Recent Posts

81

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

« Last post by exnihilfit on December 05, 2025, 12:40:27 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...

82

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

« Last post by RoadShow on December 05, 2025, 12:33:15 pm »

Hi folks,

I wanted to build a Fender Vibro-Champ w/o the vibro and "think" I've drawn the circuit correctly.  Could you please review and give me comments?

In particular, it seems the Hammond 1760C is the right fit per their website, and to match the OT to the 6V6 I would hook up the blue & red, correct?  If so, what's the proper way to tie off the 5k brown wire?  Cut it short, heat shrink the end, and tape it to the chassis?

Thanks for your inputs!!!

83

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

« Last post by scstill on December 05, 2025, 11:52:37 am »

I have chopped sticked everything, there are no microphonic/noise results anywhere

Yes all three Chassis' are grounded to each other with a bolted wire.
In reply 42 you grounded the Neg side of PSU that killed the buzz.
https://el34world.com/Forum/index.php?topic=15795.msg157568#msg157568
Is that Power Supply Unit? Isn't neg PS always grounded?
Am I missing the secret in your build that I haven't tried yet?

Thinking the hum is magnetic, I added a temp ferrous metal housing over V3/V4 - did not affect the hum
I am running a 12au7 in v3/v4 which lowers the hum and 12ay7 in V1/V2 which eliminates the squeal.
I bypass v3 which further lowers hum (and signal) but still present.
This is a very high gain amp with three stages before PI and super clean 7591 power tubes.
Maybe I just have to live with this hum,   :-( 
If there is any Good news, I have learned a lot about isolating noise during this troubleshooting
and I have found Gemini to be a valuable resource

84

Tube Amp Building - Tweaks - Repairs / Re: Transistor parts MTP6N60E

« Last post by DummyLoad on December 05, 2025, 11:35:28 am »

The same parts. 

https://www.digikey.com/en/products/detail/onsemi/NTP6N60/12126151

https://www.mouser.com/ProductDetail/STMicroelectronics/STF6N60M2?qs=DqCdCwOw4%2F5IRCJpc%2Fwudg%3D%3D&mgh=1&srsltid=AfmBOop70B0Ivq5bpKiGHps7LgImik-q6fTi3TjWAQIQAkElctNG3i9HSlY

--Pete

85

Tube Amp Building - Tweaks - Repairs / Re: Transistor parts MTP6N60E

« Last post by passaloutre on December 05, 2025, 10:54:54 am »

I'm no expert on semiconductors, but these all appear to have reasonably similar specs:

https://www.digikey.com/short/dqf8fmq3

86

Other Topics / Re: Platefire Update

« Last post by bmccowan on December 05, 2025, 10:40:32 am »

Albert Collins played with his strap over the right shoulder. Very wide strap.
I play seated and find that offset bodies are more comfortable. But I like teles, so I built an offset with tele bridge and pickups. Was a fun project. But arthritis is making it hard these days. I might need to relive my childhood and go back to air guitar.

87

Tube Amp Building - Tweaks - Repairs / Re: 2 questions- Tube values and pot value

« Last post by pbman1953 on December 05, 2025, 10:32:11 am »

What a beast!!!
You are in a great position to answer the V1b cathode resistor question for yourself because you have experience. What value did you choose the last time you rebuilt a Super Twin? How did that work out?
Or you can run a load line to see what the best theoretical value is. If you do this, make sure to calculate using the funky 47k ohm value for the plate load--not the standard fender 100k ohm.
Or you can try different values by tack soldering in 1500, 820, or 680 resistors and seeing how they sound.
Have at it!

The last 2 have the 680 but I didn't experiment. I installed them and ran with it.

88

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

« Last post by EL34 on December 05, 2025, 09:49:36 am »

Hackers and spammers are a huge problem on forumsThey all use VPN's or TOR exit nodes to hide their true IP address

If you use a VPN on the forum here I will just delete your account
I will assume you are trying to hide your IP address.

It's in the forum agreement to not use a VPN

Please read this for more info

https://el34world.com/Forum/index.php?topic=33040.0

89

Tube Amp Building - Tweaks - Repairs / Re: 2 questions- Tube values and pot value

« Last post by ac427v on December 05, 2025, 07:48:44 am »

What a beast!!!
You are in a great position to answer the V1b cathode resistor question for yourself because you have experience. What value did you choose the last time you rebuilt a Super Twin? How did that work out?
Or you can run a load line to see what the best theoretical value is. If you do this, make sure to calculate using the funky 47k ohm value for the plate load--not the standard fender 100k ohm.
Or you can try different values by tack soldering in 1500, 820, or 680 resistors and seeing how they sound.
Have at it!

90

Tube Amp Building - Tweaks - Repairs / Re: DC-coupled cathodyne and feedback

« Last post by tubeswell on December 05, 2025, 03:56:52 am »

why does this circuit of all circuits have 15k load?

Firstly, a cathodyne with a 15k plate resistor and a 15k cathode resistor = a 30k load


The resistive load determines the gradient of the  loadline. A smaller resistive load = a steeper gradient. This means all other things being equal, that there will be more tube current change per amount of input (grid) voltage change. In a cathodyne, the plate and the cathode are the current ‘sources’ for the signal outputted from the stage. More current at the source means the signal will better withstand the attenuating effect of whatever comes after the stage (all other things being equal).



(Edit: a balanced cathodyne has unity gain and low output impedance, but there is an advantage in maximising tube current by reducing the load, in that better impedance bridging is achievable for driving the following stage)