I also suggest 6550's because of the current capability and plate dissipation. With other mods like a G2 voltage of 200 and also the bias mod, a 6CD6 will work also and is $6 NOS.
I have subbed 6550's and 6CA7's for 8417's (incl bias mod) with very fine results in the Bogen MO-200.
HMMM Do you remember PTP ohms for those tubes? Height? I can look that up. Just thot you'ld remember.
Second thought -- If I remove the cabinet and mount the whole thing on a STD 19 1/2 rack panel it wouldn't matter
Good suggestions all
Al
The 6550 is a perfect match on the power side because it also wants 600V on the plate and 300v on the screen. One thing to watch is spacing between the tubes. The older coke bottle type 6550 is a lot fatter than an 8417, and the newer cylindrical 6550 is the same size or very close to it. 6CA7 are rather thin and made for higher voltage/lower current.
The 6550 can do 100W into 5K CT at the same plate and screen voltages as in the amp there.
I would not be too concerned about insufficient grid drive. When I did the MO-200, I did not have to mess with the driver due to any shortage of output voltage, but I nonetheless replaced it with a 12AU7 because the original tube was a bit low on emission and was unobtanium.
Here you can find the schematic for the Stromberg-Carlson APH-1200 which uses the same type of phase inverter driver circuit. For sure a dime-a-case 6GH8A can (and has) been substituted with minor component value changes and socket rewiring. The 6GH8A is one of the best tubes for audio (poo-poo'ers may start!!) because it is cheap and was produced in phenominal numbers, since at least three were used in every self-respecting tube type color TV set. I have an unopened case of 100 on the shelf!! I ought to make a little shrine.
http://www.montagar.com/~patj/aph1200.jpgFor the Bogen MO-200 with 6550's I used a 12AU7 for the driver-phase inverter. Comments are here:
http://www.montagar.com/~patj/bogenspecs.htmFor your amp specifically;
If you slightly increase the value of R46 in your amp and replace C29 with a 100V unit and maybe you will have to upgrade the rectifier CR3, it is easy to get a higher bias voltage.
It may be necessary to decrease the value of R47 slightly.
It should not be necessary to increase the valuse of C30, but C30 is there to act as an impedance without dissipating excess wattage and also as a DC blocker as the hot side of the transformer will have about 300V plus quite a bit of AC on it.
-So C30, R47, and R46 are the voltage divider for the AC input to the bias rectifier.
R45 is just the bleeder, but that is a heavy bleed and its value could be increased to 27K which would help reduce the load on the aforementioned voltage divider in light of the higher DC voltage produced.
It is best to make more bias than you need and use a pot to adjust. They used precision resistors instead of a pot, but who can say what is right for a particular set of tubes as they age? If a pot is going to be used, a good value for bias supply is 55-65V in that circuit with 6550's. I have had good luck adding a pot for adjusting the bias there. I usually have used a 50K or 100K pot, and put it across the C29 position. You can use a 10-turn is you want to nit-pick, but I usually use the single turn kind with a screwdriver adjust and a locking nut.
On the wiper going to the R40 & R42 connection, I usually add a 1-2uF capacitor so that any noise at the pot is removed and to keep the impedance rather low.
It is nit-picking but R41 is the balance compensator, and it is "small" for the 6550 as the value is based on bias voltage as well as cathode current. There is no real need to change it but you could put a 50 Ohm 2 watt WW pot in there. If you put a 10 Ohm resistor in series with the wiper to GND and install a tip jack, it's easy to set the bias, and you can measure any imbalance by putting the zero-center meter meter across the opposite ends of R41.
I hope these suggestions help. If you do the necessary ones, I can almost guarantee it will work properly and trouble-free.
