Big tube amp ....needs some TLC
Nov 3, 2008 at 7:38 PM Thread Starter Post #1 of 18

Pricklely Peete

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Hi fellow DIY junkies,

My old tube amp went pfffffft...today ....so I hauled it's immense bulk upstairs turned her over and found a baked (grid ? ) resistor at one of the EL34 tube sockets.

Here's a pic.

MC34-ABblownresistor003.jpg



Now the resistor in question is what 2W 10 ohm ? I'm guessing the rating. I'm sure my high line voltage is causing undue stress on the components although the amp does have autobias...nevertheless the other 7 resistors...some look like there ready to go as well . It might be wise to replace all 8 ?


I'm thinking 3 Watt Kiwame or Dale ? Maybe just a good metal film at 3+ W rating ?

What are your suggestions fellows ?

Thanks for lending a hand....

EDIT: Band code is Brown Black Black Gold...= 10 ohm.....ordered 10 x Vishay 3W 5% 10ohm....should be a straight forward repair.

Additional close up pics of resistor damage

MC34-ABblownresistor003-1-1-1.jpg


MC34-ABblownresistor002-1.jpg




Peete.
 
Nov 4, 2008 at 2:08 AM Post #2 of 18

fran

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Do you mean the bit of burnt copper wire from pin 1? It looks like the heater wiring shorted against the copper going to pin 1 - Thats where I'd be looking first. The other thing is that 10R seems very small for a grid stopper - I think normally the grid stopper for EL34 are fairly high value.

Then I might be completely wrong its 2am here as I type this......
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Nov 4, 2008 at 2:33 AM Post #3 of 18

Pricklely Peete

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I guess it's hard to see......

I tried to get a close up shot but my cam stinks. The far bottom left octal from pin 3 up to leg of 10R metal film 2W (I presume). In that cluster of 4 you can see a small spot where the insides of that resistor blew out the black residue. I haven't touched the amp yet because the caps need a full 24 hours to drain charge. I'll clean that black residue up after the new parts arrive (sometime this week I hope).

I have 2 types coming 3W IRC 1% Mil spec and 3W 5% metal Film Vishay. Both are 10 ohm. Anyhow if you look closely they are split by the ultralinear/triode switch wiring between the pairs of EL34's.

Tough to say what the original designers were thinking...I'm by no means a tube amp topology expert...just a hacker learning my way around these things
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I hope this info helps a little.

Peete.
 
Nov 4, 2008 at 3:57 AM Post #4 of 18

n_maher

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Pete if you're going to be working on this amp a lot I'd suggest searching out and building a capacitor discharge tool. They're really quite simple to build and would save you 24hrs between cycles of testing. Please note, the usual disclaimer applies, you're on you're own working with high-voltages. Even if you think a cap is discharged check it and check it again periodically as some can build residual voltage even after being discharged. WHATEVER YOU DO, BE CAREFUL!
 
Nov 4, 2008 at 4:38 AM Post #5 of 18

dsavitsk

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Quote:

Originally Posted by n_maher /img/forum/go_quote.gif
Pete if you're going to be working on this amp a lot I'd suggest searching out and building a capacitor discharge tool.


If the amp doesn't have a bleeder resistor, add one. In fact, if your line voltage is too high, add one that draws some current to lower the B+ a little (assuming your PT is up to the task). I usually add an LED inline for a visual of when the voltage has dropped. Still be careful and measure before touching, but it is one more level of safety.

Pin 3 is the plate. A 10R resistor could be plate stopper. If that's that case, under normal triode operating circumstances, a 1/2W resistor is fine. Even under the harshest operating conditions (Class AB1 PP draws 250mA) a 2W resistor is more than triple the necessary rating.

It is also common to use fusable resistors (or even fuses) in this spot in case of tube runaway. I would be hesitant about just replacing the parts with higher spec resistors as this may lead to actual damage (tubes, OPT, PT, caps, etc.) Try to come up with a schematic, or let someone who knows the circuit take a look at it.

-d
 
Nov 5, 2008 at 12:36 AM Post #6 of 18

fran

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Its a good idea to check the B+ rail with a multimeter before putting the fingers near it! I know someone who had a commercially available hybrid amp, very well respected make that he managed to blow some fuses in (part of a protection circuit). Anyway, he opened up the amp and couldn't see what was wrong etc etc, and eventualy brought it into a shop. When they opened it up and put a meter, they had the full b+ charge still stored. Fuses blowing had stopped it draining. Broskie puts it well in his booklets - there are few enough solder slinging tube lovers left - we don't need to lose another one.

************************************************** ********************

If it was me, I think I would replace the part first. Remove at least those tubes on that channel and power it up and see what happens. Maybe use a light bulb in series to limit current draw just in case.....

Assuming thats OK and the voltages check out, I would swap over the tubes from the known good channel and power up again. Do the same checks.

Lastly, before inserting that tube again, I would do a few internal resistance checks on the tube, make sure its not shorted internally. If you had a spare, use that first to verify that all is OK.

I had a EL84 once that had a cracked base that escaped my attention. Took out a few resistors when it went down. To my confusion this happened just as I was finishing up building the amp and it took me a good while to figure what happened.

How does the tube from that position look?


Fran
 
Nov 5, 2008 at 1:12 AM Post #7 of 18

RinksCustoms

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Perhaps you had the tube in question ARC over, in this case, aside from discharging the caps (could contain a few joules worth of electricity?! - enough to toss you like a rag doll!!) and replacing the resistor which is a 10 ohm 5% and looks to be a 1W. Definitely have the tubes checked before turning the amp back on!! It's alot cheaper than killing a good amp!!
 
