Reverb Resuscitation

A friend of mine brought over a nice old Fender 6G15 outboard reverb unit today and as I had my nose slightly out of joint over other matters (social media drama) I hid out in the laboratory.

This friend has a knack for acquiring stuff on the cheap and this was no different. It had a bad habit of blowing fuses, and when it was brought up on the variac it was drawing about three times the normal current.

The fix was simple, and this seems to be my week for dodgy old electrolytics in power supplies.

A refresh of all the consumables and a new power cord and all was well.

I did find that it is susceptible to ground loops as it developed the granddaddy of a hum in the living room. I soon figured out that it was on a different circuit in the house. Plugging it into the same outlet as the Cowpoke quieted things down and it'll go home tomorrow or the next day.

When Vintage Was Vintage.

What you see here is the progress so far of a two or three year quest.

I'd been reading a piece by Hartley Peavey, the Mississippi amp wizard, and in it he discussed the 5F6A Fender Bassman and how he was inspired to cook up an all tube, 100 watt tweed Bassman with reverb.

Here's a page where he discusses that. I do not think he will mind.

"So" says I to me, "What could be better than an all tube 100w tweed Bassman with reverb?" and the search to locate one started.

In between all that I built a high power tweed twin knockoff which I am still not entirely happy with and I will get to it presently.

So I'd been searching on Craigslist and fleabay without ever finding a good all tube Vintage. There were a couple but one was pickup only in the great state of Oregon, and one was a wreck in Tennessee that the owner still wanted a lot of money for. And then there was the Peavey faux tweed, which is something nobody wants.

Anyway....this amp showed up on fleabay as a bare chassis, no tubes, and it seemed to be in pretty decent shape. A deal was made and it arrived here Saturday, no glass, power cord cut off, the whole works.

It's in good enough shape cosmetically although a few of the resistance welds are broken, but that is what me and Mr. Miller are made for.

Mr. Miller is a Miller MiG140 welder that runs on household current and is good enough for everything up to 3/16 steel plate.

But the overarching consideration was to do enough to power it up and see if it held up or whether one of the transformers was finished, and that required a recap with some of the good stuff from the House of Fischer and Tausche in the great nation of Germany.

But never mind that for now.

I just finished that job and installed a new power cord, fuse holder and strain relief and flipped the switch-running through the variac of course.

It held, by G-d. The plate and screen voltage without power tubes was about 525v and the filaments look happy.

So there it is. The plan now is to repair the cracked welds on the chassis, and order up a head cabinet in blond tolex with a wheat grille.

Stay tuned.

UPDATE: I  decided to try repairing the cracked chassis spot welds with JB Weld epoxy. I cleaned everything with alcohol and an emery board, mixed up some JB Weld, spread the chassis where the welds had come apart, fed it in with a cut off zip tine and then clamped it tight. I'm letting it sit for a few days while I attend to other matters such as figuring out what sort of cabinet to order and polishing up the faceplate.

Stay tuned.

When The Lights Went Out: Repowering a Deluxe Reverb Reissue. Enter The Avalanche.

A reissue Deluxe Reverb arrived here with a complaint of "no sound. Smells bad." and a brief investigation lead to some rocket science and the beginnings of a conspiracy.

The rectifier tube was defective and the power tubes were very weak. Some more investigation revealed that the primaries on the power transformer were open and therefore a new transformer was called for.

I had a Deluxe Reverb power transformer on the shelf but the original ones have a smaller footprint than the reissue amplifiers, namely the 65 Deluxe Reverb and the 68 Custom Deluxe Reverb.

However.....however the footprint is the same as the original power transformer for a Vibrolux Reverb or a Tremolux. As it happened I had one, and comparing published data I soon realized that not only could I use it to replace the failed transformer, it had significantly better capability current wise, and was less expensive in the bargain.

I got to thinking that with this transformer, a 40w output transformer and a pair of 5881s and the existing Jensen C12K speaker, a Deluxe Reverb Reissue could deliver significantly more power out of the same package. A minor revision of the power supply capacitors might be in order as well.

I do not think the 6V6 output transformer would be optimum for this conversion because the impedance is 6.6k rather than the 4k that the 6L6 and its variants take.

It's going to be called the Avalanche. I've got a friend lined up who's going to donate a 68 Custom DRRI for the prototype. This could be some serious fun.

Back to the amp on the bench.

What had happened was a cascading failure. The 22 ohm resistor R69 in the bias chain failed open. I imagine the power tubes red plated and the rectifier tube and the power transformer both failed no doubt from overheating, which explains the no sound and bad smells.

The point here is, never miss an opportunity to check your raw voltages with the tubes out of the amp. That revealed the original source of the problem and had I not checked the voltages it might have been an expensive afternoon.

