[Pricklely Peete]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif You should be able to trace it. There are only so many ways to connect EL34's in PPP mode. What is the amp, b/t/w/. Maybe someone knows the topology already. Anyhow, as I say, I would be weary about just dropping new resistors in there as they may be part of the protection scheme. Indeed, that resistor does not look like it baked over time and finally gave up. It looks like bias was lost, a tube ran away, and the resistor blew quickly as a way to protect the rest of the amp. You may be right that it was the last set and that they weakened the resistor, but I'd test the "good" tubes before you do anything.

Agreed ......the amp is a MeiXing MC34-AB.

I bought a new octet of Winged C's from Joe at the Tube Depot. I asked him to burn the set in in for 24 hours before shipping and to match them carefully (not that it matters all that much with the auto bias circuitry). The tube set will be a known good set. I don't anticipate any other issues frankly although I may be proven wrong. I'm hoping it's a straight forward repair though and it should be from other amp* failures I've seen far worse than this come back to life with a few parts swapped. (* Guitar amps mainly)

The blue glass tubes make it harder to discover early failure of a tube...in a dark listening room this combo may not be a good idea. Another set of tubes I'd like to try in this are JJ KT77 or Gold Lion KT77...that's down the road

The SED's should give me much better reliability over the standard life span of 18 months for a tube set. I take max life and cut it by 33% to be safe. The stock tubes for this amp, and the one that failed are Chinese Jinvina EL34 blue bottles...so anyone contemplating these...don't bother they have longevity and QC issues.

I'm still waiting on the tubes and other parts to arrive....I hope I get both tomorrow so I can get my beloved amp back up and running over the coming weekend.

I'll let you guys know what happens and will watch this thread closely if further info comes to light that I need to look into before hitting the power switch.

Thanks again for all the help thus far....

Peete.

EDIT: Here's a pic of the entire amp circuit...it's not the greatest quality picture, sorry about that.....

It has a full 4 input preamp which can switched out of the circuit on the front panel via a toggle(like the Triode/UL toggle). I use it in pure power amp mode only.

 

[Pricklely Peete]

Update:

She's fixed and running great once again. Before starting in I took the advice of all and measured the caps...none had any residual voltage hanging around ready to ruin my day...so that gave me the green light to have a closer look. The Stackpole 2W 10R's I ordered were 2/3 the size of the stock metal films so my choice of 3W IRC made the most sense to me. The Vishay parts have not yet arrived and are now not needed for this repair but will make handy spares in case this happens again (knock wood it won't).

I replaced all 8 resistors with 3W IRC's (1% tolerance mil spec) checked all sockets for shorts (man that took quite a while) checked for continuity between the pairs and measured total resistance (20R bang on) across them.

Checked individual parts for tolerance before installation (all measured exactly 10R...impressive). Cleaned up the wiring that had blown resistor residue all over them and found no damage to the insulation at all. Added insulation tape to all pin 3 runs between the wring and the chassis and finally checked the continuity of those runs to ensure no shorts to the chassis. These were all triple checked ....I'm nothing if not thorough.

Right ...reassembly and testing. I took a known good quad of SED Winged C's and 4 of the stock tubes (just to test the amp) and installed them along with the needed preamp pairs (12AT7 and 6SN7) Wired everything back up...double checked that wiring then said a little prayer and hit the power button.....she sprang back to life...no issues to speak of out of the ordinary, that's a good sign...no popping, clicking, background noise, no tranny chatter nothing but inky black silence (with the volume knob on the pre at 3/4's no less)

Anyhow to make a long happy story short...I shut down after 1 hour and will wait for my octet to show up before giving it a real work out. The amp did gain a bit in SQ during that initial power up test. I wasn't expecting that at all. Sounds a little more resolved in some respects. I wonder if that's a byproduct of the weakened resistors on pins 3 of the 8 tubes being replaced with new better quality parts ? ( 2 of which were noticeably heat damaged prompting the swap of all 8).

I see no future issues thus equipped with reliable tested power tubes (fingers crossed). One upgrade now in the works though, besides a voltage device to drop line levels, is a revamp of the sections using the 1uf MKp's + 10uf 250V rubycons....that's a major upgrade and will take quite sometime to fabricate the assemblies in order to have them fit properly. The same formula as the preamp I finished 6 weeks ago (my DIY version of the Ref Audio Mod for that preamp).

I'll add another update after I've had 48 hours with the new octet of SED tubes. Hopefully there will be no more surprises for years to come (as long as I notice a tube going bad earlier than I did this time around).

Thanks guys for all the help. d I'm going to construct simple bleeder circuits like you suggested except these will be clip on devices to be taken off when not needed. Seems like a better use to me so I can apply these to other amps when I mod/repair them.

Peete.

 

[fran]

good to hear it, glad it all worked out in the end.

Happy listening!

Fran