Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Mike Sokol]

Found a 1956-57 Danelectro Tweed Leader amp in the back of my gear closet that's been sitting there for 25 years. Another gear trade for repair story IIRC. There's no power isolation transformer on this thing and the power plug is unpolarized, so there's a 50/50 chance it will kill you if plugged in backwards. That's why I never used it in my studio or sold it. Didn't want to kill any of my guitar playing buddies. I just did a quick visual and soldered a broken wire going to the speaker and fired it up (after carefully checking chassis polarity), and it works perfectly without any extra hum. Since this appears to be a collectable amp I'm going to add a small 120-120 volt iso transformer in the bottom of the case and keep everything else stock.

I have a feeling this would be a perfect match for my incoming vintage Shure Green Bullet mic. But without a proper power isolation transformer it would certainly be a death trap for any harp player. I may have to sell some of these cool old stage amps since I'm just a keyboard player who dabbles in guitar. But I'm not letting anything out of my shop unless it's properly grounded and shock safe. 

[Mike Sokol]

I found an appropriate 120-to-120 volt isolation transformer that should work for the Danelectro. See http://www.ebay.com/itm/STANCOR-P-6412-PRI-115V-50-60Hz-Isolation-Transformer/121544166404?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D29172%26meid%3D4e186e06f1cb45579fff410e5c321f72%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D391050475623&rt=nc

I'm thinking about mounting the transformer in a small project box in the bottom of the case with an attached Edison (grounded) plug on one end, connect the stock 2-wire plug from the amplifier, and tie the incoming EGC safety ground wire to a screw on the chassis. I think it will be safe and not mess with any collectors value. Any better ways to do this? 

[Mike Sokol]

I found an appropriate 120-to-120 volt isolation transformer that should work for the Danelectro.

Hold the phone.... I may not need to add an isolation transformer after all. I found a schematic that shows there's an isolation step-up transformer for the B+ supply, with only the tube filaments in series tied directly to the incoming power line. See schematic below. If that's the case, then I only need to add a grounded power plug to make this safe. But I still think that cutting off the non-polarized plug will affect the collector's value, so perhaps it needs a special kludge cable that brings in a grounded wire that connects under a chassis bolt.

Thoughts???

[Frank DeWitt]

I only need to add a grounded power plug to make this safe.
Thoughts???

Agreed, a ground is a good thing, but as is, what makes it unsafe?

[Mike Sokol]

Agreed, a ground is a good thing, but as is, what makes it unsafe?

The potential danger is leakage currents between the incoming hot line and the chassis. I'm sure it ain't double insulated, so this amp can become a shock hazard at any time. The real interesting thing is that the Danelectro collector websites call this the "widow maker" because it doesn't have an obvious power transformer. There are indeed a number of other guitar amps that are true AC-DC units with series filaments that add up to 120 volts and only use 180 volts B+ derived by the diode tube and a capacitor across the line. But this is the first "hybrid" I've seen with 120 volt series filaments and an isolation B+ transformer. I'll open up the chassis tomorrow and confirm if that's indeed the case for this guitar amp.

In any event, this is a piece of guitar amp history which should be a lot of fun to play through. Now for you young pups out there it will be a noisy, distorted, and underpowered amp. But that's the charm of messing with antique gear. IIRC Frank, you do some playing with steam engines and antique generators, so you must get the same thrill out of seeing old gear come back to life. I'm amazed that something sitting unused in a closet for so long can still be operational.

[Tom Bourke]

Hold the phone.... I may not need to add an isolation transformer after all. I found a schematic that shows there's an isolation step-up transformer for the B+ supply, with only the tube filaments in series tied directly to the incoming power line. See schematic below. If that's the case, then I only need to add a grounded power plug to make this safe. But I still think that cutting off the non-polarized plug will affect the collector's value, so perhaps it needs a special kludge cable that brings in a grounded wire that connects under a chassis bolt.

Thoughts???

I think given all your other electrical escapades an isolation transformer should be in your kit anyway.  Maybe consider an isolated variac. That way you can also soft start old gear when appropriate.

Having said that I also think a GFI device could also be a low cost substitute for gigging.

I have some thing like this on my work bench   It cuts out when power is lost so I have it and my lights on a power strip.  Turn off the strip to the bench and I have to actively reset the power to the tools and projects.

[Steve M Smith]

Is a 25L6 the same as a 6L6 but with a 25 volt heater?


Steve.

[Mike Sokol]

I think given all your other electrical escapades an isolation transformer should be in your kit anyway.  Maybe consider an isolated variac. That way you can also soft start old gear when appropriate.

Tom, I do have three B&K variable isolation transformers on my bench, one of them simple (cheap) enough to take on gigs. But I've considered getting a 120:120-volt isolation transformer with an EGC grounding clamp for use on gigs when a git-player shows up with a vintage ungrounded amp they want to play. I should probably build one and detail how it works for the forum.

[Mike Sokol]

Is a 25L6 the same as a 6L6 but with a 25 volt heater?

Steve.

While it looks similar and has an identical pinout to 6L6, the 25L6 is only rated for half the power. The Danelectro catalog says this Legend is a 10 watt amplifier and is using the pair of 25L6 tubes in standard push-pull, class A-B configuration with an output transformer. Once I have a few more of these guitar amps up and running, I may setup a distortion analyzer and o-scope to measure output power just for fun.

[Mike Sokol]

JR, so what would YOU do to make this safe for a customer? There's no stinger cap that I can find, and it's a non-polarized power plug. So plugging it in one way connects the leakage path to the neutral wire, while reversing the plug connects the leakage path to the 120 volt line and makes chassis hot. I'll do current measurements today, but let's assume it's only a few mA leakage and not terribly dangerous, but certainly enough shock to get your attention.

If I add an EGC ground wire to a stage amplifier with a ground fault leak (let's say 5 or 6 mA), then any time it's plugged into a GFCI outlet it will make the outlet trip. And that's no good since the ground lift adapters will quickly come out. I could add a stinger cap with a switch which you flip for minimum shock and buzz, which is how many stage amps worked for decades. Or I could just leave it stock and tell the guitarist to flip the power plug for minimum shock/buzz, but that seems dangerous. I could even build in a GFCI in a little box in the bottom of the case, but that will require a reset every time it's powered up.

So how to we proceed to make this type of vintage amp safe for musicians?  Is my original thought correct - adding a 120:120-volt isolation transformer in the bottom of the case with an EGC connection to a screw on chassis? I think that would make me feel the best.

[ProSoundWeb Community]