Author Topic: Power Transformer output Tube Rectification (Read 6446 times)

Johnny10 · « **on:** August 03, 2016, 01:08:35 pm »

I will have to admit I am a beginner in Tube Amplifiers.
I just picked up an old mono tube amplifier that uses a 5Y3 Tube for a rectifier.
I have the schematic for the amplifier and it is noted the output from the rectifier should be 340volts.
However I am getting 450 volt output.
Transformer center tap with 720 volts output.
5Y3 Test good in tube tester.

Why am I not getting 340Vdc?

Gyro · « **Reply #1 on:** August 03, 2016, 01:25:03 pm »

A first thought, have you got the other tubes fitted and are their filaments lit? An unregulated HT supply like that will rise a lot if not loaded.

P.S. A dire warning... It you're a beginner in tube amplifiers then you MAY be a beginner in such high voltages (apologies if not). There's a lot of energy stored in the reservoir capacitors, not to mention the direct high voltage from the transformer. It will try to kill you if you're not on your guard. Check twice that there are no lingering voltages after you unplug it and don't connect or disconnect test clips while it is powered.

P.P.S. There's a safety sticky related to Tube amplifiers over on DIY Audio that is worth reading...

http://www.diyaudio.com/forums/tubes-valves/30172-safety-practices-general-ultra-high-voltage.html

Johnny10 · « **Reply #2 on:** August 03, 2016, 01:31:50 pm »

I pulled all the tubes because the EL84 started to red plate.
Then started checking voltages.
Have never even input a signal.
But I will try your suggestion.

oldway · « **Reply #3 on:** August 03, 2016, 01:35:51 pm »

Quote from: Johnny10 on August 03, 2016, 01:31:50 pm

I pulled all the tubes because the EL84 started to red plate.
Then started checking voltages.
Have never even input a signal.
But I will try your suggestion.

For sure, the power transformer has been replaced but secondary voltage is too high.
It would be more or less 580V (2x 290V) and not 720V.

Tomorokoshi · « **Reply #4 on:** August 03, 2016, 01:42:50 pm »

In note #4, it mentions that measurements are made with a VTVM. What kind of instrument are you using to measure the voltage? In DC mode on a VTVM a 120 HZ rectified AC signal will wind up being filtered and averaged in some way this is not really what a modern RMS voltmeter will do.

Johnny10 · « **Reply #5 on:** August 03, 2016, 01:44:23 pm »

Thank you for your suggestions.
I replaced all tubes and let unit warm up for longer and voltage settled at 358.
Guess I am thinking unit was warm in less than 20 seconds that I left it on.
However after few minutes it settled in.

Thank you for the HV advice. I have been extra careful.
Even measured time for capacitors to discharge on there own.

I think learning curve on some of this old equipment is long since with point to point wiring it is not always easy to follow circuits as free terminals are used as tie points for various components.

Johnny10 · « **Reply #6 on:** August 03, 2016, 01:47:51 pm »

I was using a Fluke 87. Yes it is RMS.
However I have just refurbished an old Heathkit IM-11 for use with the tube equip.

Andy Watson · « **Reply #7 on:** August 03, 2016, 01:48:29 pm »

Because valves are rubbish

Quote from: Johnny10 on August 03, 2016, 01:31:50 pm

I pulled all the tubes because the EL84 started to red plate.

So you were operating the power-supply with minimum/no load. See the chart at the bottom of page 4:
http://r-type.org/pdfs/5y3gt.pdf
The chart indicates that the rectifier has a forward voltage drop of about 30 volts, increasing to about 150V when fully loaded. This would be consistent with the transformer output delivering \$\sqrt{2} \times 350 = 510V_{peak}\$, plus a bit extra for "regulation".

dmills · « **Reply #8 on:** August 03, 2016, 02:17:17 pm »

C7 and C9D would be by suspects for a red plate, possibly C9D needed time to reform if the thing has not been used for a while, and if C7 is one of those awful wax paper tubular things then just replace (all of them) as a matter of principle.

Personally I would replace both with modern parts, a 50V 47uF elco is a small part these days, no need for it to be part of a big multi section affair.

Do be careful in there, plenty of bits that will reach out to you if you let them.

Regards, Dan.

Johnny10 · « **Reply #9 on:** August 03, 2016, 02:32:22 pm »

Amp has been in barn for 15 years.

Ian.M · « **Reply #10 on:** August 03, 2016, 02:48:32 pm »

Swap the two EL84 output valves. If the same valve still red plates in the other socket, its a bad valve that's either got an internal short (probably heater - cathode) or has gone gassy. If the fault stays with the same socket, take Dmills' advice on which caps to check.

Seekonk · « **Reply #11 on:** August 03, 2016, 03:32:29 pm »

This should be a warning for those who want to replace the rectifier tube with diodes. The voltage will be even higher and exceed the standard 450V cap. This is the reason most of these amplifiers have grounded cathodes. They wanted voltage as high as possible without exceeding 450V of standard cap. That extra 30+ volts of bias gave that to them. If replacing the main 450V cap, consider using three 300uF 250V caps in series with 270K balancing resistors across each one. It will likely be cheaper, give you more margin and allow you to use solid state rectifiers for a little more power and take a little load off the transformer.

