Supro 1624 amp kit

[Ampzone]

I have a Supro amp 1624 kit reading some lower voltages than spec. V1 pin 1 is seeing around 100v not 200v as seen on the schematic.

V1
pins:
1 100v
3 1v
6 110


v2 is more in line
1 145v
3 1.8v
6 138v

So before the dropping resistors upstream its about 325v at the 1k 5w intersection. 280v after the 15k resistor drop. 225v after the 100k drop, pin 1 V1 - 100v after the 270k drop.

I realize the tolerances of the power transformers change, as well as the rectifier brands, wall voltage (120v here) preamps gain, all affect the overall voltage.

Any ideas why so low?

layout and schemo
http://tubeamplifierparts.com/layout..._watts_kit.pdf

http://tubeamplifierparts.com/schema..._schematic.pdf

[big_teee]

The 200v, looks suspicious.
How can V1a pin 1 read higher than V1b pin 6?
When V1a is biased hotter than V1b?
You say 270k, on V1 it is actually 270k + 100k =370k.
The extra 100k on 1a, & 1b, is why V1 Reads lower than V2.
T

[Ampzone]

The 200v, looks suspicious.
How can V1a pin 1 read higher than V1b pin 6?
When V1a is biased hotter than V1b?
You say 270k, on V1 it is actually 270k + 100k =370k.
The extra 100k on 1a, & 1b, is why V1 Reads lower than V2.
T

You think the 200v is a misprint? Based on the schematic what would you expect to see there? 'm I around 90-98 volts depending on the wall voltage on v1 pin one. There is a 370k resistance after the power supply feed.

[Enzo]

What does the amp do WRONG? What are we trying to FIX? If it works and sounds OK, then forget the voltages.

Yes, schematics have mistakes all the time, not to mention amp circuit change and the schematics do no always represent those changes.

[g1]

The V1 voltages must be wrong. You can't have more current through the cathode (V1a vs V1b), yet have a higher voltage at the plate of V1a (when the two halves have equal plate resistances).

[Ampzone]

The V1 voltages must be wrong. You can't have more current through the cathode (V1a vs V1b), yet have a higher voltage at the plate of V1a (when the two halves have equal plate resistances).

Thank you guys That's what had me concerned with the schemo. The amp sounds fine but I wanted to verify as this schemo and layout are posted all over the place.

[Ampzone]

One last thing based on this schemo would you expect channel two to have a bit more gain and be a bit louder? This amp is louder and more gainy onm ch2.

[Ampzone]

See attached. Searching around I found an original 1950's Supro Schematic, same exact circuit and again it shows 200v at V1??? I guess another typo but that's interesting.

[52 Bill]

Searching around I found an original 1950's Supro Schematic, same exact circuit and again it shows 200v at V1??? I guess another typo but that's interesting.

I think that the new version was just copied from this one. So they copied the 200 volts from the original, only changing the values of some of the components to modern values.

The chances of two amps having the exact same cathode voltages are pretty slim.

[J M Fahey]

Not a typo.
I bet you are not using V3 , the tremolo oscillator.
Or it has some problem.
V1a is biased more negative than V1b, so it must pass less current, so voltage drop across R7 must be smaller, so plate (pin 1) voltage must be higher ... and modulated by tremolo signal.

[g1]

V1a is biased more negative than V1b, so it must pass less current, so voltage drop across R7 must be smaller, so plate (pin 1) voltage must be higher

According to the cathode voltages shown (and cathode resistance values), V1a is passing 1.6mA, V1b is .5mA, so shouldn't voltage drop across R7 be larger?

Edit: I missed that V1a cathode is connected to V3a cathode, so the 1.6mA is for both triodes.

[J M Fahey]

V1b is passing 0.5 mA
V1a is passing 0.243mA , because it drops 90V across 370k .
V1a is passing less current than V1b because itīs biased higher (colder).

The cards are all on the table, face up

JPB already got it

[g1]

V1a is passing 0.243mA , because it drops 90V across 370k .

edit: Doh!

[g1]

Never mind, I see the error of my thinking.
V1a cathode resistor is shared with cathode of V3a. So there is extra current through the cathode resistor that is not present at the plate.
Thanks and apologies to Mr. Fahey.

[Ampzone]

Not a typo.
I bet you are not using V3 , the tremolo oscillator.
Or it has some problem.
V1a is biased more negative than V1b, so it must pass less current, so voltage drop across R7 must be smaller, so plate (pin 1) voltage must be higher ... and modulated by tremolo signal.

Good catch! Thanks. Thats Correct the tremolo circuit is left out. Without it does the circuit still need to see 200v at V1? Would this result of 100v less on V1 pin1 occur just by removing the tremolo tube as well?