Guitar amplifier, grid resistors burn instantly  - Page 1
Forum section: electronics / repair

author: Andbro
author_rank: Regular Contributor
timestamp: November 15, 2022, 06:00:42 pm
content: Hi,I have and guitar amplifier with vacuum tubes 6L6 at the output. The problem is, when I turn ON the amplifier, the 470 gate resistors R71 and R72 burns instantly. I have tested the tube and it's ok. I have tested the output transformer and it's ok.Thank for your help

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2022, 06:05:26 pm
content: How did you test the tubes?A simple emission tester might connect the screen grid (G2) to the plate during the test and miss a short circuit from the screen to plate.It would take a lot of current to smoke a 470 ohm 5 W resistor: 5 W is roughly 100 mA.Could there be a short on the socket from pin 4 to ground or elsewhere?

author: Andbro
author_rank: Regular Contributor
timestamp: November 15, 2022, 06:36:48 pm
content: Hi,I use a mutual conductance tester. The test compare (Plate, screen grid, control grid) with the Cathode. The TC value read is like on the sticker.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 15, 2022, 06:47:53 pm
content: Both resistors ? I would be looking for a common fault such as loss of negative bias supply for the grids.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: November 15, 2022, 07:07:04 pm
content: Both screen resistors indicate some major anomaly.Like the output valves are somehow plugged wrongly into their sockets.

author: N2IXK
author_rank: Frequent Contributor
timestamp: November 15, 2022, 08:22:20 pm
content: The resistors burn INSTANTLY, or after the tubes have time to warm up?Do you have sufficient negative bias on the control grids?Do you actually have plate voltage on the tubes, coming through the transformer primary? Screen voltage without plate voltage will result in excessive screen current...

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2022, 10:46:48 pm
content: As stated above, you can get excessive screen current if there is no plate voltage present.It would take a few times my calculation of 100 mA to blow the 5 W resistor, but your power supply can probably support that.With zero grid bias (again, as stated above), the cathode current is excessive and may have blown something in the plate supply (although you have tested the transformer, perhaps there is something in the power supply that could blow).The combination of zero G1 bias and no plate voltage could well be fatal to the resistors.The nice thing about octal tubes is that they are hard to plug-in incorrectly, unless you have broken the key on the tube base.(There are some equivalents to the 6L6 with a different pin-out, such as the 6BG6 with plate cap and different base connections.)

author: Audiorepair
author_rank: Frequent Contributor
timestamp: November 15, 2022, 11:06:42 pm
content: Zero bias should blow the HT fuse well before burning out the screen grid resistors.A shorted tube will blow the screen grid resistor, but the chances of 2 shorted tubes at the same time is quite small.It is possible to insert power tubes with the wrong orientation, I have seen it done with missing key posts and sheer brute force.I suspect though there is something here that that not yet been revealed by the OP, as this otherwise makes little sense.Like maybe one shorted tube blew a screen resistor, and then when the tubes were swapped, it blew the other one.A tube tester may give an ok reading at its low test voltage, but when put in circuit at 500v or so the screen grid may arc a short.