EML 300B-XLS, anyone using them?

[johnsonad]

Is anyone out there using the EML 300B-XLS tube or any other EML 300B in their Bottlehead amps?  With the Euro so low it seems a good time to pick up a pair.  If so, please list your amp and what other tubes you have compared them to.  

I've read some good reviews of the XLS when used with standard voltages and the increased lifespan would be nice considering the price.  Yamamato offers this tube as standard in their highest offering over the standard 300B or mesh plate.  

Thanks,

Aaron

[johnsonad]

Hmmmm, posted before researching.  PJ, you wrote earlier this year that the 1.5 amps was too much for the Paramounts in the SET Asylum.  Looking on the EML site, the mesh plates use 1.4 amps and the standard use 1.3 amps.  Doc, PJ, PB, can you use any EML 300B in the Paramount without modification? 

[Paul Joppa]

For filament power, I would not worry much about 1.3 amps, but would be uncomfortable with more, even 1.4 amps.

The EML tubes are a tough question. They are very much "new technology" when it comes to their cathode emission - in fact that is quite likely why they sound so good. But the new cathode technology is prone to damage if the start-up transients are not managed suitably, and the suitable approaches are different from what we have become used to with the classic tubes.

Formally, we do not recommend them in our amps, in the sense that if they fail, we are not going to refund the money you spent on them. Informally, I expect the 300Bs would be perfectly fine in a Paramount. (See similar comments today under "Stereomour".)

The 2A3 Paramount is direct coupled which is a severe problem with the EML technology - do NOT attempt to use them in the current Paramount 2A3 design! We have developed a slow-start driver circuit for the Paramount which, once it is in production, will probably work with the EML 2A3. But even if it does (we have not tested it yet) it still has limitations - the amp has to rest for 30 minutes after it is turned off, before you can turn it on again. I am looking into that problem as well, but it's early days to say how it will come out.

All these difficulties are quite frustrating for me, if that was not obvious. The EMLs have such a great reputation, and the few I've heard sounded so good, that I would really like to make them work in our amps. But at this point we are still saying the amps work with the supplied tubes and with NOS tubes that are in good condition; anything else is at your own risk. I am paraphrasing Doc B here, but I can't speak for him - he may want to clarify.

[johnsonad]

Hi Paul, thanks for the reply.  I exchanged some emails with Jac yesterday in regards to the Paramounts.  He thought in the circuit without modification that the standard 300B would work as a drop in.  Though he didn't directly state it, he lead me to the XLS as the prefered tube.  Apparently some draw 1.4 amps and other 1.5 amps.  I'm awaiting a reply to see if you can order the tubes which draw 1.4 amps.  He also thought that by adding a cap before the FS choke you could get the 1.4 amps needed without too much of an issue.  Would it be better to email you offline about this?

Regards,

Aaron

[pro_crip]

I went out and bought a pair of the EML 300b XLS's without even thinking there might be a problem, I figured a 300b is a 300b. Would the circuit need to be modded in order for those tubes to be used? Keep in mind, I've only just glued the base together so I'd be able to build the mods right in as part of the construction. Also, to put minds at ease I won't be asking for any kind of refund or anything along those lines. Btw, the US distributor for eml is a good guy, he lives near me so I was able to arrange local pick-up of the tubes at the UPS store. Thanks


Rich

[Grainger49]

See PJ's reply #2 above.  The soft start should be installed.  Paul will find a way to make it work since he wants to use these tubes himself.

I went out and bought a pair of the EML 300b XLS's without even thinking there might be a problem, I figured a 300b is a 300b. Would the circuit need to be modded in order for those tubes to be used? Keep in mind, I've only just glued the base together so I'd be able to build the mods right in as part of the construction. Also, to put minds at ease I won't be asking for any kind of refund or anything along those lines. Btw, the US distributor for eml is a good guy, he lives near me so I was able to arrange local pick-up of the tubes at the UPS store. Thanks


Rich

[pro_crip]

At the pace of my construction, with this heat wave and my health there aint a snowball's chance in hell of getting out in the garage to do any sort woodwork, perhaps the soft start upgrade will be available by the time I get to my paramounts. Which is my oh so subtle way of asking when, perchance, they will be available for purchase. Thanks

Rich

[Paul Joppa]

I reviewed the Paramount power transformer and filament choke designs today, and I think it is possible to draw up to 1.5 amps into a 5-volt 300B filament.

