Asteroid 60 with 6550 output tubes

[passfan]

Has anyone tried this ? Right now I am running 6l6's at a 490v B+ at 45ma's. 6550's seem to share the same Pinouts so I'm foaming at the mouth to try the matched pair I have in the drawer

[martin manning]

You would need to reduce the grid leak resistors to 100k or less, and really the screen voltage should be down around 300V to go with the 490V on the plates.

[M Fowler]

You would need to reduce the grid leak resistors to 100k or less, and really the screen voltage should be down around 300V to go with the 490V on the plates.

Martin 150k isn't low enough for the 6550 tube swap?

Also, when not having separate lower voltage tap on the PT what is the best or perhaps easiest way to lower the screen voltage down that low to 300v?

Mark

[Roe]

I tried kt88s - they worked fine. But I prefer the sound of el34s, 6ca7s and 6l6GCs

[martin manning]

You would need to reduce the grid leak resistors to 100k or less, and really the screen voltage should be down around 300V to go with the 490V on the plates.

Martin 150k isn't low enough for the 6550 tube swap?

Also, when not having separate lower voltage tap on the PT what is the best or perhaps easiest way to lower the screen voltage down that low to 300v?

Mark

The 6550 data sheet (GE) says 50k for fixed bias, so I figured 2x the recommendation is close enough for R&R.

The power supply is really not set up for the screen voltage needs of this type of tube. The load line will be way below the knee if the plate and screen voltages are similar. I don't know what the primary Z is, but 350V on the screens would be good if it is a Marshall-ish 3k4. Probably why Roe reports KT88's didn't sound too good?

[M Fowler]

I just use EL34's in my Asteroids and have not experimented with other tubes.

But in many other amps I have tried 6550"s, KT88's and KT120's.

[martin manning]

At lower plate voltages the match is better. With 400V on the plates and screens a 2k load looks pretty good, and a pair of 6550 or KT88 would do maybe 80W.

[passfan]

Thanks for the info. I may give them a listen but I'm really not expecting much now

[stephenl]

Also, when not having separate lower voltage tap on the PT what is the best or perhaps easiest way to lower the screen voltage down that low to 300v?
Mark

You can use a MosFet stabiliser referenced with zener diodes or voltage divider to drop the voltage. Since the current is so low you can use a completely insulated package and bolt it directly to the chassis. Its a good idea to add a current limit circuit.

You could also create another supply rail right off the rectifier that is choke fed. The choke fed supply will have a much lower dc level than the cap fed supply. Sorry, the formula for Vdc from a choke fed supply is escaping me right now....

[Roe]

Also, when not having separate lower voltage tap on the PT what is the best or perhaps easiest way to lower the screen voltage down that low to 300v?
Mark

You could also create another supply rail right off the rectifier that is choke fed. The choke fed supply will have a much lower dc level than the cap fed supply. Sorry, the formula for Vdc from a choke fed supply is escaping me right now....

yes, see here: http://imagizer.imageshack.us/a/img692/1581/6qz3.png, http://www.ax84.com/drums/bbs/dm.php?thread=480502 and here http://www.ax84.com/bbs/dm.php?thread=345517

[M Fowler]

Thanks guys I will give that a try sometime.