Amp gurus , PT questions and others...

[RectifiedAmps]

Are these voltages with all tubes installed?

The ratings for tubes are usually plate to cathode, so the voltage on the cathode of the PI will make a difference. Your B+ node voltages can be higher so long as the voltage from plate to cathode in each tube are below their max rating.

Also, are the output tubes biased up properly? If biased very cold, they’ll draw little current and the B+ voltages will sit up to 50v higher as a result.

[StrummersOfThunder]

Are these voltages with all tubes installed?

The ratings for tubes are usually plate to cathode, so the voltage on the cathode of the PI will make a difference. Your B+ node voltages can be higher so long as the voltage from plate to cathode in each tube are below their max rating.

Also, are the output tubes biased up properly? If biased very cold, they’ll draw little current and the B+ voltages will sit up to 50v higher as a result.

I get -37 v is that bias you’d call cold ?

[sizzlingbadger]

Are these voltages with all tubes installed?

The ratings for tubes are usually plate to cathode, so the voltage on the cathode of the PI will make a difference. Your B+ node voltages can be higher so long as the voltage from plate to cathode in each tube are below their max rating.

Also, are the output tubes biased up properly? If biased very cold, they’ll draw little current and the B+ voltages will sit up to 50v higher as a result.

I get -37 v is that bias you’d call cold ?

That voltage means nothing without knowing the other factors in the design.

[MikeC]

A bias "voltage" isn't what's measured as hot or cold. It's plate dissipation in watts. I.E. power, too much and the plate glows red or worse!

[StrummersOfThunder]

Learning so much!
Just been reading the above
Thanks team amp guru

[RectifiedAmps]

Are these voltages with all tubes installed?

The ratings for tubes are usually plate to cathode, so the voltage on the cathode of the PI will make a difference. Your B+ node voltages can be higher so long as the voltage from plate to cathode in each tube are below their max rating.

Also, are the output tubes biased up properly? If biased very cold, they’ll draw little current and the B+ voltages will sit up to 50v higher as a result.

I get -37 v is that bias you’d call cold ?

Isn’t this configured as a cathode-biased amp? What’s the voltage at the cathodes of the power tubes? Divide that by the cathode resistor value, divide by two (assuming resistor is shared between both tubes), then multiply by the anode to cathode voltage of either power tube = watts dissipated.

[Jay]

If you find RA's wording above hard to follow, then you can use Rob Robinettes XCEL spreadsheet to calculate dissipated wattage. Fill in the measured voltages, etc and out rolls the answer.

[StrummersOfThunder]

Are these voltages with all tubes installed?

The ratings for tubes are usually plate to cathode, so the voltage on the cathode of the PI will make a difference. Your B+ node voltages can be higher so long as the voltage from plate to cathode in each tube are below their max rating.

Also, are the output tubes biased up properly? If biased very cold, they’ll draw little current and the B+ voltages will sit up to 50v higher as a result.

I get -37 v is that bias you’d call cold ?

Isn’t this configured as a cathode-biased amp? What’s the voltage at the cathodes of the power tubes? Divide that by the cathode resistor value, divide by two (assuming resistor is shared between both tubes), then multiply by the anode to cathode voltage of either power tube = watts dissipated.

Yep was just reading this exact thing of Rob Robinette stuff

[StrummersOfThunder]

Sorry this must be exceedingly painful for you all
Thank you

[jeremyb]

Sorry this must be exceedingly painful for you all
Thank you

Ssssshhhhh this is great learning for the rest of us too!!

[StrummersOfThunder]

Back to this.

I think the missing pre-amp tube is v2
In the original schematic it was only used half the tube to add a gain stage to the bass channel
So I’m guessing this amp just has two gain stages for each channel

Am I right in saying that ?

[MikeC]

All other things being equal then yes.

[Jay]

Back to this.

I think the missing pre-amp tube is v2
In the original schematic it was only used half the tube to add a gain stage to the bass channel
So I’m guessing this amp just has two gain stages for each channel

Am I right in saying that ?

It appears that the second valve in the bass channel is not included in your amplifier. Apart from that valve adding a bit more gain, it also provides some tone shaping. If you are not using that channel, just build a copy without the bass channel at all. If you intend to use it as a bass amp, I would add the second valve back in.

[robthemac]

Back to this.

I think the missing pre-amp tube is v2
In the original schematic it was only used half the tube to add a gain stage to the bass channel
So I’m guessing this amp just has two gain stages for each channel

Am I right in saying that ?

It appears that the second valve in the bass channel is not included in your amplifier. Apart from that valve adding a bit more gain, it also provides some tone shaping. If you are not using that channel, just build a copy without the bass channel at all. If you intend to use it as a bass amp, I would add the second valve back in.

I'm not entirely sure Ants is aware of the existence of frequencies below 200Hz.

[StrummersOfThunder]

Back to this.

I think the missing pre-amp tube is v2
In the original schematic it was only used half the tube to add a gain stage to the bass channel
So I’m guessing this amp just has two gain stages for each channel

Am I right in saying that ?

It appears that the second valve in the bass channel is not included in your amplifier. Apart from that valve adding a bit more gain, it also provides some tone shaping. If you are not using that channel, just build a copy without the bass channel at all. If you intend to use it as a bass amp, I would add the second valve back in.

Yep
I’ve been looking over schematics. I’m wondering if I can leave it out but modify the tone stack on the bass channel to be more on keeping with the 6g6b