Hoping someone might be able to repair my beloved amp,which,after making a loud crackling noise,has died on the R/H channel. Had a quick look inside and noticed the dielectric has melted and charred on the point to point wire near the aluminium clad resistors (on both channels).Don't know if this has anything to do with it.
Anyway i would be very grateful if anyone can help and of course i will be willing to pay for work carried out.
The resistors in these amps run WAY too hot IMO, they are underspecced for the job.... you'd be better to fit a 470R / 50W in it's place (or 430R / 50W if you can get one).... you will also have to replace the solid core wire connecting to the resistor.... a bit of 2.5mm twin and earth solid core mains cable will be fine.
I could repair it for you no problems at all but better if you can do it yourself as it will save you a LOT of money shipping that ton weight of an amp... can you use a soldering iron? If so, I'll guide you on what to do.
Just had another look..... BOTH of the 430R resistors need changing, just look at the state of the insulation on the connecting solid core wires.... those resistors are running way too hot for comfort.... 430R / 50W is definitely the way to go in my opinion.
My soldering skills are very limited,would be more than happy to send the amp up to you for repair if thats ok with you?
I'm pretty busy at the moment but will fit your amp in, no problems, the "fix" looks pretty straightforward (from looking at the pics) and, if that's all it is (2 x 50W alu clad resistors and rewire) it will be about £45 to repair it.
The amp arrived safe and sound today I had a quick look and, yes, one of the 430R resistors has burnt out. The three suppliers I use don't stock the 430R value so I have ordered 470R instead, 25W types (ARCOL). They are double the size of the ones in the DarkVoice and shouldn't burn out.
Hopefully that's all that is wrong with it. I will replace the wiring and, fingers crossed, it should spring into life again
The parts will be here either Monday or Tuesday, I will keep you updated.
I had a go at fixing this amp.... replaced the 430R resistors.... amp worked fine for a few hours and then, WHAMMO!, there was a LOUD crack and the internals of the new resistor blew clean out of the metal housing
Obviously something further down the chain causing excess current to flow through these resistors. Unfortunately, I don't have the time to fault find this amp.... it would take a LOT of time and would not be cost effective.
Thing is, Maybe I've overlooked the obvious? If so, comments welcome..... I'd like to see Dennis get his amp up and running again.
had a quick search round the net but did not fine any other reports of this problem.
however when i looked at the sale info from DV on ebay it looks like they have changed the component layout. they have separated the cathode (230ohm 20W) and dropper resistors. the droppers are now relocated to either side of the case. I'm guessing they had a a reason to do that (heat or the the large PD between the droppers and the cathode resistor).
if it,s a strange one with the amp being a duel mono design and both cathode resistors overheating. it does lead me to draw the same conclusion as mike re the power rating of the 230ohm 20W resistor.
I've used the 6as7 (6080) as a reg tube many times in the past and its a real beast of a thing. looks like in this amp they are running both triodes in each envelope strapped to run in parallel. Run like this they are good for 100ma per tube but that seems to me to be a bit over kill for a HD amp. it would be useful to know the OP of the tube in order to check that those resistors are up to the job.
From what I can make out of the posted pics the amp is a basic single ended OTL amp. Because of this setup the cathode resistors probably have something like 50 to 70 Volts on them (it being a cathode follower setup with the 430 Ohm as the cathode resistor). 20W should be enough BUT it should have been cooled properly. Judging from the pics there is little contact material for proper heat conductance as it is mounted on a venting grid limiting the contact surface. The thickness of the material is not suited for conducting large amounts of heat either (at least it appears to be rather thin) and will create a hot spot (a terribly hot hotspot I might add) IF it is mounted on a better cooling surface I figure it will remain intact.
IF it were my amp I too would have chosen 50W resistors (or 2* 820 Ohm/20W in parallel or 2* 220 Ohm/20W in series) and mount it where it can 'loose' it's heat.
Last Edit: Sept 4, 2010 10:16:14 GMT by solderdude
yes i was thinking along the hot spot lines as the droppers and cathode Rs seem to be all loaded in one spot (a very hot oneunder the mains TX). could be why they moved the droppers in later models.
usually i try to spec my cathode resistors at x5 the calculated value as they can take a bit of a beating heat wise.
point taken re the heat sinking arrangements. those Ali clad resistors usually give 2 specs one with adequate heat-sinking and one without (usually the one with shown on the body because it looks better). the 50watt option looks favorite (or go for some nice thick heat-sinking) and the heat damage seems to affect both sides (hot enough to melt solder!) which does seem to point to a design problem ie under spec cathode resistors rather than any thing further up the chain. I'd expect that a problem would be more likely at switch on with the power surge an all. I'll have a look at the tube data to see what the heater / cathode voltage tolerance is (from memory its quite low and the common practice is to tie the filament center tap to the cathode when used as a reg that is) as it may give more of an idea of the OP.
a circuit diagram would be lovely but manufacturer don't like to publish especially with simple tube designs. however a little history may help to get to the nub of the question.
how old is the amp and was it from new? any mods done? how long did the amp work for
hi just checked the tube data and the heater to cathode voltage limits are + and - 300v so Mr memory has failed me again
one thing that bothers me a bit is i don't remember a component failure like the one mike had in my 20 or so years of fiddling with tube amps. something strange going on there. lets have some history it may help solve the problem.
it does seem like a heat thing or I'm sure it would have popped straight off. so 50w resistors as suggested by Mr solderdude looks like a good place to start
It was Mike's idea to put in a 50W in the first place but could not get one. I still think it needs to be mounted on a location where it can get rid of it's heat and this is why they fry. 20W should be enough when properly cooled but I myself would use 2* 20W or a 50W and mount them elsewhere. both (completely separated) channels having the same fault is unlikely so it must be a design fault. Also the discoloration of the resistors and wiring tells me it has been like this a long time. The replacement resistors simply couldn't handle the same amount of heat as the original ones because they were not properly cooled and uncooled were probably rated lower then the original ones as you commented. The resistors are not situated under the transformers b.t.w., the reservoir caps are.
Eventhough it sounds nice it remains a Chinese design...
Last Edit: Aug 30, 2010 18:14:46 GMT by solderdude
i need to find my glasses and as you say the resistors are at the other end to the tx's.
yes I've just been experiencing the Chinese delights of a little hot :-*ER i mean dot mkII mk 2 and every thing does seem a bit close to the bone spec wise.runs warm but not as hot as my regulated 300B SE mono's but it sounds OK in a creamy midrangy sort of way
by the way hows the HD amp with TX gain stage coming. a good place for transformer winding is AE Europe who i think are in Rotterdam. just down the road from you i think ?
I've asked John to chip in to give a bit more "history" about the amp.... funnily enough, he changed the resistors (again) and the one in the opposite channel blew up this time so this is not limited to one side it seems.
I didn't think it "was" a resistor thing, thought the problem may have been further down the line causing excess current to run through the resistor.... obviously not.
Well, those 20 watt Chinese resistors must be better built than the Arcol equivalents..... I'll get a couple of 50 watters and send them over to John, see how they go.