[pro_crip]

Pull off that little trick and I would be eternally in your debt. What could happen if I put those tubes in the stock circuit and there is a problem? Aside from holding myself and no one else responsible for my stupidity. Would the the tube die and/or the circuit get fried? Thanks a bunch.

Rich

[Paul Joppa]

Jac thinks the tube will survive, and he has more experience with a variety of circuits so I'd take his word over mine - and I agree with him anyhow.

I am pretty sure the amp will survive. The issues with the amp itself are power transformer temperature and filament choke saturation. For the power transformer, it does get hot even in stock form but is built to handle high temperatures. If it does not boil a drop of water after running for 3 hours, the heat is probably OK*. The first check would be the DC filament voltage, which should ideally be between 4.75v and 5.25v. If the choke is saturating (in theory it should handle 1.6 amps by my calculations) the voltage will be off.

Note that the filament voltage will be off if your power line voltage is not close enough to 120v. A filament voltage error will risk damage to the tube; most tubes can handle 10% variation but I specified 5% above for safety. If you have a power line problem we can address that separately.

If you want to be extra careful, the best test for power transformer temperature is to measure the primary winding resistance when the transformer is cold, and when it is hot. It takes about 3 hours for the temperature to stabilize. Knowing the temperature coefficient of resistance for copper allows a calculation to be made of the temperature change.

[johnsonad]

Hi Paul.  Thanks for taking the time to look into this.  So are you saying the Paramounts can output up to 1.5 amps at 5v without modification to the circuit if circumstances permit?  Jac seems pretty specific that there needed to be 5v on the filiments.  I'll measure the hot resistance today and post here; these amps have been burning in for the last 48 hours

[pro_crip]

If I'm reading you correctly Paul the main issue with using the eml 300b xls lie in the pt and the choke, specifically saturation. I was planning on getting the MQ iron upgrade with the nickel core, would that alleviate the saturation problem? One of these days I'll check what my mains voltage actually is and how it varies throughout the day. I'm assuming that if my mains voltage isn't steady there's some sort of device I can get to provide a nice steady 120v. Good luck with your resistance check johnsonad. Thanks

Rich

[Paul Joppa]

If I'm reading you correctly Paul the main issue with using the eml 300b xls lie in the pt and the choke, specifically saturation. I was planning on getting the MQ iron upgrade with the nickel core, ...

The MQ iron consists of the output transformer and the plate choke - the audio inductive conponents. The issue is with the power transformer and filament choke, which are not part of the MQ upgrade, and do not handle audio signals. Sorry for the lack of clarity!

[johnsonad]

Hi Paul.  I measured the resistances hot (after 48 hours) and cold and got 10.38 ohms cold, 11.68 ohms hot.  This was with the stock EH 300B's.  What do you think sir?

[Jim R.]

Paul,

Slightly off topic, but the three hour time to thermal stability in a pt is really interesting because I have sworn that my Fi amps sound their absolute best after being on for 3 hours.  Decent at power on, better in 30 minutes, very nice after an hour, and stunning after 3 hours.  And I've heard this while being away from the room for most of those 3 hours to come back and hear a clear difference in the sound from when I left.  I've repeated this dozens of times and it is perfectly consistent.  Thanks forgiving me some idea of why this is happening and so consistent.

Of course my dac takes 24 hours from cold to sound really good, is even better at 3 days, and slightly better still after a week.  There's no power switch on it, as there has not been on several dacs I've owned, so the designers must know this too.

-- Jim