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

[Grainger49]

When I rebuilt my ST-70 I removed the stock RCA jacks and the Stereo/Mono switch.  I put one input where the stock ones went and one where the switch went.  Good ones will not fit where the stock inputs are.  I just used the screw holes for the switch to anchor the plate that held both inputs.

Doesn't really look elegant, but works.

[syncro]

The padded "CD" input has balanced the inputs to the preamp nicely.  I used, as recommended by P. Joppa, the 80.6K resistors provided with the kit for this.  Phono and digital levels are very close to the CD now.

Regarding padding input of my ST-70:
I have installed a 47.5 K ohm metal film resistor on each input of the ST-70 as in the diagrams I posted above in this thread (where the 56K is shown.)  The result is that I have attenuated the input beyond what I would like with this modification.

Can someone explain to a lay person how these two series resistors work with the input wire taking off from the joint between them?  Is the 470 K between the  RCA input and the wire doing the attenuation?  If previously the stock wiring had no resistor there where there is now a 470K could I reverse the two resistors so I lessen the attenuation?  Suggestions are welcome, or if I should take this to an ST-70 board just let me know.  (I am having an issue with my ST-70 that I have posted to the dynacotubeaudio forum at http://dynacotubeaudio.forumotion.com/basket-f2/st-70-channel-cutting-out-t379.htm )

Suggestions for driver board updates are welcome, as it may come to that soon. 

Thanks!

[JC]

In looking at your diagram, you are basically making what is known as a "Voltage divider" where the signal Voltage gets "divided" proportionally between the two resistors in series.  Then, you are extracting the signal at the dividing point to feed the input to the amplifier.

If I understand the manner in which you have constructed your divider correctly, the lion's share of the signal Voltage is being dropped by the larger resistor with the leftover Voltage being developed across the smaller of the two.  This ratio follows the ratio of the two resistance values, so, very roughly speaking, you have created a 10:1 divider with the smaller part of the signal being input to the amp.

So, if the result is that the signal attenuation is too great for you, the solution is to make that ratio smaller; this means, of course, either replacing the smaller resistor with something larger, or replacing the larger resistor with something smaller.

If you are able to, I would suggest trying to rig a way to clip-lead various values into place before making one permanent.

In reading the linked thread about your ST-70, since the issue is a temporary imbalance between left and right, the first question that comes to mind is whether you have swapped tubes from left to right to see if the issue stays on one side or follows the tubes.  Swapping the 7199s from left to right would be the first step, I think.

I also note that you have mentioned the tube sockets, and I can only say that in my 1959 vintage ST-70, that is the second thing I replaced, after the bias diode.  I can certainly imagine one of those old funky 50s vintage plastic sockets being the cause of many an issue.  "Freeze-mist" of some sort or another is often helpful if you think that there is a thermal issue with some part or solder-joint, but you will need to be careful with it when working around hot tubes; you don't want to spray a coolant on hot glass!

I really can't say much about the replacement driver boards available for the amp; I have only ever heard one of the variety available, and I really wasn't sufficiently impressed to call it any kind of "improvement" in sonics.

[syncro]

Thank you J.C. for the description of how these resistors are working.  It has some intuitive logic, which helps.   Does that mean that before modification, when there was only one resistor (470 K ohm) to ground after the input wire in the schematic (both soldered to the center conductor of the RCA in fact) that essentially 100% of voltage went to the input wire?  If that is the case, and now I have approximately 10% of the voltage then I have an idea of where to go.

I might try a 50-50 split by two series resistors of the same value in the "voltage divider".   I'm also wondering about your suggestion on using clip leads.  Can I clip lead a smaller resistor across an in-place larger resistor to simulate replacement?  Or should I remove the resistors (all) and use clip leads and resistors on both legs of the divider?

I'm also wondering about creating a mute switch to lower the levels when desired for fine adjustment at lower levels.  I enjoyed that feature on a previous integrated amp.   Maybe I should first do some research on the pros and cons of a mute circuit.

I will try swapping tubes across the channels again, but I did this some time ago and it did not change things at that time.  I replaced only the driver tube sockets.  I have not tried probing with a pencil eraser yet.  Thanks again for the input.

[Paul Joppa]

"...Can I clip lead a smaller resistor across an in-place larger resistor to simulate replacement? "

That's what I would recommend. Parallel the existing 470K with leftover resistors from the Foreplay (360K, 180K I guess?) and see what works best. You can in fact just solder the winner in place, which might be easier than replacing the 470K completely.

