Danelectro 299

[ernest]

This is an early 60s Dano model 299 Twin Twelve that I converted into a tweed single Fifteen.  It's been recapped and new tubes. I'm having trouble figuring out how to bias the two new JJ 6L6 tubes I just put in.  This is the only schematic I'm able to find, only mine is a Twin Twelve with two output tube instead of four.
http://el34world.com/charts/Schematics/files/danelectro/danelectro_twin15_1961.pdf]
All of the other tubes are the same.  I measured 353v plate voltage then 76 and 75 amps from OT to plates.  That puts them too hot at around 26 watts for 6L6 at idle?  I can't figure out looking inside the amp where bias is adjusted.  Its a big harness of wires inside and it looks like all the pin 8 cathodes are linked from tube to tube.

[tubenit]

Have you considered simply converting it to cathode biased?   I have two amps that have around 17w and 23w respectively with 5881/6L6's.

I use 257R/10w on one & 360R/10w on the other.  I can change between 6V6's and 6L6's or 5881's without rebiasing anything.  I like being able to do that.  With your plate voltage of 353v, you could probably do the same thing IF you wanted to. 

I have found if I use a cathode cap of 60uf to 100uf range that the cathode biased approach doesn't sound too much different then fixed biased to me.  I can still get a reasonably clean tone with good tight bass response.

Just a thought to consider.   With respect, Tubenit

My favorite of all production amps I've owned was a Dano Centurian 275

[sluckey]

I'm pretty sure this amp is cathode biased, but the 12AX7 filaments will probably be in the cathode circuit of the 6L6s so that really complicates changing the bias by manipulating resistor values.

I would be wary of that twin 15 schematic. Better to just draw your own schematic. But as a minimum, trace out the circuit from pin 8 of the 6L6s and report what you find. Also trace out the filaments for the 12AX7s and report what you find. If they connect to the filaments of all the other tubes, you're in luck. But I bet you will find they connect back to the 6L6 cathodes (maybe thru some resistors).

You may just have to accept the bias as is.

[tubenit]

I wonder if the Silvertone 1484 schematic would be of use for you?  It seems to have similar tubes to the Dano Twin 15 but with only two 6L6's.

http://el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf

With respect, Tubenit

[PRR]

Please re-study the cathode system. The two 6L6 pin 8s connect to each other, but then surely go to 4,5/9 of the 12AX7s.

A cathode-bias amp WILL run near 100% of rated dissipation. 26W in new JJ should not be any problem.

As this is biased by heaters, and probably lacks the extra resistors used on the four-6L6 job, there's no simple way to change bias. The current MUST be enough to light the 12AX7s. If repeated power-bottle failures happen, then reduce the B+ somewhat.

[ernest]

I'll tear apart the harness and draw out where everything is going then report back.   Thanks for the advice!

[ernest]

I got some help finding a schematic that matches and confirmation the power tubes bias goes through preamp filaments. 


Silvertone 1394

[sluckey]

Good find! I had looked at the Silvertone 1394 Chassis 185.10300. I dismissed it because it used a 6SN7 for the PI.

Nothing to do about the bias. Just accept it. It'll be OK. 

[ernest]

Thanks!

[PRR]

> Silvertone 1394

Be real darn careful!!

That amp, if you disconnect the RCA plug, the whole control chassis and RCA plug is 300V HOT!! They depend only on the RCA plug to return the high voltage. Up-plug, YOU get 300V through the preamp tubes.

The danger may be very small because unplugged the preamp tube has no heat and will not conduct. I think the danger is if you un-plug while running. The hot tubes will conduct dangerous current for several seconds, while your hand may be between plug shell and main chassis. Or across your guitar and the nearest mike-stand or radiator.

At the risk of a new ground-loop, _I_ would hard-wire a ground that can't be undone without tools.

[tubenit]

PRR,

I am very grateful for your knowledge and willingness to share it!  Thanks for being a moderator and providing valuable information like this.

My respect, Tubenit

[ernest]

PRR,

I am very grateful for your knowledge and willingness to share it!  Thanks for being a moderator and providing valuable information like this.

My respect, Tubenit

same here. Thanks for all your help.  This forum is invaluable to me.