EL34 Tube Overheats

Thank you to everyone who gave me help to debug and fix my amp. I replaced all the bias resistors with one watt 10 ohm metal-oxide ones. I like that my voltage is my amperage now by a decimal place. Running at around .333 for now and keeping an eye on red plating and voltage. After I see that it is stable I will take it up to around 44ma. I still need to tie pin one and eight off the tube to the bias resistor. What is the impact of pin one not going to bias resistor? I plan to put pin one into the circuit, but for now I was just wanted to get it back to how it was running before I cranked the bias to high. Oh that tube sound - soooooooo sweet.

What a felling fixing it yourself:) Makes me want to build one from scratch.

Ran it for about half an hour at medium volume. Bias settled at .45 to .43 between all the tubes after half an hour and at idle. It sounded great!

Thanks again

It would be a good idea to re-tighten the socket mounting screws to ensure a good connection to ground for those bias resistors.

The screws are very tight, however some of the socket pins are lose in the base. I need to check that they are tight on the pins, but it took some effort to plug in the tubes.

I knew something was strange when you set your bias to 1.56v with 15 ohm cathode resistors, you probably could have fried an egg on your power transformer...

Sal

Perfect example of ignorance - everything I had read before coming to this site was to set the bias to 1.56v. Now I know that is for two tubes on the same resistor. The person I bought it from had no idea what to set it to. I think the voltage was just under one volt and there was a transformer hum. Now there is hardly any hum from the transformer. I played it for about six hours today keeping the voltage around .4 + or -. I made sure it never went above .44v. I was chasing the voltage at times. How much of an issue is it if it goes over .44v when playing music? I think my tubes are weak and I will need new tubes sometime soon. I keep a fan running on it and the transformer is not warm to the touch, before it was so hot I put the fan on it. Hopefully all I have done is prematurely worn out my tubes.

From another forum: Tube "sweet spot" http://www.diytube.com/phpBB2/viewtopic.php?t=5429

That settles it, I need to keep it at .4v if not under a little.

So after reading about pin one floating, it seems the difference is that I am running in tetrode mode (and EL34 is not really designed to run that way). Makes me think that maybe someone was running this with 6L6 before? Amp has been running pretty stable and cool. I have a fan on it and checking the voltage like an expectant father. I have started pushing the volume and I do see an increase in the bias voltage when pushed, so I am setting the idle bias at or slightly above .33v. So far the bias has not peaked much above .4v when pushing the volume. I plan to tie pin one to eight when I get my new tubes. From all that I have read it seems to be the best option to keep an EL34 amp stable. Having pin one open leaves me on the brink of another Chernobyl red-plate. The sound is really good though and most of what I read people described issues with the sound running with an open pin one.

Now I need to decide on which tube to go with. Not sure I want to spend the money for the winged C or go with the one of the lower cost options. I am running bi-amped with my solid state amp on the bass end, so I can go with the other tubes that are weak in the bass.

Plus to add to the instability I am running the electrostatic panels in my speakers. I have read that these can bring amps to their knees because of the low load.

There should be a jumper wire between pins 1 and 8. Some tubes have an internal connection between pins 1 and 8 (KT88 6550) the EL34/6CA7 does not.

Electrostatic panels in your speakers should be fine. Some solid-state amps have problems driving a capacitive load but your ST-70 should be fine.

I got my new tubes (Electro-Harmonix EL34) and tied pin 1 an 8 together. Sound is great as always. I will need to switch back to a 15 ohm resister at some point though. The lowest I can set the bias on the left side is .46v. I can set the right side down to .34v, but I am running it at around .44v.

Thinking of getting some Electro-Harmonix 6CA7 for an extra set. Not ready to spend the money on a set of Winged Cs yet, but soon.

if you can't set the bias low enough, all you need to do is change R39 on the VTA70 board to a lower value.
Stock is typically 10K. You can go as low as 1K, and that should get your correct bias closer to the center of your range with the adj pots.