First Build: Fender Showman Brownface (6G14)

[Tqi]

I had a flick round and couldn't see any kind of official 'make your introductions here' thread.

This is, therefore, me saying hello:

Hello!

I'm a bassist and guitarist from Europe. I've made a couple of effects pedals, but never anything particularly complicated. Recently, I decided to start playing bass at a level above the '15W Behringer Combo' level, but I couldn't find what I wanted in a commercial high-end bass amp; the closest was probably something like the Fender Bassman 100T, but they're expensive, not that well made and far too full of digital gumphf for my tastes. So, after spending some time considering older model bassmans, a young man named 'Woody' convinced me that what I need is a 1960 6G14 Fender Showman amp, a quad 6L6GC brownface design. The problem is; the £4000 it would cost me to get one of these in Europe (in the unlikely event I actually found one) would make it something I could never manage to buy, much less take to gigs and stand a pint of diet soda on it (I'm kidding about that last part... mostly). So; I reached the obvious inevitable conclusion, and I'm building my first valve amp.

I'm making a few modifications compared to the original 1960 Fender schematic:

• Since I live in a country where every socket is earthed by law, I'm stripping out the ground switch and using a 3-wire plug, naturally.
• I'm also using a slightly different preamp tube with a 9AJ pin-out, and I'm wiring my tube sockets for a panel switch that swaps the pre-amp sockets between 9AJ and 9A wiring. This way I can do tube swaps between 'specification' 7025 tubes, and the kind I'm using, without having to resort to a soldering iron.
• My nice European-voltage Power Transformer doesn't have a centre-tap, so I'm adding a Silverface-style hum balance pot as an artificial centre tap, elevated to 80VDC to reduce the Vhk stress on the pre-amp tubes in cathode follower and phase inverter positions; I don't want to be replacing them often, after all.
• I'm adding panel-mounted bias adjustment, also following the design of the Silverface amps, because it seems obvious.
• I'm using a reproduction Brownface Twin output transformer to get the sound of the oversize Showman PT, but with multiple taps so I can run different speaker configurations.

Finally, I'm building the whole thing in a reproduction Blackface Twin Reverb chassis. Brownface Showman chassis' and cabinets are not a thing that can be bought, and this seemed the efficient way to get around that; because of this I had the extra mounting points to add the Heater adjustment switch and pot, the Bias pot and the extra speaker outputs. I also have an extra pot, and a DPDT On-On slide switch on each channel, which I'm populating but leaving unconnected for the time being, as 'upgrade space' for doing any additional mods later. Conveniently, I was able to find a company in the US that would precut and press eyelet boards for basically any Fender amp schematic, so most of the 'mechanical' work is done, and I can worry about the electronic side alone for this first build.

As for where I'm at with it, I've made my tweaks to the schematic, assembled a parts list with sources. Of these, I have about a third, with the remainder of the passives, potentiometers and connective parts arriving around the end of next week. After that, all that remains are the decorative parts (cabinets, custom faceplates, etc), the transformers and the power tubes. You can probably guess why those components were left till last; i don't want to damage them while messing around with the rest of the build, and they account for just shy of 70% of the cost of the amp; if for any reason this all goes downhill, I have lost a relatively small amount - unless it's after purchasing the cabinet and transformers.

In any case, wish me luck!

[Chuck H]

Welcome and good luck

A couple of things...

First, please DO provide an image of the schematic you've modified so that any future reference to the design isn't confusing. Verbal descriptions of circuits will put down a thread faster than anything else.

Changing the filament wiring such as you propose would typically require a change from 12.6V to 6.3V. Does your transformer provide both voltages? Maybe you could indicate the tube model you propose to use for us. But more important, why use an odd tube model? Even if you have them on hand? Consider the trouble it causes. First, you'll be adding switches, complication and effort to the design. Second, modern 12ax7's have been and continue to be the most redily available preamp tube for instrument amplifiers and are available cheap and LOW IN MICROPHONICS! Something that often becomes a problem with unusual tube models used in instrument amps. Further, being as your somewhat new to this, most of the circuits you reference will be relative to the 12ax7 design. In the end, is the value of the preamp tubes you want to use worth the extra trouble?

re: Building on a Twin Reverb chassis and using the extra holes... Maybe, maybe not. Good design should dictate the control layout. Not where the chassis holes happen to be. Certain controls should, for both practical and electronic reasons, be located on the back of the amp rather than in a front panel hole. Perhaps this is unimportant to you now, but it may not always be. There are good reasons for locating bias adjustment and hum balance pots on the back rather than the front. That's why you never see them on the front.

