Started down the road.

Peter, Can the PAS 2 still benefit from the addition of a 1uf capacitor on the output like the 3X uses off the bass control? I think they have a potential DC offset issue otherwise if I am correct.

Yes, it can. As already noted.

One more thing - the impedance-matching (X-Mod) should be done if not done already:

To do the impedance mod - per the Dynaco literature:

1. Obtain two 100,000 ohm, 1/2-watt 10% tolerance resistors.
2. Unsolder and discard the two 510,000 ohm (green-brown-yellow)
resistors connected to the audio-output sockets on the back panel of
the preamplifier and replace them with the 100,000 ohm resistors.
Solder all connections.
3. Snip out and discard the two 62,000 ohm (blue-red-orange) resistors
on the PC-5 circuit board adjacent to eyelets 8 & 18. This completes
the modification.

Given the relative difference in cost, I would use 5% resistors and
screen them as well.

.

[quote="PeterCapo"]The "impedance matching" is not necessary if he sticks to the original Stereo 70 circuit.

If he goes to a different circuit in the amplifier that results in a lower input impedance than that of the original Stereo 70, then it is difficult or impossible to predict exactly which resistor value would be appropriate without knowing the amplifier's Z-IN.

It is not necessary to precisely match the Z-out of the PAS to the Z-in of the amp. The mod as promulgated by Dynaco using a 100K resistor is more-or-less middle-of-the road to then-becoming-popular transistor amps. Nor will it hurt when matching a vintage 70.

Similar to capacitance matching for phono-cartridges, to include cable capacitance. Desirable, but not absolutely necessary. Most phono-pre-amps used middle-of-the-road assumptions to this end.

.

The output of the PAS benefits from tuning. GP49 has talked about this on a few occasions.

Amen, Brother!

On the tuning end of things - I remember that the 416 was standard at 50K, and in one control-configuration was as low as 20K. I seem also to remember some warnings against tube pre-amps in that literature, but that was also 20 years ago.

But for the most part, the 100K resistor does OK until a permanent partner is chosen - then, matching becomes a worthwhile exercise.

Understanding the loading requirements is one of the most important, yet least understood, aspects of using a PAS 2/3 with more contemporary power amplifiers. Thus, it has been extensively covered in the Audioregenesis line stage and compatibilty articles. Being engineering papers, of necessity they are somewhat technical, so not everyone will be able to comprehend, but the information is there for those who can. The compatibility article is perhaps more understandable for the less techically inclined.

Basically, for flat bass response with the tone controls centered, the PAS 2/3 (not 3X) must see a 50K impedance at the output. It's a design requirement because of the way the tone controls were implemented, and the source of much confusion since it's not generally understood what the load consists of. Part of that 50K load is inside the PAS itself, while the other part is the power amplifier it is driving. The parallel combination of PAS internal load resistors and the power amplifier input impedance is the total load, which must be 50K, or close to that. When used with tube power amplifiers, as originally intended, the internal load resistors (68K and 510K in parallel) primarily determine the load since the 500K (typ) input impedance of the PA is so high. So, the total load is close to the required 50K for most cases when tube PA's are used. However, when lower impedance PA's are connected, their input impedance drags down the total load impedance to the point where the bass response is reduced, the extent depending upon the actual PA impedance. Within limits, the internal load resistors in the PAS may be increased so that the total load presented is close to 50K again preserving the bass response. The lowest permissible PA impedance would be 50K at which point the internal resistors in the PAS would be deleted entirely. Lower PA impedances than that can only be accomodated by bypassing the tone controls, or employing an external buffer stage. The 3X, of course, does bypass the tone controls at center.

Note that the case given for a 100K internal load is valid ONLY for the condition where the power amplifier input impedance is 100K. Thus, 100K internal load in parallel with 100K PA load once again restores the total load to the 50K requirement. A 100K internal load used with a high impedance tube amp would result in increased bass response. So called TUNING the PAS is simply the act of altering the internal load resistors to restore the required 50K total load impedance with a specific power amplifier, within the limits it allows. If the PA impedance is 50K or more, it will be within the limits that altering the internal PAS load resistors will allow.

