5F2A Build - Tone issues and power supply voltages

[keithpop]

Hi all,
Why is it I can build fairly complex amps with no issue but have trouble with a 5f2A Princeton build??? 

The amp does work but is not right...distortion is not good, some buzz that increases with the amount of volume.  Bottom line, it just doesn't sound good.

I used a mid-50's 'Echosphere' PA amp as the basis (transformers and chassis only) because it had the same tube configuration (5Y3, 12AX7 and 6V6).  I built my own board and re-wired the tube sockets.  I used two 100 ohm resistors for the filament ground.  That seemed to work fine but I still have hum that increases with the volume control.  This is probably wire placement related....I can work on that once the tone problem is fixed.

Findings so far:  B+ voltage is higher than the schematic calls for (around 410 volts).  That doesn't bother me too much because I am using old PA amp power transformer.  But the 6V6 grid voltage (after the 10K resistor) drops to about 190 volts and it should be around 300 volts...I think.  By the time it gets to the 12AX7 plate the voltage is down to 100-110 volts and it should be twice that reading.  I think this explains why the tone stinks.

I have the replaced the 6V6 and 12AX7 tubes, the 10K resistor (choke) and 220K resistor.  Double and triple checked the power supply wiring.

I am putting the amp in a 1930's cathedral radio cabinet.  Looks really cool so now I want the sound right.

Please let me know if y'all have any ideas?  Why the voltage is screwy?  Something is dragging it down.

I attached a picture of the radio....not sure why.  I also attached a copy of the schematic.

thanks in advance for your help!  It is really appreciated!

keith
 

[HotBluePlates]

... B+ voltage is higher than the schematic calls for (around 410 volts).  ... voltage (after the 10K resistor) drops to about 190 volts ...

410v to the OT, but 190v at the screen supply node... You have 220v dropped across the 10kΩ resistor.

220/10kΩ = 22mA
22mA * 220v = 4.84w

If that resistor is 10kΩ, and not 5w or more, it should already be discolored or outright burned.

If, however, it was a 100kΩ resistor, then:
220v/100kΩ = 2.2mA
2.2mA * 220v = 0.48w

A 1/2w resistor would be hot but fine, a 1w resistor wouldn't know the difference.

IF the resistor is really a 10kΩ (and not an accidental 100k), there would have to be a partial short-circuit somewhere on the preamp side of the 10kΩ. No way 2x 12AX7 stages are gonna pull 22mA through that resistor on their own.

In fact, the plate loads are 100kΩ, and you say you have 100-110v at the plates. There's 190v at the filter cap.
190v - 100v = 90v
90v / 100kΩ = 0.9mA

So at best the 12AX7 stages seem to be drawing about 1mA. That seems to confirm the likelihood of a wrong value for the 10kΩ.

And if it is the wrong value, no sweat. We've all done it before.

[keithpop]

Thank you very much for the help.  I swapped the 10K ohm for two 4.7K ohm resistors as a test and got the same result.  They were only 1/2 watt resistors and got really hot after a minute (expected that to happen).  I reinstalled the original 10K resistor (it's 3 watts and seems to be OK).  One thing I didn't mention is the voltages start out ok but drop within seconds to the lower values after turning the amp on.

I will go over the preamp stage.  I did install two very old (but never used) .02 mfd white "Ajax" capacitors.  I have used them in the past with good results but who knows?  Maybe one of them is breaking down. 

Any other ideas please let me know.

thanks for your help.

keith   

[sluckey]

Pull the 12AX7 and 6V6. Turn the power on and check voltage on each of the filter caps. All voltages should be high. What are your numbers for each cap?

[keithpop]

I pulled the 6V6 and 12AX7 and the voltages did jump -->

Starts at 478 volts (B+)
after the 10K resistor - 476 volts
after the 22K resistor - 474 volts

I also checked the voltages at the .022 capacitors and they are also 474 volts.

I plugged the 6V6 in (left the 12AX7 out) and the voltages were a bit better but not much.  Voltages dropped to  425 volts for B+ and after the 10K resistor - 190 volts....and so on.

So there is something dragging it down.

I really appreciate the help!

keith

[HotBluePlates]

I plugged the 6V6 in (left the 12AX7 out) and the voltages were a bit better but not much.  Voltages dropped to  425 volts for B+ and after the 10K resistor - 190 volts....and so on.

Try unsoldering one leg of the 0.02uF connected to 6V6 pin 5.

How's the voltage after the 10kΩ now?

[keithpop]

I removed the wire that connected pin 5 to the .02 capacitor.  B+ remained the same but dropped even more after the 10K resistor....down to under 100 volts.  I think it settled around 95 volts.  So it's dropping 325 volts across the resistor.

Thanks again for working with me on this.

keith

 

[HotBluePlates]

Unsolder one leg of the other 0.02uF cap.

And is the 25uF bypass cap new or used?

[sluckey]

I removed the wire that connected pin 5 to the .02 capacitor.  B+ remained the same but dropped even more after the 10K resistor....down to under 100 volts.  I think it settled around 95 volts.  So it's dropping 325 volts across the resistor.

That's not the same as what HBP requested. Unsolder one end of the cap. Leave the 220K connected to pin 5. Does the screen voltage still drop?

If so, closely examine the wiring to the 6V6 socket and the socket itself.

[keithpop]

heheheh.....oooops

I found the problem...For some reason I had the output transformer primary wire connected to pin 4 of the 6V6 tube...and then ran it right back between the 10K and 22K resistors (so I had 2 wires connected to pin 4).  Not sure what I was thinkin'....  But I guess that explains the voltage drop because the voltages are all good now.

I still have a little buzz to deal with but my wiring on this is pretty sloppy.  I'll cut, re-dress and even shield a couple of the wires.
 
Thanks guys for hanging in there with me and leading me to the right area.  Y'all are the best! 

Have a great day!

keith