MK111 Heater resistors

[Alan Lever]

Hi everyone, just a general question please if I may, I have a MK111 blue stripe, checking the chassis out for voltages etc. the two 5W 0.05 ohm resistors tied to each leg of the heater wires seem to be generating a lot of heat, if you float your hand over the top (safely of course) going over the board its definately a high heat area, is this normal?, I know there is a lot of current here so I expect heat but its very noticeable. the voltages on each leg are bang on really at 3.22v so that's ok..... it just shocked me really, i haven't noticed that before on other boogies I have had. the resistors are the factory fit ones and they 'look' ok

thanks everyone.
Alan.

 

[mark1406]

Can you supply a photo?
I had a look at a circuit diagram and I couldn’t find where the resistors are.

Regards Mark

 

[Alan Lever]

Hi Mark thanks for looking, no they don't appear on any schematics i've ever seen either.
photo attached, its these ones right on each leg of the heater wires off the PT.

 

[Jay Omega]

I assume this is a four power tube Mk III? What kind of tubes are loaded in it? If you have four power tubes that draw 1.5A each on the heaters (EL34, 6CA7, KT77) then I could see a 0.05 resistor getting pretty warm

 

[Alan Lever]

its a simul-class so centre two are 6L6's and outer two are EL34's all Mesa tubes. Im not even sure why those resistors are there to be honest.
I did wonder also about the current rating, there are 5 pre-amp tubes at around .3 amps and the power tubes at like you say around 1-1.5 amps each so its getting up to 6 or 6.5 amps of current and its the correct power transformer which I believe on the 6.3 lines its supposed to be 6 amps so yes its at its limit, but saying that the board where they join isn't burned and its the correct heater voltage going to the power tubes, so yes you can tell why those resistors are getting warm. I just didn't know if that was normal. I am sure it is to some degree and they have been in the amp since 1988 without failing... thats why I joined the group, you can read all the data you want but you cant beat experience thanks Mark

 

[Alan Lever]

Sorry, I meant to say thanks Jay

 

[Jay Omega]

its a simul-class so centre two are 6L6's and outer two are EL34's all Mesa tubes. Im not even sure why those resistors are there to be honest.
I did wonder also about the current rating, there are 5 pre-amp tubes at around .3 amps and the power tubes at like you say around 1-1.5 amps each so its getting up to 6 or 6.5 amps of current and its the correct power transformer which I believe on the 6.3 lines its supposed to be 6 amps so yes its at its limit, but saying that the board where they join isn't burned and its the correct heater voltage going to the power tubes, so yes you can tell why those resistors are getting warm. I just didn't know if that was normal. I am sure it is to some degree and they have been in the amp since 1988 without failing... thats why I joined the group, you can read all the data you want but you cant beat experience thanks Mark

My guess is they wanted to drop voltage and implemented that way to split up the dissipation over two resistors. I've never seen a factory schematic for the blue stripe though.

Don't be scared to give the factory call, they give fantastic support!

 

[Alan Lever]

thanks Jay, I will give them a call, just have to time it right as I'm in the UK. in any case thanks for giving me feedback.

 

[mark1406]

Send an email Alan, I have found that to be effective. As stated by Jay there is a lot of current going through those resistors, but the resistance is very low. Ultimately, I suspect the resistors are there to protect the transformer. A fault which can occur (happened to my Mark 1 amp) is there is a short between pins 2 and 3 on the valve socket. This fault puts the B+ voltage on the heater windings. Usually the 100 ohm resistors get blown, but this doesn’t stop the rail voltage being present on the heater windings. I would imagine the higher voltage on the heaters would blow the resistors.

Regards

Mark

 

[Alan Lever]

thanks Mark, I have literally just sent Mesa an email asking about it.... I'll check to see if there is any continuity between 2 and 3 when I get home, I don't think there is but its worth checking and will only take a minute.... thanks for the suggestions to you both.
I'll let you know what Mesa say when I get a response. I had a good look at the 100 ohm and also the 39 ohm resistors next to them which are on the board but they look fine, no burning or damage.... thanks to you and Jay for your help and experience.

 

[mark1406]

thanks Mark, I have literally just sent Mesa an email asking about it.... I'll check to see if there is any continuity between 2 and 3 when I get home, I don't think there is but its worth checking and will only take a minute.... thanks for the suggestions to you both.
I'll let you know what Mesa say when I get a response. I had a good look at the 100 ohm and also the 39 ohm resistors next to them which are on the board but they look fine, no burning or damage.... thanks to you and Jay for your help and experience.

