Some simple explanations please....

[MallyMad]

Right,here goes....I've been forum lurking recently looking for advice/reviews of valve amps and I've eventually ended up with an Affordable Valve Company EL34 which (according to prices on here) seems to have been a bargain at £165! I say seems to as its not in my possession yet...

Hopefully someone could please explain a few of the terms I've heard;

valve rolling? Seems to mean swapping out valves but what does that do?

Bias - no idea!

I'm also after opinions and worthwhile modifications(for further down the line).

As it stands I currently run an Audiolab 8000a into Mission 732s and use an iPad and sacd(seemed like a good idea at the time) as a source. I also own an Arcam 8/8P combo and a linear dynatronics mono valve amp which was pennies at a car boot sale.

How does anyone think that will shape up - the EL34 into the missions?

Hopefully someone will get a bit amusement out of all this...

Thanks!

 

[Vinyl Art]

Valve Rolling - Willy nilly swapping out of valve brands in the hope of gaing a cheap (don't make me larf) improvement (any change will do) to the sound. As opposed to buying a decent amp in the first place(not aimed at your amp). NOS (new old stock) valves are derigeur any be prepared to pay silly money for them. If you have Kondo you can ignore this as you will be beyond reproach in all aspects.

Bias - Either use this term prolifically in any cable thread. Alternatively wait to be confused by terms associated with valve bias, fixed (which isn't) and variable (which isn't) and is adjustment of grid voltage which effects annode current.

Mission 732 - Oh dear, you will be box swapping like a mad man before you know it

ps..Never take advice off a wood butcher

 

[mongeddavid]

Tube rolling means as you suggest changing tubes from different manufacturers and in some cases the tube type provided that the amp is designed to take different valve types. Valves of the same type from different manufacturers and even different eras can and do sound different and therefore you can attempt to find the ones you fid most pleasing.

Bias you can think of as a kind of balance system for the valves in order that they operate the same on both channels. Some amps require matched valves others dont, some auto bias others will require you to bias the valves to ensure an even operation between channels. Its more technical than this but thats kind of the essence as i understand it.You should speak to the AV Co re your amp and see what valves you can use and how the bias works on that amp.

 

[Tel]

.You should speak to the AV Co re your amp and see what valves you can use and how the bias works on that amp.

Good luck with that!

 

[Vinyl Art]

Good luck with that!

:rofl:

 

[Valvebloke]

You should speak to the AV Co re your amp and see what valves you can use and how the bias works on that amp.

Good luck with that!

Don't bother trying to contact AV Co about bias. I own one of their amps and I tried. If you did not buy it directly from them then they will not help you at all. It's their declared policy. In any case they will not tell you what the bias setting should be. If you did buy it from them and you think it needs re-biasing then they'll tell you to return it to them and let them do it.

When I got my amp the EL34s were showing signs of ageing and were no longer equally biased. So I bought a new set of four (a 'quad') matched Svetlana EL34s. These were similar to the ones originally fitted. At the time (last Nov) the best deal was from Musician's Friend in the US http://www.musiciansfriend.com/accessories/svetlana-el34-matched-power-tubes. Even with the shipping and import this was cheaper than I could find them anywhere in Europe, but do bear in mind that the pound has weakened against the dollar since then. I went for their medium rating (they call it 'green'). The valves arrived quickly and safely and when I tested them they were indeed well matched.

The amp will probably need to be re-biased when new valves are fitted though. This is relatively easy if you know what you are doing. But it involves working inside the amp with it switched on. There are lethal voltages in there and this is not a job for a beginner. Either hand it to a professional or at the least get someone who does know what they're doing to show you how to do it, and to stand alongside you while it's done. For that person's benefit I'll say that there are four separate pots, one controlling the bias voltage to each EL34, and there are current-sensing resistors in the cathodes. So you just have to monitor the voltages across these to check the individual currents.

As far as the actual value goes I set mine to 40mA cathode current per valve (400mV across the 10ohm current sensor). This gave relatively low distortion without running the mains transformer too hot. Do be aware though that there are various versions of this amp with different mains transformers. So I can't guarantee that this bias will be OK for your amp. (When you get it set I suggest you let it run for a few hours and keep checking the transformer temperature.) I also came across a website here http://gafferlicious.net/bias.htm where Murray Johnson (sometime of this parish) suggests 390mV as the bias setting, so very close to my value, which was comforting. That website is a very slow loader though, if it will load at all, so if you're interested you'll either have to track down a cached version or just take my word for it.

This is not a bad amp actually and is very good value. IMHO it uses too little feedback though. This makes it oversensitive (270mV in gives 25W out), over-distorted (more than 1% at 25W) and with too high an output impedance (IIRC I measured 4ohms or so on the 8ohm speaker setting). I tried a quick experiment with upping the feedback which improved all these things. But I didn't test it thoroughly. So I can't guarantee that it will be unconditionally stable with any more feedback than it's supplied with.

VB

 

[MallyMad]

Didn't want to slag the guy off..... Many thanks for the info. Can see I've a lot to read up on and learn about! Think I'm well out my depth here but I will persevere. I will post up when the amp arrives.

thanks again!

 

[Willie Garvin]

Valve Rolling - Willy nilly swapping out of valve brands in the hope of gaing a cheap (don't make me larf) improvement (any change will do) to the sound. As opposed to buying a decent amp in the first place(not aimed at your amp). NOS (new old stock) valves are derigeur any be prepared to pay silly money for them. If you have Kondo you can ignore this as you will be beyond reproach in all aspects.Bias - Either use this term prolifically in any cable thread. Alternatively wait to be confused by terms associated with valve bias, fixed (which isn't) and variable (which isn't) and is adjustment of grid voltage which effects annode current.

Mission 732 - Oh dear, you will be box swapping like a mad man before you know it

ps..Never take advice off a wood butcher

Now you tell me ;-)

 

[MallyMad]

Purite North - that's brilliant,thank you. I've ordered up those valves. I take it its not as easy as pull the old ones out and push those ones in?

I can almost feel proper wammers teeth grinding at this question....

 

[Vinyl Art]

Get in touch with a chap called Brian, aka Take5, he's an expert in that area

It is that simple, plus the biasing of course.

 

[Valvebloke]

If you've never done it before the base of the EL34s has a central spigot which sticks downwards and this has a key on it which fits into a keyway on the valve socket. So it can't go in the wrong way round. Don't be too rough with it. The spigot's only plastic, so it is possible to snap it off. When removing a valve like the EL34 the pros will hold it by the base, not the glass. There are fewer accidents that way. And if you've just switched the amp off, so the valve is still VERY, VERY HOT, the base burns you less badly than the glass. For smaller valves the only trick is to try not to bend them over any more than you have to. The pins will stand a bit of bending but if you overdo it then it is possible to crack the glass-to-metal seals.

VB

 

[MallyMad]

Yet again folks thanks are due. Just to give you an idea of the intelligence levels you are dealing with I only ordered up 2 of the recommended valves. Twat. I've corrected that now.

 

[awkwardbydesign]

I wear leather gloves to remove hot valves. No. not welding gloves!