Amplifier Troubleshooting

[Soonerorlater]

No probs, just trying to eliminate other faults such as incorrect tube type, incorrect bias, internal damage due to being dropped for instance by previous owner. If after replacing tubes it ran properly for a reasonable time then we have a platform to start from.

PS. just as an after thought, does your model have a stereo/mono switch and if it does what effect does it have.

[boywonder]

From the linked data sheet, it appears that it's SS rectification, and from the OP's previous post, it sounds like the fault is repeatable once the amp is warmed up.

Cogline: Are you comfortable working around high voltage? This amp probably runs a B+ voltage of around 300-350VDC. Have you read the safety sticky at the top of this forum? Do you have clip leads for your DMM? Clip leads make troubleshooting "hands-off" which is much safer than probing a live amp with a regular meter probe tip. Are you aware that you need to keep one hand in your pocket (or around your back) when troubleshooting a live amp?

[Soonerorlater]

Essential advice from Boywonder. A visual inspection of internal components might show up something. Charged capacitors can still give you a nasty shock even with the amp turned off.

[payloadde]

1st inspect innards - any visible damage, discolored parts, shorts, scratches, loose wires, loose strands, connectors etc

2nd disconnect pre-amp and short the power amp input terminals - any change ?

next I would be concerned about speaker cones moving in and out - you could damage your speakers on the long run - use a dummy load (resistor) instead and hook up a meter
dummy load is not necessaryily to be 8 ohms, I would possibly try 100 ohms to begin with.

now you can safely proceed with in-circuit measurements - start with power rails, are they stable and reasonable value ? !!! be careful - dangerous high voltage everywhere !!!

one source talks about "... individual output tube protection" audiocostruzioni.com
might give rise to cutting-in/out if defective or indicate overload or short or ...

pull tubes of one channel - any change in the remaining channel
same vice-versa

is there a bias pot ? make shure it hasnt been set to one end by the pre-owner

to be cont'd ...

[Cogline]

Lot of information, firstly to answer questions :

I am very aware of the dangers of tinkering with live amps and even ones not connected to a power supply. I am equally aware of easy precautions that are to be taken in order to avoid shock. Thank you for your concern though.

The tubes are definitely the correct types and I'm pretty sure they are bias correctly since I'm using the tubes that came with the amp originally.
Looking inside the amplifier no damage can be seen.. no weak solder joints no dis-colored components everything looks good.

"pull tubes of one channel" - are you sure this is safe to do won't this increase the load on the other channel?

I'll look into the information and troubleshooting tomorrow (tired and need some natural light to help see into the amp a bit better!).

Lastly, i am the only person after being manufactured to have taken a look into the amplifier so the bias is at factory Default and nothing has been changed.

once again thank you!

I am able to take pictures if needed just ask!

Cogline

[boywonder]

Originally Posted by payloadde

one source talks about "... individual output tube protection" audiocostruzioni.com
might give rise to cutting-in/out if defective or indicate overload or short or ...

Good point! Cogline: Do you happen to know the impedance of your speakers? Or even better, an impedance curve?

Is there any evidence of the output tubes "red plating" when the amp is warmed up? Have you checked the bias? Setting/verifying that the bias is set correctly would be the first thing I would do.......

[Cogline]

Yes my speakers are Sonus Faber Electa 8 ohms! very nice i play them with a SS amp i have.

I'm completely lost with this comment "one source talks about "... individual output tube protection" audiocostruzioni.com
might give rise to cutting-in/out if defective or indicate overload or short or ..."

Care to explain?

Thanks,
Cogline

[Soonerorlater]

Cogline, just noticed your join date, welcome to the forum........

[boywonder]

If your speaker's impedance drops to very low levels at some freq (not too uncommon) or your bias adj is not set correctly, you may be pushing the output tubes too hard and causing the protection circuit to kick in. Tube amps typically have higher output impedance than SS amps and like a relatively flat impedance curve for a speaker load.

Originally Posted by Cogline

Yes my speakers are Sonus Faber Electa 8 ohms! very nice i play them with a SS amp i have.

I'm completely lost with this comment "one source talks about "... individual output tube protection" audiocostruzioni.com
might give rise to cutting-in/out if defective or indicate overload or short or ..."

Care to explain?

Thanks,
Cogline

[boywonder]

Originally Posted by Cogline

......i am the only person after being manufactured to have taken a look into the amplifier so the bias is at factory Default and nothing has been changed.


Cogline

From the linked pics, it appears that the bias adj pots are located on the top deck.....easy enough for anyone to tinker with.....

Do you have any other speakers that you can use with this amp?