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

[edge59AlnicoIV]

When I get a chance I will open my Mark III up and see if the resistors are oriented as yours, could be they put them in opposite places, that might lower the filament voltages, do you have a voltage meter to test the filaments? Be careful if you do this yourself there are voltages as high as 500 vdc in these amps! The filament voltages should around 6.3 vac though the diode rectifier here makes it DC for the relay and DC filament on the first 12ax7 in some amps. I just found this pic online that shows the orientation of those resistors.

 

[Henz]

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

Ty for the info! I have a DRG C+. With 220k's. I typically use 6l6's in outer. I can't wrap my head around this negative bias. My outers idle at app -48vdc. If I put the 150k's in, being less resistance, wouldn't it raise the -48? Which means hotter or cooler? The negative sign stumps me. Based on the old school number line , any number left of zero is negative, and any number to the right is positive (higher). that said, is for example, -10 higher than -20? Going more toward zero. Am I over thinking? Shed some light on my confusion.

 

[Boogie-knight]

When I get a chance I will open my Mark III up and see if the resistors are oriented as yours, could be they put them in opposite places, that might lower the filament voltages, do you have a voltage meter to test the filaments? Be careful if you do this yourself there are voltages as high as 500 vdc in these amps! The filament voltages should around 6.3 vac though the diode rectifier here makes it DC for the relay and DC filament on the first 12ax7 in some amps. I just found this pic online that shows the orientation of those resistors.

Thank you so very much for taking the time to reply and for the details provided, this is appreciated far more than you could possibly realise!

Thanks for confirming for me that those 4 resistors on the board are of the correct value and tolerance, at least I can rule that out as the reason for the amps behaviour. It seems like you have a great knowledge of these amp with first hand experience in repairing them so I would be curious to know if based on the info that I have provided regarding the amps poor performance in Class A mode with the EL34's in the outer sockets, if you have any ideas what would possibly be causing this. Could screen resistors of an incorrect value be to blame? I have attached some photos showing an overview of the board and of the individual tube sockets that detail their screen resistors etc, does your expert eye detect suspicious? Any insight or suggestions would be greatly appreciated.

 

[edge59AlnicoIV]

Thank you so very much for taking the time to reply and for the details provided, this is appreciated far more than you could possibly realise!

Thanks for confirming for me that those 4 resistors on the board are of the correct value and tolerance, at least I can rule that out as the reason for the amps behaviour. It seems like you have a great knowledge of these amp with first hand experience in repairing them so I would be curious to know if based on the info that I have provided regarding the amps poor performance in Class A mode with the EL34's in the outer sockets, if you have any ideas what would possibly be causing this. Could screen resistors of an incorrect value be to blame? I have attached some photos showing an overview of the board and of the individual tube sockets that detail their screen resistors etc, does your expert eye detect suspicious? Any insight or suggestions would be greatly appreciated.

look at the brown wire below the green resistor, it looks like it is not soldered securely, can you confirm that is or not?

 

[edge59AlnicoIV]

Your

View attachment 4613look at the brown wire below the green resistor, it looks like it is not soldered securely, can you confirm that is or not?

very welcome, I don't know everything and always learning, but happy to share when I can. The

View attachment 4613look at the brown wire below the green resistor, it looks like it is not soldered securely, can you confirm that is or not?

It looks like the 2 resistors soldered in parallel may be the bias fix, red, red brown orange should be the 220k (actually 221k) if you are not familiar with color coded. The red, purple, red resistor on top (what it looks like) 2.7k maybe unsolder on end, leaving the other end attached and see if it makes difference with bias voltage.

 

[edge59AlnicoIV]

Your

very welcome, I don't know everything and always learning, but happy to share when I can. The

It looks like the 2 resistors soldered in parallel may be the bias fix, red, red brown orange should be the 220k if you are not familiar with color coded. The red, purple, red resistor on top (what it looks like) 2.7k maybe unsolder on end, leaving the other end attached and see if it makes difference with bias voltage.

