18Watt.com

I would like to slightly increase headroom on my 1974x and would like to know whether my thinking is correct that if I increase the current bias resistor R23 from 91Ω to say, 10% more, I get lower gain from the output tubes and less output tube distortion at any given volume setting?

Don't expect too much from it. My 18 watts run at higher B+ and are supposed to have slightly higher headroom. But the difference is negligible. It only changes the shape of the overdrive...
I would try to get the expected results with the volume knob on the guitar, that reduces input signal.. so there's less push in the front. Maybe try a 5751 tube in V1 position? and / or a 12AT7 in the phase inverter socket? And in case you don't already know.. adding 2 or 3 watts output is barely audible. A 30 watt amp is 'slightly' cleaner than 18 watts..
people are going to consider this as herecy, but adding a NFB loop in the output will give you some more clean headroom.. a SS rectifier won't help much either, for the EZ81 produces no sag anyway.

an 18 watt amp is not known for it's clean headroom, and the early overdrive is where it shines. that's why people want them, and some are even complaining it is pretty (too) loud before OD kicks in.

If you want a compareable sound but more headroom ... it's time to build a 36W !! That'll do the job very well!!!

Off the top of my head. I increased the EL84 cathode resistor to 240r 6 watt. I also mucked around with the cathode resistors to the pre amp valves (1k5 to v1 cathode IIRC). Though I can't remember what I did. My 18 watt starts to compress at halfway up the volume control and doesn't distort anywhere near to the degree it did stock.
Much more usable for it. I don't notice much difference in volume up the top end of the volume dial. But I do notice the extra clarity and control at the bottom end of the volume control

Thank you. Did doubling of the cathode resistor value affect the heat/lifespan of the output tubes?

I don't know about lifespan. I've 5 amps I gig and the 18 watt isn't one of them. I use it at home infrequently. It does better for home use

Increasing the value of the bias resistor will increase plate voltage and lower plate dissipation,so in effect yes you will cool the tubes and get a longer lifespan.the 1974X reissue has a lower B+ than your typical 18W which leans towards early breakup rather than clean headroom,but just try doubling from 92R to 180R and see if you get a bit more headroom.if it affects your breakup or you don’t like it swap back to stock.
There’s been a few posts recently with the 1974X’s so check them out.

geoff 1965 wrote: ↑

Wed 12/01/21 1:08 pm

Increasing the value of the bias resistor will increase plate voltage and lower plate dissipation,so in effect yes you will cool the tubes and get a longer lifespan.the 1974X reissue has a lower B+ than your typical 18W which leans towards early breakup rather than clean headroom,but just try doubling from 92R to 180R and see if you get a bit more headroom.if it affects your breakup or you don’t like it swap back to stock.
There’s been a few posts recently with the 1974X’s so check them out.

That statement isn't exactly accurate. Aren't the reissues also using lower secondary voltages? The only thing that matters is what the calculated dissipation is at the power tubes. The value of the cathode resistor is arbitrary.

Thanks,
Josh

Lower 275-0-275 HT is what I meant by having a lower B+ than you typical 290-0-290 HT.

geoff 1965 wrote: ↑

Wed 12/01/21 1:35 pm

Lower 275-0-275 HT is what I meant by having a lower B+ than you typical 290-0-290 HT.

Ah, okay. But they shouldn't arbitrarily double the bias resistor. They should measure and target. We're using 180R with 290-0-290 that's realistically 300-0-300 on modern wall voltage. Biasing too cold would also be bad. Personally I target 85% for power dissipation.
Thanks,
Josh

Agreed Josh,I don’t think it would give him the headroom he was asking about but trying would’nt harm anything,I think one of the 1974X’s was biased at 200R but no mention of breakup/headroom difference to stock value.

Watkins Dominator 20, the supposed predecessor of the 1974, uses 220Ω @ similar B+.

junior marbles wrote: ↑

Wed 12/01/21 5:55 pm

Watkins Dominator 20, the supposed predecessor of the 1974, uses 220Ω @ similar B+.

Sure, that could be, but it means nothing by itself. I've had amps with 470R cathode resistors, but it isn't relevant. You need to consider the PT specs, and the measured voltages / dissipation at the power tubes.

The other aspect of this is that increasing the bias resistor will create more negative bias on the power valve grids. Then the grid sensitivity is reduced, which effectively lowers the gain of the power stage. You don't get more headroom as in a higher level of max output power, but you do get more headroom in that the threshold of power stage clipping will correspond to a higher setting on the volume knob.

So the short answer to your question is actually "yes".

Change in cathode voltage at bias with different cathode resistors can change by a significant percentage, where unloading the PT a bit only changes the B+ plate voltage by a much smaller percentage.

if you don’t mind give us some reference voltages with the stock 91R bias resistor i.e. B+ at first capacitor,plate,screen & cathode voltage of the el 84’s.

Will do. How should I dummy load this, considering that the amp chassis will be off the enclosure, i.e. no speaker?

Make yourself a dummy load with a large 100W wirewound resistor the same impedance as your speaker i.e.8ohm,16ohm.

I already have one of these. Perfect.

geoff 1965 wrote: ↑

Wed 12/01/21 7:25 pm

if you don’t mind give us some reference voltages with the stock 91R bias resistor i.e. B+ at first capacitor,plate,screen & cathode voltage of the el 84’s.

With line voltage at 117.5VAC I measure:
* 329VDC @ CT
* 314VDC @ plates
* 302VDC @ pins 9
(Forgot to measure @ cathode)

Colin’s 1966 amp in the downloads shows 325V at first cap so that is very close,thanks for posting those voltages.

geoff 1965 wrote: ↑

Sat 12/04/21 10:14 am

Colin’s 1966 amp in the downloads shows 325V at first cap so that is very close,thanks for posting those voltages.

But we should also consider the Mains AC for the measurement, if trying to make some connection between historic reference amps and today's copies. Junior's mains is running a cool 117.5VAC. Most places in the U.S. run 122-125VAC and on a PT wound for 117 or 120VAC input, the secondary is going to run hot.