I don’t think you will find anything. Though it’s not a good sign if the socket is going really dark with heat particularly between pins 2 & 3. If the socket was faulty it would be very dramatic. I would check your 470 ohm screen grid resistors while you are in there.

Regards

Mark

 

[Alan Lever]

Hi Mark, had this reply from Mike B at Mesa.........
"we got transformers but the heater voltage was too much, so we added those 5W 0.05 ohm resistors. they do dissipate a bit of heat and it all sounds normal"

good enough for me

I did check the voltages all across the heater rails and there is nothing untoward, also checked the 470 ohm screens on the 6L6's, I am replacing them but they were not out of spec greater than 10% but close enough to warrant swapping out. I already changed all the screens on the two EL34's, they were out of spec (one of the 2.7k's measured 562 ohm's !! both in and out of circuit)

thanks for you help Mark and Jay.

 

[AxH]

Thanks for checking Alan - I have the same resistors in my Mark III Blue stripe, I thought they were later additions by a tech to lower the heater voltage (as someone have added three pots for bias adjustment as well) - now I know those are stock! I assume you have an export transformer as well (as I have).

 

[Alan Lever]

no worries, its purely down to a batch of transformers which put out a little too much heater voltage, I have had two other blue stripes in the past and both of those had them too, and yep, all my boogies have been export ones as I am in the UK... in any case I'm not worried about it anymore....hope you enjoy yours too

 

[user 41014]

Thanks for checking Alan - I have the same resistors in my Mark III Blue stripe, I thought they were later additions by a tech to lower the heater voltage (as someone have added three pots for bias adjustment as well) - now I know those are stock! I assume you have an export transformer as well (as I have).

I would be interested in knowing how they implemented the 3 bias pots. Are 2 of them to dial in each side of the power section?

 

[Alan Lever]

Hi AxH, would you be able to answer this one from Electric Mayhem?
my blue stripe has no bias pots.

thanks
Alan.

 

[AxH]

I would be interested in knowing how they implemented the 3 bias pots. Are 2 of them to dial in each side of the power section?

I´ll check as soon as I open the amp up, to change the main electrolytics, hopefully in a few days. If I remember correctly (wasn't now able to see in the pictures I took) there was one overall pot (likely replacing R117), and one for each side which I think both balanced the outer/inner tubes (I have simulclass) and also affected the balance between the sides.

I'll see what the pots do and draw a schematic of where the three pots are connected and post here. Will be interesting to see if there's any comments if this is a good arrangement, or if there are any drawbacks in the way it's implemented. Anyway, it was also hard to understand what I actually would aim for as idle plate current for the inner/outer pairs, there seemed to be different thoughts on that.

 

[Alan Lever]

I just did a recap on my MK111 and changed out the power resistors and all the grids and screens.
at bias point 'E' on the schematic mine measures -53vdc (simul export would be -55 according to the schematic) thats at the end of both 220k resistors at that point. at pin 5 on all the tubes I get -53 for both 6L6's and -43 on the EL34's.
my TAD bias meter reads crazy stupid readings so I didnt trust it, I used the OT shunt method and got the following after calculations
EL34's at 17W or 68% of max (assumed 25 watts for an EL34)
6L6GC's at 16W or 53% of max (assumed 30 watts for a 6L6GC)
I read lots of stuff on forums and I believe from what I read for simul amps the inner pair of 6L6's are biased on the colder side so that the EL34's don't fry as the outer EL34's are biased hotter.

 

[Jay Omega]

I just did a recap on my MK111 and changed out the power resistors and all the grids and screens.
at bias point 'E' on the schematic mine measures -53vdc (simul export would be -55 according to the schematic) thats at the end of both 220k resistors at that point. at pin 5 on all the tubes I get -53 for both 6L6's and -43 on the EL34's.
my TAD bias meter reads crazy stupid readings so I didnt trust it, I used the OT shunt method and got the following after calculations
EL34's at 17W or 68% of max (assumed 25 watts for an EL34)
6L6GC's at 16W or 53% of max (assumed 30 watts for a 6L6GC)
I read lots of stuff on forums and I believe from what I read for simul amps the inner pair of 6L6's are biased on the colder side so that the EL34's don't fry as the outer EL34's are biased hotter.

Boogie tends to cold bias tubes, inner tubes are for Class A/B and outer are for Class A. A typical Class A amp has tubes biased at 100% of plate dissipation, you can see Boogie is around 70%. A typical Class A/B tube amp will be biased around 70%, you can see Boogie is around 50%.

If nothing else, the way Boogie biases certainly helps extend tube life!

 

[Alan Lever]