Fender Champ mods part 1 - snubber cap

The Fender Champ has long been a small tube amp favorite. It's a great-sounding five-watt amp that works well not only as a practice amp, but also for recording. Its simple circuit lends itself well to beginning the study of tube amps in general. There have been many versions of this amp over the years and comparing the schematics and studying what changes were made, why, and the effects they had will reveal many principles of tube amp design and function. In this video I discuss Tweed vs. Silverface, Champ vs. Vibro Champ, and mainly the snubber capacitor present in many Silverface amplifiers.
Please keep in mind that tube amplifiers harbor lethal voltages inside and that if you do not have experience working with them or are not familiar with the proper safety procedures involved, you should not open your amplifier. The risk of deadly electrical shock is present even when the amplifier is unplugged from the wall outlet and switched off.
French subtitles available.

[@timwarneka5681]

Thanks very much for posting. Interesting test.

[@charlesdespres]

Thank you very much, Tim, I'm glad you enjoyed it! Part 2 is coming up.

[@kapioskapiopoylos7338]

can't give you slack for playing it safe, but 10 or 6v DC is fine. the problem would really be a fast flick of the switch on-off, with the rectifier charging the capacitors at above the max voltage and tubes haven't been heated yet and don't draw any current. then you really need to be carefull to discharge the capacitors. in your case since you turned the amp off with hot tubes, the residual heat made the tubes conduct the remaining charge. However it is better to discharge even empty caps, than to get shocked by 500V.

[@charlesdespres]

Thank you very much for your input. I decided to follow safety procedures to the letter because I do not have the expert knowledge you seem to have. I'm just a tinkerer and I don't want to take any crazy chances. Plus I don't want anyone watching my videos to do anything dangerous. Thanks to your explanation, I now understand why the residual voltage was so low. I assume things could be different in a different amp with a different circuit. Also, thanks for pointing out the potential danger of a quick on-off flick of the switch! I see exactly how it could lead to a potentially lethal situation.

[@johnbravo7542]

OMG who upholstered the grill cloth

[@charlesdespres]

Hahaha! Not me! It was that way when I got it. The speaker was ripped, too. But it was priced accordingly. And that's why I don't mind experimenting with it.

[@fredscott7534]

That was a great video. However I had two questions . How do you compare the Vibro champ to the champ tone-wise. And secondly is that book full of schematics and layouts for Fender amps. ? If so where do you get it?

[@charlesdespres]

I'd say the Vibro-Champ sounds the same as the Champ, it just has additional tremolo. Regarding the book, if you are talking about the second book I mention (around 18:12), it is "Dave Funk's Tube Amp Workbook, Complete Guide To Vintage Tube Amplifiers Volume 1 - Fender, First Edition". It has 252 pages of vintage Fender amp schematics and layouts. The first 160 pages are devoted to Fender amp history and how tube amps work. It is an interesting read and the nice thing about the schematics is that they are large. As you probably know, vintage Fender amp schematics can also be found online. The book can be found at online bookstores.

[@kookoogearkook]

Where’s part 2. Also where is the snubber cap? Isn’t that only for standby switch?

[@charlesdespres]

Part 2 is coming up. It is about adding a screen resistor to the power tube so the screen gets the proper voltage. I have already shot it, but I still need to edit it. I need to write this down on my "to do" list! The snubber cap goes from pin 5 (grid) to pin 8 (cathode) of the 6V6 power tube. You can see it around 5:13 in the video. I understand that some people use a capacitor across the standby switch to reduce popping noise when operating the switch, but I have not investigated that and the Champ does not have a standby switch.

[@charlesdespres]

Dear KooKoo, I have now uploaded part 2. You can view it here: kzbin.info/www/bejne/rZ20nmx9o66potE

[@jjsant3250]

Mine oscillated when I removed it via a buzz on top of lower notes. I then changed the grid wire to the power tube to a shielded type and the oscillation went away.

[@charlesdespres]

Oscillation is definitely a possibility on those amps. They put in the cap automatically so they wouldn't have to worry about it. Thanks for the fix!

[@Jonathan_Doe_]

Screened wire can act like a capacitor, so depending on where you grounded the shield, that could be the reason this worked out. Basically the capacitance of the shielded wire to ground acts as a filter.

[@wyattdoodat]

Sounds like you''re blowing the crap out of your mic preamp when the amp's turned up.

[@charlesdespres]

Actually, my setup is not that elaborate. I'm just overloading my phone mic. 😂

[@maximegrunheid4276]

Video trés instructive, super! Je possède un Champ silverface 70 également, et j'aimerais votre avis a propos du potentiomètre de réglage de Basse fréquence, il a très peu (voir aucune) incidence sur le son. Contrairement au réglage d'aigu qui lui est très perceptible. Cela est il causé par le format du HP 8" ? ou cela provient il d'un défaut de composant sur mon ampli? J'ai changé le HP Oxford par un Warehouse, j'ai changé la 12AX7 .. rien n'y fait!

[@charlesdespres]

Merci pour ce commentaire, Maxime ! La modification "Tweed" que j'ai effectuée sur mon ampli supprime les réglages de tonalité (les Champs des années 50 n'avaient qu'un bouton de volume), ce qui augmente le gain de l'ampli ainsi que les fréquences médium. J'ai donc essayé un Champ Silverface non modifié et en effet, le potentiomètre de réglage des basses a assez peu d'effet. Pourtant, lorsqu'on observe le schéma de l'ampli, on s'aperçoit que les valeurs des condensateurs d'aigus et de graves reliés aux potentiomètres de tonalité sont les valeurs standard de chez Fender (250 pF et .1 uF). Le condensateur et la résistance de preset des fréquences médium sont différents de ceux que l'on trouve dans un Super Reverb ou encore un Deluxe Reverb, ce qui peut expliquer certaines différences. Mais je crois surtout que le Champ a une trop faible puissance et un haut-parleur trop petit pour générer des basses profondes et abondantes.

[@ot4kon]

you waited to much to measure the capacitors. Probably those were discharged by the tubes or someone put a discharge leak resistor in there.

[@charlesdespres]

Yes, the caps were probably discharged by the tubes. There is no extra cap in the amp.

[@ot4kon]

@@charlesdespres ohh I meant a resistor, Sometime people put a 1000k to 470k 1W resistor between B+ and Gnd to discharge slowly the capacitors.

[@charlesdespres]

@@ot4kon Oh, OK, now I get it! But I don't think one such resistor has been added. It might be a good idea, though!