Sudden increase in B+ voltage

[riley0]

Can someone explain why I've suddenly got an extra 30v after my diodes.
-Wall voltage is same
-No changes before increase

It really got on my nerves now my voltages are all shot..
Is it the transformer?
Is the amp pulling more voltage from somewhere maybe, if so where?

Thanks in advance!

[Bieworm]

We need more information. What amp exactly? Schematic?

[riley0]

Its an old watkins amp.
Voltages have been good for days I have it on my bench awaiting new matched tubes...
The B+ went from 290v to 320v overnight without me touching anything..All resistors/caps check good.
AC after transformer is 250v.
Diodes check good..
All dropping resistors check good, tubes are good..I can't understand it...I've been over and over it looking for cause...

Question I'm asking is what could possibly cause this?

[Daviedawg]

320v is closer to what you should see for B+ if that is a Dominator or Scout. What is your voltage selector set on? For example with my Scout set on 250v the B+ is 330v with a valve rectifier. Set on 225v it is 350v. Diode rectifier might read a little higher.
If that is the age of the amp there are many potential culprits. Is this an amp you have used a lot or just bringing back into use? I can see no issue with 320v if it is a Dommie or related amp. But Westminster or other related I am not familiar with. There is a Facebook group WEM/Watkins amps where you will get plenty advice.

Dd

[riley0]

Westminster mk9 model. Not sure how to post image but schem is available online..
Schem calls for 290 B+ I was originaly getting 295.
Amps used at home quite a lot...Familiar with FB group but they're not that technical.
Subscribed to the Watkkins.com page but awaiting admin email which doesn't look like it's coming so posted here as it's Marshall 10w related amp.

[Bieworm]

I would check the bias and see if it is correctly set. A raise in bias resistance would raise the B+ voltage, but you haven't done anything to that resistor...
Have you checked and swapped the tubes yet? I suggest you take all voltages and write them down + post them. That means all B+ nodes, all plate and cathode voltages, heater voltages, grid voltages,...

[riley0]

How do I upload pics on here?

[JMPGuitars]

How do I upload pics on here?

Under the text editor box I'm typing this in, there are tabs. One of them says Attachments.

[riley0]

voltage Pic uploaded
I've got a handfull of ecl82s and none change the voltage so it's nothing to do with tubes or bias

[Bieworm]

Assuming I am doing it right on RobRob’s tube bias calculator, I took the 9W tube since there is no ECL82 in his list, you have the bias set at 82%… so that’s perfect! No matter what voltages you recorded.. this is spot on.
When you look at your primary voltage… that is 249V!!! That's pretty steep if you ask me. At my area they’re 235… that’s nearly 15 volts more you have. This explains the 260V secondary HT voltage instead of 245V spec’d. That is why your B+ is so high. I assume the voltages you took before these, when they were lower, your wall power was much lower.
Maybe you should check your DMM (battery,…) or use another one and check again

[riley0]

ECL82's 7W tube buddy...
UK mains voltage is 240 - 250v....It's usually 250 by me on a good day..This problem has nothing to do with mains voltage raising and lowering.
During the time when i was getting correct V readings, I hadn't taken the V on the secondry windings so can't say if raised or not..
This will rise anyway will it not, if more demand is asked in circuit.
Appreciate your input but seriously batterys in my dmm..

[Bieworm]

Sorry for miscalculating you man… but sometimes even genius people forget the plain stupid things like batteries of DMM’s

I wish you all the luck…

[JMPGuitars]

ECL82's 7W tube buddy...
UK mains voltage is 240 - 250v....It's usually 250 by me on a good day..This problem has nothing to do with mains voltage raising and lowering.
During the time when i was getting correct V readings, I hadn't taken the V on the secondry windings so can't say if raised or not..
This will rise anyway will it not, if more demand is asked in circuit.
Appreciate your input but seriously batterys in my dmm..

The amp wouldn't be "pulling more voltage from somewhere" - that would lower your B+ if the load was raised (consider how much higher your voltages would be with all the tubes removed).

You said "AC after transformer is 250v" assuming you mean the secondaries, the schematic says 245VAC, however, you wrote 260 on the schematic next to the 245V, so which is it? 245, 250, or 260? That variance could cause a higher rectified DC voltage, and implies the higher wall AC may be related.

Here's a thought. Let's say there was an issue that you corrected, whether aware of it or not. Maybe the old tubes you replaced were bad, maybe something was causing an oscillation that increased the transformer load, and dropped the voltage. Correcting whatever that was likely corrected the load, and now you see the results expected with higher source AC compared to vintage numbers.

If my assumption is correct (or if you just want to fix it), see lowering B+ in my signature to resolve the issue.

Thanks,
Josh

[riley0]

Thanks for the replys guys..
Altered schematic below with zeners in place-

Can someone confirm this is correct?
When placed inbetween 1st - 2nd filters does this mean the ECL82 output plates won't be lowered?
Also the other problem is, sourcing 400V+ zeners. As standard they're usually 200v max..

(Pls don't see my replys as condescending, it's easy to misunderstand or read posts the wrong way, I truly appreciate everyones feedback and this forums a great read for builders..

[Bieworm]

Zeners are best placed in the DC path between the rectifier and the 1st filter cap. The stripe has to point towards the rectifier or standby switch. Could it be that misunderstand the zener value? If you want the B+ to drop 30V… you need a zener of 30V. But a single zener is not ideal, for you can not tweak the B+ accurately enough plus you need a higher wattage type that will cost a lot of money. But if you take 5pcs 6V 5 watt zeners and solder them in series you get the same voltage drop, but a better spread of heat dissipation. I’d advise you to buy more zeners in different voltage values, so you can tweak as much as you want. Needless to say that the stripes of the zener chain all point in the same direction… towards the source that is

Forget your drawing.. the zeners are at the wrong place and in the wrong orientation. Plus you would have only dropped the G2 voltage

Don’t underestimate the heat.. they get very hot very fast!

In the attachment you see my zeners between the recifier and the 150R sag resistor. This was the only ideal free space to put them in this amp.

A5C2AB88-7674-4834-8B17-1AD17A4E4FE4.jpeg