[Tqi]

Welcome and good luck

First, please DO provide an image of the schematic you've modified so that any future reference to the design isn't confusing. Verbal descriptions of circuits will put down a thread faster than anything else.

Can do, though I'd like to tidy up my scribbles first, before I show it to anyone! The original schematic is here, if you're interested: http://www.thetubestore.com/lib/thet...-Schematic.pdf

Changing the filament wiring such as you propose would typically require a change from 12.6V to 6.3V. Does your transformer provide both voltages? Maybe you could indicate the tube model you propose to use for us.

The schematic for the Fender Showman (like most Fender amps) wires the heaters of the 7025's in Parallel, not series - so they'll be running at 6.3V anyway. I'm using a Hammond 291FEX which provides plenty of current for the heaters of the 7025 in parallel, as was done in the original design of the amp. It's literally just a case of switching pin 9 to ground, and switching Pin 5 from one side of the AC heater circuit to the other. As a simple safety alteration, I plan to mount the switch 90' from the power and standby switches, so that no-one can switch it by accident while feeling for those switches. Obviously it would be a bit rubbish to put 86.3V through a 7025 heater if it was switched in error. This is however the only risk state in the design: if the switch is flipped to 9A mode with a 9AJ tube inserted, the tube simply gets 0V across the heaters. There's a risk of cathode stripping if it was left that way for a long time, but it should be pretty obvious to the user that something is wrong when no sound comes out. For reference, the tube I'm planning to use is the 6N2P-EV, which apart from the 9AJ pinout change for the heater wiring, shares effectively identical specifications with the 12AX7.

But more important, why use an odd tube model? Even if you have them on hand? Consider the trouble it causes. First, you'll be adding switches, complication and effort to the design. Second, modern 12ax7's have been and continue to be the most redily available preamp tube for instrument amplifiers and are available cheap and LOW IN MICROPHONICS! Something that often becomes a problem with unusual tube models used in instrument amps. Further, being as your somewhat new to this, most of the circuits you reference will be relative to the 12ax7 design. In the end, is the value of the preamp tubes you want to use worth the extra trouble?

In terms of extra complication, we're literally talking a switch and an extra (ground) wire for the heater, which is the only difference between 9A and 9AJ socket pinout. I could just wire them flat out as 9AJ; the tubes are common enough that it wouldn't make a problem for replacing them down the line, but given the simplicity of the switchable heater wiring. I'm pretty happy with the technicality of doing that, even if it is 'my first barbecue'. I'm much more nervous about B+! As for why? In all honesty, because I can, because I like the idea of doing something just a little different, and because it's been done often enough for me to feel comfortable in the performance of the tube: Red Bear amps, Jellyfish Audio, et al. I'm also new to building amps, but not new to building electronic circuits as a whole, so I really don't see this being an issue. If I don't like the characteristics of the tubes, it will literally be a flick of a switch to go back to using 7025 tubes as in the original design. Even if I scrapped this change, I'll be adding an artificial centre tap and elevating the heater anyway, because the 12ax7 specifies the same 100V for Vhk(max) and I've heard a number of reports that recent valves from Sovtek have been failing in cathode follower and Long-Tailed PI positions due to the high cathode voltage. As the saying goes, they just don't build 'em like they used to!

re: Building on a Twin Reverb chassis and using the extra holes... Maybe, maybe not. Good design should dictate the control layout. Not where the chassis holes happen to be. Certain controls should, for both practical and electronic reasons, be located on the back of the amp rather than in a front panel hole. Perhaps this is unimportant to you now, but it may not always be. There are good reasons for locating bias adjustment and hum balance pots on the back rather than the front. That's why you never see them on the front.

The bias and heater balance pots will be in the original Spk/ext sockets on the chassis, to minimise the length of the wiring for this reason. After all, running an AC heater signal across next to all my nice low-voltage signal wiring would be a lovely way to induce (literally) a whole pile of noise. I assume that's part of what you're referring to, about the electronic reasons? Because there are more speaker outputs than originally used on a Twin or a Showman, these are being moved to the original reverb and reverb footswitch connections. This will extend the speaker wires slightly (about 4cm), but I'll be careful with how they're run to minimise the risk of interference or signal loss.

