Rocket with 6V6s

[MHProd]

I'm planning to build a Rocket with a pair of 6V6s. Should I change the PI Tail resistor to a lesser value? Also, should I drop the 1K2 to 470R ?

Thanks!

[bluesky636]

Not sure why you would need to modify the PI values for a change in output tubes, but you can use this calculator to determine your desired PI gain and balance.

http://ampbooks.com/home/amplifier-calc ... iled-pair/

[Smokebreak]

Doesn't 6V6 need more signal drive from PI?

[matt h]

(deleted)

[bluesky636]

Then the calculator I posted should allow you to figure these things out.

[randalp3000]

Not sure why you would need to modify the PI values for a change in output tubes, but you can use this calculator to determine your desired PI gain and balance.

http://ampbooks.com/home/amplifier-calc ... iled-pair/

Thanks, I'm gonna have some fun on that site

[bluesky636]

Not sure why you would need to modify the PI values for a change in output tubes, but you can use this calculator to determine your desired PI gain and balance.

http://ampbooks.com/home/amplifier-calc ... iled-pair/

Thanks, I'm gonna have some fun on that site

I find it very useful.

[Smokebreak]

Then the calculator I posted should allow you to figure these things out.

The calculator computes " voltage gain" from PI. Isn't that different than "signal voltage" that the PA would see? Say, 25V for 6V6?

[bluesky636]

Then the calculator I posted should allow you to figure these things out.

The calculator computes " voltage gain" from PI. Isn't that different than "signal voltage" that the PA would see? Say, 25V for 6V6?

If you know the voltage gain (per the calculator) and you know how many volts it takes to drive the power stage, you can calculate how many volts you need to feed the PI. Assume the 25 volts is what it takes to drive the power stage to maximum output. If your PI voltage gain is 10 (not in dB), than you need 2.5 volts at the PI input.

[Tillydog]

The calculator computes " voltage gain" from PI. Isn't that different than "signal voltage" that the PA would see? Say, 25V for 6V6?

As you say, the calculator won't help - What we need to know is whether the available voltage swing (headroom) from the PI is enough to drive the 6V6s.

As a rough guide, the PI needs to be able to swing at least as much as the bias voltage on the power stage.

EL84s seem to bias at about -8V acording to the Rocket voltage chart

6V6s usually bias at about -28V at typical Express voltages, but since the rocket has much lower B+, the 6V6 bias voltage may well end up a lot lower - 12-15 volts at a guess, but you could work it out from the datasheet - I think the calculation would be needed to work out the correct cathode resistor, in any case.

From an LTSpice sim, the Rocket PI will swing ~ +/- 25V before clipping, so *I* think it would probably be OK if the 6V6 bias voltage works out OK. Incidentally, I didn't see much increase in voltage swing by varying the tail resistor, etc. It would probably need higher B+ (like the Express) to get more drive (voltage swing) out of it.

Not an expert - just my 0.02p...

[bluesky636]

The calculator computes " voltage gain" from PI. Isn't that different than "signal voltage" that the PA would see? Say, 25V for 6V6?

As you say, the calculator won't help - What we need to know is whether the available voltage swing (headroom) from the PI is enough to drive the 6V6s.

As a rough guide, the PI needs to be able to swing at least as much as the bias voltage on the power stage.

EL84s seem to bias at about -8V acording to the Rocket voltage chart

6V6s usually bias at about -28V at typical Express voltages, but since the rocket has much lower B+, the 6V6 bias voltage may well end up a lot lower - 12-15 volts at a guess, but you could work it out from the datasheet - I think the calculation would be needed to work out the correct cathode resistor, in any case.

From an LTSpice sim, the Rocket PI will swing ~ +/- 25V before clipping, so *I* think it would probably be OK if the 6V6 bias voltage works out OK. Incidentally, I didn't see much increase in voltage swing by varying the tail resistor, etc. It would probably need higher B+ (like the Express) to get more drive (voltage swing) out of it.

Not an expert - just my 0.02p...

I disagree with your statement that the calculator won't help. The average output voltages of a single coil or humbucker pickup are known. You can calculate the gain of each stage through the amp to determine if the PI will provide the required output voltage to drive the power tubes to full power. You can use the calculator to change component values to get the required gain and balance at the PI output.

[Tillydog]

What we need to know is whether the available voltage swing (headroom) from the PI is enough to drive the 6V6s.

I disagree with your statement that the calculator won't help. The average output voltages of a single coil or humbucker pickup are known. You can calculate the gain of each stage through the amp to determine if the PI will provide the required output voltage to drive the power tubes to full power. You can use the calculator to change component values to get the required gain and balance at the PI output.

OK, so what is the maximum voltage swing that you can get out of the stock Rocket PI (schematic as per the version the files section)?

Will it provide 30V peak to peak?

(Assume any magnitude of input signal you wish, as there is plenty of gain in the preamp).

[matt h]

(deleted)

[bluesky636]

What we need to know is whether the available voltage swing (headroom) from the PI is enough to drive the 6V6s.

I disagree with your statement that the calculator won't help. The average output voltages of a single coil or humbucker pickup are known. You can calculate the gain of each stage through the amp to determine if the PI will provide the required output voltage to drive the power tubes to full power. You can use the calculator to change component values to get the required gain and balance at the PI output.

OK, so what is the maximum voltage swing that you can get out of the stock Rocket PI (schematic as per the version the files section)?

Will it provide 30V peak to peak?

(Assume any magnitude of input signal you wish, as there is plenty of gain in the preamp).

Ignoring the cut control, the gain of the PI is about 26 (28 dB). To get 30 volts P-P you need 1.15 volts P-P at the input of the PI. Feel free to calculate the rest of the preamp gain if you like with Spice (I don't have it).

[Tillydog]

I'm surprised Pspice didn't show any difference when lowering the tail. It doesn't increase headroom, it "decreases footroom"...

I understand what you're saying: It did make some difference, but nowhere near as big as I had expected - the TW and the Rocket headroom are within a couple of volts of each other with the same B+. If anything the Rocket PI has slightly greater headroom. Rocket bias (1k2) with Express tail (15k total) get maybe 1 or 2 volts more, but nothing earth-shattering. It would be easier to tweak the B+ to the PI to fine-tune this IMHO.

It's just a quick LTSpice model I put together, so I wouldn't trust it without further verification. All it illustrates (to me) is that there is no compelling reason to change the PI just to accomodate the 6V6 vs the EL84 (provided the 6V6s bias as guessed above).

I agree there's no substitute for trying it 'in real life', tweaking and listening to the effect.