more power from S.E.X. with 6EM7

[shelby1420]

Ok, thanks for the quick reply!! With the SEX amplifier is there a way to squeeze out a little more then the 2 watts it gives??  Going to be using Cornwalls with it.....

[Doc B.]

With the SEX amplifier is there a way to squeeze out a little more then the 2 watts it gives?? 

Not from a single SEX amp. You could buy another, and then strap the inputs and parallel the output secondaries on each amp to make it a 4W monoblock. But then of course you would have to rig up some sort of custom cable from the two monoblocks that goes to a stereo headphone jack when you want to use it for headphones. I know that the urge is always strong to turn these inexpensive kits into a Universal Solution (witness the Quickie battery preamp kludged into a headphone amp) but alas, we often run up against physics in the end.

[Paul Birkeland]

Ok, thanks for the quick reply!! With the SEX amplifier is there a way to squeeze out a little more then the 2 watts it gives??  Going to be using Cornwalls with it.....

You can do some modifications and use the 6EM7.  To use this tube, you will really need the MQ iron upgrade and the C4S boards to get it to work properly.  (Done properly, you get ~4 watts)

[shelby1420]

Thanks for the responses, to do the mods you speak of CB is it in depth or easy enough??

[Jim R.]

Shelby,

I'm building a s.e.x. amp right now and just picked up a mint pair of original Cornwalls this past weekend, so I can let you know, though I have no plans to use the 6em7 tube.

I started a thread on BH amps and the Cornwalls a few days ago in the general discussion forum of this board and there is one person who posted about the various BH amps he's used with his CWs.

I am planning to build a stereomour when it comes out and will use it alternately with my Tonian TL-D1s and the CWs, and the s.e.x. will be doing mostlyheadphone duty in my bedroom.

-- Jim

[Paul Birkeland]

Thanks for the responses, to do the mods you speak of CB is it in depth or easy enough??

In the amplifier, two 620 ohm 3 watt resistors get replaced by 1600 ohm 5+ watt resistors (this is mentioned specifically in the manual).

Two 1000uf 35v electrolytic capacitors get replaced by two 1000uf 50v electrolytic capacitors (this is not mentioned in the manual, but it is more of a safety margin issue).

On the C4S boards R1 should be 500 ohms (there are some alternatives discussed elsewhere, but I have concluded that this is easiest)

This isn't a super tough modification to do.  I believe that when I did it, I used chassis mount 1.6k resistors and just placed them against the chassis close to where the 620 ohm resistors mounted.  Sourcing the 500 ohm resistors and 1000uf 50v caps should be pretty easy from any electronics supplier. 

[shelby1420]

Wonderful, thanks for the info!!!

[pevelg]

Oh, this is exciting.  I am using a cheap Harmon Kardon as my amp right now and I really want to get a tube amp to get the full analog experience.  I would have got the S.E.X. amp already had this stereomour not had multiple inputs and slightly higher output power.

[Dr. Toobz]

Thanks for the responses, to do the mods you speak of CB is it in depth or easy enough??

In the amplifier, two 620 ohm 3 watt resistors get replaced by 1600 ohm 5+ watt resistors (this is mentioned specifically in the manual).

Two 1000uf 35v electrolytic capacitors get replaced by two 1000uf 50v electrolytic capacitors (this is not mentioned in the manual, but it is more of a safety margin issue).

On the C4S boards R1 should be 500 ohms (there are some alternatives discussed elsewhere, but I have concluded that this is easiest)

This isn't a super tough modification to do.  I believe that when I did it, I used chassis mount 1.6k resistors and just placed them against the chassis close to where the 620 ohm resistors mounted.  Sourcing the 500 ohm resistors and 1000uf 50v caps should be pretty easy from any electronics supplier.  

I now have all the stuff to do this, but one question remains: how can you get more power out of such a similar tube, esp. considering that the power triode has a lower mu than its counterpart on the 6DN7? Does the high gain of the va half and/or lower plate impedance of the power half (850 ohms?) really lead to a more efficient transfer of power, let alone a doubling of output wattage? I can't figure out how this would double the output power.

[Paul Joppa]

As far as I know, nobody has done any careful measurement of power and distortion at various output levels for either the stock SEX or the 'EM7 version, with either stock or upgrade iron. My theoretical analysis shows 2.97 watts for the 6DN7 and 3.35 watts for the 6EM7, ignoring transformer losses and tube nonlinearity. On the 6DN7 version we saw (if I recall correctly) a bit over 2 watts before visible distortion (probably no more than 5% distortion) of the waveshape on an oscilloscope, driving an 8 ohm 5% resistor - so I thought we could conservatively claim 2 watts.

The 6EM7 is a bit more efficient due to the smaller plate resistance, but it is also a bit less linear. To improve linearity, the 6EM7 is operated into an 8K load, even though it is "optimum" for about 7K. That reduces its power a bit. These are the conditions for which I have quoted theoretical numbers above.

If you use a lower impedance output transformer with either tube, you will get more power and more distortion. You should be able to get around 4 watts from either tube if you use a 5K transformer, for instance. Distortion at full power will be significantly higher, probably 10% or more. It will not sound much like a Bottlehead amp, and it will be especially intolerant of speakers which have impedance dips. I will suggest that better plate chokes and output transformers would probably make this compromise more tolerable.

Our operating conditions are chosen for low distortion and reliable performance with real-world speakers rather than getting big power numbers, and our conservative specifications reflect the same philosophy.

[Dr. Toobz]

Thanks for the info!

Probably better to leave well enough alone - I couldn't see going through all that trouble to gain 1/2 watt and risk losing sound quality. For one thing, dissipating more energy (as heat) through the 1.6k 5W resistor seems like a waste. The enhanced S.E.X. amp is great with the MQ upgrades and C4S - so a Stereomour indeed sounds like the next logical upgrade, as opposed to mucking around with the 6EM7 tubes. Power isn't an issue, though I'd like tighter bass - perhaps the Stereomour w/2A3 tubes might be a little more linear than the 6DN7's (let alone 6EM7's) and yield a different bottom end?

In any case, the Stereomour seems to be a widely anticipated product, considering that it's not even done yet!

[Paul Joppa]

... I couldn't see going through all that trouble to gain 1/2 watt and risk losing sound quality...

... perhaps the Stereomour w/2A3 tubes might be a little more linear than the 6DN7's (let alone 6EM7's) and yield a different bottom end? ...

The reason the 6EM7 conversion was considered during the design phase was in case 6DN7s dried up - that actually seemed to happen after the original S.E.X. amp had been out for a few years. Haven't seen any sign of a problem the last few years though.

I expect the stock Stereomour will have better bass than the stock SEX or Paramour II - clean to at least a half octave deeper. Now that we finally have production-spec iron, we'll find out shortly! When Mike gets an upgrade package for it, I expect it to be a killer, similar to his Paramour II package (one of his best, IMHO).

[Dr. Toobz]

After having my brother over and literally "blasting" my BH system with the knob turned up maybe 30% of the way, there's no way in hades I would bother with the 6EM7's now. On 99dB/1W/m speakers, I'm barely using any of the 6DN7's output power as it is! Amazing how having the right speakers (and a good, pre-amp driven source) really negates the need for wattage in a medium-sized room or smaller.

The tubes and parts I bought for them are stowed away now as an "emergency" kit in case 6DN7's do dry up someday and I have to change tubes. Otherwise, I now see the wisdom behind sticking with the 6DN7's.