Crate V3112 issues, Grounding and hiss in clean channel

[iefes]

Hey everybody!

I just joined the forum to find some help with my Crate V3112. I already posted these questions in the Gear Page forum but discovered the music-electronics-forum afterwards, which seems to be more appropriate to my needs :-)

So, here we go:

First of all I have to say, that I'm familiar with the potential hazards which come with working on Tube Amps, I know how to discharge the filter caps etc. I've built and designed several effects pedals during the last years, so I'm used to working with electronics and schematics.

First problem with my amp: There's always some noise on the clean-channel. It's more of a hiss than a hum and it's always present, even when there's no guitar plugged in. It always sounds the same and its level rises with the increase of the volume. In the crunch-channel there isn't this exact noise. It's a ittle bit noisy too, because of the stronger amplification, but this sounds quite normal to me.

Any suggestions on this?

Second issue: It seems like there is some problem with the grounding. Yesterday, I tried using the Line Out of the V3112 into my DI Box and record it but when I plugged the plug from the Line out in the DI there was just hum. Then I unplugged it and when I touched the plug I felt some voltage through it. I also felt some voltage on the guitar strings. Then I plugged the guitar directly in my DI Box, which was powered from the supply of my effects board. One of the effects on the board was still connected to my amp with an instrument cable so the effects power supply shared GND with the amp. I just got a loud hum through the DI Box until I pulled the cable to the amp out of the effects on my board.
The Amp itself works fine and I use it every day. Also with effects it's no problem.

Does anybody here have an idea where I could start? I disassembled the amp and had a look at the PCB and all the parts inside. They look quite normal. Two resistors look like they've gotten very hot, but they seem like they're still doing fine (R21 and R60). I also checked if the Effects line out GND is connected with the mains GND ---> yes!

Schematics are attached. This should be the right one but I am not 100% sure about this.

Btw, I am in Germany, so this thing runs on 230V/50Hz mains voltage.

Any suggestions are much appreciated!

cheers, iefes

[The Dude]

Welcome to the place!
I wasn't aware the VC3112 had a "Line Out"? Are you sure you're not using the effects loop? As far as the hum, it sounds like a typical ground loop to me. Does your direct box have a ground lift switch?

[g1]

That schematic link doesn't work, try these:
http://music-electronics-forum.com/a...-677sch_a-.zip

And Tom Carlos had a similar problem with this model, not sure if it got solved or if it was considered normal: http://music-electronics-forum.com/t36917/

[iefes]

Hey both and thanks!

@The Dude
I am not sure if the VC3112 and the V3112 are actually the same amps.
The amp has two jacks on the back called "Line Out" and "Line In". I suppose this is meant to be used as an effects loop but I thought it should be possible to grab the signal and use it as tried. The DI doesn't have a GND-lift, it's Bo Hansens Design which claims not to need one.

@G-one:
Thanks for helping with the schematic, I don't see why it doesn't work :/
I had a look at this topic you posted and I will try to bridge the loop and see what happens. I also heard that some people discovered that the noise won't be there with the footswitch-plug plugged in.

I am kind of worried because I felt this voltage through my guitar and the cable from the Line Out. I can't imagine this is supposed to be like this.

Edit: One more question, I want to swap that nasty 10W resistor with one or two aluminium ones mounted to the enclosure. How critical is this value of 60 Ohms? I can't find a 60Ohm one. Just 50 + 10 or 56, or something like this. If I would use two 120 Ohm in parallel, would I need two of them with 10W or is it enough to have 2 x 5W? Thanks!

[g1]

Two 120ohm 5W in parallel will give you 60ohm 10W.
Where on the schematic are R21 and R60 ?

[iefes]

Each of them is connected to the grid of one of the EL84's.

Do you have any other ideas how I could try to solve the issues?

[The Dude]

......@The Dude
I am not sure if the VC3112 and the V3112 are actually the same amps.
The amp has two jacks on the back called "Line Out" and "Line In". I suppose this is meant to be used as an effects loop but I thought it should be possible to grab the signal and use it as tried. The DI doesn't have a GND-lift, it's Bo Hansens Design which claims not to need one.......

Oh, my bad. I hadn't noticed the "no C" in the model number. I was thinking a different amp. On the DI, check continuity from the 1/4" input ground to pin 1 of the XLR. If they are common, that could be a problem. If Bo is using some sort of chassis isolation, maybe not an issue.

[Jazz P Bass]

Each of them is connected to the grid of one of the EL84's.

