Taming Fender Red Knob Dual Showman

[ruepselrum]

The amp has worked as stock for decades, no reason to change it now.

That's what I'd told, too, if anybody had asked me. But as the one who's questioning, I try to gain deeper insights. I hope I'm not too annoying.

Sascha

[g1]

If you increase the value of R128 (to compensate for increasing value of R129), it will affect the way the voltages sag under dynamic conditions. (Larger R128 values will cause more sag at preamp supply nodes)
I would like to think Fender tried it that way and did not like the sound. R129 is a fairly unconventional approach to setting preamp voltages and I would think they had a good reason to do it that way.
It's easy enough to experiment with the value of those 2 resistors and hear for yourself.

[ruepselrum]

I just added D202/203 and btw replaced R115/R116 with 1% types.

It`s really hard to get this high wattage resistors.

Thanks for your help and patience!

[ruepselrum]

For the sake of completeness I'm adding a picture of how I solved the high wattage resistor issue. I couldn't get 30k nor 2k7 resistors with 10 Watt or higher. As mentioned before I wanted to raise wattage to cool things down a bit and get more reliability. I finally chose two 1k5/10W and two 15k/10W in series and also incorporated Tom's idea of moving resistors away from the PCB, at least partially.
Unfortunately they are still getting VERY hot after a few minutes of operation and measured voltages are about 20V higher than stated values in the schematic at 220V mains setting. When I run the amp in 240V mains setting voltages are decreased as expected to about 10V below stated voltages.
Which setting do you recommend? In Germany we have 230V mains supply and in my case 225V coming out of the wall.

[g1]

20V high where? On 500V that is not much difference, but on 30V it's a lot!
Look at your heater voltage which should be 6.3VAC, which mains supply setting gets you closest to 6.3 ?

[ruepselrum]

I'm referring to preamp voltages succeeding the voltage divider built of R128/129 measured across electrolytic caps, C110-112, node A to C, highest of them being 397V.
Heater voltages are around 6,7VAC at 220V and about 6,2VAC at 240V mains setting.

[jalexquijano]

I'm referring to preamp voltages succeeding the voltage divider built of R128/129 measured across electrolytic caps, C110-112, node A to C, highest of them being 397V.
Heater voltages are around 6,7VAC at 220V and about 6,2VAC at 240V mains setting.

I need to purchase R129 (30k, 10W). Where can i get one??

[Jazz P Bass]

Not too sure where to get the cement resistors.
These are all wire wound: http://www.mouser.com/Passive-Compon...yzekizZ1z0wt4w

[g1]

Heater voltages are around 6,7VAC at 220V and about 6,2VAC at 240V mains setting.

Then I would use the 240V mains setting.
The 10V low in the preamp is only 2.5% of 400V, so well within tolerance. The tubes will be better off at the lower heater voltage.

[g1]

Cement resistor available here:
https://www.guitar-parts.com/catalog...30k-0026486000

[Tom Phillips]

...I wanted to raise wattage to cool things down a bit and get more reliability. I finally chose two 1k5/10W and two 15k/10W in series and also incorporated Tom's idea of moving resistors away from the PCB, at least partially. Unfortunately they are still getting VERY hot after a few minutes of operation...

This is due to the laws of physics. When you use larger wattage resistors, whether it be a single higher wattage resistor or two resistors that comprise the same resistance of higher wattage, the total energy dissipated is the same as it was with the original resistor. Therefore, the total heat emitted into the surrounding area is also the same. The difference is that the surface temperature of the higher wattage resistor can be lower if the replacement resistor is physically larger. That could reduce the possibility of heat damage to surrounding parts and PCB.

As you have observed, power resistors get really hot when they are doing their job. That's the way it works. The laws of physics again.