JCM 2555sl questions...

[Jack71]

Hey all,

I'm new to this forum and just picked up a JCM 2555sl Slash amp and had a few questions.

First of all, the amp sounds great but it seems to me that it is not very loud for a 100 watt amp. As a matter of fact it's fairly quiet until the master gets to six and beyond and then comes on like gang-busters. But still, even dimed it doesn't seem like 100 watts.

The tubes are Electro Harmonix EL34s which I believe are the stock powertubes for this amp. They look pretty old and at first I thought maybe they were just on their way out but then I picked up another 100 watt amp (Top Hat Emplexador) with a quad of these same tubes and that amp didn't seem very loud for a 100 watter either. Maybe these EH power tubes just don't put out much power?

So, I guess that brings me to my next question; what tubes do you Jubilee and Slash owners find work the best in your amps? Preamp wise I've got a JJ in V1 and an EH in V2 and V3 which sounds really good, so I'll probably stick with that, though I'm always open to suggestions.

Power amp wise, like I said, the EH's sound good but they just don't seem to pump out the watts. They are Russian tubes so maybe some JJ EL34s would work well. A lot of people seem to like the SED winged "C"s too.

Also, what is the recommended bias range for the power tubes in this amp?

Any advice is greatly appreciated. Thanks guys!

Jack

 

[steelhorse]

Retube and go with either the Winged C's for the power section or TAD EL34B's.

That's a great amp and I hope the retube brings it back to life volume wise.

 

[jcmjmp]

First of all, the amp sounds great but it seems to me that it is not very loud for a 100 watt amp. As a matter of fact it's fairly quiet until the master gets to six and beyond and then comes on like gang-busters. But still, even dimed it doesn't seem like 100 watts.

The tubes are Electro Harmonix EL34s which I believe are the stock powertubes for this amp.

With the way the jubilee and Slash amps are designed, the volume doesn't really get loud until 5-6 on the lead & master controls. Its just the way the amp is designed, which is for more preamp centric distortion than power amp distortion. You're also right in saying that the 2555 is not as loud as other 100w Marshalls. The 2555 still delivers 100w though, its just a little cleaner power. In comparison, a JCM 800 2203 100w actually puts out more than 100w when cranked into power amp distortion.

As for the tubes, EH tubes were never stock on these amps.

Just to be on the safe side, clean the effects loop jacks as signal power can be lost through dirty contacts on those jacks. When I don't use the effects loop, I usually plug in a short patch cable between the send/return loop jacks.

For preamp tubes, I like the Shuguangs and Tung Sols in V1/V3 but really, anything that is reliable, low noise and has low microphonics should be ok. V2 is where the most gain happens and a low noise tube is best in that location. I've used older 7025s with great success in V2 on the jubilees.

For power tubes, I'd go with the =C=. They are probably the most reliable EL34 tube on the market today and they sound great to boot. When biasing, check your plate voltage and bias at approx 65% dissipation.

 

[Ken]

When I don't use the effects loop, I usually plug in a short patch cable between the send/return loop jacks.

.

I never considered this. Should I do that on my DSL 50 as well?

Ken

 

[Jack71]

So is the fx loop always in the circuit even when no effects are plugged in?

What's the formula for calculating dissipation again?

 

[jcmjmp]

So is the fx loop always in the circuit even when no effects are plugged in?

Yes, not buffered like on modern amps, like the DSL. The 2550/2555 preamp signal is going through the jacks at all times via a normally closed connection.

What's the formula for calculating dissipation again?

EL34 max plate dissipation 25w

voltage=400v (this is the plate voltage, for example only -- this should be what your meter is measuring for voltage)

.7*(25/400)*1000=44ma
(% of max - 70% in this example) x (max wattage/plate v) x 1000 =bias current

 

[Jack71]

Yes, not buffered like on modern amps, like the DSL. The 2550/2555 preamp signal is going through the jacks at all times via a normally closed connection.


EL34 max plate dissipation 25w

voltage=400v (this is the plate voltage, for example only -- this should be what your meter is measuring for voltage)

.7*(25/400)*1000=44ma
(% of max - 70% in this example) x (max wattage/plate v) x 1000 =bias current

Ok, I can follow that. But how do I know what plate voltage to start with? I know I have to measure it, but doesn't it change depending on where the bias pot is set, or does it remain constant regardless? And would it be the same for all the tube sockets?

 

[jcmjmp]

how do I know what plate voltage to start with? I know I have to measure it, but doesn't it change depending on where the bias pot is set, or does it remain constant regardless? And would it be the same for all the tube sockets?

You measure the plate voltage with a meter. You should keep the meter on the plates while you're biasing because the voltage will change slightly as you adjust the bias.

