Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: johnsonad on March 22, 2011, 01:11:14 PM
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Hi Paul and team,
Spoke with Mike about plate chokes for my Paramounts. As I already have MQ iron, he does not sell the BH-6 seperately. He is going to make a pair of BCP-15's but needs to know if the rating should be 40 or 50 mA? The soft start kits are installed and working fine. Also, the parafeed cap will be increased to 10uF correct? If it makes a difference, I'm using the EML 2A3 mesh plate tube.
Thanks,
Aaron
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Choke should be gapped for 50mA.
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That was quick! Thanks Dan!
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Aaron,
I also just asked this of PJ as I decided to build my paramour IIs with 2a3s instead of the originally planned sr45, and he also said the 50 hy at 40 ma is the best choice for the BCP-15.
I just don't seem to be able to get confirmation from Mike L that I wanted to change the gap spec on these -- sure hope he gets the change before it's too late.
-- Jim
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I called Mike tonight. He is usually around in the evening time.
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All you have to do is look at the schematic and use Ohm's law to calculate what the plate choke current rating needs to be. The 2A3 in Paramount has 220V at the cathode and the cathode resistors are a total of 4K ohms. 220V/4000ohms=55mA.
ParamourII - cathode at 60V and cathode R is 1100 ohms. 60V/1100 ohms = 54.5 mA
For the 45 the current is typically more like 35-40mA.
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Popping in for a minute here.
The Doc B is correct, and I am wrong. The Paramount 2A3 need the greater current, so it normally uses the Paramount upgrade. The Paramour II and Stereomour were designed for 50mA current; the higher current may be tube to tube variation (?). I'll get back to this next week when I have my notes and files available.
It's the 45, or a 2A3 operated in 45 mode, that can use the 40mA rated choke in a Paramour or Stereomour.
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Popping in for a minute here.
The Doc B is correct, and I am wrong.
I am going to print this post and frame it.
I just calc'd the currents from the Vk spec on the maps in the manuals.
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You saw it here first, folks! :-)
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So confused!! :) I'll wait for Paul's response..... For some reason I thought the cathode was seeing around 205v with the new soft start (measured at center of hum pot) which would put it a little lower at 51mA and change. I should probably stay out of these high level discussions though :)
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That is possible. Might be that the 220V in the schematic didn't get updated in the new soft start schematic. At any rate the plate choke gapped for around 50mA is still the right choice over one gapped for 40.
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And Aaron,
gapped for 40 and 50 mA are your only choices on the bcp-15, so just go for the 50 mA version and youwill probably be ok, assuming the bcp-15 is going to work in the paramount, which I amit, is still not completely clear to me.
Ill be using mine in the paramour II, so I fel comfortable with that.
-- Jim
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Welcome back Paul! Hope you had a nice vacation! Just a bump on this to refresh your memory.
Regards,
Aaron
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OK, here's what I have now that I am back in town:
Paramount 2A3 original design: target was 55mA, can't use the BCP-15. Use the stock choke or the Magnequest BH-7. Target parafeed capacitor is 5.6uF for either, with the stock 3.3 up to around 10uF being an acceptable range.
Paramount v.1.1 (soft-start) 2A3 : Properly adjusted, the current is 50 +/- 2 mA, so the BCP-15/50mA or the BH-6 (Paramour upgrade) plate chokes are acceptable. (Note the Paramour upgrade transformer BH-5 is the same for Paramount or Paramour.) With the stock or upgrade BH-5 output transformer and BCP-15 or BH-6 choke, the parafeed cap is 10uF, range 5 to 18uF acceptable.
Paramount v.1.1 (soft-start) and a 45, using a 6K cathode resistance (two 3K resistors replacing the 2K resistors in the stock amp), and retaining the stock or BH-5 output transformer, the plate current is 35 +/- 1mA, so the BCP-15/40mA choke is acceptable. In that case, the parafeed cap would be 12uF, range 6uF to 24uF - assuming you retain the stock or BH-5 output transformer. NOTE: I separately have advised jrebman and Paully on specific changes, such as different output transformer impedances and cathode resistor values, and assuming the power line voltages at their houses. Those recommendations are not general ones, they are specific to their situations only.
Original Paramour and Stereomour 2A3: The 2A3 cathode resistor is 1200 ohms and the bias is 60 volts, giving 50mA plate current. So the Paramour II upgrade BH-6 or the BCP-15/50mA (Original Paramour upgrade) are suitable. The original Paramour upgrade transformer (not by Magnequest) was a 4K primary impedance, same as the Stereomour stock transformer, so the parafeed cap value is 5uF, with an acceptable range of 2.5 to 10uF.
Original Paramour and Stereomour 45: The plate current is in the range 35mA to 40mA, so the BCP-15/40mA can be used. Assuming the 4K transformer impedance as in the 2A3 versions, the parafeed cap would be slightly larger, 6uF with a range of 3 to 12uF. Using a different output transformer impedance would call for some other changes.
