Randall RD-5 not happy..

[Jon Snell]

Colours are; Red 6.3v AC, Blue 6.3v AC feed to diode bridge and Green DC feed to 12AX7s.
Keep it simple, which one is missing, Red is missing, from what I can gather from your fault diagnosis.
Chase the red coloured lines.

[Sea Chief]

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.

Hi Pixel,

yes that makes sense, so I think I've established that the original 6v6 wasn't working. Whether this facilitated damage furtherr along, or whether something has caused it to fail though.. I don't know.

If I put a good 6v6 in & I see some glow, then have I ruled out the possible connection issue "from 6v6 pin 2 to board" ? I'm not sure you see.

I do actually read 6.3vdc across the 6v6 pins 2 & 7 (not 6.3vac). Actually 5.7v I get. Anyway as long I get some voltage, I guess I can rule this out.

Yup seen where I can unplug CN13, thanks good suggestion/ would never have seen it, tucked away. Stupid thing!


cheers SC

[Jon Snell]

The 6V6 has AC heaters not DC!
Anything from 5.6v to 6.9volts is acceptable.
Your 6V6 is faulty by the sound of it.
This is what happens when we over think simple faults. A vertical learning curve that most of us have seen.
Have fun.

[Sea Chief]

The 6V6 has AC heaters not DC!
Anything from 5.6v to 6.9volts is acceptable.
Your 6V6 is faulty by the sound of it.
This is what happens when we over think simple faults. A vertical learning curve that most of us have seen.
Have fun.

Hi Jon,

yes its been so long since I used my DM (it has no Ac, Dc labels) so I was measuring using the wrong side on DMM. That's fine, now I understand.

Ok I put in a different, known to be good 6V6. Now I get glow inside (possibly before with the original 6v6 too.. but red leds blinding me).

But it is still not working. Nothing different.

[Sea Chief]

Colours are; Red 6.3v AC, Blue 6.3v AC feed to diode bridge and Green DC feed to 12AX7s.
Keep it simple, which one is missing, Red is missing, from what I can gather from your fault diagnosis.
Chase the red coloured lines.

Ok thanks that clears this up.

No nothing is missing. When I originally measured across pins 2 & 7 on the 6v6.. I was simply using the wrong side of the DMM. This meant I saw nothing on the DMM. This meant I assumed no voltage across. Actually There was voltage across, once Id switched the DMM over to other side of 'midday' on its dial.

(Im rusty, & just used to using the vdc side on capacitors, measuring circuit voltage etc.. plus the bloody DMM doesn't just say 'AC' & 'DC' on, I mean even the symbols are identical either side of OFF @ vertical position).
Sorry for the confusion.

thanks SC.

[Helmholtz]

VAC range on DMMs is often identified by a V~ symbol.

[Sea Chief]

VAC range on DMMs is often identified by a V~ symbol.

Yes, but the line dot dot dot symbol is the same on mine both sides, exactly same place. The V- squiggly line is tiny & amongst other things/ not clear. But yes it is what I missed obviously. Apologies its a while since I used it to measure voltage, & 99% always seemed to be DC.

Anyone know where I go now?

(Orig 6v6 not lit, no sound. New 6v6 lit, still no sound. Another good 6v6 tried too.. but still no sound.)


thanks. SC

[Pixel]

Anyone know where I go now?

(Orig 6v6 not lit, no sound. New 6v6 lit, still no sound. Another good 6v6 tried too.. but still no sound.)


thanks. SC

Just to make sure the original 6V6 has a bad heater, please measure resistance across pin 2 and 7 on the original 6V6 tube while it is out of the chassis.

Try plugging guitar into the return jack to see if you hear anything on speaker.

[Sea Chief]

Just to make sure the original 6V6 has a bad heater, please measure resistance across pin 2 and 7 on the original 6V6 tube while it is out of the chassis.

Try plugging guitar into the return jack to see if you hear anything on speaker.

Well in fact I do get continuity across 2&7 on original 6v6. This is very odd..because I cannot see any typical spots of glow whatsoever. Not even now your cn13 pulled silly leds removed. Even in a dark room! Very odd. Anyway same results with a glowing known-good 6v6.

Do some tubes have very feint glow? Its a Ruby 6v6 str.

Tried via return jack..absolutely nothing. There's no hiss or anything suggesting anything is getting to the speaker.

Thanks, SC.

[Jon Snell]

You have established there is heater voltage, what is the voltage on pin 8 of the 6V6. Zero voltage means the valve is drawing no current. If zero what is the Anode voltage, pin3 and Screen Grid voltage, pin4? Mind your fingers as there is 380volts there!
Alot of 6V6s have blackened glass.

[Sea Chief]

You have established there is heater voltage, what is the voltage on pin 8 of the 6V6. Zero voltage means the valve is drawing no current. If zero what is the Anode voltage, pin3 and Screen Grid voltage, pin4? Mind your fingers as there is 380volts there!
Alot of 6V6s have blackened glass.

Hi Jon. Warnings noted.

Ok @ pin8 of 6v6 (original one) = 0 vdv (& 0 vac)

On a known-good 6v6 = 21 vdc (& 47 vac.. if of any significance).


I presume post these odd findings before doing pins 3,4.


Thanks. SC

[Jon Snell]

That means the new 6V6 is working, so no need to test anode and screen voltage as it either were missing, there would be no electron flow from cathode to anode, so no current flow and zero volts on the cathode.
Check the speaker is working and connected as you should be getting some audible hum.

As a matter of interest, there will be no AC voltage on the cathode so I wonder what your meter is and is it trustworthy ... there are lots available on fleabay, many of reputable quality, like Fluke.

[Sea Chief]

That means the new 6V6 is working, so no need to test anode and screen voltage as it either were missing, there would be no electron flow from cathode to anode, so no current flow and zero volts on the cathode.
Check the speaker is working and connected as you should be getting some audible hum.

As a matter of interest, there will be no AC voltage on the cathode so I wonder what your meter is and is it trustworthy ... there are lots available on fleabay, many of reputable quality, like Fluke.

Ive disconnected speaker purely due to getting nothing audible/ never have at any time with it connected. Totally zilch (as if the OT were just not working).

My meter's served me well (I understand you've no confidence in my abilities after not distinguishing heater AC/DC!) but I've used it for years, built a few amps with it in fact, plugs firmly seated/ checked, so I have faith in it.

I was wondering if this pin8 cathode AC voltage signified anything? i know I'm not meant to get any here.. but if I do-?

[Sea Chief]

Jon Snell shouldn't I expect to get HV @ the 6v6 cathode though? 350v, in that ballpark-?

(edit: no pin8 should be around 20-25v I think .. if www 5F1 info as an approximate guide is ok).

thanks SC

[Sea Chief]

I get a healthy 395v @ pin3 of 6v6 anode ! (& a good 20v @ pin 8 cathode too).

So that's good: & bang on a 5F1 figure too I believe.. maybe this circuit is loosly based on one I wonder.

Thanks, SC