• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Motor Boating Caused by SS Preamp. Help!?

Robbie010

Member
2019-02-08 2:24 pm
Hi all,

I have a strange issue occurring with my setup that I am struggling to get to the bottom of.

I recently collected a pair of old Heathkit MA12 tube monoblocks after having them overhauled and recapped etc. I immediately noticed an issue with one amplifier motor-boating.

To cut a long story short, after being returned for further repair, the issue persists but I have determined that the source cause of the motor-boating is my preamp, which is an Audiolab 8000A.

If I switch the monoblock amps on and just leave them sat for any period of time the motor-boating does not occur. However, if I switch the monoblock amps on, then switch on my preamp, the left channel monoblock immediately starts to motor-boat. Alternatively, if I switch on the preamp first, then the monoblocks the motorboating does not occur immediately, but will occur after a short period of time.

The only way to stop the motor-boating is to switch off the monoblock amp for a few seconds.

Can anyone suggest what interaction between the two amps could be causing this issue?

Thanks!
 

martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
I would suggest comparing the values of the newly installed parts against the schematic. Mucking around with capacitor values is a great way to turn a stable amp into a marginally stable amp. I learned that lesson the hard and expensive way. Hopefully someone more qualified than me will respond to your question but in the meantime it will not hurt to check what you have against the original schematic.
 
Just for clarity:

If you swap the two MA12s round, does the 'motor boating' follow the MA12 or stay with the left hand output of the Audiolab?

Explain what you mean by 'motor boating'.

The MA12s take 15 / 20 seconds to warm up enough to produce any sound, so there will be a delay switching them on after the Audiolab.

Does the motor boating stop if either of the two MA12s are switched off? And does the motor boating return as the MA12 is powered back on? (Best not to switch off and straight back on by the way, with these amps always wait 20 or 30 seconds before switching back on...)

Circuit is near standard Mullard 5-10, .pdf enclosed.
 

Attachments

  • MA12_CCT.pdf
    1.3 MB · Views: 52
Last edited:
Motor boating is usually indicative of positive feedback, indicating a unwanted feedback path from output to input. You would need to check in particular any part of the circuit with a large time constant such as inter stage capacitance, but also could be transformers or power supply capacitance.

A schematic is a must, or indeed a return to the manufacturer to repair ... properly.
 
Hi all,

I have a strange issue occurring with my setup that I am struggling to get to the bottom of.

I recently collected a pair of old Heathkit MA12 tube monoblocks after having them overhauled and recapped etc. I immediately noticed an issue with one amplifier motor-boating.

To cut a long story short, after being returned for further repair, the issue persists but I have determined that the source cause of the motor-boating is my preamp, which is an Audiolab 8000A.

If I switch the monoblock amps on and just leave them sat for any period of time the motor-boating does not occur. However, if I switch the monoblock amps on, then switch on my preamp, the left channel monoblock immediately starts to motor-boat. Alternatively, if I switch on the preamp first, then the monoblocks the motorboating does not occur immediately, but will occur after a short period of time.

The only way to stop the motor-boating is to switch off the monoblock amp for a few seconds.

Can anyone suggest what interaction between the two amps could be causing this issue?

Thanks!
One cause of low-frequency oscillation aka motorboating is too large capacitors in the signal way. As these are inside a feedback loop they can cause
instability.
 
First, check your Audiolab preamp output with the volume down and use a multimeter to see of the Audiolab is passing any DC voltage. Another thing is to try a different source, any audio device with adjustable volume connected to the monoblocks to see if they exhibit the same behavior.

It sounds like one amp is only marginally stable and any, especially low frequency, signal generated by the Audiolab is sending it into oscillation. Another possibility is that the preamp connection is introducing some stray capacitance to the input of the amps which would set them off. And/or the Audiolab is acting like a giant antenna and picking up stray RF which is setting off the amps.
 

LJT

Member
2015-11-07 9:53 pm
Lots of good advice has already been given. My guess is that you will find the issue to be with your Audiolab 8000A

"If I switch the monoblock amps on and just leave them sat for any period of time the motor-boating does not occur. However, if I switch the monoblock amps on, then switch on my preamp, the left channel monoblock immediately starts to motor-boat. Alternatively, if I switch on the preamp first, then the monoblocks the motorboating does not occur immediately, but will occur after a short period of time."
The tech manual for the audiolab clearly states that pre-out is not muted and therefore should always be switched on before and switched of after the power-amp. If your amp stars serial number 89xxx you MAY have approximately 8 second mute on your pre-out. If you pre-out mutes when inserting headphones, your amp will also have this 8 second delay.

