Breakup & spkrs/ basics.

[Sea Chief]

Here's one that will make your head hurt: air distorts..

Now look here- Im not having any more of this.. Im going for a snooze!

Ok Im going to put the reins on here. Please just assume the amp IS going via an attenuator (so all speaker-related/ added distortion is discounted from the equation to hone in on the amp/ tube OD itself- its perfectly interesting for a rainy day but complicating things unneccessarily for me here).

[Sea Chief]

R.G.

I bet you have an uber-fabulous hifi, with a turntable as your fav thingy of all thingys.. and you live in scotland is my hunch? respect if so (and if not also of course!).

I think an attenuator is the way to go, if I can be assured if used correctly (load etc) it will definitely be equally as 'detrimental' as using the amp without anattenuator at all. It seems a simple and affordable build. Maybe Chuck could point me tweds what sort of dial/ rotary component I assume would be the costly bit.

thanks chaps- Chief.

[Enzo]

If Scotland is in Texas...


Do yourself a favor, put all this in a context. How can we say if an amp under performs unless we can compare it to other amps of its kind? Take YOUR guitar to the store and play it through other examples of these amps. Are other Champs also clean all the way up, for example.

[Chuck H]

The big. expensive part is just a 25W/25ohm rheostat. They can be purchased anywhere. Don't buy a real cheap one, they're extra glitchy at the ends and don't last. Mine are made by Ohmite, which is sort of mid line. The resistors are of a type that have their own aluminum heat sinks with screws for chassis mounting. If you use thermal paste, don't place them right up against each other and mount them to the inside TOP of an aluminum project enclosure you can crank your Dominator through it all day and it'll just get real warm. I get parts from Mouser. Not sure of what UK distributor might have it all on hand. It should cost about 25 BPS in parts.

[Sea Chief]

Hi Enzo-

If I could I would- nearest would be london, 70m away (a normal drive for you guys- but a helluva way here esp fuel etc). Im very rural/ don't know anyone who even plays gtr here, let alone anyone who'se even heard of a tube amp!

[Sea Chief]

The big. expensive part is just a 25W/25ohm rheostat. They can be purchased anywhere. Don't buy a real cheap one, they're extra glitchy at the ends and don't last. Mine are made by Ohmite, which is sort of mid line. The resistors are of a type that have their own aluminum heat sinks with screws for chassis mounting. If you use thermal paste, don't place them right up against each other and mount them to the inside TOP of an aluminum project enclosure you can crank your Dominator through it all day and it'll just get real warm. I get parts from Mouser. Not sure of what UK distributor might have it all on hand. It should cost about 25 BPS in parts.

Ok ChuckH that sounds like a plan- I might strip the 'Champ' and redo it, or get a dedicated Champ PT and go from there.. I just don't trust this PT tbh: I know some may say its capable but I can't think my issues can be anything but this iron's limitations.

Can you tell me doing such a build, by how much I could expect to attenuate the Dominator cranked? it stays, from what Ive read, clean up to quite high unfortunatley then goes into the goodies. Could I say attenuate it right down, at its max setting?if only 1/3rd vol this still might be too loud for use here with the Dominator exuding its goodies/ turned up to 9.

Thanks- Chief.

[Chuck H]

Well, this attenuator is infinitely variable with a wider range than many. If the Dominator is putting out about 20W cranked (this is about what a fifteen watt amp puts out at full clip), plugged into the attenuator you'll be able to adjust the power to the speaker from about 18 watts down to 1 watt or anywhere in between. One watt of clipped guitar tone is just loud enough that you need to raise your voice a bit if you want to be heard. So, If you want to adjust it to the same level as the Champ, you can. If you want it a little louder or quieter you can do that too. As to how loud to turn up the Dominator, I'll say this...

Are the tubes old?
Does it make any funny noises like excess hum, static sounds or popping spontaneously?
Does it smell funny when it's been on for awhile?
Is there anything wrong with it's reproduction of tone like a buzzy sound or odd harmonic that follows the notes?

If you answered "yes" to any of the above I wouldn't suggest clipping the amp before it's serviced and has a full bill of health.

If you answered "no" to all of the above then start where you normally would with the attenuator full up and then start increasing the volume on the amp and decreasing the volume at the attenuator until you start smiling. Keep messing with the amps volume and tone controls until your smile is really wide. If anything makes you frown, discontinue and return to a setting that makes you smile again.

Tip: once the amp is clipping hard and your at an acceptable volume level you don't need to adjust the attenuator anymore. Continuing to raise the amps volume at this point will only increase distortion, not volume. You'll get use to it pretty fast. You just need to lose the perception that the amp will be quieter at 9 than it will be on 10. If the amp starts to break up notably at 6, then 6 is about where your max volume is and anything higher is just more distorted, not louder.

Does that help?

[Sea Chief]

Hi Chuck-

you bet that helps (as do the other replies- read thru several times again).

