Laney valve amp, rebiasing

[stacman]

A mate at work has a Laney AOR 50 watt protube valve amplifier,
The pre amp has 4 x ECC83 and the power amp has 2 x EL34 valves.
He has ordered some valves as a replacement kit from a supplier who sends them stated as matched sets.
The company sending the valves says that valve number 4 is the phase splitter but Laneys circuit diagram shows it position 3.
Now I've told him that you can't just remove the old valves and replace them with the new ones as they will need to be re-biased, so is this a job for a pro, or fairly straight forward with a decent multi meter?
The odd thing noticed on the back of the amp it has a plate stating it is a 100 watt head, but that version has 4 x EL34, where this one only has 2,
It looks though the amp has been down rated to a 50watt head, there is the holes where the 2 extra valves should be, but there are no valve holders, just the holes,
The amp was bought second hand a few years ago sold as a 50watt head.
Any advice from you valve amp specialist would be appreciated, I've told him to hang fire a bit before proceeding, I don't want him making an expensive mistake,
Regards, Alan

[williamsunique]

I used to repair such amps for a recording studio. I always set the bias after changing the output valves, although I heard from other owners of amps that they didn't and no harm ever happened to their amps. Setting the bias is something that should only be attempted by someone who knows what they are doing. There are high voltages involved.

Paul.

[stevehertz]

Only the two EL34 output valves need re-biassing. Preamp valves do not need biassing. The purpose of the procedure is to set the standing current in each valve, the 'tickover' current while the amp is powered up but not being driven, used. If the amp only has one adjustment pot for setting bias, then the two valves should be purchased as 'matched', from a reputable supplier. This adjustment can sometimes be a fixed resistor on older amps, or it may even be two pots. In the latter case matched valves are not required as each will have its own adjustment. Setting bias is not a particularly difficult thing to do, but not so easy to explain to someone with limited electronics knowledge especially if they don't have the required test equipment, some of which is special for the procedure. Best to get a known, trusted, qualified person to do it. Beware! a lot of music shops may claim to be able to perform re-biassing, but call me a sceptic, I have my doubts. More info including procedure can be found here: https://www.google.com/search?q=bias...obile&ie=UTF-8

Please be aware that there are high DC and AC voltages in that amp that can be lethal.

[McMurdo]

looking at the diagrams the 2-valve 50W has an HT+ of 398V and the 4-valve has an HT+ of 458V. I suppose so long as you aren't running a 100W unit with only 2 valves in it, you'll be ok.
Looking at the 50 and 100 chassis decks, the 50W is a 100W deck with 2 big rubber bungs where the other valves would go.
But the output W is clearly marked
And I would definitely check (at least) the bias, modern valves vary wildly between batches and guitarists are very picky (no pun intended) about how the output stage sounds, re - under bias.

[stacman]

Thanks guys, It's the reason I won't touch valve equipment, The voltages involved, and a lack of experience with valve gear, Just wondered if any one local might be available?
I live in Sheffield, lad who's amp it is lives Barnsley, but works with me in Sheff, I know he wouldn't mind travelling a little way to get this done correctly.

[stacman]

Thanks Kevin, That's the odd thing, I've only seen a photo of the back of his amp, obviously hasn't brought it into work, but it definitely states 100watt,
as per the pic, there is no bungs in the missing holes where the valves would be though.

[dougietamson]

Originally Posted by stacman

Thanks Kevin, That's the odd thing, I've only seen a photo of the back of his amp, obviously hasn't brought it into work, but it definitely states 100watt,
as per the pic, there is no bungs in the missing holes where the valves would be though.

For guitar amps where low distortion is not a design goal it is 'possible' to run a 4 x EL34 100W power amp with 2 x EL34, ie 2 valves pulled out for half power but could cause damage/shorter service life.

But!

The 2 'outer' or 2 'inner' valves are removed, not 2 from the same side.

AND...

the bias voltage is adjusted, commonly the bias voltage is set to result in 70% max anode current while at idle.

AND...

you set the output impedance to double what it was with 4 valves, eg before if it was 8Ω then set it to 16Ω.

It could be that the Laney amp you have is a 'parts build' using the chassis from a 100W amp and the parts from a 50W.

Doug.

[stacman]

Hi Guys, I have a picture of the rear of the actual amp, It doesn't say Protube 100 where the logo should be, but it looks like the 100 marking has been scrubbed off, so we assume it has been downrated to a 50 watt head, any body know anywhere local that is recommended to assist, Regards, Alan

[trobbins]

The AOR protube 50 schematic (original and V2) shows the output stage cathodes are hard-wired to 0V, and there is a single fixed bias voltage adjust pot. A service shop would likely use a bias socket based tool (assuming the head-height is sufficient).

If it was my amp, I'd insert cathode current sense resistors and make them available at the rear for maintenance checking, as IMHO that is safer and quicker.

[tritone]

Originally Posted by dougietamson

AND...

you set the output impedance to double what it was with 4 valves, eg before if it was 8Ω then set it to 16Ω.

Doug.

H'mmm, it's the other way, you set the impedance selector to half of what it was with 4 valves to match your speaker. eg; if it was set at 8Ω with the four valves installed, and you pull two valves out, then you set the impedance selector to the 4Ω setting.

It would be worth seeing a couple of pics inside the chassis of this amp if possible.

[stacman]

I'll have a word tomorrow at work to see if a pic of the underside circuit can be taken.

[tritone]

Originally Posted by stacman

I'll have a word tomorrow at work to see if a pic of the underside circuit can be taken.

Yea, A few would be good if you can do that and it's not too much bother. Just want to see what the story is with the two absent valves and the 100W label is all about, it's possible that it may have been modded.

[dougietamson]

Originally Posted by tritone

H'mmm, it's the other way, you set the impedance selector to half of what it was with 4 valves to match your speaker. eg; if it was set at 8Ω with the four valves installed, and you pull two valves out, then you set the impedance selector to the 4Ω setting.

It would be worth seeing a couple of pics inside the chassis of this amp if possible.

Doh, yes, my bad

Pull 2 valves, the remaining 2 need to 'see' double the reflected load.

[Backtoreality]

Hi folks,
Having had fun with a Vox AC30 I agreed to have a look at this amplifier. I was hoping to be able to test it with the old valves in place but they had been removed.
In preparation I consulted You Tube and made a valve base tester from a dead 6K8G and an IO socket so I could check the anode voltage and current at the same time.
I've put the new valves into the amp after checking the circuit and determining that V4 is indeed the phase inverter. The new balanced ECC83 has gone in there. The 2 EL34s were matched by the supplier and both have 30 and 5.2 written on the top.

I've used an 8 ohm speaker so I had to change the impedance setting on the amp.
Testing the valves V 5 reads 405V 16.8mA and V6 reads 406V 13.1mA.
All of the controls are set to 0.
Any thoughts welcome!
David