I'm designing a amp to see the limits of a new feedback system, and I would like to know what is the limit you consider on the lowest side for the loadline.
Please consider that the DF has not to be considered here.
The amp is a PP based on a pair of EL34 at 450V B+.
Initial OPT was 6k6 with UL 23%, now I would like to test a 4k with UL 23% I have, but I would avoid to blow a pair of EL34.
The point is that the loadline is on the edge of 2x the max plate dissipation on some points (as highlighted on the image) and this considering a load of 8 Ohm, so it's even worst at mid frequencies where load is usually lower.
What is the limit you consider when you design an amp?
Is that loadline the lowest you'd use, not considering the DF in the equation?
Thank you in advance,
Roberto
Please consider that the DF has not to be considered here.
The amp is a PP based on a pair of EL34 at 450V B+.
Initial OPT was 6k6 with UL 23%, now I would like to test a 4k with UL 23% I have, but I would avoid to blow a pair of EL34.
The point is that the loadline is on the edge of 2x the max plate dissipation on some points (as highlighted on the image) and this considering a load of 8 Ohm, so it's even worst at mid frequencies where load is usually lower.
What is the limit you consider when you design an amp?
Is that loadline the lowest you'd use, not considering the DF in the equation?
Thank you in advance,
Roberto
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I look at long-running commercial amps and see what they got away with.
EL34 are often run in guitar amps near 450V at 3k6 load. Making well over 50 Watts sine and 70 watts no-too-distorted. For hours. Is your use that tough?
EL34 are often run in guitar amps near 450V at 3k6 load. Making well over 50 Watts sine and 70 watts no-too-distorted. For hours. Is your use that tough?
Thanks PRR,
Application is hi-fi.
I know Marshall used 3k4 with 460V since decades, and others just raised the voltages to around 500V keeping the same load, but I also thought that instrument amps play in a non-continuous mode (each instrument individually plays alot of pauses between notes) whilst an hi-fi amp plays continuously (all instruments summed together have way less pauses), so there’s no time to cool down “between pauses”.
What worries me is also the fact that most of the power is in the range where the speaker has lower than nominal impedance, so the loadline would be even worse.
…I have a pair of OPT with 3k5 Raa and UL at 40% that were mounted on a self built Hi-Fi I bought two years ago, but I always used them as 7k by connecting 8 Ohm speakers into 4 Ohm tap. I will give them a try too with that weird AB2-like loadline.
Application is hi-fi.
I know Marshall used 3k4 with 460V since decades, and others just raised the voltages to around 500V keeping the same load, but I also thought that instrument amps play in a non-continuous mode (each instrument individually plays alot of pauses between notes) whilst an hi-fi amp plays continuously (all instruments summed together have way less pauses), so there’s no time to cool down “between pauses”.
What worries me is also the fact that most of the power is in the range where the speaker has lower than nominal impedance, so the loadline would be even worse.
…I have a pair of OPT with 3k5 Raa and UL at 40% that were mounted on a self built Hi-Fi I bought two years ago, but I always used them as 7k by connecting 8 Ohm speakers into 4 Ohm tap. I will give them a try too with that weird AB2-like loadline.
Running them like that means they don't run "for hours", they die pretty fast.
You shouldn't be worrying about overdosing the anodes on the EL34, that just heats up the glass.
What kills them is masses of secondary emission from G2. (screen grid).
Once you see that glowing bright red then white hot, you know you are minutes away from total destruction.
The weak point in the EL34 has always been the screen grid, which is why I have always avoided them like the plague.
Thank you sarcastic1,
so a steeper loadline would be better than a flatter one?
I will use the datasheet suggested 1k screen resistors with UL taps.
G1 will be dc coupled to the driver stage, through 1k grid stoppers.
They will run in fixed bias, with 1 Ohm on cathodes to check the cathode current.
so a steeper loadline would be better than a flatter one?
I will use the datasheet suggested 1k screen resistors with UL taps.
G1 will be dc coupled to the driver stage, through 1k grid stoppers.
They will run in fixed bias, with 1 Ohm on cathodes to check the cathode current.
Years ago when I used to repair EL34 guitar amplifiers I found short bursts at full power were the best for the maximum power test as many amps could kill G2. A one second burst should be suffice to capture data.
On the simulation, with 450V B+, 3k5 Raa with 40%UL and 1k screen resistor, I have 16W peak and 6W average on screens. Datasheet ( https://frank.pocnet.net/sheets/129/e/EL34.pdf ) says 8W. Should be within limits.
Again on simulation, with 450V B+, 4k Raa with 23%UL and 1k screen resistor, I have 21W peak and 7.5W average on screens.
The former is at 50 Wrms, the latter at 66 Wrms, with average Ik of 145 mA when biased at 80% of plate dissipation (when the plate is at 80V the screen is at around 280V).
Does it seem reasonable to test a pair of EL34 without risking to kill them?
Thanks
Again on simulation, with 450V B+, 4k Raa with 23%UL and 1k screen resistor, I have 21W peak and 7.5W average on screens.
The former is at 50 Wrms, the latter at 66 Wrms, with average Ik of 145 mA when biased at 80% of plate dissipation (when the plate is at 80V the screen is at around 280V).
Does it seem reasonable to test a pair of EL34 without risking to kill them?
Thanks
You'll risk killing the screens, but it will probably work. You won't know unless you try it.
The "design centre" is 8W - peak can be higher (I used to think the "design centre" was a building).
I'm thinking of designing a similar amp - I have a pair of 4k3 OPTs and a set of EL34.
I was thinking 360V - 400V B+, 40mA - 50mA idle.
Then I decided I'd rather keep building with trioded sweeps.
Anyone need a tube, choke, and transformer set for EL34 or KT88? LOL
The "design centre" is 8W - peak can be higher (I used to think the "design centre" was a building).
I'm thinking of designing a similar amp - I have a pair of 4k3 OPTs and a set of EL34.
I was thinking 360V - 400V B+, 40mA - 50mA idle.
Then I decided I'd rather keep building with trioded sweeps.
Anyone need a tube, choke, and transformer set for EL34 or KT88? LOL
Thanks for your feedback kodabmx!
I will then test them on 4k and 23%UL that still seems within limits.
I will then test them on 4k and 23%UL that still seems within limits.
Simulation is totally meaningless, and largely useless, as were the Mullard claims to run the EL34 at 800V on the anodes.
I once tried to run a so called "hi end" EL34 amp under a square wave test, supposed to deliver 30WPC...on 1 ch only.
(a feat any 807 Ab2 amp will swallow without any worry at all, for an hour with anodes glowing).
The El34 screens glowed immediately so white hot at merely 10W output, - brown trousers followed.
I realised it came within a whisker of blowing up and melting down!
Such tests never to be repeated least of all with most "hi end" amps into a dummy load. They can't handle it.
Thanks again sarcastic, it is surprising for me, because I've always played guitar amps with 3k4 load and 500V without any issue (insert apotropaic gesture here).
Could it be a set of faulty EL34? I remember even Marshall switched to 5881 during 90s because there were no reliable EL34.
Could it be a set of faulty EL34? I remember even Marshall switched to 5881 during 90s because there were no reliable EL34.
Just watch the tubes - do it in a dim room if you can. If anything makes light besides the heater, cut the power immediately or kill the tubes 🙂 Grid wires shouldn't make light in an EL34.
FWIW, I have killed a few tubes by screen death. The plate supply was missing on one tube so the screen was the plate - screen wire looked like an old light bulb.
That's why I use a 240R to 270R 1 or 2W resistor in the screen circuit now. If the plate goes away, that resistor will burn up - saving the tube.
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Most guitar amps run the outputs as pentode though, not UL? I might be wrong though.
EDIT: Just for the heck of it, I plugged in my operating point for 6P31S strapped as triode in push pull into 2k2. The idle point is off of the graph (320V B+, 40mA idle). The tubes get HOT but they don't red plate and I get about 15W from a pair.
The Pd of a sweep tube is easily exceeded for audio - They will handle their rated power as a 15% duty cycle switching sawwave in a TV. I much more difficult service than audio IMHO. I would NOT run an EL34 anywhere near that far over it's ratings...
EDIT2: Found the 6BQ6B in the list - I run them 320V@60mA - 2k2 load.
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Which do you think will fail first, G2 or your tweeters? I have one in the pipeline and thinking similarly - I already wound up the power trafo to give me 460V unloaded, dropping to 415 at 250W DC load (600 mA). Planning on a quad of winged C’s I have for stereo. Still waffling on what OPTs to buy - 3.5k, 5k, or something in between. The speakers I’ll be using have irreplaceable Focal TLRs - I doubt I’ll be cranking this one up loud enough to worry about melting screens. Guitar amp might.
I was testing “the big one” without a load (accidentally) for some 20 or 30 minutes, and judging from the cathode currents and screen regulator (which I was scope monitoring) I was hitting G2 peaks of over 100 mA per tube on the positive half cycle. With music, run a good 6dB into clip. No sign of screen glow. I would see the cathode current spike, the regulator drop out (limit), and hear the OPT sing. Output voltage showed nice clean clipping. Same amp ran for an hour at full signal sine wave, and all day running clipped music into the dummy load. These were 6550’s not EL34’s, but it certainly was testing the screens. This amp was intended to be pushed hard driving PA type speakers, the EL34 project won’t be.
I was testing “the big one” without a load (accidentally) for some 20 or 30 minutes, and judging from the cathode currents and screen regulator (which I was scope monitoring) I was hitting G2 peaks of over 100 mA per tube on the positive half cycle. With music, run a good 6dB into clip. No sign of screen glow. I would see the cathode current spike, the regulator drop out (limit), and hear the OPT sing. Output voltage showed nice clean clipping. Same amp ran for an hour at full signal sine wave, and all day running clipped music into the dummy load. These were 6550’s not EL34’s, but it certainly was testing the screens. This amp was intended to be pushed hard driving PA type speakers, the EL34 project won’t be.
In my case, neither. Not enough power to blow the tweeters and triode strapping means thee plate is always the primary anode WRT to current.
My Yorkville has blown two sets of tweeters this year though (don't get drunk and blast your music)... I have "retired" it but not before it cost me about 400$ for the repairs to the speakers. I have Energy RC70 speakers. The tweeter is discontinued so I found a pair of RC10 on Kijiji for the tweeters. Then another pair the second time... *Grumble Grumble*
I have also run the sweep amps unloaded at power without incident. The OPT becomes a tweeter though LOL
I try and make my designs as robust as I can. I even drop test them (one inch onto a desk while powered playing music).
My Yorkville has blown two sets of tweeters this year though (don't get drunk and blast your music)... I have "retired" it but not before it cost me about 400$ for the repairs to the speakers. I have Energy RC70 speakers. The tweeter is discontinued so I found a pair of RC10 on Kijiji for the tweeters. Then another pair the second time... *Grumble Grumble*
I have also run the sweep amps unloaded at power without incident. The OPT becomes a tweeter though LOL
I try and make my designs as robust as I can. I even drop test them (one inch onto a desk while powered playing music).
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Well noted, thanks!
I have 1k on screens, that's what the datasheet suggests for EL34 in UL.
Yes you are right, only a few amps over the entire guitar amplification history went for UL: the 200W Marshall in the 60's, the 135W Silver Face Fender in the 70's, the 1492 Advance in the 90's and some Dr Z in the last decade.
But this won't be a guitar amp, will be an attempt of Hi-Fi amp implementing a different feedback (I will talk about it after the trials).
Because of the THD? On sims it's below 1% until 60W and mainly 3rd, whilst at 5% 5th harmonic is still 1/3rd of the 3rd.
Average power blows tweeters, not distortion. By the time you pump 16 watts into a pair of screens, you’re putting out a lot more than that into the speakers. If you have more than 5 or 6 of those watts going into the tweeter it’s toast (unless it’s a 2” exit compression driver). Better turn it down before then. That tends to protect the tubes too. Screen meltdown in actual hi-fi use is the least of your worries. I’d be a whole lot more worried about tweeters that cost $450 each 20 years ago than a pair of $50 EL34’s which won’t last forever anyway.
I’ve been playing with sweep tubes lately, experimenting with amps that can take all manner of abuse. If you’ve got high enough emission (hot enough cathode), the screen can be run down around 120V where the G2 currents are low even if you’re clipping the bejeezus out of it. Different kind of amplifier for a different kind of use.
I’ve been playing with sweep tubes lately, experimenting with amps that can take all manner of abuse. If you’ve got high enough emission (hot enough cathode), the screen can be run down around 120V where the G2 currents are low even if you’re clipping the bejeezus out of it. Different kind of amplifier for a different kind of use.