Carlsbro 50/Express hybrid/monster!

[ampgeek]

Hey Mark,
With all due respect to those more experienced than I, don't get too wrapped around the axel with accuracy on those 1 ohm resistors. If you are using a typical DVM to measure them, the resistance in your test leads will likely introduce more than 1% error into you measurements.

Just assume 1 ohm and...rock on!

And...if it's any consulation, I link up with a couple of old salt EE's from time to time (retired AF officer and defense contractor) who go back to the vacuum tube heyday and they always chuckle when I start discussing component and power tube biasing tolerences and such.

Their standard fallback comment to me is...."Dave..ya gotta understand that in the vacuum tube realm...+/- 10% don't mean shit!". Meaning that these systems are quite robust and have pretty wide operating range tolerences.

Personally, I start with a powertube bias that approximates 70% max dissipation with 1% cathode resistors and tweak it to taste from there. As long as there is no red plating...it's good to go.

I maintain an acquaintence's JCM 800 and he likes the tone at 90% of max dissipation. He plays out 2-3 times a week and is using the sampe power tubes (JJ's) that I installed 2 years ago.

Now...a quick favor to ask: Any chance that you could measure the voltage drop across R20 and R21 before you make any changes to the B+ string? I would like to know the screen current at your present conditions so I can do a reality check on how I am interpreting the tube performance curves. Set your DVM to ~20VDC and slap one lead on each side of the 1K resistor. Polarity doesn't matter much as we are looking for a differential voltage.

Obviously...use extreme caution here as you will be in close proximity to lethal voltages!!!

Sure wish that this thread started ~3 days earlier. I had just placed an order with Mouser for the final parts for my express build and would have gladly bulked it up for your stuff and saved a few quid in the shipping for you.

Cheers,
Dave O.

[greatwhitemark]

Dave,
I've just changed R25 to a 4k7 10watter. It brought B+ down by a small margin to 490v and B+2 to 452v.

I measured the voltage across R20 at 1.16v and across R21 at 0.76v.

Is this useful, I can quite easily put the 1k back into the equation and measure R20/21 again?

[ampgeek]

No need to put the 1K back in. I was only hoping to get another data point before upping the value of that resistor.

The R20/R21 measurements indicate that the EL34 screen grids are drawing ~1.2 mA and 0.76 mA (ohms law: V/R=I where R=1K ohm and V is the measured voltage drop) which is about a 1/4 of what I would have estimated. I am not sure what is going on there.

What are you running for power tubes? It would be great if others more experienced with this circuit could share their measurements for comparison.

As you might have guessed, the B+ dropping string excercise is all about knowing how much current each tube is drawing and sizing the dropping resistors to get the desired voltage at each node. That can be obtained by measuring the voltage drop across the cathode resistors in triodes (12AX7s in this case) and the screen grid resistors and 1 ohm on cathodes in pentodes (EL34's).

The lower than expected screen grid current draw generated a lower than targeted B+2 voltage drop across the 4K7 (to ~450 V as opposed to 425 V). But...a decent margin on the "max" grid voltage was obtained as desired (e.g., ~425 V against 500 V).

I think that the next major questions are: Where are the power tubes biased (e.g., voltage drop across cathode 1 ohm and/or bias voltage on pin 5) and, most importantly, how does the amp sound and respond? In the end, that (and not thrashing the tubes by operating way outside their limits!) is all that really matters.

You are still going to see higher than reference value voltages on the pre-amp tube plates (pins 1 and 6) which, generally, provides more headroom. That may or may not be desireable depending on guitar pick-up type, how you want the amp to feel/respond to your playing style ,etc...etc...

Cheers,
Dave O.

[Jackie Treehorn]

What's funny is that the schematic for the stock amp has a 440 volt plate voltage. Are you sure you're using the right transformer taps? It's strange that you've got 500 volts.

Just my opinion, but if you're going to go with a higher plate voltage, maybe you should scale the rest of the amp up rather than just stick the stock Express setup in the preamp/phase inverter. Also, a lot of resistance in the screens is going to induce lots of sag. I would just stick with something between the original resistor 330 and the Express 1k. I've got a Fender 75 running EL34L's at probably close to 500 volts and its stock 510 ohm resistor works pretty well.

In terms of the phase inverter, you'd want slightly more swing to match the higher voltages of the power tubes and the higher bias voltage. So, I wouldn't drop the voltage there either. I think the stock Express PI at the usual voltages swings about 100 volts or -50/+50 into tubes biased at -30. That gives quite a bit of compression which you'd probably want to duplicate to get the "clean to mean" characteristic.

Then if you want to drop the voltage at the third stage, you could bias it a little hotter to make up for any extra headroom as a result from the higher than normal PI voltages. You could just use a trim pot to get the best spot. I'd also suggest using a trim pot for determining the dropping resistor in the power supply. Another option would to use the Concorde type third stage: gain stage with direct coupled cathode follower. That setup also clips asymetrically due to the grid current and might work nicely at the higher voltages. Then you'd just drop more voltage at the V1 & V2 end of the power supply. That can sound pretty nice. Something in the 15-25k range can give a nice sag/response.

