Foreplay III New Build + adjusting listening levels + ST-70 troubleshooting

[Aljaheejus]

Took some official readings under the hood of the ST-70 while powered and playing music, and I found that most voltages were close to the design levels.  The AC voltage on EL-34 pins 2 and 7 should be 6.4 but it measures 3.3 VAC.  Same anomaly on 7199 pins 4 & 5, both channels, should be 6.5 but reads 3.3 VAC. 

Anyone have a clue as to why this would be, and why it matters?  (I'm double posting this again to the dynacotubeaudio forums in a new thread.)  Thanks in advance.

 

[JC]

This sounds as if you were measuring from each tube pin to ground.  I think if you measure AC Volts between the specified heater pins, you will get closer to what you're looking for.

BTW, it is not uncommon for Voltages to be somewhat higher than when the amp was new; in those days, the Voltage coming out of the outlet was supposed to be 117 VAC.  Now, 120 VAC is the goal, and 125 VAC is common.  With higher Voltages going in to the power transformer, you end up with higher Voltages coming out.

 

[Aljaheejus]

That is correct, Jim.  I was measuring to the grounding terminal near the quad capacitor.  Looking at the schematic, I cannot figure out which pin on each tube I would use as ground to measure otherwise. 

In the mean time I am going to temporarily solder the 180 KOhm resistors into the voltage divider parallel to the 470 KOhm.  Sounded about right with the clip leads, but I might err to the louder side when I finalize the resistors to replace this little cluster.  The Van Alstine input mod I am interested in trying will complicate things here also, but I'll cross that bridge when I get to it (with the help of the good people of this forum!)

 

[Grainger49]

David,

As Jim points out you have a good voltage for the output tube heaters.  The measurements you made add to 6.6V AC, that is fine considering the higher incoming voltage.  As it is AC you can measure with either lead on pin 2 or 7.  

Looking at the voltage divider, paralleling a 180k with the existing 470k gives you a 130k.  The resulting voltage divider will be a 2.7:1 divider.  Very different than the 10:1 you have now.  Is this what you are looking for?  

If you have a target I can give you an idea of what to parallel with the 470k to get there.

How far are you going on the FP III volume control?  The goal is to get as loud as you ever want with a click or three left.

 

[JC]

Perhaps better to look at the Heater circuit as a separate entity from the rest of the amp.  The heaters are powered by a separate secondary winding from the power transformer (or, maybe a separate winding for the tubes in each channel).  And, while the center tap of that heater winding is referenced to circuit ground for a couple of reasons, if you want to measure the AC Heater Voltage, you will need to measure across the entire winding; if you use circuit ground for one meter lead, you will only be measuring from the center-tap to one end of the winding, which will give you only ~ half of the total Voltage you are looking for.

If the Heater connections on a given tube are pins 2 & 7, for instance, one meter lead will go on pin 2 and the other lead on pin 7, with the meter set to AC Volts.  Since you are expecting ~ 6.3 - 6.4 VAC, the meter range should be set to something higher than that, if it is not an auto-ranging meter.

Again, I would expect the Voltages to be somewhat higher than specified due to an increase in line Voltage delivered to your home, as compared to when the amp was new.  Tubes were usually designed to operate well within + or - 15% or so of nominal line Voltage, since it could vary quite a lot from town to country and place to place back in the day.

What will be interesting to see is what Voltages you are able to read if the amp is acting up; they may tell you something more about what is going on.  BTW, if you haven't heard it already, it is always a good idea to keep one hand in your pocket while measuring Voltages.  This usually means that at least one of your meter leads will need to be clipped into place.  This is to minimize the possibility that you would accidentally shock yourself up one arm and down the other.

 

[Aljaheejus]

The problem you describe on the Dynaco Tube Audio board sounds like a problem with the preamp.  That is easy to figure out just by swapping your input cables at the input of the ST-70.  If it moves then it is not in the ST-70.  But you have probably done that.

Well, I thought I had done this.  Today I bypassed the new how-could-the-problem-be-with-my-newly-built Foreplay III preamplifier.  Connecting the Hagerman Ripper directly to the ST-70 there is no imbalance in volume between channels.  I must have done something incorrectly in the changing of the input resistors operation.  Guessing I should test resistances across these input resistors and compare to opposite channel/specs.  Seems like a one-notch variation, but not exactly so.  Other ideas for troubleshooting this?  Guess I need to switch the 12AT7 tubes first and see if it follows the tubes.

