Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: Fullrange on September 07, 2010, 02:41:53 PM
-
Has anybody used the JJ 2a3-40 instead of the 300b with the same operating parameters as the standard 300b except for the 2.5 a filament requirements.
-
I hope someone knowledgeable replies. I'm interested in these tubes too!
-
I purchased these tubes about three weeks ago, replacing my Shuguang Treasure 2A3-Z tube , which I thought sounded a little muffled even after considerable burn-in time. The JJ 2a3-40 sounded great right from the start and are getting better every hour. I would buy again. They also are build like a tank, and a little taller. I like them.
-
The J/J 2A3-40s are essentially 300Bs with 2.5V, 2.5A filaments. They should also work well in the Paramour IIs (and the original Paramours) as well. With their 40 Watt plate dissipation ratings, 2A3 duty should be a "walk in the park" for these tubes. In the Paramounts, in 300B mode of operation, they would require re-wiring the filament supplies and chokes for 2.5VDC (2A3) operation, and leaving everything else as-is.
Update:
The Paramount 300B's plate dissipation is significantly lower than 40W, though I don't immediately recall the exact figures, I'd guess around 24-26 Watts or so (appx 340-350V P-K @ appx 70-75 mA). I'm Just too darn lazy/tired to go dig up my Paramount manual and notes, and look up my actual voltage measurements right now ;-)
Latest update: I see that PJ has answered your question - better than my previous update.
/ed B in NC
-
Would it be necessary to rewire the amp to 2A3? Or since it is "basically" a 300b, can you leave it wired the same (dropping the voltage on the heaters of course )?
-
From the specs, I see no reason not to use these as 300Bs, switching the filament power to 2.5v.
-
Thanks Paul for your answer.
Could you please tell me how much dissipation the 300b uses, hope I am using the right term, if it is 22 watts or less it might be possible to run the 2a3 Sovtek also, the idea of using 2.5v AC filament appeals to me.
-
Thanks Paul for your answer.
Could you please tell me how much dissipation the 300b uses, hope I am using the right term, if it is 22 watts or less it might be possible to run the 2a3 Sovtek also, the idea of using 2.5v AC filament appeals to me.
Paramount runs at nominally 350v (plate to cathode) and 70mA, which is 24.5 watts plate dissipation. You can probably run a Sovtek 2A3 that way for a while - better have a spare set on hand though. Smoke 'em if you got 'em! Don't do this with anything that's out of production though, that would be a crime against posterity. :^)
By the way, Paramount runs DC heaters, not AC.
-
Paul,
Thanks again for your answer, DC on the filaments are just as fine by me, I certainly had no intention to run NOS tubes at those operating conditions just the ones mentioned because I like the 2a3. Is is difficult to bring the plate voltage and current down to about 20w dissipation to be able to run the 2a3 sovtek as well as JJ 2a3-40 without the need to direct coupling.
-
You could increase the cathode resistor to get about 57mA, that would be around 1300 ohms total resistance, and the 5v bias increase leaves you with 345 volts plate to cathode (at nominal 120v power line) for very close to 20 watts. It's an approved WE operating point, with a bit more distortion but well within the reasonable range. Or, go all the way to 1500 ohms (two 3K resistors in parallel) and 50mA, for about 18 watts. Those resistors are the same onew I spec in series for the direct-coupled 45 version. You can parallel an additional resistor to inch the current up and explore the region between 50 and 60mA. At 50mA, the ideal would be a 5K output transformer, and Magnequest makes a really good one. Depends on haw carried away you want to get... :^)
-
Paul,
Thanks for the info, you mentioned Magnequest 5k OT or others to be used in the Paramount, which are the ones to be appropriate.
Also liking the Stereomour because it looks simple to built, is it possible to increase to voltages and currents to run the OT tubes at higher operating conditions, guessing that it is limited do to the Power Transformer and OT's, looks like it is easy to wire up for one input only and owing a very good tube preamp which I like to use as source input.
-
... you mentioned Magnequest 5k OT or others to be used in the Paramount, which are the ones to be appropriate.
I was thinking of the EXO-050. Kind of overkill, but has excellent sound, at least the nickel-core ones that Doc has used. I'm not sure but I think it's a bit larger than the TFA-2004 and might be a very tight fit.
Also liking the Stereomour because it looks simple to built, is it possible to increase to voltages and currents to run the OT tubes at higher operating conditions, guessing that it is limited do to the Power Transformer and OT's, looks like it is easy to wire up for one input only and owing a very good tube preamp which I like to use as source input.
Your guess is correct. You can't get more voltage because of the power transformer limitations. It runs at 360v (300v plate to cathode plus 60v bias) in 2A3 mode, which is a bit higher voltage than the usual 2A3 amp. (You could probably run a 2A3-40 at higher current, but you'd need new output transformers - lower impedance - and plate chokes. The power benefit would be modest, maybe 5 or 6 watts at most.)
-
View Profile Personal Message (Offline)
Re: Paramount using JJ 2a3_40
-
...