Paramour II 2A3: This version of the Paramour design uses the Western Electric automatic bias (a.k.a. "split-rail") scheme, where the driver current as well as the 2A3 plate current goes through the 1100 ohm resistor. The total current is 55mA, of which 50mA goes through the 2A3 plate, 4mA through the driver plate, and 1mA through the driver C4S bias. So the Paramour upgrade BH-6 or the BCP-15/50mA are suitable, same as the original Paramour and the Stereomour. If the stock or BH-5 output transformers are used, then the parafeed cap is 10uF, with an acceptable range of 5uF to 20uF.
Paramour II 45: As with the original Paramour and the Stereomour, the current is below 40mA and the BCP-15/40mA would be acceptable. The BH-5 output transformer (3K impedance) would normally be used, calling for a parafeed cap of 12uF, range 6uF to 24uF.
Man, I hope I covered everything and got it all right this time!
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Thanks Paul! This should be a sticky! The quality of product and your dedication to all of us out here is what keeps me coming back to Bottlehead :)
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Paul,
I'll second what Aaron said! Thanks for this, and yes, this should be a sticky, but where to put it as it covers multiple amps --perhaps tech topics?
Looks like I'v got the right iron coming no matter which plan I end up following -- upgrade existing Paramour IIs, or go with paramount 1.1 with 2a3.
Thanks again,
Jim
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I'm planning 2a3 with softstart and BH-5 and BH-6 iron upgrade. As for the parafeed capacitor,
Auricap makes 400V 10uF or 7.2 uF 600V parts. Can I use the 400V capacitor?
Thanks.
Dave
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No, you cannot use a capacitor of less than 600 volt rating.
Well, you can if you want, but you should expect it to fail rapidly, possibly immediately, and possibly take out some other expensive parts - like the Magnequest iron!
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Dave, use two 5uF 600v Auricaps in parallel. That's what I'm planning on doing.
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Thanks Aaron and Paul.
I'll go with the two 5uF 600v capacitors as well.
Dave
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Dave,
There are other great caps out there that you don't need to parallel you may want to consider. I have a pair in my Paramounts so it was an easy choice for me. Some feel that parallel caps in the signal path is not a good thing and others say as long as they are identical caps you are okay. I'm not experienced enough to say either way ;)
Regards,
Aaron
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Here are a couple of my favorite reasonably priced 10uF caps (all at 630V). I have the $11.00 ones in my Paramours:
http://www.diyhifisupply.com/node/91
http://www.diyhifisupply.com/node/88
http://www.diyhifisupply.com/node/164
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Wow, thanks for the help Aaron and Grainger!
I'll check out those Obbligatos.
Dave
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Paul J.,
Another question about another configuration -- I'm assuming that the nickel tfa-2004 jr and the bcp-15 gapped for 50 mA would not work very well with the paramount built as a 300b, but with a 2.5v filament and a high-power 2a3? However, would it be possible to adjust the bias of the 2a3 down somewhat to get the plate current in the <50mA range, at the expense of a few watts?
Basically I'm wondering if the iron I have on order would be suitable for the higher power 2a3 config in the new paramounts.
Thanks,
Jim
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Paul J.,
Another question about another configuration -- I'm assuming that the nickel tfa-2004 jr and the bcp-15 gapped for 50 mA would not work very well with the paramount built as a 300b, but with a 2.5v filament and a high-power 2a3? However, would it be possible to adjust the bias of the 2a3 down somewhat to get the plate current in the <50mA range, at the expense of a few watts?
Basically I'm wondering if the iron I have on order would be suitable for the higher power 2a3 config in the new paramounts.
Thanks,
Jim
Changing the plate current without changing the plate to cathode voltage would require a different transformer impedance for good performance.
You can do it, at the price of increased tube distortion. You would have to increase the cathode resistor from 1000 ohms to 1500 ohms, approximately, and the 300B distortion at full power would probably increase from 3% to 8% or so (reading between the lines of the WE table of operating points).
The stock cathode resistor is made of two 2K/25watt resistors, in parallel for 300B operation (1K) and in series for 2A3 direct coupled operation (4K). Replacing them with 3K resistors in parallel would get 1.5K. A few people have made this switch, using them in series for 6K and running a type 45 tube. This is still a substantial deviation for the original design. :^)
In my opinion, the better alternative would be to keep the stock plate choke for 300B operation, but use the TFA-2004Jr. Then swap in the BCP-15 for 2A3 operation. The optimum parafeed capacitor would be a bit smaller with the stock choke (5 to 8uF?) but close enough to use the same cap.
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Paul, thanks again for the help. I decided that since the paramours and the stereomour are very similar aaqnimals that I will fix the tube socket on the one paramour (properly this time) and sell them, and I just bought a pair of paramounts on audiogon, so will rebuild those as 2a3s with the soft-start upgrade and the iron I'm gettig from MQ.
Thanks again,
Jim