Say What?

[g1]

According to the schematic, R21,23,57,60 are the grid resistors and they should all be 10K 1/2 W.
R22 is the cathode resistor and it is 60ohm 10W. It will normally run quite hot, which is why they use a 10W there.

As far as the voltage you are feeling, does the chassis get earthed via a 3 prong AC cord? Not sure how it is done in Germany.

[TomCarlos]

That schematic link doesn't work, try these:
http://music-electronics-forum.com/a...-677sch_a-.zip

And Tom Carlos had a similar problem with this model, not sure if it got solved or if it was considered normal: http://music-electronics-forum.com/t36917/

I put in a fresh set of tubes, check the dc voltages, and figured I got this amp as clean as I can get it. The other issue with the amp was the deteriorating Celestion Seventy 80 speaker. When I ran the amp through a fairly new Eminence speaker, that made a difference.

[iefes]

Hey y'all, thanks for your responses!

@Jazz P Bass:
As G-one said, sorry, my explanation was a bit confusing.

@ G-one:
Thanks! In Germany we do have a GND phase which is connected to the chassis. I checked it and it is all connected as it's supposed to be (chassis connected to mains jack GND pin and GND on the mains cord).

@ The Dude:
I will check this later when I get home. According to the schematic, the gnd of the XLR and the Input jack are connected via two diodes, a resistor and a cap.

@ Tom Carlos:
Thank you for the information. I will probably try new tubes (do you think I should start with the preamp tubes or are the power amp tubes more important for the issue of noise?). And the voltages will definitely be checked :-)

[TomCarlos]

Hey y'all, thanks for your responses!
@ Tom Carlos:
Thank you for the information. I will probably try new tubes (do you think I should start with the preamp tubes or are the power amp tubes more important for the issue of noise?). And the voltages will definitely be checked :-)

There is noise (white, pink, etc) static, ripple, hum, etc. I kinda rolled it all up into one big bucket. As we are reminded, we are not dealing with super quiet high fidelity equipment here and we certainly would never do a "signal to noise ratio" test on a guitar amp. But every guitar amp will have some type "noise." And we need to separate that from speaker noise(coil or paper rattle/failure in general) which can easily be tested by using another speaker.

[iefes]

Yea, you're right. I've had this amp for several years now and I'm pretty sure it was a lot quieter when I got it. Additionally it seems to be weird that the noise is just on the clean channel.

[iefes]

Small update:

I checked some voltages but not under test conditions like mentioned in the schematic. I ran it on 230V mains and didn't turn up the volume completely as I got some popping when I measured, already when the volume was at zero and I didn't want to annoy the neighbours.
The readings were always a little higher than in the schematic, but just in a range of 5 - 20V (similar to those made by TomCarlos). On V3 I didn't measure because, when I tried there was a VERY loud pop coming out of the speaker (pin 1).

I also tried to run the guitar directly in the Line In, which eliminated the noise in the clean channel and gave me a nice clean (but dull) tone. So the noise definitely has to do with the preamp. Bridging the Line In and Out didn't help with anything.

I used a pen to tick slightly against all the tubes while running the amp. On the preamp tubes I could here the noise coming through the speaker sometimes. So maybe the noise has to do with the contacts of the tubes in their sockets. I don't think they are all microphonic or anything because they aren't very old. What do you think? I could buy a new set of tubes but I don't know if it is worth the money. I will try to clean the sockets and tube-pins first and see if something changes.

The power amp tubes didn't react to the ticking even if they look quite dull but this is probably because of some dust.

Thanks a lot for your help until now, I think I'm getting closer. If somebody of you has any other suggestions what to try or if there is some common mod (beside the R22 swap) I'd be happy to hear about it :-) As I am always using the clean channel only, I'm not really interested to change something on the dirt channel.

Ah, something else. I had some noises while testing today. Just from time to time something like a minor "pop" or some hiss or hum but always just for some seconds. Hard to describe but maybe you know what I mean. This could probably have to do with bad connections on the preamp tubes as well.

cheers!

[g1]

A couple things to mention here, first, when you compare the distortion channel noise, to you have the OD level control up full? If not, try it that way, with the gain up to the point you get equal loudness as the clean when playing. Is OD still quieter?
And my question about the chassis ground was in regard to the issue of you feeling voltage.
When you say you feel voltage from the guitar strings, measure voltage with one probe on the strings and one probe on the chassis, try on both AC and DC settings.