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Another Eico ST 70 thread

ok I have a design adjustment calculation question. Because i'm retaining tone controls. I have a supply voltage issue. Jon was using a 6CG7 his (W) supply is 294v in my circuit (well in the factory its a 7247) the tronola mod makes it a 12 AX7, but doesn't change that voltage so I think i'm looking for the factory voltage of (w) 165v. So what i'm thinking the 24k 1w going to the (w) supply should be about 200K . I'm basing this mostly on the network creating the Z supply which makes 158V which i'm guessing in a bind probably would work. Can anybody tell me whether my assumptions are correct or should I be going about this another way?
I'm pretty sure I have the schematics posted in earlier post of this thread...Thanks
 
while i'm in the supply. what i'm thinking is I should duplicate the filament setup . keep the hum control in both 6.3 v with the 02 caps and do the diodes again with the filtering caps and the 3.3 resistor . pickup 3 ax7 on one side and 2 on the other. am I overkilling for nothing?
 
NJ -- I must admit to being completely confused as to your questions. I think I need to see a base schematic of what you're working with, as many of the components you reference don't exist in the original unit. I did a quick look back to see if you posted one and didn't see any, but maybe I missed it.

Dave
 
Alright I guess i'm missing apples and oranges. My goal in this project is to incorporate the improvements in the amp section from Jon's design. The power supply from Jon's design as it applies to what matches. And the improvement of the phono section from the Tronola article. The amp section is straight forward and I have it down. The Tronola mod while I haven't done that portion yet I think is pretty clear. While working on power supply I came up with one issue and one question. Because Jon has deleted the tone section his design utilizes a 6CG7. Because i'd like to retain the tone controls, That is where my problem lies. In Jon's circuit the (w) plate source to the 6CG7 is 294V. The stock Eico circuit is a 7247 and that (W) plate voltage is 165V. First i'm assuming because i'm keeping the tone section (the tronola article changes that tube to a 12AX7) That I need to make (W) 165v. Because I was following Jon's power supply drawing, my (w) was derived from (T) which was 428v dropped through a 2.2k 5w in series with a 24k 1w. Here is my assumption and my reasoning. To make (w) 165v, the 24k 1w should be changed to a 200k resistor. I came to this conclusion looking at Jon's supply the (z) 158v source starts at the same (T) 428v and goes through 4 series resistors totaling 213k. While i'm guessing I think I should be in the ballpark and I can always tweak it.
My next question, which I may just be overthinking, I have 5 12AX7s (after the Tronola mod), so if I created DC filament voltages for the first 3 why not the other 2? (basically copying the brown filament circuit to the green. (even within that the green has the hum control and the brown doesn't. At this point i'm just thinking i'm overcomplicating and just pretty much do it as shown. I'm going to try to include 5 drwgs. The HIFi engine PS drwing which easily corresponds with letter designations to Jon's. The Sams which shows the voltages clearly at those points for comparison. Jon's PS to see what i'm talking about with component values. Then Jon's amp circuit and the eico amp circuit showing where (W) source end up. (If you've followed this all through , I'm shocked) Thanks for the patience
 

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NJ -- Since you are retaining phono preamps and (basically) the original line amp (although modified for the Tronola mod), then you would want to use the original B+ dropping network to power those stages from the original Eico design. You would simply eliminate the W supply point on Jon's schematic (24K and associated decoupling cap), and use the W point from the original B+ dropping network.

When you convert filaments that were originally running on AC power to running on DC power that otherwise use the same heater winding as before, the current draw from heater winding will be increased by a factor of about 1.75 to account for the current drawn by the filter caps when they recharge themselves on each peak of the AC sine wave. Since this causes an overall greater current draw from the heater winding, you will want to reserve this feature for only the tubes that absolutely must have DC power on their filaments to minimize hum in the output. Generally, powering the phono preamps and line amp stages is all that is required to accomplish this goal. Powering any more tubes in this fashion only serves to potentially overload the heater winding even further, and cause the transformer to run at a higher temp than normal.

