Misdirected and confused

[Ned Sheats]

A Dynaco PAS2 repair turned into a rebuild. Seeing as I am going to make it as good as possible I looked up an article by Mr. Carole (CAE) about improving the RIaa equilization. Now I am going to ask a question. Is anyone willing to stick with me on this because what I am finding is what he claims were original values on the Dyna board do not correspond to either Dyna's pictorial or schematic values. If it is too much trouble to go throught the fourm and if allowed then please contact me at nsheats@aol.com using PAS2 in the subject line.

 

[peterh]

You are the victim of misinformation.
See http://www.audioregenesis.com/documents/PAS_Phono.pdf

 

[gadget73]

Its honestly not that bad out of the gate. I did the common "fix", and eventually un-did it. The only change I ended up keeping was moving the caps from the selector switch to the phono board to eliminate the routing through a switch contact. It also gives me 3 phono inputs, which I wanted.

 

[primosounds]

Misdirected and confused, you are not the only one. I feel that way all the time! I think its because politicians want us to be that way. It is how they stay in power.

 

[Ned Sheats]

You are the victim of misinformation.
See http://www.audioregenesis.com/documents/PAS_Phono.pdf

Thank you very much for your saving me from destroying my PAS2. Its bad enough I have to rip out all of the ground loops inserted by someone using shielded cable from all the jacks to the switch and other tie points. What sent me searching for help was the reputation of Dynaco and the fact that even as it is it didnt have the claimed eq issue.
Thanks again to you and Audiokarma. Now I have to check up on the power supply. I am sure there is some effect from upping the B+ and filaments when SS rectifiers are installed. As received mine had an open 10uf filter section, one open .05ufd cap and a slightly leaky .1ufd cap. I also changed all the .0075ufd caps to .0082 since the originals varied all over the place from .0062 to .0085 and I had the .0082 on hand.

 

[gadget73]

You can actually use shielded cable, but it can only have the ground connected at one end. If its not a bad install, honestly I might consider just clipping the grounds at the switch end and calling it square.

 

[BinaryMike]

Now I have to check up on the power supply. I am sure there is some effect from upping the B+ and filaments when SS rectifiers are installed.

You will encounter varying advice on the heater voltage issue, but I believe it's best to aim for the original design target of 11VDC across each 12AX7 heater unless you're certain that you will never use the phono preamp section. The reason for this low operating voltage is reduction of tube noise (hiss), which will be worse than a good SS preamp even when optimized. Anyone who advises a higher operating voltage should be asked for S/N measurements.

 

[GordonW]

It's a tradeoff between noise and frequency response linearity, with the heater voltage on the phono stage.

11v may well be quieter- but it can also cause a loss or inconsistency of gain in the tube, due to the cathode not being at full emission- which can upset the feedback loop, and therefore the RIAA EQ and the frequency response. .

Back when tubes were cheap, and you could just pick up a 12AX7 for cheap anywhere, a loss of gain wasn't a big deal- just get a new tube. Now, maybe it is.

I tend to run them on about 11.4v to 11.5v or so (within 10% of their 12.6v spec, as recommended by most tube manufacturer data for the 12AX7). That seems to be a good trade-off between noise and keeping the tube out of cathode-emission-loss-related gain errors.

Regards,
Gordon.

 

[gadget73]

The other thing that low voltage can do is make the difference between good emission and poor in a not so perfect tube.

 

[Ned Sheats]

It's a tradeoff between noise and frequency response linearity, with the heater voltage on the phono stage.

11v may well be quieter- but it can also cause a loss or inconsistency of gain in the tube, due to the cathode not being at full emission- which can upset the feedback loop, and therefore the RIAA EQ and the frequency response. .

Back when tubes were cheap, and you could just pick up a 12AX7 for cheap anywhere, a loss of gain wasn't a big deal- just get a new tube. Now, maybe it is.

I tend to run them on about 11.4v to 11.5v or so (within 10% of their 12.6v spec, as recommended by most tube manufacturer data for the 12AX7). That seems to be a good trade-off between noise and keeping the tube out of cathode-emission-loss-related gain errors.

Regards,
Gordon.

Thanks Gordon
This is becoming "quiet" (lol) a project. According to my AC VTM, (Leader) using a phono (RIAA) input of 13 mv @ 1KHz and adjusting the volume for 0db (1V) output from the entire preamp as reference; with all tone controls centered, all filters and loudness off, I have 500 uv of noise with phono inputs shorted with shorting plugs. THis is with the case off and no AC line conditioning. The aforementioned is with the filaments at 11.6 volts as a result of new caps and silicon rectifiers. My issue is with Dynacos claim that this should be 2 Uv without any description of how the test was conducted. According to my meter, if I am reading it correctly my 500 uv should be apx 65db below 0db.. Dynacos 2 uv would IMO be unachievable in tube analog circuits. Where am I in error.

 

[GordonW]

Try with the case on. It sometimes makes a significant difference in the noise in a PAS.

Also, try swapping tubes- one or more of them may be abnormally noisy.

Is it 500uv of noise on BOTH channels, or is one noisier than the other? If it's both, it's likely power supply (caps?) or radiated noise (possibly from having the lid off, at least in part). If it's only one channel, it's likely a tube or a coupling cap, or a bad connection somewhere.

