Converting to 2a3 mode

[saildoctor]

Hi all, I'm going to switch over my paramounts to the 2a3 configuration to try out some Philco 45s I bought! I had some trouble finding in 3k 25w resistors for the cathode, so I bought two 4k ones instead and will use them in series with one of the 2k ones that are already in the setup. I've been looking at my manual and I noticed something - the schematic drawing for 2a3 setup shows a 249k grid resistor (Rg) going from the driver output to ground, while the instructions later say that resistor gets left out. Which is correct? Or is there something I'm missing?

[Paul Joppa]

OK, sorry for this:

1) In the original design, that resistor should not be there; it is an error in the manual.

2) With the "soft-start" 5670 driver upgrade (and Paramount v1.1), that resistor is back, but is now 300K, is shown on the driver schematic, and is installed prior to the coupling cap not after. (It is actually installed as a jumper on the PC board.)

3) The circuit error is still there in the v1.1 manual, unfortunately - it still shows the 249K resistor for the 2A3, and it should still not be there.

I do recommend the new driver and board for any 2A3 or 45 Paramount, for two reasons:

1) It protects the output tube grid from transient high voltage at startup; this has caused arcing in a (very few) tubes. As far as I know, this is only a severe problem with the EML 2A3 (and I expect, 45) tubes which should NEVER be used without this change.

2) It allows the precise driver plate voltage to be set to correspond to your house's power line voltage, optimizing the operating point. The operating point of direct coupled circuits is usually very sensitive to the power line voltage.

If you have the original design and  you use an old-stock 45 or 2A3, or the 2A3 we supply, you are not likely to have any arcing problem. If you have the original design and your power line is close to 120 volts and your 12AT7 is close enough to the spec, then you will probably be about right on operating point. You can measure the driver and output tube plate voltages and post them here; I'll check them against my calculations and if they are not close to optimal provide suggestions for improving them. This would usually be just adding a small resistor in series with the LED on the driver cathode.

[saildoctor]

Thanks Paul! So I'm not crazy after all. I purchased and installed the soft start boards already with the intention of trying out either 45s or 2a3s.

My speakers are Tekton 8.1s- they have Fostex 207E's with Fostex 90A supertweets. With the 300bs I currently use I have a 5:1 voltage divider on the input, and i have a FP3 with 80k input pads. I will remove the divider on paramount, change the FP3 pads to something lower, and I'll report back how successful the 45s work with my speakers!

[Grainger49]

Thanks Paul! So I'm not crazy after all.   .  .  .  .   

We can not be certain of that conclusion, you are part of the community now ;-)

[saildoctor]

True Grainger, my roommate claims I've already gone over the brink.   I did the necessary circuitry changes to go direct coupled yesterday night.  All of the resitances checked out.  I decided to connect  my voltmeter to a2 and ground during the first hot test of the first monoblock - my meter read somewhere around 550V so I switched off immediately.  Not sure if something went wrong in what I did but it was getting too late to do a thorough inspection.  I'm guessing a small voltage spike upon turning on might be expected, but what I got seems much to high.  I'll go over my wiring today and report.  If it checks out then I guess my next step should be trying the same voltage test on the other amp?

I noticed that the circuit traces on my PSU pcb are a bit different than the pictures in my manual - namely I see a trace coming back into the board from the middle filament supply in the picture and none (on top) of my board.  Perhaps they've been revised since the manual.

[Grainger49]

Don't let your friends skepticism worry you.  I went over the edge sometime in the 70s.  Your friends will either embrace it or ignore it after a decade or so.

The Paramount PS capacitors are rated 135uF at 700V, that is what you get when you put two 170uF/350V caps in series.  So the power supply "floats" high when it turns on.

Think of it this way.  When the power is turned on (this is a solid state rectifier which acts instantly) the tube isn't conducting because the heater and cathode are cold.  The power supply (PS) "floats" to the output of the "voltage doubler" before the cathode starts to get warm.  The cathode takes several seconds to warm up but the PS at the plate is on instantly.  There is no current so the plate, A2 (for the 2A3), is at the same voltage as the output of the PS.

Aside - Ohm's Law: V=I*R.  When there is no current the I*R end of the equation is equal to zero (zero times anything is zero).  So there is no voltage drop in the PS through the 270 ohm resistor nor through the plate choke.  It is like the output tube plate is connected to the output of the diodes.  Which, by the way, no one tells you.  But rest assured it is higher than the 470V shown on page 9 of the Paramount manual for the output of the 2A3 PS Pi filter.  It is also higher than the 454V shown at the voltage measurements for tube pin A2 (2A3 output tube plate).

It is brief, transient, and the reason that capacitors have a surge voltage rating.  PJ has taken that into consideration in his design.  The voltage should drop in a few seconds, well under 10 seconds.

There are two heater/filament outputs from the PS board as the 300B and 2A3 take different voltages.  The filter choke, FC-1, is also wired differently for these two tubes.

[saildoctor]

Right Grainger, that all makes sense. I checked everything over and tried powering both amps. One did settle down a bit, but the other is worrisome.

Amp 1 - a1:225 a2:498 a3:166 a4:223

These numbers seem like they work although 498 still seems high. So plate to cathode is at 275 which is max for a 45. And the grid bias is -59 or so.