The MA12 input does not seem to have capacitive coupling. Personally, I would consider using this on tube amps connected to the audio lab. Especially so if the motorboating stay with the left channel output of the 8000A and the pre-amp out and power amp in of your 8000A has not been separated as per tech. manual. Depending on serial number of the unit, the implementation of pre-out / PA-in seem to differ slightly between serial numbers starting 87xxx and 89xxx.


The output of the 8000A line/tone stage has two 35v rated electrolytic to protect the power amp from DC. However the servo of the power-amp will superimpose DC on the input and may therefore also produce a certain amount of DC on the pre-out of the Audiolab.

The MA12 seem to have it's own volume control - did you try to connect the source direct? If you use the 8000A only as RIAA and source selector, you could try to connect to the tape out and use MA12 volume control.
The Audiolab RIAA seem to have electrolytic blocking capacitor rated at 50V
 

Attachments

  • pre-out_power-in.PNG
    pre-out_power-in.PNG
    52.7 KB · Views: 207
  • Audiolab_8000a_servo.PNG
    Audiolab_8000a_servo.PNG
    55 KB · Views: 209
First, check your Audiolab preamp output with the volume down and use a multimeter to see of the Audiolab is passing any DC voltage. Another thing is to try a different source, any audio device with adjustable volume connected to the monoblocks to see if they exhibit the same behavior.

It sounds like one amp is only marginally stable and any, especially low frequency, signal generated by the Audiolab is sending it into oscillation. Another possibility is that the preamp connection is introducing some stray capacitance to the input of the amps which would set them off. And/or the Audiolab is acting like a giant antenna and picking up stray RF which is setting off the amps.

I have connected my iphone to the monoblocks using a phono jack to RCA cable and the motorboating was present almost immediately, so the issues is not strictly limited to the Audiolab preamp.
 
Thanks to the many suggestions I have been able to test and troubleshoot the amps. The conclusion is that it is definitely the monoblock amplifier that is faulty.

What i cannot understand is how the techs at the hifi shop where they were repaired could not reproduce the fault!?
 

martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
..What i cannot understand is how the techs at the hifi shop where they were repaired could not reproduce the fault!?

The OPT is in the feedback loop so what you use as a load to test the amplifiers matters.

I would post some pictures including of the undersides with the covers off before I took them back. Somebody will spot the likely culprit
 
Motorboating is a pretty simple issue for a real shop to fix. For old valve gear, it's just replacing the antique filter capacitors. Who is this hifi shop that can't fix this? And, this being diyAudio, why can't we just fix this together? Ain't rocket surgery.


So, what do you have in the way of testing equipment? DVM? Scope? etc.


All good fortune,
Chris
 

Robbie010

Member
2019-02-08 2:24 pm
Unfortunately, I no longer have any testing gear, it didn’t get used enough so I moved it on.

However, I paid a local hifi shop £200 to recap the amps and install new RCA and banana terminals, so I expect them to correct the fault. I just didn’t want to go back to them with issue that could possibly be nothing to do with the MA12’s.
 
What kind of speakers are you using? Do they have powered woofers? These speakers often have a rising impedance into low frequencies which can destabilize some amps.

Also, swap the EF86 inputs between the two channels and see if the motor boating follows it. One may developed some gas which reduced its input impedance so much that it is interacting with the output circuit of the preamp, essentially coupling it to the feedback network.

Also try replacing C5, it might have gone bad. This capacitor filters the screen for the input tube and its value is often chosen in such a way as to compensate the feedback loop for low frequencies (replace with same value as in schematic).
 
Motorboating is often due to feedback through the power supply. The differential stage will reject power supply noise, as long as it's balanced. Resistor pairs R13-14, R17-19 should be matched. Likewise C7 and C8 should be matched in value. As this has an extra coupling cap vs. the standard Mullard circuit, the time constants of the two stages should be staggered (at least 3:1, ideally 10:1). The decoupling caps at the earlier stages should be at least 40 uF.
 
Last edited: