Noise in a (somewhat of a) Rocket build

[dartanion]

Also, how are your input jacks grounded? I can't see from the pictures that they are grounded, but they need to be.

[mauiboy81]

Thanks for the replies.

Dartanion- I changed over to the terminal strips because I thought they would be more secure and i had them. i was rerouting the grid wires and thought it would be a good. There is a white wire connecting between the grids on each terminal. It might look like the top of the terminal but there are wires there.

I changed to a crossline master using a 500k that I made no load. I was wanting to see if the other was causing noise and the dual pots weren't well matched. I ordered 10 of the 250k dual ganged pots and none are well matched. I scraped a bit of the carbon from the track to match it up. Anyway, I think I liked it a bit better with the dual ganged pot but I don't feel like redoing it at this point.

I took out the sag resistor and used a 1k instead of choke. I only had 500ohm 5 watts for the screens so that's what is in there. The cathode resistor is 100 ohm and the el84s are dissipating ~11w on the low viltage switch. When on high it's more like 14w! The pt is 290-260-0-260-290. I don't know how a 50-70ohm cathode resistor wouldn't burn the poor tube alive. My line voltage is consistently around 125v.

I added the two 1m in parallel for 500k as the voltage devider before the pi. Not sure if I want to keep it.

I lowered the plate resistor on the input to 120k. I have it so one input is single triode and the other input is parallel triodes.

My voltages are higher all the way throughout. ~120v on the plate if the input, ~200v and 130 or 140v on the cf(can't remember exactly and it's not in front of me), but despite the pi node being high I get ~185 at the plates.

I routed the ground wire from the caps for the input and cf to the ground by the input jack. Pi node ground over by the master volume. Plate&screen nodes grounded with el84 cathodes over by the hi lo switch. Heater ground to + of cathode.

I get white noise when the master volume is up and the pre volume is over half. Is that normal?

[mauiboy81]

Input jacks ground is connected to the sleeve. I've got star washers on there.

Thanks for looking!

[mauiboy81]

I will also note that I have the 9.1k's for the input and cf b+ both coming off the pi node. The parallel triode input stage was pulling the b+ down too much.

[Richie]

Are the mica caps leaky? Most have quit useing them or use something else cause so many have had problems with the newer mica caps.

[dartanion]

Are the mica caps leaky? Most have quit useing them or use something else cause so many have had problems with the newer mica caps.

Yep. I switched to film caps after numerous problematic micas.

[mauiboy81]

What type of film have you used instead of mica? Polystyrene? Have you tried ceramic I noticed the zwreck uses them? Although j have seen mostly mica for the 50pf treble cap.

What are your thoughts on the voltage devider before the pi? Since I have the boost switch would it be better to use the divider to keep from pushing the pi too hard? At lower volume it takes away a bit of the sweetness but I haven't been able to crank it since I added the divider because of the neighbors so I don't know if it worth it.

[mauiboy81]

Any thoughts on the white noise/ hiss with volume up past half way? What is normal for this style build? I have just heard that they should be dead quiet but I get the white noise when it's up.

Also, I turned it on at night when it was really quiet and there is a bit if a hum that I hadn't noticed before that goes up with the master volume . With master down there is nothing.

Thanks!

[mauiboy81]

The white noise is a sssshhhhhhhh that increases with volume (noticable after 12 o'clock is loud at full) and is more pronounced when the cathode bypass cap is switched on. I know at that point the amp is pretty dang loud but I'm not sure how much to expect. If I ground the grids of the input tube it goes down slightly but when I ground the grid of the cf driver (or volume wiper) it is gone.

As far as the slight hum/buzz kind of noise that I there when preamp volume is down and master is up what could it be? Dirty power? Bad cf or pi tube? Bad ground? Bad filter cap? The cap going to the + side if the pi after the 220k is super sensitive to tapping. Is that normal?

On the schematics the power tube cathode ground is kind if off on its own. Mine is grounded to the same point as the plate and screen caps. Should I move it to a different point?

[Reeltarded]

Shhhhh is probably expected. Hard to say without hearing it in person or having a comparison. Cathode bypass will increase hiss exponentially. Little while off, and then massive.