Nov 5, 2008 at 4:55 AM Post #8 of 18

Pricklely Peete

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Thanks guys ....lots to think about for sure. I'm not sure it's possible to get a schematic for this amp although I will request one from the manufacturer.
Precautions about residual voltage noted fellows.....it will have sat all week by the time the parts arrive.

A couple of points about the tubes....the tubes were a brand new tested octet of Winged C SED's.....they had about 65 hours on them before the resistor went. Saw no unusual behavor from any of them.....one interesting note is this...

I had one of the original set (7 month old Jinvina EL34 blue glass with about 3500 hrs on them) showing failure condition (bright orange red glow) from bottom right ......hey wait a minute...it was that tube that prompted me to swap out all eight for the new set.......interesting. I guess I didn't catch the original bad tube fast enough. It was glowing near the bottom (at pin 3 ) near the plate junction and plastic....all the others were fine compared to it.
Now it all makes sense..........thanks guys....DOH...I didn't notice fast enough I'll bet and the damage had already been (mostly) done...matter of time with the new set before that resistor gave out I would imagine.

Not to worry about proper voltage discharge fellas, I've worked on tube gear before so I'll measure all the caps with my meter first before touching anything...also know enough to wear rubber soled shoes and never have more than one hand touching something inside at a time...until I know it's safe that is.

I'll look into setting up a simple cap bleeder circuit ...I have some spare LEDS somewhere around here
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I guess a good idea would be to look into something to reduce the line voltage. I've looked at 4A autoformers so far...220US ...ouch...

I'll get another octet of matched and tested tubes while I'm at it. Some Vishay 2W need to be ordered now
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I still may use the 3W IRC's....

I'll keep a much closer watch on the tubes now you can count on that.

Off to order another octet.

BTW has anyone tried the new EH 6CA7 large bottles ? Supposed to be exact copies of the old RCA or was it Phillips ?

Thanks for the info gents !!!

Peete.
 
Nov 5, 2008 at 5:48 PM Post #10 of 18

Pricklely Peete

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Quote:

Originally Posted by fran /img/forum/go_quote.gif
Check out the socket as well and make sure there are no shorts within it. Other than that, do what you said above and see how it goes for ya,


Fran




Will do. I also like d's ideas greatly....all who have contributed are appreciated !!!

I have some research and some learning to do while waiting on the parts to arrive (among other things) to implement the changes (on top of some film cap replacements + Teflon bypass caps etc).

This should be a straight forward repair ( I hope). One last observation...the outboard power "humpty" that houses the two PT's the fuse had done it's job and blew....so I hope the only issue with the amp is the resistors of note.

I will double check for shorts.....the photo doesn't show how far the copper bus wires are off the chassis. A good 3/4 in. The white wire with all the resistor residue on it will be checked out ...I'm willing to bet it's fine but will add some insulation tape underneath all eight identical spots for added protection.

Hopefully I'll have a good amp back on it's feet by the coming weekend. I sure do miss it's killer sounds
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Best Regards to all,

Peete.
 
Nov 5, 2008 at 8:01 PM Post #11 of 18

fran

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Some of that thick double sided tape is great stuff for holding down wires (or should htat be holding up wires) here and there.

FWIW, the wiring inside that chassis looks nice and ordered and well put together. Looks like 8 EL34s with 2 pairs in PP for each channel? Must give quite a bit of wallop!

Fran
 
Nov 5, 2008 at 9:43 PM Post #12 of 18

Pricklely Peete

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Quote:

Originally Posted by fran /img/forum/go_quote.gif
Some of that thick double sided tape is great stuff for holding down wires (or should htat be holding up wires) here and there.

FWIW, the wiring inside that chassis looks nice and ordered and well put together. Looks like 8 EL34s with 2 pairs in PP for each channel? Must give quite a bit of wallop!

Fran



Hi Fran,

Correct ....it's PP pairs of 2, rather conservatively rated at 75 Watts per channel which I bet is about 25W per channel below what it really puts out (if bias is set at optimum level of 32 ma). It really handles my inefficient mid/tweeter ribbon panels (the bass sections are amped by beefy SS amps) with aplomb. The only drawback to this amp is the cost of NOS EL34's......of which matched octet of even the cheapest RFT's would cost multiple hundreds....So far the SED Winged C's are doing (not right now of course) a commendable job.

I have en route Stackpole 2W 10R Carbon film, Vishay 2W metal film and IRC 3W 10R...may use the IRC 3W to replace the big gray 10R's...hopefully nothing else was damaged by the resistor going....most likely I'm fine once everything is given the once over (among other planned upgrades)with those 8 resistors replaced. Otherwise I'll need to find a local tube amp tech...that won't be easy...this thing weighs a ton even with the PT's offloaded to an outboard chassis. Really heavy large trannies in this amp.....it's as quiet as any good SS amp I've ever had which is a huge bonus.

Update for d, still no word on the schematic request.

Peete.
 
Nov 6, 2008 at 1:28 AM Post #13 of 18

n_maher

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Peete,

I'm not entirely sure you're going to have much luck getting the schematic from the manufacturer, even if the amp is out of production.
 
Nov 6, 2008 at 5:19 AM Post #14 of 18

Pricklely Peete

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Quote:

Originally Posted by n_maher /img/forum/go_quote.gif
Peete,

I'm not entirely sure you're going to have much luck getting the schematic from the manufacturer, even if the amp is out of production.



Agreed.

I'd have to say my chances are slim to none.


Peete.
 
Nov 6, 2008 at 8:42 AM Post #15 of 18

dsavitsk

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Quote:

Originally Posted by Pricklely Peete /img/forum/go_quote.gif
Update for d, still no word on the schematic request.


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.
 

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