.

One could add a trick;
suppose a PA has 50k input impedance. At the same time it might have sensitivity at 0.5V for
full power, then a series resistor of 150k from an unmodified PAS will make both reasonable
happy, the PAS will "see" 200k as load, and as the pas happily emits 2V the PA will "see" 0.5V
at it's input.

peterh wrote:One could add a trick;
suppose a PA has 50k input impedance. At the same time it might have sensitivity at 0.5V for
full power, then a series resistor of 150k from an unmodified PAS will make both reasonable
happy, the PAS will "see" 200k as load, and as the pas happily emits 2V the PA will "see" 0.5V
at it's input.

Yes, but you would have to take care in where and how you implement it. A large series resistor would greatly increase the effective output impedance of the PAS, which is normally quite low (1K) "at higher frequencies". Interconnect cable capacitance, which is not normailly an issue, could become one, especially on longer runs. But, if not taken to extremes, this approach could help to extend the load range.

Corndog7- For the ST70 I do plan on getting the Octal driver for ST70 that uses three 6SN7’s
and the Auto-Bias board from tubes4hifi. Going for the updated board, easy decision after pricing the 7199’s.

Received the parts chassis and cage yesterday. Chassis has a few rust spots. I plan on getting it
powder coated. Talked to a local shop and with any of their standard colors the cost would be
around $50 US. Cage is in good condition, not sure if I will also power coat it.

Also received the preamp this afternoon. Did a quick once over and will have to replace the
power switch if I can’t free it up. It has one Dynaco 12x4 (Japan), 2 12AX7A’s (GE) and 2 12AX7’s
(GM Delco)

Pre amp ‘s overall appearance is pretty nice. Cover is faded. Thinking of also having it powder coated, maybe same color as the ST70.

Read and downloaded all the info at audioregenesis site, found it really interesting, some info I followed some I did not, will have to read it all a few more times.

FM tuner is somewhere between California and Texas.

Now all I have to do is start buying components for the ST70. Tapped out on my hobby budget so
the transformer set I plan on getting from Dynakit parts will be next item to pick up. This may be a month or two away.

Want to thank everyone for all the pointers, suggestions and other info. Really appreciate it. Glad I found this forum, otherwise I would be lost.

Sounds great.  I have the octal version of my Mk3s and I think the 6SN7s sound great.

A couple of comments you probably already are aware of:

Check out https://dynacotubeaudio.forumotion.com/t2788-octal-driver-board-power-transformer.  My Mk3 power transformers run pretty hot but they've been working fine for about 6 months...fingers crossed.

If you powder coat the chassis, don't forget to grind/remove any paint where you need a ground.  Sometimes that means tube socket mounts.

Good luck!
dave

CNCfan wrote:Corndog7-  For the ST70 I do plan on getting the Octal driver for ST70 that uses three 6SN7’s
and the Auto-Bias board from tubes4hifi. Going for the updated board, easy decision after pricing the 7199’s.

Now all I have to do is start buying components for the ST70. Tapped out on my hobby budget so
the transformer set I plan on getting from Dynakit parts will be next item to pick up. This may be a month or two away.

Want to thank everyone for all the pointers, suggestions and other info. Really appreciate it.  Glad I found this forum, otherwise I would be lost.

I picked up the octal driver board and auto bias board too. Even though I'm loving my M125's and my VTA120 is on the bench, I still want to build another ST70. You know, for fun.

I too need to pick up the transformers but I'm going for a custom chassis layout. I'm just not a fan of the cramped quarters in the stock chassis. That's the beauty of building from kits. Flexibility.

Well, I made a detour in my chassis build.  Found this on ebay so threw a bid at it and won.

