Topic review

Hi Bob,

Good to know, thank's for this very quick response!
I 'll go this way and let you know,

Regards,

Marc

Hi Marc,

Although the driver circuit on the '92 version of the ST-70 is different from the older original ST-70, the output tube pin outs are the same. Pin 3 is the plate lead and pin 4 is the ultralinear screen tap. Alteration of this ST-70 to be set up for the amp to work in triode (with or without a switch) will work exactly the same way.

Bob

Hello Bob,

I have two sleeping dynaco st70 II (1992 version)different of the original one.

Do you thing it is possible to modify this version in triode mode without many problems?

Nice site!

Regards from Belgium,

Marc

Thanks Bob.

Bugs,

An all triode driver circuit is a circuit that uses only triode tubes. Triode tubes have THREE parts > an anode, a cathode and one screen. Triode driver circuits are simpler and if properly designed are usually superior to driver circuits that use pentode tubes. Pentode tubes have five parts - an anode, a cathode and three screens, are more complex and are difficult to use in a driver circuit because they are not as linear as triode tubes at all drive levels. Dynaco used pentode/triode driver circuits because it let them get by with only ONE driver tube (a 6AN8 or a 7199 tube) in each channel. This allowed Dynaco to save ONE tube when you really need "a tube and a half" per channel if you use an all triode design. In most upgraded Dynaco ST-70 driver boards like the VTA driver board you have THREE dual triode tubes. The center tube is the initial voltage amplifier for BOTH channels while the two side tubes do the phase splitting/phase inverting for each channel.

To run the amp in "triode mode" you take a tube which is a pentode (5 section tube) like an EL34 or KT88 and you tie one of the screens to anode with a resistor making it a "quasi triode" - in other words you make the pentode output tube act like a triode output tube. The only downside is that power output is now only about 60% of what it was as a pentode BUT the amp will sound a little different. In triode mode a pentode tube will sound a little "smoother" and the midrange will be projected forward a bit. Depending on the type of music you listen to and your system components you may or may not like the sound. I like the triode mode for acoustic jazz and vocals. If you are into classical music or are a rock fan you will probably like the pentode mode since it has more power and a little bit more output at the extreme top and extreme bottom of the frequency range.

Bob