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70.187.93.113
I know the specs on the 7189 (and Russian equivalent) allow for a higher plate voltage than the 6BQ5 (400v vs 300v) while the screen voltage (300v) and the plate dissipation (12w) is the same.
I've always been under the impression that if a tube is being operated within the plate dissipation rating that the voltages are less of a concern. Am I misinformed about this? Specifically, if a 7189 and a 6BQ5 are both running at, say, 10w plate dissipation, why would it matter if the plate voltage on the 6BQ5 is too high?
Follow Ups:
The RM 10 runs its 6BQ5s at over 600 volts on the plate and 300 on the screens. The key is to keep the idle current low so you don't reach the max plate dissapation ratings.
Not sure how this works, at a 300VDC difference.
Thanks!
The difference is due to the screens running off a separate B+ supply. A separate screen supply is the best way to operate a pentode.
Not only separate, but regulated .
Screen grid to cathode potential, not plate to cathode potential, dominates in pentodes and beam power tetrodes. Regulation of g2 B+ reduces the amount of open loop IM distortion generated. NFB, of whatever form, works best when it has comparatively little to do. NFB, of some kind, will always be needed with pentode mode "finals" to realize an acceptable damping factor. However, less is definitely more, when it comes to open loop distortion.
FWIW, the most effective implementation of ultra-linear (UL) mode "finals" requires O/P transformers that contain a separate screen grid winding. That separate winding allows g2 B+ to be regulated and benefits from more than 1 source obtained.
Eli D.
Charlie
The Sherwood S5000 with 7189s runs about 425vdc on the plates and about 420vdc on the screens being an Ultra Linear output trafo.You are correct in that plate dissipation is more critical regardless of where the plate voltage is at..Now keep in mind you don't want a ridiculously high plate voltage even if you don't exceed plate dissipation because you may have arc over issues.I'm talking like 600vdc or more.. If you are at 460v as long as you are biased not to exceed plate dissipation,you will be fine.
The Sherwood S5000 I'm able to squeeze 24wpc out of them with no issues.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/30/16
Thanks Mikey, that's what I thought. Actually, that is the amp too. I have a few of them (one with slant 12AX7s and two with the vertical ones) and I just started going through the slant tube one last week. To test voltages I put in a very lightly used set of the Russian 7189 equivalents.
A couple of other voltage issues . . . First, at wall voltage of 122v I was getting around 428v on the plates but screen voltages that are a few volts higher. I may put in some screen resistors but given the low current I doubt they will drop the voltage below the plates. Should I be concerned about this?
Second, when I measured voltages the first time I measured approximately .6 volts on Pin 3 of each tube. Later, I measured again and I was getting about .58 on one pair but only .41 on the other pair. I went out of town for the weekend so I won't be able to troubleshoot it any further till next week. Any ideas of what I should be looking at?
I guess I'll swap the outputs between left and right and retest the tubes. I've got plenty of NOS 6BQ5s if the 7189s have an issue.
A couple of other voltage issues . . . First, at wall voltage of 122v I was getting around 428v on the plates but screen voltages that are a few volts higher. I may put in some screen resistors but given the low current I doubt they will drop the voltage below the plates. Should I be concerned about this?
No concern at all on the higher B+. Screen resistors aren't needed.If you do put them in,make sure they are noninductive type like a carbon comp.
Second, when I measured voltages the first time I measured approximately .6 volts on Pin 3 of each tube. Later, I measured again and I was getting about .58 on one pair but only .41 on the other pair. I went out of town for the weekend so I won't be able to troubleshoot it any further till next week. Any ideas of what I should be looking at?
Make sure your cathode resistors are perfectly matched. After that,any differences you get will be the differences in the tubes and they aren't that critical as you would think..The sleeved dual pot is what sets the balance of course. Set the hum for each channel as low as you can get it.AAMOF,if you want to get it almost perfect,an AC voltmeter set to the millivolt scale adjusted to the lowest output voltage would be best.Have an 8 ohm resistor across the speaker load.
I would be using 7189 tubes or equivalent in this amp..This is what the amp was born with and you will get about 25% more power then with EL84s
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Thanks for the reply, Mikey.
"No concern at all on the higher B+."
My concern wasn't so much the level of B+ as the fact that the screens are a few volts higher than the plates while the schematic says they should be a few volts lower. I've seen this before but I've never heard any discussion of its significance.
Of course, as we all know, manufacturers seem to have totally ignored the data sheets when it comes to screen voltages. Both the 7189 and 6BQ5 are rated at 300v on the screens.
"Make sure your cathode resistors are perfectly matched. After that,any differences you get will be the differences in the tubes and they aren't that critical as you would think. The sleeved dual pot is what sets the balance of course."
