crate palomino 32 212 crazy scifi noises update

[georage]

i started a new topic to update my old misspelled topic on a crate 32 212 that had bad problems. the original owner said it would shut down while playing. in my experience it would start making crazy startrek noises at loud volumes.... i attempted to read the bias on the 4 el84 tubes and found that the tubes were dissipating 17 watts per tube and that the b+ voltage was 345volts.
i have since started working on it again and i have placed a 50 ohm power resistor in series with the b+ line. that brought the b+ down to 314v, which is close to the 320 the schematic calls for. the amp stays on now and does not make any crazy noise(by ear)

could someone help me out with the bias procedure again?
the single bias resistor has a voltage drop of 11.22v and a resistance of 62.9...the schematic calls for a voltage drop of 10 volts btw. but is this close to the 12 watt mark that el84s are rated for? thanks everyone.

[g1]

Seems they are running around 13.5W each, depending on the actual plate voltage at power tubes.
11.22/63 gives 178mA for 4 tubes, so 44.5mA per tube. Voltage across tube would be plate voltage minus cathode voltage. So around 314 minus 11.2 gives about 303V.
303V times 0.0445A equals 13.48W.
Plate voltage will be a bit less than B+, so measure and reduce calculation accordingly.

[Chuck H]

IMHE there's no need to reduce voltage on the EL84's from 345Vp. Just increase the value of the cathode bias resistor and target about 11W dissipation per tube. I've "fixed" a couple of these amps and the way they're designed is problematic. The tubes, which are biased HOT are sitting right under the front panel control board. As is the cathode bias resistor which also gets hot. And the front panel control board shares the board mounted tube sockets and is stabilized by the board mounted pots. Take the tube shields off and throw them away so no one will be tempted to install them again. Board mounted sockets on a flexible board just can't handle the stress of putting on and taking off spring loaded tube shields. So this already questionable arrangement is being constantly overheated. What I've done is to move the cathode resistor to the vast, open area of the chassis. But leave the bypass cap where it is on the board so that it doesn't suffer heat from the resistor. I increased the cathode bias resistor value to 82ohms. I used one of those aluminum finned, screw mount resistors just to make the job easy. I detected no tonal change or detriment for the cathode resistor value change. I also added a retrofit stabilization for the control panel board (though I can't remember just how I did that) after replacing all the pots with solder lug units using flying leads so the board wouldn't be damaged by rough use of the control panel. This amp already had several bad potentiometer pads and it hadn't even been used much. I think I may have also replaced the input jack with flying leads but I can't remember. As stock I remember the front panel would get strangely hot to the touch. After cooling the bias and moving the cathode bias resistor things seemed more normal.

Surprisingly good sounding amps. But a terribly unreliable physical design.

[georage]

thanks for the refresher G1, its still too hot, then.
Chuck! is 345v really ok for the el84 tubes? the data sheet states that 300v is the maximun plate voltage...some data sheets state 500v under certain conditions, but, is that ok? you seem to have some experience. you are right about the heat! i measure 165 degrees Fahrenheit on that cathode resistor. messing around with the b+ is getting complicated anyway. i got the plate voltage down to 290v, but then the screen voltage was still at 314v. it sounded good though...

[Chuck H]

Well you don't want the screens higher than the plates for sure. And.... A whole bunch of manufacturers, including ALL of the well known ones have run el84's at or near 100% bias at idle at or even over 400Vp. They really are tough tubes and the greatest detriment to them in harsh environments is physical abuse. They suffer microphony in combo amps way before any electronic circumstances take their toll because they have those skinny bottles. Trust me, el84's are tougher electronically than they are physically. As I've said before, they're still the cheapest tubes on the market. Smoke 'em if you got 'em. Matchless has built a reputation on that premise with this tube! The VOX AC30 ran them in class A at bias (though they actually DO shift into AB in clipping) at 335Vp. Over the rated max for the tube and in class A. Of all the tubes I've incinerated over the years el84's are not among them. They may have a limited life in given operating conditions but they tend to go rattly and microphonic before any electronic failure IMHE. 345V is mice nuts compared to how they're treated by Peavey, Mesa, Marshall, Fender and Traynor. My own best el84 design and my personal amp has them running at 90% bias at 365Vp. I'm using Reflektor tubes to extend the life but it's more a matter of microphony than electronic tolerance.

