New rocket build - low volume issue

[DaveMon]

Hey guys, I have finally made it into "the rocket club" by finishing my most recent build, using a set of boards, transformers and a reverse chassis from RJ. I used the "Mike" layout from the rocket docs section here at TAG.

So, the amp sounds killer, and sounds just how it was described on TAG.....
Despite having to extend the PT and choke leads, this amp is nice and quiet.

There is one major concern..............volume, and a lack of it. I estimate the output at about 2-4 watts.......my epiphone valve junior is approx twice as loud as the rocket, with all knobs at 10, using same cab/speaker.

I have built several express amps, and I was expecting ridiculous volume and output......

I have checked voltages, they are all in line with charts here at TAG, I rechecked the layout and component values 4-5 times.....nothing.

The only other weird thing is the volume and treble knobs seem to both affect overall volume .......and it takes high settings on both knobs to get any real output.........

I will post gut shots when I get home tonight...... thanks in advance

Dave

[joeboo88]

Hi: I think I had a similar problem on one of my builds, and found the mixer resistors (2-220K - R16, R17) for the phase inverter, I forgot to ground the junction....
I'm getting old and forget easily, so maybe I'm wrong...anyways

But we can wait for gut shots..... and guys here will help no prob.

I Built a Liverpool with RJ's reverse chassis..... Its good to see people still going to him for parts
peace... Joe

[pompeiisneaks]

Yeah I had a similar problem where I forgot to ground my PI as well in a Dumble, (the layout was a bit confusing and an old one) it worked, but really weakly. Photos of the chassis will help us look and see if we see something wrong.

~Phil

[M Fowler]

Did you use the pot to chassis as the grounding point like the layout shows or a separate wire to chassis ground?

I would not rely on pot chassis ground.

Must ground the tail resistor in a cathode bias amp, in your case appears to be a voltage divider 220k/220k like the Trainwreck Zwreck does.

Mark

[DaveMon]

IMG_20180621_000353.jpg

[M Fowler]

You used a standard board for an upside down chassis which really makes it difficult to review.

The 220uf bypass cap is in backwards.

Is that 56k of the tone stack connected to the 100k slope resistor & tone caps?

Mark

[pompeiisneaks]

Not specifically about the low volume, but I see your screen and grid resistors on the power tubes are hanging loose and not attached to anything. Having those out in the wind can provide serious problems down the road, they can either move too much and break the connections, or just due to being loose and not completely 'covered' with shrink tubing, they can arc to places you wouldn't want, that can cause serious problems. Usually they're either jumped across from an unused pin on the tube socket, or connected via a terminal strip to ensure you don't let them 'move'.

The image is a bit dark, and it's also got a few areas that are hard to view due to wires covering things, etc, so it would be good if you can also get some different angles of the different sections with more light and closer in. I usually think the preamp area, power tube area, power supply, and controls as separate photos tend to help a ton.

~Phil

[M Fowler]

I adjusted the photo is this better?

Mark

Rocket build Ampgarage (2).jpg

[pompeiisneaks]

I think that's a bit better, some of the pics show a few areas that may need a bit more soldering, I see what looks like either a missed solder connection in one, and a slightly less than fully filled solder joint in 2 (may be okay) and in 3 the ball of solder seems to imply it may not be getting a great connection. I've used a crappy red arrow to point out the joints I'm referring to. Also a technique I've used often to find out how well I've jumped/soldered my connections is to get my DMM on continuity mode, connect to a good ground point and then connect to the lead part (blue arrow) with the other end to ensure I've got good grounds everywhere. Then I use the same DMM continuity mode to double check every connection across the amp to ensure it goes from a lead or socket pin to the other end via the termination point itself and not the thing connecting them together to ensure the solder is good. I'd especially check all grounds. It could be any one of the tubes not getting a good enough ground to let the tube conduct.

If you have an oscilloscope, you can input a sine wave at 400mV AC at say 1k Hz and trace it through the amp and see where it's not amplifying much.

Solder.PNG

Solder2.PNG

solder3.PNG

~Phil

[M Fowler]

That 220uf/300v should be a lot smaller voltage and it is currently installed with positive to ground.

[pompeiisneaks]

oh yeah good catch! The voltage rating won't hurt anything but is overkill for that spot, I think usually it just needs 100V rated caps, but that won't hurt anything.

The fact that it's backwards will. That means that there's no cap reference to ground there, and that is part of what gives it a decent amount of it's output/gain. In higher gain amps subsequent stages are often left unbypassed (w/o caps) to make them not get too much gain. This is you doing that unintentionally, stifling the gain of the power tubes. If you desolder that and swap it around so the ground is to the right, you may have a perfect sounding amp. Often putting caps in backwards can cause damage, but since this has relatively low voltage at the - end and the + is at earth it may be okay. You may be safer to just replace the cap, though, as often being backwards will damage them.

~Phil

[DaveMon]

Team,
Can't thank everyone enough, for their input and comments, I apologise for the delay in responding, and I'm really sorry about the crummy image......I can dedicate tonight to focusing on this, and I will reply in detail to the questions. Thanks again everyone!

[DaveMon]

I have been working on correcting these issues, I have flipped the 220 uf capacitor around, tightened all nuts and bolts, then went around the entire amp, re-soldering connections very generously. During this re-soldering, I think I realized the root cause for all my grounding issues, which is a 6 inch piece of 12 AWG copper wire which I used across the backs of the pots, I thought it would be a great grounding bus, and be pure copper. But the stupid thing is too bulky, I cant get it hot enough to solder to it…… so I am going to remove it and use a “normal” size wire like everyone else does.



Regarding those 100 ohm 5 watt resistors, dangling off of the output tubes, I had original put 1 K “Marshall type” resistors and had them neatly installed, while troubleshooting the low volume issue, I learned that lower values can be added if preferred. I stupidly figured that 100 ohm resistors may cause more output than 1K….so I ran to Fry’s Electronics and grabbed those ugly “sugar cubes” and slapped them in there. I will revert back to my original plan.

Regarding that monstrous 300V capacitor, I swear I tried to find the proper size, but was having no luck though my usual suppliers ( tubedepot, antique electronic supply, triodeElec ) I tend to avoid “non tube amp” type retailers as a rule, But I ordered the correct part from a place called Elliot Electronics Supply….. but I am not nearly as happy with their service as my regulars ( 2 days to get a track #)


I now realize that I need to take grounding MUCH more seriously than I have in the past…….. I will update soon, I plan to stay busy all weekend!

[M Fowler]

https://www.tubesandmore.com/products/c ... ectrolytic

220uf/160v while not 220uf/100v better then that large 330v caps.

Another source for resistors and caps of all sizes, individual or kits https://www.justradios.com/

[DaveMon]

Mark,

Thanks for the link, but I managed to get some 220uf 100v that were right on the money, and I got one installed now. I have now been over this amp numerous times, checking and tightening grounds and lugs, I actually have broken a couple in the process and had to replace them ( so I know I am getting them tight ) I also found that somehow I had connected the legs of the cap on the volume control pot, this seemed to correct some of the “missing” volume when I fixed this error. Finding this defect actually inspired me, building amps is a lot like debugging computer code……. The machine only does what you have wired/coded it to do, whether you intended it or not…..


One thing I have not figured out, is why are the volume and treble controls still “linked”, they both seem to affect the overall volume and gain….. i.e. Volume all the way up, but treble at 0 gives 0 output…….. treble doesn’t seem to really do anything by itself…..just a “2nd” volume knob in effect……


Big thanks to all who have chimed in on this!!