Building a Rocket - Layout Check for group build

[RJ Guitars]

Hello Guys,

Here is a look at the work that Lonely Raven supplied for our Rocket Group Build Project... these are awesome!

The second pair are the 1829 Rocket... an interesting story if you google it.

After we fill the first dozen group build requests I'll make the remaining faceplates available to the forum builders.

rj

[Fischerman]

Faceplates look great!

Some other things I've been thinking about: I've seen different values for the first stage cathode resistor and even the plate load resistor. It's always either 100k or 220k for the plate and I've usually seen 1k5 for the cathode BUT very often (like in an AC30) that 1k5 is shared between two triodes...which makes it's effective value for each triode about 3k. Which may be why I've seen 3k3 in some schematics (some Mark Abbott ones) where that Rk isn't shared. But in an AC30 the supply voltage there is ~280vdc and the plate voltage is ~170vdc and I think ours will be much lower using the TW power supply. RJ has it as a 'variable' value in the schematic. I usually shoot for ~.8uA or .0008A and go from there.

The series 220k at the input to the PI...if you were to try and 'clone' the AC30 preamp (which the schematic we're using appears to be doing) then you would also need a resistor to ground there as well...to simulate the 'other' 220k summing resistor in the AC30 which then goes to ground (or B+ which is AC ground) depending on where the 'other' volume control (Normal channel) is set.

I've never experimented with it but I've heard others say that sometimes swapping the OT primary wires can make a difference even though we have no NFB. So I intend to try it both ways and see.

And FWIW, I intend to compare our Heyboer 'TW spec' OT with a Mercury Woden clone OT (in the mail) and a Heyboer JTM45 OT that I have in another amp. I'll try to make comparison clips but won't guarantee anything.

[dansamp]

Fischerman,
regarding the shared cathode resistor I believe you have it wrong
if you were to split the cathodes between v1a and v1b each side would need a resistor of half the value of 1k5
that value is not available so usually you will see a resistor of 820 ohm
because that is the closest value
all things the same (plate voltage) if you used a value of 3k3 that would bias v1a pretty cold
but then again what do I know
Dan...

also just as a note:
I built an AC30 type amp using the Hoffman circuit with a normal and TB section I fed them into the PI using two 220k resistors and it sounded fine but then I ungrounded the PI and fed each channel into either side of the PI and that made a huge difference, it sounded much better louder and more grind
the original Vox AC30 has one side sharing one end of the PI and the other side of the PI has the tremelo/vibrato section feeding it
IMHO I think that is a major part of the AC30 sound

[Fischerman]

regarding the shared cathode resistor I believe you have it wrong
if you were to split the cathodes between v1a and v1b each side would need a resistor of half the value of 1k5
that value is not available so usually you will see a resistor of 820 ohm
because that is the closest value
all things the same (plate voltage) if you used a value of 3k3 that would bias v1a pretty cold
but then again what do I know

I think you have it wrong. Two triodes means twice the current...twice the current through a resistor = twice the voltage drop across that resistor (so cut the resistor value in half to get back to your original voltage drop). 1uA (i.e. split) through 3k = 3v but 2uA (i.e. shared) through 1.5k = 3v. So for two triodes sharing you halve the Rk value. Look at old Fenders...820ohm when shared and 1.5k when not. We're just biasing the tube so the voltage drop across the Rk is the issue.

Regarding the way the AC30 PI is set-up; I agree. If I choose to just do it 'regular' I plan to experiment with 91k/100k plates and such. The 47k tail should give us pretty good balance...we'll see after I build it and measure things. We won't have a second channel to connect to the second PI input. Did you retain the series 220k resistors when you switched to using both PI inputs? If not that could explain the 'louder and more grind'. Even if you kept them...can you see that there is a path to AC ground (through the 'other 220k and volume control'...and remember that the B+ supply is AC ground just like true ground is)? I think it's in the 330k-effective-to-ground range but I haven't calculated it. In any case, you likely had a voltage divider there before and don't now. I had intended to try my amp with and without that series 220k and with/without a 330k to ground there too.

[dansamp]

[/quote]I think you have it wrong. Two triodes means twice the current...twice the current through a resistor = twice the voltage drop across that resistor (so cut the resistor value in half to get back to your original voltage drop). 1uA (i.e. split) through 3k = 3v but 2uA (i.e. shared) through 1.5k = 3v. So for two triodes sharing you halve the Rk value. Look at old Fenders...820ohm when shared and 1.5k when not. We're just biasing the tube so the voltage drop across the Rk is the issue.
[/quote]
yeah, like I seaid what do I know
I had it back wards
sorry

Regarding the way the AC30 PI is set-up; I agree. If I choose to just do it 'regular' I plan to experiment with 91k/100k plates and such. The 47k tail should give us pretty good balance...we'll see after I build it and measure things. We won't have a second channel to connect to the second PI input. Did you retain the series 220k resistors when you switched to using both PI inputs? If not that could explain the 'louder and more grind'. Even if you kept them...can you see that there is a path to AC ground (through the 'other 220k and volume control'...and remember that the B+ supply is AC ground just like true ground is)? I think it's in the 330k-effective-to-ground range but I haven't calculated it. In any case, you likely had a voltage divider there before and don't now. I had intended to try my amp with and without that series 220k and with/without a 330k to ground there too

I did not retain the 220k resistors and I did run them right into the PI with out any voltage divider
are you saying that the un-used channel volume control has and affect on the channel that is being used ??

