Mesa Blue Angel slightly down on power

[rf7]

I've got a Blue Angel on the bench that is down about 4Vrms on the output at all 3 Simul-Class settings. New tubes did not make a difference. The power supply is down about 10V from what's on the schematic. It looks like the PI may be clipping before the Power Amp, as they clip simultaneously, with or without Feedback connected.

I fed a signal into the PI via a cap and got the same results. I'd like to feed the power tubes directly, but I would need a differential source, which I don't have.

Any ideas?

[Jazz P Bass]

Curious, is the customer complaining about the amp being down 2 watts?

Or this simply an observation?

If the B+ is low, the output will be lower.

[Chuck H]

Jazz, he said 4Vrms. Doesn't that equate to about 15W less output into an 8ohm load?

With the voltage low and the output power so low I would pull the tubes and see if the voltage meets spec. There could be something drawing excess current that isn't a power tube. With the PI clipping before the power tubes I would guess voltages are low on that tube as well. It may be that circuit that's drawing too much current. Did you check voltage on the PI as well? Or try a different PI tube?

[J M Fahey]

I've got a Blue Angel on the bench that is down about 4Vrms on the output at all 3 Simul-Class settings. New tubes did not make a difference. The power supply is down about 10V from what's on the schematic. It looks like the PI may be clipping before the Power Amp, as they clip simultaneously, with or without Feedback connected.

I fed a signal into the PI via a cap and got the same results. I'd like to feed the power tubes directly, but I would need a differential source, which I don't have.

Any ideas?

That statement means nothing, sorry.

Please post expected and actual voltages at the onset of visible clipping , also stated load impedance.

I guess you are using resistors and not actual speakers.

Also tubes used and actual +V voltage.

A schematic here would help.

Thanks.

[Jazz P Bass]

Jazz, he said 4Vrms. Doesn't that equate to about 15W less output into an 8ohm load?

Huh?
P= VxV/R

[Chuck H]

Not as it stands alone but as a ration of the voltage for a 40W amp. I was looking at this:

[J M Fahey]

"4 VAC less" means nothing by itself.

Examples:

1) suppose a 4VRMS@4 ohms amp, or 4W RMS .
4V less is 0V out, which happens to be 4W less but should be better described as no output.

2) now a 10V/4r or 25W amp.
4V less is 6V/4r or 9W ; a 16W loss

3) now 20V/4r or 100W
4V less is 16V/4r or 64W ; a 36W loss

as you see, "4V less" is absolutely meaningless without stating expected and actual voltage, and load impedance.

[Chuck H]

Since the Blue Angel has multiple power and impedance options this is certainly true. I went nominal with the assumed 40W stated output and 8 ohm load.

For the Blue Angel design at any output and impedance combination a 4V loss of AC at the output is considerable.

Semantics aside... We need to hear more from rf7 before we can make any determinations.

[rf7]

Thanks for the responses!

This is what I measure when running a 16 ohm load:

EL-84 only: 19Vrms, which is 22.6Wrms. Rated at 33Wrms, which would be 23Vrms output, 4Vrms difference

6V6 only: 10.7Vrms, which is 7Wrms. Rated at 15Wrms, which would be 15.5Vrms output, 4.8Vrms difference

Simul-Class: 22Vrms, which is 30.3Wrms. Rated at 38Wrms, which would be 24.7Vrms output, 2.7Vrms difference

Yes, I am using load resistors.

The tubes are all Mesa tubes, marked STR417 for the 6V6's.

I've tried JJ's as replacements and there was not much of a difference.

I have swapped the PI tube and there was no change.

The power supply voltages drift down over time as the amp warms up. The output stays low, regardless of where the power supply B+ is sitting.

After being on for 15 minutes:

Point A is 307V, should be 315V
Point B is 305V, should be 313V
Point C is 294V, should be 306V
Point D is 255V, should be 265V
All points above referencing the HI-V supplies on the schematic

Jazz has posted the schematic.

The amp belongs to a friend and he wanted me to check it out for him. It's only a 21% drop for Simul-Class operation, but it's a 50% drop for the 6V6's alone.

[J M Fahey]

Ok.

Tube amp rated RMS power should be always be taken with a grain of salt ... or a spoonful, unless it's an old Ampeg , Hiwatt or an early 560V +V Marshall
The 135W Twins might have slightly surpassed 100W`RMS, which is a lot anyway .

So, in a nutshell, and beyond the 6V6 mismatch , which I attribute to horrible impedance matching, the rest looks fine, specially the simulclass one.

I usually feel sick after seeing many Mesa "designs" , and this one is a prime example of that.

It makes me believe that R.S. plugs random resistor and capacitor values in his breadboard until it sounds good to somebody and then he transfers exactly that to a board.

82k + 10k + 1k .... really ?
Not a single 93K 1% resistor there if value is so critical?

100 ohms in series with a plate derived signal?

3k3 in series with 680r instead of just 3k9?

A cathode follower driving nothing?

Sorry, don't want to puke my keyboard.

[rf7]

Thanks Juan. Yeah, the 6V6 rating is really off.

[The Dude]

...... 82k + 10k + 1k .... really ?
Not a single 93K 1% resistor there if value is so critical?

I doubt that's even necessary. We're talkin' tube amp here- not med tech. Why not just go with a 100k and be done with it. I agree with you, Juan. Mesa does some goofy shi+.

[Chuck H]

As a novice, digging into a Mesa product confused the $h!t outta me because I couldn't understand why the design used a long string of resistors in series, with no intersecting circuits, where one resistor would do. I thought there was something I didn't know. After all, Mesa is a respected product among novices

[Mick Bailey]

When you measure the power supply voltages do they come close to the schematic when cold? Check the AC supplies out of the PT aren't dropping.

Did you check the heater voltages? very low and you lose too much emission and power. Check them at the destination as well as the source

Double-check the PI and output tube voltages.

Re-check your load resistance and impedance setting

Is the clean output undistorted? A shorted turn in the OT will sap power and distort the clean signal.

Measure the power tube idle current for each side with no signal applied. I measure the OT primary DC resistance then power the amp and measure the voltage drop across each half of the OT primary. Ohms law then gives you the current. If your idle current is very low the screen supply could be suspect.

I would check the screen supply under signal conditions too. Low screen voltage means low output. Again, check the voltage at the screen resistors as it may be getting dropped along the way.

[Pedro Vecino]

The power supply voltages drift down over time as the amp warms up. The output stays low, regardless of where the power supply B+ is sitting.

Have you tried using a new GZ34?
A while ago I had a Blue Angel that did not develop power and the source of this was the rectifier. At idle the voltages were apparently good but with power they decreased.
Of course the sound had an improper distortion relationship clearly perceptible.