5F6A Preamp w/ Little Wing Output?

[The New Steve H]

Looking at a Fender "The Twin" schematic, I see that they have two coupling caps going to four tubes.

[The New Steve H]

Wondering if anyone can explain the huge resistor between the output tubes on the Little Wing. By "huge," I mean physically huge. The resistance is small (420 ohms), but the resistor is a 10W brick.

The capacitance is also small: 25uF.

I assume we're trying to cut back on the bass here. Is the resistor so huge simply because the low resistance causes it to pass a lot of current? Also, how come we don't rely on the preamp bypass cap instead of putting this stuff on the power cathodes?

I managed to jam this stuff onto the circuit board. Pretty freakish.

[The New Steve H]

The chassis is taking shape.

I wonder what other people use aluminum channel for.

[John_P_WI]

Wondering if anyone can explain the huge resistor between the output tubes on the Little Wing. By "huge," I mean physically huge. The resistance is small (420 ohms), but the resistor is a 10W brick.

The capacitance is also small: 25uF.

The power amp section is "cathode" biased. That is what the "huge" resistor and cap are for.

BTW, I have not read the whole thread, but IF you are using more or different power tubes, you will most likely need to change this resistor value to bias the output section properly.

Also, the bias resistor would be much happier against the chassis where the chassis could absorb the heat.

[The New Steve H]

Someone on another forum explained it. Looks like the actual wattage is about 1.2, so the resistors are oversized.

Why would I need a different resistor for more tubes? I assumed the resistance determined the voltage at the cathode, so the additional current coming from more tubes would not make any difference, and since I'm only going up to 2.4 watts, I thought a 10W resistor would still be way beyond adequate.

[10thTx]

IF you are using 6BM8 tubes, a pair of them is around 7 watts. And it is a LOUD 7 watts. Look at DaGeezer's schematic at the top of the page.

I used an amp with a pair of 6BM8 with a band with keyboards, loud drummer, bass and an acoustic guitar with 3 singers. The amp was mic'd into the PA, but I could hear it fine and used the amp sort of as a monitor for myself.

And the amp had VVR in it also. It was my 56T amp.

with respect, 10thtx

[surfsup]

10thTX, was your amp fixed or cathode biased?

[John_P_WI]

Someone on another forum explained it. Looks like the actual wattage is about 1.2, so the resistors are oversized.

Why would I need a different resistor for more tubes? I assumed the resistance determined the voltage at the cathode, so the additional current coming from more tubes would not make any difference, and since I'm only going up to 2.4 watts, I thought a 10W resistor would still be way beyond adequate.

IF you double up the output section to 4 tubes and the power transformer is up to it and IF you only use 1 cathode resistor for the 4 output tubes the current will be doubled through the single resistor.

V=IR, so IF the current is doubled through the cathode resistor, the cathode voltage in theory will be doubled upsetting the bias point....

It certainly does not hurt to oversize the cathode resistor power rating to minimize heat effects. Sandbox power resistors are cheap.

[The New Steve H]

Today I realized I had to come up with a place to put the caps. I cut a piece of phenolic, added turrets, tapped 4 holes in the chassis for #6 screws, and turned 4 standoffs from 6061 aluminum! I love it.

I couldn't go through the chassis and put nuts on the other side, because the transformers were in the way, so I turned the chassis into a set of nuts. The precise height of the standoffs prevents the screws from exiting the box on the top.

[Structo]

Did you mill that chassis out of solid billet?

Or was it already in the U shape?

[The New Steve H]

First, I mined the bauxite ore in my back yard...

Actually, it's aluminum channel. If the photo was better, you would see that I thinned out 1/8"-thick panels in the front and back for the pots and jacks. I was going to use a Dirty Dawg box, but this thing was already sitting in my garage.

I'm wondering if next time I should mill out recesses so the heater wires could be covered with a piece of plate. It might not do anything, but it would be cool.

[The New Steve H]

Here is my latest conundrum.

I'm hooking a 5F6A preamp to a Little Wing output section. The voltages coming from the 5F6A PT and the Little Wing PT are very different. Example: the Little Wing PT is going to give me something like 263 V where the big PT provides 325 V, so when it gets past the resistors and so on, it will be below the values on the Fender schematic. The DESIRED values at the tubes are below the 275 V max coming from the PT, so I assume the PT is capable of providing the right voltages if I mess with the rest of the circuit.

Should I screw with the resistor values downstream from the power caps in order to get the same voltages Fender specifies?

[The New Steve H]

I got no answer on the last thing, but I think messing with the resistors has to be the way.

New question: should I get rid of the presence control? I'm putting in a PPIMV, and I've read that the purpose of the presence control is to restore high frequencies you lose at low volume. Supposedly, the PPIMV will bring them back out.

It's too late to take the presence knob off this amp, but I thought I'd ask in case I build more of them.

I didn't realize a PPIMV uses a dual pot, so I have two knobs to turn! Knobs make an amp look more impressive, so I guess that's okay.

[Structo]

Usually the presence control is a by product of the negative feedback loop.

So if you eliminate that, you may end up with an unstable amplifier.

[10thTx]

Yes, you will experiment with lowering the dropping resistors in the B+ between the power tubes and the preamp to try to the voltages up in the preamp.

The PPIMV renders the negative feedback loop somewhat ineffective.


With respect, 10thtx