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When we bias a tube amp what are we measuring?

Archguy said:
Whew, thanks for that. I was really mystified by that one!

And another upvote for this entire thread, which is civil and very educational--even for those of us who need a life preserver in this end of the swimming pool.
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Thanks to everyone for keeping this going and on topic. :)
 
dcgillespie said:
6 --.

To keep the thread on track then, a change in bias voltage that moves a design from Class A operation to Class AB operation, can in fact ultimately result in a significant increase in power output. But it is hardly created by the shift in bias voltage alone. As previously stated, the bias voltage that a given tube requires in a given application is merely a means to an end. It is no different in this case, either. The shift in bias voltage allows the use of greater B+ voltage and ultimately a lower load impedance OPT for the tubes to operate into, that together creates the increased power output developed.

Dave
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This is quite true and this I did to get the RM-10 to produce 40 watts from one pair of EL84/6BQ5s. The B+ is about twice the standard and the idle current is therefore half. The load is 14,000 ohms instead of the typical 8,000 ohms plate to plate. However to do this you must make the load impedance higher not lower..
 
maxhifi said:
Ok Ok, I have a question... Of course after seeing you on this site before, I had to make myself familiar with some of your products. You can probably anticipate what I am going to ask, but I will ask it anyway. 35W from a pair of EL84? HOW? What are the operating conditions? It's incredible this is possible. Are you running them at 500V and barely any idle current? I've never seen anything like that before, it makes me wonder how much power one could get from 6L6GCs if the same sort of design was applied.
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You can get 100 watts from a pair of 6L6s, its on the GE data sheets.

As for the RM-10 I gave a talk on that at Burning Amp 2018, just this month. Here is the link. The RM-10 amplication is near the end... I wanted to first have the audience understand a bit about data sheets. If you like put youself on our mailing list at the school.

http://berkeleyhifischool.com/having-fun-at-burning-amp-2018/
 
Selmerdave said:
I'm still not clear on the bias-does-not-affect-maximum-power thing, so if someone is willing to indulge me to help me understand I'd appreciate it.

My understanding is that class of operation is largely a function of bias (in conjunction with B+ and primary Z), and for example with a pair of PP EL34s biased in class A it's not *possible* to get more than about 25 or 30W. In AB though, it seems easily possible to get 60W or more with low distortion from the same pair. So what am I missing with this equation?

Edit: is it that the lower quiescent current of the AB bias allows for higher B+ without exceeding plate dissipation? Thus only with the higher B+ is higher output obtained?
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Excellent Question. Your edit correctly answers your question. It is indeed the dissipation of the chosen output tube that determines class A power in both single ended and PP amplifiers. The higher B+ brings many advantages the most of which is lower saturation voltage and greater swing. Consider the saturation voltage (knee of the plate curve) as a percentage loss of the total B+. In the RM-10 the saturation voltage is only 15% of the total where in typical circuits it is 30%.

Remember in a AB amplifier we are allowed to take the idle current as low as we like. In classic McIntosh amps they set theirs so low that the tubes are almost cut off. Its so low they dont every bother with a bias pot.

I covered some of the EL-34 aplications in my lecture. There is one that is 100 Watts per pair. http://berkeleyhifischool.com/having-fun-at-burning-amp-2018/
 
6DZ7 said:
No. It goes from 20mA to 70ma @ 350v on an EL34. It stays under 25W Pdiss. And if you continue to send 35v of signal on a -35v bias changed to -25v bias you don't get full power, you get ##$@$#.
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While I agree it goes from 20 to 70 ma I do not agree with the rest. Have you tried this on the bench? I have. How much do you estimate the power loss to be?

If there is no feedback the volume control will have to be turned down a few dB yet the power will be the same, only the gain changed. In a feedback amplifier the gain will stay the same and the amount of feedback will increase a few dB. In either case the tube will still reach full current at 0 volts on the grid and thus the same power.
 
maxhifi said:
You sure about that? I've never seen it for the 6L6GC, but I have seen the 100W rating for EL34s.

Anyway, 14k load line, with B+ exceeding data sheet ratings. Clearly you have proven it reliable, that's cool.

http://www.ez260.com/ge-6l6gc-datasheet/
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Here is the 807 data sheet showing 120 watts in an audio application which is ICAS (intermittant, not continuous full power, which is what we do in audio). It does require some grid drive power making it AB2. Granted the 807 is 5 watts more plate dissipation rating but I think this illustrates the point well. The 807 is just a 6L6 with a plate cap, same plate size and other ratings.

The RM-200 is AB1 to 100 watts and AB2 to over 130 watts, mainly so it can handle difficult loads. The 200 uses KT88s mainly because its hard to get audiophiles into 6L6s.

https://frank.pocnet.net/sheets/084/8/807.pdf see page 3 far right.


RM-10 Reliable? Yes there are 500 of these in the field some of which are over 22 years old. I hardly ever see one that needs service. Please watch the presentation. All the numbers are there.

http://berkeleyhifischool.com/having-fun-at-burning-amp-2018/

As to high voltage, the data sheet is not the final word on this at all. At Beveridge I learned that as long as a tube is well pumped out, and they all are, the plate voltage can be many thousand as long as the screen is low and the socket can take it. Most sweep tubes are rated a 500 volts B+ but 5.000 peak. I currently make 5,000 volt direct drive amplifiers for Acoustat, Quad, Sound Labs and my own ESL. I am looking at one right now that is over 5 years old.

Perhaps my early years at TV tech helped me see this.
 
