Recent Posts

71

Tube Amp Building - Tweaks - Repairs / Re: Testing Bad Filter Cap with a scope for 120HZ hum

« Last post by wsscott on December 05, 2025, 04:23:19 pm »

The jack is isolated.  Do you want me just to take the jack’s ground lug out of the circuit by un-wiring it?

72

Tube Amp Building - Tweaks - Repairs / Re: Fender Vibro-Champ w/o vibro build - OT hook up & review please

« Last post by RoadShow on December 05, 2025, 03:41:16 pm »

Yep, that transformer works, but you also might consider the Hammond 1760CP, which has a 4 Ohm tap rather than 3.2 Ohm, since new 3.2 Ohm speakers aren't common (but it's a minor difference anyway).

I built a no-trem Champ (the AA764, as exnihilfit pointed out), and added a NFB switch, which is a really useful mod if you are looking for additional tweaks to the circuit. I also add a mid/raw pot (from Rob Robinette's site) to give more tonal flexibility that can send the sound into more tweed territory.

1760CP, yes, but the 1760C has 8 & 16 ohm options, CP doesn't have 16.

What resistors did you use for NFB switch?  For the AB763 example he uses 820/1.5k whereas the Champ starts with 2.7k.

Thanks

73

Tube Amp Building - Tweaks - Repairs / Re: Testing Bad Filter Cap with a scope for 120HZ hum

« Last post by shooter on December 05, 2025, 03:36:55 pm »

Removing the RCA cable at the Reverb Input jack kills the hum.

before you head for the cap;


remove the Ground-part of the cable but leave signal part connected, 120hz??

74

Tube Amp Building - Tweaks - Repairs / Re: Fender Vibro-Champ w/o vibro build - OT hook up & review please

« Last post by RoadShow on December 05, 2025, 03:23:49 pm »

why not use the Champ AA764 schematic/Layout?
https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schematic.pdf

For the 5K lead on the transformer, I usually just shrink wrap the end and then loop together and use a larger shrink wrap for the rest of the lead to make it a stub, I don't like to cut in case I may use it in the future...

Yep, that's exactly what it is.
Thanks

75

Tube Amp Building - Tweaks - Repairs / Re: Testing Bad Filter Cap with a scope for 120HZ hum

« Last post by wsscott on December 05, 2025, 02:58:26 pm »

I checked it again and I've got a 120hz signal at the Node B filter cap, and that signal also shows up on V4, Pins 1 and 6 plates which are the Vibrato Oscillator/Driver circuit, and V6, Pins 1 & 6 plates which are the Reverb Driver circuit.

Removing the RCA cable at the Reverb Input jack kills the hum.

It seems that it must be a bad Node B, 40uf, filter cap, since the signal seems to be originating from there, at the cap, and goes on out from that point through the wiring directly to V4 and V6.  Nothing is between the cap and the connection on those plates.

So I'll give that a try.

76

Tube Amp Building - Tweaks - Repairs / Re: Please do not use VPN's

« Last post by EL34 on December 05, 2025, 02:10:46 pm »

In the UK, since September, we need a vpn to view any photos hosted by imgur.
I've not got one sorted out yet, but Merlin advised that's how to work around this current imgur issue.
Having Merlin active on this forum seems like a massive asset to me, deleting that account seems like it would be a regrettable decision.
Dunno if the imgur thing also applies to the rest of Europe 

I doubt any of that will happen.
You should be able to access the forum without using a VPN


It's a constant battle with spammers and hackers every day trying to access this forum
It's way more important to keep the forum running without having to deal with all that comes with that non sense.

77

Tube Amp Building - Tweaks - Repairs / Re: Please do not use VPN's

« Last post by dogburn on December 05, 2025, 02:05:43 pm »

If my computer restarts automatically when doing updates, the VPN starts up automatically, so by accident, I have occasionally tried to get to the site with it running. I will try to change the settings on the VPN, but I hope not to get deleted in the meantime.

But I completely understand the reasoning.

78

Tube Amp Building - Tweaks - Repairs / Re: Fender Vibro-Champ w/o vibro build - OT hook up & review please

« Last post by dogburn on December 05, 2025, 02:00:49 pm »

I built a no-trem Champ (the AA764, as exnihilfit pointed out), and added a NFB switch, which is a really useful mod if you are looking for additional tweaks to the circuit. I also add a mid/raw pot (from Rob Robinette's site) to give more tonal flexibility that can send the sound into more tweed territory.

79

Tube Amp Building - Tweaks - Repairs / Re: Please do not use VPN's

« Last post by pdf64 on December 05, 2025, 01:57:47 pm »

In the UK, since September, we need a vpn to view any photos hosted by imgur.
I've not got one sorted out yet, but Merlin advised that's how to work around this current imgur issue.
Having Merlin active on this forum seems like a massive asset to me, deleting that account seems like it would be a regrettable decision.
Dunno if the imgur thing also applies to the rest of Europe 

80

Tube Amp Building - Tweaks - Repairs / Re: Fender Vibro-Champ w/o vibro build - OT hook up & review please

« Last post by dogburn on December 05, 2025, 01:52:59 pm »

Yep, that transformer works, but you also might consider the Hammond 1760CP, which has a 4 Ohm tap rather than 3.2 Ohm, since new 3.2 Ohm speakers aren't common (but it's a minor difference anyway).

And I put heat shrink on the end of the unused wire, roll it up and secure it with a zip tie and tuck it out of the way.