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multimeter is reading very high plate current

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Posted on February 20, 2017 at 11:44:50
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 I have been struggling with measuring the plate current on my ECC99 tube. I break the circuit and use my multimeter and get measurements well above 100mA. So, I put in different tubes (12BZ7) and adjusted the plate load resistor and cathode bias LED (I use LEDs instead of a resistor/cap) accordingly. The 12BZ7 tubes also measure very hot plate current, once again well above 100mA.

So, I am wondering about 2 things:

1. Is my multimeter measuring current correctly?

2. If the multimeter is OK, then how do I get the plate current significantly lower?

Schematic is attached.

thanks for any input.

***************************
UPDATE:

When measuring mA, I needed to put the multimeter lead into the AMP plug-in instead of the V/Ohm plug-in. My amateur mistake. that said, I got very nice feedback regarding voltage swing and will be upping the bias voltage to -4V.
Thanks to all who responded!

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

UPDATE #2 2/21/17

I think there is something wrong with my multimeter. After reviewing all of the plug-ins on the multimeter, there are actually 2 spots where "current" can be measured. 1) 10A and 2) 200mA. I was using the 200mA plug-in and was getting the bad readings. I think its time to get a fluke.

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


Pat



 

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RE: multimeter is reading very high plate current, posted on February 20, 2017 at 12:06:59
Stuben
Audiophile

Posts: 669
Location: Guber Ohio
Joined: December 30, 2005
 Might be a stupid question....do you know the DCR for the LED? Never used one...I'm sure those who have will chime in.

Stuben

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 12:09:41
Palustris
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Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 Install a 10r resistor between the diode and ground. Measure the voltage and use ohm's law to determine the current.

Draw out the operating point on the plate curves and you will see that there is no operating point that conforms to your schematic. A plate supply of 325V with a load of 22k laid out on the plate curves shows a plate voltage of about 90V and plate current of about 11mA when the bias point is -2V. You will only have about 40V peak to drive the 2A3 and you will likely need about 60V so you may want to use two LEDs to bias the ECC99 at -4V so as to give you much more voltage swing. -4V bias should give you a plate voltage of 125V and about 9mA plate current with a voltage swing of about 80 V peak.

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 12:47:28
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 There is no way you can pass 100ma though a 22K resistor so there is either something wrong with your meter or your hookup procedure. (were you bypassing the 22K resistor when you did this test?)

all you need to do to meadure the plate current is measure the DC voltage across the 22K resistor at idle then use ohms law. In your schematic you have 322V B+ and 106V on the plate so that is 216V across the 22K resistor. Ohms law tells us that 212V / 22,000 = 0.0096A or 9.6ma.

dave

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 12:54:41
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Palustris - goint point regarding the swing voltage. I will try upping the bias to -4V. Thanks

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 12:57:01
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 I must have something amiss with my meter or measurement hookup process. Regarding my process, I disconnect the 22k resistor from the tube pin. Then, I clip one lead of my multimeter to the tube pin and the other lead to the bottom of the 22k resistor. Turn on the power and measure mA. I think this is correct, but let me know otherwise. I still get a very high 100+ mA reading.

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 13:38:05
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave, My mistake. I did not put the multimeter test lead into the AMP plug-in instead of the V/Ohm plug-in. My bad. Pat

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 21:48:43
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Hi Pat,

I'm curious whether you are now getting the expected reading?

I strongly suggest you perform the measurement exactly how Dave suggested.

91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: multimeter is reading very high plate current, posted on February 20, 2017 at 21:51:28
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Pat, that is not how I would do it. I would perform the measurement precisely as Dave suggested.

91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: multimeter is reading very high plate current, posted on February 21, 2017 at 05:24:46
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Dave's method is what I am using going forward. It gives the appropriate current reading.

I think there is something wrong with my multimeter. After reviewing all of the plug-ins on the multimeter, there are actually 2 spots where "current" can be measured. 1) 10A and 2) 200mA. I was using the 200mA plug-in and was getting the bad readings. I think its time to get a fluke.

Pat

 

RE: multimeter is reading very high plate current, posted on February 21, 2017 at 06:21:45
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 I would test the meter with an external 9V battery and a 1K resistor to confirm its function.

dave

 

65uF capacitor?, posted on February 21, 2017 at 09:31:41
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 One question - is the 65uF capacitor really connected as shown?

Agree with others about measuring voltage across the 22k resistor, you don't want to mess with using your meter as an ammeter.

Check the manual for the meter, they can give weird readings if you're not hooked up to the right terminals. I'm sure it's fine.

I agree that the current is too high though, because the plate voltage is too low. Try and add more diodes in series, to get it to around 180-220V. With the plate voltage so low, you won't get a lot of output before the driver clips.

 

RE: 65uF capacitor?, posted on February 21, 2017 at 10:01:36
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 The 65uF cap is really connected as shown. This is connected in the UltraPath method. As I understand it, the UltraPath method eliminates the cathode bypass capacitor from the signal path.

 

RE: 65uF capacitor?, posted on February 21, 2017 at 10:06:13
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004



If you connect the 65uF capacitor as shown, it's going to be effectively in parallel with the output transformer... this is BAD, it's going to kill the high frequency response of the amplifier. Old radios would put a much smaller value cap in this location to roll off the highs.. I can't honestly believe it even works with it connected as shown.

