Red Plating and the phase inverter.

[kc2eeb]

After an exhaustive trouble shooting episode including switching output
transformer and the power amp board in my RM50, one major cause of
overdriving the output tubes and making them red plate was a JJ 12ax7 in
the phase inverter/driver position.
Switching it to a GE5751 and changing nothing else dropped the idle current
1.5 ma. Corrected the bias and played the amp wide open. (JJ 6V6s)
No red plate. The usual tube used for many years in Fender and many other amps for PI was a 12AT7, and in some others, a 12AU7.
Maybe one of the causes of the rash of EL34's that red plated last year
was the PI letting the control grids of the EL34s swing into positive.
That'll do it.

 

[m0jo]

Did the problem occur with several JJ 12ax7s? (ie. is it a structural problem with that model?)
Or did you leave the same one in?

 

[sweetrock]

In fact , I have read some good reviews about JJ ECC83 for PI position .

There must be some tube failure . :?


Hakan

 

[m0jo]

In fact , I have read some good reviews about JJ ECC83 for PI position .

There must be some tube failure . :?


Hakan

That's why I asked if it was one particular tube that caused this, or JJ 12ax7s in general ..

Although it could be the combination with 6V6s .. they can handle less power right?

 

[sweetrock]

In fact , I have read some good reviews about JJ ECC83 for PI position .

There must be some tube failure . :?


Hakan

That's why I asked if it was one particular tube that caused this, or JJ 12ax7s in general ..

Although it could be the combination with 6V6s .. they can handle less power right?

You are right .
6V6 s have less output power and only the JJ s can handle the 500 V plate voltage .
I want to try them in RM100 but still affraid to ruin my amp .

I can use them in my THD BiValve but it has an additional low voltage setting for 6V6 s.

Hakan

 

[kc2eeb]

The JJ 6V6s that red plated with the JJ 12ax7 did not when the JJ 12ax7 was changed to the GE 5751. I don't think this is a problem with tube quality.
The JJ 12ax7 tested good before and after on my TV-7 and my Longhin Lo460. It's more the particular combination.
First, the red plating occurred with all the master volumes full up.
I think it's simply all the gains adding up. The typical tube for the PI position
until the end of the tube era was a 12AT7 or a 12AU7. Both have higher current not gain. When the grid of the output tube(s) swings too positive
it will draw too much current. An easy way to see what's going on is to leave the multimeter connected to the amp after you set the bias and play the amp as loud as you will be in use. The meter will show the current draw for the output tube.

 

[kc2eeb]

An update.
Just finished playing throught the amp with a pair of new JJ 6V6s with the
a 5751 as the PI. All good. I will post pictures as soon as I get them onto photobucket.com unless anyone knows how to post them directly.

 

[sweetrock]

An update.
Just finished playing throught the amp with a pair of new JJ 6V6s with the
a 5751 as the PI. All good. I will post pictures as soon as I get them onto photobucket.com unless anyone knows how to post them directly.

Could you please tell us the pros and cons of 6V6s over 6L6s besides the output power ?

Hakan :wink:

 

[kc2eeb]

The basic difference is the tone, and being able to push the output tubes into overdrive distortion which is very different than preamp distortion, at
a lower volume level than EL34s, 6L6s, etc. From what I read in this forum, most players form their tone from the preamp and then adjust overall level with the master volume. Using 6V6s (JJ) is more in the school of turning the volume of a pre-master volume amp like a Fender Deluxe or Tweed Bassman full up. Different tone and sustain characteristics.

 

[sweetrock]

I see ,
What is your biasings for JJ 6V6s ?

20-25 mA ?

And does the RM Amp platform ready to use that way ?

Because there is no info about 6V6s in the manual .


Hakan

 

[JKD]

I don't think you should bias JJ 6v6 over 20mA due to the high Plate Voltage on MTS amps...but that's just from memory and mine is quite often faulty.

 

[sweetrock]

I have read from somewhere in this forum that ,
It should be ,

6V6 Bias 19-21 mV DC

Plate voltage is 500 V

But I've never tried and don't know if the amp accepts that settings ? :roll:

Hakan :wink:

 

[kc2eeb]

Actually, I bias them at 18ma, because I'm going to push them fairly hard. The recommended range is 19-21ma. In actual use they will draw over 100ma under load, i.e. playing loud.
Something also to take into account is PI distortion and over driving the
output tubes. That's why, in troubleshooting the red-plating issue I was having, I switched to a 5751 for a PI.
From what I can deduce from the way most of the forum members use their amps, very few, especially the RM100 users turn their master volumes all the way up.
The RM50 is fine to use 6V6s with provided the particular 6V6s can take 500-515 volts on the plates. JJ's can. [/u]

 

[Mike361]

I have run jj6v6 with my rm50 biased at 19 or so. Sounded good but only with lower gain modules. I like to push the master higher than the preamp to get a more open sound and I did like the v6 but eventually went back to 6l6. The problem I always have is I want the amp to sound it's best from Fender To Marshall and to vox ect... and it's like trying to tap dance tubes. I guess that would be one advantage to the rm100, Mixing power tubes.

 

[kc2eeb]

And then God created........the RT2/50