Blown C5 Cap

[77seriesIII]

ok quick test w/o 300Bs no hum. My wife cleaned up while I was out and I have to find my black test lead...

[77seriesIII]

Shannon,
for TP1 and 2, just testing across the two is what I did. AC=0v DC=5.3mv. No hum. hmmmm remove the OP tubes no hum, seeing a trend.

more voltages later. Dog is screaming WALK ME NOW.

/e

[77seriesIII]

ok as promised here are voltages WITHOUT 300B's installed.

V1 6SL7GT New Issue Tung Sol Pins:
1) 0v
2) read 200v Supposed to be 147v
3) read 2.3v Supposed to be 1.8
4) 0v
5) read 208 Supposed to be 147
6) read 2.3v Supposed to be 1.8v
7) & 8) read 6.29vac

V2 6AX5GT CBS
1) 0v
2) 6.18vac
3) 332v Supposed to be 324vac
4) 0v
5) 330v Supposed to be 324vac
6) 0v
7) 6.18vac
8) 432v Supposed to be 333v

V3 & V4 300b. I'm just testing the B+ and heater
1) & 4) 7.3v REMEMBER NO Tube, just measuring across with probes
If anywhere this is where I may have screwed up
OPT is wired to Pin 2 if you are looking from the bottom up or Pin 3 if you are looking from top down V3 = 432 V4 = 431 There are NO 300B tubes attached/installed during these measurements

PREVIOUS MEASUREMENTS from EH 300B INSTALLED
Pin 3 w/SS rectifier V3 382v V4 380
Pin 3 w/6ax5gt V3 and V4 = 294v
V3 & V4 Pins 1) & 4) = 4.6v
Pin 2 51.6mv BOTH v3 & V4 w/ SS rectifier forgot to measure w/ tube rect
Across R19 & R20 pin 1) 4.29 & 4.25 Pin 2) (otherside) 4.17 & 4.14

J1 & J2 0v

J3 & J4 the only pins I am getting accurate readings on 1) & 2) 6.4v each

J5
1) 432v Supposed to be 333v
2) 433v Supposed to be 327v

J6
1) 331vac Supposed to be 324vac
2) & 3) 0V
4) 331vac Supposed to be 324vC

J7
1) 431V Supposed to be 319V
2) 430v Supposed to be 319v

Almost all of the readings are not even close, but I have a feeling that they will not be based on no OP tubes installed. Any suggestions ideas for more problem solving would be really cool. Thanks for all the help so far!

/e

[Shannon Parks]

ok as promised here are voltages WITHOUT 300B's installed.

V1 6SL7GT New Issue Tung Sol Pins:
1) 0v
2) read 200v Supposed to be 147v
3) read 2.3v Supposed to be 1.8
4) 0v
5) read 208 Supposed to be 147
6) read 2.3v Supposed to be 1.8v
7) & 8) read 6.29vac

These numbers are A-OK. The plate voltages are higher without the main load attached (ie the 300Bs).

V2 6AX5GT CBS
1) 0v
2) 6.18vac
3) 332v Supposed to be 324vac
4) 0v
5) 330v Supposed to be 324vac
6) 0v
7) 6.18vac
8) 432v Supposed to be 333v

Ditto. These numbers are also A-OK. The B+ is about where I would expect it without the 300Bs.

V3 & V4 300b. I'm just testing the B+ and heater
1) & 4) 7.3v REMEMBER NO Tube, just measuring across with probes
If anywhere this is where I may have screwed up
OPT is wired to Pin 2 if you are looking from the bottom up or Pin 3 if you are looking from top down V3 = 432 V4 = 431 There are NO 300B tubes attached/installed during these measurements

Again, this looks normal considering no 300Bs. The rest of your voltages look to be normal, too. Thanks for taking the time to list those.

The 900 ohm bias resistor network biased those 300Bs a little to aggressively with the SS rectification. Even though you had switched back to the 6AX5GT rectifier, the damage could have been done already and one may failed, thus killing C5 with the B+ across it. One shorted 300B would drag down the whole system, so try one 300B at a time, and swap sides with one at a time, too. I am guessing one tube will bias up normally. Record the voltages at TP1 and TP2 when you do this test. But if it makes a racket, pull the plug immediately.

Check out Lee's thread in this forum. An almost identical event happened to him.

[77seriesIII]

Shannon,

Thanks for the quick reply and vote of confidence on the volts I captured.

Ok, I purchased two new tubes while waiting to have my EH's tested. I place one tube in the socket and turned it on. As the voltage built the hum started so I turned it off. I placed the tube in the other socket and the same thing happened. the hum is not immediate nor instant loud. It builds as the amp warms up.

During this test nothing was connected to the RCA inputs.

You mentioned the 900 ohm network, do think one of those went? Also, the hum does travel with the tube, both tubes. Is it possible I blew the OPT or inductor? It appears everything before the 300bs is working normally. Once they are connected and current travels the remaining circuit, HUM begins.

