First Build: Fender Showman Brownface (6G14)

[mikepukmel]

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

It would be an exciting first time power up sequence. or maybe, the speaker would act like a microphone, and sound would be pushed back into the guitar pickups? Nahh.

[Tqi]

Well, it took me twelve hours to upload this photo, so I hope you enjoy. (380kB! Why is my internet speed worse than Antarctica's all of a sudden!?)



Diodes are now facing the right way, with thanks to Eschertron and g1. You can also see my completed filter pack! Note as well the two 220K bleed resistors I've added, one on the reservoir side of the standby switch and the other across the first filter cap (B+). Hopefully that should reduce the chances of horrible electrocution further down the line. I'm not 100% convinced that the value is high enough despite what the intarweb suggests, I'm worried about messing up the voltage dividers. Oh well, if the multimeter gives me bad news when I actually test the amp, I'll order some 1M's to replace them. I'm actually adding a 47K resistor across the standby switch too, so I guess two bleed resistors is also overkill. I'm mollycoddling my tubes a bit, I know.

I started dry-assembling my main circuit board, too. Things I've LearnedTM: I should have ordered some random small ceramic radial caps as well. I've now got half a dozen hilariously awkward-looking places where axial mallory caps are inelegantly mounted vertically Not a desperately big deal, but I'll see if I can find some radials at a store I'm visiting today. More importantly, check parts before hand. Innocent (and now corrected for future orders) mistake by one of my vendors, the cap kit I ordered contained 3x25uF and 1x50uF instead of two and two. So I'm waiting for the replacement to arrive.

Now for a thought experiment!

I had a look at solving my 80V heater problem. As a reminder, I've made two modifications to my heater design (yeah, I know, keep it simple stupid) that are harmless by themselves but risky in concert. Firstly, I'm adding switching between 9A and 9AJ 6.3V socket wiring. Secondly, I'm elevating the heater voltage to 80V to protect the Vibrato and Phase Inverter tubes. This is all fine, except in the one configuration of 9AJ wiring and 9A tube, in which case you get ~86V across the heater. Which is obviously bad.



This isn't a big deal, because I'll know not to hit the 'death' switch, but since I'm building this, I thought it would be a fun learning exercise to try and work out how I could solve this if I wanted to. The best way I can see to do it, is to either switch the 80V elevation separately from the valve type, or add some kind of fuse that fails to open when more than say 7V is dropped. The problem with the first option is that it still allows one to kill their tubes if both switches are set wrong. So the best option would be some kind of fuse.

Following some research, the obvious solution would be to use some kind of Zener diode-plus-fuse combo that would break the circuit, except I'd need two of them, right? One for each leg of the AC circuit - unless I also built a full bridge rectifier and ... yeah, no.

It then occurs to me - the 80V only enters the circuit at one point, through the elevation. And in theory, it should never, ever have any meaningful amount of current going across it - it's just a voltage reference. So, if one was to put a very small in line fuse between the voltage divider and the center lug of the humdinger pot; something like 50mA or 100mA fast blow, that should protect the preamp tubes in the case where someone sets the tube mode to 9AJ while a 9A tube is inserted. Best part, if this happened at a gig, the heaters would just float - which would get hummy, but I could, in theory, keep playing.

How's my logic?

[eschertron]

How likely are you to want to change tube types in the middle of a gig or rehearsal? I ask because I'd think the safest "simple" way to do your switching is to either 1) mount the switches where they can't get moved without the utmost of intention, or 2) put jumpers on your eyelet board that allows you to make the change, but only when you open the amp up for testing/troubleshooting. Sure, #2 takes the fun out of it, but also removes the agony of replacing blown parts. Or playing with a "hummy" amp if the fuse blows spuriously.

[Tqi]

I'm pretty happy to just not take any safety precautions other than the one I already have (the death switch goes side-to-side, so can't be mistaken for the up-and-down power/standby switches while fumbling). I don't desperately plan to ever run 7025 tubes in there and I have enough 6Н2П-ЕВ tubes here to last a lifetime, it's just kind of fun designing for the option. I'm not adding the overvoltage protection, because no-one apart from me will ever touch that switch.

I really mean 'thought experiment', I'd like to build my electronic skills even further through this project, and there's things like Varistors and Zener diodes that I've never really understood until I started considering how to fix my suicide-switch.

If I made a 'production run' of amps using this socket-switching, your suggestion of putting the switch 'out of the way' is bang on the best one. I'd put them on the underside of the chassis, next to the tube sockets. It would also be the correct place to put the bias pot and the humdinger as well - so that every part of routine maintenance could be done without opening the chassis, but not be 'interesting' for non-experts to mess with. If I was building an amp that way, I'd also probably put use mini-toggles and have individual switches on each socket, so that a mix of tubes could be used (6Н2П-ЕВ in the preamp, and some nice tough Shuguang's in the Vibrato and PI positions. For example.)

In any case, thanks for the reply!

[Tqi]

I'm not adding the overvoltage protection

Freaky. So, today I got this!