In other words, 345Vp is fine regardless of the published spec sheets because the physical limitations of the tube preceed the electronic limitations.

Smoke 'em if you got 'em. Probably my favorite tube to work with.

[Helmholtz]

The Vox AC30 is a hot biased (47mA/120%) class AB amplifier.
At low output it runs in class A and at least above half-power in class B.
Most likely class B operation starts considerably below 50% because of bias shift.

Full class A operation would require center-biasing at around 70mA plate current.

[Mike K]

EL84s sound quite good at "normal" operating conditions, even if the likes of Mesa run them way high. Manufacturers typically push them in a war of headroom and power. They'll actually sound very good at less than 250V if you want a low headroom amp. And still stay bright and clear if the rest of the power supply and OT are well designed.

I'm most surprised at how much abuse the screens will take, and to me, this is where the tone is. I tend to use screen resistors because I like the smoother tone they give when pushed (by pulling down the screen voltage), but I've certainly run them with none. If you want headroom, then go low and run the screens hard. Personally, I think EL84s can sound amazing driven, and the fact that they have such low headroom (and high gain), makes them ideal for that. I tend to push them in this direction, but to each their own. If I want more headroom EL34s do that nicely even at moderate (very safe) voltages.

[georage]

what does the screen do? and why does it have to be 12 volts below the playe voltage? this amp was running with screen voltage aboclve plate voltage and it sounded really good...wouldnt be surprised if it was about to explode tho.

so i took out that b+ resistor and changed the cathode resistor to a 100ohm. the cathode voltage went up to 12.9v . and the b+ changed back to 324v. so that should be.....129ma/4=(32ma)*(324-12.9)=9.9 watts.

that seems pretty spot on to 90percent or so ..the amp stayed on for 15 minutes without going warp drive. its sounds a lot different now....it was very clean and pretty sounding...now it is distorted even on the clean channel and the hiss is noticable now...i never heard it hiss before. but whatever...its getting there

[georage]

its really great reading about your experiences with el84 amps. wish i could upload pics and schematics, but this amp looks similar to the ac30

[georage]

the new resistors came in. the results are...
cathode resistor is 81ohms but when warmed up is 94ohms, 12.57volts across it, 320v on the plate, for a result of 10.17 watts per tube....
the amp stayed on and sounded good, but, it lacked a little sparkle and had great distortion on the gain channel.
i turned it off because the cathode resistor kept climbing and it was getting very hot! i didnt have the thermal camera on this time, but, it was almost too hot to touch...so probably around 160 degrees maybe more.
i used 3 265ohm resistors in parallel @ 5 watts each to try to average the heat disapation and get the heat down without a heat sink...its still roasting though and it keeps climbing and the heat is maybe causing the resistance to go up and up.

[georage]

why is the cathode resistor getting so hot? at 12.57 volts and 94 ohms the resistor is dissipating about 1.68 watts. each of the 3 resistors is rated for 5watts. i thought that would divide between them for a average of .5+ watts and be cooler than a single 10 watt resistor....
the tubes are dissipating 10 watts a piece..so does it need a 40 watt resistor?

[The Dude]

Tube plate voltage rating is from plate to cathode, so if you are measuring to ground, you need to subtract cathode voltage for actual plate voltage. If resistors are running within their rated temperature spec, they should not change value. At 10.17 watts per tube, the resistors shouldn't be getting that hot, unless they are being heated by external sources. Did you move the cathode resistor(s) away from tubes as Chuck mentioned in post #3? Also, just an FYI, three parallel 265 ohm resistors would be 88.3 ohms.
Edit: Have you checked the cathode cap?

[g1]

If the voltage across the cathode resistor keeps rising, I would expect a bad power tube, or some voltage getting to the grids.
You do not need a 40W resistor, a 10W should be fine there.

my calculations (based on your info) show a plate dissipation more like 18W per tube

Did you calculate for 4 power tubes?

[The Dude]

I f'd up, apologies. No schematic in thread, so here's a link. I'm going to edit my above post. Thanks for the heads up g1.

https://schematicheaven.net/newamps/...2_palomino.pdf

[georage]

The resistance of the resistor shou!dnt change during operation, should it? Maybe something tricked the meter? I better change it out and check the capacitor too. The tubes have been super stressed too. I better bench test those resistors they could be fakes...
thanks for the schematic dude. I can t upload stuff