Recent Posts

31

Tube Amp Building - Tweaks - Repairs / Re: Hum and Buzz in Cordovox Build

« Last post by scstill on October 22, 2025, 06:56:55 pm »

I had reassembled the amp into its cabs to see how it sounded complete vs on bench. No improvement, so I put it back on the bench to keep trouble shooting...

Tried a couple of other ideas:

1) High filament Voltage
received some insight from another thread that maybe the heater voltage was too high because the Cordovox Tone Generator with ~60 tubes was no longer part of the circuit.
The heater voltage measures 6.8v, when I added a dropping resistor to get it closer to 6.3v nothing really happened to the hum.

2) Power Tube Fixed Bias with only reservoir cap
The original Cordovox bias circuit only had one filter cap before the dropping resistor and no filter cap at the node.  Another conversion design (noisemakers) added a filter cap at the Vbias node. When this was tried it did nothing for the hum. The attached current schem show the original Bias and the revised bias (although I only jumpered in the filter cap to test did not add the adjusting pot)

The sound bite link below is the hum with the MV turned up and the Tone adjusted in the middle of the bite and the Vol adjusted toward the end of the bite. Seems to be a 60hz hum (LMK if you think otherwise)> All pots have a click when they are at full (not sure why but the new CTS pots do have a resistance skip at max as well).
https://drive.google.com/file/d/1gSXfBiblGQhzKY9qJP8gj2QojfC7alxJ/view?usp=sharing

32

Tube Amp Building - Tweaks - Repairs / Re: Adding gain stage

« Last post by Wonder2121 on October 22, 2025, 05:40:32 pm »

Alright...Its been a while but finally got around to dropping in a switchable channel... Sounds great but haven't had oppertunity to turn it up full tilt yet
The tube driven fx loops officially out...The new inline one works great..

Can anyone see any madness that might happen here...I'm concerned both channels sharing gain pot might be a problem, noticed on plexi circuits they have what looks to be 470k mixing resistors...Not sure if I need them or not...The circuit does produce a slight hiss..Cant seem to nail it down..

33

Tube Amp Building - Tweaks - Repairs / Re: AB763/6V6Plexi amp kit questions/advice

« Last post by pdf64 on October 22, 2025, 03:45:06 pm »

How about the heater voltage, ie what V AC do you measure across the 6.3V heaters?

Thanks; getting 6.8v; should I drop to 6.3v? would take 0.25ohm @ 2w
running (2) 12ax7, 0.6a and (2) 7591, 1.6a

6.8V isn't a bad as I feared, but yeah, bringing it down to 6.3V can't be anything but beneficial.

I have been struggling with a bit of hum...

I suspect that'll be unrelated to the heater voltage being near its upper limit.
Is it hum or buzz, mains frequency or twice that?

34

Other Topics / Re: Meme of the day

« Last post by shooter on October 22, 2025, 01:39:27 pm »

...

35

Tube Amp Building - Tweaks - Repairs / Re: AB763/6V6Plexi amp kit questions/advice

« Last post by scstill on October 22, 2025, 12:00:16 pm »

How about the heater voltage, ie what V AC do you measure across the 6.3V heaters?

Thanks; getting 6.8v; should I drop to 6.3v? would take 0.25ohm @ 2w
running (2) 12ax7, 0.6a and (2) 7591, 1.6a
I have been struggling with a bit of hum...

36

Tube Amp Building - Tweaks - Repairs / Re: Hum in a sortof 18W amp

« Last post by TexasTone on October 22, 2025, 11:04:46 am »

Issue is resolved.

the 60 Hz hum was resolved by changing out the Mojotone Channel Switcher board and DC fan supply for the Hoffman Amps relay power supply board (for both the fan and the relay) and relay board, along with a Cliff jack and LED.

The 180 Hz hum was traced to the LTPI tube.

Thank you to all who responded.

37

Tube Amp Building - Tweaks - Repairs / Re: AB763/6V6Plexi amp kit questions/advice

« Last post by acheld on October 22, 2025, 10:12:17 am »

Speaking sailor is a specialty I've developed over the years, aided and abetted by building guitar amps.  And especially by not following my own advice at times!

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Tube Amp Building - Tweaks - Repairs / Re: AB763/6V6Plexi amp kit questions/advice

« Last post by pdf64 on October 22, 2025, 04:11:50 am »

If you don't use much of the rated current, you will need to drop the voltage with big dropping resistors in power supply.

Doesn't the circuit just use the current it needs?

Kinda, but as voltage is increased, current draw will follow suit, unless circuit impedances are raised.
Using those 60W Concert transformers in a 15/20W amp circuit will result in a super stiff HT voltage, up around 500V, and the amp will be more like 30W, with the potential to overstress the output valves.
https://el34world.com/charts/Schematics/files/Fender/Fender_concert_ii_83.pdf
Remember he's a bedroom player, excess voltage here is a pain to deal with.

I'm curious because in my Cordovox Conversion project I use the original PT but its not driving ~60 Tone Generator tubes anymore. The B+ is fine, but the current draw is way less as a guitar amp than as an accordion amp.

How about the heater voltage, ie what V AC do you measure across the 6.3V heaters?

39

Tube Amp Building - Tweaks - Repairs / Re: AB763/6V6Plexi amp kit questions/advice

« Last post by scstill on October 21, 2025, 06:12:31 pm »

If you don't use much of the rated current, you will need to drop the voltage with big dropping resistors in power supply.

Both the 290ex and 290bx have the same B+. So what are you dropping?
Doesn't the circuit just use the current it needs?

I'm curious because in my Cordovox Conversion project I use the original PT but its not driving ~60 Tone Generator tubes anymore. The B+ is fine, but the current draw is way less as a guitar amp than as an accordion amp.

40

Tube Amp Building - Tweaks - Repairs / Re: AB763/6V6Plexi amp kit questions/advice

« Last post by shooter on October 21, 2025, 06:06:09 pm »

It's all in the specifications -- can it supply the current you need at the required voltages without introducing noise?

guessing the "specs" will say yes but the builder, they will most likely speak sailor words many many times 


IF... you decide to use a switch-mode PS, I suggest you give it it's own shielded box, then run B+ into the typical tube-amp PS filtering section.  AND...have on hand a typical PS transformer n diodes to replace the switch-mode IF...you start speaking sailor.