I would make su

re that pin 3 wire (Brown most likely) is soldered properly as it is a high voltage plate wire from the output transformer, it could be a cold solder joint, causing the issue.

 

[edge59AlnicoIV]

View attachment 4614

I would measure the negative voltage before and after you raise one end of the 2.7k resistor, without the outer tubes and see the difference, then in standby put the outer tubes in socket, warmup and check your negative bias voltage. Hopefully the it will show more negative voltage @ pin 5. Back in the 80's if I remember correctly the outer sockets were set to run about -55 volts with 6CA7's that my Mark III came with. Should say on the schematic. Well I was off a bit, here is the voltages on the schematic... The -55 was for an earlier Mark IIB that ran a hotter bias. Mesa cooled their bias with the mark III so I understood, as to save the power tubes through the warranty period of 6 months back then.

 

[edge59AlnicoIV]

Ty for the info! I have a DRG C+. With 220k's. I typically use 6l6's in outer. I can't wrap my head around this negative bias. My outers idle at app -48vdc. If I put the 150k's in, being less resistance, wouldn't it raise the -48? Which means hotter or cooler? The negative sign stumps me. Based on the old school number line , any number left of zero is negative, and any number to the right is positive (higher). that said, is for example, -10 higher than -20? Going more toward zero. Am I over thinking? Shed some light on my confusion.

Hello Henz, 150k should run cooler for 6L6 use, 220k cooks 6L6's more but works with EL-34's/6CA7's. If I remember me III Green stripe came with 150k's (mine came with 6L6) and was wired Pentode mode, louder and faster attack than Triode mode. The Mark IVB had a switch to be either Triode/Pentode. My Mark IVB had 220k outer socket resistors and came with EL-34's in the outer socket pair

 

[edge59AlnicoIV]

Your

very welcome, I don't know everything and always learning, but happy to share when I can. The

It looks like the 2 resistors soldered in parallel may be the bias fix, red, red brown orange should be the 220k if you are not familiar with color coded. The red, purple, red resistor on top (what it looks like) 2.7k maybe unsolder on end, leaving the other end attached and see if it makes difference with bias voltage.

View attachment 4616I just paralleled a 220k with a 2.7k and I got approximately 150k. So with all resistors in place, the 2.7k in parallel with the 220k, it would seem that it is biased for 6L6's in the outer sockets, so how do the 6L6's sound in the outer sockets? I think you mentioned it here somewhere So if you want to try EL-34's I would raise one end of the 2.7k, leaving the other end soldered, so you can put it back should you prefer the 6L6 sound. I hope this helps, but definitely check that brown plate wire that looks iffy, looks as it had been tampered with at some point

 

[edge59AlnicoIV]

Here's a more clear pic of what I am suggesting, being very careful with high voltages here! Only unsoldering the one end of the 2.7k per outer socket.

 

[edge59AlnicoIV]

Ty for the info! I have a DRG C+. With 220k's. I typically use 6l6's in outer. I can't wrap my head around this negative bias. My outers idle at app -48vdc. If I put the 150k's in, being less resistance, wouldn't it raise the -48? Which means hotter or cooler? The negative sign stumps me. Based on the old school number line , any number left of zero is negative, and any number to the right is positive (higher). that said, is for example, -10 higher than -20? Going more toward zero. Am I over thinking? Shed some light on my confusion.

Yes, it is confusing... So with a higher - voltage, it is actually cooler cause its a negative voltage, the lower the - number, the hotter the bias. I know there is a better description out there for this, but as you will see in the schematic I posted, the inner 6L6 pair (Simul-Class) -56 and -47 export, and -45 and -37 export in the outer Class-A sockets.

 

[Joey1970]

Back when I used my Bias-Rite, the STR 415's would run about 42Ma in the outer sockets of the 105 C+, and the STR-416 would run in the mid 20Ma in those same sockets. No changes made to the circuit. Let it eat. If you are trying to beef up your sound by increasing the bias on the Class-A sockets, you are pissing in the wind.