Valves,Tubes.......

[Mark]

Derek from Watford Valves is a helpful guy, I've bought many sets of valves from Watford. Re: ECC83; a nice Mullard ECC83 (NOS) in the first valve position is a good move. For a nice condition, low noise Mullard valve you can expect to pay good money. There's plenty of "rubbish" on the market, so be careful!. But, if you can, buy one just for this position. Billy, Mullard was a British manufacturer, they had several valve manufacturing factories around the UK, most of which closed during the 70's. Genuine Mullard valves were only manufactured in the UK.

Mark

[Zumbilly(Murdoch)]

Thanks for the help Mark,
I need all the help I can get,that's why I post here and have always received great answers to My questions.
Perhaps You can answer again.
The "first"valve (previously mentioned) was an 802 for which Derek suggested, and supplid an ecc 82,the second vale was 12AX7
Being pretty ignorant on electronics I would ask if replacing an 82 with an 83 in the first position is good practise?
I would always err on the safe side and replace like for like.
What life can one expect from valves in terms of usage hours? how ling is a piece of string.
Best regards
Billy

[Mark]

ECC81's (12AT7), ECC82's (12AU7) and ECC83's (12AX7) are all pin compatible and will work ok in the typical first valve position. Changing the valve type will affect the gain of the input stage. By lowering the gain of the first position valve, you reduce the possibility of the input stage distorting. However, you also reduce the input sensitivity; so like most things in life it's a compromise. Luckily most steel guitars generate a high output so a little reduction in sensitivity still means you have plenty of output.

The ECC83 or 12AX7 has the highest gain, the ECC81 or 12AT7 has a medium gain, and the ECC82 or 12AU7 has the lowest gain: yes they don't go in order! The requirements for the first position valve are quite demanding, it's job in a typical amplifier is to significantly boost the signal level ready to be processed by the tone controls. It's required to operate at full gain over a wide frequency spectrum whilst generating very little of it's own noise. The RCA 7025 valve used by Fender is a low noise version of the ECC83 and works nicely in the first position. So what makes the old Mullard so good, I hear you say? Well, you just have to try it! The bloody things even sound great when you distort them!

I note Billy that you had an ECC802 valve in the first position. If this is Telefunken valve, it's a great valve. Telefunken ECC802S valves are highly prised by hifi people, they are a super version of the ECC82. If it gives you enough gain and it's in good condition it should be superior to any ordinary ECC82.

The 12AX7 should be fine in the second position; noise is not such an issue unless you are into heavy metal playing with massive gain levels..... and probably not even then!

Pre-amp valves usually last several years, especially if they are treated nice, i.e. not too much vibration and overheated. They don't tend to go "pop" like a power-amp valves. More than likely a pre-amp valve will go a bit noisy at first, or become microphonic, you might then notice a lack of gain or treble in your sound. First position valves, reverb driver valves or valves in distortion circuitry usually shows the first signs of age.

Hope this helps Billy

Mark

[Zumbilly(Murdoch)]

Thanks again Mark for the informative reply and for taking the time to post.
Billy

[Paul Brennan]

Can anybody answer this question:

When you buy two matched pairs of 6L6's for a Fender Twin, in what position do the matched pairs go?
Let's call the power amp valve positions 1, 2, 3, 4 looking at the back of the amp from left to right.
Do the pairs go in positions 1, 2 and 3, 4?
Do the pairs go in positions 1, 4 and 2, 3?
Do the pairs go in positions 1, 3 and 2, 4?
Over the years I've had people give all the above answers. I've never been able to get a definitive answer. Does it even matter. Can somebody please put me out of my misery?

[Basil Henriques]

Valves 7 and 8 matched and 9 + 10 Likewise

[Mark]

Paul,

It's very unusual for Baz not to get the correct answer, but on this occasion he is incorrect.

The correct answer is: It's best to buy a matched quad. By using a matched quad, the current as set by the biasing will be equal in all valves and no single pair of valves will be stressed.

If you have purchased two matched pairs, you can use them, it's not an ideal situation because one pair of valves will be stressed more than the other pair. But this is how to fit them: Using Baz's drawing for valve numbers - viewed from the rear of your Twin, valve 10 is the on the extreme left hand side. Heading to the right it goes 10, 9, 8, 7 and then onto the preamp valves.

Put one matched pair in positions 7 and 10. Put the other matched pair in 8 and 9. In this way the summation of bias currents in each half of the output will be the same and you will get a symmetrical output. If you fitted the valves as Baz suggested, your output would not be symmetrical. At higher volumes this will cause a few problems! Unless of course the valves are all matched!

That's another good reason for buying a matched quad, you can put any valve in any position.

I usually go one step further and measure the bias current of each valve and select the valves so the higher bias current valves are placed in the cooler positions. To do this requires special equipment. But it does raise an importance point; keeping your valves cool greatly extends their life. A nice little quiet fan will work wonders!

I hope that helps to put you out of your misery!

Mark

[Basil Henriques]

Now I know why I didn't like the sound of my guitar through a twin reverb..Thanks for putting it right Mark, a bit late in my case, I got rid of the twin reverb in 1973.

[Mark]

LOL: That Twin would be worth a few quid now! Just ask JD..

[Paul Brennan]

Mark, thank you for this information. Like Basil I thought, from referring to the circuit diagram, that the pairs went side by side (1, 2 and 3, 4). However, I had a suspicion that it might be 1, 4 and 2, 3. So, I take it if you want to reduce the power of the amp by removing power valves you'd remove either the inner pair or the outer pair.