This revealed that I had voltage up to the 22 ohm resistor and nothing coming out the other side and emerging at the grids. You should know what pin it's on.

When all was said and done I had 425 volts on the plates and screens, and all that was required was a matter of changing the bias resistor R59 from 22k to 10k which is what is called for in the schematic.

A heat soak for about four hours while I was watching the Super Bowl festivities got me to a surface temperature of 133 deg f on the exterior of the power transformer and there it stayed. A bit warm but tolerable. A small fan like the ones from Kmart or Target or Amazon come to think of it might just hit the spot. Or, perhaps, a metal bracket with a muffin fan? Maybe.

Don't expect this to stay here forever. If it looks like I can make some dough on this you won't see it any more.

Trade secrets, y'know.

Fun On A Budget

I was listening to some Patsy Cline sides the other day and later on chatting with my pals over at the Basement of Broken Dreams while Bob was plaing some slide on a Telecaster.

That's when it hit me.

A simple lap steel guitar oughtn't cost too much cash and might have some potential for new and interesting sounds and add to my generally skimpy repertoire.

It was the work of a few minutes to find this on the GC page and I immediately placed an order:

It arrived here today, and it was the work of a few minutes to get it unpacked. Shipping was free and even though I had to pay the state of Illinois some four or five dollars in sales tax it was worth the doing.

I have a notion that lap steels are making a modest comeback from their heyday in the thirties, forties and fifties, as people are returning to music that is more authentic and unfiltered and unprocessed.

The Rogue lap steel comes with three furniture style extendable legs that screw into three mating sockets on the underside of the guitar.  One was loose and the guitar was kind of wobbly because of the following.

As the parent wood is kind of soft, a few of the screws were stripped out. However, the cure is found by breaking off a toothpick in each hole that is stripped, putting a dab of Loctite cyanoacrylate glue in, and running the wood screw back down to hold the sockets permanently.

That took all of five minutes. It pays to be handy with tools.

It took a few minutes to find a tuning video on youtube which has come to be everyman's encyclopedia of everything. A C6 tuning gives you a C major on the lower three strings and an A minor on the upper three.

The tone is nice and mellow, and I think it will be just fine as it is. It came with knurled knobs, which could be fun for doing swells if I didn't have the big assed Ernie Ball volume pedal someone gave me.

I will have to obtain a steel bar and some finger picks and learn a few basic riffs. If I'd gone and bought a lathe and milling machine I could make my own steels, but I didn't. Dammit.

Stay tuned as they say.

Why You Need A Tube Tester

I've been around this line of work for a good ten years or so, and if you count the radios, since 1983. Here are images of the three different tube testers I use in amp repair, and each one has a specific purpose.

The Sencore is used only for testing ten pin base tubes such as compactrons like the 6C10 and 6K11 plus the 7868 power tube. It is a basic emission tester.

The Hickok 532 is a mutual conductance tube tester, and the one I use is older than I am, having a build date of 1946. Nonetheless is is a durable piece of kit, and if it ever fails me I have another on standby-even though it was not modified for 9 pin Noval base tubes so I'll have to do that if push comes to shove.

These two testers have one feature which makes them indispensable, and that is a shorts test. They both have neon lights which will illuminate brightly if a shorted tube is encountered.

What does this mean to you? Simple. A shorted tube will render your amp inoperable and if it is a power tube can cause serious damage to associated components.

Both testers will tell you if the tube or grid circuits are open. Either it won't illuminate, or no results will display.

A lot of folks say "Well. The only real test of a tube is how it performs and sounds in your amp." That's true as far as it goes, but the purpose of using a proper tester can save you a lot of work, and it can prevent damage from occurring that can run up the repair bill.

As an example today a Marshall JCM2000 DSL 100w amp came in today, with no output. I knew the amp because it doesn't get used much and I'd marked the tubes with the date when I repaired it two years ago.

If I didn't have a tube tester chances are I'd still be down in the shop pulling my hair out trying to figure out what had failed, and it would be running the customer's repair bill up as I hunted and pecked and hoped to get lucky.

As it turned out it took five minutes to find a preamp tube with an open filament which was all it needed to get it back running again.

Now. The third tester is a Maxi Matcher II made by the fine folks in Seattle at Maxi Matcher which just goes to show there are more things of value that come out of  that fine city besides coffee and big airplanes. It tests a certain range of power tubes for mutual conductance and plate current and it allows the operator to match tubes-which makes for a nice quiet and well balanced performance.

And, if it detects an overcurrent situation-a short-a LED illuminates and you get an overcurrent shutdown, thus preserving your tube tester.

In fact, while matching some brand new tubes the other day, it detected a brand new tube which was shorted, and that prevented an expensive failure and some rework I would not get paid to do.