The current plan for the heater balance is to have it adjacent to the standby switch and connected to the rectifier side of this, which is live with a 430V slightly-filtered DC, then filter it further as part of the voltage divider I'm using to generate the 80V for the elevation. The circuit I'm using for this is taken from this page: The Valve Wizard, under the section 'heater elevation'. This way I can minimise the amount of additional distance that the heater wires are running, and hopefully minimise the amount of additional noise they pick up - obviously the hum balance and heater elevation should both reduce noise, so it would be a shame to add more back in as part of that circuit! The Showman and Twin designs use a Cap pan for the filtered voltages, so I'd be running some longer wires to get a more heavily filtered B+ for the heater elevation. I'll do it if I have to, but I'll see how much noise is produced by using the standby switch as a DC source first. Here's my loose, embarrassing sketch, if that helps:



The reason I picked the Twin Reverb chassis, is because it's the same chassis as the AB763 Silverface Showman. It has a few 'extras', like the additional sockets for the reverb channel; but everything that was on the 6G14 Showman is in the same place on the AB763 chassis. Compared to the 1960 amp, the only thing that's moving are the speaker sockets; for the reasons I mention above. I do not have any way to access the equipment necessary to press and prepare my own chassis - so this was the best way to do this.

As for the free spaces on the front, they're being populated with unconnected slide switches and unconnected 1meg pots. The only 'live' controls on the faceplate are the controls from the original 6G14 faceplate: Volumes, tone controls, vibrato controls and Presence. The 6G14 design leaves half of a pre-amp valve disconnected, so I was considering adding a simple gain stage and low cut filter inspired by the Dallas Rangemaster circuit, and a master volume control; but those are things that I'll think about later if I decide to do so. For now, the intent is to build an amp that (tube heater and adjustable bias notwithstanding) follows the 6G14 specifications.

I hope that explains in a little more depth!

[Tqi]

Yesterday I ordered almost all the remaining parts. The components that I still have to purchase are:

• The power transformer (Hammond 291FEX)
• The output transformer (TAD 195860 - Brownface Twin, but multitapped)
• The cabinet (Tube Town Custom build for a Twin Reverb Chassis)
• The output tubes.

And the rear plate, which is going to be interesting - the Mojotone model will fit, but will need stripping and refinishing. I'll probably end up ordering that from TAD with the OT and stripping it myself.

It's actually been quite fun and challenging designing the aesthetics of the amp. It's a brownface circuit, but in a blackface chassis and with a few modifications away from original (the heater circuit, Bias pot and ground switch removal, to name a few). The plan is to make the aesthetic design fundamentally brownface, but throwing in a couple of nods to the Blackface era as well: The dogbone handles are terrible anyway, so it'll have a blackface chrome-ended handle on top, and obviously the control layout is closer to the Blackface design. Thank goodness for companies making custom heads and cabs!

On the subject of cabs, my first instinct is to get an Aguilar DB210 cabinet in white hot/oxblood, which is their closest to the blonde tolex. I've liked the sound of Aguilar's DB-series 10" cabs and this one seems the best bet for me. Open to suggestions though! And chances are I'll be picking up something cheap like a 90's Ashdown 4x10 in the meantime - I don't want to wreck a good cab until I have the amp working perfectly!

[mikepukmel]

Good deal. Any idea who makes the TAD transformers? TAD makes nice custom laser marked plates, if you're not going for that complete retro look, but you must know this already. I bought a JTM45 type chassis and plates from Valvestorm. The chassis is fine, but the plates are pretty crappy looking.

[Tqi]

I'm going pretty retro.

But unfortunately the TAD front plates won't quite fit my chassis. The rear one will fit, but will have gibberish markings; so I'll probably have to repaint it. The front plate came with the chassis, but needs painting.

For the moment, both plates are made with cardboard and sharpie!

I was wondering about the TAD transformers. They don't appear to be Hammond, and they're way to cheap to be Mercury. If there's any detail on the transformer once I've got it I'll let you know.

[Tqi]

I should post some assembly shots. Well, let's start with a nice overview:



As you can see, no PT or OT as I mentioned earlier! But, if this goes well...

Here's a close up on my pile of components. Most of them came from three sources: AmpMaker provided the passives and wire - well, most of the passives. A few more came from AmpRepairParts, who have recapping kits for pretty much every Fender amp ever, with actual 20/600 caps for a sane price. They also provided the pots (except the weird tapped pots, which had to come from a private seller in Australia of all places), cap can and a few other things. TubeAmpDoctor provided most of the sockets and a handful of other things. As for the rest, a few things came from Mouser of course, but as I mentioned to someone else, a huge shoutout is Watts Tube Audio, who built my fibreboards to order.



The Chassis is from a guy called Zach MD Hunter, who sells a variety of cool Chassis' on eBay. Brownface Bandmaster, anyone? As I said before, his faceplates are sometimes a mil or so out from Mojotone's, so he includes his own where he needs to.


