[Seano]

I'm in the process of assembling a GK-1M and the process has raised in me a few questions.

Most have been directly straight to Hugh (like 'why won't it work?' - cause C12 and C14 are around the wrong way) and he has been most prompt, tactful and supportive in his responses too.

But I still have some for the wider community.

1. Tubes. Playing with the tubes supplied has bought up some mad variations in measured responses during the testing procedure.  Now I know naff all about tubes (they glow, they get hot, they can look quite pretty) and I was quite surprised at the differences in circuit voltage that they generated. Hugh tells me this is quite normal and that tubes can vary up to +-15% between two tubes of the same value. And since they look like a glorified lightbulb I don't find this to be a difficult concept to grasp.

But obviously there must be the same bang for buck prospect in terms of tubes. Some breeds or makes of tube must stand above the others surely. Ones that are better built, have less variation in properties and performance within a batch etc.

Is playing around with tubes a legitimate upgrade path for the GK-1? And if so has anybody yet had a fiddle or have you all been so stoked with the initial outcome that you haven't been driven to experiment.  Given all the talk of upgrades to wire, resistors, caps, binding posts and RCA jacks (to name a few) that has occured here before I find this very hard to believe.

2. Mute switch.  Why would you want/need a mute switch? I know what it does so don't go getting clever.  But I can't see the need for one (isn't that what the volume control is for?) and since I'm not keen on drilling yet another hole and finding a spot for yet another switch.......but if someone can explain to me why I 'might' need one then I would reconsider.

I assume (Hugh?) that if I don't want the mute switch then I simply jump the mute posts on the board?

[Tinker]

1. Is playing around with tubes a legitimate upgrade path for the GK-1?


2. Mute switch. Why would you want/need a mute switch? Isn't that what the volume control is for?

1. Changing tubes does change the flavour of the GK-1 and much of this is personal tast, however, I would not recommend changing them until the standard issue GK-1 parts have been well burned in. If you want to play then, check out some of the earlier threads on tube rolling in this forum, such as
http://www.audiocircle.com/index.php?topic=7379


2. Yes, you will want it. You might have the perfect volume setting, then have to mute the thing to take a phone call. You can then recover the perfect setting with the flick of a switch instead of fiddling with the knob, getting up and down until you have it right again.

Cheers,
 T.

[andyr]

1. Changing tubes does change the flavour of the GK-1 and much of this is personal tast, however ...

Hi, Tinker,

WRT Seano saying he "was quite surprised at the differences in circuit voltage they generated. Hugh tells me this is quite normal and that tubes can vary up to +/-15% between two tubes of the same value."

Given we go to all sorts of lengths to get various things in the AKSA amps matched (like bias currents or zener diode voltages) wouldn't it deliver a "higher fi" solution if the GK-1 tubes were matched so they delivered the same voltages?  After all, Hugh supplies matched transistor pairs in the AKSAs?

This ain't quite the same as the tube "rolling" (swapping) which was in that other thread you referred Seano to, but is simply a matter of selecting a pair of tubes so that their operating parameters match?  Is this a waste of time or should it deliver a sonic advantage?  (In which case, there's the first candidate for the GK-1 Nirvana ... "Tubes carefully selected by an ageing Zen-master to the bucolic sounds of kookaburras screeching!!"     )

Regards,

Andy

[Tinker]

Hi, Tinker,

WRT Seano saying he "was quite surprised at the differences in circuit voltage they generated. Hugh tells me this is quite normal and that tubes can vary up to +/-15% between two tubes of the same value."

My apologies to Seano, I misunderstood "playing around with."

T.

[Seano]

Apology unneccesary!

The question tried to ask what upgrade options anyone felt was available.

That means both within tube types and between tube types. I think

I have zilch experience with tubes and no grasp at all of the fundamentals or nomenclature involved. And no great desire to get caught up in it. Even from the outside that prospect looks terrifying.

I'm just curious to see if I can 'go to school' on anyone elses experiences.  By whatever means available.

[jules]

Seano,

Michael Percy stocks matched [+/- 5% in RAM Labs] pairs of tubes. I don't know if the GK-1 types are available from him but I've found it useful to keep a downloaded pdf of his stock on file.

jules

[RonR]

Seano,

I had one 'out of spec' valve delivered with my GK-1. Although Hugh offered to replace it, the distances and postage costs involved led me down the path of NOS valves.

Understanding the nomenclature surrounding the valves used in the GK-1 is not a trivial task! There are at least 3 different naming conventions (European, US, Russian) for the same type valves. They come in Standard, Industrial, and Low noise versions, with steel or gold pins, with 6.3 or 7 volt heaters, with fixed or variable 'mu' (not sure what that actually means, can anyone help on this?), and some people will have you beleive that the different 'Getter' disk/halo/D shapes affect the sonics in different ways.

According to a thread I saw a while ago, variable-mu valves are said to work best in the GK-1.

