6sl7 vs 12ax7 and 7199 in ampegs

[ernie_jr]

Info question
I plan on building an Ampeg b15 etc and I am looking for recomendations on using 6sl7 or 12ax7 or 7199
as the phase inverter and 12ax7 or 6sl7 for the preamp sections. if  switch from 6sl7 to 12ax7 to I have
to change  the circuit any or can I use the same values for the resistors.
thanks,
Ernie

[ampgeek]

Hey Ernie,
I have built a handful of 5F6A Bassman circuits over the last couple of years with all octal (6SL7) preamp and PI tubes.  Although I have tweaked values countless times, I always seem to end up with nearly identical values to the standard nine pin schematic.

So...I have to believe that you will have no problem starting with the exact same values going the other way and tweaking from there.  Just make sure to rewire the pins accordingly.

Sorry.  No experience with the 7199 to offer.

Good luck!
Dave O.

[jjasilli]

7199's are very expensive, and a good hi-fi tube.  Due to price the 6GH8A is a reasonably priced substitute, with a different pin-out.

[ernie_jr]

Thanks
I know 7199 are expensive, I plan on using 6an8 or 6gh8. My main question concerning the 7199 is to use a  pentode/triode
or the twin triode (12ax7a/6sl7) as the phase inverter.
Thanks again
Ernie

[sluckey]

My main question concerning the 7199 is to use a  pentode/triode or the twin triode (12ax7a/6sl7) as the phase inverter.

Look at any of the old Sunns. Pentode as gain stage, triode as split load PI.

[ernie_jr]

Thanks,
I have built several Sunns with 6an8's, but I dont have much experience with a twin triode for the phase inverter.
Any advantages/disadvantages with either? Different sounds?
Thanks
Ernie

[jjasilli]

My theory is that PI type affects tone when the PI itself is overdriven; and that the LTP provides a smoother overdrive tone, when the PI is overdriven.  The reason is that when a tube stage is overdriven, the top and bottom portions of the signal wave are treated differently and become asymmetrical.  When an LTP is overdriven, the 2, simultaneous, mirror-image drive stages eliminate this unbalanced signal  condition.  Other types of Pi's have a driver stage, then a unity gain follower stage.  When the driver stage is overdriven, it yields an unbalanced signal, which is passed-on by the unity gain follower stage.

With hi-fi, or before overdrive tone in guitar amps became desirable, this factor made less difference.  So I guess the choice is to duplicate the PI of a vintage circuit, or "modernize" it with a LTP.   Some people prefer the unbalanced OD tone, which to me is more Tweed-like than Blackface.

[xm52]

If you want your B15 to sound as close as possible to a real B-15, then I suggest that you use the 6SL7's in your build. I think that the octal 6SL7 has a warmer and richer tone that is different to that of the 12AX7 in these amps.

The B-15's 6SL7 based PI design seems to work well in lower power 25W or 30W amps. Ampeg switched to the 7199 based phase inverter in higher output power designs like the B-18 with the 7027A power tubes. The 7199 drives the 7027A tubes better than the 6SL7 can without distortion or stability issues.

The other important thing to consider is the power supply voltages. Ampeg used a higher B+ than you see in many other amps. It costed more to do this. The design places the quiescent operating points in a position that optimizes headroom and minimizes distortion.

[ernie_jr]

same amp, new question; on the 1962 b15n the 6sl7 phase inverter has 47K's from the B+, the 1968 has 120K. They both run the same PT with the same dropping resistors. What would be the reason for such a large difference.
Thanks
Ernie

[sluckey]

Probably because one PI is driving a cathode biased power amp and the other PI is driving a fixed bias power amp.

[xm52]

With the 47K resistors the plate voltage on the 6SL7 is listed as 310V and 309V on the '63, cathode bias schematic. With the 120K resistors the plate voltages are listed as 235V and 225V on the '66 fixed bias schematic.

Ampeg used Sylvania 6SL7GC tubes in these amps. The data sheet lists the max plate voltage as 275V. Not that that means anything as manufacturers would sometimes exceed the published limits without any problems.

There are a lot of early model B15's that run the higher voltages without any problems. It is common to see original tubes in these amps. Mine is one of them and it is a well used amp.

[ernie_jr]

On the clone of a B15 that I plan to build, on the caps feeding the grids of the 6l6's, B15 calls for 0.022, b25 calls for 0.33, 5f6a call for 0.1. How does this affect tone and what would be best for a bass amp.
2nd ques-- The bass and treble pots are 1M, Can I use 500K as I have several/
Thanks,
Ernie

[panhead]

You may find problems with microphonics with the 6SL7. If you can get a quiet one, I think they are great sounding tubes. Like xm52 said, they have a richer, warmer tone than 12AX7.