And finally... That feeling when you carefully hotshot your fancy 20/600 capacitors onto your fibreboards, then it turns out all of your solder is missing.

[mikepukmel]

Nice parts so far. I used a chassis from zachmdhunter as well for a deluxe build. Very solid chassis. A Hammond power transformer dropped in to the hole with no modifications. Didn't realize before buying that the CTS standard pots were too short to bolt through a brass plate, the chassis and faceplate, so I had to drill out the faceplate and hold it in place with the pilot light, and 4 input jacks. Worked out ok.

Haven't seen ELUM caps before.

This guy sells really nice wire (you already got yours for this build) https://www.ebay.com/sch/skipmalley/...1&_ipg=&_from=


I used Hammond transformers in my deluxe build (build #1). I think the power transformers are ok, not so sure about the output transformers. The amp sounds good, but not great. Could be lots of things. Used Mercury transformers in a JTM45 type thing, not completed yet.

What are your ideas on speakers?

[mikepukmel]

As a complete amateur, still fiddling with my first amp build a fender Deluxe type thing, the most useful suggestion I can give you is: really be careful with your wire routing, they call "lead dress". Its a lot easier than I had understood (starting at zero) to get these amps to oscillate. I had started collecting any photograph of original chassis that I could find on google, bing, or yahoo images. One connection with a wire a little bit too long, running too close and parallel to another wire could cause an oscillation.

[Tqi]

Nice parts so far. I used a chassis from zachmdhunter as well for a deluxe build. Very solid chassis. A Hammond power transformer dropped in to the hole with no modifications. Didn't realize before buying that the CTS standard pots were too short to bolt through a brass plate, the chassis and faceplate, so I had to drill out the faceplate and hold it in place with the pilot light, and 4 input jacks. Worked out ok.

No brass plate for me..? But they were certainly tight. One washer each, no spacers.

Haven't seen ELUM caps before.

Me either. But at the end of the day, they're caps. They're the right rating, and they haven't got holes in them! it'll do.

This guy sells really nice wire (you already got yours for this build) https://www.ebay.com/sch/skipmalley/...1&_ipg=&_from=

Looks like about the same stuff. For now, I think I'll be either buying from Barry or just buying whole 100m+ reels from Mouser. The international shipping gets silly on short lengths of wire that you don't want super kinked.

I used Hammond transformers in my deluxe build (build #1). I think the power transformers are ok, not so sure about the output transformers. The amp sounds good, but not great. Could be lots of things. Used Mercury transformers in a JTM45 type thing, not completed yet.

That's encouraging! My hammond 194B choke fitted without modification, but was a little tight on the holes - Zach would benefit from a few tweaks on his CAD files. Still though, it was a third of the price of the Mojo, and though a couple of things were a little tight, I'm much happier with the investment I made. Support private entrepreneurs, etc!

What are your ideas on speakers?

That's a tricky one. So far I've got the following in mind:

The Aguilar DB210 (White Hot) is a nice sounding 2x10, I've tried the 4x10 and loved it. Ideally I'll find some 4ohm ones. That way I can have 1 cab, 2 cabs, or 4 cabs stacked using the different OT taps.

I'd love to get my hands on a [URL="https://www.zzounds.com/item--FEN2215200"] Fender 215 Pro. Seems like a really great cab, and obviously a closer match for the Showman, semantically. Sadly, long since discontinued - but there's a chance I can find a used one.

Any other suggestions?

[eschertron]

Do the diodes on the rectifier card look like they're facing backwards to anyone else? I could be facing backwards, after all...

[Tqi]

No, it's definitely you. Grey strip is cathode, and the orientation matches both the schematic and the layout diagram!

I had the hot iron in my hand when this flashed up, don't frighten me like that!

[g1]

No they are backwards. The schematic & layout has an error where they use + and - marks on the wrong sides. At least the 6g14a schem. I have.

[Tqi]

No they are backwards. The schematic & layout has an error where they use + and - marks on the wrong sides. At least the 6g14a schem. I have.

My apologies to Eschertron.

Also,

FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF

Edit:

Also, bloody hell I can see the icons the other way around from the symbols now. How the hell did I miss that!?

[g1]

I'm not sure if it's an old convention or not, maybe others can verify, but I was taught that any (single) symbol shown on a schematic for a diode signified the cathode end, even a + sign! They used it as a trick exam question.
However, in this case, both ends are marked, so that doesn't really apply.
But I believe the old selenium rectifiers were also shown that way on Fender schematics.