The Amp Garage

Fixed bias in SE creates the potential to drive the tube into cutoff if the signal is strong enough. "Class B" SE does not sound good. Self bias/cathode bias ensures that this can't happen (with a reasonable value for Rk) since as more current flows in the tube it creates a larger voltage drop in th...

Check your grounds. A dodgy input jacks ground can do exactly this sometimes. When you flip switches, hit loud power chords, bang on the amp, etc., the signal spike can punch through whatever unintended resistance is present. This how those Christmas tree light string fixers work.

What rectifiers are in each? Originals were RCA-branded Blackburn Mullards.

There's something intuitively wrong with putting the power switch on the volume pot: hummy wall current right next to a grid wire -- and even with the lead shielded, the resistive element itself must still be basking in that 60 cycle glow (as would the bright cap, too). If you look at vintage Champs...

The thing I see that might be an issue is lead from the 100K PI plate is in phase with the input and the tone circuit. I can't make out how close together they are from the overhead shot but it might be worth experimenting there. Maybe that lead shouldn't route back to the front where its signal cou...

WRC34 wrote: ↑Wed Apr 19, 2017 11:44 pm Firestorm, pardon my ignorance, but how would I attenuate the post/pre PI signal?

Attenuate just like you would with pre-PI and post-PI masters, just with fixed resistors instead of pots. To look for parasitic problems a gut shoot would help.

It might help to isolate the problem to a specific stage: attenuate the post-PI signal to see if it's exclusive to the power stage; attenuate the pre-PI signal to see if it's just in the preamp. That said, it sounds very like a somewhat less noticeable effect that occurs in the reverb channel of man...

What are the plate and screen voltages on the EL34s?

If this is a non-reverb Deluxe, you're missing the (small) gain of the reverb mix stage, which makes a difference especially considering the loading of the trem circuit. It's not as bad as the non-reverb Princeton, but it won't get up and dance around.

There's not all that much gain in Blackface circuits. The tone stack throws away a lot of first stage gain, the tone recovery stage puts it back and the reverb mix stage has its input set for is set for very little net amplification. One of the best places to control output volume in Fender Reverb a...

davebolden44 wrote: ↑Sun Mar 19, 2017 9:15 pm scratch that I was readin the wrong end of the damn thing, I have the same 330 v at the end of the two main filters

A Twin has the first two caps wired in series. You should not see the same VDC at both anodes. Something is wrong with the main filter configuration.

Ceramics are also compact in 1KV or 2KV ratings. This may help minimize leakage which can hurt the oscillator's performance.

I've used the CTS concentric pots to great effect. You can even make the knobs almost match cosmetically by cutting two stock Fender knobs, one above the set screw, one below it. When the two parts are installed on their respective shafts, it looks like one too-tall Fender knob. The upper piece for ...

A little more info about the amp might help (if the speaker is original). There was a President amp made in Canada. Any indication the amp might be Canadian?

A simple way to tweak the headroom of a DR is to reduce the value of the 220K grid return/bias feed resistors on the output tubes. Subtle but noticeable. As I understand it, putting cathode followers between the PI and the output tubes can prevent overtaxing the PI when heavy drive signals are requi...