Ahhh- I re-read the Dynaco manual, and I see the misunderstanding. It's 2uv "Equivalent" - referring to a noise level at the output, equivalent to the preamp output at 2uv of phono input level. That would be 2uv compared to the PHONO INPUT level (NOT the output level)- which in this case, is 13mv. That calculates out to about 76dB. That's still a pretty lofty goal for a vintage piece, but believable. Yours, at 65db, is noisier, but not UNBELIEVABLY noisier. And that 10dB of noise (or so) could definitely be caused by stuff like noisy tubes or having the lid off.

Regards,
Gordon.

 

[gadget73]

I ran across this the other day, looking for something else actually but it may be worth a read.

http://conradhoffman.com/PAS3X.htm

Cliffnotes: noisy resistors in the line stage.

Its something I can honestly believe. Carbon comp resistors do tend to get noisy over time.

 

[triode17]

Its honestly not that bad out of the gate. I did the common "fix", and eventually un-did it. The only change I ended up keeping was moving the caps from the selector switch to the phono board to eliminate the routing through a switch contact. It also gives me 3 phono inputs, which I wanted.

I did the same mod from CAE to move the EQ caps. off the switch and on to the board where they belong. It worked well. EQ caps should never be far from their termination point or loss of treble will occur.

 

[Ned Sheats]

Thanks guys. I am getting closer to completing this "love". Everything I do pushes me to something else. I replaced the selenium and the old caps which upped the filament voltages to 12 across each tube. So since I am after low noise and accurate eq I split the boards added a resistor and second filter (2000 ufd) before the phono board. Final voltages were 12.2 on the output board and 11 on the phono board. It dropped the AC component of the filament supply from .4 volts to .05 on the phono board.

Replacement of the 12X4 with silicons and 10 ohm surge resistors gained me nearly 15 volts of B+ so a resistor before the first filter brought those all back in line and added additional filtering. NOTE for people working on PAS2 CE Dist has a 30 30 30 10 @ 475 electrolytic with the same twist lock case size in stock. Nice fit mechanically and electrically.

Replaced the vol control with original value without loudness tap and reduced a varyig channel balance of +/- 3 db while rotating to practically 0. Now to a time consuming issue. The RCA jacks are so loose that there are static issues if you even breathe on the cables. Even those gold plugs that I have to almost hammer into my yamaha just slip right off. I have not tried to locate the strips of jacks but found individual jacks that can be used to replace the old clamp in ones after they are removed and the hole in the mount is slightly enlarged.

While on the jack subject.. can any one tell me why Dyna put a ten ohm ceramic resistor between the ground for the outputs / chassis and high level input jacks? I was thinking ground loop removal but its all high level and both sides end up being grounded at the same point and why a wire wound? what current could they be expecting.....lightning protection...AC bypass cap failure in some external unit? in 50 some years I've never seen that .

Thanks for all your input. If I'm violating the rules with my verbosity let me know.

 

[Ned Sheats]

It's a tradeoff between noise and frequency response linearity, with the heater voltage on the phono stage.

11v may well be quieter- but it can also cause a loss or inconsistency of gain in the tube, due to the cathode not being at full emission- which can upset the feedback loop, and therefore the RIAA EQ and the frequency response. .

Back when tubes were cheap, and you could just pick up a 12AX7 for cheap anywhere, a loss of gain wasn't a big deal- just get a new tube. Now, maybe it is.

I tend to run them on about 11.4v to 11.5v or so (within 10% of their 12.6v spec, as recommended by most tube manufacturer data for the 12AX7). That seems to be a good trade-off between noise and keeping the tube out of cathode-emission-loss-related gain errors.

Regards,
Gordon.

Since I have extra filament voltage I will play with it after I complete the rebuild and see what the optimum trade off is for the JJ 12AX7s
Thanks

 

[arts]

Thanks for all your input. If I'm violating the rules with my verbosity let me know.

The fact that you can correctly use the word ''verbosity'' entitles you to prattle on at will; please carry on

I really like the JJ 12AX7S for phono stages.I believe it to be one of the quietest new-production tubes available in this type.

 

[BinaryMike]

While on the jack subject.. can any one tell me why Dyna put a ten ohm ceramic resistor between the ground for the outputs / chassis and high level input jacks?

I have been led to believe that it limits ground loop current flow, which could reach pretty high levels if multiple earth-grounded components are connected to the preamp -- when those earth grounds are separated from one another.

 

[Ned Sheats]

Thanks. I guess I cant picture equipment in one cabinet being connected to power outlets that could be connected to different circuit breakers or grounds. I'll check the noise level with and without them in the circuit.. Thanks for your response.

 

[Ned Sheats]

The fact that you can correctly use the word ''verbosity'' entitles you to prattle on at will; please carry on

I really like the JJ 12AX7S for phono stages.I believe it to be one of the quietest new-production tubes available in this type.

Thanks for the response. I have had so much bad luck with NOS that I wont go that route anymore

 

[arts]

Thanks for the response. I have had so much bad luck with NOS that I wont go that route anymore

I know how you feel Ned.At this point in the game,unless I just ''happen upon'' some NOS in person,I won't waste my money.I firmly believe that much of the available NOS is all of the stuff that has been previously tried and rejected for various issues. Unless I can test a tube on my own equipment before buying,forget it.

Another excellent modern 12AX7 offering is the Sovtek 12AX7LPS. This and the JJ are my ''go-to'' tubes for this type,and they both seem to get a lot of love from other techs too.