Sunn Sceptre

[Cups]

I've repaired a Sunn Sceptre. It's got a solid state rect instead of a tube one (can't find a schem for ss rect) B+ on the schem shows 500v. So lets' assume another 50v or so with ss rect? I'm measuring 560v for B+. Not worried about this because 6550 can take 600v on the plates. The problem is that it's an Ultra Linear amp and the screens are seeing 560v as well. The 6550 is rated for 400v on the screens (G2) 

I'm thinking it's a desing flaw as even with 500v g2 would be stressed.

Should I put some resistance on g2?(not that I could bring the voltage down to 400v with the resistor but it might add some protection) Should I leave well enough alone? It's not my amp or I might consider giving the screens it's own node.

What do you guys think?

[plexi50]

With tubes made these days i would put at least 1K 5 watters on the tube screen pins. Can hurt. Would be some regulation vs none

[Cups]

I don't have much experience with Ultra Linear and was wondering if screen resistors are used in that kind of topology. Havn't traditionally seen them with UL.

[plexi50]

Maybe some will chime in that knows a little more about that than i do. I have bypassed the ultalinear taps before and just wired the screens off the B+ node and used screen resistors.

[sluckey]

I have a Sceptre also. Just put a GZ34 in it like it should have and don't worry about the 500+ volts on the plates and screens. It's OK. And don't put any resistors on the screens either. They are fine.

PS... Be sure your first and second filter caps are rated 600VDC or better.

[Cups]

I am not punching another hole in the chassis, thank you very much. 

Like I said, it's not mine and I don't think the guy wants to pay an arm and a leg to get this fixed up. It's sounding fine. I'm just worried the amp will eat up 6550s (expensive tubes)

[kagliostro]

I've a schematic that uses GU50 in UL (560v on plates and 295v on G2)

it uses a mosfet + zener circuit between UL tap and G2 as to lower the voltage

but I really don't know much more about this

K

[sluckey]

I am not punching another hole in the chassis, thank you very much.

Sorry. I thought you meant there was a plugin SS rectifier in the socket. I don't recall ever seeing any with individual rectifier diodes. Did that come in the later years?

[DummyLoad]

I'm just worried the amp will eat up 6550s (expensive tubes)

just stay away from the chinese junk - e.g. valve art, etc. winged C and sovtek hold up.

--DL

[navdave]

I have bass amp I made using Marshall Major iron from Heyboer and the b+ is almost 700vdc on the plates and screens. The amp has a quad of Sovtek 6550's and is operating in UL mode since 2008 with no problems yet... I used 3k 5 watt resisters for the screen taps but that doesn't do much to knock it down. I have the amp bias at 30mA per tube so I say let it roll it will be fine.

[SoundmasterG]

Many of the Sunn tube amps after around 1972 used solid state rectifiers stock with no hole for a tube rectifier. These do have higher voltages.

You may have to add some caps in series in the first two stages (along with parallel resistors) in order to get enough voltage rating.

Greg

[Cups]

I upped the capacitance a bit by putting two 100uf/350v in series for 50uf 700v. Did that for the first three nodes. The amp hums a lot less.

Gonna do the ol' zener in the center tap thing and add resistance to the screens. These tubes are way too hot and started to smell like something was cooking. Better to err on the side of caution, and I don't want to work on this amp anymore. 

[SoundmasterG]

Did you adjust the bias in the correct range? I've found in the Sunns that biasing on the cooler side of things still sounds fine. That said, the 2000S did use resistors in the screens and plates so it can help in extreme situations.

Greg