PRECISION ELECTRONICS S 100 question about power

[Colas LeGrippa]

Hola amigos !

Last week a musician brought me a PRECISION ELECTRONICS amplifier which is similar to a Bogen ( who copied who ? ). I got the schem direct from the company. The power tubes are 6GT5, preamp 6EU7.
The problem was the main switch but I checked the bias and all the voltages. I find very strange that given the fact that ALL THE VOLTAGES correspond to what the schem shows, the power tubes draw only 10mA @ 450V , for only 4.5 w each, aproximately. The tube data says 17,5V Pa and 2,5V Pg2. Exactly the same even with a new ( NOS ) quad of tubes. Bias voltage is -37, which is perfect.....Is htere something I am missing here ?

Colas

[eleventeen]

There isn't much going on there in the output section, looks very ordinary. What could it be other than an output transformer where the internal connection to the center tap is bad? It lets voltage through but acts like a big resistor and will not allow any current. With power off and e-caps drained, try measuring ohms from the center tap to either side/either pair of tube plates. Should be 120, 150, 180 ohms, something like that.


Esa es mi mejor suposición, senor!

[HotBluePlates]

Before doing all that, how does the amp sound?

[Colas LeGrippa]

Muy contento que hables espanol, Eleventeen. Eres latino? Yo estoy aprendiendo con mi novia, es Cubana.
The amplifier sounds great though not as loud as we would expect from a 100W amplifier.
J'ai probablement remplacé les lampes de puissance inutilement. Yes I will measure the resistance of the OT and get back to you guys, thanks !
Grazie ! Danke ! Gracias ! Thanks ! Merci ! Tchinwingtang wong ! ("$$%()*&%&?!/"$%$  ! ( martian )


Colas LeGrippa

[HotBluePlates]

... The amplifier sounds great though not as loud as we would expect from a 100W amplifier. ...

Well, those output tubes are only rated for 17.5w dissipation (30% less than a 25w EL34), so if it sounds as loud as you might expect from a 60-70w amp, you're probably fine.

[Colas LeGrippa]

Hola HBP, you said I would probably be fine If it sounded good. But 10 mA of current isn't enough and it could sound even better !!!!!


70 %  X  17,5 W =   11.94 w


11.94W  divided by 450V =  26mA


I can understand that the amp has been biased very cold at the factory and to get the max power of the tubes,
I got to modify the bias circuit to get approx -25V, which is very high ( compare to other tubes ). But I have to keep in mind that  these tubes ( 6GT5 ) are much more different than 6L6's which could not live a long and happy life with -25V at G1 and 450V at A .

Cheers !


P.S.: spring arrived last night at 12:30h..........we're gonna get it

[HotBluePlates]

Have any of the tubes been on a tube tester? Do you know if they have proper transconductance (Gm)? Have you measured the OT resistance as Eleventeen asked?

Hola HBP, you said I would probably be fine If it sounded good. But 10 mA of current isn't enough and it could sound even better !!!!!

70 %  X  17,5 W =   11.94 w
...
I can understand that the amp has been biased very cold at the factory ...
I got to modify the bias circuit to get approx -25V ...

70% is a recommendation, related to People asking folks like Gerald Weber & Ken Fischer, "Okay, so if we bias by current, then what current should we bias to?" To which they replied, "Well, it depends."

Class A would be 100% dissipation at idle. True Class B would be 0% dissipation at idle; however, tubes are hard to cut off precisely so Class B idle winds up being some higher %.

Your amp is idling at nearly 26% dissipation, deep in class AB and closer to Class B. If it idled at 70%, it would be a hot Class AB and closer to Class A. Power output would be down.

The design bias voltage on the schematic is -34v. The factory designed the amp with a certain bias voltage to result in a specific power output with an applied signal. Right now, you're biasing hotter than that at -37v, so plate current will be reduced somewhat. The 6GT5 data sheet shows the tube will idle at 70mA with -22.5v bias and 150v on the screen; -42v is enough bias to essentially turn off plate current (down to 1mA). I figure your idle current with -34v should be something between 13-20mA.

So do check the OT, do adjust the bias (but -25v is probably going very hot unless it's adjustable bias voltage), and do check the tubes' Gm. The amp could probably be biased better, but will probably make the most output power at its designed -34v bias and less (meaning distorting earlier) at 70% dissipation.

[Colas LeGrippa]

Buenas tardes,

I measured OT resistance which seemed fine, put  NOS 6GT5, did not touch the bias. The owner of that thing tried it, liked it, paid it and took it away. The power tubes will last very long I guess .
One thing is sure: I didn't understand well this amplifier ( like my wife occasionnally  ).

Cheers !

Colas LeGrippa