Dave
 
I was planning on working on filament circuit later today. I was going to bring up with a DBT. My question, because I have mostly new power supply. In order to check voltages (DC) I was going to tack in a diode and leave tube out initially. For sake of protecting tubes will I get relatively accurate filament voltage and DC supply voltages without tubes installed or will I need the loads ? also Dave had said there was a thread for setting ac balance with a vtvm.
 
nj pheonix said:
I was planning on working on filament circuit later today. I was going to bring up with a DBT. My question, because I have mostly new power supply. In order to check voltages (DC) I was going to tack in a diode and leave tube out initially. For sake of protecting tubes will I get relatively accurate filament voltage and DC supply voltages without tubes installed or will I need the loads ? also Dave had said there was a thread for setting ac balance with a vtvm.
Click to expand...

Bringing the voltage up is not a problem with our consumer grade amplifiers. What you may be referring to is cathode stripping which is a problem with high power RF amplifier tubes. Keep in mind that Fisher, Scott and Sherwood had silicon rectifiers in their amplifiers nearing the end of the vacuum tube amplifier era. It wasn't a problem then and isn't a problem today...
 
Ok upon auditing again, which is somewhat difficult because i'm mixing apples, bananas and pineapples ( I think this has cocktail potential ) I found I missed something (Actually if its right the first time through i'd be a little surprised, my design as I understand it might need tweaking), Anyway a little more I can do and then wait for the UPS man to bring me my Mouser delivery....Stay Tuned
 
nj pheonix said:
Ok upon auditing again, which is somewhat difficult because i'm mixing apples, bananas and pineapples ( I think this has cocktail potential ) I found I missed something (Actually if its right the first time through i'd be a little surprised, my design as I understand it might need tweaking), Anyway a little more I can do and then wait for the UPS man to bring me my Mouser delivery....Stay Tuned
Click to expand...

Add a little rum to the above mixture... Oh darn, I digress.

Will be awaiting the results. Thanks for the info!
 
Hey Kv, I don't think I was clear on my test question. What I wanted to know, because filament wiring was altered and all the high voltage dc wiring was altered, can I measure these accurately (or reasonably so) without the tubes in loading down the circuits, or will they be way off without the load? At least i'd be able to look into major sins without risking damage to tubes?
 
Good morning everyone,
Does anyone know if reproduction ST 70 chassis exist? I've scoured the net and have come up empty handed.

Thanks,
Ray
 
I can't say I've ever seen one. If you going to migrate everything any way, i'm pretty sure you can get something off the shelf. There is a running thread of a custom Bob Latino VTA ST 120 thread, where the building fabricated his own chassis or someone made him something to work with. You could PM him. While I like the insides, its a bit of an ugly duckling this might be a good opportunity to spruce it up a bit. Sorry I couldn't be more helpful. http://www.audiokarma.org/forums/showthread.php?t=524347
 
Ok Somewhat encouraged and somewhat concerned. I was able to cobble together 2 resistors to at least let me test. So theoretically PS is complete. That being said. Powered on DBT no smoke or flames so. Powered straight up no tubes, diode tacked across rectifier. My digital meter crapped out and new one is on back order until end of month so making readings from analogue meter. Bias voltage looks good. High voltage, starts nice (S) 425vdc (T) 390vdc (U) 380vdc (V) 370vdc (this one bothers me its awfully high) (W) I haven't checked because I know I have to change, (Y) 350 vdc (this is also awfully high) (Z) 300 vdc (If (Y) was high this one had to be.) still a problem but obvious issue first is (Y). Filament voltages look good. The DC on the 12v on my meter looks about 14. I'm guessing filaments will drag that down when I install tubes. Ok question time Are my high reading due to lack of tubes in circuit to load down or should I be concerned?
 

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Dave (dcgillespie) have I not had the whole supply built I would've went back to the factory resistor network. I wasn't disregarding your suggestion. So any ideas on how to address this?
 
Is that last schematic you posted of your PS as it stands? Could you direct me to this phono mod? 165 v is way more than I ever run on phono tubes.:scratch2:
 
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