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Old 18th August 2006, 07:03 PM   #1
Molloy is offline Molloy  Europe
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Default 2A3 grid DC voltage

Hi,

I've just finished an 2A3 monoblock. Schematic is from JE Labs.
http://members.myactv.net/~je2a3/JELsc-ST2A3DX.jpg

I have a problem, no matter which coupling cap i use i always end up with 1.3V DC at the 2A3's grid.
When i turn on the amp everything is allright, the DC voltage takes about 5 minutes to reach 1.3V and then it remains like that.

I have 52 volts from cathode to ground (1119 Ohms resistor), 46 from grid to cathode, 280 from anode to cathode. The tube works within its operating points. Sound is poor.

The 2A3 is an used RCA, still, i did paid 60 euros for it. The owner said it was electrically perfect. :-)

The 220k grid resistor is grounded, no doubt about that. What can be the problem other than a defective tube? I have to try everything before i email the seller.

Any ideas?




Al.
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Old 18th August 2006, 07:22 PM   #2
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You've verified it's not the coupling cap, and the grid resistor is intact.... sounds like the tube is pulling/leaking grid current to me.

For kicks you could replace the 220k with something lower (47k or so) and verify that the voltage drops accordingly.

I assume that with the tube unplugged the voltage stays at 0 volts?

Do you have another (maybe current production and cheap) 2A3 to try?

Pete
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Old 19th August 2006, 12:38 AM   #3
kevinkr is offline kevinkr  United States
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I have a couple of thoughts..

Idle current is pretty low at 43mA, imo 2A3 sounds much better around 60mA.

Voltage plate to cathode at 280V may be a little high, particularly at higher currents. 240V - 250V seems more like it..

Grid current is about 4uA, which may not be atypical of an older 2A3 like this with some hours and 60yrs to get a little gassy. I prefer grid resistors in the range of 100K with older tubes like this.
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Old 19th August 2006, 05:54 PM   #4
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Hi,


Thanks for the replies, they have been helpful.
I did perform a seies of tests:

First i pulled the tube and read 0 volts at the grid. Next i changed the 220K resistor to 100K, the DC voltage dropped from 1.3 to .8 Volts. With a 50k grid resistor the DC voltage only goes up to .5 volts and i did raised the current a little (changed that 1119 cathode resistor to 1k).

Then i followed Kevin's suggestion and raised the current to 57 mA. Before any sonic evaluation was made i found i was running the 2A3 beyond its power rating - with a 820 cathode resistor the 2A3 was dissipating 15.5 Watts- (.57mA x 272 plate voltage). With this operating point the 2A3 had 1.4DC voltage at its grid (100k resistor). Back to the 1k cathode resistor then.

I think lowering the B+ (easy enough as i am using a choke) wouldn't be a good idea because the driver stage needs as many volts as possible. In fact i did found out that the 6SN7 sounded much better with a B+ of 550, though perhaps the reason for the big improvement came from the fact that i was feeding the 6SN7 with its own power supply. This was when i was breadboarding the amp, now both stages are running with the same power supply.
Main reason for using this driver stage is that i need a .5V RMS input sensivity.

BTW, the amp has started to sound good. I'm hearing it as i'm writing this... All i did was to change last night the .22 uf FKP WIMA coupling cap to a "vintage" .33uf cap i had at hand -came from a WWII receiver and surprise! same 1.3 DC not leaky at all.:-) and had all the night the amp playing music.. Don't know if it has been the cap change or the burning in process but the amp sounds much better now, could also be the 50k grid resistor? Could it also be that the grid current has lowered?

The amp it's listenable now yesterday it wasn't. I expect it will improve in the next week if not i'll change the driver stage to SRPP and use a preamp. Or replace the output tube.

I have been hearing so much about the myth of DHT's that i had to try. Sound is only ok at this moment but i'm hearing things i've never heard with any of my other tube amps.

So with a steady .5 DC at the grid, will this tube last long? The RCA looks great (i should post a pic), solid base, grey plates with no stains, no blue glow at all; it looks healthy. Then you may have a cancer and look healthy.
I will really like to keep the tube as i know the chinese 2A3 Golden dragon would be the substitute.

Is that amount of grid current a big problem? Will the tube last a year? If it dies after a year of good service it will be ok.
Anyone knows what to expect?



Offtopic:

Pmillet, i did build your 6sn7/6as7 headphone amp....sounds great. :-)
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Old 19th August 2006, 10:49 PM   #5
kevinkr is offline kevinkr  United States
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Hi Molloy,
Reduce the plate voltage slightly and make sure your dissipation is under 15W and you will be fine. With 12W dissipation your 2A3 should last more than a few years with normal use.

I am running a used pair of 75 yr old UX245 in one of my amps at 3/4 of their dissipation rating and they have lasted 10yrs so far.

Note that some grid current is perfectly normal in these early dht, and I don't believe that a few uA of grid current is an issue. Also once you drop your plate voltage a little more I think you will find the grid current will drop as well.

I'm not sure why your driver stage requires 550V to drive a 2A3, my latest implementation which is all dc coupled requires only 300V on the driver stage (a choke loaded mu follower) and this is to satisfy required dc conditions at the 2A3 grids. The 2A3 needs less than 100Vpp to drive it to full power. More information?

Glad it is working better.
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Old 20th August 2006, 04:31 PM   #6
Molloy is offline Molloy  Europe
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Hi kevin,

The driver stage has a B+ of around 330V , a bit less than the JE schematic. What i did say is that i tried the driver stage with a B+ of around 500 and sounded better, that B+ came from a separate supply that i was developing for an amp still unfinished -a 6av6/6as7 all SRPP amp.

Anyway the amp is starting to sound really good, i am happy and will keep the tube. Your info about some grid current being ok was all i wanted to know.
Thanks!




Al.
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Old 3rd September 2006, 07:19 PM   #7
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1.3V/220000 ohms = 5.909uV
this is the cap's leakage current, dont worry, simply to do is replasement with better one...
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Old 3rd September 2006, 09:49 PM   #8
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Actually in this case it is grid current, not cap leakage, with the tube removed there was no voltage across the 220K resistor at all according to Molloy. I can't vouch for a specific tube when it was made, but after 70yrs of sitting around most are just a little gassy, in many cases with some use the getter will take care of most of it. Current production dht's from most vendors are not that hard (vacuumwise that is) and usually draw several uA grid current minimum even when new.
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Old 4th September 2006, 06:36 AM   #9
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Probably... I have a batch of russian 6S4S tubes from 1957, GM70 from 1984, but there is no problem with grid current...
2A3 is a nice triod, if it is original, not chinese fake production...
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Old 4th September 2006, 07:05 PM   #10
Molloy is offline Molloy  Europe
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Yes, Kevin was right, it is grid current, only 0.01 mA. But i have to say i love this amp, from now on no more pentodes or pseudo triodes for me , thanks.

I will post a pic soon, it is still looking ugly.
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