Grant Fidelity Audio - Read Only

[kennyo]

My A-88 has a low-pitched hum coming from it. Not from its outputs, or I would call it the classic 60hz grounding hum, but from the unit itself. The speakers are completely 'black' and noiseless. The hum is very low but my listening room is very quiet (at night anyhow!) and I listen to a lot of music that has 'rests' in it so I do hear it occasionally.

It's the amp itself. Could it still be a grounding issue?

[igrant]

You should check your BIAS (at least once a month). Under BIAS (<0.55v) will cause hum from the unit. Other things to check is your power supply, is it stable between 110 and 125v. Other things on the circuit can cause the voltage to fluctuate and do you have a video monitor/tv too close to the amp? Is the hum present in both Triode and UltrLinear modes? Roghly how load is the hum?

If it is a faulty power transformer it will get louder over time, if it does, then it should be turned off and returned for warranty.

Best,
Ian

[kennyo]

Thanks for the reply Ian.

I have a Monster AVS2000 power conditioner (not a surge protector) that keeps the power constant from 115v - 120v and the amperage as well so hopefully that is okay.

I do have a 50" Plasma that is within 3' (1m) of the A-88. However it is always turned off when I am using the A-88.

So far I have only tried the A-88 in Ultralinear mode. It's very difficult to change it to Triode mode in my rack even with the cage removed because of how my rack is built (not because of the amp).

As to biasing the amp - I wouldn't have a clue how to do so. I'm quite prepared to go out and purchase an ammeter, etc. but wouldn't know how to use it on the amp. I am fairly electrically aware - I have a computer science degree and build my own PCs and do my own home wiring - but don't know what to do to bias the amp. Unless you can tell me .

Thanks.

[igrant]

You must try it in Triode Mode !!, that is the sweeter sound. You should have enough clearance above it to switch it. I use a pencil. You should have at least 4" of clearance top, sides and back around the amp for air circulation too.

Does the hum increase with volume or remain constant? How loud is the hum roughly? Have you made sure the tubes are secure in the sockets?

To BIAS the KT88's you need a basic digital multimeter (analog works too, but you want to match the bias exactly) with long lead probes. Read your multimeter manual. With everything hooked up and power on for around an hour and volume set to zero, follow these steps:

Set multimeter to read 1 Volt range.

Insert Negative, then Positive probes into V1 (tube 1) test points above tubes, read and adjust bias with supplied scewdriver to between 0.55 V and 0.60 V (.55 softer sound, longer tube life, 0.60 more power, shorter tube life) up to you. Note that it is a very 'screwdriver turn' fine adjustment, be careful not to turn BIAS into damaging range

Repeat for 3 remaining tubes, match BIAS to within one or two points.

The tubes are hot, so careful about burning your pinkies.

Let me know.

Ian

[kennyo]

Thanks Ian. The hum remains constant with volume, no change. The tubes seem to be secure, I wear cotton gloves when I touch them and, trying it again (I'm at home today), they seem to be firmly in their sockets.

Ummmm. No supplied screwdriver. Sorry.

Thanks for your help.

[igrant]

oops, things like that happen with samples. I'll send you a BIAS screwdriver, it is a rubber coated small Robertson (square) Orange (#00) Size No. 1 & 2 screws. Please email me your address again (work or home?)

Is it possible for you to try it on another power circuit, with just the speakers connected?

Best,
Ian

[kennyo]

By "other power circuit" do you mean one on the power conditioner or direct from the house power? Thanks.

[igrant]

From house power, with or without your power conditioner. I'm out and about for the long weekend, we can do this over the phone too.

Best,
Ian

[kennyo]

'Kay. Just got back from the long weekend myself. Tried house power, conditioned power, and a feed (via an extension cord) from a completely different circuit. The results are all the same.

The hum is *very* low. My wife said she couldn't hear it when I checked to see if I was insane... when she got within a foot she was sure I was insane - until I turned the amp off. Then she said 'oh' - you notice the hum more by its absence than its presence (if you know what I mean). I can hear it from 10' away... but I admit that it is very, very low.

Tomorrow I get an ammeter and I'll try to bias it to see if that changes things. One last thing - I'm reasonably / pretty damn sure that it didn't hum when I first started using it. So it may well be the bias.

[igrant]

excellent, i appreciate your efforts and hope you enjoy this type of follow up service. All the woman in my life know I am insane and you got a chance to prove to yours that you aren't, this time .