A good place to start to scratch the surface is: This Pdf from Aopen. They made an 'Audiophile' motherboard with a tube sound stage that used the same type valves as the GK-1. There's good info from page 26 onwards.
Tubes Asylum has The 6DJ8, 6922, 7308 Saga, Part 1 and Part 2.

I have used: Philips miniwatt PCC88s, 1960's Telefunken PCC189s, and 1960's Philips gold pin E188CCs, and each have their own character. Next on my list is a pair of Amperex (to be decided). I'll try to post my impressions when I've listened to a larger range of valves in the GK-1.

Happy Valve Hunting,

Ron.

[Seano]

Michael Percy stocks matched [+/- 5% in RAM Labs] pairs of tubes. I don't know if the GK-1 types are available from him but I've found it useful to keep a downloaded pdf of his stock on file

Who the heck is Michael Percy?  With all due respect, I know a few people on this planet, but I don't know this bloke or how to find him.

Is a link out of the question?

[AKSA]

Let me offer a few words on matching tubes.......

The GK1 uses its 6ES8 in cathode follower (CF) mode;  that is, no voltage gain, but very high current gain.  The preceding SS stage gives a small amount of gain, the signal passes to the level control, then to the grid of the output tube.  The output of the tube is then capacitively coupled to the output RCA jack.

A CF produces around 5/32 of the distortion of a conventional plate loaded triode, which is commonly used in all tube amplifiers for its voltage and current gain.  When the tube is used in plate loaded configuration, it is extremely susceptible to imbalance, particularly in the phase splitter, where technically we like outputs in each phase to match in amplitude.  In cathode follower form, this changes however, because there is no voltage gain and thus mismatches do not result in differences in output.

The GK1 uses two triodes wired in identical CF mode for each channel.  They operate in antiphase.  While technically they should produce identical outputs such that their combined currents sum to a constant, this property is exploited only for the relatively non-critical bass frequencies and removes the influence of the power supply.  However, the midrange and top end quite deliberately utilize the interaction of the tube with its power supply.  So using tubes with matched sections here is not necessary to the all important mid and high end bands.

However, this is audio, and there are undoubtedly subjective differences.  I'm told by DSK (whose killer system is a jaw-dropper) that the cat's meow is the Seimens gold pin 7308, which is around $US80 per matched pair.  You can use the following tubes in the GK1:

6ES8 (US) or ECC189 (EU)
7ES8 or PCC189  (7V filament)
6DJ8 (US) or  ECC88 (EU)
6922 (US and RU)
7308 (German and US)

Those who know their tubes will recognise that these are all high transconductance frame grid tubes, around 12.5 milliseimens (mA per volt).  Frame grid tubes are very quiet and were the penultimate development of this technology, the last being the Nuvistor.  Plate impedance of these lies between 2.5-3.2K.  Correct plate operating current in the GK1 is 8.2 - 9.5 mA;  but it's not particularly critical.

There is no doubt different tubes from the many manufacturers do sound different.  However, this is a subjective minefield, because there are only seven or eight factories, and with tubes there is a great deal of re-badging, so you often don't know what you are getting or where it was made.  Large tube dealers often had their own 'house' brands;  this clouds the issues somewhat.  Seimens, Amperex, RCA, AWA, Sylvania, Mullard, Brimar and Tesla are all very good manufacturers, but the nature of the GK1 design plays down differences because the tube is not used as a voltage amplifier.

The variable mu issue is interesting.  I originally cast around looking for a suitable tube for the GK1 with a gimlet eye on cost.  The 6DJ8/6922 was my original choice, but these were very expensive.  The sound was extremely clean, almost transparent, and not quite what I wanted - a little color to aid engagement was needed.  Since the intrinsic distortion of the CF is so low, to achieve this color I decided to use the variable mu variant of the 6DJ8 - the 6ES8 - and this has been extremely successful.  This tube is readily available, not overly expensive, has a secondary use in compressors for recording studios (one of the best audio compressors in the market is made in the States and called the ES8!!), and was originally designed for Automatic Volume Control elements in AM radios where the variable gain function (changing gain with operating point) in plate load is essential.  

The HT bypass simply bypasses the entire GK1 - SS and tube sections combined - for connecting up a HT unit, which of course has its own source selection and volume control.  It has often caused confusion but may be omitted if desired.

Hope this clears up the debate!

Cheers,

Hugh

[jules]

I completely agree ... leaving out links is totally unacceptable  

try www.percyaudio.com/

Apparently what RAM labs do is to buy thousands of tubes and match them by computer ... makes sense I guess. This would be hard to do with small numbers.

jules

[SamL]

6ES8 (US) or ECC189 (EU)
7ES8 or PCC189 (7V filament)
6DJ8 (US) or ECC88 (EU)
6922 (US and RU)
7308 (German and US)

Just a slight off topic but related question.
Are all these replacement/substitute tube the same height & width as original? Depend on the enclosure, some might not fit?

Sam

[AKSA]

Sam,

All share the same pinout and base dimension, of course, but height will vary very slightly by about 5mm.

So give it 5mm more clearance and all these tubes can be rolled into service......

Cheers,

Hugh