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Old 12th November 2012, 02:55 PM   #1
wicked1 is offline wicked1  United States
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Default Tube Frequency Response

I've just received a bunch of 12sn7's to retube my aikido phono amp. They're what I think are called coin base. These new ones are JAN tubes. I replaced some RCA tubes which appear to have the exact same construction. Same plates, same heaters, same mica supports, etc.
BUT..... these new tubes do not have bass! It's not subtle. It's like a filter has been put on my system. They get down to the kick drum level, but none of the rumbly low bass, or decay on bass notes, etc are there.
I put the old tubes back in, and it sounds good again. Other than the distortion when I turn it up... (Old tubes are several years old.. Volume knob needs to be up at 75%. New tubes, volume needs to be at about 25% for the same loudness, and there is no noticeable distortion)

I don't understand what happened to the bass, though! I wouldn't think there would be such a large variation in the frequency response in the tubes.
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Old 12th November 2012, 03:16 PM   #2
DF96 is offline DF96  England
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As all valves work down to DC the most likely change is an increase in treble which is then interpreted as a reduction in bass. However, in some RIAA amps the exact network response can be affected by the valve anode impedance, which can increase with age as the transconductance reduces. Other things being equal, it is more likely that the new valves give the correct response but you had become used to the old sound. This assumes that the network values were carefully calculated and not, as some seem to do, adjusted 'by ear'.
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Old 12th November 2012, 03:24 PM   #3
wicked1 is offline wicked1  United States
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Thanks! that was my assumption as well. I'm listening to the old tubes now, and there is definitely less information in the highs.
My riaa resistors and caps were calculated based on my tubes via a formula in the aikido phono instructions. I'm tempted to adjust them by ear now, but I should probably let my ears re-adjust, instead.

-I guess if it keeps bothering me I could easily change the loading on the input to my phono amp, to roll off the highs a little.

Last edited by wicked1; 12th November 2012 at 03:26 PM.
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Old 12th November 2012, 03:34 PM   #4
DF96 is offline DF96  England
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Double-check your calculations, just in case you made a mistake. Apart from that, I think you should give it a while and see if your ears get used to it, as you say.
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Old 12th November 2012, 04:01 PM   #5
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Hi!

You should check the frequency response with a signal generator and a scope. Only then you really know what's going on and what to change if necessary. For phonostages i recommend to measure the linearity of the circuit by removing the RIAA components. Replace it with a 1:10 voltage divider to avoid overload.

If the gain stages work linear, then add the RIAA circuit again and check it with a iRIAA network at the input

If you don't have a scope, borrow one or get a used one

Best regards

Thomas
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Old 12th November 2012, 04:44 PM   #6
kevinkr is offline kevinkr  United States
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Do you have tube tester? I would check to see whether the transconductance of these new tubes is in the expected range as a way of verifying that they are actually 12SN7.

Another thought would be to build a simple amplifier circuit and measure the gain.

Good 6SN7/12SN7 should last for a very long time at moderate currents - I've had to replace very few.

As a quick simple check measure the frequency response of the phono stage with original tubes and then the new ones. Gain at 20Hz should be approximately 20dB higher than at 1kHz, also measure at 20kHz and verify the gain is about 20dB less than at 1kHz. ** An easy way to do this with a scope would be to set it up at 1kHz for 200mVpp output, at 20Hz you would than have approximately 2Vpp, and at 20kHz 20mVpp - this should be within the range of accuracy of your scope and eyes. All of this presupposes you have a decent scope and sine wave generator. Set the scope so that 4 vertical divisions are used for each amplitude measurement, do not adjust the generator once set.

Note:
** These are approximate values, refer to the RIAA curve for the exact numbers, but with a scope the resolution is insufficient to make this level of accuracy necessary.
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Old 12th November 2012, 04:52 PM   #7
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There is something funny going on here. There should not be a three to one variation in apparent gain between tubes of the same type. It sounds to me like you used to have 6SN7 tubes in there and you have replaced them with 12SN7 types so the heaters are being severely under run. If it is not that then the tubes you have been sold are suspect.

Cheers

Ian
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Old 12th November 2012, 05:30 PM   #8
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Quote:
These new ones are JAN tubes.
Are they JAN Sylvania or Philips ECG (green or blue ink)? These guys were notorious for stuffing the envelope with whatever guts they had to fulfill whatever contract they were bound by at the end of the vacuum tube era. The tube was tested to work in whatever equipment that the contract stated, but often bore no similarity to the type number stamped on the glass. This was not an issue until the tubes appeared on the surplus market.

There are some 6BG6GA's that actually contain 7027A guts. These are good for far more power than a 6BG6 could ever make. There are some 6B4G's (a DHT) that contain 6AV5 guts (a an indirectly heated sweep tube pentode). I have seen two different "6CW5's" that contained unusual guts. Both melted quickly in a Tubelab SPP amp that uses 6CW5's.

Your tubes could contain a mismatched set of components that may not have a Mu near 20, but may have worked OK in whatever military equimpent specified in the contract.
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Old 13th November 2012, 01:25 PM   #9
wicked1 is offline wicked1  United States
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Thanks everyone.. KevKR, those are good instructions for testing. I have a very old scope and an analog modular synthesizer. I believe those will do. The scope isn't exactly calibrated, but as you said, it should be good enough to see the difference.
I know they're 12sn7's and replacing 12sn7's. There is actually one pair of 12sl7's in the mix, but I haven't replaced those yet. (Aikido.. 8 tubes for the phono amp alone!)
And Tubelab, they are green ink. Date stamp says 87, so definitely beyond the end of the vacuum tube era.

I also have a 12sn7 preamp in my basement, so I put these tubes in there... They were light on bass there too. Not quite as bad, but that's only one gain stage, vs the two in the phono. I'm also beginning to doubt post #2 and 3, saying maybe there is just more treble now. There is some more, but unfortunately the bass issue is like a relatively sharp filter. I turned up my subwoofers, but there is simply nothing there for them to amplify!
Well, I didn't want to get quite this deep in to the project, but I think you are all on the right track, and the only way to proceed w/out driving myself crazy is to use some test equipment.
(or just buy a different batch of 12sn7's I'm pretty confident at this point, that it is the tubes. Buying tubes on ebay is like going to vegas)
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