Mark 111 with extra resistors

[willy barden]

Hi - I have a Mark 111 (red stripe DR marked) in for repair with a variety of faults which I am fixing. However, at some point someone has fitted a pair of 10-watt 470 ohm enamel wirewound resistors in parallel (making a 230 ohm 20-watt resistor) between pins 3 and 4 of the outer pair of output tubes/valves. These heavy items have been secured by some kind of silicon sealant which seems to have restricted the heat flow away from the resistors because they were smoking like a steam loco on an uphill pull. Can anyone suggest why they were fitted? My only suggestion so far is that maybe someone wanted to fit 4 EL34s - but why 230 ohms and why 20-watt?

 

[Restless Rocks]

Here is the schematic for the simulclass version of the mark III http://www.schematicheaven.com/boogieamps/boogie_mkiii.pdf

Sounds like a dilatant tryed to modify the amp. I would recommend putting the amp back stock. If you have a 100watt output version of the amp (non-simulclass) the outer pair will be paralell to the inner pair (like a fender) and you'll know what to do. If the amp IS a simulclass version it is possible to run 6L6's in the inner pair and EL34's in the outer pair with the stock arrangement.

 

[willy barden]

Hi - this Mark III is a Simul-Class/Class A. It seems to have had a hard life and the circuit board components are coated in tobacco film and just plain dirt. There were two self-tapping screws loose inside the amp casing and an broken-off output valve locating spigot. I am beginning to think that I am the lucky first technician to peer under the bonnet since about 1985! The only tech's sign is from "Tina" who I seem to remember was one of the assembly line checkers. All of the valve sockets are full of crud and some corrosion so I am having a great time going from point-to-point cleaning then checking circuit components to solve a variety of faults. Anyone fancy some dirty pics?

 

[willy barden]

Me again - have cracked the basic problem - the two outer tube bases have loose pin sockets so giving the whole damn thing a good shake takes it from miracle tone to a bad sound city clone complete with bangs and whistles - so they will have to go. Funny thing is that they have been replaced before and seem to be have been reasonably good quality ceramic items. I suppose this is the problem with suggesting that the customer swaps tubes - although the Owners Manual for the MkIII discourages this activity.

Thanks and regards Willie