Vibro King Issues

[mikeboone]

My V/K is oscillating when the volume is above 1/2 and really gets bad when it is dimed.

I first thought it might be the vibe circuit but it is switched off and the lamp is not pulsing

One know problem with this amp is the reverb circuit is in-operable. Not sure if it is the tank, transformer, or tube. Could problems in this circuit cause this?

Could it be weak power supply caps?

I was hoping for a little direction prior to shot-gunning.

mike

[Alex R]

Is it the amp with the EL84 reverb driver, or the later 6V6 type? The EL84 type gives loads of trouble & weird noises thro the reverb, though not oscillation I don't think. If it is an el84 see if twisting the dwell control affects the oscillation, just to get the reverb issue out of the way.

Microphonic preamp tubes can cause oscillation, as can a bad preamp filter cap. Check for these by tapping preamp valves, and by (cautiously) bridging the filters with a good one. C34 and 35 are prime suspects.

[mikeboone]

It's an EL84 rev driver. I was looking at the schem and it looks like I could input a signal in the effects return, it'll bypass that circuit all together. It's worth a try.

And the term oscillation may not be the best word to describe the issue. Its a slow repeating drop in level along with popping and an occassional high freq squeal. (to be elementary - a "whop-whop" kind of sound)

I will definitely look into the filter caps as well. The amp is 10+ years old from what I can tell and re-capping it would be inexpensive and my well be some low hanging fruit.


Thanks for the input... I will work on it tonight and let you know what I find.

Mike

[Satamax]

Motorboating is usualy from failing electrolytic capacitors.

[Alex R]

...yep, likely filter caps - but just try pulling out that EL84 and see what goes away. I think that Fender actually did a recall on the vk reverb driver - there's too much voltage on that tube and it's hard to drop it. Someone here will have better experience to offer I imagine.

[mikeboone]

Update...

I put signal directly into the effects return and the out output works fine at all volume levels.

I removed the EL84 no change. I swapped out V1 & V2 one a a time and no change. These are all in the stage that was eliminated in the above mentioned test.

The reverb dwell (or others) control do not affect the problem just make it sound slightly different.

Observation: With the guitar plugged in either input and the volume at 10 the problem is there, but it is input level dependent. As I roll back the volume on the guitar the problem clears up. My guess is the EL 84 or the transformer.?.?

The output side of the reverb tank measures open, mystery solved there. I dont think the tank is affect the issue as the problem persists with the EL84 removed.

Comments?

Mike

[Alex R]

Sounds like an input stage problem then (or a guitar problem!). This amp has an unusual input stage - a cathode follower, V2A. There are quite a few components around the input that might be causing this, either by putting a bit of DC on the guitar pot or forcing the stage to find ground reference through it, or some other weirdness. So check C19 (0.047uF) and C2 (0.1 uF), and R45 and R46, both 2.2megohms.

The filter caps on that stage, C34 and C35, are still prime suspects by the way.

[mikeboone]

OK Guys...

I have been though all the steps you guys have suggested. The resistors were taken out of circuit and measured. I have changed all 4 of the caps (C34, C35, C19, C2 - schem attached) one at a time and no change. I also repaired the reverb circuit. The tank, the 9p socket, and EL34 were all shot. Its working now. I have also changed V1, V2 and V6.

To recap the issue... When the amp is turned up past 8 there is a very noticable oscillation (suspiciously like trem effect). If I plug the guitar into the effects return (bypassing the main input and rev circuit) there are no problems.

I can now say with reasonable certainty that the trem circuit is affecting it, but I don't understand how. The trem lamp flickering is in time with the issue and can be affected with the trem controls. When the trem is switched out, the problem is not as bad but when it happens, the trem lamp flickers. The trem circuit works fine when plugged into the eff. return.

Thanks,

Mike

[Alex R]

Hello again Mike. Because this kind of 'motorboat' issue is usually caused by poor decoupling between stages I'd carry on and do the rest of the filters. C31, 32, 33, 40 anyhow. But first a quick and dirty test is to take a new cap, clip the -ve end to chassis ground, and poke the +ve end to the +ve of each cap in the amp with it to see if the motorboating stops. There will be a few sparks... it does often helps pinpoint the problem. Be careful, obviously.

[robertsk]

Hi Alex - I just bought a VK shipped from US (I'm from Singapore) and is using a stepdown transformer output of 500 watts. What is the recommended wattage output for my transformer? I find the vibro king one of the worst amps I've ever played. I cant get a good clean tone at higher volume. If I wanted a distorted sound I wouldn't opt for Fender. True? And it's so
expensive I could've gotten a Soldano for that price. Some one posted a thread that by changing the power Groove tubes
could solve this problem. I tried, spent some extra money and still the same. Currently I own Twin reverb, Peavey 5150,
Marshall Plexi 1959 and Mesa Mark 1. Kindly advice what should I do. Selling is difficult as you know the price is not easy.

[Enzo]

Hi robert, welcome to the forum. Just want to point out that the thread here is from over 3 years ago, so hard to say if those guys are watching.

I don't know if they ever resolved the issue. SOunds to me like trem bleed, not "motorboating." And a quick thing to check would be to move the plate wires from V5B, the trem "bug" driver triode, as far as possible from the grid wires of the tube next to it.

Robert, for your amp on 240v, the mains fuse is either 2A or 1.6A depending on the version you have. That is very close to your 500 watts at full current. I would probably use a larger step down.

HOWEVER, as far as I know, all the V-K amps already have a universal transformer in them, so instead of a step down, you should be able to change the connections inside the amp, and then plug it directly into your mains outlet.

[robertsk]

Hi robert, welcome to the forum. Just want to point out that the thread here is from over 3 years ago, so hard to say if those guys are watching.

I don't know if they ever resolved the issue. SOunds to me like trem bleed, not "motorboating." And a quick thing to check would be to move the plate wires from V5B, the trem "bug" driver triode, as far as possible from the grid wires of the tube next to it.

Robert, for your amp on 240v, the mains fuse is either 2A or 1.6A depending on the version you have. That is very close to your 500 watts at full current. I would probably use a larger step down.

HOWEVER, as far as I know, all the V-K amps already have a universal transformer in them, so instead of a step down, you should be able to change the connections inside the amp, and then plug it directly into your mains outlet.

Thank you so much for replying. I'm no amp tech so the plate wires are alien to me. However, there's a
webpage from guy named John Shannon from Fender and said there's a bias knob under the chassis beside
the power cable. I found it and did exactly told but sounds to me just increasing or decreasing the gain.
Distortion still there. Guess I've to bring to somebody knowledgeable as you.
May i ask another question. My 5150 when powered on the power tubes are lighted. But when standby
switch is on there's no sound and i found the power tubes remained the same glow. What happened?

[Enzo]

Your 5150 could have any number of problems, we'd have to get in there with a volt meter and find out what is what.

As to the VK amp, remember that the voltages inside tube amps can KILL YOU, so if you cannot identify plate wires and so forth, I really must suggest taking teh amp to a competent tech. I don't think the problems you describe will be difficult to repair for him.

[robertsk]

Thank you for yr advice again. Now I realised that the main problem with the sound is that the power tube closer to the power cord (right tube if amp facing you) is always over heated and turns amber color. Kindly advice. Also I've changed the connections to 240v.
should I also change the amp's mains fuse to T1.6 or T2?

[Alex R]

Not sure why you assume that an overheating valve is the cause of your distortion. It's a fault, but there is not necessarily a connection between that and the distortion.

I agree with Enzo - you're not going to be able to resolve this safely; the amp needs to go to a tech.