Hot Rod Deluxe re-valve/biasing - help please

crunchman

Fender's spec in the manual is low.  If you calculate it as the conventional 70% of maximum plate dissipation it comes in higher than that.  I can't remember the exact value but I think you are ok up to the mid 80s mV.  I don't remember the exact number - it's a few years since I sold mine.  If you google it you can find out how to measure and calculate it, but to do do it properly you would need to measure the plate voltage on the power valves - around 430V so be very careful.

It does depend on the voltage you are getting from the wall.  I measured 246V AC on the primary of the mains transformer in mine at my old flat.  That made the plate voltage on the power valves about 26V above what the spec on the circuit diagram said!

ICBM

You don't *have* to bias up to 70%. Contrary to popular belief, amps don't always sound best with the valves as hot as safely possible (or hotter in many cases). 70% is a guideline limit, not a target. As long as you know where the limit is, just use your ears.

For example I find there's a 'sweet spot' with vintage Marshalls with the EL34s running at 15W, which is only 60% of the rated dissipation. It's quite consistent from amp to amp too, so I set that as a starting point and then adjust if necessary, if there's audible crossover distortion.

BenSirAmos

I could tell a long, long story about this - about how many times the valves were swapped around with no reduction in hum - but I took it to Fender who said it was a hummy V2, charged me a total of £7.80 for a new valve and said it was all to spec, I didn't believe them but I got it home and no hum. £7.80 for a complete cure is pretty good value. 

Emp_Fab

Download Justin Holton's old "Unofficial HRD" site as PDF documents from here:- http://www.stratopastor.org.uk/strato/amps/hrdx/hrdx.html

Awesome resource.

Emp_Fab

...A better organised version is available here :- http://www.maxxplay.us/z-jp-hrdx.html

markslade07

Nice resource, cheers @Emp_Fab

BenSirAmos

I'm a bit too nervous to do any mods to it just now

jpfamps

crunchman said:

Fender's spec in the manual is low.  If you calculate it as the conventional 70% of maximum plate dissipation it comes in higher than that.  I can't remember the exact value but I think you are ok up to the mid 80s mV.  I don't remember the exact number - it's a few years since I sold mine.  If you google it you can find out how to measure and calculate it, but to do do it properly you would need to measure the plate voltage on the power valves - around 430V so be very careful.

It does depend on the voltage you are getting from the wall.  I measured 246V AC on the primary of the mains transformer in mine at my old flat.  That made the plate voltage on the power valves about 26V above what the spec on the circuit diagram said!

The higher HT is almost certainly due to the amp being wired for 230 V operation.

Also if you clip the negative lead of your multi-meter to the chassis (as you should to be safe), then you will get a voltage reading at the bias test point about 10 mV too high (which is on the safe side).