Adding second channel to Rocket

[chopstuck]

I built my Rocket with TS power and Heyboer A470 clone OT.

I redid the chassis to accept a 6BR7 pentode channel ala' Stingray.

Question is how to best tap off the PS string to get enough voltage to feed the pentodes' screen and plate resistors.

I'm using the lower end taps to get 300 VDC to the EL-84 plates but that puts the string too low to get proper voltage on the 6BR7 at the last node.

The best I have done so far is tap off the choke voltage and add a extra 47uf node just for the 6BR7. The only way to get the plate and screen voltages right was to go from the tried and true 220k plate and 1 M screen values to 150K plate and 580K screen to get about 100v plate and 73v screen.

Problem is I have too much bass and not enough gain from the 6BR7.
It doesn't distort even when dimed. The volume level is puny compared to the Rocket preamp side.

Any advice ? Do I increase the V+ and current supplied to the 6BR7 by tapping of the other HV winding ? With an extra choke and PS ? or raise the voltage of the whole amp ?

[paisley_tele]

Ala stingray z has that amp setup to be fairly clean.

[chopstuck]

Yes I have built two of them as a single channel amps and they are great.
The preamps have quite a lot more drive than this one.

[chopstuck]

Alright, I had a little time to look over the amp and found a mistake with my pentode grid cap and did a little mod here and there. I got my voltages closer to the 106v on plate and screen. Actually 105 plate and 94 screen. The amp really roars now with both channels. The pentode has the Zodiac tone control and sounds great with plenty of mid scoop for clean Tele lines. The Rocket has more mid focus and gain. Both useful, different tones.

I still have too much muddy low-mid/bass. I.m using a 20 mfd /2k2 node hung off the screen supply and the Rocket channel seems to like that fine.
I'm thinking of compensating for the boom by changing the pentode plate .01 coupling cap to .0047 or smaller.
Are the lower plate voltages on the PI and PA responsible for the lack of bass vs the higher voltages of the stock one channel AC-30 type amp?


Should I try switchable HV/ LV taps for better punch on the clean
channel ?

Any insight ?

Slightly less stuck than before.

[Jackie Treehorn]

I've got an EF86 channel on my rocket. I ended up with 1,680pf coupling on the EF86. It's still got plenty of bass, it could go lower. I also put 500 pf across the 220k resistor into the PI to add a little sparkle. What's the zodiac tone control?

[chopstuck]

The Zodiac tone control is the mid shift control used on 60's Selmer Zodiac amps and later on the Stang Ray.

Thanks, I will try to piece that cap value together to see how it sounds.

[KellyBass]

Chopstuck,

Just wondering how your amp turned out and what you ended up using for your plate voltage. That zodiac circuit is very interesting.

Jackie, on your ef86 rocket channel, did you end up going straight into the PI? Do you also have a normal rocket channel as well? I'm building a ef86 channel on a rocket and I'm sort of stuck as to where the optimal place to "bring it in" is. The PI seems to sound the best, but the gain isn't where it needs to be as of yet.

Thanks!

[Jackie Treehorn]

Yes, I've got both channels. I did it standard vox/matchless by using the other side of the PI as the input so both channels are in phase when used together. There are some nice tones using both channels. So, each side of the PI has a 220k input resistor, the EF86 side has a 500pf bright cap across it. I took the B+ off the screen supply of the power tubes, 22k dropping resistor to a 20 mf filter cap. I also used a 1.2 meg screen resistor on the EF86 instead of the standard 1 meg. Both of my channels are balanced pretty close, breakup starts around 11 o'clock.

Are you not getting enough gain from your EF86 channel? How are you handling the inputs to each channel?

[chopstuck]

I solved my problem a little differently. At first I toyed with the different cap values and played around with the voltage source.
Now I don't understand load lines well, I had a happy accident. At fist I tried to keep my B+ high by keeping my dropping string resistors low and the sound was flabby and bassy. I decided to put a switch on my two HV taps on my TS power Trans. (260 and 300 approx). I then put a larger resistor on that node - a 27K 3w. Bam the percussivenes was right although the voltages were still too low. I got 309v at the node but with the 220k on the plate and 1M on the screen I got 92v plate and 55v screen.

With a little trial and error I settled on 164r plate for 104 and 680r screen for 79v, for now.

When switching to the rocket voltages both values only fall about 10v but the sound changes dramatically.

I feed both channels to opposite sides of the PI .

So low-v for Rocket and Hi-V for the 6BR7 pentode sounds best to me.

[chopstuck]

double post sorry

[KellyBass]

Thanks for lining me out on the PI, Jackie! It sounds MUCH better. Now, I'm going to play with the plate voltages and coupling a bit and see what happens. Right now I think I'm pulling the HT from the B+4 node. But, I'll measure and make sure. After your findings, Chopstuck, I'm thinking I need more V.

[KellyBass]

Did some voltage checks tonight on the pentode channel and just as I suspected, it's way low. 67V on the plate and 56V at the screen...and the channel sounds darkish...with not much of the true character of the ef86. Still, I can tell it's gonna be very cool when I get it sorted out.

I'm gonna play with a couple of resistors and aim for your voltages, chop.

[daz]

Sounds cool. Is there a schematic floating around of what you guys have done? I'm got some vox iron sitting here and looking to do stangray and maz gt in one amp, which is not too far off what's been done here.
Cheers!

[Richie]

https://tubeamparchive.com/viewtopic.ph ... us&start=0

[rooster]

Richie - Eh, no EF86 in that link. I miss something? Still, a fun link to read and see, thanks.