Danelectro/Silvertone 1335 - Challenger 89 bias problem

[Saransk]

When a really nice condition Danelectro Challenger Series D Model 89 chassis set showed up for sale I had to have it.  Not only was it from the year I was born but the control panel was in great shape.
Of course it needed all of the work a 60 year old amp needs, and then some.  Basically it was a rebuild.  Because it was very open it was easy to photo everything is series and then draw up a wiring diagram.  The power transformer put out the listed voltages and seemed in good shape.
When I started doing voltage checks without the 6L6's everything seemed okay.
Put the output tubes in and nothing came out of the speaker.
While the power tubes are warming up it doesn't appear that they are putting out the 19vdc on the bias so the two 12AX7's are not coming on.
The heater voltage may also have an issue for both the 6SN7 and the 6SJ7 - it looks like it is dropping with the power tubes in.
I've checked both the layout of the two parallel legs of the heater voltage and the continuity/wiring of the 12AX7 heater / bias and cannot find any issues. 
At this point I'm not sure what could be the issue.  The bias circuit is pretty simple - Pin 8 from both 6L6 tubes to pin 4 of the 12AX7 on the chassis.  Pin 5 of that 12AX7 to #4 on the upper chassis connector which connects to Pin 4 (on mine) of the preamp 12AX7 with Pin #5 to ground.  The Preamp ground connects via the audio connector and a dedicated Ground cable.
What I wonder is if the Power Transformer is just worn out.  Could it be putting out the voltage but can't handle the actual load of the 2 output tubes, even though they warm up.  I have B+ on the preamp tube that matches the schematic so I assume the output B+ is good.
Any ideas would be very appreciated
Schematic is attached - both are in Schematics library as well, as is the Airline GDR8516a which is also the same circuit.

[sluckey]

Measure resistance from pin 8 of each 6L6 to chassis ground. Probably about 200Ω. If not, check your filament wiring for the two 12AX7s. If all this is correct...

Measure voltage on pin 3 and 4 of both 6L6s. Should be 300v or more. Without plate voltage the tubes cannon conduct. If the tubes can't conduct, there is no current flow through the filament circuit and no 19v will be produced at pin 8 of the 6L6s.

If no voltage on pins 3 and 4 you need to look at the power supply... PT, 5Y3, filter caps.

[Saransk]

I know the filament resistance isn't 200 from Pin #8
The circuit runs through the 2 12AX7's in series so it reads closer to 40 as each tube reads about 20 from pin 4 to pin 5.
In all of the schematics for this circuit there is no other resistance in the line from Pin #8 on the 6L6 tubes to Pin #4 on V3, Pin #5 on V3 to #4 on the connector to Pin #4 (or 5) on V1 in the preamp, Pin #5 (or 4) to ground.
I noticed that the Silvertone 1396, which is this circuit with 4 output tubes, there is a 220 ohm cathode resistor which puts 27 volts down the 12AX7 heater line which has a 39 ohm dropping resistor.

But a large cathode resistor would be hard to miss in the 1335 chassis and I've not seen one in any of the photos, nor was there one in my chassis.
I'll check the B+ as suggested but I did a voltage check without the tubes and had both plate and screen voltages at the socket that matched the schematic values.

Thanks

[sluckey]

Yeah, I forgot the resistance would be lower when the tube is cold. The resistance of a hot 12AX7 filament should be 12.6v/.15a = 84Ω. So 20Ω cold is reasonable. Did you actually measure about 40Ω from pin 8 to chassis?

Don't throw the 1396 into this discussion because that will just cause confusion.

[jjasilli]

Ditto.  For testing purposes & to eliminate variables you could temporarily: i) disconnect the 12AX7 heaters from the power tube K's & ii) install a K resistor for the power tubes: 170 to 200 R.  The power tubes & PI should light up and pass signal.  You can inject signal into ea power tube and/or into the PI.

[Saransk]

In most amps this would work but...
In this circuit, to use the tremolo circuit which is sort of a bias modulation design - there are two stages in the PI.
V3 - the 12AX7 on the main chassis appears to be the Actual PI with V4 (the 6SN7) is another amp stage which drives the grids of the 6L6 tubes and allows the tremolo on the grids - like if there was a "fixed" bias circuit.
So at least one of the 12AX7's has to be in the circuit.

What I can do is put the resistors in the 12AX7 sockets and see if I get the voltage for the heaters on that circuit.

It looks like a very convoluted way to get DC voltage on the preamp tube heater.  Maybe Danelectro scored a warehouse of the power transformers that didn't have the usual 6.3 heater windings.

[PRR]

Off-topic car talk moved to:
https://el34world.com/Forum/index.php?topic=24694.0
Let's let Saransk fix this amplifier; a 289 is not the answer.

[2deaf]

Off-topic car talk moved to:

Thanks

[jjasilli]

In most amps this would work but...

Good Points.  You're idea is a good possible test solution.

Car talk aside,  I would not gut this venerable amp, but get it to work.

[2deaf]

I would not gut this venerable amp, but get it to work.

I agree.

[Saransk]

Turned out to be a true "senior" moment - sort of like "Duh"
I was testing it with the incandescent surge limiter and forgot that with tube rectifiers sometimes it causes a voltage drop on the AC line.
I had this same issue with a AC15 build when it turned on but was very distorted.  Didn't like trying to run on 90 VAC.

I plugged the the line cord directly into the power strip and viola, it works.
sounds great, remarkably quiet, and has a really nice nice strong tone - I love the sound of 6L6's.
I do have to redo the lettering for the input plugs but otherwise is works.

I did add a switched pot for the tremolo intensity so I could turn it off without a switch, and an output jack.
Otherwise, it's just like the original.

Thanks for the help, it got me looking and thinking.

[jjasilli]

 

[DummyLoad]

i had one of those. i think mine was 66 model - mine was called the panoramic - it had two johnson plugs for slave units with a signal and tremelo outputs. schematic attached - pretty much identical to yours except for the slave plugs and the 6AU6A for the trem osc. in place of your 6SJ7. i ended up giving it away to an acquaintance as no one wanted it for 200bux: he was supposed to record some sound clips for me. never saw him again.   


--pete