Looks to be in pretty good shape, if anyone sees something I should address please let me know. I will have to get a bottom cover
for the Power transformer.  Seller said it is about 2 years old.  Pics are the ones that seller has in his ebay listing.

Reason I went with it is it has the components that I had planned for the parts chassis and still do, it will take me
a bit longer but it is still on the workbench.

The only thing this does not have is the autobias board that I had on the list for the parts chassis build.

I quote "You know, for fun", so Corndog7, I am going to put this on you when my wife ask why I bought it. I'm going to say
You know, for fun,  will see how far that will fly,

Photo's here:  https://dynacotubeaudio.forumotion.com/t3817-photos-of-new-st70#36733

this is just an FYI post for those who are not military - the avitar that CNCfan has is a military ribbon for Vietnam service (1958-1975)
and is for actual Vietnam veterans, as opposed to people like myself, I'm a Vietnam "era" veteran as I was in the military during that time period, but I was never IN Vietnam
CNCfan, thank you for your service!

Thank you tubes4hifi, appreciate it. Also thank you for your service.

Yep, was in a place called Phu Lam, Nov 69-Jun 71,  it was a Army communications site. Did not see any action.
Guys doing guard duty in the towers would get shot at every once in a while but nothing serious. Never happened when I pulled guard.

CNCfan wrote:Thank you tubes4hifi, appreciate it.  Also thank you for your service.  

Yep, was in a place called Phu Lam, Nov 69-Jun 71,  it was a Army communications site. Did not see any action.
Guys doing guard duty in the towers would get shot at every once in a while but nothing serious. Never happened when I pulled guard.

I was in B-52s flying out of Thailand 1970-71.  Arclight -- you may have heard or felt some of our work

dave

Desert Storm veteran here. Only got shot once but it was minor.

Mail man handed me a box that looked like someone rolled it down a staircase a few times.

Lucky that upon unpacking all I would was a tube that was a bit bent over and two that
looked like they were ready to fall out of the sockets.

Did a quick once over, connected it to my varac.  Also setup the preamp and FM3.  First
time power on for everything.  

Make a long story short I am very happy with the amp, preamp and FM3.  

Did a short test by hooking up to a set of cheap speakers I keep in the garage. Only tested
for a few minutes. The preamp still has the selenium rectifier in it and all the caps look to be
original so did not want to let it run to long.

Putting the parts chassis on hold until I get the preamp and FM3 recapped and anything else
to insure they do not go up in smoke.

Pic of my Dynaco system at the link below.

Did I say I was VERY happy

Sorry for all the clutter in the background but was eager to see everything working.

https://dynacotubeaudio.forumotion.com/t3821-st70-arrived#36792

I have a very faint 60 cycle hum. With nothing connected to the amp input.   I have to put my ear right next to the speaker to hear it and then only when nothing is playing.
I have a 3 prong 120v plug on it and the inputs have resistors to ground.

Just wonder just how much of a hum is normal on an ST70? Absolutely no hum or the amount you are prepared to live with?  

On checking the bias the left channel was .94v and the left 1.1v, set both to 1.4v. No plans on loud music so figured this would help
the tubes last longer.  If I am wrong or it would not matter, let me know please.

Found a few what I consider cold solder joints and re-did them.

The ST70 has this power option: http://triodeelectronics.com/capboards.html and the driver board here:  http://triodeelectronics.com/st70boards.html
Wondering if this is compatible with the auto-bias option here:http://tubes4hifi.com/ST70.htm?
It also has the Triode power and output transformer(s).


Sorry for all the basic questions, this is the first tube amp I have owned.
Thanks for any input anyone would like to give?

Some mechanical hum from the power transformer may be unavoidable. There is no reason why you must have audible hum through the speakers. To paraphrase Orwell... three prongs good, two prongs better. I am assuming the hum is the same in both channels. Only one device in your audio chain should have a grounded three prong power plug. Try powering this way to see how it impacts your hum.