Hum/buzz is lead dress between the input and volume somewhere. Pull the first tube. Did it stop? Probably input wire then.

Don't pull tubes with the amp running!

[mauiboy81]

Thanks reeltard.

I had tried pulling tubes and the sssshhhhh goes away completely when I pull the cf tube. When I pull just the first preamp tube it is lower but still there. When I pull the pi the slight hum goes away that was there when the master is all the way up.

Can you see anything that would be a problem?

[mauiboy81]

I have been trying to track down this noise and it has been driving me nuts.

The noise is kind if a light hum with a dirty buzz/hiss kind of sound. I can hear it when the master is all the way up and the preamp volume is at zero. It goes down with the master vilume and is pretty quiet at about 3'oclock. The tone controls affect the noise but not the preamp volume.

I have tried everything I can think of to get rid of it. When pulling tubes it is gone when I pull the pi. I had the pi grid wires running along the chassis and the plate and cathode wires flying so i made the grids fly and plate and cathode run along the chassis. Shortened the wires from the treble pot to the 210k feeding yhe pi. Replaced the treb pot. I have rebuilt the board like 3 times and changed caps and grounds. Rewired the power supply with new caps and resistors. Removed the master then put the "larmar" master back in. Swapped tubes. Rewired sockets. Replaced wire from volume wiper to grid. Replaced volume pot. And probably more I can't think of at the moment.

From the wiper of the treble pot to the grid of the pi(wire,220k,.047 cap,wire) is super sensitive. Buzzes and makes a ticking noise when I put my finger on the cap.

I have done everything I can think of and it is still there. Please help before I throw it at the wall!!!

Thanks.

[Colossal]

I have been trying to track down this noise and it has been driving me nuts.

The noise is kind if a light hum with a dirty buzz/hiss kind of sound.
I have done everything I can think of and it is still there. Please help before I throw it at the wall!!!

Thanks.

Don't trash the amp out yet. I concur about the region at the entrance to the PI as being sensitive. I ran into this with my last Rocket build. Make sure your OT wires are away from this area, ESPECIALLY the OT B+ center tap.

Second, what are your plate and cathode voltages? Are you running your EL84s well past 100% dissipation at idle like the Vox AC30?
[EDIT: I see you are biased at 100R but SS rectified, so maybe you offset the bias for the increased voltage due to higher rectification voltage)

Third, have you injected a signal into the amp and tuned it to the frequency of the hum you are getting? If so, what is that frequency?

Thanks,
Dave

[surfsup]

The noise is kind if a light hum with a dirty buzz/hiss kind of sound....the sssshhhhh goes away completely when I pull the cf tube

Are your filaments elevated? Could this be noise from a large voltage potential difference between the filament and cathode if the filaments are not elevated on the CF tube? Is that a 12ax7 or 12au7?

You can also try snubbing the filaments with a 110R resistor and 0.068u in series between the filament secondary of the PT.

[mauiboy81]

Thanks guys!

The transformer is 260/290v and solid state rectified. I had a sag resistor there before but I didn't like how hot it got. They are biased around 11w with the 100ohm and with the sag resistor they were around 9w I believe.
The noise is a little bit louder when I switch to the 290v taps and the tubes are biased around 14w.

My power transformer is a little loud... When on standby it hums a bit and the vibration can be felt when touching the chassis or components. I knew this before i built it because i could feel and hear it when laying out the pt and ot doing the headphone thing. I thought that the slight hum i'm getting could be from that but it goes away with the master down so I don't know about that.

I can't really tell what the hum frequency is but it has a slight buzzyness and little bit of pink noise.

My ot secondary's enter the chassis right under the pi input cap but the primaries enter through a different hole closer to the pi plate resistors.

Edit: the 6.3v center tap is connected with the power tube cathodes to the + of the cathode bypass cap. I moved it to ground with the ht center tap last night but I didn't notice a difference so I moved it back to the cathode. T
They are all 12ax7's.
Would snubbing the filament secondary be in addition to elevating them?

Sorry about another picture but this is where it's at now.