Yeah, there are those two 12 ohm resistors that go to the concentric balance pot which sets the balance between the tubes in each channel (L&R) not between the two channels. Correct? The measurements I mentioned on pin 3 were taken after I set the pots in the middle (equal resistance to each side) which resulted in a reading of -19v on pins 1&2 of each output tube. The resistors in mine are not matched, they read 12.8 and 13.1. I don't remember which pair of tubes were connected to which resistor but the difference between a reading of .58v and .41v at the same plate voltage (428v) would be a difference of roughly 3 watts of plate dissipation (~6.8w vs 9.8w) per tube if I'm doing the math right. So the total dissipation on one channel would be roughly 20w while the other would be 14w. This is a difference between the L & R channels, not between the two tubes in one channel, so the balance pot can't correct it. Are you suggesting that that is not all that significant?
I need to retest the tubes when I get back home. I'm thinking something significant changed after they had been at working voltages for a while. Maybe one of the tubes has gone wambly.
"I would be using 7189 tubes or equivalent in this amp..This is what the amp was born with and you will get about 25% more power then with EL84s."
Why would a 7189 produce more power than a 6BQ5 if they are both being operated at the same plate voltage and the same dissipation? I didn't think they were all that different - maybe heavier duty plates on the 7189. I can see how the 7189 might have some advantages if the dissipation is pushed to the 12w limit or over, but at more reasonable levels does it really matter?
And even if it does result in 25% more power, how significant is that really. As I recall, twice as much power results in a 3db increase in volume. So 25% would equal an increase in only .75db and an equally minor increase in headroom at less than full power, which is how most of us actually listen. I'm inclined to think the difference would be almost unnoticeable.
It's actually more significant than you think because you have to consider that you are getting much more peak transient power,especially under dynamic conditions..You don't want to be pumping 430vdc into an EL84 because you will shorten the life of the tube and then you won't even make the rated power of an EL84 after a few months.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/02/16
Michael Samra, I disagree with what you said,"..You don't want to be pumping 430vdc into an EL84 because you will shorten the life of the tube and then you won't even make the rated power of an EL84 after a few months."
As long as the plate and screen dissipation are withing the rated spec, the tube will last just as long with the possible exception of an arc over due to the lower, 300v plate voltage rating.
I'm confused. You warn against "pumping 430v into an EL84", yet earlier both you and others agreed that plate dissipation is more important than plate voltage in determining how hard a tube is being run and, I would assume by extension, its expected lifespan. Both 7189 and 6BQ5 have dissipation ratings of 12 watts.
I don't doubt that 7189s "take abuse better", as Doug put it. My earlier response was: I don't doubt the 7189 responds better to "torture". Would you characterize running a tube rated for 12 watts plate dissipation at only 10 watts as "torture"?
I can see how the 7189 would be the clear choice if the dissipation is pushed to the 12 watt limit or over, but at more reasonable levels does it really matter?
Say both tubes are being operated at the same points - running at 430v on the plates but only 10 watts plate dissipation, is there any compelling reason to use a 7189?
Fla Charlie, you said, "Say both tubes are being operated at the same points - running at 430v on the plates but only 10 watts plate dissipation, is there any compelling reason to use a 7189?"
Yes, because the 7189 has a higher plate voltage rating (400v) so it would be less susceptible to arc over.
Would you characterize running a tube rated for 12 watts plate dissipation at only 10 watts as "torture"?No I would not and don't misunderstand,you can run EL84s in your S-5000 but keep in mind that we are running the full 425vdc across the tube being we are using fixed bias with a small value cathode resistor.
When you rebuild your Sherwood,did you put a 35uf film cap off the GZ34? That makes all the difference in the world for stability,deep bass,and increased power bandwidth.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 08/04/16
Concerning higher than max ratings, it is true that many vintage amps use 6BQ5/EL84s beyond their design maximum values. Some manufacturers went way beyond these rated voltages and even abused their tubes. The revered Acro 2020 is a great example of this abuse. They did not specify the heavy duty 7189; but, Sherwood did specify the "stronger" 7189.
It is also true that some 7189s were simply "specially selected" 6BQ5s. If you look inside the two types of tubes from the same manufacturer, I bet you cannot "see" any differences between the same mfr. 6BQ5/EL84 vs. 7189.
However, the 7189 is allegedly "up to the task" better than the "normal" 6BQ5/EL-84. Way back when tubes were made in the USA, UK and European plants, their testing and ruggedized proving techniques were confidential; but very significant. If they guaranteed a tube branded a 7189 was a 7189 even though it looked identical to their 6BQ5, their reputation was a big corporate concern. When they tested their tubes, quality control was definitely important. Learning from the industrial grade, mil-spec JAN and Western Electric manufacturing techniques, their proven techniques of "breaking-in" and "endurance testing" tubes were important ingredients. In development and mfr. testing, there is a high probability that a 6BQ5 worthy of enduring 400 Volts continuously, before being branded a 7189, was temperature tested as running cooler than their "stock" 6BQ5. If not, there must be special ingredients that more than meet the eyes. Their reputations depended on their reliability.
