VOX VR30 Guitar Amp

[CJ]

just a short blurb on this nice sounding amp,

hybrid circuit, one 12AX7a in the mix,

this amp is used by a "player" so they wanted a quick turnaround,
input jack again, plastic jacks are the #1 failure on modern amps, next to red plate 6L6 tubes from designers that want to be a little louder than the other amps on the showroom floor,

probably the best hybrid i have heard,

 

[CJ]

here is a chassis shot lifted off the net,

 

[CJ]

could not find the schemo in jpg form,

probably for the better as this is a fairly complex circuit and would not fit on a single 800 x 600 sheet,

schemo on page 3 and 4 of the pdf,

"Valve Reactor" technology, kind of a cool marketing name,

easy to use amp, not too many bells and whistles, just  one clean channel and one overdrive, no effects processor, just a nice reverb pan,

read on a forum that a couple of EE's could not understand the circuit, VOX must have stole someone from the space program to do their design work,

pwr amp is a LM3875 chip,

http://www.blueguitar.org/new/schem/vox/vr30.pdf

LM3875 datasheet, 56 watts all day it says,

http://www.ti.com/lit/ds/symlink/lm3875.pdf

tube vs transistor part, those are 47K plate resistors,>

 

[Whoops]

Hello CJ,
I'm working on a Vox VR30 amplifier at the moment.

The Pots, at least the volume Pots are bad, loud thunders when rotated, probably humidity and use.
They will be re-lubbed or replaced.

There's also a Loud low end Hum (Europe) coming from the amp, with all the controls at zero, and without anything connected to the input.

I tried o tap components with the chopstick, and nothing improved or change.

PSU electrolytic capacitors have good looks, but I will replaced them because they can be the source of the Hum.

I will replace the Chinese 12AX7 anyway for a proper tube.

If you have any other ideas  of what could be the source of the hum or advise on service this amp please let me know?

 

[CJ]

put a patch cord into the effects loop,

pwr supply hum will be 100hz,not 50,

knowing what you are hearing can narrow the problem down,

compare  it with a signal generator into the input,

cheak the heater supply and output tube balance,

lytics s/b ok    bridge with a 47/500 to find out  be careful on that one,  one hand in the pocket,but don't be playing pocket billiards

 

[Whoops]

Thanks CJ, hum is centered around 150hz , with dime harmonics also in tge octave up and otave down. I measure it with my phone because Im at home now and not at the shop. Pic attached.

I will do as you told.

What do you mean by "output tube balance"?

Thanks

 

[Whoops]

put a patch cord into the effects loop

Just to let you know theres no effect loop on this amplifier.

The output jacks are for footswitch, phone out and external speaker

 

[mjrippe]

Thanks CJ, hum is centered around 150hz , with dime harmonics also in tge octave up and otave down. I measure it with my phone because Im at home now and not at the shop. Pic attached.

Neat!  What software/app is that?

 

[CJ]

hum is cancelled in the push pull circuit

hum canceling will therefore be tied into tube matching

try to get equal plate current

Fender has  a pot for this,  check the Bassman 50 circuit

 

[Whoops]

The app is called "iAnalyzerLite", I have the Free version and its great.

Tried a lot of diferent RTA apps for iOS, and this was and is my favorite

 

[Whoops]

hum is cancelled in the push pull circuit

hum canceling will therefore be tied into tube matching

try to get equal plate current

Fender has  a pot for this,  check the Bassman 50 circuit

This is a solid state amp CJ

 

[CJ]

oh boy, you are right,

might have to track the hum with a scope

too many amps, not enuff time,

 

[Whoops]

No Worries mate, because it was your thread I thought you might remember the amp, but it waa 2 years ago and upu probably had a lot of diferent amps in your bench.

I replaced all the Eloctrolytics in the PSU and power boards just to be safe.
But that was not the source of the constant hum.

The "thunders with scratchiness" were from humidity residues inside the Pots.

The constant hum even with nothing connected and all pots at zero was from humidity residues and bad contact in the Master volume Pot alone, the pot that has Valve reaktor written under it.

Caig Fader Lube in spray fixed all the Pots (with rotation of the pots) fixed everything at least until the Pots reach to a point one day where all need to be replaced. For now and hopefully some years it will be fine.

Just for the sake of it I tested the cheap chinese 12AX7 tube that comes with these amps, with an Hickok tester, and it tested fine.
For young people that might read this thread in the future would just like to say, having one 12AX7 tube  running at 15v inside an amplifier doesnt make it a guitar tube amp. This is pure marketing hype that allows companies to use the "Valve or Tube" word.
This is a completely solid state amplifier, as is the Marshall Valvestate series, Art Tube MP preamp or any other starved plate design.

I would not even call this an Hybrid amplifier.
Examples of Hybrid amplifiers are the Roland Bolt series and Music Man amps, where you have a solid state preamp feeding a real and complete Tube Power Amp.

 

[Whoops]

Forgot to mention the amp is quiet now without guitar connected and with pots at zero.

Once you increase the Master volume theres some low end hum, something you can live with, theres also an hum increase when increasing the reverb knob.
This hum is caused by electro magnetic interference from the Power transformer inside the chassis. Solutions to this are to mount the power transformer outsidd the chassis, you can even use the transformer actual chassis holes and mount it outside the chassis facing them (like inverted to the present position)
You can also use transformer metal isolation sheed around it.
Or replace it for a Toroid trabsformer, rotate/position it for the least amount of induced hum, and then still use a trafo iso meral sheet around it. this will probably be the quietest

 

[CJ]

we pull the knobs off every amp that comes in so the face plate gets cleaned and the nuts get tightened

just about every amp has loose hardware, speaker and input jacks, speaker bolts...

 

[Whoops]

we pull the knobs off every amp that comes in so the face plate gets cleaned and the nuts get tightened

just about every amp has loose hardware, speaker and input jacks, speaker bolts...

yes I also had to tighten the pot's nuts

The transformer has 2 screws to the chassis, those were quite loose, so transformer was balancing inside.

This is just a cheap practice amp, but it was used on the road for the last 6 years.
When doing a lot of KM's in travels packed in a Van, the vibration of the car will make the screws loose after a while

I'm using Women Nail Varnish to seal and glue the nuts and screws, works great.
I like Dark Red, but you can use any color you like, the sound will be different though.