Mark III simul-class - Issue in Class A mode with EL34's in outer sockets.

[Boogie-knight]

Hi there
First time poster here and I'm hoping that someone can assist me in diagnosing an issue that I am experiencing with my Mark III.

I have recently replaced the power tubes in my Mark III simul-class (red stripe) and following the advice and instructions given in the user manual I have purchased two 6L6's for the inner sockets and two EL34's for the outer sockets.

However, when taking the old power tubes out of the amp I noticed that instead of having two 6L6 tubes in the inner sockets and two EL34's in the outer sockets, the previous owner had 6L6's installed in all 4 of the sockets, something that the manual advises against.

With the new tubes installed, although I am able to play my guitar trough the amp when in Simul-Class mode, when I switch to 'Class A' mode where just the 2 outer tubes (EL34's) are in circuit there is a significant drop in volume and when I turn the volume on the amp up to compensate for this, the sound is still low and also distorted and breaking up in an unpleasant way, even when on the Rhythm 1 channel. However, when I put the old 6L6''s back in the two outer sockets there is no issue when switching from 'Simul-Class' to 'Class A' mode.

Could it be that the amp has had a bias mod that has resulted in it not being able to function properly with EL34 tubes in the outer sockets as originally intended for this amp and as described in the user manual? And If the amp has received such a modification, how would i be able to visually confirm this when looking inside? Any ideas or insight into this would be very much appreciated.

 

[Boogie-knight]

Since my original post above I have tried replacing the two El34's in the outer sockets with a pair from another amp so that I can rule out that it's not the new tubes that are at fault and causing the problem when the amp is switched to 'Class A' mode, but the issue with the amp remains the same even with these tubes swapped out. Anyone have any ideas or suggestions?

 

[GJgo]

The outer Class A slots can accept 6L6 or EL34, however it's important to have them within a reasonable bias range. It's possible yours are not.

That said, it might be time to have the amp serviced.

 

[DavidMenke]

Hi there
First time poster here and I'm hoping that someone can assist me in diagnosing an issue that I am experiencing with my Mark III.

I have recently replaced the power tubes in my Mark III simul-class (red stripe) and following the advice and instructions given in the user manual I have purchased two 6L6's for the inner sockets and two EL34's for the outer sockets.

However, when taking the old power tubes out of the amp I noticed that instead of having two 6L6 tubes in the inner sockets and two EL34's in the outer sockets, the previous owner had 6L6's installed in all 4 of the sockets, something that the manual advises against.

With the new tubes installed, although I am able to play my guitar trough the amp when in Simul-Class mode, when I switch to 'Class A' mode where just the 2 outer tubes (EL34's) are in circuit there is a significant drop in volume and when I turn the volume on the amp up to compensate for this, the sound is still low and also distorted and breaking up in an unpleasant way, even when on the Rhythm 1 channel. However, when I put the old 6L6''s back in the two outer sockets there is no issue when switching from 'Simul-Class' to 'Class A' mode.

Could it be that the amp has had a bias mod that has resulted in it not being able to function properly with EL34 tubes in the outer sockets as originally intended for this amp and as described in the user manual? And If the amp has received such a modification, how would i be able to visually confirm this when looking inside? Any ideas or insight into this would be very much appreciated.

Running the amp without simul will give you 15watt's only using the EL34, when switching to simul all tubes are running. By Mesa's information, you can use all 4 6l6gc in a simul amp. The lower wattage with just the El34, do not give the same sound as running all 4 tubes. Again the bias may be off. Another question, did you buy Mesa tubes from Mesa Boogie site. They are made for Boogie amps, and bias is set for Boogie. Although a bit more expensive, but may be worth it! Cheers, by the way I have a simul SOB amp. Made in 85' and also had a Mark III which left the collection.

 

[mark1406]

I think the first thing you need to know is the bias of the output valves. Typically, 6L6’s require a larger negative bias voltage, this would mean the EL-34’s aren’t running in class A. The screen is tied to the plate so this will result in less wattage from the valves.

It doesn’t account for ugly distortion though. Are you able to service the amp yourself?

Regards

Mark

 

[Boogie-knight]

Thank you for all of your replies so far, really appreciated.
Even though various sites online and the manual clearly states that the mark III Simul-Class should have two EL34's in the outer socket and two 6L6's in the inner sockets, I have just seen in the Mark III manual that:

"Series 300 amplifiers with Simul-Class have a different tube layout than the regular 75-watt Simuls. In the Series 300 Simuls, the EL34/Class A sockets are the middle pair of sockets - that is, the 3rd and 4th sockets as you count across. The other sockets (1&2 and 5&6) use 6L6's."

I find this interesting being that my amp clearly is not liking the EL34's in the outer sockets but is ok with the 6L6's in these locations. I see no reference on my Mesa Boogie Mark III to it being a 'Series 300' which I assume would be made obvious but would this be the case. How can I tell if it is a 'Series 300' as I'm finding very little info of these online?

 

[Joey1970]

Left to right a series 300 should be 6L6 1&2, EL-34 3&4 ,6L6 5&6. At least that is what I understand for a Mark 3. Series 300 Boogies are a bit rough on current production tubes.

 

[Boogie-knight]

So all Series 300 amps would have 6 power tubes. No Series 300 amps were ever produced with just 4 power tubes?

 

[electric mayhem]

So all Series 300 amps would have 6 power tubes. No Series 300 amps were ever produced with just 4 power tubes?

Correct- aka coliseum

 

[Boogie-knight]

Correct- aka coliseum

Ah, that definitely rules out mine being any kind of 300 series as it only has the 4 power tubes.
Just out of interest though, I did try putting the EL34's on the inside sockets and the 6L6's on the outside and with this configuration I am able to switch between 'Simul-Class' and 'Class A' without any problem but as soon as I put the EL34's in the outside sockets and the 6L6's in the inside sockets as shown in the manual I am then getting the significant volume drop when in 'Class A' mode and with the turn volume turned up, the sound in breaking up and sounding horribly distorted.

 

[electric mayhem]

It's always possible someone made some changes. I like 4x6L6 so just plug and play is the easy no solution. Could someone have changed the outside screen R's? I believe those are 2.7k (red purple red [silver/gold]). But a guess is someone changed the bias to the outer tubes..

 

[Henz]

With my experience, SC is a travesty. Just open her up and take A peek. If you aren't comfortable. doin this and are really not sure what to look for, take it to someone who knows. Blessings.

 

[gts]

Send it to Mesa to get it fixed. Just an idea....