Musicians Roadhouse

Has anyone got round to trying any other model of tubes in their 2555x ?
If so what did is sound like?
I have some sovtek 5881 lying around and thought I might give them a go ,also got a quad of gold lion KT66 in the cupboard worth a try also.
Anyone??

Why? It wont make much difference. You must do a bias adjust and maybe swap some resistors and caps also.

Mats A wrote:Why? It wont make much difference. You must do a bias adjust and maybe swap some resistors and caps also.

I think your wrong there Mats
The 2555x can run El 34 ,kt77 ,KT88 ,6L6 , 6550, 5881, kt66 According to the back panel ,maybe bias but that's it .

Yeah the jubcan run many different types, get swapping MK and let us know the results

Strange on most Marshalls you have to change some components to get it right. On a VM you can't even fit EL34 because of the OT. Still maybe if you push the power amp you'll get a difference. When i had the AFD100 i compared it with 6550 and EL34 and even did some recordings. The difference is very small. I couldn't tell wich just by listening.

Power amp valves being changed is more of a feel thing, 6550s will usually sound stiffer than 34s, 66s are a bit brasher, 88s give more headroom and more bass.. And so on

If you push it maybe. On normal volumes the difference is very little even in feel. High gain amps gets it's tone mostly from the preamp.

On the afd i guess since you can push it at low volume with the epa you can prob hear the tone difference without blowing the windows...

Nope i compared on an AFD100. When you really push the power amp you push the OT also. With the EPA you only push the valves wich in some electronic way gets less voltage. I guess it's a chip or something. If you use 0.1W the OT only gets that. That doesn't sound the same. Try the AFD100 with an attenuator and the EPA and master on full volume you'll see. The EPA is kind of a simplified version of the London Power Scaling. It's like the YJM100. Sounds fantastic with an attenuator and the EPA on full. If you only use the EPA to turn the volume down you lose the tone and distortion.

Hi all,

Havn't been around for a while as I started a new job in Oct and it's been full on since. But here we are again hope everyone is well., I swopped out the tung sol EL34 that came in the amp for some sovtek 5881's the other day ,sounds nice a bit less volume but smoother and fuller sounding , only did it as 8 of my other marshall's have EL34 and fancied a change, I set the bias to 76mV per side a little hot I know but just a test run really . I take it I am correct in saying that the 76mV equates to both tubes on that side as in 38mV each . 38 x 2 = 76 . On the bias info on the chassis it says 'SET METER TO mV- mV - mA . Don't get the last bit mV - mA ? What mV is equal to mA ? I don't think so.
On a meter you get two very different readings if you set the meter to mA instead of mV .

Anyone biased yet and got any info? The reason for asking is I have a weber bias rite and the difference between the weber and the chassis points readings is quite large.

Good to be back!!

Welcome back!
I am afraid i can't help as know nothing about biasing... But glad to hear you are trying different combinations!
Still love the jube?
I have so much gas for the mini jube... How does your jube sound at lower volumes? Think a mini jube could be usedat home volumes and give good tones?

I am not all that familiar with a 2555x but does this amp have a spot on the back with pots and pins for adjusting the bias? If it does then there is already a 1 ohm resistor installed for each pair of tubes. If the amp is set up this way using any bias probe with a one ohm resistor in it will throw your readings way off.

Im not sure why the chassis instructs to change the meter from Mv to Ma but the conversion is simple with Ohm's law which states: one Mv across one ohm equals one Ma. So for example if your Mv reading is 65Mv that becomes 65Ma. Then divide by two to get the approximate for each tube when measured in pairs.

Thanks fellas,

The amp sounds warmer and more compressed than the EL34, according to the blurb a 5881 is around 3watts lower per tube than EL34 so not huge volume diff.

ANT, that makes sense if a resistor is already fitted ,but look at the EUROTUBES vid on youtube regarding bias points the difference between the two is quite big and really I would like to know if anyone has info regarding the most accurate method. I think the mV - mA bit must be including the resistor as you said.so maybe don't use the bias rite and stick to the points on the chassis. After it was nice and warm I set the bias with the points and switched the lights off and a nice glow and the blue gas glow in the tubes was all present so I think it should be all good . Nothing seemed too hot just nice and no extra noise.

Thanks for the info guys.

I am assuming you have Russian ( Sovtek ) 5881's not NOS 5881's, correct? If they are Russian the max idle dissipation at 70% for 5881 is 24 watts the same as an EL34.

Not NOS mine are quite new ones . But it is quieter defo. Not as harsh as well . might try some kt66 soon just for kicks .