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RogerMod said:
Perhaps my early years at TV tech helped me see this.
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I always thought the issue was the breakdown voltage of the sockets, hence most horizontal output tubes in TVs, and transmitting tubes like the 807 having a plate cap. On the other hand, damper diodes in TVs are subject to the same voltage peaks as the horizontal output, and they have no issues with socket arc-over. Years ago I built an amplifier using Chinese 6550 tubes, and one of them arced from pin 3 - 2 with about 650V on the plates. The arc actually carbonized the base of the tube, and formed a carbon track. I scraped away the carbonized part of the socket, and used them for many more years at a reduced voltage.
 
maxhifi said:
I always thought the issue was the breakdown voltage of the sockets, hence most horizontal output tubes in TVs, and transmitting tubes like the 807 having a plate cap. On the other hand, damper diodes in TVs are subject to the same voltage peaks as the horizontal output, and they have no issues with socket arc-over. Years ago I built an amplifier using Chinese 6550 tubes, and one of them arced from pin 3 - 2 with about 650V on the plates. The arc actually carbonized the base of the tube, and formed a carbon track. I scraped away the carbonized part of the socket, and used them for many more years at a reduced voltage.
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The plate cap on horizontal output tubes is for the high peak on retrace which can be many thousand volts (5000 typical rating). To make TVs reliable they just put on a plate cap, no big cost in that. The 807 has a plate cap more for keeping capacitance down at 30MHz in a transmitter.

Though the dampers are rated for 5000 V also, they don't get hit that hard because they are on a lower tap than the plate and feeding into the boost which is less than 1000V.

I have said to many who want to learn about tubes. "Read the whole section in RCA manual on TV set electronics". There is so much there because TV sets have many systems, each of different nature and something to learn.
 
RogerMod said:
The plate cap on horizontal output tubes is for the high peak on retrace which can be many thousand volts (5000 typical rating). To make TVs reliable they just put on a plate cap, no big cost in that. The 807 has a plate cap more for keeping capacitance down at 30MHz in a transmitter.

Though the dampers are rated for 5000 V also, they don't get hit that hard because they are on a lower tap than the plate and feeding into the boost which is less than 1000V.

I have said to many who want to learn about tubes. "Read the whole section in RCA manual on TV set electronics". There is so much there because TV sets have many systems, each of different nature and something to learn.
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Every TV I've worked on has warnings on the schematic telling you not to measure the voltages on the damper, or you will fry your meter. Perhaps not 5kV, but if the octal (or compactron) damper socket can take that voltage, it provides excellent empirical evidence that a quality 9 pin socket EL84 socket can easily handle the voltages your RM-10 is putting on it. Agree about RCA tube manual, it's fantastic. You can probably guess by my avatar picture, I'm something of a fan of all things RCA.
 
maxhifi said:
Every TV I've worked on has warnings on the schematic telling you not to measure the voltages on the damper, or you will fry your meter. Perhaps not 5kV, but if the octal (or compactron) damper socket can take that voltage, it provides excellent empirical evidence that a quality 9 pin socket EL84 socket can easily handle the voltages your RM-10 is putting on it. Agree about RCA tube manual, it's fantastic. You can probably guess by my avatar picture, I'm something of a fan of all things RCA.
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Yes Ive seen that warning. Depends on your meter doesnt it? On the Simpsons with 5000 volt scales no problem, but then one might turn it to 500 volts to get the average yet the peaks are high and might indeed burn things up. Sams is going to err on the cautious side. When I want to know what going on I get out my HV probe and look on the scope. I dont use the standard ones. I make one with a 10 meg HV resistor and a 10K to ground for 1000 to one probe which ends having frequency response beyond 20 KHz. Most DC probes are all over the place for AC. I do have to look at response and distortion of 5,000 volt P to P audio on direct drive amps.

I got over my fears of High Voltage in my teens and never looked back.

Nice RCA badge, what exactly is it and where did you get it?
 
RogerMod said:
Yes Ive seen that warning. Depends on your meter doesnt it? On the Simpsons with 5000 volt scales no problem, but then one might turn it to 500 volts to get the average yet the peaks are high and might indeed burn things up. Sams is going to err on the cautious side. When I want to know what going on I get out my HV probe and look on the scope. I dont use the standard ones. I make one with a 10 meg HV resistor and a 10K to ground for 1000 to one probe which ends having frequency response beyond 20 KHz. Most DC probes are all over the place for AC. I do have to look at response and distortion of 5,000 volt P to P audio on direct drive amps.

I got over my fears of High Voltage in my teens and never looked back.

Nice RCA badge, what exactly is it and where did you get it?
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Totally agree with all the above.. My 260 is an 8P, and only goes to 1000V. I haven't had a TV issue I needed to dig that deep into. High voltage needs respect, but not fear.

The badge is from an MI-12214 amplifier. I'm restoring it right now. It's basically an MI-4288, with mic preamps on the same chassis. Gonna set it up to play 78s, and mono movie sound. It's a real tank, I very much enjoy working on old tube equipment from the 40s and 50s.
 
RogerMod said:
You can get 100 watts from a pair of 6L6s, its on the GE data sheets.

As for the RM-10 I gave a talk on that at Burning Amp 2018, just this month. Here is the link. The RM-10 amplication is near the end... I wanted to first have the audience understand a bit about data sheets. If you like put youself on our mailing list at the school.

http://berkeleyhifischool.com/having-fun-at-burning-amp-2018/
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Roger, I happened to watch your Burning Amp talk from last month just before viewing this thread. I wanted to also provide a link for the group here to your Burning Amp talk from 2015, which goes into even more detail about the RM-10 "magic" [science]:


I've actually watched the 2015 presentation a few times over the years -- as an EE, I really enjoyed it, as it covers some great technical topics.

I also own an RM-10 mkII and confirm that Roger is not pulling anyone's leg when he says he can get that power output out of a pair of EL84s!
 
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