For those folks needing heavy duty ability, many manufacturers stepped up their techniques with the 7189A. And, yes, these can "look" identical to their generic 6BQ5s and EL-84s.
When Mike was talking about 25% more power, he must mean his "real time" durability testing revealed better amplifier power output results when using 7189s, not simply instantaneous results. Unless you have a huge stock of 6BQ5s, why push a 6BQ5 beyond their limits when the 7189 can easily endure more torture ?
IT,
In addition to the important points you raised, consider money and economies of scale. If, for whatever reason, reputable tube maker X had to produce 7189s, a quantity greater than that needed to fill a contract, etc., would be constructed, with the "excess" being sold as 6BQ5/EL84s.
It's both ethical and legal to deliver a product that exceeds specifications, as is the case of 7189 vs. 6BQ5. If doing so was profitable, you can bet that some tubes sold as 6BQ5 were really 7189. However, you can't count on any particular NOS 6BQ5 specimen as being a 7189. Date code research of 7189 production runs might provide an indication.
Eli D.
"When Mike was talking about 25% more power, he must mean his "real time" durability testing revealed better amplifier power output results when using 7189s, not simply instantaneous results. Unless you have a huge stock of 6BQ5s, why push a 6BQ5 beyond their limits when the 7189 can easily endure more torture?"
Actually, I believe I read a post by Mike in the archives that said he had measured his S-5000 at 1000 Hz, and I think it was a steady output, not a momentary peak. But, as I said, 25% only equates to an extra .75db, so really not much to be gained in sound level output. Measurable, yes, but probably unnoticeable in a typical listening situation.
I don't doubt the 7189 responds better to "torture". Would you characterize running a tube rated for 12 watts plate dissipation at only 10 watts as "torture"? If the tubes aren't being run over 12w dissipation my guess is that either tube should be fine. I do have quite a few NOS American BQ5s - mostly GE, RCA and Sylvania.
Like a jackass I sold two of my NOS/NIB quads of Syl 7189 black plates for 115.00 each quad..This was in 2006 tho..
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
I'd say you're probably right, that the plates of some well-made old stock 6BQ5/EL84's could withstand higher voltages. However, I would still be weary about running a 6BQ5/EL84 into that range (forget new production tubes). Unless otherwise specified through rigorous (published) testing, I would respect the data sheet.
And, it's not the be-all end-all.
The 7189 designation (and others like it) indicates some level of quality control and military/industrial purposing (like the 5881, 7025, Red Bank tubes, etc). Looking at the data, I *think* the 7189 was intended for use in ultra-linear circuits or setups with regulated screen supplies.
Also, we should consider the *7189A*. The 7189A by GE is much more interesting than a regular 7189. Plate goes up to 440V. The screen can operate at 400V. The maximum plate and screen dissipation are up a little. And, there are extra internal pin connections (1+2 for control grid, and 6+9 for the screen).
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
May your tubes be lively, warm, and long-lasting. Holy be thy heater.
If arcing does not occur, voltage (potential difference) is of little consequence. What really matters is power dissipation by the plate and, especially, the screen grid. Fundamentally, power is defined as I 2 R, but it can be expressed, via Ohm's Law, in other terms.Exceeding power dissipation limits, not mere volts, is what wrecks tubes.
See my remark about OS tubes labeled 6BQ5 being "truly" 7189.
You are making an "apples and oranges" error in comparing 7189A to 7189. The GE 7189A datasheet uses the Design Maximum system, while the TungSol 7189 datasheet uses the Design Center system. Increase Design Center numbers by approx. 10% to get their Design Maximum equivalents.
Fact , the current production EL84M, which is really Russian 6p14p-ev, is both very tough and good sounding. Perhaps the SCARCE and costly NOS is slightly better, but the reasonable price of well performing current production tubes (IMO) ends the discussion.
Eli D.
Edits: 08/03/16
Exactly! You cannot compare tube specs unless you know what rating system was used:
1. Design-center - the most conservative and most commonly used; the most tube-friendly of the three systems
2. Design-maximum - middle level, but needs to be heeded. Look at the specs for a 7591A for instance, they are design-maximum specs on the excellent Tung-Sol datasheet.
3. Absolute-maximum - rarely used, but I can promise you that running a tube close to the absolute maximum is almost always going to be trouble.
Also, the JJ EL84s are pretty stout tubes! They certainly have not trouble with 400 volts and they seem to work well and survive (I'm not a JJ fan, but this is a good tube!) though nothing approaches the EL-84M/6p14p-EV for ability to take a pounding. It is every bit as durable as any NOS IMHO.
So there are indeed current production tubes that are quite happy at 400 plus volts.
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