Reducing Noise in Tweed 5F1 Champ Kit
by Mike Marsh
How can I reduce the noise in my Tweed 5F1 Champ Kit?
There are three mods that I found very helpful to reduce the hum level of a stock 5F1 champ and most other tweed series amps as well.
** Note: These mods will only be effective If you have checked that the shorting contacts of your jacks are working properly and that the ground connections in your amp are solid. Check these two functions first or you are wasting your time with the mods.
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A mod to the filament circuit designed to have it inject less noise into the signal path of the amp. This requires you take 2 pieces of green filament wire twist it tightly into lengths of 4.5 inches (for connections between 6V6 and 12AX7) and 4 inches (for connections between the pilot light and the 6v6 filament socket pins).
Then take the two green wires from the Power Transformer (6.3VAC supply) twist them and connect one wire each to the pilot light tangs.
The pilot light and the filament circuit is no longer grounded on one side, instead you take two 100 Ohm resistors connect one end of each to the pilot light tangs with the filament supply (from Tranny) and Feed (to tube) the other end of each resister get connected together and then to ground.
This is commonly called a virtual center tap. If done correctly it virtually eliminates Filament hum. It it not required in amps that have transformers with a filament supply center tap.
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Using shielded wires replace the stock cloth wires on the input jacks, Volume control and first grid of the Preamp will serve to make the preamp section resistant to interference from the filament supply or any RFI in the area. typically 1.5 foot length is enough to do the two input jack connections to the 68K input resistors, The Volume control input from the first stage coupling cap and the return to the grid of the second half of the Preamp tube.
The sheilded wire should have the shield grounded at one end only. (I typically ground the Volume control leads to the casing of the pot or left most terminal, the input wires to the sleeve terminal on the jack and the 1st stage grid lead to the tube socket mounting screw.
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Since the stock filtering values were selected when the standard line voltage was 105-110VAC, it is now commonly believed that the tweed era amps and some Blackface models are under filtered. We now enjoy AC line supplies of 120VAC in some areas in can actually be closer to 130VAC. In some, but not all cases, the stock filtering is not strong enough and results in an unacceptable level of 60 cycle hum.
There are two ways that this can be tested one is to eliminate the cause by putting the amp on a variac set to 110VAC. If by doing this the amp hum comes down to a normal level then additional filtering will likely help. If it does not, then it is more likely that the noise or hum is casued by filament interference, improper biasing (too much Gain), a noisey component, etc.
The recommended filter changes for Diode Rectified models are to up the main filter to 100uf/500 (this will only improve low end tightness) and screen supply to 40uf/450 (no real change in tone). I do not recommend changing the preamp filter as it will change the tonal characteristics of the amp. Sometimes it's a good change sometimes not, use your ears to determine if it's worth it.
I heard a lot of debate on the internet about the values and brands of caps for this type of mod. I have found that the difference in brands and their ability to filter out hum or ghost notes is quite remarkable. I try to stick to the following brands for filters: Ruby Caps, Sprague Atom and Tech Cap.
I'm not saying there aren't others that work as well or even better, only that these are the ones I have had success with.
For tube recified models The main filter cap values can be up to 40uf. This will again result in tighter bass response over model with lower stock values.