are these voltages correct ?

[FatGaz]

the first stage in my preamp is a 12ax7 where I have connected the two triodes in parallel.

I have a 220K common plate resistor, a common 1.5k / 22uF cathode, a .22u coupling cap and a 68K grid input.

the b+ is 260V, but the voltage on the plate is only 80V ? this seems too low ? I was expecting around 150-ish V.
the bias/cathode voltage is about 0.9 volts.

I have seen these component values on a couple of schematics (a m'tchless combo, maybe a t0p-hat one too I think).

I swapped the tube but the voltages are the same.

Do i need to change the plate and/or cathode resistors ? or is there something else causing this ?

Ta,
Gareth.

[big_teee]

the first stage in my preamp is a 12ax7 where I have connected the two triodes in parallel.

I have a 220K common plate resistor, a common 1.5k / 22uF cathode, a .22u coupling cap and a 68K grid input.

the b+ is 260V, but the voltage on the plate is only 80V ? this seems too low ? I was expecting around 150-ish V.
the bias/cathode voltage is about 0.9 volts.

I have seen these component values on a couple of schematics (a m'tchless combo, maybe a t0p-hat one too I think).

I swapped the tube but the voltages are the same.

Do i need to change the plate and/or cathode resistors ? or is there something else causing this ?

Ta,
Gareth.

Sounds low to me.
Can you not drop the b+ Plate res down to 100k, real common on a lot of amps?
Also got any schematics, or layouts of the build.
What is the B+ Voltage before you start dropping along the B+ Rails?
We need more Info?
T

[soundguruman]

The voltage on the plate is correct. The voltage is being dropped across the 220K resistor.
If you want a higher plate voltage use 100K resistors.
A 220 K plate resistor produces a lot of gain, and usually too much, the stage will ring and oscillate. If you want all that gain from one stage, other design considerations must be employed.
Usually the largest value you will see is 100K. There is a good reason for that. Too much gain = too much noise.
.22 uF coupling cap is avoided, because it will usually cause low frequency oscillations, "motor boating." .047, .02, .002 is the preferred values. You will notice in other amp designs, that larger values are avoided. There is a good reason for that too.
In Hi Fi amps you will notice larger values, to obtain subsonic bass frequencies. The stages MUST be very carefully designed to avoid the low frequency oscillations. The bigger the coupling caps, the more difficult the design becomes.
What guitar amp uses .22 for a coupling cap? There are not any. Reason? Now you know better.

[loudthud]

See post #9 in this thread: http://music-electronics-forum.com/t28209/

Numbers are for 300V B+ but should be close enough for the 250V you have now.

[FatGaz]

thanks for the advice

See post #9 in this thread: http://music-electronics-forum.com/t28209/

Hi, thanks for that - real data about biasing parallel triodes !
(sorry about the double posting - I though I'd put too many questions in my other thread for things to be clear.)

What guitar amp uses .22 for a coupling cap? .

Oops, sorry,typo! it's 0.022u.

If you want a higher plate voltage use 100K resistors.

I think I'll consider this with the 1.5k bias/cathode resistor as the above data suggests and see how it sounds.

Thanks !

Gareth.