If you connected it instead to the other end of the output transformer primary (the high voltage supply, as opposed to the plate of the 2A3), this would properly bypass the cathode bias resistor of the 2A3.

I assume the attached image is what you meant to do, right?

 

RE: 65uF capacitor?, posted on February 21, 2017 at 10:07:50
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 I have it connected to the B+ that is going into the OPT. So, good call on your part as it needs to be redrawn. Thanks!

 

RE: 65uF capacitor?, posted on February 21, 2017 at 10:14:17
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 I thought it had to be a drawing error :)

I notice there's only 2V on the bias diode of the ECC99, this explains the high current. If you add a couple diodes and get this to about 6V, I think everything will balance out nicely and you should be ready to go.

 

RE: 65uF capacitor?, posted on February 21, 2017 at 10:15:23
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 Yesterday I added a 2nd diode and got the bias to 4V. I will give 6V a try as well. thanks again for the input.

 

RE: 65uF capacitor?, posted on February 21, 2017 at 10:21:20
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Sounds like you're on top of things. I strongly suggest reading the front section of any RCA tube manual, where it is explains how to design resistance capacitance coupled amplifiers, and how to draw load lines onto tube curves. It's all very intuitive, and graphical.. no heavy math or theory involved, and you will have a way better idea how all this works.

 

It would have to be in the multimeter., posted on February 21, 2017 at 12:30:04
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 At 100ma,that would mean one triode section dissipates 10 watts and we know it can't do that. If you have any stability issues,I would can the LEDs and go to a high quality cathode resistor.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: 65uF capacitor?, posted on February 21, 2017 at 14:13:55
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 This is a great suggestion... and something I have not done. Taking a quick a look through the RCA Radiotron Manual (version 10), pages 1-21 look like a great introduction. I will be reading this as a refresher over the next few days.

I find endeavours more rewarding when I have some knowledge of the fundamentals/ basics. From that point I can build, question, experiment... and can develop understanding.

My tube learning was messy and my knowledge remains gappy. When I started seriously considering building amps as a hobby, I read the NEETS training manual (just the first few modules related to electricity, not the tube part) then goodness knows how many posts and tube-specific articles of various types... then attempted an early edition of Morgan Jones, which was a challenge. I learned how to calculate various types of circuits (and even designed some) and worked through converting schemas to layouts... all before building/modifying my first amp. I'm certainly not suggesting other beginners take this approach(!) - there are more efficient ways.

I still have so much to learn, but time is limited and becoming more so.

91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: 65uF capacitor?, posted on February 21, 2017 at 15:15:13
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 You can downloaded RC-30 too, It's a bit newer than the '10. It really help to have the basics in your pocket. The rca tube manual isn't going to turn anyone into a prodigal amplifier designer, but it will give you an idea what's what and why.

 

RE: 65uF capacitor?, posted on February 21, 2017 at 16:16:30
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Thanks for the info max!

I think I have most of the very basics sorted... or at least know when to look, where to look to find out, and hopefully how to ask questions. Actually, maybe not so great on that last point ;)

Still, a refresher is in order. I just found RDH4 & 10, RTM30 and a heap of others stored on my HD... and also have a few more recent texts in hardcopy. I won't be reading them all in their entirety, but the key sections of the RCA manuals would be a good start.

Setting up and using a basic test rig will be next big challenge...

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: multimeter is reading very high plate current, posted on February 21, 2017 at 16:24:50
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Just when you say "appropriate current reading", do you 1) the current expected form the circuit (per Dave's post) or 2) the very high mA you were initially reading? I assumed the former, but some might be assuming the latter...

91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: 65uF capacitor?, posted on February 21, 2017 at 16:30:41
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009
 "The rca tube manual isn't going to turn anyone into a prodigal amplifier designer..."

I'm still trying to get my brain around this one!!!! :) :) :)

Chris

 

RE: multimeter is reading very high plate current, posted on February 21, 2017 at 16:50:22
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 By "appropriate", I am expecting around 9 mA just like dave and others have stated which is supported by the math.

 

Get a RDH3, posted on February 22, 2017 at 07:27:18
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 They show up on ebay and the used book sites for about $12 and have all the important stuff in a much easier to digest form that its big brother #4.

dave

 

RDH4, posted on February 22, 2017 at 08:20:12
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 I think RDH4 is pretty great, I especially like the section about power supplies and hum mitigation. I believe it's all online now too. The stuff about hi-fi is very dated, but still useful.

There's also all sorts of basic textbooks out there about tube electronics which were intended for vocational level radio repair courses, from the 1930s - 1960s. I have a variety of books of this nature - all of them describe how to draw AC and DC load lines, and find operating points, etc. Some are better written than others but all are sufficient for an introduction and the depth is a little better than the tube manual, it's just the RCA tube manual is so ubiquitous. Keeping eyes open in used bookstores is a good idea.

Negative feedback isn't covered very well in most such books, but understanding the subtleties of negative feedback design isn't totally necessary to troubleshoot and repair amplifiers.

 

Thanks Dave - found pdf! I'll start here. nt., posted on February 23, 2017 at 19:37:21
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 .
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

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