Mesa Triple Rec Mains Current at Idle

[tdlunsfo]

I have a triple rec on the bench with the complaint of loss of low end, thinner and overall output loss. All tubes test good in my hickock. Filters pass the ESR check. 6L6GC's bias around 31mA at 432V or ~46% dissipation. Screen resistors are good. Customer has everything in diode rectifier mode and "bold".

When running through my 8 ohm dummy load I am looking at 1.8A idle current and I can only get around 50W out of it at 3A , at 4ohms we are looking at 70W at 3A. My variac fuse is only 3A so I have to stop there for now, but this seems like low output watts for the current draw, especially with the mains fuse at 4A and the amp supposed to be 150W. There is certainly plenty of tubes in this thing, but I don't work on these enough to know for sure. Thoughts?

[Helmholtz]

Please post schematic and scope pics at full output.

- Did you measure each tube for idle current?
- What are screen voltages at each tube socket?
- What is B+ at full output?

[tdlunsfo]

Here is a close schematic: http://diy-fever.com/wordpress/wp-co..._rectifier.pdf
All screens measured 450Vdc, all bias voltages on pin 5 were -51.9Vdc. Obviously tube #5 stands out a little. B+ at 3A output is 409Vdc. I can't get to full output unless I bypass my variac and go with my ammeter across the fuse.

6L6GC idle currents:
1. 32mA @ 452Vdc
2. 21mA @ 453Vdc
3. 20mA @ 453Vdc
4. 22mA @ 453Vdc
5. 54mA @ 451Vdc
6. 22mA @ 451Vdc

[g1]

When running through my 8 ohm dummy load I am looking at 1.8A idle current

What does that drop to in standby mode?

[Helmholtz]

No sense measuring power, voltages or currents with a variac (or bulb limiter) before the amp.
If the amp doesn't blow fuses you don't need the variac.

[tdlunsfo]

No sense measuring power, voltages or currents with a variac (or bulb limiter) before the amp.

It's at full mains power. In this case it is being used for the isolation transformer and ammeter.

[tdlunsfo]

What does that drop to in standby mode?

~1.1A

[Helmholtz]

6L6GC idle currents:
1. 32mA @ 452Vdc
2. 21mA @ 453Vdc
3. 20mA @ 453Vdc
4. 22mA @ 453Vdc
5. 54mA @ 451Vdc
6. 22mA @ 451Vdc

Tube current spread of 2.6:1 is huge.
Are voltages measured at screen pins?

Scope pics show heavily distorted output. Will full output I meant output at the onset of clipping.

If your variac is an isolating type it is likely to have some sag at high amp output.

[tdlunsfo]

Variac is a Heathkit IP5220. Channel 3 on the triple rec is pretty distorted but I should be able to clean it up with the gain knob. Screens where measured on the screen resistors but will check the pins to verify pin integrity. Yes 6L6 #5 is an oddball. I'll get you some better numbers tomorrow.

[tdlunsfo]

Bypassing the variac for all readings, B+ at onset of clipping is 434Vdc measured at the standby switch.

Also,
6L6GC Plate, Screen and Grid voltages measured at the pins @ idle.

1. 467, 465, -53
2. 465, 463, -53
3. 464, 462, -52
4. 463, 462, -53
5. 463, 461, -53
6. 463, 462, -53

New Scope Pic.

[Helmholtz]

DC voltages look ok. What is heater voltage?

As I can't read your scope settings...

- What is signal frequency?
- What the rms output voltage at full clean output?
- What is your load resistance?
- Does the output clip symmetrically?

[tdlunsfo]

Scope is at 5V/Div and 5mS/Div

- What is signal frequency? 500Hz with all controls at 5 out of 10 with the exception of Gain which was at 1 in order to clean up the signal.
- What the rms output voltage at full clean output? 90W
- What is your load resistance? 8ohm
- Does the output clip symmetrically? Yes

[Helmholtz]

Scope is at 5V/Div and 5mS/Div

With these settings I would read a frequency around 70Hz and a voltage of 31Vpp corresponding to 11Vrms from you scope pics?
Are the vernier knobs in there calibrated positions?
What rms voltage does the Fluke measure?

What is your load resistance? 8ohm

Did you measure it?

If your signal generator is able to provide a few volts, feed your signal to the PI input before the coupling cap (make sure to measure 0VDC there). Turn all MVs fully up and repeat output power measurement.


BTW, the display of your Fluke seems to start self-destroying.

[tdlunsfo]

BTW, the display of your Fluke seems to start self-destroying.

8.8ohm measured with the cable.

It fell off the bench the first week I owned it and cracked the screen, and I just haven't gotten around to fixing it. I love working on amps but despise working on test gear... weird huh?
30.6Vacrms on the fluke at onset of clipping at 500 hz test signal. (30.6*30.6)/8.8 = 106Wrms. This correct? My watt meter is homemade so looks like I need to work on the calibration.

Checking scope calibration with my audio generator and it's pretty far out.

[Helmholtz]

It fell off the bench the first week I owned it and cracked the screen, and I just haven't gotten around to fixing it. I love working on amps but despise working on test gear... weird huh?

Lately had a similar display defect wih one of my DMMs only without any mechanical damage. Dark zone slowly increased over months.
You'll need a new display.

30.6Vacrms on the fluke at onset of clipping at 500 hz test signal. (30.6*30.6)/8.8 = 106Wrms. This correct?

Yes, 106W is correct.
That's what I'd expect from 4x6L6s.
Are you using the correct 8 Ohm output?