In the mean time, the ST-70 has a new voltage divider configuration with the 470K + 360K + 180K (all three) parallel on the input side and the 47.5K to ground.  I measured 90.6K across the former (from memory) and the volume is about right.  I will wait to sort out the FPIII preamplifier issue before thinking about changing this in the ST-70.

 

[Grainger49]

  .  .  .    Today I bypassed the new how-could-the-problem-be-with-my-newly-built Foreplay III preamplifier.  .  .    I must have done something incorrectly in the changing of the input resistors operation.  Guessing I should test resistances across these input resistors and compare to opposite channel/specs.  Seems like a one-notch variation, but not exactly so.  Other ideas for troubleshooting this?  Guess I need to switch the 12AT7 tubes first and see if it follows the tubes.

In the mean time, the ST-70 has a new voltage divider configuration with the 470K + 360K + 180K (all three) parallel on the input side and the 47.5K to ground.  I measured 90.6K across the former (from memory) and the volume is about right.  I will wait to sort out the FPIII preamplifier issue before thinking about changing this in the ST-70.

I chuckle because this sounds like something I would have done.  The best laid plans...  and all.

Sounds like you have the input of the ST-70 sorted out at about a 2:1 ratio.

 

[Aljaheejus]

Okay, I finally sorted out my mistaken placement of the input resistors in the Foreplay III.  Naively following only my intuition I had installed the resistors symmetrically, which looked good but padded the left channel of input 1 and the right of input 3.  Since I kept changing inputs and wires, thinking it was the ST-70, it was a moving target!

So, back to the ST-70, yes, I have about a 2:1 voltage divider padding the inputs now and I'm pretty happy with that.  I'm looking at modifying the inputs per the Van Alstine page mentioned above, so I'm not sure how this divider will work with that, but in the mean time I would appreciate a recommendation to optimize or simplify this voltage divider with just two resistors on each channel.

I am also curious, having bypassed the preamp in the troubleshooting, why my phono preamp (Hagerman Ripper) when connected directly to the amp is, or can be, very loud compared to the Foreplay.  It seems that the Ripper is designed to do double duty as phono + pre-amp.  It appears about the lower half of the volume steps are attenuating the inputs, and above that they're amplifying.  Is this happening with my CD input, and is this normal? 

 

[Grainger49]

I looked at the AVA page.  The two filters on the input of the amp will attenuate the overall sound as well as the frequencies that they don't intend to become part of the feedback loop.  I don't know how much these filters will attenuate the midrange which is untouched by the filters.  Most likely more than you have right now. 

According to the AVA site the assumption is that, "the driving source has a reasonably low output impedance."  Certainly the FP III is "reasonable" in that it includes a cathode follower stage so you can use longer than average cables.  Three feet is average. 

With their suggested circuit you shouldn't need the 2.7:1 attenuator you have now.  A two resistor per channel attenuator for you now would be 33 k where the paralleled resistors are now and 10 k where the 47.5 k resistor is now.

As for the Ripper, it has full gain with no attenuated inputs, the FP III now has attenuated inputs.  Remember back to your post #16, you were rocking the house at three clicks.  With the stock 33 k ohm resistors it is considered unattenuated.  Now that you have attenuated the inputs and you use more of the volume control.

 

[Aljaheejus]

Thanks again for the advice.  So, I don't need to approach the 470K ohm resistor-to-ground of the original spec input of the ST-70?   If that's the case, that lower resistances are suitable and the ratio is the design requirement, then I'll sort through my stock of resistors and come up with something - or just implement the AVA modifications and take it from there.

On the voltage divider I now have 96k and 47.5k to ground (= 2.0:1 ratio.)  Your suggestion of 33k and 10k to ground (=  3.3:1) would yield more volume, correct?  

BTW, I'm happy with the sound of my system, thanks to the bottlehead designs/products and approach to sharing.

 

[Grainger49]

No, 470k is not necessary.  The spec for the FP III says a load of 10 k ohms or greater.

The 2:1 ratio (you are at 2.7:1 now).  The 3.3:1 would yield a higher volume.  But not by much, not discernible.  I was looking for a load under 100 k ohms.