Single Channel 6G3

wallybob

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As NTC mentioned, red-blue wire is for fixed bias voltage generation purposes; it has nothing to to with a tube rectifier
 

Che_Guitarra

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Alright... slow progress, but progress nonetheless.

Made the mistake of cutting the IEC hole on the wrong side, effectively making this a left handed amplifier 😅

Got as far as can be seen with the wiring today. Will give it a rest and come back to it tomorrow... I know i'll make a mistake if I try and do it all in one sitting. Doing things backwards has added a degree of complication I could do without.

PToI0Vj.jpg
 

DaveNJ

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Alright... slow progress, but progress nonetheless.

Made the mistake of cutting the IEC hole on the wrong side, effectively making this a left handed amplifier 😅

Got as far as can be seen with the wiring today. Will give it a rest and come back to it tomorrow... I know i'll make a mistake if I try and do it all in one sitting. Doing things backwards has added a degree of complication I could do without.

PToI0Vj.jpg
Super clean work - nice!
 

Che_Guitarra

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Thanks!

So got it all together today. Initial voltage checks OK. One step further and plugged in the tubes, plugged in a cable and touched the tip... not an iota of sound from the speaker, not even faint hum.

Ahhh! It wouldn't be amp building if everything worked first time! Let's see if I can work this out over the week.

Not a hint of sound - let's start looking at the OT wiring and see what we find.


IriDkeQ.jpg
 

Che_Guitarra

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So... troubleshooting phase one.

Pulled out the multimeter and prodded around. Found a loss of continuity in the shielded preamp wire. Great... maybe. Pulled it out, prepped up a new length, tested OK for continuity, so in it went. Tested again after install - once again no continuity. Not going to try a third time - need to find better wire. Might even ditch the shielded wire altogether.

Also - a little concerned I got no sound (I mean some kind of noise floor) when I fired the amp up. Assuming the PT is fine seeing 6.3V and 5V reading OK. OT getting readings in the 160 - 300 ohm range on the primary side which, no continuity through to second. Not sure how to otherwise trace issues on the secondary side.

Alternatively, maybe my GZ34 is kaput? No backup GZ34 to check, is it OK if I use a 5Y3 just to see if I can get this amp to make some noise?
 

Uncle Daddy

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So... troubleshooting phase one.

Pulled out the multimeter and prodded around. Found a loss of continuity in the shielded preamp wire. Great... maybe. Pulled it out, prepped up a new length, tested OK for continuity, so in it went. Tested again after install - once again no continuity. Not going to try a third time - need to find better wire. Might even ditch the shielded wire altogether.

Also - a little concerned I got no sound (I mean some kind of noise floor) when I fired the amp up. Assuming the PT is fine seeing 6.3V and 5V reading OK. OT getting readings in the 160 - 300 ohm range on the primary side which, no continuity through to second. Not sure how to otherwise trace issues on the secondary side.

Alternatively, maybe my GZ34 is kaput? No backup GZ34 to check, is it OK if I use a 5Y3 just to see if I can get this amp to make some noise?

The 5y3 draws 2A on the filament; your PT is specced at 3A so yes, as long as you kept the stock pinout. It'll drop the B+ a bit.
 

Che_Guitarra

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5Y3 has confirmed a trickle of noise through the speaker. My GZ34 must be cooked. But no guitar noises just yet.

Troubleshooting phase two. Double-checking my PCB - all voltages seem proportional to what they should be, given i'm using a 5Y3. PCB design has been checked yet again with no faults to pick. Preamp section is confirmed OK. Control wiring looks fine. Phase inversion is wired up correctly. Rectification seems OK. Output/OT wiring looks OK.

Turning on, i'm getting a pop through the speaker after a few seconds and then just regular noise floor sound. Rectifier and power tubes glowing and getting hot way quicker than my 6G2. Not sure if I should read into that, but seems like where I should be starting.

Otherwise no burns observed on the components or PCB. No smells that concern me. No shorts, no cold solders.