This was not the first time, either. A customer brought in an amp for service and he provided tubes he'd obtained from an eBay vendor-always a risky proposition. On the Maxi Matcher they went and they were not matched at all-there was a 15 ma split which was unacceptable. So more postage back and forth was required, and a careful reading of the guy's ad on fleabay revealed he did not have any idea what he was talking about when he advertised the tubes as "matched".

Now. You do not have to spend a lot of dough here. A reasonably good quality emission tester like the Sencore, or a Knight KG1, or an Eico 625 and many many others can be had for less than fifty bucks.

If it sorts out one bad tube before it causes damage you're money ahead. And that is something the people who insist "the only true test is in your amp" do not seem to be able to comprehend.

Heating Up A Blackstar

One of the kids at the shop has a Blackstar HT40C which had a persistent "no volume" problem. and because I'm a nice guy and they do provide a lot of work for me I agreed to tackle it.

It seemed that the power tube filaments were intermittently on the fritz, with the result that no joy was to be had. This was the second go around because the last time I'd thought "What's the issue? It works, dunnit?"

Well, it didn't.

Once the problem was localized to the power tube filament circuit, I measured the raw output from SP14 and SP15 and found that unloaded they were quite different-54 and 64 volts respectively.

Pulling the main board for inspection I desoldered pins 2 and 7, redid them with silver solder  and tightened the connectors. On reassembly with a new set of JJ E34Ls I found that the filament voltage was low-about 5.5 and 6 volts. Scrounging through my stash I found some Valve Art EL34s that yielded 6.0 volts on pins 2 so that was good.

The schematic shows the return path as going straight to ground but as I've illustrated it doesn't. It goes through a 100 ohm resistor and a diode in parallel, and then the entire ground goes through a pair of diodes and another 100 ohm resistor and capacitor in parallel before it reaches home base.through SP3. This goes a long way toward explaining why measuring pin 7 to the chassis is not completely grounded. Also, the ground portion of the bias system which contains a diode and a 1 ohm sampling resistor dumps into the above chassis potential "ground" labyrinth described here.

Because of this quirk I will be adjusting these amps to 50mv across the sampling resistor as is noted on the circuit board and the Blackstar HT Club 40 test specifications rather than using a bias probe as has been my habit.

I think the problem is pretty much resolved with the general cleanup

UPDATE 1: I revised the shop sketch here to show the complete return to ground path which includes the paired diodes D42 and D34, the resistor R223 ,and the capacitor C116.

I've no idea what they're supposed to do but they go a long way toward explaining why ground on the power tube pins 7 isn't really ground although the schematic from the factory says it is. Go figger.

Putting Life Back Into A Silvertone 1335

One of my old pals from Metuchen, the Bench, and Port Jervis runs on the Beezer has reinvented himself three or four times and now holds sway as an auctioneer in the Easton, Pennsylvania area.

He's a grand fellow and a superior auctioneer.

Anyway, he alerted me that this amplifier was coming up in a large estate he sold. As the story goes the estate belonged to two spinsters who'd decided to liquidate their late parents' estate, part of which included a wood shop that had been closed up eighteen years ago. The Silvertone was found under a pile of junk in the back of the shop. Between the purchase price and the shipping I was out about four hundred bucks. The shipping was a little pricey but the people who packed it did a good job so it got here in good nick.

It's an interesting circuit, mostly because of the use of the filaments of the two 12AX7 tubes as the power tube cathodes for biasing purposes. I don't know if anyone else has tried it but Nat Daniel always had some interesting ideas for circuitry.

As it happens I know this circuit pretty well, having worked on a number of old Airline amps that had the same or similar setups. It's good and clean for a while and the low headroom makes for pleasing tone.

I spent the better part of today replacing every capacitor in the lower chassis, and I'd previously redone the upper. They're all wax paper of varying makes and they were all likely to be bad. The folks at Danelectro didn't go cheap on the transformers-the power transformer is a Freed and the output is a Utah made transformer. The fifteen inch speaker is from the House of Rola.

The tubes were all pretty much garbage but I've got a good stock of glassware for these applications.

A good cleanup with lots of spit and polish got most of the musty smell out of it, and I'll give it a road test tomorrow or the next day.

I expect magic.

The fake wood grain shelf paper doesn't look too bad, although the original tweed is underneath. It's tempting.

UPDATE: It lives, but one channel has a lot of white noise-but since that's detachable I'll only have to pull the upper chassis. The vibrato didn't work until I remembered something and started switching tubes for similarly based numbers. It had had a 6AC7 which I pitched thinking it was a hole stuffer but maybe the last person in there installed it for a reason. A 6SH7 got the job done where the 6SJ7 on the chart would not. I may play around with power tubes as well. It still smells kind of musty but that's to be expected.