Randall RD-5 not happy..

[Jon Snell]

I assume you have the chassis out and are reading from under the chassis?

[Jon Snell]

Are you measuring on the pins, from the underside of the chassis?

[Sea Chief]

Aha I do see signs of life in the 12ax7's! great.. a tiny glow (I got 3 bloomin red very bright led's, bizarrely, located under the chassis pointing into the cabinet.. I think cosmetic bllx.. but make it very hard to see any tube activity).

Definitely nothing in the 6v6 though.

Progress though.

[Jon Snell]

If you remove the valve and read 6.3volts if there is a bad connection you won't know!
Are the 12AX7s lit?

[Jon Snell]

Try a new 6V6.
Yes lights to make the valves look 'good'.

[Sea Chief]

Are you measuring on the pins, from the underside of the chassis?

I have the chassis out Jon yes, reading on the solder junctions, across pins 2 & 7 of 6v6. Without desoldering this pair of wires I think its the next best way to get a reading-? I'm not poking into the 6v6 socket!

So signs of life in the 12ax7's as per my post just now.

thanks. SC

[Jon Snell]

If the three LEDs are lit, you have one side of the AC so I would definately replace the 6V6. Anything will do to test; 6L6, KT66, EL34 etc

[Sea Chief]

Try a new 6V6.
Yes lights to make the valves look 'good'.

If I get glow in the 12ax7's, surely this means that the 6.3v winding IE both wires, is in good health? so their two wires are sending & receiving 6.3v respectively, yes?

If yes, then how beforehand in the circuit/ IE before the 12ax7's, am I not reading 6.3v across the wires @ the 6v6 mini pcb (regardless of whether the tube is shot or not) ?

[Sea Chief]

If the three LEDs are lit, you have one side of the AC so I would definately replace the 6V6. Anything will do to test; 6L6, KT66, EL34 etc

Replaced 6v6. Notice two spots of glow very top centre, & very btm centre of the tube (bloomin red led's blinding me).. & it gets hot.

Plug gtr in.. no sound at all tho.

Hmm.. a sideways progress step then.

[Jon Snell]

The heaters in a valve never go short circuit, they always go open circuit because that is how they are made.
Voltage on the heaters of the 12AX7s is DC in this case.
Follow the schematic and the heaters connect to a pair of capacitors, smoothing some of the AC born ripple. They are fed from a bridge rectifier that is fed directly from the heater supply.
I have no idea where that is except the diagram shows it coming from pins 2 & 7 of the 6V6.
In conclusion, the transformer must be alright therefore if the supply to that bridge rectifier comes from the 6V6 base, the 6V6 is definately faulty ... but I know Chinesium diagrammes are notoriously incorrect.
Good luck, keep logical. Beer time for me.

[Sea Chief]

Jon Snell many thanks for you're help today Jon- a good forwards step taken. Enjoy your beertime!

(JurasSic-?)

[Pixel]

Aha I do see signs of life in the 12ax7's! great.. a tiny glow (I got 3 bloomin red very bright led's, bizarrely, located under the chassis pointing into the cabinet.. I think cosmetic bllx.. but make it very hard to see any tube activity).

Is there a plug at CN13? If so you can disconnect those bothersome leds.

[Sea Chief]

Is there a plug at CN13? If so you can disconnect those bothersome leds.

Hi there Pixel.. yes indeed/ that's impressive of you to find that out, a connector CN13 going to the 3 led's. So no harm to rest of circuit to just unplug it-?

thanks for that. SC

[Sea Chief]

Could anyone else lend a hand?


I do get 5.7 (vdc) across pins 2 & 7 of the 6v6. Also all 3 valves are lit. But no sound from the speaker.

Jon mentions possibly a connection issue from 6v6 pin 2 to ''the board.. but Im a bit lost, as to which board (the 6v6 sits in its own one).

I don't see any voltage test points on the schematic, so unsure where to start to diagnose.

Thanks if so.. SC

[Pixel]

Could anyone else lend a hand?


I do get 5.7 (vdc) across pins 2 & 7 of the 6v6. Also all 3 valves are lit. But no sound from the speaker.

Jon mentions possibly a connection issue from 6v6 pin 2 to ''the board.. but Im a bit lost, as to which board (the 6v6 sits in its own one).

I don't see any voltage test points on the schematic, so unsure where to start to diagnose.

Thanks if so.. SC

According to the schematic the 6V6 heater is AC, the 12AX7s are getting DC. I think Jon's mention of possible connection issues on 6V6 pin 2 were when the heater wasn't lit.

You can unplug the "bling" leds at CN13 if you want to.