Hot Rod Deluxe mods

[Curranproducer]

OK I got a scope on this bad bear, and the wave on the tube plates are a distorted mess, the PI is clean and then we get into the output tubes and it is super low output and bad wave forms all over the place. I got a buddy helping me with it a little bit and he is starting to think the OT is flaking out
one of the test point on the PI is reading 26v instead of the 37 the schematic says. Any help will of course be wonderful!

[Curranproducer]

found an open resistor in the PI the 82k resistor to be exact,so I am getting new one tomorrow I hope that is it!!!

[billyz]

found an open resistor in the PI the 82k resistor to be exact,so I am getting new one tomorrow I hope that is it!!!

Replace both of them, 100k and the 82K . They had a run of crap and they often open up. you may have one of the 91K models too.

[Curranproducer]

it was made in 96 so what is the 91k thing?

[billyz]

They used a split load on one leg of the PI plate. 91k/9k = 100k.

[Curranproducer]

and what was the point of that? to help clean up the distortion or something?

[martin manning]

The '96 schematic has the usual 82k/100k loads on the PI. The difference in the PI plate load values is there to lower the gain on the input triode so that the outputs are balanced.

A split-load on the input triode would reduce the gain on that side while keeping its overall load the same. I have not seen this, and it doesn't show up on the HRDel/Dev III schematics (2010) either. Is this a new thing?

[Stevem]

Any plate load resistor like those PI ones with more than 325 volts hitting them need to stepped up to 1 watt from the stock 1/2 watt which is a issue with far too many mass pumped out the factory amps these days!!!

[Curranproducer]

yeah, the volts coming off of those tubes are about 431VDC, I will bump it up to 1 watt on the PI resistors

[Curranproducer]

HAHA!!!!!!!!!!!! rage success!!!! the amp is running wonderfully! so I modded the four power caps to 47uf from 22uf and the 47uf to 82uf!!! I also added a second 82uf to the cap section on the PCB with a jumper over the R70. I do not know if it has improved the tone at all BUT it is working awesome!!! I biases the 6v6's to 50mv. NOW I want to tighten up the low-end even more and take out the harshness in the distortion circuit

[Firestorm]

Any plate load resistor like those PI ones with more than 325 volts hitting them need to stepped up to 1 watt from the stock 1/2 watt which is a issue with far too many mass pumped out the factory amps these days!!!

A lot of amps have been running with 1/2 watt plate resistors for 50 years. What's the issue? (Just being pedantic.)

[Curranproducer]

431V is a lot of voltage for a 1/2 watt resistor to handle. I could see according to OHM's law this being pretty close to the resistors full temp limit. So a 1 watt resistor gives your amp more room to breath and run cooler in that resistor.

[Stevem]

It's also a white noise issue when you run them that hard.

[martin manning]

Nothing wrong with Ohm's law, but you have to use it correctly. Rated voltage for resistors refers to the voltage dropped across them, and for 1/2W CF or MF a rating of 350V, with a max overload of 700V is common.

At idle the plate current for a 12AX7 might be 1.5mA, which means a 100k plate resistor is seeing a voltage drop of 150V, and it is dissipating 0.225W. Since this is a Class A stage, that will also approximate its average power dissipation under signal conditions. A half-watt rating is >2x what is necessary, which is a good rule to follow for reliability.

The maximum current flowing through the tube will always be something less than 2x the idle current, but if it were to reach that level the voltage drop across the plate resistor would only be 300V.

Some people like to use higher wattage resistors for lower noise, and that might be a good idea for CC or CF, but with MF it isn't necessary.

[cbass]

I like to use bigger resistors cause I think they look purty