Forte Model 1a Class A amp runs ice cold

[rsquare]

An earlier post by someone with the same Forte amp mentioned one side being cold and a response suggested the problem might be the bias. Well, my unit runs cold on all sides, but it didn't used to. Brought it to a local audio store and they checked it out, said it sounded good to them, and had no idea why it would run cold.

Any thoughts much appreciated, and thanks in advance.

 

[Nelson Pass]

Since the channels are independent of each other, the likelihood
of identical failures is pretty small. You will want to measure the
bias across the emitter resistors.

😎

 

[rsquare]

Thanks, Nelson, appreciate the reply. I don't work on my own equipment so am not sure what to do here. Should I just bring it to another audio shop and mention what you said? Thanks again.

 

[Nelson Pass]

Find somebody confident with a DC voltmeter, and see what the
voltages and resistances are on the Emitter resistors.

😎

 

[codname]

Hello , i have the same problem mentionet earlier , my forte audio 1A is cold after runing for 1-2 hours , i have measured the bias in this way : the minus of the voltmeter i put it to the base of the transistor and the plus on the resistence conected to the emitor of the transistor . my readings where initialy 460 mV on left and 126 mV on the right then i have ajusted them to 500 mV aprox.

it is correct how i measured the current of the transistors ? if yes why it is still cold at 500 mV.

thank you

 

[HVfanatic]

You have to measure the voltage drop across the emitter resistors. You can then calculate the current. Afterwards you compare this value with the value given in the service manual.

Note: I guess, the service manual gives the current summed over all devices...

Good luck.

 

[codname]

thank you ,

I have only a scheme of the amplifier uploaded in another thread , but the scheme has no voltage markings , after looking more careful i think that i have measured correctly - (minus) of the voltmeter at the base (b) and + (plus) of the voltmeter after emitter resistor of each transistor .

actual current at the transistors is 500 mV , but the amplifier is cold, no shadow of warm

one more mention is that the right channel at initial current (126 mV) the sounds where distorsioned , but when i rised the current (bias) the distorsions disapeared almost 100% but not totally .. it is normal ?

 

[HVfanatic]

You have to measure across the resistor. Not the base pin. R=U/I, I=U/R. Who knows the correct idle current???

 

[HVfanatic]

Attention: Current is in amps, not in mV. Of course you'll hear bloody distortion: 126mV means the silicon is switched off. Don't turn the pot, if you do not understand what you're doing. Otherwise you'll blow your amp..

You measure a voltage, because you know the resistor value, you calculate the current through it...

Please ask BEFORE killing Nelsons baby...

Dirk

 

[jacco vermeulen]

idle current???

40W in Class A for the Model 1 usually corresponds to 1.6A idle current, divided by 5 output devices per rail is 320 mA.
50W in Class A for the 1A is 1.8A, divided 5 makes 360 mA.

 

[codname]

thank you all for the answers ,

well, applying the formula given by HVfanatic and substituting the variables it give's me an voltage of 169 mV across the resistor ,

i have also seen that on other threads peoples said that the factory setting is 120mV across the resistor ?

i have done the calculation wrong?

 

[HVfanatic]

No. It simply means, you're not full class A... ;-)

So change bias to 12omV across the resistor, and you're fine.

BUT: Be very slow turning the pot!!!

Regs, Dirk

 

[HVfanatic]

And: Check the value after warmup!!!

 

[codname]

thank you Dirk , i have raised the bias to 120-130 mV across one resistor , i have measured 3 resistors on each side and they where between 120 -130 mV

now the amplifier is getting hot , i am not used to amplifiers getting hot so rapidly in 10-20 minutes , i hope that i have not made a follishness

 

[Nelson Pass]

As long as you can still put your hand on the sink, it should
be OK.

😎

 

[jacco vermeulen]

i am not used to amplifiers getting hot so rapidly in 10-20 minutes

Just think how hot the heatsinks would be if you set it to the Full Class A numbers i posted.
The heatsinks in the Forté power amp models barely reach a thermal resistance number of 0.35C/W (25x5.5x15 cm, LxWxH, 20 ribs)
0.35C/W allows a max dissipation of some 90 Watts/ch, if you are not in the mood to fry eggs or other pets.

The 5-pair 2SA1302/2SC3281 output stage on the heatsink of the 1a does have enough dissipation headroom to raise the bias level to Full Class A operation in 8 Ohm, without an unhealthy life expectancy of the output devices.
A 65C heatsink temperature is more than most folks enjoy, though.
Goes to show what level of hardware is required for a 50W Full Class A amp, heatsinks are sooner undersized than too large.
A True Class A level of 50W on ~36Vdc rails corresponds to +125W dissipation, for which you'd need heatsink sizes as the ones in the Threshold SA/3 model, twice as big.

 

[codname]

Thank you Jacco Vermeulen for clearing for me some aspects about the heat and pure class A operation , also thank you Nelson Pass .

After setting the bias on different values to see the temperature , i will leave the bias at 110 -120 mV because in summer temperatures reach 35- 40 degrees celsius in my country

thanks all for the help

 

[gosvit]

For this model bias on resistors must be 25mV, maybe 30, but no more!!!

 

[Badbrad]

Still seeking an actual schematic for a Forte Model 3. Does anyone have a link. I can see a few others, but not the model 3. Thanks in advance!

 

[Nelson Pass]

As I recall, the 3 is almost the same circuit, just different power supply and
bias figures.

🙂