Voltage and bias questions

[Rob Livesey]

Hi Chaps,

Just setting up the Ceriatone Express and I have a question about bias.

I have a pair of XF2 Mullard EL34's, about 2ma apart in terms of current draw. I have about 365v on the plates at 42 ma current draw. My calculations tell me I could go up to 48ma current draw and still be within 70% of max dissipation at idle.

Where do you guys bias your EL34's and what's your plate voltage??

Cheers,
Rob.

[Ron]

Rob -

I don't think you will have a problem with plate dissipation at that bias point. I think you can go to about 53ma/tube and be at 70% of the 27.5-watts max plate dissipation (I'm not sure what the precise limit for Mullards is).

If you started at 50ma/tube, you can adjust downward in idle plate current for the best sound and leave it there.

Remember, the disspation limit assumes open, still air at a modest temperature; a confined cabinet or intentional air flow over the tubes will change the limit.

Ron

[Allynmey]

Hi Rob, congrats on the new amp. 365V on the plates seems a little low to me. Most Tw Express amps and clones were putting about 400V on the plate give or take a few. I think Ken recommended 30V Bias measured on the grids and leave it there. Of course, try and see what sounds best to you.

Allyn

[Rob Livesey]

Hi Rob, congrats on the new amp. 365V on the plates seems a little low to me. Most Tw Express amps and clones were putting about 400V on the plate give or take a few. I think Ken recommended 30V Bias measured on the grids and leave it there. Of course, try and see what sounds best to you.

Allyn

Hi Chaps, Thanks for the input. I'll go a bit higher than the 42 ma I've got it at at the moment and see how it sounds.

365v seems low to me too, maybe I'll talk to Nik about it. It doesn't seem to adversly affect the amp tho, man this thing is LOUD !! I mean LOUD. It may be louder than my Train 45 but I'll check that out when I can really open it up.

One interesting observation. On first firing up, I thought I had either a problem tube or a parasitic going on, couldn't get the volume past about 10 o'clock without noise and whining. Changed tubes, bias, put 6V6's in there, nah, must be a parasitic oscillation problem. Then.........tipped the amp on it's end and it vanished, ah ha !! I stuck a sheet of aluminium between the amp and the cab and all is well, no adverse noises and the amp will crank to the top.

Ahs anyone else had this going on, do you guys put shielding in the bottom of your head cabinets?

Cheers,
Rob.

[Allynmey]

Oh yes!!!! You can't lay a TW chassis open on a cabinet without all hell breaking loose. Francesca was like that too! You need to use a metal shield.

Allyn

[ScottW]

Rob -

I don't think you will have a problem with plate dissipation at that bias point. I think you can go to about 53ma/tube and be at 70% of the 27.5-watts max plate dissipation (I'm not sure what the precise limit for Mullards is).
Ron

All the data sheets I've seen show the EL34 max plate dissipation at 25W at idle and 27.5W when the grid is drawing current. Since we normally set bias at idle, I think it's better to do the math with max plate dissipation at 25W max. I suppose you could bias the tube as hot as you want, though, as long as the plates don't go cherry....

[Ron]

The 70% factor is a rule of thumb. Some thumbs are larger than others!

[philmanatee]

I've seen a few other posts and can attest to this myself, you can bias an express a lot hotter than other amps without having your plates go cherry red. I swapped some tubes in mine without checking the bias, did a gig and checked them later and they were drawing 70ma. I brought them down to about 42ma at 390v and haven't messed with tubes for a couple months now. I know if I biased any other amp that hot the tubes would be red before I even played it. I'm not recommending you try biasing that hot, it just is a bit more forgiving. Phil

[Rob Livesey]

Quick update.

Thanks to Nik at Ceriatone I now have 399v on the plates of the EL34's.

I've spent the evening swapping Mullard ECC83's in and out of the amp and now have the definitive set of three.

I STILL have not been able to crank the amp in the house, I'm using a hotplate set up as loud as I can get away with without the neighbours banging down my door. Having played the amp for a few hours now I can say that I'm very very happy with it, it truely is a beast. All the touch sensitivity is there, the note bloom, the clean backed off, the wail when you roll up the volume. I'm using a 2002 R7 at the moment, through a '70 basketweave cab with '77 G12M's..........oh baby.

Many thanks to Nik at Ceriatone again. He solved my little voltage query in less than 24 hours.

Cheers,
Rob.

[philmanatee]

How does it compare to the Train 45? Phil

[Rob Livesey]

How does it compare to the Train 45? Phil

I have not done a direct comparison yet. Without playing one then the other at volume, I would just have to say that they are very similar, but I'm feeling that the Ceriatone amp may be louder and a little more aggressive than the Fuchs amp, which I didn't think was possible !! The Fuchs amp may be smoother and certainly has a lower noise floor than the Ceriatone amp, but I spoke with Nik about this and I did want the exact Express clone warts and all, Nik explained that there are mods to take out noise etc but I just wanted the amp to be pure Trainwreck to see what it was all about.

I'm going to struggle to choose one over the other when it comes to gig time, both will be with me though as I don't travel without two amps in tow.

I haven't played with tubes in the Fuchs yet either, so the Ceriatone amp has got the added benefit of a full set of Mullards and the Fuchs has it's stock JJ's and Tung-Sol's.

Andy, if you are reading this, please would you tell me the positions on the chassis of V1, V2 and V3 in the Fuchs amp, they aren't laid out in a linear fashion like the Express?

Cheers,
Rob.