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Old 14th September 2012, 04:50 AM   #21
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Join Date: Dec 2011
I changed all the Cathode Caps for V1 and voila'. No more distortion. Just sweep amp Sound!
Just to be fussy. The trem has a small amount of low end tick especially when fast. Any thoughts on this?
Thanks so much for the help!
Enzo- Your words on fixing the amp without blindly replacing random parts Rings So True. I replaced the Filtering caps for nothing. Well I guess they are solid for a couple more decades.
Thanks so much Comrades!!!!!!!
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Old 14th September 2012, 06:00 AM   #22
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Location: DC
I replaced the Filtering caps for nothing.
Measure the old ones for leakage under full voltage and you'll know if this is true.

The shelf life on the twistloks and cardboard tubulars was 10 years, i.e. finished over 35 years ago.

I have 4 Ampeg (1963-73) amps in this room. All needed new filter caps. I measured the multisection caps on a Tel Ohmike if the amp wasn't humming too bad. Leaky. They all work much better with new filter caps.
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Old 14th September 2012, 04:02 PM   #23
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What is your practice for measuring? Check negative end for voltage or lift one electrode and test for capacitance. I am just wondering.
Also The new JJ 7591-S Power Tubes are a bit funky. When the amp is in the last 75% of the volume control and the trem is on fast I see some pulsating in one of the power tubes?????????? And only in 1 of em'. The other tube upon powering up has a bright flash inside the tube near the base. Bunk tubes???? They are matched and I must say that they sound pretty good, ills and all.
Also the trem tick I described is directly related to this pulsing plate/heater/?? in the first power tube.
What the Heck is that?
When comparing the JJ and the ampeg 7591s data sheets it seems the jj is rated for not nearly as much voltage. Could this design need more headroom in the power tubes???
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Old 14th September 2012, 07:00 PM   #24
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I use a Sprague Tel Ohmike tester to check caps for leakage under full voltage. It puts a resistor in series with the + terminal and measures voltage drop across the resistor, i.e. current going through it. Since the cap is not connected to anything else, any current is leakage.

Same procedure for coupling caps, which should have no or almost NO leakage.

Look it up: Checking capacitors for leakage.

Trem ticking may be caused by the oscillator signal getting back into the signal line. Did you replace the power supply decoupling capacitor on the trem supply line? Might be part of the multi section cap in that amp.

Did you replace the cathode bypass cap on the oscillator tube? I would. I'd look at the coupling caps too.

It is hard to chase this problem over the internet on a theoretical basis.

Enzo has a real good point about randomly changing stuff, words of wisdom there. But parts do go bad and some, such as filter caps, have limited useful lives.

Based on long experience fixing 50s and 60s electronics, I will stick with my earlier suggestion that many of the coupling caps types used in Ampegs are notorious leakers, but not all. If you see any red Tiny Chiefs or green Tropicaps, replace! [But don't mess with the phase shifting caps in the oscillator yet if it is oscillating, try to fix the tick first.] The above-mentioned caps are leaky as NOS in original packaging, 9 times out of 10!

Be careful that you don't mess the amp up more working on it though! Old things get fragile!

70s Ampegs featured a circuit to add ticking on the trem...they called it Afterbeat!

JJ tubes are fine. Don't worry about the light show.
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