Tube question...?

[Ironjose]

Heya guys... iīm thinking about changing the tubes on my amp it uses EL34 but i was wondering if i can put in it other tubes? without modifying anything?
and another one, besides having 4 power amp tubes the (EL34) it has 4 other 12ax7t tunes in it, but 3 of them are bare naked and the 4th is with a silver plated cover? whatīs the deal with it?
thanx you guys
JJ

[Al Heeley]

Re: Tube question...?

If you put in anything other than another set of EL34's you'll need to get it rebiased, to make sure the valves are fed with the right current, else: kapow!
On the pre-amp valves, I have 7 in my Marshall, they are all covered with a little tin tube. Maybe your amp is second hand and the previous owner used them to grow yogurt cultures in?
I think its not worth losing any sleep over

[Ironjose]

Re: Tube question...?

ok i know that now... a friend of mine told that just after i post the message here, but anyways i wanted to have a second and more powerfull opinion... since i dont want to biased the amp coz i dont really know how... and i think that nobody here in my country knows how to do that.... i'll put new EL34 on it, i've also search it that there are more powerfull EL34's that can drive more the amp... about the preamp valves like i said b4 there are only 4 of them but only one of the has the metal cover... and i bought brand new... and this is the second time i opened the amp plate... its kinda cool
JJ

[greendy123]

Re: Tube question...?

If you put in a new set of el-34 tubes you still need to bias unless you the old tubes are based on a rating system like Groove tubes has and you can plug in the same ratings of tubes.

[casblah]

Re: Tube question...?

I just put a new set of KT66's in my 2204, with good results. They have similar bias specs as EL34'S, as well as the proper "pin-outs". The KT tubes are very ruggedly built, and can withstand over-biased conditions with no problem. I have mine biased at around 68 mA. Physically, the KT's are much larger than the EL's. , so check the hole spacing on your chassis to see if they will fit, if you see this as being an option.

[Lake Placid Blues]

Re: Tube question...?

When changing out tube types there are some things to pay attention to, beyong biasing and pin configurations.

One important matter is the current consumed by the heaters. Some tubes consume more, and this can put a strain on the power transformer. For example a 6L6/5881 consumes 0.9 amps, but a KT66 consumes 1.3 amps. Nearly a third more per tube.

Some ratings:
6L6 - 0.9a
KT66 - 1.3a
KT88 - 1.6a
6550 - 1.6a
EL34/6CA7 - 1.4a

The EL34 and the KT66 are rather close, so going to KT66 from EL34, or from KT66 to EL34, shouldn't present to much of a problem. However, installing a KT66 into an American 6L6 amp may be problematic, despite the fact that the KT66 and 6L6 are similar tubes, and usually listed as a direct replacement. However, another consideration is the impedance matching with output transformer primary.

Despite what tube manuals may list as plate to plate resistance specs, it kind of depends on how the circut operates and the voltages. Generally, the EL34 types and the KT88/6550 types have similar impedance specs in the same circut. That's why we see Marshalls that run EL34's usually switched over to KT88/6550 instead of other types. On the other hand, the KT66 will want optimately, to be matched to a different output transformer than one designed for EL34, 6550, or 5881/6L6. Many people don't know this, but the Marshall plexi amps that ran KT66 tubes, prior to mid to late 1967, had KT66 specific transformers. After the the switch over to EL34, they used slightly different transformers optimized to the EL34.

The original JTM45 is another interesting case. While very similar to the Fender bassman, they didn't use anything like the Fender Bassman transformers. This is one reason why the JTM45 has a unique sound that is neither modern Marshall or Fendery. It close to vintage Marshall but a little rounder. The actual OPT used on the originals delivers quite an impedance mismatch with standard 5881 or EL34 tubes. It was probably all an accident, but that was probably one of the things that made the original JTM45 rather unique.

[Ironjose]

Re: Tube question...?

ok thanx you guys i didnt know that if i change them into another EL34īs i had to rebiased the amp... however i dont know anything about that... and i also think that no one here on my country can... thatīs another bummer situation for me and my amp, i think that if i go to the store i bought it from they canīt either and thatīs what you called a really bad luck. i rather learn myself the biased thing and change them myself, coz thatīs what happend to my pups when i bought them, no one could change them and now i can
about Lake Placid Blues said i didnt understand most of the post, but could you be more specific, iīd be so thankfull... thanx
JJ

[Lake Placid Blues]

Re: Tube question...?

about Lake Placid Blues said i didnt understand most of the post, but could you be more specific, iīd be so thankfull... thanx
JJ

Sorry....

In short, your best off using the tube model your amp was originally designed to use. There's less potential problems that way.

[Ironjose]

Re: Tube question...?

yeah i get it now, but i was reading that it still needs to be re-biased to fit the new tubes, is it true?
or i just have to buy the tubes of the same model but different brand?
JJ

[Lake Placid Blues]

Re: Tube question...?

yeah i get it now, but i was reading that it still needs to be re-biased to fit the new tubes, is it true?
or i just have to buy the tubes of the same model but different brand?
JJ

The bias should be checked with any retube. It will usually require an slight adjustment even when using the same type and even the same brand of tube.

For example, I have a pair of Sevtlanas EL34's that will run 56ma if I put them in one of my 50 watt Marshalls, that had another pair of Svets running 38ma, on the same bias setting.