Matthias, Were you able to find the operating points for the AudioNote Quest? I was looking on line and could not. |
Matthias, Thank you for sending the schematic. |
Thank you Charles and Brownsfan. |
I agree with the above. Thanks Charles for this and the advice you have given me in the past-Jet |
Well now you have me thinking about these tubes Charles... Shame on you my wife would say :)) i'm going to contact the elrog folks and ask for a data sheet. I would like to know the tube's preferred operating points and if it likes AC or dc heat. |
Sorry to hear about the failures guys. |
The franks have a very high operating point!?
- 525V operating voltage with some drop (25V) over the OPT
- Makes 500V at the Anode
- because the 1kOhms cathode resistor we have 86.8V at the cathode
- results about 413V across the plate of the Elrog.
If that's the case its way too high for the Elrog. Its probably too high the the Tak's and any other standard 300b tube. It could make for a very sweet operating point for the EML 300b XLS.
Its hard to believe that is the actual operating point for the Franks since Israel Blume has said he runs the franks conservatively.
If there was actually a 525V operating voltage coming from the power supply there would have been a warning to only use 300b tubes that could handle a plate voltage of 400 to 425v. Something is not quite right here -Jet |
Charles I agree there is some confusion going on. A call to Israel will solve this. I would guess the Franks Power supply is outputting more or less 425v. And if it is somewhere around 425v no adjustments to the Cathode resistor would be needed. |
If the power supply is outputting somewhere around 425V that would mean the 300b tube would see a more conservative 325 to 350v acoss the plate depending on the cathode resistor value. |
Charles you may want to call Israel Blume. I find that this operating point would be hard to believe. It would fry any mesh plate tube and push any standard 300b to its limits:
"1kOhms cathode resistor we have 86.8V at the cathode at anode current of about 87mA." |
I did not expect to hear that the Franks are indeed running 400v on the plate. That puts them towards the upper range and not a conservative operating point. I would think that's the upper range of what the Elrogs can handle. No wonder the EML 300b XLS sound so good.
A more typical operating point for the Western Electric 300b is 350v on the plate @ 60mA according to the Tube Data Sheet |
...and if the franks are set up to run at 413v @ 87mA. Many would say that's running 300b tubes pretty hard. |
Matthias also brings to the table an interesting observation about the current draw of 1.5 amps for the EML 300b XLS. It is 25% higher than a standard 300b which is 1.2amps. This is something I have mentioned in these pages to folks who were considering buying the EML's since they are considered drop in replacements for standard 300b's. I have over spec filament transformers in my amp that can handle the extra current so it is no problem but the bigger concern is that 25% extra current draw reduces the voltage accordingly. I noticed this when I first installed my new EML's. No problem since I can simply add or subtract the dropping resistors in my filament supply and I check my voltages regularly since I am a DIY person. This is not so easy for a person who is not.
All this to say that when considering different types of non standard 300b tubes it is very important to do the research to make sure your amplifier can handle this tube properly -Jet |
This makes buying any amp and not just the franks super important that one knows the actual operating points of that amp so when it comes time to buy tubes one can do so with confidence. Also know the current draw of the tube one is buying and confer with the builder that all will be well when using that tube. High quality output tubes are so expensive this research is necessary and when both tube and amp manufacturer agree that a tube is safe to use one has mitigated one's risk. |
I have measured this in my amp. Since my EML 300b XLS filaments draw 1.5 amps I have to adjust the dropping resistors so I still get 5 volts because voltage gets pulled below 5 volts. Other 300b tubes that draw 1.2 amps makes the voltage rise above 5 volts making me re adjust the dropping resistors. It's ohms law in practice. I heat my filaments with a/c. Jet |
Al-
when I got home this evening I measured my Brooklyn line voltage with my fluke meter and got 124v which is more or less normal. My EML300b XLS filaments measured 4.97v and my Sophia mesh measured 5.06v. That's about a 2% increase. I am using 5v filament transformers that are over spec. They are rated at 3amps. |
Al - yes I installed a 100ohm resistor on one leg of the primary side of the filament transformer. And I did not change that value when changing from Eml to Sophia.
My vintage Stancor filament transformers are rated for 117v.
These trannies have a center tap so I can bias through it and avoid a bad sounding hum pot and still use a/c heat. I have no 60hz hum but if I put my ear to the speaker I hear what I believe to be a harmonic of 60hz which is 120hz. At least that is what I think I am hearing. The ELM tubes are so well balance a hum pot was not needed. But I was not so lucky with the Sophia which I could not use with out installing a hum pot.
When I removed those hum pots this amplifier came to life.
I would not feel comfortable running these tubes much above 1% which is 5.05v. |
Its amazing to touch a 12w Mills resistor that is dissipating only 3-4 watts and it be too hot to touch. No one tells you these things when you first start out in DIY. Best thing to do is to look at lots of schematics and see what ratings for the caps and resistors they use in the various positions. Charles I'll be interested to find out what brand of resistor you eventually go with. Jet |
Sorry to hear that Charles |
Charles, The 300b cathode resistor is actually one part in my amp that I have not changed so I have no experience how different brands would affect the sound. They have to dissipate lots of heat so are pretty large. Maybe Grannyring or Salectric would have some experience. Also try the Audio Asylum Tube DIY thread for a good sounding 300b cathode resistor. My cathode resistor is pretty cool. It has various lead outs which give different resistances. The one time I noticed an improvement in sound was when I used to use Sophia Electric Mesh plate 300b tubes. I increased the resistance which lowered the stress on the Sophia Mesh and it did sound a bit better, so its possible with some tubes you may notice a sound change. |
Charles, I would suggest a resistor at least 20 watts in this position. The heat you are dissipating could be from 6 to 8 watts. For instance the original Joe Roberts WE91a schematic calls for a 50watt resistor in this position. You could also go with two Mills 2.4k 12 watt resistors in parallel. That would give you a 1.2k / 24w resistor. I believe a 10 watt resistor will be burnin hot and too close to its max tolerance. |
Mesh Plate 300b's might not do to well in the Coincident Franks 300b amps. They would probably be run too hard and not have a long life. If I were running 300b Mesh my operating points would be conservative. 350v across the 300b plate at 60mA. I think the Franks are running over 400v across the plate and maybe 80-90mA?? A good way to fry a mesh plate tube since they don't dissipate as efficiently as a solid plate tube. Jet |
