Biasing a valve amp

[Dorisandjess]

Hi
I am seeking advice regarding the setup of my first valve equipment. I bought a pair of 6g4g monoblocks from Vinylnvalves almost two years ago and embarrassing have only just gotten around to setting them up. There is some hum from the amps even after warm up and I understand that this could be due to the bias not being set correctly. I have read/watched various biasing guides online but they reference parts that are not present on my amps. I have an 1 external pot for adjustment but two output valves, does this alter the idling current to both or alter the balance between the two? Is a shunt probe (valve seat insert) necessary as there are no dedicated bias points? Sorry if this seems like elementary stuff, but I get the impression that I may break something/someone/me if I start poking around in an uninformed fashion.
Any help would be greatly appreciated
Steve

[Blackdog]

First off, be careful. Not having external bias points means you must open up the amps to get at the points. There will be lethal voltages, so if you are not comfortable doing this get help from someone who is familiar with tube amps.
Having one pot for each pair is sub optimal, and means your tubes need to be matched for current gain. Regardless of bias if the tubes aren't matched you may get hum due to imbalance of the output transformer.
Before you can continue you need to know the recommended procedure, bias points and the current.

Did the manufacturer give you a bias procedure? If not contact them for it. If they won't or can't then it needs to calculated and this opens a whole new set of procedures.

[Dorisandjess]

Thanks for the quick supply Dan
The bias pot is external which is the only reason I am attempting to do this myself. There is no-one in the Teesside area that advertises amp repairs, plus I think knowing how to bias the amp will pay dividends in the long run. The amp appears to be made by an enthusiast sand as a result there is no manual. The previous owner carefully labelled each valve with its corresponding socket which indicates that they are indeed matched pairs. I suppose my question is then is there a generic biasing procedure for TWO power valves and ONE bias adjustment pot and no bias points? and can it be explained to a numpty?

The specs for the 6b4g valves are as follows
http://www.r-type.org/pdfs/6b4g.pdf

[Ali Tait]

Is this an SE amp or PP?

[Ali Tait]

Are you sure this is a biasing point?

It may be a hum bucker. Does the hum alter if you twiddle it?

[Jazid]

Upload a picture before you do anything. We don't know if this is a monoblock, stereo, SET, PP, PSE, really it's just a shot in the dark so far


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[Dorisandjess]

It’s these
https://theartofsound.net/forum/showthread.php?56604-For-Sale-Pair-of-6B4G-mono-blocks&highlight=6b4g

[Lawrence001]

Given the history in that thread hopefully something can be worked out for you [emoji16]

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[AJSki2fly]

Hi
I am seeking advice regarding the setup of my first valve equipment. I bought a pair of 6g4g monoblocks from Vinylnvalves almost two years ago and embarrassing have only just gotten around to setting them up. There is some hum from the amps even after warm up and I understand that this could be due to the bias not being set correctly. I have read/watched various biasing guides online but they reference parts that are not present on my amps. I have an 1 external pot for adjustment but two output valves, does this alter the idling current to both or alter the balance between the two? Is a shunt probe (valve seat insert) necessary as there are no dedicated bias points? Sorry if this seems like elementary stuff, but I get the impression that I may break something/someone/me if I start poking around in an uninformed fashion.
Any help would be greatly appreciated
Steve

Just a thought you say you only just set them up, were the main output valves marked left and right and/or which amp they came from, or were they transported to your home with the valves left in the amps? If they were marked and you know which pairs are which make sure you have them in the correct sockets(left/right) as the bias would have been set up for them in that configuration.

If you have twiddled the hum pots then what I am about to suggest below may not work or be worth doing. Having had a pair of push/pull 300B's it can be tricky to get the lowest hum. But in my experience if you know which is the bias pot (possibly the one on the top between the two output valves) you should be able to hear it lower as you tweek it. If you can get advise form the designer then this would be best.

If you do what I suggest below whatever you do make sure you switch off before removing the valves and swapping them, and remember they get too hot to handle so take your time.

Swapping them may improve the hum issue, but is likely to be hit and miss unless you are methodical. There are several permutations as you have 4 valves and two amps, i.e. 3*2*2amps*2(left and right), so you could start with one amp and select one valve, mark each A,B,C and D. So the process would be put A in left, B in right and power on and assess asses the hum, switch off and put swap A and B from left to right, switch on and asses hum. Then A in left and C in right, and repeat as before, then with A and D. Then repeat with B and C, B and D, finally C and D. Hopefully you will find the pair in the correct configuration which are quietest. You will then have two valves for the other amp that can go in only two ways, and in theory one way should be relatively hum free.

If you already have twiddled the pots and did not not mark or know the original pot settings then it will be a matter or setting the bias and as others have said.

[Dorisandjess]

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?

[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
https://i.postimg.cc/59B51hxg/8-A27-FA38-88-B5-4005-A5-EF-EFCD1806-E332.jpg

[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/threads/audio-innovations-valve-amps.47929/

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[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/electronic-jobs/vintage-valvetube-audio-sevices/.

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

[Dorisandjess]

Thanks for the suggestions. I took the amps along with my entire hifi setup to HifiSound in Stockton. I am awaiting their diagnosis. I was fortunate to have a look around their showroom and had to be dragged away. When I pickup my amps a second look around awaits.