5E5 build questions

[stephenhartley]

Hi - having built 2 5E3 based amps, I'm now building a 5E5 with switchable cathode/fixed bias (effectively making it a 5E5/5E5-A type hybrid).

On the 5E5-A the resistor between the transformer tap and the diode is 10k, but on the deluxe reverb, it's. 470r. Why such a big difference? I'm using a Hammond 291CEX PT (which has a 50v tap). What's the best value of resistor to use? I'm using a 22k resistor with a 50k trimming pot from the other end of the diode to earth.

Also, I'd like to able to use the normal channel for bass guitar in fixed bias mode - are there any changes I can make to make achieve this?


Stephen Hartley

[Chuck H]

On the 5E5-A the resistor between the transformer tap and the diode is 10k, but on the deluxe reverb, it's. 470r. Why such a big difference? I'm using a Hammond 291CEX PT (which has a 50v tap). What's the best value of resistor to use?

The resistor is part of a "voltage divider" that determines the final bias voltage and is dependent on the supply voltage from the transformer winding. There is no standard voltage for that winding. Further there is also no standard design for bias supplies. Some may use higher resistance values than others. A voltage divider provides a ratio. A 100k series resistor with a 100k load resistor provides the same voltage drop as a 220k series resistor and a 220k load. This doesn't take current into account. Divider values are typically chosen to accommodate current requirements analogous with voltage requirements. Ideally a bias supply will only draw significant current from it's circuit resistance and not from the tube grids. Because the impedance of the winding is very low resistance values in the 10's of k's are sufficient to avoid over taxing the current available from most bias supplies. Your values of 22k/50k seem typical and reasonable provided they supply the voltage you will need. On that note, many modern 6L6 type tubes seem to require more bias voltage than NOS types. I might use a 10k series resistor and a 50k pot as a variable load on top of a 22k fixed load resistor. This design should allow for ample adjustment and the added fixed resistor makes it impossible to dial the bias to zero, reducing risk to the tubes.

Also, I'd like to able to use the normal channel for bass guitar in fixed bias mode - are there any changes I can make to make achieve this?

To play bass guitar through a six string guitar amp the biggest considerations are in the power amp design and the speakers. Altering the circuits of a tweed style amp here would significantly change it's relative goodness as a regular guitar amp. I don't know of any preamp mods or adjustments that would make significant improvement for bass guitar use. It's JMHO but I don't think a tweed style amp is a good choice for bass guitar performance. Perhaps for low level recording only but stage/gig use would likely prove disappointing.

[stephenhartley]

Thanks very much for that - that's very informative. I've been through most of the circuits in Weber's book and there's quite a variation in the bias supplies.

[Enzo]

The thing to do is look at the larger picture. Don;t focus in on one resistor. it is the circuit that matters, not the parts. The parts do not operate in isolation all alone, they work with other parts, and as Chuck described, often the relation between those parts matters a lot more than the parts themselves. If I make a voltage divider with a 22k and a 47k resistor, the results would be the same if I doubled each of those.

[stephenhartley]

Thanks - I have to be honest and say that although I'm learning fast, my electronics knowledge is still rudimentary. I read threads on here and scour the circuits in Weber's book and learn bit by bit - it's taken me hours working out how to understand fixed/cathode bias switching.

I understand that fixed bias provides a negative voltage supply to the grid and that a trimming pot allows the grid voltage to be adjusted, but that's about it. I don't fully understand what the diode does and I don't understand why some bias circuits have a resistor between the tap and diode and others don't. I presume that the cap serves the same function as in the cathode bias circuit - i.e. making the tone crisper.

[Justin Thomas]

The bias supply is kind of like a miniature version of your power supply... the bias tap provides an AC voltage. The diode is like your rectifier tube - only passes current one way. But since you want a negative voltage, you put it in backwards. So it only passes the BOTTOM half of the AC wave; you now have pulses of DC @ 60x/second. The cap is a filter, just like your filter caps in the power supply - its charging and discharging smoothes out that pulsating DC and makes it more level. If that cap wasn't there, you'd be getting some pretty awful hum! The resistor before the diode, if present, sets the voltage level to whatever the tubes need, but like Enzo said, there's no single part that doesn't work as part of a system.

[stephenhartley]

Thanks - it now all makes sense. I'm using Russian military 6L6s and hopefully the 50k trimmer will give enough scope to set the correct bias.