Speaking of OPTs and UX4. When you look at the schematic for pin layout on any tube the correct way to perceive this is from the top down look correct? I want to make sure I have the OPT wired into the correct pins. I cant think I would have it in there and would work normally for 1.5 months and then decide, "SHIT, I connected incorrectly! Time to Punch out!"

Still in Audio troubleshooting mode.

/e

[SDS-PAGE]

Erick,
Apparently EH 300Bs are prone to intermittent grid shorts, causing them to unexpectedly go into runaway. FWIW, install 0.10A fuses in the ground lines of both 300B cathodes and an additional 0.25A fuse in the ground
of the power transformer secondary. This ought to protect your investment.

My question for you was, do you get the hum when the inputs are connected? They will act hum antenas if you don't.

Also, make sure that the plate and grid on your UX4 sockets are connected right one more time. You probably have it right, but you never know. Here is a link to a tube data page: tdsl.duncanamps.com. Hope you will soon spend more time listening to the amp than troubleshooting it. -Min

[EWBrown]

I've run into issues with some Russian tubes (the last time, it was a 6550C) that must have gotten mishandled, or dropped, while still in the box (otherwise it would have shattered) or during shipping.

What happened (Svetlana 6550C) was that one of the grids got mis-aligned and may have shorted out against an adjacent element inside the tube. So I no longer had a matched quad... Yellow_Light_Colorz_PDT_21
In this particular case, the tube's biasing was very unstable and tended to drift high until the current became excessive. Replacing the tube cured the problem...

The Sovtek / EH 300Bs, 2A3s, etc use the "harp" style filaments and that can sag or otherwise get mis-aligned during shipping, and short to the grid, and kapow! goes the bypass cap...

I'm sure that Russian tubes aren't unique in this problem, it's just that I've seen it most often with them... :o

/ed B in NH

[77seriesIII]

Min,

Good tip on the fuses, I'll have to work that one out, especially when I start building the Je lab version.

On the rca connects, they were disconnected on the first test, and then I connected them on the second. yep they were connected, I just ran the wires to make sure that I didnt connect a second set of rca connects and just had them dangling.

Ed,
This is almost the same conversation I had with guys at the Tube Amp Doctor in Worms yesterday. Of the problem tubes he sees more problems with JJ than any other tube, EH follows close behind and then Chinese tubes. He really dislikes dealing with JJ and EH because they will not accept a tube back once it has been shipped from the factory. From what TAD says JJ and Eh blame the customer or the shop owner for being stupid. Whereas the Chinese guys take everything and all back. Across the board he thinks there is roughly a 5% failure rate give or take depending on the month, day etc.

When he took a look at my 300Bs which are in line for a stress/volt check (sorry got no tester other than a vm) he said the filaments look fine BUT he quickly stated its only visual, so who knows.

Based on today's run with two new tubes, and same problem I think there is a problem with the OPTs or something in that path. Could an inductor go south? Anyway to test? The inductor is common to both OPTs

Looking at the schematic I am wondering if I just need to break down and make a new order to Mouser for all the other parts, desolder the board and start over.

Is there anyway to test the inductor and OPTs?

/e

[77seriesIII]

Eliminating the UX4 as the problem. Schematics show the wiring from the top view correct?

/e

[TerrySmith]

Have a look at my 6A3 GSG, you can see a correctly wired UX4 socket. BTW, sockets are numbered clockwise looking at the bottom.

[EWBrown]

It takes a lot of "effort" to kill a choke or an OPT, and usually there is a very obvious and ominous odor of "electrical death" present. :parp:

From my experience those Trafomatic power transformers are just about indestructable and they do have a fairly low winding resistance as compared to Hammonds, PA-774s, etc.

THis one's starting to baffle me...

/ed B in NH

[77seriesIII]

Ed that is what I figured, kinda like an ion storm with a burnt rubber/plastic mixture. Blew enough blackboxes while in the army I know that smell and figured it would be same if I blew an OPT or inductor.

Now would I have gotten the voltage results if other caps had let loose? I would expect the cap to bulge and release some smoke but if it went bad but...

Maybe pull the tubes and measure the voltage drops across the resistors?

I like a good challenge and am famous for saying "You only have to be 10% smarter than the object you are working on, but 1% helps" I am afraid to say there are some inanimate objects out there smarter than me...

I am starting to get stumped, it is possible that I bought into the 5% of the bad tube lot but...who knows. A friend did swear that if it wasnt for bad luck he would have no luck at all.[/i]

[77seriesIII]

terry,

just caught your post. Thanks for the help, my UX4s are wired up correctly. BTW where are the pictures of your 6A3, went looking but cant seem to find a picture of your amp. I think I came across it before but have since lost the link.

/e

[TerrySmith]

Try this: http://diytube.com/phpBB2/viewtopic.php?t=1557

[Shannon Parks]

77,

Try measuring your transformer resistance between your filament taps (all three) and the secondary. Do all three read the same? High impedance?

Also, I just remembered the outputs are proprietary Trafomatic, too. Might be worthwhile to disconnect them and compare them to each other (all the taps DC resistances).