Whats a good voltage to run 7591's cathode biased

[cbass]

The way the show on the datasheet cooks the shit out of em.I don't know why a manufacture would suggest running them like that.To sell more tubes I guess.

I'm gonna build a 7591 rocket eventually.

[cbass]

No you are supposed to do all the work and then tell me what sounds best.

I'm gonna shoot for around 400 VDC on the plates and take it from there.I'm using the transformers I already have because I'm a cheap bastard.

[martin manning]

I think with 400V on the plate, you should use 6k6 load (so typical TW OT), 15V on the cathode (~300 ohm Rk, 42mA plate current per tube). That means 385 Va-k, and I think 350V on the screens is good, so 365Vg2. You'll have to adjust the screen dropping resistor to get there. I'm guessing 6mA screen current, so depending on what the preamp current draw is maybe 3k9 on a shared branch (or maybe half of that on the power supply dropping string between plate and screen nodes, depending upon the preamp current draw) and 1k for each screen grid mounted on the sockets.

[cbass]

Thanks Martin.I think the OT I have is a 4k 8ohm but I plan on running 8k 16 ohm.I wasn't sure if 7591's really need screen resistors.I've never seen a design that uses them but I guess most old amps didn't use them anyway.

Do you think its really nescessary to drop the screens that much?I have an amp that runs about 480 on the plates and couple volts less on the screens It still has the original tubes from the 60's It is fixed bias though.

I've seen a couple amps that were designed around the Cathode bias example on the 7591 data sheet.And the tubes were completely cooked as well as everything around it.That could just be a coincidence though.

http://www.hebertech.com/datasheet/sylvania/7591.html

[martin manning]

Dropping the screen voltage is about keeping the screen current in check. IMO, 4k is too low, 8k is too high. If you want to run both plate and screen at about 385 (400V on the plate and screen and 15V on the cathode), 5k OT primary would be pretty fine. The Sylvania data sheet is the one I've been looking at.

[cbass]

Thanks Martin.I meant to post the sylvania data sheet not sure waht happened.

I'm kinda confused because the data sheet Says 9k while running 450 on the plates 400 on the screens.200 ohm cathode resistor.So i was thinking 8k would be pretty close.If I ran near 400 volts.

You don't have to get to in depth for me but why do you recommend a 5k OT?

[martin manning]

If the plate load puts the load line much below the knee of the 0V grid curve, then the screen current will become excessive when the plate voltage is pulled down at the signal peaks. The load line and 450V operating point in the data sheet is probably OK because the screen voltage is 50V below the plate, and even though the load line is well below the knee that means it has some significant resistance between the plate node and screen. That will put a lot of sag in the screen voltage, which pulls the knee down and limits the screen current. If you were to run 400V on the plate and 8K, that would probably be OK too, as long as you have enough resistance to drop the screen down to 350 or 360 so it would sag in a similar fashion. With similar plate and screen voltages, a 5k load puts the load line through the knee, so you wouldn't need the screen voltage sag to stay out of trouble.

[cbass]

Thanks Martin.I tought myself to halfway understand load lines a couple years ago.Now I have totally forgpt how to read them again .If I don't refresh myself now abd then it falls out the back of my brain. I need to study again.I'm just prettty much putting all my physical and mental energy into work right now.

[martin manning]

Getting a handle on load lines is time well spent. Valve Wizard's site has both a SE and PP article, and between them they cover the essentials.

[Diablo1]

My old Capitol is a stereo amp with a quad of 7591s. It's cathode biased, and runs the plates at 450V. One side uses a 3.9K resistor in the B+ filter to a pair of screens, and the other side uses a 1.8K resistor for the other screen pair because the string also powers five 6EU7s for the preamp and one side PI. The tubes are VERY old and still work fine. I have no specs on the output transformers, and there is no schematic available for the amp. I haven't measured the screen voltage, but I calculated it should be around 405V at idle. Originally, the amp came with a pair of 190 ohm cathode bias resistors. I bumped them up to 250 ohms to make the tubes live longer. The amp really sounds great, and the screen voltage droppers add a lot of compression when you crank the volume and dig in.

[gingertube]

Doing a restore of a 1968 Ampeg Echo Twin ET1-B right now. It uses 7591.

Measured:
B+ to OT centertap is 400V
Screen voltage Vg2 is a couple of volts lower (1K dropping resistor to both screens, 20uF cap plus the preamp B+ is tapped from here too ).
Cathode biased with common 140 Ohm 10 Watt resistor (measure about 15 volts across that so 53 to 54 mA per tube = just under 21W dissipation).
6K6 Raa output trannies.
Cheers,
Ian

[martin manning]

Thanks for the data point, Ian. Idle dissipation is probably just a little under 100% without the screen current, and the 6k6 Ra-a puts the load line a fair bit below the knee at that plate voltage. I see a another version of that amp runs 6V6's.

[cbass]

Thanks Ian Boy they run them little ol tubes hot.

[gingertube]

Thanks Ian Boy they run them little ol tubes hot.

Yes,

Part of this is that Ozzie mains volts is 250V AC and the amp is set for 240V (Its max voltage tap). We down under folk often have reliabilty problems with tube amps because of this. Everything tends to run 5 to 10% higher voltage than intended by the designers so I tend to be conservative. Similar to you US folk running 115V gear on what is usually more like 120 to 125V.

For a new design (at B+ = 400V) use a standard resistor value of 150 Ohms rather than the 140 Ohms for a common cathode resistor. The cathode bypass cap in the Ampeg is ONLY 20uF which is just what was left in the 40 + 20 + 20 + 20 uF cans they used. 20uF is a bit light for a common cathode resistor of 140R.

If using separate cathode resistors (one each tube which I RECOMMENDED) then see if you can get 300R resistors. If not then 2 x 560R in parallel for 280R or 2 x 620R for 310R would be the go These could be 2W or 3W metal oxide. Each would then need a cathode bypass cap. 20UF each would probably be fine BUT the last amp I built was a cathode biased PP 6CA7 with separate bias resistors (Trainwreck Clone) I ended up with 2 x 100uF = 200uF bypass caps on each (similar to the Hotcat). So You may want to experiment with 22uF 47uF and 100uF bypass caps. Build it with the 22uF, you can tack the extra across it for testing purposes and then if necessary change it for bigger cpas if tests indicate it. You are listening for bass "tightness".

Cheers,
Ian

[Randall]

Not sure if this will help, but I just went through a 50 year old HH Scott amp that runs a fixed bias quad of 7591. It has 450v on the plates and idles at 36mA with the original tubes.