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    Found this posting on ebay...

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    JunkyJan

    Posts : 103
    Join date : 2008-12-09
    Location : BC, Canada

    Found this posting on ebay...

    Post by JunkyJan on Mon Jan 19, 2009 5:01 pm

    Good Day all

    In view of danf's quantitative analysis results published on this forum, I could not help but smile at this recent posting on eBay for a 7199 - based replacement board (IOW, the original). The bit that got me was the "not an unknown "new improved" circuit" part:

    Extract Starts here:
    This is an exact replacement for the original Dynaco® PC-3 board, fabricated from 0.062" FR4 material with plated-through holes with pads and solder mask on both sides pre-assembled with a complete set of audiophile grade components using silver-bearing solder. If you are serious about getting the absolute best sound from your Dynaco Stereo 70, either this board or our board kit is a must. It uses the 7199 circuit from the original, not an unknown "new improved" circuit. Its mounting holes and numbering are identical to the original Dynaco PC-3, allowing easy replacement. It is easy to install using the included installation instructions.

    All the components on this board are of the highest quality brought to you at a reasonable price. I had to shop several vendors to find the quietest resistors at the best prices. All components have been carefully chosen to support the high voltages found on the ST-70 board.....

    Blah blah blah
    ...End of Extract

    -- JunkyJan

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Found this posting on ebay...

    Post by Bob Latino on Mon Jan 19, 2009 8:19 pm

    Jan,

    Dynaco devised that 7199 circuit as a cost cutting measure because it allowed them to use just TWO driver tubes instead of THREE like most of the replacement driver circuits for the ST-70. When you save one tube on 350,000 amplifiers you have saved a lot of money. Remember - the design goal back in 1959 was to sell the ST-70 kit amp for $99 USD. In order to reign in some of the circuit deficiencies they had to use excessive amounts of feedback (20dB) which is pretty high by todays standards. The VTA driver board in comparison uses only 13 dB of feedback. You usually need some feeback in most push-pull designs to keep the amp stable. You can use even less feedback if you run the amp in triode mode.

    If you replace your ORIGINAL ST-70 driver board with another ST-70 ORIGINAL driver board replacement you will still be stuck with the 7199 tubes. Two things about 7199 besides their IMHO mediocre sound. (smooth, mellow, non offensive but not particularly accurate) 7199 tubes are not made anymore by any company anywhere in the world. As the present stock of 7199 tubes dry up, the price on these tubes will continue to rise. Secondly, there is NO DIRECT REPLACEMENT tube for the 7199. You can rewire the socket to accept 6GH8A tubes OR purchase 7199 to 6GH8A adapters from www.dynakitparts.com to allow you to use the more readily available and cheaper 6GH8A tubes.

    The demise of the ST-70 started in 1974 when the FTC mandated a new standard for comparing amplifier power output. Now all the inadequacies of the ST-70's power supply and driver board limitations came to light and Dynaco was forced to change the specs of the ST-70 from 35 WPC to 20 WPC and the power bandwith was reduced to 50 - 10,000 Hz. If you own a stock ST-70 today that is what you are looking at > really just 20 WPC and a power bandwitch of 50-10,000 Hz. See the page below from the 1974 Dynaco product brochure in which Dynaco explains the new standards by which they now have to advertise power output and distortion figures.

    Bob



    JunkyJan

    Posts : 103
    Join date : 2008-12-09
    Location : BC, Canada

    Re: Found this posting on ebay...

    Post by JunkyJan on Tue Jan 20, 2009 12:49 am

    Hi Bob

    Just to make sure that you understood the intent of my posting, I quoted the eBay entry merely for its "Entertainment Value" ;-). I have seen many threads at other forums (for instance diyTube) regarding alternatives to the original 7199 driver board because it is almost universally regarded as not acceptable by modern standards. The consensus is pretty much that the VTA board is right up there among the best.

    -- JunkyJan

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Jan - I understood the intent

    Post by Bob Latino on Tue Jan 20, 2009 9:39 am

    JunkyJan wrote:Hi Bob

    Just to make sure that you understood the intent of my posting, I quoted the eBay entry merely for its "Entertainment Value" ;-). I have seen many threads at other forums (for instance diyTube) regarding alternatives to the original 7199 driver board because it is almost universally regarded as not acceptable by modern standards. The consensus is pretty much that the VTA board is right up there among the best.

    -- JunkyJan

    Hi Jan,

    I fully understood the intent of your post in that it WAS for "entertainment value" ..

    Too often though people regard the older original Dynaco circuit with some "reverence" and in all fairness to Dynaco sometimes it is hard to judge a 50 year old product by TODAY'S standards. A '57 Chevy in it's day was a good car but the autos of today are a lot safer and more reliable. In it's day, as it was constructed, the ST-70 was a good amp that provided a lot of musical enjoyment for audiophiles that couldn't afford more expensive gear. You have to remember, though, that the main signal source of the day, vinyl records, only has a 30 dB or so dynamic range and as such does not really tax the very weak power supply of all the original Dynaco tube amps. Many of the deficiencies of the old ST-70 were "hidden". Most modern digital music sources have a dynamic range of over 90 dB and really tax an amp's power supply and driver circuits in their ability to respond quickly to amplitude changes.

    It is hard to believe that Dynaco used the SAME 30, 20, 20, 20 uF quad cap in all their EL34 and KT88 based power amps. While the Mark II, Mark III and Mark IV are single channnel amps, the ST-70, a stereo amp, had to run TWO channels with the same woefully small amount of capacitance. Years ago, back in the 1970's, I had a PAS-3 preamp and an ST-70 amp. When I would play the amp loudly it would "crackle" on musical peaks. At the time I thought to myself "Hey - my phono cartridge is mistracking?" - I adjusted and readjusted the tracking weight a few times but it always seemed to happen no matter what. One day I was playing the same selection but I had the volume down - hey - no crackling on the peaks! I opened the amp up and on the suggestion of a friend added a little more capacitance (maybe 20 uF or so ? - I can't remember exactly) to the second section of the quad cap - the one that fed the two output transformers and I immediately noticed that the amp would now play a little louder before the onset of that crackling noise (distortion).

    Over the intervening years I learned that one of the first things you should do on a stock ST-70 is pull out that original quad cap and replace it with a newer quad cap with larger storage sections. The CE Distribution 80, 40, 30, 20 quad cap is a good cap replacement for Dynaco's original cap. It is very well made and has almost double the power storage of the original Dynaco quad cap. On an ST-70 you face the "80" section towards the back of the amp and all the other sections will be in the proper position.

    Bob

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