I was using your amp (thats how I test them) and didn't have any noticeable hum. All good tube amps have slight hum, from the transformers which you have about 45lbs of, but this hum should not interfere with your listening at listening position.

If biasing doesn't work then there is one last tube(s) check we will do before determining which transformer is the problem. Thanks for mentioning that is was good when you got it, it helps me understand what is going on.

Best,
Ian

[kennyo]

I very much appreciate your efforts Ian!

While waiting for the screwdriver I decided to measure the voltage and the first 3 tubes are all 0.64 while the 4th tube is 0.59. So, assuming I am measuring correctly the bias does appear to need adjustment.

a) I assume I use VDC due to the '-' symbol underneath the 'V'. Besides, the VAC measurement makes no sense at all... Can you please confirm (or deny)?
b) I'll try another multimeter just to make sure nothing is screwy with mine.
c) the multimeter needs really LONG probes.

Thanks.

[igrant]

Yes it does, i belive i mentioned watch your fingers. The - is fine and the you may just have the leads reversed. 3 tubes are running too hot and can be causing the hum. This is common on a new amp that the voltage increases across the tubes as things break in. Also the voltage will change a bit after turning it on and letting it warm up for an hour or so.

Getting them into proper bias will noticably improve the sound of the amp, also swithing it into triode mode will in my opinion greatly improve the sound of mellow music. I like ultralinear for movies, rock etc.

Has the hum gotten any louder?.

Ian

[kennyo]

I received the screwdriver today and biased the tubes (using the VDC scale) all to 0.550 +/- 0.001 (it's really hard to get it exact). No change in the hum volume, there never has been.

I'm starting to think I'm insane . I have a test I use all the time for system quietness - Miles Davis' "Sketches of Spain". If I can hear anything through the castanets at the very beginning then the system makes too much noise. I can hear the hum very, VERY, faintly. Very.

So maybe it's normal?

And triode mode is smoother, but not by a lot. It sounds less "clipped" in things (like horns) that are brought to a volume and then stopped.

Thanks for all your time and help.

[igrant]

Thank you too. As soon as it arrives I will test it for external hum. It would be great if you posted some of your private messages to me here, especially your comments on the sound quality compared to your other components.

Best,
Ian

[igrant]

Ok, I've tested it and compared it to 4 other A-88's here and they transmit a very slight buzz from the 4 transformers that can be heard up to about 2 feet from the front. It is basically inaudible unless you focus on listening for it. I went to a few dealers in town and all the tube amps we tried do the same thing, some better, some worse. The ones that where better are enclosed in cases.

I have another shipment coming next week and will compare. I will also go inside and see what possible mods can be made. It is definately not faulty.

Best,
Ian

[kennyo]

I suspected as much and said so in my review in the "Amps" section. You've been great about it and I understand if nothing further can be done.

Let me know when you're tired of it and we can have it shipped back to me (at my expense of course).

Thanks again.

[kennyo]

Ian,

Given that you can hear the 'buzz' I was wondering how you would compare it to any buzz you can hear from the A-34? I'm not prepared to give up on GF at all... I think you're on the right track and have terrific value. PM me if you wish.

Thanks.

[igrant]

Just tested it. The A-34 is quieter having one less power transformer, same with the A-330B. I can't measure the noise in my studio, the room lights give off more noise and my sound level meter only goes down to 49 dBc, which is more than the noise of the amps. So this is purely subjective.

On all of my test albums, I cannot hear this buzz once I have any type of music playing at the lowest possible volume setting on the amp.

I have one more check to do then I will send your amp back, probably tomorrow.

Best,
Ian

[igrant]

Great news, I found another sample unit where the transformer buzz does not travel past 2 feet from the amp. The tranformers for the A-88 (and A-34, A-330B, P-307) are hand made in the factory, so each will have slightly diifferent characteristics, the sum of the 4 tranformers seems to cancel out past 2 feet. I'll send this one back to you.

Best,
Ian

[kennyo]

WOW! I'm major impressed !

I've been following some of the thread on CAM on Chinese amps and I've noted your posting there for the A-34. I'll pop on some positive feedback (a lot of CAMmers won't buy unless they see some) if I can and just want to ask you not to get discouraged, no matter what "Yauqui" says...

Thanks again!

PS. For anyone else reading this - go to my review under "amps". As you can see there, I really wanted the A-88 back!