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output tube blocking distortion problem
output tube blocking distortion problem
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Old 8th September 2009, 01:41 PM   #1
voivodata is offline voivodata  United States
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Default output tube blocking distortion problem

My custom guitar tube amp’s PI and output section is the same as most old Fenders but uses EL34 tubes…here’s a schematic: http://www.schematicheaven.com/fende...b763_schem.pdf . I’ve had the amp for about two years and it played great. It was very loud and some minor output tube clipping could be heard only when the Master Volume was set above 9. Then the amp began gradually to go into distortion earlier and earlier, to a point where it couldn’t be played with the volume set at more than 2 or 3 …after that it would go into a really nasty breakup. From descriptions I’ve read on the net I believe it’s something called “blocking distortion”.
So I opened up the amp and played a bit with the fixed bias trim pot. Biasing the amp colder (less negative grid voltage) resulted in some insignificant improvement – it would distort say at 3.5 instead of 3.
Then, since I’ve never had changed the tubes (RFT EL34), I decided that they’ve gone bad, replaced them with JJ/Tesla EL34L’s and biased the amp as per manufacturer’s specs. There was some improvement, but still the amp breaks up much earlier than before - around 5 or so.
I found a nice article on blocking distortion solutions here http://www.aikenamps.com/ but I’m hesitant to implement any of those because they are related to changes in design, and the amp was obviously well designed since it didn’t have such problems before. I suspect that the problem lies either with the tube type, tube biasing, or another part defecting. So here are my questions:

1. How important is tube type and quality? Is it possible that the old stock RFT (before going bad) were so much better that the JJ/Tesla’s so as to go into breakup much later and thus provide almost twice “clean” power?

2. Could this early breakup be caused by a defective resistor. All resistors in the output and PI sections are really old carbon comp type. They measure correct values in ohms, but I’m not sure if this is the only criteria. Is it worth replacing them, and where should I start? Which resistors are likely to cause early distortion.

3. Can such problem be caused by fault in the output transformer? (overall the amp is as loud as before, it only distorts much earlier)

4. Have you any other ideas or suggestions on how to approach this problem?

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Old 8th September 2009, 04:57 PM   #2
m6tt is offline m6tt  United States
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I'd replace the carbon comps for sure, and check voltages at the PI plates. Some carbon comps will only measure wrong warm...occasionally they will "warm" up to a few megohms. That might be causing either a bias point shift in the PI or some strange grid impedance problem for the driver. Make sure your coupling caps aren't leaking DC too, perhaps replace them.

Are the EL34 cathode biased?

Have you tried swapping around preamp tubes?

Have you checked for incidental shorts (if only all wires were solid core...)?

For good measure, you could post a schematic or voltage readings.
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Old 8th September 2009, 05:45 PM   #3
Wavebourn is offline Wavebourn  United States
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output tube blocking distortion problem
Did you try to replace rectifier tube?
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Old 8th September 2009, 09:37 PM   #4
Rod Coleman is offline Rod Coleman  United Kingdom
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output tube blocking distortion problem
Fenders, old and new, have problems like this very often now.

The problem is usually the power supply capacitors, which are electrolytic type. The old ones (from 1950s to 1970s) might last as much as 25 years, or even more, but they sound really bad by then.

The newer Fenders, including re-issues are a lot worse. They use Illinois Capacitor (I-C) axial electrolytics, which seem to show very bad lifetime. I have fixed 10 or more recently - one example was made in 2005, and needed new capacitors to stop it howling and squealing.

Don't try to replace them with more IC axial (TTA series) since the quality seems so bad.

Choose replacements with care though. Measure the HT voltage. If it's 470V or less on the first capacitor, use a Panasonic TSUP or TSHA 500V 47uF as the first cap after the rectifier. If it's 475V or more, wire two 100uF 400V TSUP/TSHA in series for the first cap. Put a 100K 5W wirewound across each capacitor to balance the voltages across each cap. Use the same cap or series combination for the second (screengrid cap).

! Caution ! Do not leave out the balancing resistors, or a capacitor might explode.

Use Panasonic or Samwha HJ series for the other preamp caps, usually 22uF/500V.

You must change ALL the electrolytic caps in the amp, or the problem may persist. Use Panasonic ECA series 16V 22uF for the 12AX7 cathode caps.

When you have changed all the caps, put a temporary resistor in series with the rectifier (eg 50K 10W), remove all the valves, and switch on. check the B+ is at least 200V. Remove the test resistor.

Fenders rebuilt this way sound splendid!
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Old 9th September 2009, 12:45 PM   #5
voivodata is offline voivodata  United States
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Default problem solved

Hi !

Thanks for the tip on resistors. I checked again all carbon comps and .... BINGO... one of the anode resistors of the phase inverter was shot ...no resistance at all. Replaced both anode resistors and the amp is back in business ...loud as hell ....the JJ/Tesla EL34's actually sound pretty good. I will be replacing now all cc resistors in the amp, or at least where high voltages are running.

Thank you all for the replies.

Since the amp is opened anyway I will look into some mods as suggested to make the amp more reliable.

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Old 9th September 2009, 04:23 PM   #6
m6tt is offline m6tt  United States
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If you live in a humid place, carbon comps are terrible. They absorb moisture after a decade or so and start hissing and popping. You'll spend hours looking for grounding problems or high-voltage arcs when it's just a bad 25c resistor. Anything vintage in the pacific northwest is apparently guilty of this issue!

It's amazing how many musicians assume all tube amps hiss because of vintage guitar amps with CC plate resistors. Especially at V1.
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