6550 tubes in Rocket Reverb Question

[M Fowler]

Question for my fellow amp builders. I want to run two 6550 tubes in my Rocket Reverb amp.

Currently this is a TW Rocket Reverb with two EL34's cathode bias amp.

I changed the bias resistor to 350R with 220uf/100v bypass cap.

I decreased the 220k V1a plate load resistor to 150k for more headroom and replaced the GZ34 rectifier with SS plug in to increase voltage.

Should I change the 220k/220K load resistor to 150K/150K as Marshall did for conversion from EL34 to 6550.

The amps sounds so good right now but I don't want to over stress the PT or shorten the life of the 6550 tubes.

I am going to take some voltage readings today and bias readings.

[Firestorm]

The data sheets say 250K is the limit for grid circuit resistance with cathode bias (versus 50K for fixed bias, but obviously Mashall got away with 150K), so you're okay at 220K. That said, you may find you like the sound of the 150K better, especially if you are trying to pick up a little headroom.

[jelle]

The 6550 in cathode bias mode can handle up to 250K grid to ground. So the 220K's can stay. However this is per GE datasheet. If you run current prod tubes, this might be different as the current tubes are not the same as the superior GE 6550.

You should be able to hear the distortion this generates.

[M Fowler]

Firestorm thanks,

Yeah I have a guy that loves the reverb of this amp very subtle reverb but needs more headroom and volume so I decided to go this route.

I could change out to 150k's and change to adjustable fixed bias as well to pickup a little more headroom right?

Mark

[M Fowler]

Jelle thanks,

These are the Chinese 6550's from mojotone. I had intended to just have some 6550's around. Not sure if they are Shino or whom makes them no marking on box or tube.

I like the increased low end and mid range tone I am getting though.

Mark

[Firestorm]

Firestorm thanks,

Yeah I have a guy that loves the reverb of this amp very subtle reverb but needs more headroom and volume so I decided to go this route.

I could change out to 150k's and change to adjustable fixed bias as well to pickup a little more headroom right?

Mark

Fixed bias will take away any compression induced by the cathode R, but I don't know about headroom, per se. The tube will see the full voltage from Va to Vk (0V), but the difference may be too subtle to really hear. It could totally change the feel of the amp.

Try the 150Ks; works wonders in high voltage Deluxe Reverbs that break up too quick. Maybe you could find an easy way to goose the reverb signal to get more of it in the mix.

[M Fowler]

Well we will see what the guy thinks after he plays it this weekend.

[M Fowler]

Thanks guys for confirming the changes we discussed your a great group of people to hang out with.

This is what the customer said so far: The Reverb Rocket sounds great. A lot of head room, great natural overdrive on the boost channel, awesome subtle reverb. I have been toying around on it quite a bit lately and I think I have a pretty good tone set up on it.

I was sweating it because my style of playing is so different from his and he has a pile of modulation effects he dances around when playing. Liked my Badland's 60 (Asteroid) but wanted reverb. I suggested a Holy Grail pedal

He said he was going to record so if I can get it I will post.

[Ian444]

Always nice to hear a good outcome, I was wondering how it went... I was gonna say +1 for fixed bias but I'm glad I bit my tongue

[M Fowler]

I wanted to try the fixed bias but the guy wanted the amp back right away and I was anxious to hear what he thought as well.

I have another octal rocket with SS rectifier that I will experiment on with fixed bias, 6550, and add a foot switch boost.

I want to lower the load resistors way down to hear what that sounds like. A pair of trimmers need to go in.

Mark