Can I use the EML 300B mesh for my Paramount

[Nutube]

I currently use a TJ 300B for my mid base duty Paramount. I thought to use the eml mesh to get a stiffer mid base .
It appears just from readings that the mesh vs solid plate  ( has similar tight bass yet smoother and more open sound due to the mesh. How is the reliability currently of this 300b mesh? Any suggestion of other brands given my above mentioned goal?

My Tweeter and mid range is using the EML 45 solid  plate. I also have a TJ perf plate 45 ST style ( which I won at the craftsmans room back in 2003); a NOS National union ST and another 45 solid used pair
EML.....
Description quote from EML site caution me to ask
"The 300B-mesh can replace a standard 300B in several cases, but not in all cases. So please read this data sheet carefully.

Do not simply plug & play a 300B-mesh instead of a 300B. Overheated anodes may result if you do not check the following conditions, and such a damage is not covered by the guarantee. The 300B mesh can replace a normal 300B tube, when you can meat following conditions:

Bias and Plate dissipation must be reduced, to 22 Watt, maximum 28 Watt
Filament current of minimum 1.4 Ampere can be supplied The larger length of the tube is no problem. (like with closed amplifiers)"

What and where do I probe with my DMM to determine if my Paramount  is in the 22 to 28 Watt range? I have ahunch that I need to use ohms law to interpolate from my measurements.
In Parampount product describtion it mentions 8 watts.....I got a feeling we are not talking the same type of wattage here.
Please enlighten....

 

[Paul Joppa]

The Paramount dissipates nominally 25 watts on the plate, at 120v power line. It is designed to provide 1.25 amps of heater current, maximum.

I think you are in the grey area, where it will probably work but neither Bottlehead nor EML will make any promises or provide any guarantees.

 

[Nutube]

thanks Paul
How do I
1. Measure to derive the 25 Watt
2. What component do I change to ensure that my amp put out say 22 or 24 watt to be safer

 

[Paul Joppa]

thanks Paul
How do I
1. Measure to derive the 25 Watt
2. What component do I change to ensure that my amp put out say 22 or 24 watt to be safer

You can measure the DC resistance of the cathode resistors (approximately 1000 ohms for the two in parallel), but they are 5% parts so it's worth checking. It will take some time for the bypass cap to charge up - use clipleads and be patient. Then in operation, measure the voltage at the hum pot wiper and calculate the current (voltage over resistance). Then you can measure the voltage across the tube - hum pot wiper to the plate, A2. Multiply current times voltage to get dissipation. The actual value will vary from one tube to another due to individual variations in parameters (gain and transconductance), and with the power line voltage. The nominal values are 70mA and 350v.

To reduce the dissipation without changing  the circuit, you need a lower-voltage power transformer. As it happens, I have two power transformers with lower voltage - they were prototypes from when we were developing the transformer.

To power the 1.4 amp filaments, you would need a filament choke that can take that much current without saturating the core. We could have our supplier produce a special run of the filament choke with a larger air gap.

An alternative would be to increase the cathode resistor value (to reduce current) and replace the output transformers with a higher-impedance transformer (to avoid increasing the distortion). You could replace the 2K cathode resistors with 3K for a parallel 1500 ohms, giving about 50mA instead of 70mA; this would call for a 5K output transformer. Magnequest makes two, the DS-050 and the Robin Hood; both would require some new chassis holes to mount them - I think the RH would fit, but the DS-050 would have to go on top and would extend up to or even past the edge of the existing chassis plate.

I believe these changes would work, but of course they have not been built and tested. Power output would be reduced, probably to about 6 watts.

 

[Nutube]

Thanks Paul
Learning as I go appreciate your responses...
Given the additional power Transformer swap etc and clearly a slight drop in output wattage...
My decision is to go with the solid plate the one that eml calls a direct standard replacement.

In addition I would like to try PLLXO ahead of the Paramount. Can you please tell me the INPUT IMPEDance of my original Paramount 300B? the line level signal comes from my Dahlquist LP-1 which is a high pass through to the Paramount. LP-1 is an older unit and I was given to understand that I can still use a PLLXO to replace the large xover components right before my Mid woofer. Thanks

 

[Doc B]

Hi Tony,

To measure the input resistance on your Paramount touch the black lead of your DMM to the outer shell of the input RCA and stick the red probe down the center of the RCA and touch the inner conductor. You should see about 249K ohms.

 

[Nutube]

thanks doc !