Valve Amp Tube Replacement Questions

[bennyboysvuk]

I have a JCM800 head which I've had for about 4 years now and in that time I have never even looked at changing the valves. Last night I was having a fairly loud blast through the amp and I think the sound has changed slightly and it made me wonder if the valves were deteriorating, so my questions are:

1. How often do you replace your tube amp valves?
2. Do you do it yourself? If not, how much do you pay to get them changed and who do you entrust your amp to?
3. Do you adjust the bias and the crossover distortion (if that's the correct term)?

Thanks in advance.

[gbbird]

My DSL401 has needed new valves in the past - they started ti pop and crackle when warm. I initially looked at doing it myself as valves per se are quite cheap - there is a comnpany called ??Watford Valves?? who sell them.

However, after reading up on it things started to appear a bit too complicated for my non electrical brain (ie adjusting the bias etc etc). I then realised that Marshall HQ in Bletchley is only about 5 miles away from where i live . They offer a full repair service, so i get them to do it for me. I cannot remember the cost, but i do recall it was very reasonable.

g

[paulmurr]

bennyboysvuk said:

1. How often do you replace your tube amp valves?

When they explode.

bennyboysvuk said:

2. Do you do it yourself? If not, how much do you pay to get them changed and who do you entrust your amp to?

Nope, revalve and service is only a hundred quid from my local amp man. I use a guy called Dennis Marshal but unless you're near Fife I'd imagine there would be people closer that could do it.

bennyboysvuk said:

3. Do you adjust the bias and the crossover distortion (if that's the correct term)?

You've lost me... My 800 is fairly heavily modified so I assume that it's all been taken care of.

[bennyboysvuk]

Thanks for the responses guys.

They say a little knowledge is a dangerous thing...however, I understand the setting of the bias and I'm happy to take that on, but the adjusting of the crossover distortion on an oscilloscope is something that would require an oscilloscope and a dummy load and I guess some skill and experience too, none of which I have.

This is a link to a vid about doing the above, but it still looks tricky: http://www.ehow.com/video_4467501_correct-crossove...

[Zad]

If it is a Class-A amp (possible but unlikely) it won't have any crossover distortion.

[bennyboysvuk]

Zad said:

If it is a Class-A amp (possible but unlikely) it won't have any crossover distortion.

I see...I must read up some more on this subject. As I said, a little knowledge is a dangerous thing indeed.

After having a quick look inside the back cover it looks like there's no bias adjuster or anything else like that. It looks like plug 'n play.

Edited by bennyboysvuk on Wednesday 10th February 18:23

[Tangent Police]

I used to have whole sets with my Leslie 147 organ speaker. They sound different, I was using 2x6550/KT66 output tubes (which I did bung an EL34 in when a tube went on me). The bottle shaped output tubes have a smoother tone than the harsher "tube shaped" one. I also had ECC83/12AU7 input tubes which I swapped around, they made a lot of difference. The mullard ones had a nicer sounding overdrive.

http://www.jacmusic.com/KT88/kt88.htm

Have a read of that.

My leslie was 40w and used 2 6550's but would also run EL34's (smaller envelope-harsher sound).

Do a bit of research and see if you can find anyone fitting bottle shaped 6550/KT88/6L6G's to those heads. If you are running some half decent cabs, it will sound beautiful.

Edit, you'll know when they start going and if you look, you'll see all is not good. The sound starts breaking up. I always fitted matched pairs and swapped when one was toast.

Edited by Tangent Police on Wednesday 10th February 21:47

[rich24a]

Most Marshall amplifiers are class AB, except for the new Marshall class 5, which is class A. A JCM 800's bias control is inside the amp on the PCB, and it's a variable resistor that you use to control the bias voltage for the output valves.

This is a good link to biasing a JCM 800

http://www.eurotubes.com/euro-Generic-Bias.htm#

I don't think you have to worry about crossover distortion, as long as the amp sounds good when you replace the output valves...