Carvin V3M power mode question

[Randall]

Carvin V3M head with 50w, 22w, 7w power selector that makes no audible difference in output. Customer says it is stuck in 7 watt mode. Here's where I get thrown. 325v supply voltage C comes in at conn H4A. In the 50 watt position I have 325v CP at the PI plate resistors R48/49, and plate voltages of 235/242v. This produces a 12vp-p sine.

Here is what I get in the other two positions:

22 watt:

R48/49 = 69v
plates 58/56v
10vp-p sine

7 watt:

R48/49 = 47v
plates = 41.7/41v
8 vp-p sine

So it appears the switch is working by lowering the PI voltages, but why does the output power not vary by more?

http://carvinimages.com/schematics/v3m_blocked.pdf

It also calls for: Set P27 for 80mA across standby in 50 W mode. It was set at 150 mA, I lowered it to 80mA.

[xtian]

What load are you using? If you're using a 4-ohm load, and you see 12v p-p on the output, that's only 18 watts. So maybe it's not even operating properly in 50w mode?

[g1]

Agree with xtian, probably some other problem other than the switch. The switch seems to be doing it's job and varying the PI supply voltage.
What are your voltages at the power tubes and power supply nodes?

[Randall]

I'm using an 8 ohm load, but I'm not driving it to full output, so these are arbitrary output powers. I just don't see much of a change. The node voltages are all right on. The plates are 403v and the screens are 398v.

[Enzo]

I don't know that amp, but the idea in my mind is this: If I am driving my car down a 30mph road to the store, it kinda makes no difference whether it is a Volkswagen or a Camaro IROC. But stomp on the gas pedal and open them up...

Try cranking the thing in 50 watt mode to see what it can do, then back off to 7 watt mode and see if it reduces the output.

Don't think of 50 watts as a boost, rather think of 7 watts as a a limiter.

[g1]

Don't expect to see much change if the amp isn't working right.
The power switch is just a pseudo-PPIMV, it just limits the signal, it doesn't reduce what the power section is capable of doing. Once you can get full power, the switch will probably be more noticeable.
If CP is connected to C when switch is in 50W mode, you can forget about the switch and move on.

Or have you verified the amp can do 50W when switched to that mode, and not much reduction in other modes? Think of the switch as a limiter on the signal to the power amp, when amp is driven to high levels.

edit: just like Enzo said when I wasn't looking.

[Randall]

I understand the 50 watt mode is the default mode, and the 22 and 7 watt modes are reduction modes. I will drive it to limit and report back. But my point is, the customer said the modes used to work, and now they don't. My question at this time is, how can PI plate voltages ranging from 235v to 41v yield such a barely perceptible result in output?

OR, if the switch is doing it's job, which it appears it is, why doesn't it make much difference in the output?

[g1]

My question at this time is, how can PI plate voltages ranging from 235v to 41v yield such a barely perceptible result in output?

The signal at the PI plate has to be much more than 41V p-p before the switch will make much difference. Maybe something ahead of it is creating a problem where the signal never gets that large at the PI.

[Randall]

I don't understand the above. The PI plate is at 235v in the 50 watt position. It is only 41 v in the 7 watt position. But still not much difference in output.

[g1]

Think about how much signal swing there can be when the PI plate is 235VDC. And how much less signal swing is available when you drop the PI plate to 41VDC.
That signal swing is what determines how much signal gets to the power tubes.
Better than think about it, look at it on your scope.

But have you checked yet if you can get full power on the 50W setting?

[Randall]

"Think about how much signal swing there can be when the PI plate is 235VDC. And how much less signal swing is available when you drop the PI plate to 41VDC.
That signal swing is what determines how much signal gets to the power tubes.
Better than think about it, look at it on your scope."

OK, this is getting frustrating. I understand the concept of the switch, and it appears it is working, what I am trying to get across is why I don't hear a large difference in output. I had just picked a moderate level to work with and didn't see a big difference on the scope. I have since re-done tests at full power. I get:

55 watt: 50vp-p into 8 ohms for 39 watts
22 watt 25vp-p for 9.7 watts
7 watt 15vp-p for 3.5 watts

This is after I measured and adjusted for the 80mA bias current as directed on the board.

This tells me the circuit has more of an effect at higher levels. It also seems overall output is low, correct? Or could this just be attributed to tired tubes?

[Dave H]

I think it will have a more audible effect at higher levels. When you did it at a low level it only reduced the power by a factor of two which is not a big difference in volume.
I deleted that other post. It was confusing. It confused me anyway.

[The Dude]

......Don't think of 50 watts as a boost, rather think of 7 watts as a a limiter.

^^^^ Yes- that! At low volumes, with low drive, you won't hear a difference because you have not reached maximum clean tube gain. Don't think of it as a master volume control. As Enzo says, think of it as a variable limiter. Nothing changes until the threshold is reached.

[Randall]

So, I am chasing a non issue?

[Jazz P Bass]

I think that you need to have your customer come by and describe 'not the same'.

By lowering the PI voltage, the amp should distort quicker.
Call it a Brown setting.

From reading about the issue on other sites, it appears that a massive volume change is not what the circuit is all about.

A couple of questions from a new V3M owner. - Carvin Forum