what about this:

http://www.davidsonamp.com/sf/images/twin5c8.gif

isnt that bypass cap too small, according to what youre saying?

QuoteNow the trick is to figure out if it was caused by a tube currently in the unit, or was the damage done long ago.

one of the tubes was red plating when tested. any 'trick' that will tell me? ill replace those resistors anyway -

Quotedamage to the heater "hum dinger" (as these resistors or balance pot are known), where there has been a flashover between the anode pin 3 and the heater pin 2.  This only has the OPT primary in series and being DC can be a sustained arc which carbonises the valve base, socket, or both (and once carbonised they really have to be replaced).

that definitely sounds like what happened here. except these are 6L6s

really??

i calculated for a 40 Hz-ish cut-off - transconductance of 6BG6 is about 8mA/V - the cathode resistor is 250 ohms. 47 uF seems to be just right for that cut-off.

i didnt realize i was worried about ripple current there....and i thought a bigger cap would have created bias excursion problems.

am i missing something?

QuoteUnless they are black (suggesting a major problem) I'd leave them.

oh they are black alright!

the tube socket (of the tube in question) was all gunky inside - hard to even squeeze the tube in and out. i ordered a new one.

i got some better cleaner and cleaned the pcb looks good now.

those missing pads are for the screen grid stoppers. they often fall off in these particular amps, i think, which is why those resistors get resoldered to weird spots.

inspecting stuff i noticed two more fried resistors (r80 and r81 in the attached schematic). 100 ohmers in the heater circuit - what are they for??

cool!

put in a 47 uF cap and got more clean power as well as a squar-er full power overdrive (at least on the scope) - and more power overall, actually.

we'll see what happens when i put new tubes in -

alright seems good -

one question: the cathode resistor is unbypassed - its at about 30V. i get a great signal swing from the PI, but the power tubes are only putting 13 watts or so into 4 ohms. im probably going to replace them (they are quite old, too), but am wondering about the unbypassed cathode resistor. does that seem odd for a push-pull 50 watt-ish design?

i have a question about circuit board (potential) damage:

someone gave me a hot rod deville to fix up - one of the tubes is red plating the other barely glowing - figured just might be a shorted tube. i opened it up to take a look, though, and saw some bad looking light brown ash kind of residue over the area on the PCB where the red-plating tube is.

i cleaned it with some alcohol but there is still some dark brownish residue i cant quite get off

(see pics: http://dropcanvas.com/#68fj9h9Dk788l6)

1) any tips for a better solution to clean this crap?
2) i think Roly mentioned before that a burnt PCB can become resistive. the owner of the amp has said its had problems before and im wondering if a resistive PCB in the power amp section might be shorting tubes for him....any thoughts?

QuoteThe only reservation I have is that all the Maganatone circuits I looked at showed "8 ohm" outputs, so I'm wondering if that's the original OPT fitted, or maybe blown and changed some time in its long history.

hm hard to say, i guess. a very long history.....

but either way probably a safe bet to get two 8 ohms speaks in parallel, right? i did try this through a 4 ohm cab and it sounded great!

alright i did the test:

on the secondary i put in ~6.8VAC

on the primary i got ~288VAC

so thats about 42:1 ratio

42^2 = 1764

if we take the 5.6kplate to plate

that gives me 5600/1764 = ~3

so were looking at 4 ohms then, huh?

i found a PT at a swap meet on sunday and used that for the input voltage, so its 50/60 Hz - my results seems okay?

wait, so are you saying to feed the magnatone OPT with the the OUTPUT from another amp? just make sure its at a low-ish voltage?

(i was thinking just to use the heater taps from another amp to avoid any possible grounding mishaps - the magnatone would be unplugged from everything - safety earth included)

ya the 6BG6 has the plate tap on the top of the tube -

Quote
2) Yes, but be careful of the grounding on the heater circuit and the OPT secondary - it may be one wire simple, or you may need to also lift the OPT secondary grounding, or it may be easier to drive it from an external 6-12V transformer (depending on what you find).

you mean be careful that the OPT doesnt have a separate wire running to ground, aside from the two wires feeding the load?

would it be safe to drive it from the PT from another amp, that way i dont have to turn the magnatone on at all? i dont have any random little transformers laying around to give me 6VAC....

its a magnatone M-198-8D

cant find a schematic for that specific one anywhere...

it uses a pair of 6BG6's for the OP

not sure what you mean by lineup - push-pull

but in the rest of the amp,
the valve lineup is: 6SJ7(3) 6SL7 6N7 6BG6(2) 5U4

a few questions about the test:

1) its okay to turn on the amp without a load if the OP valves are pulled?
2) i just hook up the two heater wires to the two secondary leads of the OPT?
3) doing this with everything else turned on in the amp is OK? the pulled OP valves isolate the output stage from the rest of the amp, presumably?
4) measuring across the primary do i measure form one end to the center tap? or across both ends?
5) okay to measure with a good DVM or should i use a scope?

the amp is all good -

tried it out with a 4 ohm bass cab i have and it sounds great!!

issue is:

the original speakers measured about 4 ohms each and were hooked up in parallel - so im thinking maybe the OT is looking for a 2ohm load. but i have no way of knowing that the speakers were wired right. before i tell my buddy what speakers to buy, i was wondering if you guys know of a good way to determine output impedance for an unknown transformer?