Measuring the plate voltage on only 1 tube should be sufficient.

 

[GibsonSlash]

If I don't plug a short patch cable when this loop is not in use, what would happen? I haven't tried it though, just wondering how it would effect.

When I don't use the effects loop, I usually plug in a short patch cable between the send/return loop jacks.

 

[jcmjmp]

If I don't plug a short patch cable when this loop is not in use, what would happen? I haven't tried it though, just wondering how it would effect.

You'll get a better connection for the signal path with a patch cable. Those jacks get tired after a couple of years where they don't close as well as they once did and can get dirty. If you don't have any issues, I guess you don't need the patch cable but I've seen brand new jacks cause problems on the effect loop of the jube. I just do it out of habit now and never had a problem with volume swell, flickering noises and whatnot since.

 

[GibsonSlash]

Thanks for the explanation.

jcmjmp, I just noticed my MXR M108 (pluged into loop) show signal clipping (Volume and Gain slider blink) when I turn up either the volume or gain slider up, is that normal?

 

[Jack71]

Thanks for the biasing help guys. I just got a quad of Valve Art EL34's that I'm going to throw into the Slash tomorrow. We'll see how it goes. I've heard good things about the VA tubes.

Btw, what's the suggested method of cleaning the fx loop jacks? Can I do it from the outside or do I need to remove the chassis from the headshell?

 

[steelhorse]

Thanks for the biasing help guys. I just got a quad of Valve Art EL34's that I'm going to throw into the Slash tomorrow. We'll see how it goes. I've heard good things about the VA tubes.

Btw, what's the suggested method of cleaning the fx loop jacks? Can I do it from the outside or do I need to remove the chassis from the headshell?

I'm sure others may be more elaborate but all I do is spray some contact cleaner on one of the cable ends and run it in and out a few times.

 

[GibsonSlash]

will excess of the contact cleaner drip inside the jacks and short circuit the amp?

 

[jcmjmp]

will excess of the contact cleaner drip inside the jacks and short circuit the amp?

Let it dry out before you turn the amp on. Also, Steelhorse's method will not work for the effect loop jacks on the Jubilee.

Take the chassis out of its box and spray contact cleaner with a cable inserted right on the contacts where the little metal tabs lift up with the cable in.

 

[GibsonSlash]

jcmjmp, I tried pluging a short patch cable in effect loops jacks, and I noticed a slight volume drop compared to nothing plugged to the efect loops jacks.

so what does it mean?

 

[jcmjmp]

jcmjmp, I tried pluging a short patch cable in effect loops jacks, and I noticed a slight volume drop compared to nothing plugged to the efect loops jacks.

so what does it mean?

It means that either your jacks are dirty (not the normally closed tab), you have a cheap cable, too long a cable or maybe a tired jack that needs its ground re-tensioned. I use a 12" (inches) patch cable.

 

[steelhorse]

Let it dry out before you turn the amp on. Also, Steelhorse's method will not work for the effect loop jacks on the Jubilee.

Take the chassis out of its box and spray contact cleaner with a cable inserted right on the contacts where the little metal tabs lift up with the cable in.

I'm not exactly disagreeing with this posting but rather asking for clarification.

Also what's different about the fx loop input jacks of the Jubilee that any other amp of that era?

I'm trying to find logic in this post because running a jack in and out several times with contact cleaner on it vs. taking a chassis out inserting the jack and spraying the contact cleaner on then, seems very close imo. The only parts that get cleaned are the points where the jack makes contact anyway.

Apparently the logic behind removing the chassis is to clean non contact points, which would be more thorough possibly but I'm not sure any more effective, otherwise each time we had to clean sockets or inputs we'd have to remove the chassis and spray inside the amp and that's probably overkill.

 

[jcmjmp]

I'm trying to find logic in this post because running a jack in and out several times with contact cleaner on it vs. taking a chassis out inserting the jack and spraying the contact cleaner on then, seems very close imo. The only parts that get cleaned are the points where the jack makes contact anyway.

Steelhorse - The effect loop on the jubilee is not buffered. When nothing is plugged into the effect send/return, it is the jack's normally closed connection that's letting the signal through from V2b to V3a. The jack itself has a built-in switch, hence the normally closed term I've been using. When nothing is plugged in, the jack's switch is closed letting signal through. That is considered the normal state, which is why its called a normally closed switch on the jack.

With full access to the switch's internals and a cable inserted, you can now spray contact cleaner right on the switch's contacts because they are now open (because a jack is plugged in. Cleaning those contact points are very important on the jubilee. It is the #1 source of intermittent behaviour and volume drops and swells.

 

[steelhorse]

ok, that makes sense.