Strategy 400 Tube Mod?

[maeshuggah]

Hi, I'm considering getting my strategy 400 modified the accept all el34's
(12 total) as I prefer the el34 sound. also a deep/depth knob mod per side to
adjust low frequencies. Has anyone done any mods to their strategy's?

I know that the Mesa triple rectifier solo 150's can take all el34's, so my
strategy should be more than able to use all el34's by changing some components?

I called MB about the deep mod, they said it would be arround $150-250.00
Anyone care to comment?

 

[fatboy135]

the mod is very easy, you need: paper, pencil and an hp48g :lol:
ok being more serious, it is an easy mod, for anyone that knows how to fix a bias point with resistors, 6l6 and el34 has different features and different values to fix the bias ( the bias is the working point of the tube, the DC voltage and DC current that polarizate or fixed where the tube must to run ) 99% of guitar amps runs on classical AB zone, so thinking about the 400 watts you are running and maybe the enrich of your electrical company will do with you I recomend to you to fixed the bias as low as you can ( the lower values allowed for the tubes, the typical - volts that appears on the grid ) even you can do another mods, thinking that you must to mod all the circuitry I think that you could have to bias point switchable externally with a relay. for example power off some tubes an polarizating the others on high bias point to get a class AB1....
you could do a lot of mods, thinks that electronics allows you to do all your mind could imagine.
After this long answer topic I will give you some ideas to do the mod:
first of all you must to download the el34 datasheet, then you must to fixed the bias point that you want, fixing on the graphics of the datasheet the exactly value, then a voltage grid and an anode current is shown to you as the result to the bias point that you fixed on it, the last thing to do is solve the classical circuit equation of the DC bias point of the tubes, with resistors on anode, cathode and grid, then you obtain the values of the resistors to replace in your original circuit. don´t forget the decoupling cap on the cathode. and pleas dón´t try to get more watts that the el34 could give, because you could blow the tubes, good luck it is a very excite job for anybody that loves electronics

 

[fatboy135]

wait I answer you with the mod to get a 6l6 to el34 circuit :?
I don´t know anything about the low frecuencies that you say, maybe we must to research on it

 

[maeshuggah]

Thanks for the reply

I know that the depth boost knob is pretty easy to do on marshall amps.
I'm hoping it wouldnt be too hard on the Strat 400.


I took a look inside my strategy and found the the 4 tube sockets
at the rear of the amp are running 5 watt 370 ohm resistors?
(manual specs 6L6gc/el34 for these.) while the rest of the sockets
are running 2 watt's (manual specs 6L6gc for these.)
so I'm assuming that replacing these screen resistors to all 5 watts
would allow me to run all el34's.



From what I've read the net el34's require 5 watt 370 ohm screen resistors while 6l6's require 2 watt 450 ohm.


Currently I'm running the following tube compliment. 6l6gc x8 - KT88 x4

If a triple rectifier can run all el34's SO CAN I :twisted:

 

[fatboy135]

oops so i answer a part of the question, nice.
ok the first step is to get the strategy schematics and then send it to my email, i will evaluate wich is the best mod for you, anyways think that you must to replace anode, cathode and grid resistors, not only the 5 watts resistors, at least 3 resistors per tube

 

[prs1999]

Did Mesa say the Depth mod would be foot switchable (as the 2:90 amps)
and /or able to run thru and be switched via the TriAxis?
I have a 2:95 simul power amp that I might consider the depth mod.
Thanks !

 

[maeshuggah]

fatboy135

Here is the Mesa strategy 400 schematic:

http://www.tubefreak.com/str400.gif

prs1999

I had a 2:95 simul, great amp!
Mesa said the deep mod would be a rocker switch for both sides, but I'm sure
they could make it footswitchable. Give them a call their pretty easy to talk to.

 

[prs1999]

hey thanks man. I just didnt want it looking basterdized with anything extra on the front.
Did you do the depth mod on your 2:95?
One more question...do you take out a small loan to re-tube
your Strategy400?
I just re-tubbed ;
TriAxis,
The 2:95 power amp,
and my D.R. Ractifier.
My son is still working in that sweat shop overseas to pay the glass bill...lol

 

[fatboy135]

what nice people are boogie crew, they label the point to measure the bias as bias.
so as I could see on the schematics there are 4 preamp tubes and 6 tetrode power tubes, so I also could see the typical negative feedback to stabilize the tubes.
the resistors you need to change are labeled as 100 kohms just near the bias test point, also the 2,2 kohms, and it is posible that you need to put one resistor just in the cathode ( the terminal that goes directly to ground http://www.allaboutcircuits.com/vol_3/chpt_13/6.html ) also the two resistors that forms the voltage divider on the output of the bias supply, these voltage divider sets the voltage on the bias test point, and the resistors labeled as 470 ohms, 2 watts and the others that appears as 2700, 2 watts, anyway you need to ask the real schematics to boogie

 

[maeshuggah]

Fatboy135, Many Thanks for the info man. I'll see if I can get another schematic from boogie.

Prs1999, Wow that must of cost several benjamins. I try to avoid the urge to
retube, It hurts . I currently use svetlana's in the power amp 6L6gc's &
kt88's. Next time I retube I'm going to retube everthing in my rack with JJ's.

I tried NOS but that was a big $$$ pit. the good part is that sometimes you
find some great NOS and can "donate" them to others more deserving than yourself :wink: