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SurfGuitar101 Forums » Gear »

Permalink Build Log: Slightly-modified European version of a 6G14 Showman, for bass use.

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There's a guy here in the Netherlands who understands transformers. He might know what you need. http://www.marble-amps.com/transformers/

Frank

dragonsurfer wrote:

You could still use a step-down transformer to run a 120 volt amp on your power grid. It's what major tours do all the time when they go to 240 volt countries and bring USA gear.

Which would be silly, unnecessary and would still not get me a 'period correct amp'. But again, thanks for the suggestion.

Gilette wrote:

There's a guy here in the Netherlands who understands transformers. He might know what you need. http://www.marble-amps.com/transformers/

Thanks for the suggestion, but I'm pretty happy with the transformers I've found. Big Grin Again, I'm not trying to make a perfectly period-correct showman. The resistors I plan to use are 5¢ modern carbon composites, not $1.50 or $5 'NOS' or 'Reproduction' ones. The same goes for capacitors, tubes, sockets, potentiometers - and I plan to run this amp into a modern bass cab with modern drivers, not a 1960 reproduction with original JBL's! Smile

I was concerned that the low-mass blackface output transformer was going to be insufficient to pass the bass frequencies through correctly - after all, bass is the reason I'm doing this build. But now that I've found the TAD reproduction brownface twin OT, I am content and confident that this won't be a problem. Groovy

Again, the power transformer is well within spec for the amp, including for the tube heaters - so it should do the job admirably!


One more update:

image

The freaky 350K/70K tapped potentiometers arrived for the treble controls! Now, the only 'weird' parts I have left to obtain are the tremolo pots and axial picofarad capacitors - everything else is commonly available.

Whoops, sorry, didn't read the long copy. Thought you were still looking for transformers. Great score on the tapped pots though...

Frank

Gilette wrote:

Whoops, sorry, didn't read the long copy. Thought you were still looking for transformers. Great score on the tapped pots though...

Don't sweat it. I'm unnecessarily wordy! Smile And thanks! I managed to find a source for 10M RA pots, too - so the only components which will be woefully out of spec will be the 4M speed pot (3M is the closest type available) and the tubes which are definitely not what would have been used in 1960's America. There's also a 30nF±20% capacitor which isn't readily available, but putting a 22nf and a 10nf in parallel brings that part back in spec!

As soon as AmpMaker are back open, I'll put in my bulk order with them, which will include the PTFE wiring kit, most of the caps and resistors and some rather important hardware for mounting useful things like valve sockets and transformers...

Hey Redfeather, while continuing to learn I found a useful thing!

https://robrobinette.com/5F6A_Modifications.htm#Half_Power_Switch

So, you could have two switches, and have a half-tube switch and a triode/pentode mode switch. 25% power would given you something close to half volume, and running in triode mode massively reduces power amp headroom from my understanding?

Last edited: Apr 15, 2018 11:59:38

Yeah, I looked into how my Carvin achieves its 50% and 25% power modes and it turns out that it doesn't actually drop any tubes to do it. (I think) I read somewhere that the 25% setting switches to triode operation and it certainly succeeds in dropping the volume and headroom. I'd describe it as way less than half volume. At full power, full volume the thing shakes the entire building but in 25% mode it's totally "bedroom" level even cranked.

In short, I think you're onto something there! I haven't read through that entire webpage yet but it's still in the queue.

And in other news, a package just came in from Down Under...

image

Congratulations! After my original seller had run out, I got them from the same seller. They seem pretty decent! (I got six, instead of ten. I can't see myself needing to build more than three full brownface preamps in the next few months!) You should grab the eyelet boards next, they're cheap and a strong encouragement to get building. Smile As several people said in your thread, once you've got the circuit together, the hard part's done - and it's both the cheapest and most unique part of the amp. After that, everything is more or less just off-the-shelf assembly!

Talking of building more preamps... I got a little carried away looking at that Bassman Micro design I linked. Given the multi-tap transformer, pulling a pair of output valves on my amp wouldn't be a big deal, but if I don't fry myself building the Showman clone, I'll be hard tempted to try designing my own amplifier. It would be nice to have something 'small' but all valve to replace my Behringer combo down the line; much as I adore that amp - and the idea of a Micro Showman appeals. There's a few ways I can think of to go about it, which makes it fun - copy the bassman for 40W from 2 5881's (That's basically a bandmaster, right?), copy the later Princeton power amplifier for 2 6V6's at about 20W, echo my Vaporizers' EL84 pair (but with fixed bias) for 15W, or go for a 'Quad' of 12BH7's for 10W. I kind of like the last idea, because no-one seems to have used a pair of 12BH7's for a quad-triode power amp before, one can still apply a tube-cut switch to it for 10W/5W, or both tubes can be swapped to 12AU7 for a 4W/2W amp.

The heater current of even the 12BH7's is only 600ma per pair of triodes, too - so with a total of 3A it's easy to find way over-spec PT's and appropriate OT's seem cheap and easy to get, even if I build another full six-tube preamp or go for the more novel output stage. Would be nice to have a somewhat more overkill OT to match the ridiculously big one on the full amp though.

Anyway, that's for another time...

I was slightly tempted to add either a triode switch or a tube-cut switch to my amp, but given the other mods I'm adding it seems a little unnecessary - I can always just pull tubes and use a different speaker tap. Seems more appropriate for your build!

