Best tubes for the Rocket

[rooster]

Thanks for this 2tone. That's gotta be one loud Rocket! Whoa, Daddy! Kinda similar to what I'm sitting on over here with my AC30 and HEY, IT IS LOUD! Haha! Who needs friendly neighbors, right?

My PT here is a Schumacher (with world-wide multi AC voltage taps on it but only one secondary for the plates), so I don't have the option you have with that dual tap PT. If I did, I would try the lower voltage taps and bridge that last 9.1K power supply resistor with a 3.3K resistor. I just did this (actually swapped out the 9.1K for a 2.2K) with my AC30 build and it brought things up almost 5 volts DC - which doesn't seem like much but it really tightened things up quite a bit with my V1 sitting now at 90VDC.

[andresound]

FWIW, I built a rocket with 2 secondaries (250 and 280). Replaced the standby swith with a toggle for the two. As long as your PT has the current capability, playing with voltage by using a 5U4G vs the GZ34 is also interesting.

Voltages briefly with the 5U4 installed (on 280-0-280 tap) are,
Cathode to plate of 286.4v
Voltage drop over cathode resistor (66 ohm) of 11.02
Plate (pin7) of 294

I tried the MULLARD GZ34 again (on 250-0-250 tap)
41.9ma
12.5 watts
104.2 %
Plate (pin7) of 311.2

Although the higher dissipation (GZ34) sounds "bigger/bolder" and louder, I prefer the sound of the 5U4 installed. Seem to bloom and is rich in harmonics!!

Just my 2cw

[Mark]

I saw this and it looked interesting. They quote the rail voltage as 345vdc.

https://www.ampbooks.com/mobile/classic ... power-amp/

[2tone]

most Rockets had a plate voltage of around 300 to 308 volts, more of what Ken liked in the plate voltage range..for the Rocket.

[rooster]

Wow, I love this info Mark, I had not seen this before. Thank you for the posted link.

2tone, yeah, in those VG columns Ken talks about the Z amp coming to his shop originally at slightly over 400 plate volts. Apparently he suggested to the Dr. that he reduce this and the DR. did, creating a much better sounding amp in Ken's opinion. I have no idea what the Zwreck plates are seeing but I suspect it higher than 308VDC. Does anybody here know?

And BTW, here are my output section's AC30 voltages. I dragged my feet here taking this thing apart (sorry Mark) because it's like pulling the piece of cheese off a cheese burger while driving a car. I added a 8/16 ohm switch that isn't stock making things more complicated (as far as removing the chassis from the head case).

Note: My choke is probably something that none of you are using. It's the Hammond 194E choke, 30H/100mA/521 ohms DC resistance, made by them as a clone of the VOX AC30 choke. (It will account for the reduced voltage at my plates vs. the stock Rocket spec'd choke that I used to run.) I used to run the typical Hammond choke (158L) as described in the build guide, and apparently used by Ken, but made the switch when this version was offered.

340VDC @ the center tap of the OT (OT is a MM Albion version)
335VDC @ the plates
11.9VDC at the cathode/cathode bias resistor value is 65 ohms. (previously WRONGLY reported as 56 ohms!)

[M Fowler]

I used to run my Rockets with high B+ and now I took advise of many to lower the B+ and glad I did the tubes like that lower plate and screen voltage.

Use 250-0-250 or 260-0-260 PT's

[2tone]

I used to run my Rockets with high B+ and now I took advise of many to lower the B+ and glad I did the tubes like that lower plate and screen voltage.

Use 250-0-250 or 260-0-260 PT's

On my Rocket which had 350 plate volts (higher V tap) the 6n14n tubes lasted 2 years, no red plating, no blowouts. 68 ohm cathode R. I changed the tubes just because that was long enough for any set played almost daily ..

[Mark]

Did you ever measure the plate dissipation for the individual EL-84's. I put 1 ohm resistors between the cathodes and the cathode resistor to measure the dissipation. I did have to subtract the screen grid current of course. I did encounter a tiny positive voltage on the control grids which persisted no matter what I did.

[andresound]

FWIW.. revisited my Rocket today and definitely prefer the lower plate voltages. Much preferred 274 vs 308v

[Mark]

The Rocket is brighter at 308VDC over 274VDC. I did the Zener diode B+ reduction mod after making the amp to see what it would sound like with a lower B+. In my case I stuck with my B+ voltage of 315VDC (Classic Tone power transformer).

[andresound]

What is your cathode resistor value?

[Mark]

I'm using a 50 ohm cathode resistor.

[rooster]

...still waiting for the Saratov 6P14P/6n14n quad from Russia before doing some recording. It's a long ship time apparently.

This said, I'm enjoying the current Sovtek tubes. Both quads sound great and are holding up fine and very quiet at idle.

[Mark]

I have the Saratov 6P14P valves in my Rocket.

[rooster]

My Russian ebay buy of the 6n14n quad arrives on Monday or Tuesday! I'll do some sound samples then.