Original Dynaco 70 circuit mods/upgrades?

[kegger]

Thanks ELI, if I was to use an original "type" replacement board I
have considerd the EF86 one, I do like those tubes and have many.
Good to know it is a good working board.

In the past I've had good results with the Tubes4hifi/VTA board as
an excelent choice for totaly changing everything, I also redo the VTA
board to use 12bh7's and other 20ish mu tubes then I do away with all
intentional feedback including the loop, works rather well and sounds great.

On this I'm gunna experiment with what's here and see what I can do.

My thinking is essentially parallel to SY's. I like the Triode Electronics replacement driver board equipped with 2X EF86s and an ECC99, as the OEM topology is retained. However, the active devices are better than those in the 7199. The EF86 is (perhaps) the best small signal audio pentode ever and the HIGH gm of the ECC99 is protection against slew limiting in the "concertina" phase splitter.

In amps that have a NFB loop around the O/P trafo (including the ST70), I think that rolling off infrasonic trash at the I/P is important. Rolling the junk off protects the O/P trafo cores from saturation due to a massive NFB error correction signal. Set the "corner" freq. just below 20 Hz.

Increase the value of the coupling caps. between splitter and EL34s to make the high pass pole fall at or slightly below 5 Hz.

 

[Johan Potgieter]

Burnedfingers just sent me the requested Audio Basics article in full, for which I am much obliged. I presume no values were changed other than the connections for the 6GH8. I am still wondering in what way the reported significant improvement could have taken place, as the characteristics for these tubes are quite similar as said before. Perhaps the make of tube; it is known that characteristics for the same tube number can vary by up to 20%.

I would have liked to see electrode voltages for the 2 tubes tested; information I have indicates a rather startling variation in distortion for the 7199 with slight variation in electrode voltages. (This is to some extent true for all pentodes.) But enough of that; I do believe that the 6GH8 will render good performance, and if availability was a factor as indicated in the article, reason enough for the change.

Regards.

 

[Johan Potgieter]

In amps that have a NFB loop around the O/P trafo (including the ST70), I think that rolling off infrasonic trash at the I/P is important. Rolling the junk off protects the O/P trafo cores from saturation due to a massive NFB error correction signal.

Amen to that!

This is often neglected: Those expensive turntables that don't rumble, bumps that don't occur because everything is mounted sturdy, etc. ..... I regularly build a low-cut filter into my pre-amps cutting at up to 18 dB/octave below say 20 Hz, apart from looking at the main amplifier. (It mostly does not need many extra components; simply proportioning things right that are there anyway instead of just making them "large". And it costs a great deal less to improve rumble performance by a filter than buy the equivalent turntable achieving that mechanically.)

If designers would only realise that amps may be well designed to cope with the audio band, but stuff coming along outside that (both low and high frequency) will encounter a reactive response and can cause all sorts of rubbish. Filter there, and one would be rid of some of the reasons why amplifiers create listener fatigue.

Regards.

 

[Burnedfingers]

Johan,

Not a problem at all. I thought you might enjoy reading the article.

I presume no values were changed other than the connections for the 6GH8. I am still wondering in what way the reported significant improvement could have taken place, as the characteristics for these tubes are quite similar as said before. Perhaps the make of tube; it is known that characteristics for the same tube number can vary by up to 20%.

It may seem that the author enjoys getting his article printed more than he cares about backing up any claims. You made good valid points. Tube maker/make also plays an important factor in performance. I don't know if there is any performance gain to be made here as you stated. I just wanted to offer an alternate to the 7199 if anyone were interested.

 

[Johan Potgieter]

Yes.

My personal impression with articles like those is increasingly that someone is trying to sell something. Just look at cables..(oops, I should not start that! ) I avoid discussions about "the sound of" whatever. Goodness knows, in my experience there has been enough contradictory witness to that sort of thing. To be kind, one cannot go poking about the brain of the listener with a probe to measure what he was actually hearing. One can at most perform a series of hearing tests to identify normal or "standard" listeners and use them for hearing tests. This has been done quite extensively at some Scandanavian universities (possibly also elsewhere) for the purpose of correllating amplifier characteristics with human hearing. The explanation for the diversity in hearing is fairly simple - but that is another subject.

All in all there are several candidates for perfectly useful pentode-triodes for this sort of service. As said, I have read (no experience) that the 7199 has been ruggedised or whatever against adverse effects as a pre-amp input tube; characteristics that are not so important in power amplifiers. In such cases I, like others, look at availability/cost in choosing a suitable tube if nothing else rules.

Regards

 

[Cobra2]

http://www.audioasylum.com/forums/dynaco/messages/10446.html

Arne K