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?

 

[user 41014]

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.

 

[user 41014]

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]

deleted.

 

[gts]

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

 

[Boogie-knight]

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

Unfortunately I don't think this would be a viable option as I'm based in the UK. From what I understand Mesa don't even have a distributor here, and when I recently got in touch with Mesa asking about a supplier of their Official Mesa tubes here in the UK they were unable to help.

 

[Boogie-knight]

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..

Many thanks for your reply and insight, this is really appreciated. Apologies for my delay in replying to your post but I have been caught up with some family matters the past couple of months and I'm only just getting back to trying to solve the mystery with my Mark III.

Upon further testing , this is what I have found...
With EL34's in the two outer sockets and 6L6's in the two inner sockets, as is meant for this amp, I have the problem as describes above; a large drop in volume when in 'Class A' mode and when playing a guitar through the amp, any loud notes or chords tend to distort in an unpleasant way.

However...
With 6L6's in all four sockets, no problem when switching between 'Simul-Class' mode and 'Class-A' mode.

And surprisingly...
When I have the EL34's in the two INNER sockets and the 6L6's in the OUTER sockets (opposite to how this amp is suppose to operate), again I'm finding no issue when switching between 'Simul-Class' mode and 'Class-A' mode.

Im hoping that this information is able to provide some clue to someone that has far more knowledge than I clearly have in this matter, as to what is going on here. What would this suggest if 'Class A' mode only works properly and without the volume loss when the EL34's are in the inner sockets but not when they are in the outer where they are suppose to be? Would this simply mean that the tube sockets on my Mark II are just in the wrong location and that I should use the EL34's in the inner positions or could it be that there is something more at play here? I would be most grateful if anyone is able to shed any light on this for me.

 

[Eevil]

Many thanks for your reply and insight, this is really appreciated. Apologies for my delay in replying to your post but I have been caught up with some family matters the past couple of months and I'm only just getting back to trying to solve the mystery with my Mark III.

Upon further testing , this is what I have found...
With EL34's in the two outer sockets and 6L6's in the two inner sockets, as is meant for this amp, I have the problem as describes above; a large drop in volume when in 'Class A' mode and when playing a guitar through the amp, any loud notes or chords tend to distort in an unpleasant way.

However...
With 6L6's in all four sockets, no problem when switching between 'Simul-Class' mode and 'Class-A' mode.

And surprisingly...
When I have the EL34's in the two INNER sockets and the 6L6's in the OUTER sockets (opposite to how this amp is suppose to operate), again I'm finding no issue when switching between 'Simul-Class' mode and 'Class-A' mode.

Im hoping that this information is able to provide some clue to someone that has far more knowledge than I clearly have in this matter, as to what is going on here. What would this suggest if 'Class A' mode only works properly and without the volume loss when the EL34's are in the inner sockets but not when they are in the outer where they are suppose to be? Would this simply mean that the tube sockets on my Mark II are just in the wrong location and that I should use the EL34's in the inner positions or could it be that there is something more at play here? I would be most grateful if anyone is able to shed any light on this for me.

Sounds like a trip to a mesa tech would do wonders

 

[Boogie-knight]

Sounds like a trip to a mesa tech would do wonders

Unfortunately I'm having difficulty finding any Mesa tech remotely close to where I am based that is able to work on it. My feeling is that at some point a previous owner of the amp has had it modded in some way and if I was just able to establish the nature of this mod and why this was possibly done then that would be most helpful to me.

 

[Boogie-knight]

Following on from my post above, I have taken a look inside at the board and wiring to see if there are any obvious clues and this immediately looks suspicious. Bad soldering aside, can anybody tell me if these resistors are of correct value and tolerance?

 

[edge59AlnicoIV]

I have owned and worked on various Mark II, III, IV and V's over the years. The earlier Mark IIC+, III, IV were setup to run either 6L6 or EL34 (In the Simu-Class amps only!) in the outer sockets but were more biased to run 6CA7/EL-34's. I found later in my Mark V that they did not recommend running EL-34's in the outer sockets due to the iffy quality of EL-34's at the time and so the V was setup to run all 6L6's. I noticed on the outer 6L6 sockets the bias feed resistors were 150k opposed to 220k found on the earlier IIC+, III, IV EL-34 equipped outer sockets, the 150k resistor sets the bias on the 6L6's to run properly, where the 220k would run warmer bias (bigger value 220k, more negative bias, lower value 150k, lower negative bias.) for EL-34/6CA7's. A friend of mine owns a 85' IIC+ that he runs all 6L6's and the outer 6L6's running a bit cold, it had the stock 220k bias feed resistors, so if he had EL-34/6CA7's it would have been in the ball park. I swapped out the 220k's for 150k that I seen in the Mark V all 6L6's, and it works great! I also did a Mesa NO-NO! added a bias trim to bias different manufactured tubes for fine tuning, Mesa prefers us to use their select power tubes which I understand is to make it easier for the user and not to have to bother with bench fees for biasing etc. just buy their tubes plug and play, and for the most part that's great, your sound should be pretty spot on when replacing with their recommended tubes, but in this case if your amp was setup from the factory to run EL-34/6CA7's, and you have 6L6's in the outer sockets, they will not bias properly without the 150k bias feed resistors. The inner pair should be 6L6 unless it was modified to do so, EL-34's may work but the screen grid 470 ohm is too small and will eat the power tubes quicker. They should be 1k screen grids to run EL-34's in the inner pair, though I would not do this, leave the stock 470 ohms there if they have not been changed and run 6L6 there. If you want to run EL-34's effectively you need to make sure the outer pair bias feed resistors are 220k, 150k for 6L6. I hope that makes sense and is helpful

 

[edge59AlnicoIV]

Following on from my post above, I have taken a look inside at the board and wiring to see if there are any obvious clues and this immediately looks suspicious. Bad soldering aside, can anybody tell me if these resistors are of correct value and tolerance?

In your above pic, these resistors correspond to the resistors on the schematic in the red circle. I have seen these carbon film resistors come from the factory, looks like someone took them out for some reason and put them back in? The burnt orange wire...