wider band in which it is less linear, but the distortion is on relatively pleasing even harmonics.

So a transistor amp which delivers 30W with acceptable THD will sound really nasty at 35W. A valve amp which delivers 30W at that same THD might well go to 60W and merely give a "warm" sound that guitarists might even want.

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Re: Thermionic Valves

Post by Diminished » Fri Mar 09, 2018 6:54 pm

Thank you. Twenty years on, I finally get it.

The reason I was confused is because solid state amps nowadays feature "tube emulation" circuits which are intended to mimic the sound of old valve amps (and, to be fair, they are pretty good nowadays). I'd assumed these circuits would introduce the same degree of clipping-induced compression as you'd get by overdriving a valve amp, and so I thought that if you were using a similar degree of distortion on both types of amplifier, they should still have the same loudness for the same output power.

But, I missed a trick. The tube emulation circuitry in a transistorised amplifier is entirely in the *preamp* stage, with the power stage having to be linear, since (as noted) you can't overdrive a transistor amp without very harsh clipping distortion. But in a valve amp you can overdrive everything -- preamp, power amp, and even the output transformer. As you say, the hard wattage limit imposed by the power stage of a transistor amplifier just evaporates in a valve amp; you can go well beyond it.

I also now realise that if you prefer very clean tones, you might actually end up getting more volume out of a 30W transistor amp than you would out of a 30W valve one, since the valve one will start to introduce significant harmonic distortion sooner than the transistor one will. So this piece of guitarist folklore is actually only half true.

Good job, Stardot! There's no way I'd have understood this correctly if I'd let the goofballs on your average music forum answer the question. :lol:

Interestingly, the amplifiers which are traditionally considered the best option if you prefer glassy, clean tones are Fender ones. This is probably because Leo Fender was originally a radio engineer, and of course in radio sets, distortion was considered the enemy.

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Re: Thermionic Valves

Post by crj » Sat Mar 10, 2018 2:21 am

Diminished wrote:Good job, Stardot! There's no way I'd have understood this correctly if I'd let the goofballs on your average music forum answer the question.
For reasons I've never fullyunderstood, the audio industry is full of bizarre folklore, superstitiion and outright snake oil. The bits I've been involved with have been rather more scientific about the whole thing. (-8

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Re: Thermionic Valves

Post by Coeus » Mon Mar 12, 2018 1:34 pm

crj wrote:For reasons I've never fullyunderstood, the audio industry is full of bizarre folklore, superstitiion and outright snake oil. The bits I've been involved with have been rather more scientific about the whole thing. (-8
The piece of snake-oil that sticks in my mind was a salesman claiming that very expensive Ethernet cables used to carry digitally sampled audio sounded better than cheaper cables in the same application. It is hard to believe this would be true unless the cheap cables were very poor because with digital, once the bit pattern arrives correctly at the other end, no further improvement is possible and some cases error correction can compensate for a few errors. The same trick had been used by popular electronics warehouses in selling HDMI cables.

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Caution: digression about digital audio

Post by crj » Mon Mar 12, 2018 2:46 pm

Actually, Jitter is a real issue for digital audio, as are various nasty voltage fluctuations. Remember that ultimately you have to send the audio through a DAC, and analogue effects on the power supply or signal you feed it can be imparted to the resulting signal. A sobering rule of thumb is that 3ns jitter on a 44k1 16-bit S/P-DIF signal is equivalent to corrupting the LSB. (The tininess of jitter that would disrupt 192k 24-bit audio is just one clue to how implausible it is that all those bits will ever see the light of day in the analogue domain.)

The difference with digital signals is that such problems can be corrected, whereas when an analogue signal is damaged, the damage is irreversible. However, in designing audio equipment it's important not to be so complacent that you cause such problems and don't correct them.

So, while there's a lot of snake oil, there are sound engineering reasons why AES/EBU XLR is better than S/P-DIF TOSLINK which is better than S/P-DIF RCA for transmitting digital audio. There are reasons why cable quality matters. There are reasons why a bad PC sound card's digital output can sound worse than a good PC sound card. There are really good reasons why you should generate the clock right beside the DAC and feed it backwards through the system to the source.


Having said all of which, packetised audio over IP over Ethernet over Cat5 is so abstracted from the actual sound that the quality of your network cable is almost completely irrelevant. Whatever presents that to a DAC is going to have its work cut out anyway, and the people making it hopefully understand that. If you made a network cable with a 10ohm short across the TX pair, or that attached antennae to the signal wires, or whatever, you could cause trouble, but anything conforming to the spec should be fine.

Even shielded Cat5, although on paper it could help, seems not to in practice. There's plenty enough other stray RF near your hi-fi and it turns out a bit extra from unshielded Cat5 doesn't make much difference.

So yes, "audiphile" network cables are snake oil. Trouble is they're not obviously snake oil, because it's easy to make valid arguments against the superficial claim that bits are bits.

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Re: Thermionic Valves

Post by Lardo Boffin » Tue May 01, 2018 9:46 pm

Having moved and now being at the unpacking stuff stage I have managed to sneak this out of a box and I only had to build two Billy shelves from IKEA as penance! Cable management to follow... :oops:
E9DFD184-1ABE-4515-B652-3CB8CEB45AA1.jpeg
The money shot:-
80A9045F-2A1D-4317-B6FA-AD2A19219D56.jpeg
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Re: Thermionic Valves

Post by Coeus » Wed May 02, 2018 4:14 pm

the one at the back looks interesting. What is that called? It reminds me of an old reel to reel tape recorder that used something similar as its record level meter.

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Re: Thermionic Valves

Post by BigEd » Wed May 02, 2018 4:20 pm

That's a Magic Eye, I think. A kind of VU meter.
https://www.youtube.com/watch?v=Zt3IY6Jxtjs

I saw one on a domestic reel to reel recorder in the early 70s (probably). Fascinating!

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Re: Thermionic Valves

Post by Lardo Boffin » Wed May 02, 2018 8:19 pm

Here are a couple more photos of the back one:-
23400397-EABD-4A37-9387-2408CF8740EB.jpeg
CE70F68A-772A-4B3C-989F-94D9936CE569.jpeg
The amount of ‘glow’ varies with the volume.
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Re: Thermionic Valves

Post by 1024MAK » Thu May 03, 2018 3:49 pm

Lardo Boffin wrote:The amount of ‘glow’ varies with the volume.
So acting as basic VU meter then :mrgreen:

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Re: Thermionic Valves

Post by Lardo Boffin » Thu May 03, 2018 6:51 pm

1024MAK wrote:
Lardo Boffin wrote:The amount of ‘glow’ varies with the volume.
So acting as basic VU meter then :mrgreen:

Mark
Yep. :^o

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Re: Thermionic Valves

Post by flaxcottage » Thu May 03, 2018 10:34 pm

That is a nice indicator. What is the valve designation? It is a new one on me.
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Re: Thermionic Valves

Post by Lardo Boffin » Fri May 04, 2018 8:59 am

flaxcottage wrote:That is a nice indicator. What is the valve designation? It is a new one on me.
I believe it is a 6E2 (EM87) but will check when I get home tonight.
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