Amp 2 - a1:296 a2:556 a3:186 a4:294

A2 does not settle down, even after several minutes, it stays up around 550-560. I measured across a1 and a4 and got 2.6v. I set the grid/driver voltage a3 to 1/3 of that. While plate to cathode is 265v, the grid bias is -108 this way. Any thoughts?

I measured my powerline voltage across the IEC also, it read about 120.3v FYI.

You know, I just realized this saildoctor tag I chose could be easily taken the wrong way. I am in fact a doctor of sails (sailmaker), not a doctor who sails!

[Grainger49]

Ok, first swap the output tubes and take the measurements.  If the high voltage follows the tube it is the tube.  Then if it stays with amp 2 swap the driver tubes and see if the voltage changes. 

If swapping tubes changes the amp that the high voltage is in then contact Eileen.  Her email is on the contacts page.

If not, post back.

[saildoctor]

Ok! I switched tubes and again checked out amp 2. Again a2 was a steady 555v and a1/a4 were around 300v. I didn't have time this morning. I have noticed that in both amps I get 6kohms (my cathode resistance) across all 3 pins of the 8vct of the transformer- measured strait off the pins. Should I be reading this on the +DC pin? Just curious.

Thanks for all the info btw! I've been reading this forum for awhile now and I've learned quite a lot from all the posts. I've even built my own hybrid tube/MOSFET headphone amp using some leftover parts of my fp3 (I changed over to 3 6sn7's.) you know, to bide the time until the smack comes out?

[Paul Joppa]

Something is wrong here. The voltages on the tube, and the power supply voltage, indicate that the tube is cut off (not conducting), but the cathode voltage indicates 50mA through the 6K cathode resistance. These cannot both be true at the same time.

There may possibly be an intermittent connection somewhere in the cathode resistor circuit. I can't think of anything else right now - at least that's a place to start.

[saildoctor]

Loose cathode connection it was, I feel like such a dope!  The wire I have connecting the 2k to 4k cathode resistor had one end crimped but not soldered.  Thank you for the help.  Apologies for the for causing anyone grief.

Amp 1 - a1:223 a2:495 a3:165 a4:221

Amp 2 - a1:222 a2:492 a3:164 a4:219

I have it playing right now, it sounds great so far!  And plenty of volume with my speakers to boot - using the stock 249k input resistor setup I still have excess gain.  I've been thinking I might eventually drill a hole in the top plate and install a rotary switch to make a variable voltage divider on the input of each amp.

[saildoctor]

It's alive!

[saildoctor]

After ~20 hours of play time I've redone the voltage and resistance checks.  Here are my results, the (45) plate voltages are still pretty high compared to what I was expecting.  When I had everything set up for the stock Electro-Harmonix 300b's my a2 volages were 436/440 ~3% above spec.  My multimeter is from Radioshack, model unknown. I had trouble finding the 25watt 3k resistors in stock online, so instead I ordered a total of 2 4k's which I put in series with one 2k from the previous 300b setup.

Resistance Check:  (L amp / R amp)  (*=variable, -=NC)
1 */*
2 -/-
3 0.2/11.6
4 -/-
5 5.99k/6.00k

6 6.1k/6.12k
7 284.8k/283.6k
8 11.0/38.2
9 284.8k/283.6k
10 0.2/0.1

11 36M/33.4M
12 -/-
13 0.2/0
14 -/-
15 -/-

16 */17M
17 0.2/0
18 0/0
19 284.3/283.4
20 100.0k/99.9k (I have a 2.5x voltage divider at my input using 249k in series w/source and 100k across terminal 18/20.)

A1 6.01k
A2 */*
A3 284.7k/283.6k
A4 6.00k/6.02k

B1 0.2/0.2
B2 7.17k/7.67k
B3 100.3k/100.2k
B4 284.4k/283.3k
B5 0.2/0.2
B6 283.9k/283.6k
B7 can't reach
B8 can't reach
B9 can't reach

Voltage Checks (L amp / R amp) - One one amp I measured all terminals, not just ones outlined in the manual - that's why some have two values and some only one.
1 498/492
2 *
3 0
4 -/-
5 223.8/224.7

6 223.8/224.8
7 168.0
8 0
9 168.1/169.1
10 1.8mv/1.3mv

11 229.6/229.5
12 160.7/161.1
13 1.6mv/-
14 93.0/92.0
15 24.6/23.5

16 513/507
17 2mv/0.7mv
18 0/0
19 168.5/169.2
20 -/-

A1 225.4/226.0
A2 499/496
A3 168.0/168.4
A4 222.8/223.4

B1 3.25vac/3.2vac
B2 3.7/3.9
B3 0.6mv/3mv
B4 168.4/169
B5 1.4mv/0
B6 299.5/299.0
B7 can't reach
B8 can't reach
B9 can't reach

[Paul Joppa]

The 45 draws much less current than the 300B (!) so the power supply voltage will drift up. Yours is 4-5% higher than I would expect for a 120v power line; I speculate that your power line is a bit high, closer to 125v. If true, then the 495v measurement is reasonable. Reducing the plate to cathode voltage by raising the driver plate voltage will increase the plate current, so it won't reduce the strain on the 45. What it will do is move to a non-optimum operating point for the output transformer.

Your present operating point is about right for the power line voltage.