bias issues (Laney AOR) advice needed

[rashayritto]

Hey guys. I have an '87 Laney AOR 100 watt (8 knob version). Just retubed it with JJ's i ordered from eurotubes along with a bias probe. I measured the plate voltage to be 501 and biased it at 25ma (27.9 being the upper threshold for 70% dissipation).

Anyway. after doing this the amp played just fine--but when i turned it on again later it farted out after playing a few notes with the master cranked up to around 2 o'clock. It had worked fine at the same setting the first time after biasing.

I re-biased and turned it on to find that it played again, only to have it fart out during the session after--again. After the fart out the amp sends no signal to the speakers, but when i pluck the guitar the tubes glow blue in the center and emit some noise.

I'm about to set the bias again and i'm sure it will play a third time--but what exactly is happening on my second playthroughs? Any insight would be great!

 

[USAPatriot]

Re: bias issues (Laney AOR) advice needed

It sounds like one of the tubes is bad. Do you still have the ones you took out? Put them back in and see what changes, if anything. Also, if you know how to check the plate voltage directly and don't have a problem doing so, give that a try and see if you come up with the same answer as the bias probe. It's possible that it has a problem. Just out of curiosity, does the amp's board have a bias voltage printed near the pot? If so, shoot for that instead of the 70%. If it's there, there's a reason for it -Rod-

 

[rashayritto]

Re: bias issues (Laney AOR) advice needed

Actually i got the plate voltage directly (voltage pin on tube socket, chasis for ground). I'm not sure if there's a printed voltage somewhere but I'll be sure to check. This is actually my first time firing up the thing in almost a year i think so no previous tubes to refer to. I guess i could order one to swap out one by one and see if it makes the amp stable.

thanks for your suggestions!

 

[Pierre]

Re: bias issues (Laney AOR) advice needed

Try to find a schematic for the amp. Those AORs are really cooking up the tubes in there: my 30W combo puts out more than 400V to these 6V6s, so I biased with this in mind and it sounds gigantic when loud.

At lower volumes than, basically, cranked, they won't sound very warm in my experience. But with this headroom and sound they make a lot of sense when played loud.

 

[USAPatriot]

Re: bias issues (Laney AOR) advice needed

25 just seems cold to me. My HT-40's EL-34's were 401DCV and 402DCV respectively on the plate voltage and just popping the Tungsols in in place of the stock TADs, the bias voltage was 32. The board itself though specifies 50mv so that's what I shot for. Maybe recheck your math? -Rod-

 

[donnievaz]

Re: bias issues (Laney AOR) advice needed

25 just seems cold to me. My HT-40's EL-34's were 401DCV and 402DCV respectively on the plate voltage and just popping the Tungsols in in place of the stock TADs, the bias voltage was 32. The board itself though specifies 50mv so that's what I shot for. Maybe recheck your math? -Rod-

His math's good, 27.9 is 70% with a 501v plate voltage. I'm thinking bad tube(s) as well.

 

[USAPatriot]

Re: bias issues (Laney AOR) advice needed

His math's good, 27.9 is 70% with a 501v plate voltage. I'm thinking bad tube(s) as well.

LOL. I'm no good at math, haven't been since I laid down a bike when I was 15...umm...41 years ago according to my calculator :welcome:

Interesting thing...and you can do the math on my figures here if you like but the Tungsols I put in measured out at 401 and 402DCV. They initially biased at 32.2mV. Now, I forgot to the check the plate voltage on the TAD's before I took them out but I did check the bias on them and they were at 51.3 I think. I have a photo somewhere. Too hot, even by Blackstar's own specs which pretty clearly says 50Mv. So that's where I biased the Tungsols out, at 50.1. If you do 'the math' that 50Mv is way high. Strange but true and the HT-40 certainly isn't red plating or sounding over gained or doing anything unusual. In the case of this particular amp, tubes sitting there sputtering is really odd. All bad? Or just not enough juice to rev them up properly? The amp is likely, hopefully, setting the plate voltage and I think some research is in order to find out what it's supposed to be, plate voltage and bias both and set the bias according to specs rather than math. JMO! -Rod-

 

[glassman]

Re: bias issues (Laney AOR) advice needed

I re-biased and turned it on to find that it played again, only to have it fart out during the session after--again. After the fart out the amp sends no signal to the speakers, but when i pluck the guitar the tubes glow blue in the center and emit some noise.

That is really not a good sign.

The first thing I'd check would be the speaker connections, then the impedance selector. Also check the HV fuse (1 amp).

Pull 3 of the power tubes and see if you have output. Move that tube around to other sockets and sub in different output tubes to verify which sockets work and which tubes work. At least one combination should produce sound; if it doesn't, you may have witnessed the death of your output transformer. I would definitely have this verified by a tech before throwing expensive parts at it.

One way to check if you have a blown output transformer using your bias probe is to plug in your guitar and hit a chord; if the transformer has given up the ghost, the current will jump from bias (25mA in your case) to 180mA+ with little or no sound output. If the current stays right around your bias setting, you have an open screen resistor. Other possibilities exist but these are the most common. Good luck.