Mark IV with 6V6's Update

[Monsta-Tone]

Last night I was playing my Mark IV. I've had a lot of family at my house for the last week and haven't had much time.

I was running the Mark IV with 4 6V6's. It kept bothering me that I couldn't use all of the functions and that I had to pay attention to the Power Switch position.

I was planning of modifying the amp so that I could use all of the functions.


Last night I had a hard time getting harmonics and really thick distortion. It was one of those rare moments when all I wanted to play was Metal.

I decided to put the 6L6/EL-34 combo back in and try it out.

I loved the tone with the 6V6's in it, but the EL-34's (JJ E34L Blue) had the meanest distortion, and MANY more harmonics.

I think that this amp needs to have the EL-34's in it for now. I'll still be modifying the amp to add Bias Control for each set of tubes. This way I can run any tubes that I want.

I might sell some gear and get another Mark IV just so that I can run 6V6's in it. Great Blues tones.

 

[screengrid]

Hi, nothing against a bias pot modification with a Mark IV. But one must consider the fact, that the power stage is simul class, IMO ! The class A section run's hotter ( more idle current draw ) than the class AB section, althought I wouldn't say that the class A section is true class A, more class AB 1 IMO. But that doesn't matter, at least for me, tone is what counts and the MK IV`s tone is superb also without any modification, IMO.
Also one should be aware about the different power modes , the MK IV offers.So of course the idle current draw is higher in Full - than in Tweed Power.
For example here some measurements I did with my MK IV some time ago. To keep the outlet voltage stable, I used a variac.
the power tubes were 4 x GLG GC STR 430 from Mesa Boogie, color code of the outer pair was gry and the inner pair was blue.
All current values in mA
Full Power Simul Class / 230 V~

outer left / inner left / inner right / outer right
39,2 / 25,0 / 26,0 / 38,5

Full Power Simul Class / 220 V ~
35,8 / 24,1 / 25,1 / 34,6


Tweed Power Simul Class / 230 V~
28,3 / 16,6 / 17,3 / 27,3

Tweed Power Simul Class / 220 V~
26,2 / 15,5 / 16,1 / 25,5

BTW, my experience is, that the color code also doesn´t matter regarding the idle current draw, although somewere in the net one can find those color code scales.
Some time ago I asked Mesa Boogie directly and I got the friendly answer, that the color code is for matching purposes only.
All Mesa boogie tubes are selected and matched with a medium idle current draw, say around 25-30mA for a 6L6.

 

[Monsta-Tone]

Wow, the Outer tubes do run hot in Full Power, but it is also relevant to the Plate Voltage & Current. So without those readings, it is hard to tell just how hard the tubes are really being pushed.

I had fully planned on adjusting the Cathode Bias resistors of the outer tubes as well.



I really don't like the tone of the Mesa tubes very much. They seem to last forever, but they just sound sterile to me.


I only want to adjust the bias so that I can experiment with different tubes. Having an Amp Repair Shop means that I get obscene deals on modern production tubes & speakers.

I appreciate the fact that you wouldn't mod your amp, but it's not a Vintage Fender, so I personally don't mind.



Mesa told me the same thing about their tubes. They kind of hinted that the range of the colors would be between Groove Tubes" 5 & 6, depending on the color.

 

[screengrid]

Hi, the 49,5 mA for the left outer tube in Full power was a typo, sorry. But you are right, fearly hot, ar least for a boogie. A lot of people are the opinion, that Mesa amps like to sound a little bit steril or lifeless ( mainly clean sounds in comparison to a good fender ) because they would be biased too cold !
I measured also the idle current of other Mesa Boogie tubes with my MK IV and I noticed some variations. For example with the 6L6 STR 420 ( green / blue color code ) I had a reading of max 28 mA in the class A section / Full Power / 230 V ~ .
The "hottest" readings I got with a pair of Svets / 6L6 STR 454, around 45 mA in the class A section ( Full Power / 230V ~ !
At time I use a quartett of 6L6 STR 425 5881 Yel color code. With this tubes the idle current in the class A section is around 35 mA, also Full Power / 230 V~
With the Svets I had the best clean sounds for my personal taste, but with the 5881 I'm still very satisfied with the tone in all modes of my MK IV.
BTW, I never had to open my MK IV until today, so I don't know exactly, what components are used. Could you tell me, what kind of resistors are used, I guess no carbon composit resitors but metal film ones ?

regards

 

[Monsta-Tone]

Every brand, and every tube will have a slightly different Bias voltage.

This is due to the Transconductance and the internal impedances of the tubes themselves.


That is a very good justification for Mesa wanting us to use their tubes only, but I still want to experiment. I spent tons of my money on my amp, and I want to use the tubes that I want to use (sounds fairly juvenile doesn't it?).


You need to get a Bias Tool that will also read the Plate Voltage. The Bias Current (Voltage across a 1 ohm resistor in the tester) should always be set according to the Plate Voltage and the Power Handling Characteristics of the tube itself. The general consensus is that the Bias should be set at 70% of the maximum Plate dissipation the tube can handle.

Oddly enough, as the Plate Voltage goes up, the Bias Voltage setting should decrease.


The Dan Torres book, "Inside Tube Amps," has Bias Charts in it. They show the Plate Voltage and Bias Voltage (which is a range, this allows for experimentation) for most of the common Power Tube types. The book is not very well written, or rather, not very well edited, but it does nave some good information in it.







As for the Mark IV's components. I haven't spent too much time looking at the board, but mostly it looks like Orange Drop (715P?) capacitors, and metal film resistors.

I haven't seen any Carbon Comp resistors in Mesa amps, but I also haven't had the pleasure of looking in a Vintage (pre-1990) Mesa amp.

 

[screengrid]

Hi, I know I know....... of course you are right. Normally, when I´m repairing a tube amp, I like to do it exactly the way you mentioned. But as I already said, until today there was no need to carry out the chassis of my MK IV and just for fun it was too much effort for me.
But as you already said, regarding Mesa tubes all the selecting, matching and testing is already done by Mesa themselves, around 7 tube parameters as far as I could read, the mutual transconductance included.

But can we still believe it, do you ? Oh, stopp................ I don't wan't to start a wild discussion here, although a honest discussion about this topic would be very interresting. To say it clear, I like Mesa Boogie high quality amps still so much over a long time already, I like my MK IV, never would give him away, but nevertheless from a technically view, a fair factual discussion should be allowed.

In this sence :wink:

 

[Monsta-Tone]

Everybody was gone yesterday, I had the house all to myself.

With the amp on Triode, Class A, & Tweed Voltage, it was incredibly loud when the Output level was on 4.

I can't wait to unleash this thing. We had our 1st son 3 1/2 years ago and I haven't had much playing time since then. It was really great to play loud, even if it was for only 30 minutes.



The Weber Thames speaker sounds incredible, as do the JJ tubes.