I also used this set up on an 18watt TMB one side of the PI had the normal channel and the other side had the TMB going into it
ref: the Richie schematic on the 18watt forum
I originally had it going into one side of the PI with some 220k resistors but the drop in volume between the TMB side and the normal channel was too drastic

Dan....

[Fischerman]

Look at the schematic the way you had it vs. the way you have it now. See how the old way had those 220k 'summing' resistors forming a voltage divider? Doesn't matter which channel you used...the voltage divider is still there. The other side of the 'grounded' 220k doesn't necessarily go directly to ground...it might go through a pot but it still goes to ground. And the pot-to-ground might be in parallel with the 220k plate load resistor of the preceeding stage-to-B+ (which is AC ground). That's probably your difference in gain.

[dansamp]

Fischerman,
Thank you for the clarification
I think I follow what you are saying now,
Would there be any advantage to having a resistor in series with the input to the PI ??
I some to recall seeing a schematic of an amp circuit that had a single 220k resistor before the PI input cap
it would seem to be that it would knock down some of the gain

Dan....

[RJ Guitars]

Hello Rocket Fans - I am especially pleased to mention that our group build project is not only going well, but going all over the world. We have kits going to Canada, Australia, Croatia, and many here in the USA. I am going to have some residual parts that I will inventory and make available after the first wave of kits goes out so there will be some continued opportunity to build along with us. Shipments will begin today.

I am greatly pleased with all the support from the store houses of amp knowledge that have made this fun and interesting. This forum provides an awesome opportunity for promoting the advancement of our craft and development of great friendships.

I have great hopes that the AmpGarage forum will continue on in this open sharing atmosphere that allows us all to build and learn from each other. I also say THANK YOU" to Omar for his great idea and years of hard service. May the future be as kind to Omar as he has been to us.

rj

[Fischerman]

I don't know of any advantage. With the 220k it should be at least a little less gain than without. Easy to try.

[angelodp]

Sorry if this has been covered..... are there plans avail for either a head cab or a 12" combo cab for this amp yet? Or is anyone making cabs that these chassis will fit into.

Ange

[nickt]

Sorry if this has been covered..... are there plans avail for either a head cab or a 12" combo cab for this amp yet? Or is anyone making cabs that these chassis will fit into.

Ange

https://tubeamparchive.com/files/headcab.pdf

In the files section but here's the link anyway

Wrecks aren't supposed to be combo friendly but given this is a rocket I suspect that doesn't hold. Personally thinking: combos are cool - one arm for guitar case and one for the amp - a clean getaway!

[RJ Guitars]

Wrecks aren't supposed to be combo friendly but given this is a rocket I suspect that doesn't hold. Personally thinking: combos are cool - one arm for guitar case and one for the amp - a clean getaway!

I'm glad someone brought this up.... I've been busy getting some Rocket chassis packages out but I wanted to make a couple posts along these lines.

How about cabinets - heads or combos. Does anyone know what is available and is there anyone who would want to be part of the Group Build Project and provide a few cabinets for a Wreck Chassis? I don't know how many guys will need them, but I'd sure like to see what is available. I'm also a bit partial to combos although the tubes up layout isn't altogether combo friendly.

The spirit of this Group build has been to order in quantity and pass that on to the participants and have everyone gain from the quantity buying discounts. I know some vendors have a narrow margin on the profits and can't offer anything special to the Group Build crew.... we're cool with that.

If there is someone out there that has an interest in providing cabinets much the same way that Lonely Raven has provided faceplates, PM me and we'll try to figure out something.

rj

[Sven]

DELETED. Look at the schematics and layout posted on earlier page, dated june 9, 2008. That is my best. Hopefully someone posts final layout.

Sven

[Fischerman]

Sven,
I just took a quick glance and to me it looks like the power tubes will be running when in Standby and the HT fuse is in the wrong place. I think you need to move the OT center tap connection to the turret where the choke is connected (the connection on the right...NOT the B+2 turret). I'm not sure how the originals were but I was gonna go straight from the rectifier to the HT fuse...THEN to the first filter caps. Then from the first filter caps to the SB switch...then the other lug of the SB switch to that turret that has the choke and OT center tap (once you move it).

Now (done the way I described) when you turn on the amp (but in Standby)...the first filter caps charge up and they are fused...but the power tubes and rest of the power supply are OFF until you flip the SB switch.

Can someone else please verify?

[nickt]

Hi Sven,

I believe the second triode should be used in the first 12AX7 (ie pins 6,7,8). Also others have stated that the 2nd gain stage uses the other half of the first tube leaving the second half of the second tube unconnected.

just sayin'