MkIIB Plate Voltage

[Harry the Horder]

Still no answer to my first post ref:Help mkIIb.
I'm still trying to work it out if my plate voltage of 444v is correct then I think my Bias is ok here is my reasoning
Max plate dissipation for a 6L6GC=20-5 watts
plate voltage on my amp reading 444v
Bias in Ma= 20-5X500Divided by 444v
so my BIAS should be around -23Volts
Is this correct or is my plate voltage to low?
Once again Happy New Year. I hope someone replies to my postings soon
Harry the Horder.

 

[Boogiebabies]

Well, the PT should put out 448V to the plates +/- 10%. So a -23 draw only pulling 4 volts extra of the PT sounds fine to me.
The higher the tubes natural plate current, the more it tends to stress the PT. Is tour PT the 100?

 

[Harry the Horder]

Wow! I'm humbled having you reply to my post is an honour!
I'm not sure what you mean when you ask "Is tour PT your 100"
All my specs are in my earlier posting; "Help MKIIB "except I forgot to mention I'm U.K. so we run on 230v-240v Hope this answers your question.Harry.

 

[Boogiebabies]

No problem. Is it an export transformer with the multiple voltage switch on the back or the US version. The 100 is just the part number of the PT.

For expamle, a IIC+ with 478 volts to the plate will draw -22ma on a #6 GT 6L6GE and the plate voltage will be 473V. With a stout Svetlana 6L6 mine draws -35 ma and the plate Voltage may only sag to 470. Under load I am sure that the plate voltage will go down as the tubes draw more voltage to amplify the signal. The Schumachers Mesa used were pretty decent units.

On the IIB Schematic is is like 450 actual plate volts and 448 volts on the screens. You should be just fine. A standard bias for 6L6's at your plate voltage would be 37.5ma. A true 6L6 or modern 5881 is a 25 watt tube.
Yours is actually running where Boogie wanted it to.

 

[Boogiebabies]

I can't seem to get a clean sound on my amp it is ok until I play my bottom strings and then distorts or sounds dirty. I have changed the valves and checked the speaker. Is it a caps job next? and what does LDR stand for?
I've just pulled it apart to get more details.
It is a mkIIb circa 1980 fitted with Altec spkr.Hardwood/Wicker.
Readings I'm getting in 60watt mode plate voltage=452v
in 100watt mode plate voltage=444v
The output valves fitted are 6L6GC (Mesa) 40 AR Yellow
The Bias Readings from Left to Right looking into back of Amp are
V1= -23-9 V2= -26-7 V3= -28-0 V4= -22-4 using Bias tool and VOM.
Any Ideas would be more than welcome I read somewhere that the Bias should be between -48 to -50 in that case it is way of or is it?
Not bad for a first posting Heh! Happy New Year to all.
Harry the Horder.



The best I can remember, Mesa used a relay in the IIB and not an LDR
( Light Dependent Resistor/ Optocuopler) to switch into lead mode.
Running a cooler bias is really the way Mesa designed these amps. You may find them to get muddy near 65-75% MPD. If you are getting distortion on your clean channel, I would make sure all the pots are clean, tube sockets and the insides overall. I would then play test it. If I still had the problem I would try lowering my Vol 1 and see at what point it cleans up. The taper on most of these old pots was all over the place. Your 7 may be my 9. Then I would visually inspect the filter caps for leaking. They can only be tested for the loss of capacitance by removing them from the circuit. In a few cases, a 220uf cap would read 208uf after 20 years and I would keep it. If it reads 140uf it is time to replace it. Mesa caps last an unusually long time, but the amp needs to be played. The filter to ground and discharge rather quickly after shutdown. I would still check them with a voltage meter before I touched them. Look for gooey leaks or bulges.

 

[Harry the Horder]

I see what you mean. it's the export model with the volt switch.
So my Bias is fine. what do I go for now
when I play on my bottom strings it distorts like a fuzzbox at any volume or setting I swapped the valves around but still get it.
Harry

 

[Harry the Horder]

Thank you for all your advice I shall go through the caps next I did notice written on the board RELAY REPLACED and a Date I know it pops when I change from rythm to lead I have read this could be valve 2 and also a ground connection issue. anyway I will give it a more thorough going over gigs permitting .Once again many thanks. I think these forums are great keep up the good work.
Harry.

 

[Boogiebabies]

The relay pop is normal. Back then, there was no way to silently switch.
Later the LDR's came in and silently switched channels without the pop.

 

[Harry the Horder]

I thought that was the case, I don't mind that as at the moment dare I say it I gig with a Vox AC30 and use different effects for my boost and colourations I shall still keep plugging away at the Messa until I get that sweet tone singing.

 

[Boogiebabies]

Oh, a Vox AC 30....

I have been dying to get one of those forever.

 

[Harry the Horder]

I know what you mean (boys and their toys)
Ref my Vox. well I'm still talking vintage Amps,It's about a 78 Model Topboost with reverb they say they are the worst ones they made but it really sings and I've got no complaints I bought it from a guy who never gigged it for £200.00 For ease I just double it so about $400.00 I also have: Vox AC30 "T" (1965)
Fender Twin Reverb (silverface) soon to be blackfaced
Roost sessionmaster 50w combo (valve)
Selmer Treble & Bass 100w (the big square silver faced one .valve)
Kelly slave Amp lovely workmanship (valve)
Messa mkIIB
And Lastly an old and I mean ancient GramPiAn Valve P A that I don't know a thing about
My biggest regret is letting my JTM45 go for about $600.00 as it was one of the first built everything was hand written on it it was that early a Marshall then again at least it didn't sit and rot .Catch you later Happy New Year!!!!! Harry the Horder.

 

[Boogiebabies]

My biggest regret is letting my JTM45 go for about $600.00 as it was one of the first built everything was hand written on it it was that early a Marshall then again at least it didn't sit and rot .Catch you later Happy New Year!!!!! Harry the Horder.


How could you !!!!

This defies your name... You un-horded a $ 6000-8000 amp for change.

I understand...

Happy New Year !!!

 

[Harry the Horder]

And when I got Married $300.00 for my Les Paul Custom!!
You can see why I have since become a Horder! "Aint Love Grand"
Harry the Horder.