I just finished my first-run parts spreadsheet for my build, which obviously points to UK or EU sources for almost everything, with the exception of the eyelet boards, 350T70K pots and 10M-RA pot, which can only be had elsewhere. First-pass lower bound estimate for a complete from-scratch 6G14 is about £975, but the actual final cost will be higher because of my time, and shipping and import costs. Of that lower bound figure, about £300 are the 'structural' components like chassis, cabinet and control plates, and about £370 is the transformers and tubes. (The £75 of my tube selection is relatively cheap, so I would expect this figure to be higher in more historically-accurate builds.)

Cost just to build the pure board set: of boards, caps, resistors, diodes and wire is about £150, of which I already have about £50 invested. So again, my next plan is to order the few remaining 'weird' parts (some pots and the tube heater mode switch), the mounting hardware, and the complete set of caps and resistors so I can finish populating the boards. Smile I'm expecting that to be about a £120 outlay, which isn't too bad!

Oh, and most importantly: I managed to give away my little 5W 'SubZero' head to a friend who hadn't ever owned a tube amp before! Smile

Last edited: Apr 16, 2018 21:10:52

It looks like my next batch of parts should be arriving around the end of next week!

Excitingly, I've also now filled the last hole in my rear panel.


I've mentioned several times now that I'm planning to use slightly different tubes from the norm on this build. I was reading the datasheet a couple of days ago when I realised something interesting - the 6G14 was going to run one of them way out of spec:

image
Well they're yer problem right there!

The oscillator tube in the vibrato channel has 165V on the cathode of the second half. Big deal right? Well, kind of: tubes have a Vhk(max) rating, or the maximum safe difference between the reference voltage of the heater and the cathode. Above this voltage, the tube lifespan will be dramatically shortened. On my tubes, this voltage is 100Vdc. Naturally, my first thought is 'Just use normal tubes, you lunatic!' Except, when I checked the specs of the 12ax7, that also shows a Vhk(max) of 100V. Our Showmans have been running these things out of spec since 1960! In fact, many guitar amps have. This has never been an issue, because pre-amp tubes are cheap, and in the old days they were so overbuilt that it didn't matter. The truth is, my tubes are also likely so overbuilt that I could just ignore this entirely and take the ~1000h lifespan hit without noticing.

Buuuuuut, that would be boring. There's also been a handful of anecdotes among amp builders that current-build FSU tubes are much less hardy than they used to be, which will matter if I ever flick the heater mode switch to '12ax7'. And I also noticed something else while I was researching this: The Hammond 291FEX doesn't have a centre tap on the heater winding.

Again, this isn't a big deal. The risk from having a floating heater voltage is an increase in hum; so I could get away with not having it, hypothetically. Still, for the sake of a few milliamps of heater current, better to ground it through an artificial centre tap of 200R resistors from each leg. Or; for the sake of a few milliamps of A+ current as well, elevate it to about 80V. By doing this, I should make an even bigger hum reduction compared to just grounding it, but more importantly I'll reduce that Vhk(max) down to about 80V (or down to about -80V on the others. The Vhk(min) is -200V, so even less of a problem.) We'll get our 80V with a small voltage divider, taken from the Standby switch for simplicity.

But, this doesn't explain what happened to our last rear panel hole...

In later AB763 amps (including the Twin RI), Fender removed the centre tap tranformers and replaced them with an artificial centre tap like we're using; except that they used a 100R screw driven pot, externally identical to the bias pot. This allowed a technician to tweak the relative position of the centre tap between the two legs of the circuit, in order to further reduce noise. So that's what I'm going to do here. I'm keeping the 220R resistors; this is to reduce the heater current loss and to add a little safety in case the adjustment pot ever fails to a dead short.

image

So TL;DR, elevating the tube heaters to ~80VDC to protect the vibrato tube and reduce hum, with a hum balance pot added to the rear panel, next to the bias pot. Total cost, just under £11. Nice.

Edit:

I've also decided to remove the Vibrato footswitch jack and replace it with a vintage style RCA. I actually don't want to do this, because it means using a £30 Fender vintage repro footswitch, instead of any of the three Vaporizer Mode footswitches I have lying abandoned in a storage box in my wardrobe, but now that I've filled my remaining hole, the Vibrato footswitch is the only 6.3mm jack on the rear panel, that isn't a speaker connection; and it's directly adjacant. Better to have them different types, so I can't accidentally short my output transformer by plugging it into a footswitch.

Last edited: Apr 20, 2018 22:30:23

I forgot how much I hated assembling parts for this kind of thing. Mouser wanted to charge the value of the Choke in shipping costs, so I ordered that now, along with the 47 bulbs, a few random spares - in order to make up to the free shipping limit. Similarly, the shipping for Studio Sound was hefty; so I ordered a few extras from them as well, including a couple of decorative parts and a Cap Pan Cover - both of those will save a fortune compared to buying them locally or in a separate order. That order includes all the pots, F&T caps and resistors for assembling the Cap Pan. When the order arrives I'll be able to wire 2/3 boards - with the last one requiring the rest of the passives to complete.

And my pre-amp tubes are on the way for good measure. A big box of 6N2P-EV tubes that are the same age as me. It's obviously a bit of a gamble using NOS tubes, but there are enough tubes in the box to have a few spares in case a few of them are faulty.

After work, I'll place the big order for the passives, wire and mounting hardware from AmpMaker - then it'll be a cabinet, two transformers, the 6L6's, some form of speaker and... $1 worth of fuses. Funny how things end up.

And the passives are ordered, along with the wire, mounting hardware and most importantly: the cream brownface knobs.

I've decided I hate the blackface knobs, so this build is going to be 'supersonic-esque': themed like a Brownface amp, but obviously different on the second look.

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