[greatwhitemark]

Dave O - I've got EL34M's in the output, not sure what the M denotes. I agree that ultimately it's the sound that matters. I've just stuck a few old valves in it to get it up and running and already I like what I'm hearing. As you state, my major concern was running the valves within their safe operating range.

Jackie - Yes I wondered why the voltage would be higher than on the original schematic. I haven't touch the wiring of the PT primaries but it wasn't exactly virgin territory when I got the amp so I'll check that out. Good point regarding sag in the drop resistors, I haven't had a chance to crank it with the 4k7 so can't comment.
The clean to mean capabilities isn't of major importance at present and a bit more headroom wouldn't be that bad for how I work.
When I can chance I'll post a full voltage chart and hopefully you good people could tell me of any potential issues. Cheers.

[greatwhitemark]

OK, I put the 1 ohm resistors in place and set about adjusting the bias. The 4k7 R25 is still in place by the way. The max voltage I could get across the 1 ohm was 26mv with the bias trimmer up full, while increasing this voltage the plate voltage decreased to 488.
So, 488x0.026=12.7w static dissipation.
So I’m thinking if I can increase the bias voltage then this will bring the plate voltage down a bit and also get the el34’s running nearer to their 70% suggested level. Does that make sense?
By bias circuit is running off its own tranny winding at 40vac across its winding. I discarded R29(220k) in the early stages to increase the bias voltage nearer to the correct levels and would appreciate any suggestions to increase it a bit more?

I also measured the voltages of where the amp is at right now:

V1 -
plate: cathode: heater: plate: cathode: heater
Expected: 210, 1.9, 6.3, 182, 1.5, 6.3
Actual::::: 244, 2.1, 6.1, 200, 1.6, 6.1

V2 -
plate: cathode: heater
Expected: 267, 2.8, 6.3
Actual::::: 280, 3.3, 6.1

V3 -
plate: grid: cathode: heater: plate: grid: cathode: heater
Expected: 190, 28, 42, 6.3, 193, 27, 42, 6.3
Actual::::: 217, 15, 42, 6.1, 220, 14, 42, 6.1


V4 -
heater: plate: screen: Bias: heater
Expected: 6.3, 415, 397, -33, 6.3,
Actual::::: 6.1, 492, 450, -33, 6.1,


V5 -
heater: plate: screen: Bias: heater
Expected: 6.3, 415, 397, -33, 6.3
Actual::::: 6.1, 492, 450, -33, 6.1

[ampgeek]

Hey Mark,
Yes. That makes perfect sense.

A common approach for doing just that is to increase R31 (47K) in the bias circuit to 56K.

Also, the screen grid current contributes to what you are measuring at the cathodes. So...take another reading across R20 and R21, subtract that from the cathode total before calculating plate dissipation.

How is it sounding? Having fun yet? Planning your next amp build already?!?

Cheers,
Dave O.

[greatwhitemark]

Dave, I'm definately having fun but no plans to build another amp just yet. Would be nice to actually play some guitar inbetween this endless search for tone!

It's hard to give a full appraisal of the amp using the speaker I have at home, and it's so loud that it'll have to wait 'til I'm back at the rehearsal room.
I sorted the bias circuit, ending up with a 22k in R31. I thought the 5k R25 introduced too much sag as suggested by Jackie Treehorn and reverted back to the stock 1K.
I've put in a zener string to bring the voltage down as I've also realised that the filter caps supplied by Ceriatone are rated at 450v. I've now got 431v on the plates and 423v on screen when biased at about 70% - the volume of the amp has come down quite a bit and I could find the gain sweet spot without putting a hole in the wall.

Tried a few different valves and opened up a whole new can of slippery decisions. The reason I'm attracted to something like the express is it just 'does', I don't like having too many options or messing with controls. I guess I need to take my valves etc and see what works when a full band is getting in the way of my perfect tone 'cos what sounds best on it's own doesn't always cut through the sonic haze.

Thanks again

Mark

[ampgeek]

Good to hear Mark!

Interesting that you ended up with a 22K at R31. Did I have something backwards? If so, I apologize. What was the original R21 value and what is the value of the adjustment pot?

Just need to keep adding to my "database" of understanding.

Almost done drilling all of the holes in my Express chassis....so....I am not too far behind you!

Dave O.

[greatwhitemark]

The original R31 was 47k and raising the value to 56k had the opposite effect of giving me less bias voltage. So I went the other way, with the 22k getting me in the zone. As for the trimmer pot I was assuming it was 25k, it's marked on Ceriatone layout as 22k, so I just had a look and found it to be a 50k.

Good luck with the build.

[ampgeek]

My bad! I recalled that in the Express reference documents section (BOM I think) there is a note to increase R31 to 56K to get a hotter bias.

When you said that you needed "more" bias voltage, that resistance increase made sense (e.g., "more" meaning further away from zero either positive or negative).

I failed to think it through as I know darn well that reducing negative voltage increases current flow through the tube.

Sorry about that. Hopefully Ron W will update the reference documents accordingly to keep us novices on the straight and narrow.

Cheers,
Dave O.