Bias Problems in a New Build

[Just Mike]

I built a Ceriatone layout JCM-800 2204 and I can't get the bias voltage close. The best I can get is 12-20mv range with tubes installed.

With no tubes installed I have 499v on pins3 of the tube sockets and -35v on pins 5. On the anode side of the bias ckt. diode, I get -45.8mv. Between the 15k and the 56k I have -36mv. The trim pot varies the voltage from 0 to -25mv. Doesn't look like enough range to me. I've doubled the 220k bias range resistor with minimal effect. It gets me another few mv. Any ideas? Thanks.

[g1]

Are you getting proper screen voltage at pin 4 of the power tubes?
The layout shows a 47K after the 15K, but you mentioned 56K?
Can you post a proper schematic rather than the layout?

[Just Mike]

Don't have a schematic. I built it from this layout. 47k is correct. I've been messing with values here and at one point it was a 56k. Voltage at pin 4 is 495 with no tubes installed.

[g1]

In your first post there are a few places you said mV where I think you meant volts (-45, -36).
You are checking bias by measuring the DC (mV) across the 1 ohm cathode resistors of the power tubes?
How about measure the DC volts at pin 5 with the bias pot set to min, then set to max.

[Just Mike]

With tubes installed and the 47k bias resistor;
pin 3 489v
pin 4 487v
bias pin 1 7mv max. Brand new EH EL34's.

[g1]

power amp section schematic:

[Just Mike]

pin 5 I have a range of -29.7 to -52.6v.

[g1]

Make sure those cathode resistors are 1 ohm, and connected in series between pin 1/8 connection and ground, as shown in layout.

[Just Mike]

They are. FYI, the amp works, passes audio and the front end sounds like it works properly. I get gain with the pre volume cranked, but the output is very low...like a 1 watt amp.

[g1]

Are the heaters lit up good on the power tubes?

[Just Mike]

What do you mean by "good"? You know this got me to thinking so I measured AC volts across pins 3&7 and I'm reading 3.2 VAC. I have the heater center tap grounded...is this the problem?

[g1]

You mean from pin7 to pin2 (not either pin to ground) ? You should have 6.3VAC there, if it's 3.2, that is definitely a problem.

[Just Mike]

Yes and I just discovered that I miswired the heater secondary. I ASSumed black was the CT.
http://www.classictone.net/40-18023.pdf
Hope I didn't fry the winding.

[g1]

Before you fire it up with the proper heaters, set bias pot to max resistance (-56V at pin5).
Also you mentioned you had "doubled the 220k bias range resistor", which I think is the one feeding the bias diode. Make sure to put it back to stock value.

[g1]

Bet that sounds better now, dunnit' .