Rhythmic Hum/Oscillation/Tube " flashing"

[Ervikingo]

It also may happen that one amp is oscillating and the problem transfers to the other through common line impedance. Try one amp at a time to proof my theory.

Osvaldo, one amp usually starts the oscillation shortly after powering up (on its own no other equipment powered up). The other starts quietly and after 3-4 minutes it starts to oscillate. I will try your suggestion further and comment.

 

[jan.didden]

Jan, I run them with a Lexicon MC-1 for volume control but the noise is present with the interconnects disconnected at the amp's input.

And with shorted input?

Jan

 

[Ervikingo]

And with shorted input?

Jan

Jan, not sure what you are referring to??? or how to do it.

These amp have a switch for the type of input being used (XLR or RCA) I am using RCA and have disconnected them and also flicked the switch to XLR so the RCA is off but not the XLR.

 

[wiseoldtech]

Schematics are copyright protected. There are some on the web but they are of modified Snappers: http://www.hbaudio.eu/Manley-Labs-snapper-reformed.html

If those schematics are representative of what's inside those amps, it's clear that they're overly complex, over-engineered messes, designed by crazy people.
Tube equipment doesn't have to be so over-built to perform in a satisfying way.

 

[jan.didden]

If there's some feedback in any way through the source, to kill it is short the input and see if it goes away.
Use a shorted RCA plug or something like that. No audio diy-er should be without one ;-)

jan

 

[petertub]

Peter, I agree with you and thus why I sent them back to the factory. To my ears, those amps are perfect as they come from the factory. No need to mess with them. Odd to say that on a diy forum.

All caps measured OK. I gave them a blank check to replace anything needed to bring them to full factory spec. They tested and said no changes were needed except a couple of tubes.

Speaking of tubes, I have swapped tubes between amps (no change) and also replaced the EL34 with new ones, no change.

If problem persists, what does the mfg say ?

This kinds of problem may occur when coupling between stages are unsufficiant i.e. when power surged in the
output stage loads the psu enough that voltage sinks, and this voltage drop reached a previous stage causing
even more power surge, a typical oscillator with a period defined by the storage caps.

 

[wiseoldtech]

If problem persists, what does the mfg say ?

This kinds of problem may occur when coupling between stages are unsufficiant i.e. when power surged in the
output stage loads the psu enough that voltage sinks, and this voltage drop reached a previous stage causing
even more power surge, a typical oscillator with a period defined by the storage caps.

Have a look at the PSU in the link - I doubt that motorboating is caused by the PSU.
It's over-built like the rest of that mess.
The so-called designers were "parts-happy" - the more the better in their view.

 

[petertub]

Have a look at the PSU in the link - I doubt that motorboating is caused by the PSU.
It's over-built like the rest of that mess.
The so-called designers were "parts-happy" - the more the better in their view.

I guess you are wrong. And we do not know if the actual amps corresponds to the schematics. Only a real
evaluation will show, nothing we can do on a forum.

Motorboating in general comes from interaction of the amp with psu according to my experience.

 

[jan.didden]

Motorboating often also comes from more than one time constants in the loop, especially if they are similar. Like the RC in the feedback loop and an RC like a coupling cap in the forward loop. Two similar TCs give a 2nd order loop which can build up enough phase shift to cause motorboating.
Really, without a schematic it's just trial and error.

I've anyway always operated on the idea that it is smarter to try to find a cause than throwing out random suggestions, so I'm out of here.
Wish you all good luck.

Jan

 

[Ervikingo]

SPECIFICATIONS
Input Impedance RCA: 475 Kohm

Input Impedance XLR: 15 Kohm or 600 Ohm switchable

Input Sensitivity RCA: 750mV input = 110W output

Input Sensitivity XLR @ 15Kohms: 1.5 V input = 110W output

Input Sensitivity XLR @ 600 Ohms: 2.4 V input = 110W output (w/600 Ohm source)

Gain RCA: factory set for 31 dB; Range = 29.5 to 34.5dB

Gain XLR: factory set for 25 dB

Negative Feedback: factory set for 9dB of global NFB

Maximum Output Power into 5 Ohms: 110 Watts (1.5% THD @ 1kHz)

Maximum Output Power into 8 Ohms: 100 Watts (1.5% THD @ 1kHz)

Signal to Noise Ratio Ref. 1W: Typically 90 dB A-WGT 20-20K

Noise Floor: Typically 105μV = -77dBu A-WGT
Typically 388μV = -66dBu unweighted

Dynamic Range: 98dB

THD+noise @ 1W: less than 0.1%

Frequency Response at 110W full power: 15 Hz to 40 kHz FLAT

Frequency Response at 5W into 5 Ohms: 10 Hz to 65 kHz FLAT, -3dB @ 100KHz

Recommended Speaker Load: Optimized for 5 Ohms

Actual Output Impedance: 1.5 Ohms

Power Consumption (idle): 170 Watts (1.4A @120VAC)

Power Consumption (at Full Power 110W): 336 Watts (2.8A @120VAC)

Operating Mains Voltage: Factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.

Operating Mains Voltage: changeable with power transformer re-wiring and fuse value change.

Mains Voltage Frequency: 50~ 60Hz

Tube Complement (per each chassis):

Input Vacuum Tube: 1 x 12AX7 (front 9-pin tube)

Driver Vacuum Tube: 1x JAN NOS GE 7044 or 5687 (equivalent) (middle 9-pin tube)

Output Vacuum Tubes: 4 x Tungsol EL34B

B+ Voltage: 570V DC

Output Tube Quiescent Standing Current: 30mA

Set Bias for: 300mVDC measured across 10 Ohm cathode resistor

Lamp type: "FuseLamp" size 1/4" X 1 1/4", 12 volt, 0.15 amp; Units built after 5/2003 use white LEDs

Fuse values & types for SN#MSNP848 and below:

120VAC operation: MDL or MDA 4A / 250 Volt SLO-BLO size 1/4" X 1 1/4"

240VAC operation: MDL or MDA 2A / 250 Volt SLO-BLO size 1/4" X 1 1/4"

Fuse values & types for SN#MSNP849 and above:

120VAC = 5A/250V SLO-BLO, 5 x 20mm

240VAC = 3.15A/250V SLO-BLO, 5 x 20mm

B+ FUSE: MDA 1/2A Ceramic SLO-BLO size 1/4" x 1 1/4"

Dimensions: 15" deep x 13" wide x 8.75" tall

Shipping weight each: 45 pounds

 

[hbc]

But you really think that is causes changes in brightness of the tubes?

Could happen, the blue glow that is, due to relative time constants of the HT supply and the biasing. The amp bias circuit could be marginally stable with respect to mains fluctuations.

 

[Chris Hornbeck]

Or, it could be that somebody at Manley had carnal relations with a canine. As posted early on. Happens.

All good fortune,
Chris

 

[Osvaldo de Banfield]

If a design is too excesivelly dependant on the brand of their components, it is a dreadful design.