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Another Eico ST 70 thread

OK I have something resembling a trouble shooting method in mind. I didn't realize I had Jon's amp drawing downloaded to phone. So upon observation, there aren't that many pieces in the bias circuit. I'll pull the 7591s and the 6sn7s. And test passive components (they're new but I didn't test them before they went in. Maybe bag mislabeled or I missed a decimal point. Just trying to eliminate the obvious). Assuming they check OK. I'll take some voltage readings at the pins of the tubes. Maybe that will be telling. I don't know the impedance of the tubes im circuit. So to me they are the variable.
 
The output tubes are not supposed to affect the bias,but I have heard if they are gassy they can have leakage,but someone needs to confirm this,my memory is not that good.The 6SN7's are isolated by the orange drops,so should have no effect on the bias.

I heve seen new resistors with colors thare "off" so that oranges can be confused with reds,blues with greens etc..4 band resistors had me messed up for a while.
I usually check them as I install them.
 
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ok checked all values of passive components. Remove the shunt resistors across the 68ks. I even changed out the 3 electrolytics in the bias circuit to the factory spec ones. I can balance the sides. I only get about -100vdc off bias diode (80vac in). I can get bias voltage up to .22 on one side and .19 on other. I ordered a UF15J to try and change out the UF4007 that I have in now. I'm back to i'm stumped. Drwgs say I should be getting -110vdc out of diode. I don't know whether diode in dropping circuit or circuit is dropping diode output.
 
If the current readings taken at the cathode test points cannot be brought up to the proper level, then there are four basic circuit areas where the problem could lie -- understanding that the problem could be in any one area, or a combination of them:

1. Low Screen Grid Voltage (pins 4&8 on each output tube). The screen voltage sets the current flow capability of the output tubes at any given control grid (pin 6) bias voltage level. If you are measuring an honest ~ 400 vdc between the screen grid terminals and ground, then this is not the problem.

2. The value of the 10 ohm cathode resistors. If these are honestly 10.0 ohms as measured from pin 5 (the cathode) to ground, then a .22 volt reading across them in fact indicates a current flow of only 22 ma. However, if the 10 ohm resistors should really read only 5 ohms (for example), then current flow across the resistors is actually 44 ma, while you are still "thinking" it's only 22 ma. The value of these resistors must be verified at pin 5 of each output tube.

3. A problem in the bias circuit. Making the voltage more negative at pin 6 of the output tubes only causes the output tubes to draw less current. Jumping the 68K resistors did just that, which should have made the problem worse, and it did. For this exercise, the output tubes could care less HOW the bias voltage is developed and delivered. They just know they want to see about -16 vdc (or so) at pin #6 to produce the target ball park current draw based on the other voltages present. Whatever accomplishes producing (about) this amount of negative voltage at pin 6 of each output tube should cause them to draw the correct amount of current. Since there is (potentially) too much bias voltage, I would be more inclined to double check the value of the 33K resistors at the bias pots, and the 100K resistors at the balance pots. If any of these are significantly high in value, then that would tend to produce the problem you are having.

4. Low heater voltage. If the AC voltage across pins 2&7 are lower than 6.0 vac, then current flow can be a problem if weak tubes are installed. Check the voltage across these terminals with the amplifier biased as close to normal operation as possible.

If ALL of these areas are verified to be as they are supposed to be, then the only other possibility left is that of the tubes themselves. However, since the problem seems to be present equally in both channels, it implies a mistake common to both channels, or a similar mistake made in both channels.

I have no doubt that you will find the issue somewhere within these possibilities. Good luck with it!

Dave
 
I need to wear the pointy hat for a while. I found mistake....GOD I FEEL STUPID!
going to eat dinner now. I'll clue you in , in a bit.
Ok , testing voltages, first the regular high voltages a little low....really close though. so for sake of argument, I jumped cl-80. That pretty much made up the difference (only a couple of volts across the board. It dropped the line from 115.3 to around 112. So I look for screen voltage of 400 (i'm starting with 390v so that doesn't seem likely but certainly in the ballpark. with the cl in between 270- 291. Jumping it got us to to 283 to 299. still way low. I go back to where it comes from (i'm starting between 378 and 390 depending on whether the CL is in circuit and my line is between 115-120. So what's going on. In the original Eico drwg its straight in. In the modified drwg its a 47 ohm resistor. What do I have 47K DOHHHHHHHHHHHHHHH. Ok I have them, pop them in and viola maybe 375 or so. Flip it over run the bias and balance
.38 vdc no problem. Thank you all ......Dave you description was a road map to my problem.... Now that I got it sorted out I know the EFB module will change everything but YES! ..... Playing beautiful music right now.....Thank you
Is that CL really protecting anything? Would I be better off with the slightly higher voltages?
 
if it's the original circuit,look at the tronola website for the loudness mod,Eico did it differently than all of the other manufacturers,I think the "problem" goes back to the HF 85,and Hf 84

Actually the loudness mod is not on the Tronola website,but there is a HF-85 mod in the last link in my next post.
 
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Hey Scott,
I ran a couple of searches on that article. I came up on the periphery of it. Do you by any chance have a link?
Just an observation, amp sounds really good. In the very brief sampling its not missing anything. One note, it has both a low level hiss and ac hum (you have to listen for it with no source. It's not related to volume level and can't' be heard even at very low listening levels. Not complaining, I just like a silent bottom.
 
I'm starting to feel cursed. I thought I was in the back stretch. A little tweaking a little house cleaning. I rechecked bias. One channel sort of runs (the bias) (oscillates) It wont hold steady to set. I tried swapping tubes in channels. The it oscillated in both. I reflowed the points thinking maybe a cold solder joint. Calling it a night. I'm also curious why the DC voltages in the supply are different at different times of the day. Sometimes they're spot on and sometimes almost 5% low. I did try some tube rolling. 12AX7 and 6SN7. could the tube rolling cause the oscillation. I'll play again tomorrow. I'm shot.
 
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Scott, I incorporated the line and phono portions of the Tronloa article. I built amp section like the schematic I posted which was Jonathon's build , On Dave's recommendation. The Loudness switch I gad planned to address, I have to look at schematic and read article and then take notes and look at physical layout to see where things are and what has to move where. Thanks for the link. I would really like to resolve the bias oscillation issue first. I can't understand why it was fine for several hours and then it kicked in. Maybe its somewhere else in the amp and that's just where its showing.
 
Can't hear it, You can watch the needle swing on an analogue meter. On the digital meter the numbers roll (they jump higher than where bias was set) I would guess almost .2v on the digital (but I think its confused) i'd guess looking at needle on analogue maybe .02v or so. The rest of supply voltages are solid, no source. I tried swapping output tubes channel to channel. Problem didn't move with tubes. I also tried rotating pairs of tubes out but it didn't disappear.
 
nj pheonix said:
Scott, I incorporated the line and phono portions of the Tronloa article. I built amp section like the schematic I posted which was Jonathon's build , On Dave's recommendation. The Loudness switch I gad planned to address, I have to look at schematic and read article and then take notes and look at physical layout to see where things are and what has to move where. Thanks for the link. I would really like to resolve the bias oscillation issue first. I can't understand why it was fine for several hours and then it kicked in. Maybe its somewhere else in the amp and that's just where its showing.
Click to expand...

Which post is that schematic on?
 
amp drawing is this one (not including 6cg7, I kept tone controls so from that portion back plain eico drawing). Oh I didn't use CCS either (Jon and Dave didn't think it improved the sound). Just cathode resistor
 

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