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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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It is currently 27 Oct 2020, 02:05

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PostPosted: 23 Aug 2020, 02:15 
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Joined: 23 Feb 2017, 02:02
Posts: 677
Zero idea on why it would arck to case, unless you have something connected inappropriatley or too little space between HV and low volt.
Otherwise should not happen.

Maybe tube socket is poor and becomes coundcutive?

For clipping I would first look at the voltages at the pimary of OPT and at plate of 6SJ7.


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PostPosted: 23 Aug 2020, 12:01 
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Joined: 03 Jun 2020, 11:06
Posts: 83
Location: France
@gofar99,

Some suggestions, coming from dozen years of troubleshooting tube amps... :)

1 - search for a mistake in wiring somewhere, poorly routed or too long connection... That kind of bug is not rare on kits (done by novices :D )
2 - insert a 1K to 2K2 in serie with screen grid : the screen is sometimes prone to trigger oscillations at high volume on EL84.
3 - insert a 4K7 in serie with grid n°1 : same reasons as point 2 above.
4 - add a 470K to 1M between grid n°1 and GND : if the pot shows a poor/intermittent wiper contact, the EL84 can go up dangerously.

It's always difficult to help at distance for troubleshooting... :blush:

But it's me, OK ? ;)

-tbln

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PostPosted: 10 Sep 2020, 21:13 
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Hi, For the moment it seems OK. There is a 0.01uf from anode to cathode and a ceramic socket. Time will tell. Thanks for everyone's input.

Good listening
Bruce

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PostPosted: 26 Oct 2020, 13:12 
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Joined: 26 Nov 2019, 02:09
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Location: The Netherlands
A practical 'translation' of what Suncalc already pointed out earlier in this thread (= the EL84 is biased way too cold and because of that comes into cut-off when driven; as a result high voltage spikes develop at the anode because of the inductive nature of the output transformer) could be this:

Put a resistor of 820 Ohm parallel to the 500 Ohm cathode resistor. With this, the total value of the combined cathode resistors becomes 310 Ohm. With this value the EL84 is biased a lot better than with only the 500 Ohm resistor.

If you try this, than let us know what the voltages are at the anode, screen grid, and the cathode of the EL84 (all measured with respect to ground). Depending on the result you might even go a bit lower with the parallel resistor (with 680 Ohm as the parallel resistor, the total value of the combined cathode resistors would become 288 Ohm).

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