EML 300B mesh plate suitable?

davidnow · 1696

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Offline davidnow

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on: August 11, 2020, 11:21:49 AM
Hi all,

I have build several BH kits thanks to the precise and documented manuals but I must admit I don’t understand how these marvellous pre amp, head amps, power amps work...

I would like to know if EML 300b Mesh Plates would work in the Kaiju amp. Specs say the recommended anode dissipation is 22W and absolute maximum is 28W.

Thanks for your help!
David



Offline Paul Joppa

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Reply #1 on: August 11, 2020, 01:34:56 PM
In Kaiju, the 300B dissipates about 25 watts, between the recommended and maximum values. EML also specifies a fairly tight tolerance on the filament supply voltage, so you would want the DC Filament upgrade which provides a regulated supply.

Paul Joppa


Offline davidnow

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Reply #2 on: August 11, 2020, 06:57:28 PM
Thank you very much Paul. I installed the DC filament upgrade so it will be OK.
David



Offline Paul Joppa

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Reply #3 on: August 11, 2020, 07:05:52 PM
Forum member Deke609 has some experience with Kaiju and EML, hopefully he will see this post and chime in.

Paul Joppa


Deke609

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Reply #4 on: August 12, 2020, 08:15:17 AM
Chiming in. I've not used the 300B mesh, just the regular EML 300B. But both tubes have higher than normal filament current requirements, so they require the same kind of tweak to the Kaiju filament supply resistors (labelled "Rdrop" on the schematic).  The stock value is 0,51 ohms. That value will need to decrease.

. EML specifies that the filament of the 300B mesh draws approximately 1.4 amps. In my experience the "approximately" qualifier is an important one. Unless you happen to get tubes that by happy coincidence draw exactly 1,4A @ a 5V drop (+/- 4%), you'll need to experiment a bit to find the right resistor value.  Use Ohm's Law to get a starting point: R = V/I.  I'd measure the output DC voltage from the DC filament board, subtract 5V, and then divide by 1,4 to get your ballpark figure. [edit: and do this for each pair of DC outputs on the board - in my Kaiju, there is a slight difference between the voltage output of each side]. Example: (6,3V - 5V)/1,4 = 0,93 ohms. Note this is the combined (sum) value for both filament supply resistors, so you need to divide by 2 to get the value for individual resistors. So 0,46 or 0,47, or a combination of both is a good starting point. After installing them, you'll need to measure the voltage dropped across the filament and tweak the resistor values as necessary to get within the specified 5V +/- 4%.

In my experience, the filament current draw of the EML tubes takes a while to stabilize -- settling in to the final value after 5+ minutes.  For the first measurement, I'd wait as much as 20 min just to get a sense of how long it takes the filaments to fully warm up.

when I did this I chose to order a bunch of values of 3W resistors -- e.g., 0,40 - 0,5 ohms -- b/c this was cheaper than paying $20 shipping for each new order from Mouser or Digikey.  And with all resistor values on hand, I was able to find and install the right value in an afternoon.

Hope this helps.

cheers, Derek
« Last Edit: August 12, 2020, 08:22:57 AM by Deke609 »



Offline Norbert

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Reply #5 on: October 13, 2020, 01:58:27 AM
In addition to decreasing the Rdrop resistors to 4.7 Ohm I increased the cathode resistor by putting in 3 pcs. 3.3kOhm Ohmite Audio Gold 10W resistors in parallel resulting in a 1.1kOhm/30 Watt cathode resistor. This brings the anode current down to roughly 65 mA and the anode dissipation closer to the 22W target. I can afford that maximum power loss of a few hundred milliwatts ...
Regards, Norbert