Replacing 6505 power tubes

[MetalJonesy]

hey guys, just a quick one,

i know alot of amps have their tubes configered in pairs.. how are the 6505's configured if looking at it from the rear of the amp? are they the two outer and two inner, or literally paired next to each other?

 

[Force666]

Yes they are the two outer and two inner. But if it's a stock 6505 you should buy a matched quartet.

 

[::XeS::]

Buy a matched quad and you're ok. It's way better to have 4 tubes with the same bias current

 

[MetalJonesy]

yeah ive bought a matched quad, just waiting on their arrival! cheers guys!

oh on a side note, i won't need to open the amp up to rebias will i? as im replacing the tubes with the same model etc.

 

[Metaltastic]

Nope, just stuff 'em all in! (DVDA style)

 

[MetalJonesy]

awesome!

 

[mburton21]

the 5150 and 6505s are auto biasing amplifiers, just stick in the right tubes and it's ready to go.

I use the JJ eurotubes high gain matched set and they tear.

 

[streetwise]

the 5150 and 6505s are auto biasing amplifiers, just stick in the right tubes and it's ready to go.

Um, no... It's a fixed bias amp (not cathode bias, which is kind of an "auto bias", I'm not really familiar with those though) and should normally be biased when changing tubes. However it's biased so cold from the factory that it won't do any harm to the tubes if you don't bias it. Personally I preferred the warmer biased sound when I modded a friends 6505+.

 

[MetalJonesy]

when i can get the amp back home ill chuck the oscillator and multimeter on it and maybe bias it a bit hotter!

 

[Force666]

when i can get the amp back home ill chuck the oscillator and multimeter on it and maybe bias it a bit hotter!

If it's not a + version, you can't bias it unless you do the "bias mod" on it. It is a fixed bias.

 

[barricade]

@MetalJonesy: How did you replace those tubes in your amp? Just turned it off, opened up the back, took the old tubes out and gave the new ones in? Is there anything special to be aware of?

 

[MetalJonesy]

If it's not a + version, you can't bias it unless you do the "bias mod" on it. It is a fixed bias.

oh, so its just a bias resistor in place not a variable pot? i remember my JCM900 havinbg a pot inside to turn to adjust the bias. i think i have the info on the bias mod somewhere

@MetalJonesy: How did you replace those tubes in your amp? Just turned it off, opened up the back, took the old tubes out and gave the new ones in? Is there anything special to be aware of?

i haven't put the tubes in yet, just about to do it now, and yeah its pretty much as you have written!

 

[::XeS::]

@MetalJonesy: How did you replace those tubes in your amp? Just turned it off, opened up the back, took the old tubes out and gave the new ones in? Is there anything special to be aware of?

This, and use a pair of gloves when you touch the tubes.

@MetalJonesy
You have to sold a resistor and a pot.

 

[MetalJonesy]

i use a lint free cloth (or something that my dad gave me to deal with such electronic components) when replacing tubes, its a must not to get fingerprints or grease etc on them as the cause hotspots.

XES; ive just found the 5150 fja bias mod pdf document, so will have to give that a go when i take it home next (so i have the ossciloscpoe and multimeters) also i need to replace the stupid hardwired mains lead with a removable one, really shit design!

 

[::XeS::]

Yeah it sucks. Fortunatelly I bought an used 6505+ with a removable cable by default.

 

[53Crëw]

its a must not to get fingerprints or grease etc on them as the cause hotspots.

This myth seems to be floating around on the internet a lot lately. This is a potential problem for halogen light bulbs, but installing vacuum tubes with your bare fingers will not harm them.

 

[Metaltastic]

This myth seems to be floating around on the internet a lot lately. This is a potential problem for halogen light bulbs, but installing vacuum tubes with your bare fingers will not harm them.

Yup

 

[::XeS::]

It's not a big sacrifice using gloves for 10minutes

 

[TheWinterSnow]

If it's not a + version, you can't bias it unless you do the "bias mod" on it. It is a fixed bias.

fixed bias does not mean that the bias is unadjustable, it implies that the actual bias of the stage does not change depending on the strength of the incoming signal, cathode biased is one of those setups where as the input current increases, the gain stage will actually bias colder and as the input decreases the bias gets hotter.

Preamps = Cathode Biased
Poweramps = Fixed Biased

Even if the bias for the power section has a pot it is still fixed bias.

ive just found the 5150 fja bias mod pdf document, so will have to give that a go when i take it home next (so i have the ossciloscpoe and multimeters) also i need to replace the stupid hardwired mains lead with a removable one, really shit design!

the non removable power cable is lame, i will do that mod to my 5150 soon. Unless you have a bias probe, a multimeter won' help you at all, just use the O-scope connected to the speaker outs (make sure you have a dummy load or your cab plugged in) and just use the scope to bias it until you are out of crossover.

 

[Force666]

For sure. That part of my reply was to it being "auto-biasing".