Rocket Combo Takes Off

[rawnster]

Sweet looking amp Ron!

Love to hear some clips when you get around to it.

Thanks Tom. It sound's pretty cool. Yet, it breaks up a little to quick for my tastes. And I'm not liking the .47uf in the fat/normal/bright section. It sounds like it boosts around 900-1k hertz. A little too strident. I'd like to get the frequency boost a little higher up the scale, say around 4k or so. Any cap recommendations?

[Structo]

Are you talking about the switchable bypass caps?

If the .47uf isn't right then wouldn't you go to a smaller value?

Most of the time when I have messed around with that kind of mod, I find I have to make a significant change to hear much of a shift in tone.

So I would guess a .20uf or smaller.

[rawnster]

Yes, smaller would be in the right direction. I was just asking you, knowing how much you like to tinker.

[rawnster]

I was able to put some more work into the amp. This build does not have as much clean headroom as my earlier "traditional" rocket build. It also is not nearly as loud. It feels like a low wattage amp. I'm building this for a friend who plays primarily an older gibson 335. I've got the guitar, and boy does it ever push this amp into hairy tones. So, I've still got some tweaking to do to get some more headroom with this guitar. I've was able to coax a bit more by adding a split plate load resistor mod to the first plate off of V1. Split the 220k resistor into 68k/150k. Unfortunately it dropped some volume too. hmmm. Anyway... Come to think of it, this mod cleaned up the amp just about the same as using the low input jack. So, maybe I'll just put the 220k back, and use the low input jack with the 335. Geez talk about over thinking things.


Here are the voltages for those who are interested:

B+2: 371
B+3: 273.3
B+4: 233.3
B+5: 222.5

Notes on usages:
V2: half is used for typical tone stack, and half is used for the reverb return
V3: entire tube is used for reverb send
V4: PI

------------------------------------

JJ-12AX7 V1: (p1: 100.1) (p2: n/a) (p3: .83) (p4/5: 6.6) (p6: 138) (p7: n/a) (p8: 1.09) (p9: 6.6)
JJ-12AX7 V2: (p1: 234.7) (p2: n/a) (p3: 137.3) (p4/5: 6.6) (p6: 158.8 ) (p7: n/a) (p8: 1.39) (p9: 6.6)
EH--12AX7 V3: (p1: 368.1) (p2: n/a) (p3: 5.4) (p4/5: 6.6) (p6: 368.1) (p7: n/a) (p8: 4.9) (p9: 6.6)
JJ-12AX7 V4: (p1: 212.6) (p2: n/a) (p3: 58.6) (p4/5: 6.6) (p6: 210.5) (p7: n/a) (p8: 58.8 ) (p9: 6.6)

KT-66 (JJ's)
V5: (p1: n/a) (p2: 6.6) (p3: 370) (p4: 368) (p5: .02) (p6: 370) (p7: 6.6) (p8: 31.1)
V6: (p1: n/a) (p2: 6.6) (p3: 370) (p4: 368) (p5: .02) (p6: 370) (p7: 6.6) (p8: 31.1)

JJ-GZ34 recto: (p1: n/a) (p2: 372) (p3: n/a) (p4: 299) (p5: .n/a) (p6: 299) (p7: n/a) (p8: 373)

Some more pics:

[IMG500]http://i209.photobucket.com/albums/bb15 ... -verb3.jpg[/img]
[IMG519]http://i209.photobucket.com/albums/bb15 ... t-verb.jpg[/img]
[IMG537]http://i209.photobucket.com/albums/bb15 ... -verb6.jpg[/img]
[IMG599]http://i209.photobucket.com/albums/bb15 ... -verb7.jpg[/img]
[IMG712]http://i209.photobucket.com/albums/bb15 ... -verb4.jpg[/img]
[IMG672]http://i209.photobucket.com/albums/bb15 ... -verb5.jpg[/img]

[Structo]

Ron what kind of voltage are you getting out of the PT?

I'll bet those KT66's aren't even breaking a sweat at the voltages you have on them.

Probably not in the cards but if you got those tubes up around 450 or more you would most likely get a lot more headroom.

What kind of impedance are you getting out of the OT primaries?

You should be able to get 50 watts or so out of those tubes.

[Normster]

I think I see part of the problem with your reverb and volume issues. The reverb pot should have both wires going to the wiper and another wire added for ground. The way you have it in the photos, you don't have a ground reference for the bottom of your PI.

Edit: Just saw that you do have a ground wire. Still, move both red wires to the wiper.

[Structo]

Good eye Norm!

[rawnster]

I think I see part of the problem with your reverb and volume issues. The reverb pot should have both wires going to the wiper and another wire added for ground. The way you have it in the photos, you don't have a ground reference for the bottom of your PI.

Edit: Just saw that you do have a ground wire. Still, move both red wires to the wiper.

Yeah, originally, I had both wires going to the wiper, as you recommended. But when I was getting no volume from the verb, I switched one to the input tab. Didn't help. I just forgot to put it back. Thanks for reminding me.

I'll add a couple of components to it when I get home from work, namely 27k/2W resistor where the b+ meets the reverb tranny; and add a .1uf cap at the junction of this b+ and resistor, and then send it to ground. Hope this helps.

[rawnster]

I know I should let things break in, but the amp's tone is a little dull to me. It really does not sound close to my rocket build at all. I know kt66's sound different; I know different trannies sound different; I know I've got to finish tweaking out the verb. But still I'm wondering if I simply don't like the sound of those vintage Sozo caps. I've read where they don't have the sparkle of other caps. hmmm. I've got a bunch of other brands laying around. I may just pull those babies and try something else.

Somebody stop me!

[redshark]

I understand that sozo caps have a breakin period, I know they are good quality caps. You should give them time.
Do you have a way to measure current on each kt66? if the amp is cathoded biased still you can change the resistor going to ground that regulate how hot the kt66's run. I know they sound good around 35mA in class ab, because as someone said before the voltage seems kinda low for what those tubes can handle....maybe around 50 mA per tube in class A could be fine.

[Structo]

I think it was somebody at the Metro forum that was talking about the difference between the Vintage Sozo's and their regular film caps.

If I recall they said the regular ones work better for a Marshall style amp.

And you probably thought you were doing the best thing in buying the vintage ones.....

[paulster]

I think it was somebody at the Metro forum that was talking about the difference between the Vintage Sozo's and their regular film caps.

If I recall they said the regular ones work better for a Marshall style amp.

John at Sozo said the regular ones were more suited for a more modern build like a JCM800, whereas the vintage ones would better suit a plexi, a Vox or that kind of era, so shouldn't be bad in this application.

I think they'll be fine, but the vintage series take loads of breaking in.

Stick a CD in the front on repeat, pull the power tubes, set the gain high and leave it for three or four days and see how you feel then!

[rawnster]

Paulster, yeah, I bet that'd do the trick. Some ZZ-top for two days would cure any ailment.

[M Fowler]

Rawnster just get some god aweful players gathered together and jam out for about 4 hours that will fix that amp or not.

The whole time you wouldn't even notice there was a problem with the amps tone, keep them all off key.

[dartanion]

Lose the JJs MAN!

Besides flat tone, you don't want tube shorting out. Seems that their QC has lapsed dramatically and the failure rate has skyrocketed.

If you've got some good 6L6s, try those to see if you get an improvement.