Dennis Had Inspire Amps

[FloridaBoy]

Originally Posted by Bombadil

I know you guys are upgrading, which isn't likely to be an issue. Dennis recently told me that a problem he is having is people modding amps, and then selling them. The new owners then contact him and he can't recognize the circuits. People have to send the amps back to him. He has found some significant problems in the modded gear.

I don't consider changing capacitors a mod. It's just like changing tubes and easily reversible.

[Rosco65]

Originally Posted by FloridaBoy

I don't consider changing capacitors a mod. It's just like changing tubes and easily reversible.

I would agree. It is also perhaps the most accessible change to an amp other than tubes and a power cord, and it is a very obvious change.

If I were going to ask for more substantive changes such as a circuit modification, different attenuator, or enhanced PS I would certainly send to Dennis. For a simple coupling cap change, I don't think it is worth risking the two-way shipment of the amp or preamp.

However, since Dennis is known for his evolving designs he might recommend some other improvements while the caps are being upgraded. It may very well be that if someone is going to spend $200 on coupling caps, he may feel there are other areas in which the sound may be improved as well.

[Bombadil]

Some people are making significant circuit mods. Hard for me to wrap my head around that. Buy an original hand built Dennis Had designed amp and then heavily modify it? That is something I might think of doing with a Magnavox console amp or an old Dynaco.

[x3workshop]

Originally Posted by Bombadil

Some will say it is acceptable, others will say it is not even close.

I wouldn't do it.

Dennis has said to me that it is absolutely fine to sub a CV 181, 6sn7, or 6sl7 into the LP-27a instead of the 6BX7. I do not know if it would be the same in one of these amps.

[FloridaBoy]

Originally Posted by x3workshop

Dennis has said to me that it is absolutely fine to sub a CV 181, 6sn7, or 6sl7 into the LP-27a instead of the 6BX7. I do not know if it would be the same in one of these amps.

It is OK in the LP27a but not in another circuit.

[Rosco65]

Originally Posted by FloridaBoy

It is OK in the LP27a but not in another circuit.

What does that mean? If it is ok in the LP27a, it may be ok in other circuits and it may not be ok in other circuits. I have at least two other applications in my possession where it works and works well.

I think we've established that in some applications we can satisfactorily substitute higher gain small signal tubes if the design can accommodate them. The LP27a is not unique in that regard.

[FloridaBoy]

Originally Posted by Rosco65

What does that mean? If it is ok in the LP27a, it may be ok in other circuits and it may not be ok in other circuits. I have at least two other applications in my possession where it works and works well.

I think we've established that in some applications we can satisfactorily substitute higher gain small signal tubes if the design can accommodate them. The LP27a is not unique in that regard.

Exactly. Technically, Bombadil is correct and so are you it is not a direct sub for a 6SN7 but will work in some circuits. I just did not want someone to try it w/o checking with the builder/manufacturer. I'm not going to use a BX7 in my amp.

[Rosco65]

Dennis just sent me this (unsolicited) comment:

"Hello, hello from sunny Cary, North Carolina. I hope all is well with you today. One of my audiophile buddies brought to my attention a few days ago a controversial subject of various dual triodes specifically in use with the Inspire gear. Please have a look at a short dissertation on the subject as it pertains to the Inspire stuff:

In short order let me address the 6BX7, 6SN7 and 6SL7 issue. Under normal design parameters in the outside world of tube operation the 6BX7 will not be suited as a sub for a 6SN7 or even a 6SL7. My design is done in such a manner that, yes you may use either in the Inspire amplifiers. The filament supply I use is AC/ 6.3 nominal. The filament windings are more than capable to accept the addition filament current requirements from the 6BX7. If one were to sub a 6BX7 in a conventional circuit designed for 6SL or 6SN7, the power supply may not have the current available. Then on to the plate voltage and current. In my designs I run regulated plate voltage and also regulated positive cathode bias voltage on the driver section. The 6BX7 will draw considerably more current vs the 6SN or 6SL. This regulated supply in the Inspire amps is designed specifically to accommodate the additional current draw. Also the plate load, dropping anode voltage coupling resistor to this DC voltage must be considerable greater in dissipation wattage ratting for the 6BX7. These resistors in the Inspire are 3 watt value. The 6SL7 would only need a 1/2 watt resistor, the 6SN7 should have a 1.5 watt …. so once again one is covered. Now as far as the sonics … all there choices will stay within a linear range driving these beam power tubes. As you have discovered each of these dual triodes will have a different sonic presentation … but ,… and keep this in mind … all three will perform in a linear fashion and have the proper components associated with this resistive coupled circuit. Outside of this operating circuit the 6SN7 produces the absolute most vertical linear curves. Also another factor to keep in mind … the choice of rectifier tube does not change the operating parameters of the drive section. The nominal regulated driver voltage stays the same 250 VDC potential with a 5Y3 or a 5AR4 or even a diode rectifier substitute module.

To my ears, the 6SL7 produces top end sparkle …. the 6SN7 produces the most even sounding full bandwidth and the 6BX7 offered the lower midrange weight need in many systems that are lean sounding.

Just as with the output tube section screen grids … the key is the regulated low impedance regulated power supply delivering the DC operating voltage to the anode and cathode of these dual triode input drivers. The long and short … either of the three will work great. Oh, you may also like the CV-181 drivers as well.

The Inspire design is not at all common with other single-ended amplifiers or preamplifiers in many respects.

Talk to ya soon and once again thank you for your continuing interest and support

Dennis J. Had"

[FloridaBoy]

Well you can learn something new.

[Bombadil]

This is interesting, as most circuits designed for one tube type are significantly off for other very different tubes. And even for the Inspire amps, Dennis is saying that the frequency curves for these tube types are different. Dennis says the 6SL7 will have more "top end sparkle" which means their frequency graphs will be accented on high frequencies. The 6BX7 will have emphasis in the lower midrange, and perhaps upper midbass. While the 6SN7 will produce the most linear curve - sounds like the circuit is optimized for this tube.

Some amps will even go into oscillation by swapping these same tube types.