New tubes

[Bern_397]

Hello,

if you must put in new tubes EL34(power amp) or 12AX7 (pre amp) what is important to remember? What is the cost of those tubes?

Many thanks

[Andy H.]

Hi Bern 397.
Right, the biggest difference is that with most amps, including almost all that use EL34s, each time you replace the power valves you must have the amp 'rebiased'. This needs done by a good amp technician, and is a quick but important job. What it does is set up the amp to deliver the right amount of idling current to the valves. This will vary as individual valves will draw different currents. This is normal and is due to the difficulty of manufacturing them <i>exactly</i> alike. If the biasing isn't set right then the tubes will be run either too 'hot' or too 'cold', both of which lead to different problems affecting sound and reliability.
With some amps like the AD30s, you can get away without rebiasing, due to some clever circuitry inside.

Pre amp valves are easier as there is no need to worry about biasing. Just replace them at will, and find ones that sound best to you. They should last several years of normal use.
Often it is recommended that you use the 'best' or least noisy valve available for the first one, as it will have the biggest impact on how noisy hissy the amp will sound. Also if you're being really fussy, you could use a 'dual matched' preamp valve for the final stage to provide a nice balanced signal for the power amp to deal with.
Costs? Vary greatly depending on quality, and how carefully they are selected by the suppliers. Quality costs more...

Andy.

[irish_admiral]

Different valves have also got different tonal characteristics... so have a think about what you might want to get. Take a look at the www.watfordvalves.com test review section to get an idea of some tubes different capabilities.


Don't tell me the best way, just tell me the cheapest way...