Old Marshall JCM900 found with 6L6 tubes

[DesertRose]

Does the amp need re-biasing or rewiring to replace back the original EL34s ?

All advice are welcome, thanks a million to all

[ErikH]

Re: Old Marshall JCM900 found with 6L6 tubes

What model is it? None of the JCM900's came with 6L6's stock. EL34's and 6L6's have different pinouts. 5881's and 6L6's have the same pinout. If it's a MkIII, it came with EL34's and someone modded it for 6L6's. If it's a Dual Reverb 4100/4500 or an SL-X 2100/2500, it came with either EL34's or 5881's (depending on the year). Do you have the serial number?

[Zerberus]

Re: Old Marshall JCM900 found with 6L6 tubes

Does the amp need re-biasing or rewiring to replace back the original EL34s ?

All advice are welcome, thanks a million to all

Yes... 6l6 can be put into a Marshall (had them for years in my 2204, but the amp has to be biased colder or the tubes will live for only a few seconds.... conversely, putting EL-34s in an amp biased for 6l6 is possible (though if the amp was designed for 6l6 it needs a small mod to the wiring), but it will "fart out" and not have the clean power or headroom it should.

[DesertRose]

Re: Old Marshall JCM900 found with 6L6 tubes

model is 4101, the amp is still at the store though

sigh*

[ErikH]

Re: Old Marshall JCM900 found with 6L6 tubes

The 4101 is a Dual Reverb 100 watt 1X12 Combo. It came with either EL34's or 5881's, again depending on the year.

Either way, it will require a rebias like Zerb said and some wiring modifications because of the different pinout between EL34's and 6L6's. I'm betting it's one of the models that came with 5881's since 6L6's match up pin wise with them

[DesertRose]

Re: Old Marshall JCM900 found with 6L6 tubes

is it better to leave it with 6L6 or advisable to swap to EL34s if the amp is really fitted with 6L6 tubes ?

[ErikH]

Re: Old Marshall JCM900 found with 6L6 tubes

I have never personally played a Marshall with 6L6's so I can't say. As for the difference between the 5881 and EL34 models, the ones with EL34's had the tone. Once they switched to 5881's the tone went south, and is one of the reasons for the bad reputation of the 900's.

[wixedmords]

Re: Old Marshall JCM900 found with 6L6 tubes

model is 4101, the amp is still at the store though

sigh*

Try it as it is DR.

You may find out you like it. Can you play it at the store?

[DesertRose]

Re: Old Marshall JCM900 found with 6L6 tubes

yeah, i tried it at the store.
All i can say that it sounded almost like a DSL401, perhaps slightly better because of the presence available

[slightly_rude]

Re: Old Marshall JCM900 found with 6L6 tubes

A 5881 is really a lower power 6L6 and they are pretty interchangable. The bias pot in this amp may have enough range to get EL34s within acceptable limits but not likely. If anything the EL34s would be running cold and not sound as good as they could. A resistor change would be needed to give the bias pot the right sweet spot for EL34 tubes.

[Lake Placid Blues]

Re: Old Marshall JCM900 found with 6L6 tubes

A 5881 is really a lower power 6L6 and they are pretty interchangable. The bias pot in this amp may have enough range to get EL34s within acceptable limits but not likely. If anything the EL34s would be running cold and not sound as good as they could. A resistor change would be needed to give the bias pot the right sweet spot for EL34 tubes.

Good post. Almost any Marshall is wired (at the pin sockets) so that it can run either 6L6 or EL34 as long as the bias range is correct.

A 6L6 Fender however, will require at least re-wiring of pin sockets to run EL34's.

[ErikH]

Re: Old Marshall JCM900 found with 6L6 tubes

There's an additional connection on pin 1 of EL34s that is not connected on 5881's but moved to pin 8. 6L6's are the electrical equivalent of 5881's so if the amp is wired for those tubes, it's going to take more than a rebias and resistor change to run EL34's. The socket needs to be rewired as well. It's plain as day on the schematics and tube datasheets, unless I'm reading it wrong.

[Lake Placid Blues]

Re: Old Marshall JCM900 found with 6L6 tubes

Most Marshalls, even those that came factory with 5881, have pins 8 and 1 externally connected. Chances are good that this Marshall can accept EL34, and the 6L6's were just put in later in lue of 5881's.

Of course it's good idea to make sure, and not just assume that is the case.

A socket wired for EL34 doesn't matter to the 6L6, because the internal connection to pin 8, and no connection to pin 1.

The grid resistor ..ect.. usually used for the EL34 works with a 6L6 too.

[DesertRose]

Re: Old Marshall JCM900 found with 6L6 tubes

so to cut the long story short, if I were to get the amp do I hafta send it back for rewiring in order to put those EL34s back right ?

[Lake Placid Blues]

Re: Old Marshall JCM900 found with 6L6 tubes

Probably not. You may need to open it up, and look and see if there's a wire connecting pins 8 and 1 on the sockets. If there's a wire connecting the pins, then it's EL34 compatiable as long as it can be biased properly.