Biasing a valve amp

[AJSki2fly]

Thanks again for all the replies, your advice certainly isn't taken for granted.
The four valves an sockets were labelled v1 v2 v3 v4, the previous owner had taken care to make things as fool proof as possible. They obviously hadn't foreseen me. I haven't touched the bias pot yet as I was conscious of my own ignorance, I this something that can be done by ear?

I have looked at the pictures and cannot see where the bias pot is on the photos. If it is the one to adjust the voltage between the two output valves then the answer to can it be done by ear is yes, but this is subjective. When I had a capacitor replaced on one of mine the chap that did it check the bias by putting a voltmeter across the speaker outputs and adjusting the bias pot down to as low as it would go. This is very low voltage and you need an accurate meter to do so. I did adjust mine a couple of times once by ear which is surprisingly accurate. I also did so by downloading a decimal meter onto my iPhone and placing it about 1 foot from the speaker, I then adjusted the pot until the meter registered the lowest reading, you obviously need a quiet room and house.

[Firebottle]

Can you take a close up picture of the components on the output valve sockets (with the power off for safety).

[Blackdog]

I had a capacitor replaced on one of mine the chap that did it check the bias by putting a voltmeter across the speaker outputs and adjusting the bias pot down to as low as it would go. This is very low voltage and you need an accurate meter to do so. I did adjust mine a couple of times once by ear which is surprisingly accurate. I also did so by downloading a decimal meter onto my iPhone and placing it about 1 foot from the speaker, I then adjusted the pot until the meter registered the lowest reading, you obviously need a quiet room and house.

That is not bias. At best the is offset or balance. It does not change the operating point of the tubes, which is what you need to accomplish with biasing. You cannot do it by ear.
I agree with Alan. We need more info. Turn the amps off and take pictures of the bottom, particularly the components around the output tubes.

[Dorisandjess]

I have been reluctant to remove the wooden base encasing the chassis as it was a very tight fit.
Despite a great deal of cringing came out easier than expected
I am also able to identify what exactly they are, Audio Innovations 800 5watt class A

[AJSki2fly]

That is not bias. At best the is offset or balance. It does not change the operating point of the tubes, which is what you need to accomplish with biasing. You cannot do it by ear.
I agree with Alan. We need more info. Turn the amps off and take pictures of the bottom, particularly the components around the output tubes.

Ok then I was miss-informed or did not understand what the engineer told me when I watched him do it. My understanding was is adjusted the voltage to the output valves until they were as close as possible and stopped hum.

[Lawrence001]

I am also able to identify what exactly they are, Audio Innovations 800 5watt class A

I'm not sure they are as that's an EL34 stereo power amp but I found this on PFM

Although a few French self builders were following an underground Japanese trend for amplifiers using Directly Heated triodes, Audio Innovations were the first company (in modern times) to commercially manufacture/market DHT power amps when they launched the 6B4G equipped First Audio (7.5W) and Second Audio amplifiers (15W). At that time, in 1987, it was unheard of to be trying to sell amplifiers with such low power output but both models received good critical acclaim often being partnered with the Snell range of loudspeakers the company were also distributing.

https://pinkfishmedia.net/forum/thre...ve-amps.47929/

Sent from my POT-LX1 using Tapatalk

[Firebottle]

Difficult to see with 100% certainty but it looks like it is an auto bias circuit.

On the left hand valve base there is a large green ceramic resistor with a larger diameter blue electrolytic capacitor connected across it.
On the other output valve base the second resistor/capacitor is connected to a potentiometer above (in the photo) that I guess is adjusted to minimise hum.

The blue capacitors appear to be touching the green ceramic resistors, this is not good as they will feel the heat from the resistors. Try to force a small gap between them.

[Dorisandjess]

Mmmmm I appear to be in a little trouble then, does anyone know of an amp tech in the Northeast?

[Hudz]

Try having a chat with Simon at Hifisound (2ndhandhifi)in Stockton, I'm sure he could point you in the right direction, if not then try The Maughan Amplifier Company in Wallsend, bit of a trek but worth it.

[Moneypit]

This chap may be worth a try. He diagnosed and repaired my Alamarro A318 a couple of years ago ( faulty valve base ) for a very reasonable charge. He's based in York , and his website is

http://www.radtronix.co.uk/electroni...audio-sevices/.

Despite appearing not to have been updated since 2011 , the website is live. Highly recommend him.