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Old 19th February 2012, 10:55 PM   #1
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Default Selecting the correct output transformer/primary impedance

Hey everyone, there have been a couple questions on this topic but non that quite answered my problem. I designed a bass amp with 4 7027A power tubes using a 100 Watt JCM-800 style output transformer (1.9K primary) and it sounds great. I was dealing with a pretty reputable transformer manufacturer and got a new bass specific transformer that has a 1.5K primary. I was kinda skeptical but they said it was all fine. I know that's not a huge amount of difference but since I've installed it I've had a couple blown fuses and even a power tube failure. All of this could be due to usage but I'm wondering if any of this could be attributed to the lower primary of the output transformer??? My 4 7027As are running at:

Plate: 485 volts
Annode Current: 45mA

Doing calculations from other strings I come up with somewhere around 2.7K for a "suggested" primary...please let me know if anyone has any tips/insights. Thanks.
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Old 20th February 2012, 12:38 AM   #2
dgta is offline dgta  United States
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Cathode or fixed bias?
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Old 20th February 2012, 03:02 AM   #3
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Fixed. Can't remember the voltage off the top of my head but I'm pretty sure it was right around -30.
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Old 20th February 2012, 05:43 AM   #4
dgta is offline dgta  United States
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A couple of thoughts. The datasheet (RCA) I looked at does suggest that your plate voltage/bias/current are pretty much in line with a published operating point. Assuming this is a normal pentode AB1 stage, not ultralinear. However, the suggested load at that operating point is 6k for a pair of tubes. So yes I would think your primary impedance is low for a quad.

Nevertheless, I don't see how that would either blow a tube or a fuse. Without the schematic and the full story as to when/how/etc. those incidents happened, it's hard to say.

When you say "bass" do you mean this is a guitar amp, or a hifi amp restricted to low frequency use? Do you overload it and clip regularly?
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Old 20th February 2012, 12:48 PM   #5
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So yes, sorry - this is a bass guitar amplifier and it does get driven pretty hard so I would assume it is getting overloaded/clipped regularly (we can get square waves as an output from a sine wave input at certain settings) but I'm not sure if that's completely power section overloading or more from the pre-amp. In general, how does an OT primary effect the overall bandwidth? I can't seem to find enough info on the relationship between output tubes/output transformers although I'm sure there's plenty of it around...
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Old 20th February 2012, 03:30 PM   #6
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Your asking questions that entire books are written on. However, here's a couple of things to consider.
First, it is not so much the impedance of an output transformer that affects power bandwidth as it is how it is designed. That is, it's inductance, winding capacitance, core size and material, etc. It is true that higher impedance transformers represent greater design challenges regarding high frequency performance, but the point is, the tubes know none of this. They only know the load presented based on the actual load connected to the amplifier, and how well (or not) the OPT couples that load to the tubes. Assuming the load remains a constant, as the coupling becomes compromised due to the transformer's lack of performance at certain frequencies, then the impedance presented to the tubes will start deviate from optimum, further compromising overall performance.

Therefore, you don't adjust the plate-to-plate load to optimize a given output stage for a given frequency range. However, if the stated secondary load varies from its nominal value at a certain range of frequencies (as loud speakers do), then you may very well adjust the OPT impedance from "optimum" so that the actual load presented at the frequencies of interest still reflects the optimum load for the tubes -- again, because the optimum load for the tubes is the optimum load for the tubes, regardless of frequency. Therefore, the alteration of impedance is not due to the needs of the tubes at certain frequencies, but due to changes in the load at certain frequencies.

For classic operation of 6L6 class tubes (of which the 7027 is a member) in production amplifier applications (fixed bias, 450-475 plate AND screen voltage), then the optimum load is 3800 ohms for a push-pull pair, or 1900 ohms for a PPP setup.

While it's unlikely that a 1500 ohm transformer would blow fuses (unless they were poorly specified), such a load (assuming that he secondary load actually reflects that number to the tubes) would definitely be harder on the tubes. Assuming the power supply can hold up (and the fuse doesn't blow!), such a load is asking the tubes to pass significantly more peak currents, which will clearly take a toll on their life.

I hope this helps shed some light on this topic for you.

Dave
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Old 20th February 2012, 04:47 PM   #7
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Yes, this definitely helps...I really appreciate it. I think I may have been misleading in my original post; the tubes and fuses weren't going out at power up or after a couple minutes of playing. It was after maybe 15-20 hours worth of hard usage. And actually, the fuses only blew after one power tube "broke down" and internally shorted the bias voltage, which made that side of tubes glow red. After that another fuse went out randomly without the tubes failing. The tubes and fuses both had seen some action so it's not completely unexpected that they failed but it was just strange that all of the sudden there were failures in the amp and I was wondering if the 1.5K was compromising tube life, which it sounds like it was. OK, so what do you think about putting 200 ohm power resistors between the output transformer leads and plates? That would adjust the plate-to-plate load to "optimum" 1.9K but would it have a negative impact on the output transformer's performance? I've seen some amps do this before but it was more like 47 ohms rather than 200. Thanks again.
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Old 20th February 2012, 09:57 PM   #8
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If I have a "5K" transformer, is that plate to plate, or plate to center tap?
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Old 20th February 2012, 11:54 PM   #9
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You generally would not want to put resistors of that size in the plate leads as it would reduce usable power output, and dissipate the lost power as significant heat in the resistors.
Yes, this approach will (obviously) limit peak currents in the tubes, but in this case, the cure is likely worse than the disease.

When small resistors (i.e. 47 ohms) are used in the plate leads, it is to suppress any parasitic oscillation tendencies in the output stage -- they are not there to help "match" the tubes to the primary impedance of the transformer.

Push-pull transformers have their impedance rated from plate-to-plate.

Dave
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Old 21st February 2012, 12:21 AM   #10
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Ahh, yeah - that makes sense. Thanks for all your help!
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