(1959) 2 output tubes died now theyre back, whats happening?

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(1959) 2 output tubes died now theyre back, whats happening?

Postby AntoAmpeg on Sat Jun 03, 2017 2:32 pm

yesterday I was using my 1959 while all of a sudden I heard a small sound like frying (lasted only millisecs) than a tiny volume drop and a darker tone, I looked at the power tubes and the pair on the right was dead and the pair on the left was completely redplating. I switched off amp leaving troubleshooting to today, this morning I opened it up and there was no sign of strange stuff so without touching anything I tried it again and to my surprise everything back working flawlessly, I tried it for an hour without issues.
As the quartet of EL34 are vintage mullards (XF2 single getter) that I put on a month ago (had modern Marshalls before without any issues) and its 3 years I have the amp (no issues), should I worry about something or do some other checks?

Thanks for any help
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby LD50 on Sat Jun 03, 2017 5:43 pm

The fact that a pair of tubes was misbehaving by shutting down and the others red plating does concern me.

Your amp was pushing and not pulling (or vice versa!)

If you look at the wiring you will see that the only thing that feeds a pair is the primary wire from the OT to plate (to pin 3 of each of the inner (V5 and V6 with a short link between that wire and from V5 to V4 and from V6 to V7).

You talk about left and right which does not help so you need to refer to tube number. You also need to tell us any rewiring or modifications. What year/Model is it - there are a lot of different 1959 amps from 1967 to present day!

The other potential area that the tubes are separated is the bias feed, the signal is split in the phase inverter and then each pair is fed via a 0.022uF coupler which blocks dc coming onto the grids (with grid leak resistors - typically 220k that are hooked up to the bias feed). Your problem could be a pair of tubes losing bias or a leaking output coupler, but those things would not normally be intermittent.

Check the connections of the wires from the board (green and orange) to the swamp resistors that go to pin 5 of the output valves (grids). Maybe you have a poor connection? Hoefully it is not your OT with an internal short that presents itself when hot.
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby AntoAmpeg on Sat Jun 03, 2017 7:34 pm

thanks for your reply, its a stock 1959HW, tubes 3 and 4 were dead (not glowing) while 1 and 2 were redplating entirely but now everything is working and I did not touch anything, connections and all the parts seem fine. Im no tech I dont have a clue what happened, I was guessing that vintage tubes could fail easily but I decided to post here to have some help ID like to avoid my amp to die... thank you very much
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby LD50 on Sat Jun 03, 2017 9:09 pm

I don't think you REALLY read my post.

Its not the tubes
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby AntoAmpeg on Sun Jun 04, 2017 8:40 am

Ive read it but as all wires and components seem fine Iwas trying to understand if I could have another option other than my OT risking to die. Your reply its pointing me to the fact that my OT is a risky business... can you please tell in what I could incur if it dies while playing? can it damage also PT, tubes etc?
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby Gooch on Mon Jun 05, 2017 2:23 am

If the tubes were "not glowing", it can only mean one thing, heater voltage not getting to the tubes. If there is a common connection for those two tubes only, start checking around there, may be a cold solder joint on a tube socket.
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby LD50 on Mon Jun 05, 2017 7:56 am

Gooch wrote:If the tubes were "not glowing", it can only mean one thing, heater voltage not getting to the tubes. If there is a common connection for those two tubes only, start checking around there, may be a cold solder joint on a tube socket.


The heaters for these are in a chain, V7 gets first, then V6 etc, so if the preamp tubes are getting ac heater juice then V4 and V5 will too.

The pair of tubes is the clue.
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby Dean Jr. on Mon Jun 05, 2017 10:37 pm

+1 with all the LD50 wrote.

This amp needs to be in the hands of a tech. I would suggest not running it until it has been fully evaluated...and repaired....because there is definitely a problem. One would not want to burn down that original OT in an original Super Lead. That would be an expensive repair and an even more expensive loss of value, ime.

Just to be sure here, AntoAmpeg....by 'not glowing' do you mean that the heater filaments in those two power tubes were not functioning?
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby AntoAmpeg on Tue Jun 06, 2017 4:47 pm

weel dont know the exact name but the central part was not lit like this:
Image
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby Dean Jr. on Tue Jun 06, 2017 5:40 pm

Yes, in that picture all four tubes have good heater filament activity. IF the heater filaments are glowing, then the tube cannot pass current. That problem is not directly linked to the excessive current draw---the redplating---on the other side of the OT primary--the other two power tubes. Perhaps LD50 has an idea how the loss of heater filament on one pair might cause biasing problems for the other pair.....I don't have an explanation at this time. +1 on what year this amp was built and what service it has had done over its lifetime.
Tech time, imho.
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby pdf64 on Tue Jun 06, 2017 7:18 pm

The symptoms described indicate that the amp would probably have 2 separate problems.
I suspect that may not be the case, and that the 2 'dark tubes just, in the heat of the moment, seemed dark in comparison to the pair that were heavily redplating.
What was the idle VHT and plate / cathode current with the old tubes, and the new tubes?
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby Dean Jr. on Tue Jun 06, 2017 7:35 pm

Okay...I reread and see that this is a modern HW MD 1959. Odd problems.....
+1 with pdf64....voltages and biasing should have been taken with the old tubes and the new tubes. One would want to know today what those voltages are, too. I am thinking you may have some solder joints that are suspect.....and/or some heat related malfunctions.
With all due respect....opening the amp up and putting the eyeballs on the components in the circuit is a valid exercise, but it is no substitute for approaching the problem in a full-on technical manner. Imho and ime, to 'play on' as if this odd occurrence never took place is asking for expensive trouble. :old:
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby AntoAmpeg on Tue Jun 06, 2017 8:35 pm

I dont know the voltage, Ive only taken current draw from each tube, every one was in the range of 26-33mA (stock Marshall and vintage mullards), is this helpful? if not let me know and I'll try to get whats needed

PS: V6 and V7 were dark no heat of the moment
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby Dean Jr. on Tue Jun 06, 2017 8:58 pm

Those current draw numbers are acceptable if that is a 'per tube' number. I suppose you have some sort of a 'modern' bias tool that reads each socket, right? Plate voltages are necessary to really know what the plate dissipation is. Say at 30ma per tube at 450VDC on the plates.....that is around 55% of max plate dissipation....workable situation, imho. Some like it warmer than that....
But.....when those tubes were redplating, the current draw was much, much higher than that. The reason for that needs to be ascertained, imho....as does the reason for the failure of the heater filament on that cold side. IF you play the amp before the problems are found, react to any change in sound....take a look at the power tubes.
I don't know if you are prepared to take voltage measurements on a live circuit safely. IF you are, then if and when the amp fails again is the time for investigation...while the amp is hot and malfunctioning. IF you don't do this kind of work on the amp, then as I said....it is tech time. And...intermittent problems are trials.
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Re: (1959) 2 output tubes died now theyre back, whats happen

Postby pdf64 on Tue Jun 06, 2017 10:51 pm

AntoAmpeg wrote:...PS: V6 and V7 were dark no heat of the moment

Ok, in that case there must be a strange fault in the heater circuit affecting V6 & 7, and a fault in the bias etc circuit causing V4 & 5 to redplate.
I don't see how the 2 faults might be inter-related, so 2 separate faults.
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