Mesa Blue Angel slightly down on power

[Alex R]

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

I remember having exactly the same experience. Voltage dropping resistors, like 10 in series. I forget which model. hey maybe Mykey was right after all

I may or may not be alone in this but my instinct faced with this issue would be that there is essentially nothing wrong with the amp. Sorry to be so bald about it but normal losses and tolerances probably account for this? Just an experienced repair tech's instinct, very ready as always to be proved wrong . Maybe another way of putting it would be that this could never be an economical repair.

[Mick Bailey]

....my instinct faced with this issue would be that there is essentially nothing wrong with the amp.

7w out of a pair of 6V6 running 314v plate voltage would need some major losses. That doesn't look right to me, even in a badly designed amp.

Then again, Ettore Bugatti commented that Bentley must have been an engineering genius to get so little power from such a large engine.

[Leo_Gnardo]

Just had a look at the output schemo, and I'm with Juan in post # 10. Gag me with a spoon. Mix-n-match output tube kit, switch off your EL84's then you have a power reduction as expected, plus a bad impedance mismatch to knock power down even more. I'd say take your 7 watts and enjoy it. Do it sound good? If yes that's all that matters. I wouldn't spend a second trying to panel-beat this goofy "deliberately lame" design into delivering another watt.

[Jazz P Bass]

hey maybe Mykey was right after all

Shhhh!

[J M Fahey]

Fact is, 6V6 can easily push 15W and up to 20/22 with some difficulty, such as rising +V as much as they will stand, carefully matching impedance, etc. BUT here we have a load impedance optimized to get some 40/45 real Watts from three tubes per side and then we switch off two of them, without switching load impedance, which is as gross a mismatch as can exist.

Even 7 clean watts out of there is a miracle, and attributable more to resiliency of tubes than to design wisdom ??? [huh?]

[pdf64]

'It looks like the PI may be clipping before the Power Amp, as they clip simultaneously, with or without Feedback connected.'

Note that any amp does that. Well, maybe a couple of bass amps that can operate in class 2 grid current mode, ie able to pull the power tube control grids above the cathode.
The clipping point of regular class 1 amps occurs when the signal V + bias V >0V, above that the grid to cathode impedance falls from near infinity to a kohm or two, and the signal gets clamped (just like an MXR distortion).

Beware of trying to analyse waveforms inside a feedback loop, as the error compensation mechanism can create some weird signals!

[Chuck H]

'It looks like the PI may be clipping before the Power Amp, as they clip simultaneously, with or without Feedback connected.'

Note that any amp does that.

Maybe, I've never noticed it perhaps. I design amps that ALWAYS clip the power tubes ahead of the PI. Other than a PPIMV this should always be the case with any properly considered amp design.

[g1]

Chuck, I think he means how it would appear on the scope. Even when designed so the PI has extra headroom, once the power tubes clip their grid impedances drop so low it clamps the output of the PI.
You can't see the fact that the PI clips later unless you pull the power tubes.

[Chuck H]

I must have missed that. And I never considered it either. It's not the sort of thing that lends itself to visual evaluation with a scope. I always see my power tubes starting to flat the waveform when the PI is still round. Of course by the time the power tubes are clipping hard the PI is too. Something I can play with and try to quantify for it's tonal contribution later

[pdf64]

'I always see my power tubes starting to flat the waveform when the PI is still round'
Bear in mind that the signal at the LTP plates is 'buffered' a little from the power tube grid clipping by grid stopper resistors.
RG describes power tube grid clipping, and how to overcome it, here http://www.geofex.com/Article_Folder...osfetfolly.htm

[J M Fahey]

Fact is that power tubes in AB1 put out maximum current possible when grid reaches 0V.

They may and do pass higher current, if grids are driven positive ... problem is that no standard PI , be it LTP, Cathodyne or those where the 2nd triode gets a padded signal from the first can provide any significant current and they will clip trying to do so.

Using larger than usual resistors in power tube grids just masks that, and may be good to minimize grid rectification and extreme bias shifting, even blocking, but they add not add one iota current or voltage to such grids.

The PI signal may look better now ... so what?
What matters is what actually reaches PT grids, and that will be severely clipped.

Of course, it's good Engineering to have a PI to be able to produce more than what's needed , so as to be able to cover tolerance of parts and parameters, but that will be produced unloaded ... once power tubes are hot and in their sockets, you are adding a clipping diode at each grid.

[rf7]

When you measure the power supply voltages do they come close to the schematic when cold?

Yes.

Check the AC supplies out of the PT aren't dropping.

They drop 5 to 6 volts at clipping.

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

Heater voltages are ok.

Double-check the PI and output tube voltages.

All ok.

Re-check your load resistance and impedance setting

All ok.

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

Yes, it's clean until it clips.

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 calculated 90.3mA for one side and 97.3mA for the other side.

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.

Screen voltage drops 18V at clipping.

Have you tried using a new GZ34?

Yes, it did not make a difference.