Watkins 14w Westminster/ dominator /marshall 18w tone and trem issues

[Metro]

Hello,
Here are the schematics: http://web.archive.org/web/200711012..._dominator.pdf

Here is a picture of the same model just out of interest: Alan Vale's Watkins Westminster 14 Amp

I am stumped by 2 issues in a 1962/63, 2 x EL84, Watkins westminster combo. Which is virtually the same as the 18w Marshalls and the Watkins Dominator.
The amp appears to have had zero work done in its life and appears to have had a yellow print Mullard EZ80 rec tube installed, poss from new?, instead of the higher speced EZ81 shown on the schematic. After one new EL84, one 12ax7, cap, res, deOxit work it sounds really great but there are issues.

First off, the power filter multicap was spitting fluid so was replaced with new as were most of the 0.01uf 400v HUNTS caps and a couple of .005 uf as many tested over, and occasionally under, 20% of the rated specs, including a couple of resistors. It is now lovely and quiet on both the trem and norm channel. However the trem bias oscillation wont be stopped with the depth pot fully counterclock or even grounded.
Secondly the Normal channel tone seems to do nothing until it is turned fully clockwise and then from 8-10 gets more muted/bassy.
I have tested the 500klog, 1k and 2M pots (vol, tone, depth and speed) as good and have checked the wiring many times.

I wish all of this forums contributors and readers the best in all their endeavours.
I have a replacement EZ81 on the way and wonder if the voltage ratings may be at the heart of this issue.
Any wisdom will be appreciated,

Have a good xmas everyone, Metro

[Jazz P Bass]

Did you do any work, replacements on the Tremelo circuit?
That footswitch grounds the V2B grid.
Does it? Check with a multimeter that the gridis infact being grounded.
With a grounded grid, ideally, oscillation of that stage should stop.
I would go back & check that all connections, from component to component, are indeed true & valid.
As to the tone stack, that is one goofy circuit.
It is nothing like the Tremelo circuit.
Kind of like a mid cut.

[Metro]

Thanks JazzP,
I should have pointed out that the only difference between this amp and the schematic is that there is no footswitch on this model although grounding pin 7/2 (grids of v2) does silence the trem; which seems like progress, thank you. The Normal tone circuit is confusing to me also, Im very glad Im not alone. The Trem tone works well though. The Normal channel is the same as many other 18w marsh clone schems I have seen but I still dont understand this issue. Its seemingly simple and yet Im still in the dark as to this little circuit, and this problem.

What mild work has been done to the trem circuit has not been real modiifications; simply replacing a few out of spec .01 caps with new Wima 600v rated poly. I did replace the .05 from plate of V2B to ground with a .047 sprague drop. The amp was showing these trem and tone issues before replacing the aforementioned components.
Its 16 degrees centigrade in my flat just now, and I have the oven/stove door open. Brrrr. All the best, M

[pdf64]

The schematic you linked to is for a Dominator, whereas your amp is a Westminster. The EZ80 may be correct for your amp. I worked on one of these, a V Front, last year, a lot of the black coupling caps were open circuit.
Ampaholics' Watkins Westminster V-Front
What value is the cathode resistor on the EL84 pair?

'However the trem bias oscillation wont be stopped with the depth pot fully counterclock or even grounded.'

I think that's normal, it needs turning off at the footswitch to be completely off.

[Metro]

Thanks for the response pdf64. This model Westminster is the 62/63 14w one, so almost the same as the V front Dominator except for the cab and footswitch. See this page: The Watkins Westminster
You may be right about the EZ80 although Im seeing some slightly low voltages at the filter caps. i.e. 308/288/260 instead of 325/310/280.
Im interested that you say the trem wont turn off completely without a footswitch as its the same conclusion that I was trying to put off. It doesnt make much sense to have such a great sounding channel with a constant bias oscillation going on in there. oh well.
The EL84 cathode resistor/s are a 430r and 220r paralleled so should be around 146r but tests at 168r. Which shouldnt be a problem as the EL84's test at just under 9w plate diss. and sound great.
Unlike the coupling caps in the Dominator you worked on, this ones had mostly just drifted pretty high along with some resistors.

Do you have any ideas regarding the weird Tone knob behaviour?
All the best everyone, M

[Steve Conner]

Its 16 degrees centigrade in my flat just now, and I have the oven/stove door open. Brrrr. All the best, M

Somewhere in Scotland, eh?

Looking at the schematic, it seems like the tremolo could indeed run all the time, and the depth pot would not reduce it to zero. If you want it to stop, probably best to replace the depth pot with a switched one that does what the footswitch did.

Have you checked the .005 and .01 capacitors associated with the tone knob? Maybe one of them is duff. But again, it could just be the way that tone circuit works. I think it's supposed to be like the "Big Muff" one- a bass cut in one direction and a treble cut in the other.

[Metro]

[QUOTE=Steve Conner;193525]Somewhere in Scotland, eh? QUOTE]

Aye, Bonny Bonny Scotland

I hope you've got the answer as I have spent too long on this. I would replace the Depth pot with a switched one if it was mine, but its a "get it fixed to sell" job. I'll mention the issue to the owner and see what they want.
The .01 and .005 caps on the tone were replaced to no avail, so reinstated for originality. Its maybe just how this circuit works, or doesnt. Really ugly and undynamic tone control.
Thanks a lot for your thoughts, respect earned, M

[Alex R]

I own a Watkins Westminster which I have had since the late 60s. Pale blue early 60s suitcase job, as in the pics. I know it had the original tubes as I knew the original owner. I can confirm that these early Westminsters used an EZ80 as standard. Can't quite swear to the Dominator but the two V-fronts I have seen lately both had old Mullard EZ80s so it looks likely.

My amp has a footswitch and I confirm that you need it to stop the trem completely. Yours may simply have been removed.

The EZ81 can stand a higher transformer voltage (not relevant to these amps) and a slightly higher current draw (100mA as opposed to 90mA), but it draws 1A of heater current whilst the EZ80 draws 0.6A. This probably meant that Watkins could spec a lighter transformer and thus it may well be best to use the EZ80 and not the EZ81 in early Watkins Westminster and Dominator amps.

[Metro]

Very helpful, thank you!
As mentioned before this one (Possibly 1963, pots seem to date end of 62 to early 63) is different to most 60's westminster as it has the Dominator 2x El84 circuit, sans footswitch. A footswitch could possibly have been removed but I doubt it as all the solder has oxidised and has the original dark brown flux. Looks very much untouched since the factory, so that adds up with the EZ80 you specify. The PT seems a decent size for this little amp, they may have used the same ones as the Dominator since they had them in stock anyway.

Alex R, could you tell me does yours have this norm channel tone issue I have described above?
M

[Alex R]

- well of course I have messed about with it a lot, but I did draw out a schematic before I did that and I will have a look around and try to dig it out.

Yes those little black cylindrical Hunts caps are generally to be replaced, same in AC30s, they are not long-lived.