Watkins Dominator junk box build

[nosaj]

I'm building a watkins Dominator and referring to the schematic there are 2 caps which I am not sure on the polarity of I pretty sure they're electrolytic .

Thanks, nosaj
c20 50uf/25v
c1 25uf/25v

[Justin Thomas]

Yes, they'd be 'lytics. Negative end goes to ground - both are cathode bypass caps.

Justin

[nosaj]

Yes, they'd be 'lytics. Negative end goes to ground - both are cathode bypass caps.

Justin

Ok if the cap C20 is the bypass cap, R30 200 ohm sets the bias of the el84's? Right?

What are common areas in the schematic where you should use higher than half and 1watt resistors?

Thanks,
nosaj

[Justin Thomas]

First question, that is correct.

As far as where to use 1W resistors, in such a tiny design, I doubt any are really NECESSARY, beyond R30 and the droppers in the power supply... without the voltages posted or listed on the schem, you'd have to do some math... figure I through the Resistor = Vdrop/R, then take that I x Vdrop for the power dissipated in the resistor. Hard to say precisely w/out knowing your B+, etc.

Personally I don't like burning resistors so I go overkill (within reason) as space permits... I also build mostly PTP & so like the thicker leads on 1W resistors.

I just followed the suggestions on the Matchless Spitfire schem when I built mine...
You could also follow a Marshall 18W kit, since they're basically the same amp (according to Jim himself).

Justin

[Jazz P Bass]

What are common areas in the schematic where you should use higher than half and 1watt resistors?

Figure out the dissipation that the resistor will have to deal with.

That is the answer.

Be carefull of overkill on resistor wattage ratings.

Depending on the fault condition involved, sometimes you Want a resistor to fail, thus saving other parts.

You could build an amp with 100 watt resistors everywhere only to find that a tube failure took out an expensive transformer, rather than a 10 cent resistor.

[Justin Thomas]

Well, that would also require a chassis 100x the normal size, to fit them all in...

I guess heater center taps and screen grid resistors come to mind?

Justin

[nosaj]

Well, that would also require a chassis 100x the normal size, to fit them all in...

I guess heater center taps and screen grid resistors come to mind?

Justin

I finally got some more time on this build, got the rest wired up. Still being bedeviled by input wiring jacks.

here's where i have a question on the tremelo side, trem switch can be open or closed to get the same effect. I can't seem to input a signal at either grid. I can input signals at plate or cathode and hear it through the speaker.
pin1 getting 82vdc pin3 and 8 89vdc pin6 98vdc.
C9 wasn't sure about orientation so I put negative end to trem intensity pot.
nosaj

[Justin Thomas]

Hoffmann' s site has actual pictures of how to wire input jacks, both single and double. That said, that upper channel confuses me... two identical jacks, one to each grid, but the rest of the tube (categories and plates) are both paralleled? Doesn't seem like there's any difference between the two; I'd just eliminate one, and have it go to a paralleled triode stage.

To anyone else: is there any difference between those two jacks? Why would they do that? Or is it a schematic error?

Justin

[Dave H]

Hoffmann' s site has actual pictures of how to wire input jacks, both single and double. That said, that upper channel confuses me... two identical jacks, one to each grid, but the rest of the tube (categories and plates) are both paralleled? Doesn't seem like there's any difference between the two; I'd just eliminate one, and have it go to a paralleled triode stage.

To anyone else: is there any difference between those two jacks? Why would they do that? Or is it a schematic error?

Looks like a schematic error to me. I wouldn't trust that schematic. All the other schematics I have seen have the Normal inputs wired for Hi and Lo sensitivity. Lo uses only one triode and Hi uses both in parallel (See attached scm).

Also on the Trem channel inputs there is a 'blob' missing, the right end of R20 should be connected the the left end of R19. And I'm not sure of the EL84 grid leak resistor value. They are usually 470k but on this schematic they are 47k (R31,R32).

[Dave H]

Perhaps it isn't a schematic error? On the Dominator schematic I posted it looks like the Normal inputs are in fact identical so I thought it must be the Marshall 18W which has the single/parallel inputs but no they are the same too! I checked the Marshall 1974x manual and it does in fact say the Normal inputs are identical Why do the schematics on 18watt.com have single/parallel inputs? Which is 'correct'? I've had the single/parallel configuration for years. I would never change it.

[J M Fahey]

No schematic error at all.
They had 2 triodes available so they used them: one jack to each grid.
To mix signals they joined plates on same load resistor, current through it mixes and adds up, coupling cathodes saves one cathode resistor and cap.
Having 2 equal , same gain, same high impedance, non interacting inputs is the proper way to do it, remember amps were expensive and meant for more than one instrument.

Joining 2 (or 3 or 4) jacks into a single triode is the cheesy cheap way to do it, only we got used to it and were told that mutual attenuation was a "feature"

http://www.prowessamplifiers.com/sch...isc/kay720.pdf

Funny because active guitars , pedals and rack processors and even high output overwound pickups were unheard of in those days, so nobody would have designed for them, multiple jacks were not designed for attenuation but for multipleinstrument use.

I metioned a few days ago I am reading Keith Richards` autobiography.
Among tons of very interesting and eye opening stuff, such as that they got not a cent from the first album , they got paid "a salary" (think McD, not General Motors CEO) for the first 18 months of playing, sometimes even 2 shows the same night, etc. , he remembers they got Bill Wyman in "because he had an AC30 amplifier" and for the first shows they plugged 3 instruments into it.
Previously they had been plugging into modded radios and Richard Mother's tape recorder (which he ruined).
So a multi input amp was a bonus and the Dominator way to achieve that was the better one.

[Justin Thomas]

Thanks, Juan! Maybe a way to get more versatility out of a Spitfire clone...

Justin

[nosaj]

Thanks, Juan! Maybe a way to get more versatility out of a Spitfire clone...

Justin

So did we decide the schematic I am using is ok? If so, without an input jack connected to the grids on the trem side, i should be able to injected signal at the grid and hear it in the speaker correct? because I'm not hearing a thing on the trem channel. Normal channel passes signal that way( Ikinda bypassed the input jacks until I could get them worked out. so I was trying to still move forward even though a section was stalled.)

nosaj

[J M Fahey]

Basically yes ... IF itīs wired exactly s shown.
But check it yourself, measure resistance from tremolo channel grid to ground,then if >0 inject signal again.

[nosaj]

Basically yes ... IF itīs wired exactly s shown.
But check it yourself, measure resistance from tremolo channel grid to ground,then if >0 inject signal again.

Measuring like 187k So Next time I get a chance i need to verify resistors connected to that socket. It's like I can inject at the cathode or plate and hear it in the speaker but not at the grid. I'm thinking grid is not grounded so maybe the load on the plate or cathode is incorrect and not pulling electrons across the grid?

nosaj