My old wem. I love her, she just has s'thing 'right'. Maybe its only that she's a proper tube amp, not a reissue or a diy clone (even though Ive never turned past 2, and its a 'bass' amp/ dominator mk3). Id answer no to your 4 Q's.. but smell: I do get a strong vintagey tube smell but Im fairly sure is a healthy niff.. does that sound right? I had a new'ish vox ac15C1 for a while which had a similar (really quite gorgeous isn't it?) 'el84' niff.

I bought her for £115 with a channel down and grill awol. I quickly diagnosed/ replaced 1 dud grid(?) resistor and fender oxblood grilled (this looks fab- better than orig which looks like granny's cardigan!).. so Id love to hear her at her best at last.

One thing worth mentioning is the OT: it was replaced before I bought not with a wem OT (n/a) but a decent 50w OT (here's the same Co's similar but 25w OT specs, mine has similar 2ndary lugs http://www.oep.co.uk/pdf/V78A01F%20issue%205.pdf ). I didnt get anwser to why replaced/ what happened but assured by seller & Co who did work (looks vg). Oddly its hooked up as follows from OT: blue wire from C lug as expected + 8ohm lug > to the orig 16 ohm Greenback 15". That's 8ohm tap to 16ohm spkr. I queried this again by work Co, assured again..

Hi Chief,
Yes the OT is fine for this amp, and it sounds like it is wired up correctly.
It has been connected for twice the normal primary impedance. In other words, by connecting the 16 -ohm speaker to the 8-ohm transformer tap, you get twice the primary impedance, so the 4k primary taps effectively become 8k taps, which is correct for a pair of EL84s.

This transformer was specifically designed to cope with this sort of operation.
Hope this helps,
Merlin (valve_wizard@hotmail.com).

As its been in and played enough times albeit @ 2 ish, I guess its happy but I'd appreciate thoughts as to why hooked up like so for reassurance before I continue with an attenuator.

Another Q I was going to ask relates here: when an amp's turned up into OD etc, is it working harder than @ 2? if s'one were to answer 'of course- whaddya think dummy?' I accept.. but might it not be so simple? IE if my amp is seemingly in good health @2, does that mean using an attenuator correctly/ carefully with her cranked should be no different? no more demanding? (maybe bar the tubes: I think Ive read tubes work harder as being a cert/ so taking into account periodic tube changing then).

Ok good stuff- most grateful, Chief.

[Chuck H]

It sounds like the amp is good to go. That bigger OT will actually be a bonus for higher power operation in the event of a mistake. It's usually the OT that get's it. I'm using a sturdy OT in my personal amp and I have mistakenly operated it momentarily without a load and suffered no failures. Still, take it slow at first.

About the amp working harder, idle or conducting signal at it's highest level. The answer may surprise you. It's 'maybe'. It depends on your bias at idle and other circuit parameters. Most Brit amps that used EL84's idle well into class A. That means the power tubes are conducting at or over 100% of their dissipation limit just sittin' there. It's not unusual to see EL84 amps that red plate at idle and but cool off when signal is applied. Now, because of the style of bias and the level of drive signal to the grids there is a shift that cools the bias into class AB1 when the amp starts conducting signal. It's usually close to a wash. The reality is that the amp is likely to get a bit warmer when conducting signal and therefor the tubes are working harder or at least doing the same work in a less hospitable environment.

[Sea Chief]

Hi Chuck,

cheers for reading my rambling rose of a post- worth posting/ concerns and Q's about my lovely dominatrix all came to fruition there. Good to read the OT is ok, and great you say its capable poss moreso than orig maybe re cranking her.

When you say 'idle' (forgive me) does this mean no gtr plugged in, and vol turned to 0? I guess its fixed bias would you know? Actually another minor Q/the tubes: the orig mullard el84's are awol (maybe shot with the OT issue), generic cheapos in place instead, as the seller would to get it ship-shape at least expense. Orig Mullard preamp tubes in tho. Are the cheapo el84's ok to get going with do you think?

[Actually the orig el84's had a slightly different pin config, and Mr. Watkins himself was obviously reading and emailed me worrying about his amp hadnt been addressed for new el84's! what a lovely thing- imagine LeoF sending you an email! I reassured him (one pin rewired to accept new el84- easy job/ done with the OT work)].

Next: build the attenuator then/ a new thread methinks. Thanks Chief.

[Chuck H]

I started writing a REALLY long post about bias and class of operation. Pointless.
Cheap el84's are fine because they're all cheap.
Idle refers to how much power your tubes are using when they're not even making sound and is controlled by the "bias".
Class A, class AB1, and single ended vs. push/pull are more in depth discussions that really require more explanation to be understood than I can give a novice in a whole day, much less a single post. The quick and dirty version is that in class A each tube makes it's full power, full time and in class AB1 each tube makes twice it's power for half the time. Your amp does both depending on how you're using it. It's possible that your amp conducts almost as much current at idle as it does at full power.