Gretsch Fury Major B+ Voltage Drop

[JunkDr]

Hello All,

I have a Gretsch Fury Piggyback head model 6169 made by Valco. There is one (and one only that I can find) schematic reverse engineered by a gentlemen in California named Tracey Norton from which I have been working. My problem is similar but more extreme to an issue in a post by Waynard started April 14 2016 that came to a dead end.

Overview. This amp was poorly stored for years before I got it with moisture damage to the cab and surface rust all over the chassis. It is also evident that someone has been in the chassis before from some none stock appearing resistors and wiring. It has dual OTs and a huge PT that also had a decent build up of rust on the exterior. I began dealing with this thing several years ago in small steps as time and / or energy allowed. From the old advertising that I have seen on the web it was claimed that this amp was 70 Watts per channel. Here is what I have done so far:

- cosmetic, all rust cleaned up as best as possible, thin coat of Tremclad applied.
- complete recap except ceramics using better quality (F&Ts, Japanese, not Taiwanese) caps.
- 3 loose preamp sockets replaced with new Beltons, all others were cleaned and retensioned.
- all CC resistors checked, all drifted high in most cases ranging from 5% to 25% high.
- burnt 5 Watt resistor for reverb supply replaced.
- two wiring errors corrected.
- coupling caps to output tubes replaced with new.
- all new tubes, all verified a couple of weeks ago as strong and good.
- all new, higher capacity diodes in rectifier.

All voltages appear fine until the power output tubes are plugged in. I see 410 VDC at the main rail and corresponding acceptable voltages at all distribution points with those tubes out. When those tubes are applied B+ drops to 254 and all others go down the toilet. Filament voltage drops to 4.5 and the primary reads about 85 VAC under these conditions. Bias was checked using plate current method and all are around 9.5 Watts which is about half. I have tried increasing the cathode bias resistor from 200 to 300 ohms on one pair but that only brought voltage up about 5 volts. The OTs seem ok and the amp actually plays but is noticeably lacking. I am reluctant to leave it on very long under these conditions.

The power output stage is pretty straight forward and I have gone over every square inch of it and cannot find anything that would be causing this. Gerald Weber goes on and on about rusty laminations in transformers and he has begun to make me think that may be where the problem lies, i.e, eddy currents eating up the available power. Any suggestions on where to go from here??

[sluckey]

When those tubes are applied B+ drops to 254 and all others go down the toilet. Filament voltage drops to 4.5 and the primary reads about 85 VAC under these conditions.

Are you sure the primary drops to 85V? That's very unusual. That would indicate a resistive line cord, or power switch, or fuse. Or if the amp is really drawing so much current that the line voltage drops to 85v I would expect to see room lights dim and or circuit breaker tripped.

If I dismiss the 'primary drops to 85v' statement, I would guess you have some bad 6L6s or the power transformer is bad. Maybe try only one pair of 6L6s and see if the voltage still drops.

You can test the PT under load by pulling all 6L8s and connecting a 5000Ω 50W resistor directly across the first filter cap. A 5000Ω resistor would put an 80mA load on a 400v supply. This should be a light load for this amp. If the B+ voltage still drops significantly the PT is likely bad.

BTW, it's always a good idea to attach a schematic.

[JunkDr]

Thanks for the reply Sluckey. I wrote the post before running out the door to work and missed a few things. I had all the 6N3C Russian tubes tested by a reputable fellow last week - they all came back good and at 75% or higher. I know a tester can miss some things but these are all brand new old stock and am aware that even so, there could be a problem with them. No problems are apparent by tap or wiggle testing. Before that, I put the tubes that were in the amp back in, same results. Also, I just received a brand new quad of 6L6GCs and put them in briefly, same results.  The power cord has been replaced and appears in good shape. The master power switch (the only one actually besides standby, also new) is brand new and of high quality. As far as the primary PT, that is the value my meter presented, hard to tell about lights dimming as I have fluorescents and LEDS in my shop but the slightly overloaded wall receptacle holds without tripping a breaker. I am also running this through a incandescent lamp current limiter (150W bulb) and Variac. The limiter first starts to brighten on power up then drop to off (as the caps charge I assume) and stays that way until the filaments light up and the tube starts doing its thing then the limiter illuminates to what I would estimate to be about half its normal brightness. I have tried the two tube thing and I get the same sort of indication on the limiter but not as bright. I didn't check drop while doing that though. It really looks like a current draw but doesn't seem related to output tube bias as stated. If I can drum up 5K ohms at 50 Watts I'll try that. As it is commonly stated, the problem is rarely with the PT and I am trying to convince myself of that but it kind of points to it. I am building a Bassman as a side project and have a new PT for that here. It's 325 volts a side though instead of 300 as is in place but perhaps enough to briefly check the condition with the existing PT out of the loop?

[DummyLoad]

the current limiter is the source of your AC and B+ drop. it's doing what it's supposed to do, as you place more current demand through the limiter, AC and B+ will drop significantly.


plug the amp direct to wall outlet and measure B+ with and without signal. use a PC or smartphone for signal generator if you don't have one.


--pete

[sluckey]

I am also running this through a incandescent lamp current limiter (150W bulb) and Variac.

Get rid of the lamp limiter and use the variac to set the line voltage to 120VAC. Then you will have some meaningful voltage readings.

[JunkDr]

Wow, do I feel dumb now. That never even occurred to me. I have not worked on anything this big wattage wise and thought the current limiter was trying to tell me something. Have never even seen it light up on the smaller stuff I have worked on. Once I get up to the shop I hope I can put this job to rest. I don't know what we would do without the internet - thank you both for your kind advice.

[sluckey]

Join the club!   

[PRR]

> incandescent lamp current limiter (150W bulb)
> tube starts doing its thing then the limiter illuminates to what I would estimate to be about half its normal brightness.

The full-power suckage of a four-6L6GC amp can be in excess of 150 Watts.

As an ignorant guess, a 150W bulb and a 150W amp will split the 120V 50:50, each gets half. Which sucks for both light and sound.

(In fact "half bright" on a lamp is a lot more than half of 120V, so the amp was getting a lot less than half of 120V.)

Get a 500 Watt lamp. Or a 1000W toaster. The limiter is mainly there so you have "some" power without any unexpected mistake burning-up the house.

But if 50%-80% of 120V hasn't released the Magic Smoke yet, like they say, lose the lamp.

[JunkDr]

Oh that was definitely the problem guys. Should have clued in when there was "no smoke"! Now, with decent voltages other things are showing up in this old brute but I'm getting them one at a time. Thanks again for the help gents!