Gyro · « **Reply #12 on:** August 03, 2016, 05:46:10 pm »

Well it looks as if it's in pretty good condition if it's been in a barn for 15 years.

If only one EL84 is red-plating then I suspect it will either be C7 or C8 (whichever one relates to the hot tube) with high leakage. Also check that R13 / R14 haven't gone high resistance. The cathode bias decoupler C9D is shared so would affect both tubes.

As Dan said, it is best to replace them us, use 33nF 400V metallized Polypropylene (MKP). Replacing C9D would be prudent too (drying out would affect bass response and stability, shorting would kill the EL84s) just remove the connection to the C9 terminal and add a 47uF 50 or 63V electrolytic.

One other possibility is a poor socket connection, anything that causes the grid pin to float high will cause excessive anode dissipation. This could be an open pin connection or excessive leakage to one of the higher voltage pins.

Luckily this is a cathode biased design so you don't have to start debugging fixed bias supplies, dodgy bias adjustment pots etc. Everything is local to the EL84s.

Another Edit, sorry: Looking at the photo again, the wires going through the two chassis holes in the middle look distinctly dodgy. It looks as if the insulation is rubbing against the sharp edges, you need to reinforce the insulation there. It would also be prudent to add 100R 1W resistors in series with the EL84 screen grids (pin 9), this would improve stability and give some protection against over-dissipation.

Johnny10 · « **Reply #13 on:** August 03, 2016, 06:26:17 pm »

Yes, inside looked quite good for barn storage because the bottom cover was intact.

Switched EL84's and the hot spot stayed with same tube socket did not move with the tube.

I am in the process of ordering new Caps.
Also I am planning on keeping this unit for HiFi use not switching to Guitar amp.

Bought an old Grommes Custom mono amp with this one but it is in pretty rough shape inside.
I think they call it Music Lovers Hi-Fi with 6V6 tubes.

SeanB · « **Reply #14 on:** August 03, 2016, 07:18:14 pm »

Change those caps and change the 470k resistors, using 1W carbon film or 0.6W metal film types. Every one of those black film capacitors needs to be changed now, do not even think of leaving them in place as they will kill tubes or transformers. I will also bet that every resistor other than the wirewound 150R one has drifted high as well.

C13 can be replaced with a regular 47uF 450VDC capacitor, commonly used in SMPS stages. I would suggest adding a series 33R 5W resistor in series with the positive supply from the rectifier V5 to keep the low ESR ( compared to the original ones likely high ESR from new of around that) from causing the rectifier to exceed it's plate current limits.

Johnny10 · « **Reply #15 on:** August 03, 2016, 07:33:55 pm »

I appreciate all the suggestions.

Adding a better schematic:

Johnny10 · « **Reply #16 on:** August 03, 2016, 09:51:46 pm »

SeanB could you explain that 33R 5 Watt resistor and ESR again I am not quite sure I understood that.

tautech · « **Reply #17 on:** August 03, 2016, 10:05:54 pm »

Quote from: Johnny10 on August 03, 2016, 09:51:46 pm

SeanB could you explain that 33R 5 Watt resistor again I am not quite sure I understood that.

Should you replace C13 with a commonly available 47uF cap that is likely better suited to SMPS application not a HV linear PSU, the ESR will be much lower than a cap better suited to this application.
Sean correctly suggests a resistor in series with that cap can overcome the lower ESR of SMPS suitable caps.

Study of some cap datasheets will give you the "heads up" and remember the ripple on this PSU is only at 2x mains frequency, not at 10's of KHz like SMPS where suitable caps ESR can be rated @ 100 KHz.

Cubdriver · « **Reply #18 on:** August 04, 2016, 07:07:24 am »

Another thing I didn't notice having been previously mentioned is that you'll likely see higher B+ voltage these days as the amp was probably designed for operation at 110-115Vac and our line voltage now is typically 5-10V higher than that. Based on that, your loaded B+ reading is about what I'd expect to see.

-Pat

Cubdriver · « **Reply #19 on:** August 04, 2016, 07:14:39 am »

And I'll second the comment about replacing the black caps with the yellow lettering - they're old paper in oil caps, and likely leaky as hell.

-Pat

Johnny10 · « **Reply #20 on:** August 04, 2016, 04:46:16 pm »

Would you use these caps in the Amp I am rebuilding.
West-Cap or Aerovox
Problem is only the .047s

Found these at the same seller as amps.
There were no .033 lying about.

Gyro · « **Reply #21 on:** August 04, 2016, 04:53:36 pm »

Those look like NOS parts, I'd go for new ones unless you're aiming to keep the 'look'.

Johnny10 · « **Reply #22 on:** August 04, 2016, 05:13:36 pm »

Some of these are pricey online.
Maybe I should sell them to help finance the restore ?
;-)

Seekonk · « **Reply #23 on:** August 04, 2016, 05:19:49 pm »

I've sold many of them on ebay for a pretty price, but I would never use them. Go with polypropylene.


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Author Topic: Power Transformer output Tube Rectification (Read 6446 times)

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