The Amp Garage

... My question...Just what are those critical lengths of wire that Dumble uses to tune his amps? Is he adjusting circuit capacitance using shielded cable? A better question... what kind of cable did he use? A shielded cable is also a cap but with very unconventional dielectric material (pvc, teflo...

Ben it seems from your drawing that your CFs are cap coupled to the PI but DC coupled to the power tubes grids, is that right?

Teo

Plate resistor is part of the "load" seen by the tube, which is actually the plate resistor in parallel with the impedance of the next stage. Changing the load of the tube changes many things but mainly gain and available swing (headroom). If you look at plate resistor from a different viewpoint you...

I like the look of the pot up at 2 oclock instead of down at 9 oclock... Just some Feng Shui!! LOL Yes you are right, always keep only stairs that go up :wink: 30% tapers would be sweet!!!! Too bad they are so rare. Fender uses some in their current production amps, but buying from them as replacem...

And i would like to think bad transformer also if it wasn't putting out 330+ on each tap... I agree with you, it's last thing to swap but it could be faulty in a way that it's ok with no load and then sags too much under even the minimal load. Before swapping first cap have you tried to measure how...

Bob given the schematic your amp sound exactly as expected you run all the preamp through the 3.3Meg/10p of the reverb mixer which give the characteristic bright Fender sound so: Fender channel sounds "Fender" -> this is ok Dumble clean sounds almost "Fender" -> this is nok you loose typical D clean...

If you want to balance the PI not having the (power amp) feedback loop in place is even better because the loop tend to reduce any imbalance!

250K lin for treble is better that audio taper, old fender amps used audio taper but it was a 30% taper, not the common 15% type

Teo

Ok but new diodes are 1N4004 or 1N4007 ? Then a leaking first cap can be the problem, change it too with a new one, if still unsuccesfull try to remove all the basic supply components from that chassis (trans, diodes and first cap) and test them separately. If diodes are new and 1N4007, cap is new a...

250K-A midpot because has more range, it still works well on 6-10 settings and at 5 still equals 100K-L on 4 so nothing is missing. OD ratio is 100K-L (with 150k resistor in front), audio taper here doesn't give enough signal to balance clean till 7-8, with 100K-L you usually stay at 4-6 which is go...

Have you checked your diodes? Still using 1N4004?

Sorry Icetech I missed the fact that your PT is a 22814 which is supposed to output 330 VAC rms!

You have to use 1N4007 diodes (mandatory), 1N4004 cannot withstand the back voltage!!

Teo

Icetech is your AC meter a "true rms" meter?

It seems that your 344 VAC is not "rms"!! It sounds more like 285 VAC rms which explains why you end up with only 404 VDC after (solid state) rectifier.

Teo

Although Robben says his two amps are identical, the combo is a late (90) model while the head is an early 80 model that "maybe" has been upgraded to later specs. I think the two amps are still in their original status with only minimal tweaking. Does anyone knows exactly the sn of the two amps? Teo

If you go with pedal leds in series with the relay coil I suggest to choose 5V relays because the leds eat 1.7 volts.

Stabilized supply (zener) is not necessary.

Teo