Paul, coming back to what you said, is it possible to use the TFA-2004 and an appropriate plate choke in the Stereomour to be able to increase the current for the JJ-2a3-40. How many watts would I be able to increase to approx., a 2 to 3db gain to 5 to 6 watts would be perfect.
Power predictions are difficult. Looking at the WE table of operating points, the stock 2A3 Paramour should make over 5 watts before clipping; I still rate it at 3.5 watts and we generally see about 4 watts before visible distortion of the waveform. Dropping the load impedance to 3K (with a current increase to keep the distortion low) ought to give you about 4/3 the power, or another 1.5dB. Whether that means 4.5 or 6.5 watts is an unanswerable question until someone tries it and does the measurement. A lot depends on how strictly linear the individual tube is.
But I think the change is possible. My calculations indicate the power transformer temperature would not exceed a reasonable maximum if you raised the current to 60mA per tube, which would take about 1000 ohms cathode resistor (use 2700 ohms 5W in place of the 4700 ohm 5W, paralleled with the 1600 ohms 10W). I have of course not tried this - I suggest that if you do it that you do a temperature measurement to be sure. It's easy if you have an accurate ohmmeter and some patience, but should probably be in another thread :^)
The TFA-2004 will not fit on the chassis plate. You don't need a lot more room; if you leave a 5/16 space between power and output transformers, then they will stick past the plate about 3/8" on each side - still within the outside of the wood base. The EXO-003 plate choke would be ideal electrically, but I'm afraid I don't have dimensions and am not sure it would fit. The Paramount plate choke is a slightly smaller alternate. I would expect about 1.5dB more power from such an arrangement - and I would also expect the better iron to improve the quality of sound.
Probably the best option would be a custom plate from Front Panel Express, 12" square and giving an additional inch on each side. Current pricing is $75 more or less for a panel of that size and complexity. That way, a wood base can easily be assembled from existing stock pieces, there will be good room for the iron, and the proper mounting holes can be made at the same time.
Clearly you have me thinking - I wonder if the increased room would let me make a PSU board with all film capacitors? ... could become a high-priced "Signature" version ... maybe a slow-start board?
-
Paul, what you are thinking of would be absolutely fantastic for an integrated se 2a3 amp with the larger plate for versatility.
All my tube amps are integrated stereo PP as well SE which I prefer because of space considerations, not necessarily weight.
Having on loan one of your first generation stock SEX amps the last 3weeks (also light weight, not getting any younger)
which sounds quite fabulous.
Would it be possible to get the stock Steremour with larger 12x12 top plate with holes punched for stock OT as well for the larger OT
and wood chassis to accommodate the top plate.
I am willing to pay for the extra $ required to have this flex build in, for future upgrade.
-
Seems possible - I'll talk to Doc after the Christmas rush and headquarters flood is resolved. The idea is quite attractive. This would basically be a stock Stereomour, with flexibility for a variety of Magnequest products as upgrades or modifications. There's no way for us to make a full set of manuals of course, and the customer (you!) would have to acquire the iron from Mike. But I could see it being attractive to a few experimenters - and we LIKE experimenters, it's what we do ourselves after all.
I'll want to think about a few other possibilities as well, I'd rather do it right that do it over!
-
Looking forward to your answer in the new year, I will be the first in line for the larger chassis Stereomour to build in stock form and possible option you come up with, will be fine to start.
Thanks for your answer.
-
Reminder - this thread has drifted off topic and is now about using high-power 2A3s or 2.5v 300Bs in the Stereomour.
I talked a little to Dan, and to Mike at Magnequest. It looks like the only suitable and currently available output transformer is the TFA-2004. It can be used with the stock plate choke or the Magnequest BCP-15/50mA or the BH-5, without changing the circuit or tube. It should give a bit more power, 1 or 2dB, depending on the linearity of the tube used. At a guess, no more than 5 watts. Distortion will likely go up at full power, from 3% to 5% if the tube is highly linear (most 2A3s and 300Bs are).
Alternatively, you can use the TFA-2004 with the EXO-003 plate choke running 60mA by using a high power 2A3 or a 2.5v 300B - dissipating 18 watts per tube. This will not increase the power any further, but it will bring the distortion back down to the 3% range.
In small quantities it does not make economic sense to use our regular laser-cut plates. I can have Front Panel Express make you a larger (12" square) plate, drilled for the TFA-2004/EXO-003 combination as well as the stock combination - the cost would probably be $100-$150. Bottlehead could probably sell a larger base kit as a separate item. I don't know the cost, but let's say $50-$100 for a total around $200.
This is not now a standard product, and probably won't ever be. Magnequest may at some point offer specific upgrade iron for the Stereomour, and we may come up with circuit upgrades (I am always working on them!) or new versions, and they may or may not be compatible with this chassis idea. Just wanted to be clear about that!
-
My take on this is that if you want an amp that performs like a Paramount, buy a Paramount. I'm going to respectfully remove myself from involvement in this custom one off Stereomour based project beyond offering to put together a kit that is without the chassis plate. We could also supply it with four 12" wood base pieces rather than two 10s and two 12s.