Hi, I'm bringing back to life an old 60's Sears/Silverstone 5 XL which were store for like 30 (or more) years. A cousin gave it to me when I was a kid and never worked so I put it aside and stored in somewhere at my parents house...    ...days ago I found it, searched the schematic and worked on it - replaced old capacitors, some carbon resistor..     ....and it's alive!!!

I have checked the voltages and seems to be ok according the schematic (almost everything) but I need help trying to understand how to bias the 12AU6 pentode. I've been reading in some places (even valve wizard site) but I just found information about biasing with a kathode resistor (self bias) and this is not the case.

Seems jfets has are very similar topologies I see the 12AU6 in this schematic is a kind of fixed bias.

Trying to find out the 12AU6 dissipation:

ANODE: 390 K / (103 V - 66 v) =  .0948 mA
GRID2:   2.2 M / (103 V - 20 V) =  .0377 mA
                     Total = 1.3259 mA

Plate voltage = 66 V

Plate Dissipation = (66 V * 1.3259 mA) = 0.00875 Watts

Isn't this too low value?, do am I making some mistakes?

I am posting the schematic and the 12AU6 datasheet, I'll be greatful to have some help to make me clear this kind of bias, I am starting with vacuum tubes so I'm just trying!. 

Don't bias the preamp tubes. Anyway, that amp is a widowmaker. You need to install a isolation transformer.

https://robrobinette.com/Widowmakers.htm

Maybe that's the reason my wife was smiling after watching me making this amp to work...  ...uhmmmm..

Thanks for that information (sad for my wife)..

Anyways, I would like to know how to bias this kind of topologie if somebody knows..   ..just for knowledge.

Thank you

Deal with the widowmaker issue first.

We don't need to consider plate dissipation with preamp tubes, that is for power tubes. The 12AU6 is grid-leak biased, which works due to the natural grid current which flows into the grid. It is rather variable between samples so tbh I would just change it to a 1Meg grid leak resistor and install cathode bias instead.
But if you're set on keeping it original, just measure the plate voltage and if it is close to 66V you don't need to change anything, it is biased. Otherwise reduce the grid leak resistance to pull the plate voltage down, or vice versa, depending on which way you need to go.

Preamp tube do not need edge-of-meltdown bias like we do to power tubes.

This type Grid-Leak Bias tends to get the MOST gain out of a tube, and tends to overload not-too-ugly. It avoids an electrolytic which was a perennial trouble point (OTOH the ceramic grid-cap is needed to work good). Most early 1950s guitar amps biased this way. US schools and vet-clubs were filled with Bogen mixer/amps with 2, 4 even 6 grid-leak input stages. They can be "designed" by copy-cat, by extended searching in old data books, or trial/error. The ratio Rp/Rs will be typical for the pentode in use, the values will be 2 to 8 times higher than you'd use for "good" audio. The very low screen voltage reflects the high-gain condition.

Yes, this is a kid-killer (or widow-maker) but not the worst sort As Long As You Do Not Work Inside 'Live'!! Late-Code T2 protects from heater-cathode insulation breakdown. R3 R4 R5 throttle the maximum Line-to-Player (panel, jack) leakage to under 2mA. It will tingle, even make you jerk stumble and hit your head, but heart-healthy persons (speed kills) often survive 4mA or 10mA.

This form of fixed bias is called contact potential and it comes from electrons being intercepted by the grid and causing the voltage to go negative.  The screen grid voltage of 20 volts is low and the plate voltage is getting down to a region where linearity is not that great.  This is a tube that usually runs 100 to 250 volts on the plate through an RF or IF transformer with less than 100 ohms of resistance, so this is starved-circuit operation as shown here.

I will repeat what other people have said - this amp is dangerous and it only takes 11 mA from hand to hand to stop your heart.  If you are using it, don't touch anything that is grounded and keep the circuit in an insulated case.

Well, I think better I put my work on making this amp secure. I have downloaded some schem using the isolation transf. Sure I will do it soon some way, meanwhile I will put it away from my kid.

Another thing I have to change is the speaker. It seems somebody before changed it for an 8 Ohms (normally it uses 3.2 - 4 Ohms). The tone I hear is kind of dark, so maybe this speaker is affecting the tone.

PRR / AMPTRAMP, I appreciate the explanations in here about the 12AU6 topologie and safety. Searching on the web many old amps uses almost the same schematics.

Saludos!!

Quote from: percyhornickel on October 22, 2024, 07:40:04 AM8 Ohms (normally it uses 3.2 - 4 Ohms). The tone I hear is kind of dark, so maybe this speaker is affecting the tone.

Not because 8><4. Yes, a high-price mellow tone speaker may have more highs; in particular, speakers tuned/selected to give good AM radio reception may be all doped-up to cut shrill static.

*WHEN* you can do it without exposed conductors, wire a nice Twelve to it. EVM-12 might play some clubs with wide tone. A Marshall Full Stack could call the cops to your garage.

I tried with another 8 ohms speaker I have at home and the sound is so much better so the one is installed right now is not in good condition. I am about to get a celestion 4 ohm speaker 8" and I hope to keep good highs with this one.