[JC]

Yes, you are on to it!  When only the 470 kOhm resistor was there from "signal hot" to ground, then the entire signal voltage being fed into the amp was developed across it and thus fed the control grid of the pentode.  When you added the smaller resistor from the previously grounded end of the 470 k, you created the divider and the Voltage across the smaller resistor became the one fed to the grid.

Here's an easy experiment which may give you a ballpark to work from: If you obtain another 470k resistor and clip it across the existing one (parallel to it), you will essentially cut its value exactly in half to ~235k.  This will change your Voltage divider ratio from roughly 10:1 to roughly 5:1.  In other words, it will reduce the attenuation to about half of what it is now. If that moves things closer to where you want to be, then you can measure the resultant paralleled resistors with your Ohmeter (approx. 235k), and clip a similar value across the combination.  The typically available values of 220k, 240k, or even 270k will be close enough for this third parallel.  Anyway, you will have reduced the Voltage divider ratio (and, the attenuation) by approximately half again.

Once you find the combination you like, then you can measure everything with your Ohmeter again and easily calculate the Voltage divider ratio you like.  Once you've done that, you can decide what you want to do about making the divider permanent.

Two things to keep in mind are:  First, the total resistance of this Voltage divider to ground is essentially the input impedance of your ST-70.  While the original spec calls for 470k, you can get away with quite a bit of deviation around that value, providing that you don't lower it so much that you overload the output of your preamp.  IIRC, the output of the Foreplay is around 600 Ohms, and the general rule of thumb is that you want to drive at a minimum 10 times that, so 6000 Ohms is your lower limit for input impedance, and I seem to recall that Mr. Joppa wouldn't mind something higher.  The upper limit is pretty arbitrary, but you can go higher than 470k for sure.

The second thing is that this input impedance is where the signal is developed for your amp, so it never hurts to make this a nice-quality resistor or combination of resistors.  Metal-films are my favorite for this duty, but carbon-film could work very well, too.

As far as your other issue is concerned, I wonder if this gain imbalance is a condition you can create on demand?  Can you make it happen and keep it that way long enough to take measurements?  That will make it a lot easier to hunt down, I'm thinking.

[Grainger49]

JC and PJ have given very good advice.  If you use jumpers across a larger in place resistor you are paralleling the two values.  The resulting equivalent value is the product of the two divided by the sum of the two.  It is always smaller than the smalest resistor.

R₁*R₂/R₁+R₂

Right now as of the Reply #31 above, are you not able to get enough volume at the highest volume setting on the FP III?  It is fine to use all of your volume control, probably best to use it all.

[syncro]

More problems have arisen in the ST-70 after installing the voltage dividers to attenuate the inputs and the new tube sockets.  I have been posting to http://dynacotubeaudio.forumotion.com/basket-f2/st-70-channel-cutting-out-t379.htm#1944 where I got (along with, of course, advice to get a new driver board) a suggestion to replace the 1.5 MOhm resistors on the PC board, and checking them I find they have drifted high. 

I found some encouragement to stick with the stock board on the Old Bottlehead Forum site from a Gary Kaufman who has some good dynaco resources on his site http://www.the-planet.org/.   Since the original PC board is in pretty good shape, even after I have re-soldered most traces and connections and replaced the 7199 sockets, I am getting new parts to rebuild it following the Van Alstine recommendations from 1982 now posted at http://www.avahifi.com/index.php?option=com_content&view=article&id=149&Itemid=172&74a0ad6b5f7a1df0ef4ab98b8fffbb41=6ba9f832ffb94d9aa84ef8d686e656ca

Any advice from this forum is most welcome.

[Grainger49]

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.

A component going thermal makes no sense.  But a solder joint that is "ohmic" is more likely.  I suggest you first inspect every solder joint you made under a magnifying glass.  If you find nothing obvious, like one with no solder on it, then rewet  all that you have gone over.  If this is every solder joint on the driver board, don't do all of them in the bad channel at the same time.  Give the board a rest.  Do some on the board, do some at the inputs, drink a Coke, then proceed.

[syncro]

Today, now that I have ordered new components to the tune of $40 - the system sounds normal - good!   I appreciate your opinion of the "component going thermal" idea.  Well, when I complete the rebuild I will know it is all it can be.  The new FP3 preamplifier is still working correctly as the switching of inputs from that have no effect.  There still may be some imbalance between channels in the ST-70 since the old 470K resistors padding the ST-70 inputs have some variation between them, likely directly affecting this?   While I wait for the new parts I think I'll continue experimenting with the input voltage divider to allow more signal through.

[syncro]

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.

[syncro]

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.