This country's last surviving high street electronics parts chain just went inevitably under. It wasn't a bad shop, but it had the same model as every other example of the business: gouge on everything, stock lots of crap and hope for suckers. Like all these businesses, if you needed a particular resistor or a hammond box in a hurry, they were there; but any well-organised hacker would be using next-day delivery so rarely needed to use them. Unfortunately, they were the exclusive resellers for a bunch of useful things, like the Fine Offset weather stations. Their staff were all really nice, too.

Still, best to take advantage of the situation. I left it a bit late, but they did have half a dozen old abs-and-steel drawer cabinets, that were being sold as 'bundles' with random components. Most of the bundles included resistors, that they were selling off in strips of a thousand at a time. Unfortunately, even at closeout prices, their resistors are about 5 times the price of Mouser, and they're only 1/4W which makes them pretty useless for anything I would want to do. However, they had one cabinet that was cheap, with just 'weird' stuff in. I only really wanted the cabinet, but figured hey - a 'grab bag' is always fun. One of the staff kindly swapped the more badly broken drawers for 'nice' ones, and now I have a decent parts cabinet, at long last!

As for the random stuff, there's about 20 pots, all useful values, but also all through-hole. About 20 wire-wound resistors of varying values from 3W to 10W, some weird gigantic cuboid capacitors, a few PIC chips (How 1997!), but also a couple of axial LED/Optocoupler packages, useful if I ever decide to try my hand at a FET Showman, some power transistors, a bunch of LED's, some random diodes and a pile of other weird stuff. Unfortunately the drawer labelled '5mw laser modules' proved a disappointment.

Drawers now contain every electronic component I own, plus 123 hotsnot sticks, a bunch of screws from amp parts kits, a Fender '63 amp logo, a bunch of spare ziploc and antistat bags, a dozen or so raspberry pi's, 7 Fender single coils, two NXPresso boards and a rather ridiculous number of miniature light bulbs.

And... 10, fast blow, 100mA fuses.



Thanks, Maplin. Now I'm questioning the existence of an inescapable fate.

[Tqi]

Just for fun, I got some post from Ukraine today!

Fifty brand new 6Н2П-ЕВ tubes, made September 1988.

[Chuck H]

Unless you sell your builds you can't go through all those in a lifetime. Even after cherry picking the quietest ones for V1 use. Do we know if the parallel filament is hum cancelling? And should the shield on pin 9 be used in any special way?

[mikepukmel]

Just for fun, I got some post from Ukraine today!

Fifty brand new 6Н2П-ЕВ tubes, made September 1988.

[ATTACH=CONFIG]48728[/ATTACH]

[ATTACH=CONFIG]48729[/ATTACH]

Holy cow, that's the mother lode.

You should tubify everything in your house.

[Tqi]

Unless you sell your builds you can't go through all those in a lifetime. Even after cherry picking the quietest ones for V1 use. Do we know if the parallel filament is hum cancelling? And should the shield on pin 9 be used in any special way?

As far as I understand, the parallel filament is exactly the same as the 12a*7 series parallel filament, just hardwired. As far as the shield on pin 9, just hook it to ground. One could actually just ignore it, and it shouldn't affect anything, but if it's there...

As far as going through all of them in a lifetime...

• Extra-long-life Soviet military tubes.
• Heater elevated to lower Vhk.
• Leak resistor across the standby switch to provide idle current and reduce plate wear.

After all that, if I go through my first set in a lifetime I'll be annoyed.

[Tqi]

After DiodeGate I've gotten nervous. There are two 50u25 (50u50), two 25u25 and one 4u25 (4.7u50) on my main board that are not given a polarity in the schematic.

Based on the AB763 schematic, I'm confident that the 25u and 50u caps are negative-to-ground. The 4u capacitor on the ¿Tremolo Amplifier? (That weird tube in the vibrato circuit that looks almost like a long tailed pair) is also connected to the cathodes, like the 25u and 50u caps - so I'm assuming it should also be negative to ground?

But if anyone wants to correct me (or tell me the AB763 showman schematic has the caps written backwards) please feel free.

[Chuck H]

Yes, the 4u is - to ground. The tip off is the positive voltage indicated in the schematic at the ungrounded end of that cap. Polarized caps don't like reverse polarity, so the "positive" end will always be the +end There will only be two polarized caps in that amp with the + to ground. The 8u caps in the bias supply. Notice on the bias supply caps that there is a negative voltage WRT ground. Using the same principal as above then, ground (at 0V) is then more positive than that negative voltage. Therefor the bias filter caps + ends go to ground.

[Tqi]

Thanks Chuck, that makes sense.

Last question before I start soldering is just one of style. I'm using Axial Mallory's for most of my caps because they were the cheapest caps I could buy individually - but it means there are a few that are mounted vertically where the cap wouldn't fit horizontally. My ...call it 'OCD', though it isn't, wants me to order some normal radial caps to put in their place. Am I just making myself work, here?

[g1]

I think you're just making extra work. Unless they are real big. I've seen plenty of production amps with axials standing up. One end is usually flush to the board.
Oh, I see in the pic above. You can't get flush like pc board because of the eyelets. I think I'd prefer to see them laying down along side the eyelets rather than upright like that.

[Tqi]

Thanks, g1.

I tried mounting them parallel at first, and couldn't make them fit cleanly; I'll give it another shot, but if it doesn't work I think I'll roll with them as they are. They should be fine!