prulup 11/14/2017 3:09 AM
Laney TF700 tubefusion - DC on the output

I have a problem with LANEY TF700 (Tube fusion), schematics here: amplifiers head combo drawings/LANEY/TF700.pdf |

There is a positive DC on the output when the load is connected (tested with Variac, low AC voltage, big current), Without load it seems to be ok, no DC on the output. I tested the PWR resistors, changed output pair, drivers, MPSA42, still the same problem.

Here are the voltages (no load):

TS8 MJ15003 B: 0,6V, C: 44,2V, E: 0.02V
TS6 TIP31 B: 1,2V, C: 44,2V, E: 0,6V
TS9 MJ15003 B-43,3V, C: 0,02V, E: -44V
TS7 TIP32 B -0,5V, C: -43,4V, E: 0,02V

TS5 C:-0,6V, B: 0,02V, E: 0V PNP
TS4 C: 1V, B: 0,02V, E: 0V

TS3 C: -0,5V, B: -41V, E: -41,5V
TS2 C: -41,9V B: 0V, E: 0,6V PNP
TS1 C: -41,3V, B: 0V, E: 0,6V PNP
TR1 C: 1,1V, B: 0,1V, E: -0,5V

I am overlooking something... I would be very glad for any help here, thank you!
darkfenriz 11/14/2017 4:51 AM
20mV? It's nothing.
prulup 11/14/2017 6:31 AM
these are voltages without the load. If I connect the speaker the DC on the output rises with the power supply on Variac (I have cca 1.5V DC on the output with 20V AC from the power supply).
darkfenriz 11/14/2017 7:30 AM
Normal speaker load?
The problem might (taking no responisibility) go away when the variac is set to higher voltage.

Many of such amp designs (especially with no input stage current source and no current mirror, just plain resistors) are not supposed to work properly at a fraction of their normal supply voltages.
J M Fahey 11/14/2017 7:39 AM
Donīt worry.
MANY amplifiers "wake up stupid" (hey, me too) and if current/voltage limited in some way (variac set very low or lamp bulb limiter) and they also have to push significant current through a load, they simply canīt.
Now you turn them on straight plugged into mains while load connected and there is no trouble.
To be more precise, there is one , a very minor one: that initial current surge is heard as a turn on "thump" on the speaker.
If you look at the cone, youīll see that it significantly moves forward or backward and stays there for a fraction of a second end then returns to normal.
If current/voltage limited in many amps it stays there and rails never reach proper voltages.

Test your amp both ways: turn on with load and plugged into mains (it should thump and then work properly) and limited, without speaker, anc connect it only after it has full voltage, it should again work properly, this time without thump.

EDIT: darkfenriz beat me to say the same
prulup 11/15/2017 6:26 AM
you were right, I already repaired it before. Now tested with the bulb and the load, everything is OK, no DC on the output. Man always learn something new:-) How would you bias the amp, how many mV across the 0.22R resistors? Thank you for the help!!
darkfenriz 11/17/2017 3:32 AM
Make sure bias generating transistor (TR1) has some good contact with heatsink and anything in the 3-20mV range drop on 0.22R resistors should be good.
prulup 11/17/2017 5:37 AM
Yes, I set 10mV yet.