Open to suggestions to where to start picking this one apart 🤔
 

Che_Guitarra

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Scratch that, I think I see the problem. My red pair went to the power tubes when they should have done to the PI. And my brown pair sent to the PI should have gone to the power tubes.

Damned left handed amp!
 

Che_Guitarra

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Hoped to have this running and ready for my Saturday practise, but not to be. I swapped the obvious wires I mixed up, plugged it in and gave it a quick strum - not a breath of noise from the speaker.

Pulled the tubes and put them in my 6G2, plugged it into the same cab - nope all fine - worked straight away. Tubes or speaker not the problem.

Starting to think my OT might be dead? Will have to read up to see how to get a rock solid answer on that one. If anyone knows how to check - i'm all ears.


Anyways - plan on making the 6G3 a head, so cut out the front facing panel just to get an idea of how it will look.

LHUZHFJ.jpg
 

Che_Guitarra

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Any suggestions on how to test my output transformer for faults? While still in the amp? I've been googling but not finding any clear troubleshoots to follow.

Not a whisper of noise coming from my speaker. Just want to isolate the OT not being the problem.
 

Che_Guitarra

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OK - no variac at my disposal, so i'm out of options.

My last hail mary - can anyone see any issues with this layout? The schematic used is below it.

H3OfcYT.png


DqlS6pP.png
 

BigDaddy23

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What’s the 15K resistor in series with the 100K plate resistor to V1 pin 6 for? I can’t see that on the schematic.
 

BigDaddy23

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You mentioned earlier that you had no continuity in your shielded input wire - did you replace it? Have a look for tiny wire strands off that shield at your input jacks or the V1 socket. Something might be shorting your signal.

Some good photos of each section would be useful to help you out. I see you have a switch near V1 as well so can you elaborate on any other deviations from the schematic above?
 
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Che_Guitarra

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What’s the 15K resistor in series with the 100K plate resistor to V1 pin 6 for? I can’t see that on the schematic.

Hi Big Daddy. I can confirm that definitely needs to be there. It's on the original 6G3 schematic, must have been accidentally omitted in the schematic above (6F10 is a photoshopped design that came from a fellow TDPRI member).

Switch on V1 is a means to shunt a bit of preamp gain to ground, as the 6GX amps are very hot at this stage.

Preamp wire was replaced. Continuity confirmed.


But the nature of the problem means the issue must be in the power or output section - i am not getting an iota of noise through the speaker. Any working amp is going to generate some kind of noise floor through the speaker when turned on. So the problem has to be in this photo somewhere.

hl5U4AI.jpg
 

2L man

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Any suggestions on how to test my output transformer for faults? While still in the amp? I've been googling but not finding any clear troubleshoots to follow.

Not a whisper of noise coming from my speaker. Just want to isolate the OT not being the problem.
Remove rectifier- and power tubes and connect AC filament voltage to OT anode circuits. On Octal sockets you can use solid 6A...10A wire bent to U-shape. Or tack solder pieces of wires to bottom side. Other filament phase on first socket and opposite phase on other. On Octals these come between 2 - 3 first and 7 -3 on other socket if you have wired filament phases to same socket pins. Remember now it is mirror view on tube side ;)

Then switch mains On and you should hear mains frequency from loudspeaker. This should take only few minutes if you find solid wire which diameter somewhat match tube pins but do not use too thick wire. If you find only much thinner wire bend its tips so that it make some contact which is fine when there flow practically no current to very high OT input impedance.

If there is loudspeaker sound the OT, and everything after is fine. You can test all impedance selector settings now because drive is so low. 16 should be loudest and 4 quietest. Then take speaker plug out and measure exact filament AC voltage and all output voltages and you have OT winding ratios.

But before this examine output jack! Just few days ago one builder found that speaker plug did not open the switch there was on jack he had used. Ohmmeter does not help here because OT secondary is less than 1 ohms and so is the switch. Perhaps in your case you can measure from loudspeaker cable and try to find if there is no contact on impedance selector when you slowly turn and hold between notches and then MM show higher than < 1 ohms. If you have 8 ohm speaker reading might be 6 ohms or so.
 
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