Orange TH30 with bias issue

[blindboybenton]

Hello all

This orange has had me puzzled .
Came in for sounding bad. Found on the EL84 been gassed. Tested all the pre amp tubes and replaced all the EL84
Sounded good. On Bias test i found that one test point was 10.40v TP6 but TP7 is at 8.20v
So i tried 2 other new sets of new tubes all the same
So tested with 2 x New sets of TAD EL84 and 1 x New set of Marshall selected JJ EL84 ( all New and Matched )
All the same issue.. Checked over the board for soldering or cracks on the pcb. Tube bases look fine
Orange_TH-30 Twin Channel Amplifier.pdf

So i changed both 220uf and the 120R, didn't expect there to be any change and there wasn't
Ive had the amp on and working all sounds ok, But there is a problem there

Taken some voltages reads of bias and Plate

Full power with 4 tubes

V6 10.4v - 313v
V7 8.3v -314v
V8 10.4v - 309v
V9 8.3v - 309v

Full power with 2 tubes

V6 21v - 330v
V7 8.7v -330v
V8 21v - 324v
V9 8.3v - 324v
I have checked the 2-4 tube switch that's ok. all outputs are working, with a brief test all sounded ok

Im a little baffled to whats going on


many thanks

[pdf64]

Does the schematic match the board? I can’t find TP6 on it, TP7 is on heater circuit leg, and there are two TP8s.

[blindboybenton]

well spotted, They are labelled at TP6 and TP7 on the board but i have just seen they are labelled different on the map
I think its a typo as ive seen spelling mistakes on it
The test points are correct and all connect to the 120R resistors.
I had to call orange this morning about my orange valve tester being faulty. i told him the fault. he said he's going to ask the designer, but he's on hols till next week
I will call them later/ tomorrow about the correct map
bbb

[Jazz P Bass]

Is it me or is that a weird output section?

Output tubes 6/ 7 & 8/ 9 have there plates tied together.
But the Cathode connections are staggered.
6/ 8 & 7 /9.

[blindboybenton]

yes its a bit of a weirdo one . nasty

[nickb]

yes its a bit of a weirdo one . nasty

It's just to accommodate the 2 vs 4 tube switch. Note that Orange chose to (correctly) change to load impedance at the same time by switching the secondaries.

[blindboybenton]

Just been told by orange that the schematic is correct. There is only one for this model. It must be a typo mistake.
I have just removed the board for inspection underneath. all perfect
The tube bases are the porcelain type. Thinking of replacing all four.
Orange tech says the guy who designed the amp is on holiday this week and will be back on monday. i'm to call be then if not resolved
Hopefully i will get another amp with 4 x EL84 in before then so i try the new tubes in, but with 3 x new matched sets all the same,



BBB

[nickb]

well spotted, They are labelled at TP6 and TP7 on the board but i have just seen they are labelled different on the map
I think its a typo as ive seen spelling mistakes on it
The test points are correct and all connect to the 120R resistors.
I had to call orange this morning about my orange valve tester being faulty. i told him the fault. he said he's going to ask the designer, but he's on hols till next week
I will call them later/ tomorrow about the correct map
bbb

Have you checked that the ground side of the 120R's are actually ground?
Tested for a leaky 220uF cap?

[blindboybenton]

Yes nick. I changed the resistors and caps, one of the first things i did after i changed the tubes out.
BBB

[nickb]

Yes nick. I changed the resistors and caps, one of the first things i did after i changed the tubes out.
BBB

...and measured what is happening a the grounds when the problem is happening with a DVM?

[blindboybenton]

Hi Nick. I have removed the board at present and still not back in the chassis. I have removed the bias components . Going to refit them and ad fit a few tinned copper wires to the tracks. as i suspect the amp has been dropped at some point. I will change the tube bases possibly as well

I can't remember if i checked the grounds with a DMM when on. I'm sure i did. I also removed the 120R and 220uf and made up 2 more and soldered directly to the tube base and to ground . it was still the same.
I keep thinking it must be a tube issue but with 3 new sets and my test used set in always the same
Might have to wait to speak to orange on monday, when the designer is back

Im baffled. BBB

[nickb]

You can always throw it in the back of the car and trundle on over...

[Pedro Vecino]

I have a TH30 here and two of the four EL84 operate at lower level than the other two. I have used a new quartet of EL84 TAD and I have measured the intensity directly through the plate in each one. There is an appreciable difference (8 or 10 mA approximately).

[Dave H]

Do the screen voltages and screen resistor values measure the same for all four tubes?

[blindboybenton]

I would look to nick. But i don't own a car at present. I have no need for one. my workshop is based 2 doors away from where i work.
Do miss the car on a sunday, fishing or long walk over those purbeck hills , but hayhoe