If you are concerned about floating grounds and safety... I suggest you avoid amplifiers that have exposed high voltage on their input boards  

.

CNCfan wrote:I have a very faint 60 cycle hum. With nothing connected to the amp input.   I have to put my ear right next to the speaker to hear it and then only when nothing is playing.
I have a 3 prong 120v plug on it and the inputs have resistors to ground.

Try shorting the inputs (while off, of course) and then listen for the hum.
I second the motion to get rid of the chassis ground-to-receptacle. Use a non-polarized two-prong plug so that you are able to reverse it in the socket for the least hum.

AND WHEN YOU DO THIS: take your trusty VOM and measure for _any_ voltage from the chassis to a known-good ground. If you get more than a very few millivolts look for any internal bad caps, sorts or potential sources for such leakage.

Note that ALL components should share one (1) ground so that ALL components are at equal potential. It is not a good bet that any two different grounded receptacles have grounds at the same potential. The reason is obvious to an electrician, perhaps not so much to others. Point is that, historically, wiring grounds have received short shrift by code. So, in an older house, apartment or other domicile, that ground may be:
a) One gauge smaller than the main conductors.
b) The casing on BX cable (MC Cable).
c) Daisy-chained to multiple receptacles - each connection being a minuscule source of resistance.
d) Bundled at the panel.
e) Even, all of the above.

So, the dedicated receptacle *here* has same-gauge home-run straight to the panel, but the undedicated receptacle *there* is at the and of a 7-device daisy-chain. So, any device plugged in *there* will be 'above ground' relative to *here*.

Peter W. wrote:

CNCfan wrote:I have a very faint 60 cycle hum. With nothing connected to the amp input.   I have to put my ear right next to the speaker to hear it and then only when nothing is playing.
I have a 3 prong 120v plug on it and the inputs have resistors to ground.

Try shorting the inputs (while off, of course) and then listen for the hum.
I second the motion to get rid of the chassis ground-to-receptacle. Use a non-polarized two-prong plug so that you are able to reverse it in the socket for the least hum.

AND WHEN YOU DO THIS: take your trusty VOM and measure for _any_ voltage from the chassis to a known-good ground. If you get more than a very few millivolts look for any internal bad caps, sorts or potential sources for such leakage.

Note that ALL components should share one (1) ground so that ALL components are at equal potential. It is not a good bet that any two different grounded receptacles have grounds at the same potential. The reason is obvious to an electrician, perhaps not so much to others. Point is that, historically, wiring grounds have received short shrift by code. So, in an older house, apartment or other domicile, that ground may be:
a) One gauge smaller than the main conductors.
b) The casing on BX cable (MC Cable).
c) Daisy-chained to multiple receptacles - each connection being a minuscule source of resistance.
d) Bundled at the panel.
e) Even, all of the above.

So, the dedicated receptacle *here* has same-gauge home-run straight to the panel, but the undedicated receptacle *there* is at the and of a 7-device daisy-chain. So, any device plugged in *there* will be 'above ground' relative to *here*.

This is good advice. The importance of a single ground point cannot be overstated in keeping electronics quiet.

Fwiw, I have been running my VTA ST-70 with a 3-prong grounded plug and it has no hum whatsoever that I can hear at the speakers. I personally feel unsafe with an exposed chassis that is not grounded.

j beede wrote:

If you are concerned about floating grounds and safety... I suggest you avoid amplifiers that have exposed high voltage on their input boards  

Yeah, then there's that...

Reminds me I really should get a cover for that driver board.

Yeah, then there's that...

Reminds me I really should get a cover for that driver board.

<--------------- That is Murphy, a 20 pound Maine Coon. He, his companion Maine Coon Mix Seamus, Tucker, the golden and Maggie the scottie, as well as four grand-kids are the reason that no tubes (or driver boards) are exposed in our household. Murphy has been known to jump from the floor to the top of the refrigerator - so no location is high enough, either.

The kids are very nearly all old enough to know better, but the animals are not quite, yet.