Blown C5 Cap

[77seriesIII]

Bummer, really excited took my amp over to Hembrook's house and we were going, going being the key phrase, have an amp taste test. I have 300Bs, simply changed the r19 and r20 resistors to 1.5Ω. I was running SS rectifier but had switched back to tube a few days earlier, I like glowing tubes.

Anyway plugged in the amp as as it was warming up hum started on the speakers, and got louder as the everything warmed up...Me staring at my amp thinking WTF I dont get hum at my house. I hit the power switch to turn it off and just before getting there C5 let the magic smoke out. I was able to replace c5 there, but the hum is still there, real loud like, both channels. I removed all leads from the board and then I tested all the resistors and got accurate readings for each one but had to remove R21 to get a good reading. but once removed r21 read ok. Freakin useless now but I know it is good. I have a spare so no worries.

What is the next step? What are some other things that may have let the magic smoke out? Should I replace the c6 as well? I have no way of testing my 300bs but may be able to find a place that can, is it even worth testing based on c5 letting lose? What about OPTs would they have been damaged? I touched both OPTs and power tranny just after the cap let loose and none of them were even warm. Doesnt mean much but...

The amps in pieces now, checking all solder joints, crimps, etc will be piecing it back together over the weekend.

Any help out there? Ideas on what to check next?

Much appreciated

erick

[SDS-PAGE]

Erick,
If your C5 was blown, there was excessive voltage (>100VDC) across it. For some reason your 300B was really aggressively biased causing the smoke. Did you check your R15s? You need to check your tube. I had this happen to me once before on power up. I didn' wait long enough before I turn the amp on again. Luckily nothing happened. A thermistor like CL-90 could help with w/ safe power up, especially if SS recifier is used.
I would also check your heater diodes. These might have gone bad due to high voltage.
Good luck! -Min

[77seriesIII]

min,

ok you got me, what are the Heater diodes?

I wasnt running ss so I do not think that it was a flash start, I had the 6ax5 in so everything should have started normally. The only that was different was the location.

I am afraid I could be burying two 300b's they were new for this project, how depressing. They were EH standards so not too bad but, ugh. drown my sorrows in a beer.

The Tube Amp Doctor is just up the road, they may have a tube testor, I may have to pay but who cares. I think I know the answer.

thanks /e

[SDS-PAGE]

Check your D1, D3, D6, and D7. Hope your tubes are healthy. I would also replace C5 with one with a higher voltage handing.

[77seriesIII]

oh the ones labeled diodes...Ima genius.

Never tested diodes before, power on/off and values. In the circuit or out of it?

Got a pretty fancy meter with a diode symbol on maybe I should find the manual. gotta be around here somewhere...


/e

[Shannon Parks]

Make sure all eight diodes (D1 thru D8) are good. They should read equal voltage drops - maybe .4 or .5 volts (these are Schottky diodes). Are you using a Trafomatic power transformer? What values did you have on the R15 network?

[77seriesIII]

Shannon,

yep using a trafomatic psu. I have everything removed from my box and am checking connections etc, my static (power off reading) of the r15 and r16 networks are pretty much the same, 901Ω for each resistor, which looks to be about right.

I'll post the diode results later today my time. I'm of to a local store to buy a few r21 and while I'm at it some more diodes/caps/resisitors just in case. Maybe rebuild that sections. I did write down my hot readings on a sheet of paper that may take some time to find. At the time ever, a month ago maybe and just before putting tubes in is when I tested everything and then did further testing with ss vs 6ax5.

/e

[Shannon Parks]

One theory:
Possibly the internal resistance of the Trafomatic is much lower than the PA774 power trannies we are using over here. Could you measure all your internal resistances? On startup, the inrush current might have damaged a filament rectifier, leaking onto that circuit and then doing strange things (like blowing C5). If this was the case, after making the repairs, we could add some resistance in the primary with either a power resistor or a IRCL.

[77seriesIII]

How do you want me to measure internal resistances, simply test across each of the resistors testing ohms or I think what you want is the hot readings and voltages across various points of the circuits?

[crispycircuit]

I had a pair of Dared monos 300b amp that ran off a single seperate power supply. I also had a start up smoke problem. ... It turned out to be the EH 300b shorting on start up. I returned the tubes and have been running JJ 300b with no problems... But a slow start tube rectifier would have probably solved the issue. When I had the amps on the bench, 1 EH300 b would short the b+ to the cathode heater about every 4th or 5th start up. I swapped 300b tubes and amps the problem followed this 1 tube. So I'd try a slow start tube rectifier first...

[77seriesIII]

All the solder joints checked voltages checked. Without tubes everything that I could check came out with pre-smoke release checks. I put the tubes in and I have to turn it off rapidly, the Hum just HUMMING, no audio in. without tubes, no humming.

I am having a little difficulty measuring the tube voltages, I dont have the best method for testing. I do have decent volt meters, just no real safe way to measure the tube voltages. the UX4 measurements are easy just lift the socket a bit and that one is easy. the other two...not so easy.

I am off to find a tube testor tomorrow, I am hoping a local tube place (business primarily for Guitar amps) should have a tube testor and should test my 300Bs for a nominal fee, probably getting two new tubes tomorrow.

/e

[SDS-PAGE]

I am having a little difficulty measuring the tube voltages, I dont have the best method for testing. I do have decent volt meters, just no real safe way to measure the tube voltages. the UX4 measurements are easy just lift the socket a bit and that one is easy. the other two...not so easy.

I don't get it. Why can't you just flip the whole amp over with tubes and inputs plugged in and measure voltages by probing leads on tubes and ground? You're ok so long as it's your probes that's probing, not your fingers. No need lift anything. You might run into accidently shorting hot tube leads by lifting and cause more damages.

Your amp will pick up hum when if inputs are not connected. What's the hum like? High or low freq?

[TomMcNally]

I am wondering if you have the UX-4 style socket connected
correctly. A lot of the problems you're having could occur if
the pins were connected wrong. Are the 300B's lighting at all ?

Tubes don't generally "go bad" ... on a new build, suspect a
wiring error.

... tom

[Shannon Parks]

See if you can get voltages and no loud humming with just the rectifier and 6SL7. Also, additionally measure AC at TP1 and TP2.

[77seriesIII]

Tom,

funny you should mention the tube lighting...I had forgotten that when the cap blew hembrook and I were both noticing that the filaments on the 300bs did not seem to light up. All of this was going on in a few seconds as I was reaching for the power switch. I think as I turned it off or shortly before, is when the cap blew. I dont think me turning the system off has anything to do with the cap releasing its smoke, I think it had an expire date already in mind. Of course in the rapid decline of the day with the pffft noises and smoke rolling around, I had forgotten that aspect. the tube heaters are pins 1 & 4 and they are showing 4.7vdc. In laymans terms the fat pins.

Once the cap was replaces the filaments do light on all tubes.

Here is the crazy thing, I was listening to this amp for close to 2 months with no problems. I move it to take it over to Hembrooks's place (he just built some TWQT, 8Ω checked that as well) and we were going to do a listening test. I did not run the amp at my place in the morning, but did the previous night. I am using a computer as a glorified cd/mp3 player but it suffices for now.