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Help with modded Dynaco ST-70?

kind of why I was curious what the supply to the board was. trying to determine if the tube is drawing a lot of current, or if the supply is just very low for some reason.

a full set of voltages for plate, grid, cathode on each tube section would be useful here. Would be even more helpful if some of that wasn't cut off the schematic but in-circuit measurements might serve as a sanity check if nothing else.
 
I'll take a closer look in the morning but was getting proper voltages with the 2 good "original" preamp tubes. But throw in the new tubes, either the E88CC or the new 6H23 the supply voltage goes way down. Cathode grid is some weird network like in the schematic from a few posts ago.

I figured maybe the E88CC were different enough from the 6H23 to disagree with a weird circuit but that didn't do it either and now I have almost $300 into tubes that don't work in this thing.

And both channels behave the same.
Will get more voltage readings tomorrow.
 
So I seem to be getting a lot of voltage drag down with all the tubes I have. Sound seems better with the originals though.
Here's a full set of voltages.
V1 (input) V2
P1 44V 32V
G1 1.6 2
C1 2 2
P2 2.1 2
G2 -1.3 -1.9
C2 0 0

Heater voltages all 3VAC.

I have 190V coming into the board. So each 15k resistor (2 per tube) is dropping 76V.
 
you'd have to measure across the entire heater supply, it won't give meaningful measurements to ground.

so 190v coming into the board, schematic calls for 400-420 so thats not right. How much voltage is dropped across the resistor between the output trafo center tap and the board, and what value is that resistor? Also whats the output transformer to ground voltage?

1781647169923-png.3784134



Raccoon1400 said:
I have 190V coming into the board. So each 15k resistor (2 per tube) is dropping 76V.
Click to expand...
I'm guessing you mean thats a pair of 15K in series to make a total of 30K? so 152v / 30k = 5mA per triode? Schematic calls for 5.8mA so that value is actually not so far off if so. If the 30K plate resistors have been swapped for 15K, then 76v / 15k = 10mA per triode which is too much.

Shouldn't have positive voltage at the grids of V1 either. it should be zero. V2 probably does have positive voltage.
 
gadget73 said:
you'd have to measure across the entire heater supply, it won't give meaningful measurements to ground.

so 190v coming into the board, schematic calls for 400-420 so thats not right. How much voltage is dropped across the resistor between the output trafo center tap and the board, and what value is that resistor? Also whats the output transformer to ground voltage?

1781647169923-png.3784134




I'm guessing you mean thats a pair of 15K in series to make a total of 30K? so 152v / 30k = 5mA per triode? Schematic calls for 5.8mA so that value is actually not so far off if so. If the 30K plate resistors have been swapped for 15K, then 76v / 15k = 10mA per triode which is too much.

Shouldn't have positive voltage at the grids of V1 either. it should be zero. V2 probably does have positive voltage.
Click to expand...
Getting a negative voltage on V1 gate now, ~-1-2V. Not sure why I got it wrong last night. Voltage is 0 with no tube.
And the plate voltage on V1 is about double with nothing connected to the input. I had my ipod connected for the above measurements.

I'm getting almost 400V to the input board with no tubes installed. The supply to the board is a bit hard to follow. There's a 27k resistor dropping about 260V with 2/4 preamp tubes installed.

And yes, each tube currently has 30k of plate resistance.
 
is the grid resistor to ground open? It shouldn't be changing plate voltage unless the source has DC on it. That will screw things up.

so about 10mA (260V / 27K) with half the tubes installed, thats about 5mA per plate which jives reasonably well with the schematic values. Problem is with 4 tubes installed, its going to pull more current and drop a lot more voltage.

With no tubes installed there should be no drop, so the almost 400v is probably what the output transformers are getting. Something about that seems low, though the schematic does seem to indicate a tube rectifier and the stock CLC filter arrangement upstream of a regulator. Feed into that regulator is probably in the 430v range, which would be stock for an ST70, and the regulator has to drop something so maybe its supplied with ~400

no clue how they could possibly expect 400v into the board if its only getting 400v on the other side of that resistor though. By math it seems like it ought to be closer to 350v. Assuming 175v at the plate of V2, 5.8mA across 30k is another 175v so 350v.

dunno, best thought I have is parallel a second 27K across the existing one, pop a full set of tubes in there and see what you get for voltages.

also confirm the ipod doesn't have any DC on the output or its going to cause issues.
 
Grid to ground resistor is fine.

I should also add the plate voltages start high but then go down as the amp stabilizes after powerup.
 
I guess I'll try less plate resistance. I'd dropped it to 10k in the first post but that only brought the plate voltage up to 60V with the E88CC. What voltage should I be aiming for? Around 200V?
 
Leave the plate resistor alone, drop the resistor feeding into the cap that supplies the driver board. Looks like the board should get about 350v based on schematic current and voltages listed
 
I put a 10k resistor across the 27k resistor, which gets me about ~80V to the plates of the tubes with the 4 E88CC installed.
 
What's B+ voltage on the supply side of the plate resistors on the E88CC tubes, now, when it's running/warmed up? I would like to see what the voltage drop is on those resistors, so we can see what the current is through the tubes.

Regards,
Gordon.
 
GordonW said:
What's B+ voltage on the supply side of the plate resistors on the E88CC tubes, now, when it's running/warmed up? I would like to see what the voltage drop is on those resistors, so we can see what the current is through the tubes.

Regards,
Gordon.
Click to expand...
Each 30k plate resistance is dropping ~200V. (leading to about 65V on the plate for that test)
Not leaving it on for long as the 2W resistor I tacked across the 27k one is not very happy with me.
 
Should I lower this pre-board resistance more, or lower the plate resistors too? Seems a pretty big reduction in resistance is needed.
 
Do I understand correctly that it ran properly on the 6H23 tubes for quite some time, and the voltages run at expected/normal values if you put those tubes in the amp?
 
6.66 mA per-plate if its dropping 200v across 30K, it calls for 5.8mA so you should be dropping about 175. I wouldn't change the plate resistors on this to fix that, I'd figure out whatever is causing the tubes to pull too much current.

Can you get the plate, grid, and cathode voltages for each tube section on the driver board?
 
whoaru99 said:
Do I understand correctly that it ran properly on the 6H23 tubes for quite some time, and the voltages run at expected/normal values if you put those tubes in the amp?
Click to expand...
I think it was always way too low but the new tubes dragged it down just a bit more to where it was very audibly not sounding right.

I can get new voltage readings when I get this thing back on the bench. (I posted readings a few posts up with original resistor values and 2 6H23 tubes)
 
At least it's a lot closer to the ballpark now. We now need to know what the grid and cathode connections are, and what voltages are there. May just need a bit of a cathode resistor tweak, or something in the feedback loop may be a bit askew, and may be overly loading the cathode.

Regards,
Gordon.
 
i'm sorta wondering if its an issue on the negative voltage supply side of things. Unfortunately we don't have a full set of voltages, but just based on the zener here I'd expect -8v at this point. If the zener is shorted it will skew things and it might be enough to shift the bias on the tubes enough to draw a little extra current

1784570660683.png

I'd expect about +19v at the cathodes, also based purely on a presumed -8v across the left-most zener, plus the 27v zener diode.
 
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