Tube bias and brand question (Bogner XTC Classic)...

[Red_Label]

I seem to have a knack for buying used tube amps with worn-out tubes in them. Last April I bought a Shiva that had tubes so worn-out, it has ZERO clean headroom. After replacing all the pre and power amp tubes (power tubes specifically), I had an awesome amp in mint condition. I'll be shipping that amp out in a few days to a buyer, with great, low hour tubes in it.

I recently bought an Ecstasy Classic and it appears that it's tubes are shot as well. Just doesn't sound right. The power tubes are =C= Svetlanas, and with the bias pot at full, they show 30-32ma -- which is what Bogner recommends (I think). But I can tell they are toast. So I threw a set of nearly new regular Svets in there today, and with the bias control at full, they were reading 17-20ma (the more it warmed-up, the closer it got to 20). They seem to sound pretty good, lots of presence there (the Winged Cs that came with the amp have very little presence -- the clean channel sounds pretty dark). It's not as smooth in the OD sound as I would expect, more roaring, raw Marshall than smooth Bogner.

So do you think the newish Svets I threw in there are too cold? Can that cause an issue with the amp? I will probably just buy a new set of Winged Cs anyways, but am curious about this, as Bogner says that regular Svetlanas are fine in the XTC as well as Winged Cs. I also replaced the preamp tubes with assorted ones I had laying around.

Anyways, I'm just curious as to the thoughts of others regarding running tubes at such a cold bias reading, as well as what tube brands others have had good luck with in the XTC, and the Classic in particular.

[Gearjoneser]

Re: Tube bias and brand question (Bogner XTC Classic)...

The factory bias is 35mA and maybe best at 36 - 38mA.

I've had Svets installed that wouldn't bias LOW enough, not high enough, so that sounds odd. My JJ EL-34's have always biased up in the middle of the bias pot's travel, so I've stuck with them since.

I don't know how old your amp is, but it could probably benefit from having someone inspect the amp for it's cold biasing on Svets. It's travel should be something like 30 - 40mA.

I'd hate to assume that the amp needs an instant checkup, but it's possible. It's odd to me that you got it at half price. You never know if he sold it because he was too lazy to fuss with it. Maybe it's one resistor away from being in mint condition?

[glassman]

Re: Tube bias and brand question (Bogner XTC Classic)...

There's cool bias and then there's cool tubes...cool tubes (ones with a PC rating on the lower end) fill out at lower bias settings and are generally a lot tighter than a hotter tube with a hotter bias. A cooler tube with a hotter bias can sound muddy. A hotter tube with a cooler bias can sound thin and shrill.

Manufacturer recommendations are generally based on a bogey tube that rests somewhere in the middle of the range. Mesa Rectos sit at 17 to 18mA with a new set of thier mid grade tubes.

Setting your Bogner where you have it set at now may sound very good and it certainly does no harm to the amp...the only effect will be longer tube life. The only issue I see is that you have run out of adjustment room to touch up the bias as the tubes wear.

[Red_Label]

Re: Tube bias and brand question (Bogner XTC Classic)...

The factory bias is 35mA and maybe best at 36 - 38mA.

I've had Svets installed that wouldn't bias LOW enough, not high enough, so that sounds odd. My JJ EL-34's have always biased up in the middle of the bias pot's travel, so I've stuck with them since.

I don't know how old your amp is, but it could probably benefit from having someone inspect the amp for it's cold biasing on Svets. It's travel should be something like 30 - 40mA.

I'd hate to assume that the amp needs an instant checkup, but it's possible. It's odd to me that you got it at half price. You never know if he sold it because he was too lazy to fuss with it. Maybe it's one resistor away from being in mint condition?

Thanks Joe. I know a very good tech and may just take it in to him to make sure everything looks good. Sounds pretty good, with either set of tubes in it, but I've had enough tube amps to know when tubes are worn-out and when they aren't running in their preferred bias range.

[Red_Label]

Re: Tube bias and brand question (Bogner XTC Classic)...

There's cool bias and then there's cool tubes...cool tubes (ones with a PC rating on the lower end) fill out at lower bias settings and are generally a lot tighter than a hotter tube with a hotter bias. A cooler tube with a hotter bias can sound muddy. A hotter tube with a cooler bias can sound thin and shrill.

Manufacturer recommendations are generally based on a bogey tube that rests somewhere in the middle of the range. Mesa Rectos sit at 17 to 18mA with a new set of thier mid grade tubes.

Setting your Bogner where you have it set at now may sound very good and it certainly does no harm to the amp...the only effect will be longer tube life. The only issue I see is that you have run out of adjustment room to touch up the bias as the tubes wear.

Good info. I will have to play with both sets of tubes some more tomorrow in light of your post just to make sure I everything straight.

[zzmoore]

Re: Tube bias and brand question (Bogner XTC Classic)...

What does the amp sound/play like if you turn the bias up so the tubes are drawing closer to 30 mA..?