two 6550 or KT88 tubes in an ODS?

[goodtone77]

I am going to build a ods but was thinking of using A set of fender twin reverb transformers with two 6550s or KT88s.

PT ratings
320-0-320 400ma
6.3 6 amps

OT Ratings
2k - 4 ohms

What do ya think?

thanks

[908ssp]

I would think 7581 tubes for lots of clean headroom and an American sound.

[goodtone77]

im not familar with 7581 tubes?

I would like to be at 75watts though. two 6550 or kt88 can do that. a twin reverb output transformer is 2k and can handle 80watts( this i have ).
maybe play with the speaker loadto get 4k on the tubes?
I Dont know how to set this up as far as what output trans ratio to use or a good voltage.

[groovtubin]

im not familar with 7581 tubes?

I would like to be at 75watts though. two 6550 or kt88 can do that. a twin reverb output transformer is 2k and can handle 80watts( this i have ).
maybe play with the speaker loadto get 4k on the tubes?
I Dont know how to set this up as far as what output trans ratio to use or a good voltage.

[llemtt]

I am going to build a ods but was thinking of using A set of fender twin reverb transformers with two 6550s or KT88s.

PT ratings
320-0-320 400ma
6.3 6 amps

OT Ratings
2k - 4 ohms

What do ya think?

thanks

Don't know what's the ideal load for a pair of 6550s or KT88 but 2k seems to me a little low, this is from 6550 datasheet:

Push-pull class AB1 tetrode connection, fixed bias
Plate voltage 600v
DC Screen voltage 300v
DC Plate current, idle 100 mA
Plate current, full power 270 mA
Grid bias -55 v DC
Load resistance, plate-to-plate 5000 ohms
Output power 100 watts
Total harmonic distortion at 100w out 3.0%

I think you can go higher with PT voltage (this is what this tubes are made to), something like 360 - 0 - 360 giving you around 500v plate, and lower with the OT staying in the 3400- 4200 ohms range.

Teo

[casajr]

FWIW, my Dynaco iron that I use for all my 50-60W 6550, KT88, EL34 and KT66 toys is 4300 ohms.

[BluzMike]

There is a rumour that Dumble plays a OD-75W with 2 x GE6550As. Dynaco iron would certainly be a good choice for this. But 6550ies need also to be run at higher voltages than the Twin PT would deliver. You want around 600 to 700 V on the plates - tough stuff. And maybe it comes out too stiff for your taste.
Check out SVT and old Dynaco schematics for some power amp inspirations.