Delayed swell with standby on Rocket

[sonicmojo]

I'm new to playing around with the Trainwreck circuit. I just acquired a Rocket clone on trade and I'm working through tuning it up. When I turn standby off on the amp, it takes about 4-5 seconds for the amp to reach full volume. This is after the heaters are definitely warmed up. If I turn standby back on/off for anything more than 2 seconds, it acts the same way again. I'm assuming this is not how the Rocket circuit should work since I've not experienced this on other amps but figured I'd ask for advice before doing anything major. It has PT, OT, and choke from an old Baldwin. I've swapped tubes and done some checks on the filter caps and I don't think those are part of the reason. I feel like some component in the amp is struggling when high voltage is hitting it.

Thanks!

[xtian]

If the standby switch comes before the reservoir cap(s), AND the reservoir cap(s) have bleeder resistor(s), that could explain what you're experiencing.

[sonicmojo]

If the standby switch comes before the reservoir cap(s), AND the reservoir cap(s) have bleeder resistor(s), that could explain what you're experiencing.

The 80uF (40x2) caps are before the standby switch and there are no resistors on them. They are holding a charge (400V standby on, 335 standby off).

[sonicmojo]

I've solved the main volume swell problem when coming off standby. There is still a minor swell but less than a second. Apparently simple cold solder joints can cause this. I also replaced the noisy/failing PI coupling cap but it was pretty much just solder that was creating this issue. I've been going through and making better mechanical crimps and resoldering all the wiring to be sure. The amp has more issues to sort, being:

1. I get slight speaker hum even without the rectifier tube. I think the old Baldwin PT is inducing it through the steel chassis. I tried moving the center taps to a different lug slightly away from the PT but it didn't help. It's not that bothersome but it would be nice to eliminate if possible. The PT seems to be fully operational otherwise.

2. Some cap is probably leaky. I'm getting static on the amp and guitar volume controls when I turn them. I've tried a couple of guitars. There's no significant measurable voltage on the pots but something must be bleeding through.

3. I'm getting a little bit of 120Hz hum. The steel chassis is either powder coated or painted gray and I need to improve grounding. I'll Dremel a bit behind all the pots. I assume it is better if the pots are well grounded. I already hit behind the input Jack and added a shielded wire (ala Dumble style) to V1 grid and it helped a little. It was also missing the heater center tap so I added two 100 Ohm resistors to ground.

This amp is full of Orange drops. If anyone has any suggestions for nice quiet caps to use instead, I'm thinking I'll replace them all to be sure.

[Littlewyan]

1. Try putting a grounded shield between the PT and OT. Are they orientated correctly?

2. I think these amps are prone to that. Try different valves in V1.

3. This could be grounding. Have you got any photos?

What orange drops are they? They shouldn't introduce noise. Even if they've gone bad I don't think they'll introduce noise.

[sonicmojo]

1. Try putting a grounded shield between the PT and OT. Are they orientated correctly?

Thanks for the tip. They are too close and a shield does help about 50%. The hum is minor and I'm not going to worry about it. For sure it is the closeness factor on the trannies. There is no room to move them easily. The PT is through chassis and the other is stand up. Is there a preferred configuration for these amps?

2. I think these amps are prone to that. Try different valves in V1.

Interesting. Yes, different tubes act differently. A Sovtek LPS is pretty quiet there. JJ ECC83 and ECC803S are equally noisy.

3. This could be grounding. Have you got any photos?

I think this is actually mainly my guitars. I don't get anything significant with the jack unplugged and grounded.

What orange drops are they? They shouldn't introduce noise. Even if they've gone bad I don't think they'll introduce noise.

They are 716PS mostly. I probably won't change them unless I think they are bad. I suspect one more but I think the solder originally used in this amp may have been crappy or just not hot enough. I'm touching up everything. I'll post a couple of pics when I get a chance. It is sounding a lot better.
Thanks for the help!

[sonicmojo]

Pics. The transformer leads were extended and shrink-wrapped with clear wrap. Not ideal but they seem solid.
[img880]http://www.sonicmojo.com/rocket/inside.jpg[/img]
[img830]http://www.sonicmojo.com/rocket/top.jpg[/img]

[xtian]

Transformers can be really close together, and still not hum. They just have to be aligned properly. Looks like yours are not. If you loosen the mounting bolts on both, you may be able to push and twist them into a better place with less hum.

[RJ Guitars]

I agree with all of what has been said... mostly.

The steel chassis will potentially couple with the PT... but that would make the chassis hum and you wouldn't hear that in the speaker. Not that it helps you but KF used aluminum for his chassis.

The alignment can couple the OT to the PT and that can hum... but even with "perfect" alignment I can always hear a little bit of hum using the headphone trick. You will hear this in the speaker and you'll know it's transformer coupling if it occurs as soon as you apply AC power to the PT and the tubes aren't warm yet. Interestingly, there is an intentional offset alignment in most if not all the KF Wrecks I have seen. I've found the hum to be minimal with the transformers on a common centerline and that is the way the official chassis drawings posted here on TAG are shown.

Since you have this hum with no rectifier tube in, it's not in the amplifier circuit and direct coupling from the PT to the OT is the most likely cause. You can fix it easily but the good news is that it won't get worse when you turn the volume up and might actually be less hum when you are playing than when the amp is sitting at idle. As Xtian mentioned you might be able to loosen the screws on the OT and twist it and change the hum. In some cases it doesn't take much.

Reflow all those solder joints while you are in there and I'd be curious what your voltages are and how you think it sounds. That OT in particular may sound very different than the others commonly used in a KF clone Rocket build.

[RJ Guitars]

...
2. Some cap is probably leaky. I'm getting static on the amp and guitar volume controls when I turn them. I've tried a couple of guitars. There's no significant measurable voltage on the pots but something must be bleeding through....

I've suffered from this a bit in the past and there is a fundamental flaw in the Rocket architecture that allows for some minimal DC in the control circuit. You might have to dig for it but there are a couple threads that review this topic. I think Mark Abbott might be the guy that brought this up. If not him then it was one of the elder statesman from the first generation of the TAG forum that have a lot of knowledge of the Rocket build.

[BLT]

I'm not sure what I'm seeing, but do the cathode bias cap and resistor have 2 grounding points? I also question the grounding points for the filter caps. The artificial center tap should probably be re-flowed.

[sunnydaze]

I'm not sure what I'm seeing, but do the cathode bias cap and resistor have 2 grounding points? I also question the grounding points for the filter caps. The artificial center tap should probably be re-flowed.

Agree with checking the filter grounds. It looks like the preamp and power supply filters are grounded at the same place. Usually better if those are separately grounded.

Hope this helps.

Mike