Laney GH100L bias question

[HumBuzzThing]

Hello,
I am trying to bias a Laney GH100L for a friend.
It was suppose to be a simple install of new EL34 tubes and a bias adjustment.

I should mention that the circuit board is version 9050_2.
I have a schematic for 9050_3.

First off, the "Matched" tube set was not a great match.
Not ideal, but would work if I was able to bias it a bit colder.

The readings from my bias probes were 42mA, 33mA, 41mA, and 34mA.
That was as far as I could adjust it with the trimpot.
Pin 5 on the EL34 tubes all measured -42V.
Plate voltage was 461V.

The bias adjust circuit has R33 - 27K and the Trimpot (PS1) is 22K.
Total bias range is 26.7K to 45.2K.

I know that components C18, R39 as well as the resistors R34 and R38 (PI) have
an effect on the bias circuit as well.

The question I have is, is it as simple as increasing the R33 value from 27K to 33K?
Or would I have to adjust(change) some of the other components I mentioned.

Hoping someone can suggest the best way to deal with this.
Cheers.

[Latole]

EL34 are 25 watts power tube
Bais ; at 461 volts X ,042 ma = 19,36 watts
It is more than 70% power dissipation. I never bias more the 60% , often between 50 an 60 %

Tubes matched under 10 ma are ok ,no need to be a 0 ma

Showing schematic may help.

To have colder bias ; a must
Raise value of R33 ; 27 K
or / and low value for R40

[HumBuzzThing]

Thanks Latole,
Adding a resistor in parallel with R40 would be the easiest option.
Saves me having to desolder some wires to remove the circuit board to change R33.
If my math is correct a 150K in parallel with R40 would give me 19K, a 220K would give 20K.

Am I correct in thinking that R40 shifts the range and that R33 & PS1 work to set a value within that range?
Thanks again for pointing me at R40. I was looking at R39 (15K).
Cheers.

[SEL49]

Look closely at the entire schematic. There is a bias switch that allows you to run EL34 at -35V or 5881s at -45v. The bias switch puts R41 in parallel with R40 to provide the higher voltage required for 5881s.

Ensure the switch is in the correct position.

Laney GH100L schematic

[AlNewman]

Look closely at the entire schematic. There is a bias switch that allows you to run EL34 at -35V or 5881s at -45v. The bias switch puts R41 in parallel with R40 to provide the higher voltage required for 5881s.

Ensure the switch is in the correct position.

Laney GH100L schematic

That's a good catch.

[HumBuzzThing]

Hi SEL49,
Yes , the bias switch is set to EL34.
R40(22K) and R41(33K) in parallel would be 13.2K. Probable too much for EL34s.

Basically all I am trying to do is get the hotter tubes down to a reasonable bias level.
The readings from my bias probes were 42mA, 33mA, 41mA, and 34mA.
Not a very good matched set, and no room left to bias it colder.

[SEL49]

Set the bias switch to the 5881 position. The higher bias voltage will allow you to adjust the hot tubes cooler. But with that much difference between the tubes, you may just need a fresh set of better matched tubes.

[Latole]

  But with that much difference between the tubes, you may just need a fresh set of better matched tubes.

I will not say that.
Anyway it is not my money

[HumBuzzThing]

Hi,
Interesting, I flipped the bias switch to 6L6/5881.
It worked.
Wall outlet was 119/120.
Plate was 461V
Measured P31 and P32 =  -45
After warming up tubes finally settled at 26.5mA, 33.3mA, 33.6mA, and 25.1.
This was a test to confirm that I could get the tubes in range.
So if you have a crappy set of matched tubes this could save you if you have no options.

My friend has ordered a new set, apex matched and pre-burned.
I will flip the switch back to EL34 and see what I get with the new set when it arrives.
Thanks for the tip SEL49.
Cheers.

[Willabe]

After warming up tubes finally settled at 26.5mA, 33.3mA, 33.6mA, and 25.1.

I know your friend ordered a new matched quad set of tubes, but, flip the 26.5mA with the 33.3mA and flip the 25.1mA with the 33.6mA tube.

Any closer? Sometimes that helps a little more.

 

[HumBuzzThing]

Hi Willabe,
By the time I saw your post I had already pulled those tubes.
Thanks for the tip. I will have to remember that.
My friend also has a couple of 50W amps that he can use these for. Two matched sets :)

The new tubes arrived yesterday and he is dropping them off today.
Cheers.

[Willabe]

It will work for just 2 tubes also.

Set the bias, write down the current draw on each tube, then flip the 2 tubes socket for socket.

Take readings again, if a little better leave them, if not put them back the way they were. 

I always do this.

[shattered]

Hi, I have a similar experience with a GH100L. It came really hot with bias set to 57ma or so (didn't measure plate voltage yet), turning the bias pot fully counterclockwise the minimum bias setting I got is 37mA-34mA (running on just two tubes).

[HumBuzzThing]

Hi shattered,
Like SEL49 said, try flipping the bias switch to 6L6/5881. That should get you lower.
I measure the wall voltage, plate voltage and use bias probes to see the current on each tube.
I then use a variac to check the plate and currents at 120V, 122V, and 124V.

So you are running two tubes?
I have heard that if you are running at 50W (two tubes) they should be in sockets V5 and V8,
and that if you are connecting an 8 Ohm cab you would plug it into the 4 Ohm output.
Can anyone here confirm that, I've never done that so I don't know.
Cheers.

[SEL49]

I have heard that if you are running at 50W (two tubes) they should be in sockets V5 and V8,
and that if you are connecting an 8 Ohm cab you would plug it into the 4 Ohm output.
Can anyone here confirm that, I've never done that so I don't know.

That's correct.

[shattered]

Hi HumBuzzThing, thanks for the tip. I got a proper DMM (rated 1kV CAT III) so I'll be able to measure voltages from now on.

Power tubes in this amp are installed at V5 and V7; as far as I know (and please correct me if I'm wrong) you can put one in V5 or V6 and the other one in V7 or V8. A 8 ohm speaker is connected to the 1x4 ohm output from the amp.

[Willabe]

Power tubes in this amp are installed at V5 and V7; as far as I know (and please correct me if I'm wrong) you can put one in V5 or V6 and the other one in V7 or V8.

Yes.

[shattered]

Hoping it's not a problem to bump a 2 month old thread I just want to report success flipping the bias switch to 5881. I contacted Laney first commenting my problem and they said "it is perfectly safe to swap over the bias switch as long as you are setting it correctly".

I replaced the existing tubes with a quartet of TAD EL34-STR REDBASE with similar transconductance (just a bit hotter) and had to put the bias pot past noon to achieve ~60% dissipation. Plate voltage was 479V and now bias is ~30-32mA.

Thank you for your help!