Can I drive the output tube harder?

[p_e_r_r_y_]

I'm building a TubelabSE and it's already sounding great on my DIY fostex FE26En. This loudspeaker doesn't go very deep so I figured I don't need any low frequencies below 70 hz. So I put in a high pass filter using a 15nf coupling capacitor instead of the standard 0.22uf. My question is. Since I don't drive my output tubes with any frequencies below 70hz, is there a way to make them go louder by driving them harder?

 

[rknize]

The TSE can already drive the grids positive due to the MOSFETs, so the only way to get more power is to increase B+ or go to a bigger tube. What tube are you using?

 

[p_e_r_r_y_]

Russ. I'm using 45's. I'm not really an expert (to put it mildly), but what I mean is. The lower the frequency the more power is needed for a certain level of sound pressure. So lets say we have a total spectrum music signal thats in fact, just about driving the grids positive. My assumption would be that taking out all frequencies below 70hz, would result in a signal way below the point of a positive grid.
In terms of sound pressure level, isn't it possible to benefit from a high pass filter?

 

[Evenharmonics]

In terms of sound pressure level, isn't it possible to benefit from a high pass filter?

I would say that depends more on the speaker. Your Fostex may have easier time dealing with > 70Hz signals thus less distortion (benefit). As for the high pass, I would go with line level filter like this and augment the low end with subwoofer.

If you want to increase the output wattage of TSE, you would want to change to 300B tubes and possibly output transformer if the existing ones are inadequate.

 

[rknize]

The driver doesn't care about the frequency. By using a high-pass filter, the amp as a whole has more reserve for the frequencies that it amplifies. How much is a complicated picture that depends on things like changes in the speaker impedance over frequency, etc. If the driver is clipping the output tubes, it doesn't matter if it is clipping them at 20Hz or 2kHz. It's still clipping. The power supply and output transformers are having an easier life, so the peak output of the amp at your frequencies will rise a little
in response.

You are looking for more dB. To get double the sound level you need approximately 10x the power. So whatever change you make it needs to be a significant one.

 

[chrisb]

which fostex model exactly? there's a digit missing from the model number, so it could be a 126 or 206 - which are quite different animals when it comes to acheivable bass extension

of course the enclosure will make quite a difference as well with any of the FExx6 series.

yes an appropriately selected HP filter (in line or otherwise) will allow more SPL from the driver while keeping it below its suspensions Xmax limit, but at the premium of possibly requiring replacement of that output with separate woofer(s)

 

[p_e_r_r_y_]

which fostex model exactly? there's a digit missing from the model number, so it could be a 126 or 206 - which are quite different animals when it comes to acheivable bass extension

You're right. I made a typing error. It's a FE126En and I'm quite happy with it. Indeed like you and others remark, it's wise to go for a subwoofer. That's a part I left out, because it wasn't directly related to my question. In fact I have a sub (Klipsch RT12). And I plan to add a pre-out for this.

...and possibly output transformer if the existing ones are inadequate.

Ouput transformers are the suggested Electra Prints.

You are looking for more dB. To get double the sound level you need approximately 10x the power. So whatever change you make it needs to be a significant one.

I think that's an answer I was looking for. I can stop worrying about the circuit. If I really want more power I'll go for a 300b. However this would only be 3x the power.

To put everyting in perspective. The sound is very good and I was just looking for a possible cheap way of gaining a bit of power.
I want to thank you all for taking the time for this.

 

[chrisb]

2A3 would give you approx twice the power of 45, with most the character you've probably come to love about the latter

but then after a few brief flirtations, the 300B has never been my favorite DHT in SE mode

BTW, what enclosure are your126s in? While well implemented supporting woofer(s) are almost never a bad thin, there are some excellent designs for this driver quite capable of performance well into the upper 50s/lower 60s.

 

[wrenchone]

SE amps are inherently limited in output power by their B+ and their standing bias current. The tube can never swing more than its standing bias current in the negative direction - once the tube swings down to zero current, that's it. Driving the tube more positive may yield a little higher swing in the other direction, depending on the B+, but this will just result in asymmetric clipping if you push it. If you want more power, as has been said, you need to tinker with the B+ and the quiescent operating current, making sure that both the tube and the output transformer can handle this. The output transformer current rating is important, as it can saturate if too much current is drawn through the primary. Whether or not this happens depends on the current rating, and on how much operating margin is designed into the transformer. Just like anything else, some designs will have margin, some will tweeter on the edge of saturation at their max rating.

 

[p_e_r_r_y_]

Chris. These 45's sound lovely, but I must admit this is my first experience with tubes. I definitely will be trying 2a3 or 300b in the future but for now this seems absolutely right for me. I went for the universal version and even went against George's advice and made a handy internal switch that gives me the opportunity to easily switch to 5V filament.
About the enclosures. I know there are a lot of fine options and I'm aware of the fact that in the right enclosure FE126E can go pretty low. But I decided to let a sub do the work. Before this loudspeaker I listened to my DIY transmission lines for about 20 years and I wouldn't want to miss out on the lowest frequencies. So I decided on a BR-enclosure that worked out fine. Sounds good and has a good WAF
Hope it isn't too much off topic but here are a few pictures.
Regards, Perry.

 

[p_e_r_r_y_]

SE amps are inherently limited in output power by their B+ and their standing bias current..... If you want more power, as has been said, you need to tinker with the B+...

Thanks for the explanation. I build the universal version of TSE. This means 45's are operatng at approx. 320V/27mA, so no room left to tinker with.
But no worries. I'm fine as it is, but only wanted to get the max out of it. I was under the impression that a substantial part of the volts driving the output tubes was related to the lower frequencies. So compressing the signal on the lower end would give me the opportunity to elevate. Now I understand that, if at all, this would only give a marginal effect.

 

[chrisb]

Perry, I think you'll find that without crossing high enough to require stereo woofers (i.e. not "subs"), the FE126 won't deliver its best in a BR. It's of course hard to be impartial about designs for this driver the evolution of which I've attended over the course of the past 5 yrs or so, but several of note would be the FrugalHorn3 and Woden Valiant. The former is quite compact and can certainly be built with WAF, while the latter is more physically imposing, and IMO an overall better performer.

In any case, if the TSE can support 2A3, I'd heartily recommend trying them with the FE126. Current production EH Gold Grids are a fairly reasonable price for this type.

best of luck