Ampeg G12: New JJ ECC83S Kills the Tremolo

[Russ_L]

Hi all, new to the site and have a question. I have a great running 1968 Gemini 1 (G-12) with original tubes except for the 7199 PI. It has superb reverb and tremolo. After the original 7199 died, I decided that after 55 years I’ll splurge for a new set of tubes, so I put in all new JJ’s. Sounds great except the tremolo went to almost nothing. I found that if I put back an original “Ampeg” 12AX7 in V7, tremolo returns to normal. I have three new JJ ECC83S tubes and all three weaken the tremolo to almost nothing when installed in V7. Then I put back an original “Ampeg” 12AX7 inV7, tremolo is back strong. Also, with the JJ ECC83S tubes installed in V7, I get a noticeable thumping with the tremolo on but very weak tremolo. Any Comments? Suggestions?

Note that the amp is all original except I just replaced the two multi section filter caps, made up a new rectifier with four 1N4007 diodes and added a new power cord. The original 7199 tube started to hum and was replaced with a 6GH8A+Adapter.

[martin manning]

In theory an ECC83S should replace a 12AX7, but it's possible the JJ's are a little weak, and a strong tube is needed to get the oscillator to start and run reliably. If the old tube works well, there is no reason not to use it. You could check for DC between the three 0.05u phase shift caps, and replace them if they are leaky.

[Phil_S]

I suspect the new JJ tubes simply don't have enough gain to trigger the desired oscillation. I can't remember the source, but I clearly remember reading that a good strong 12AX7 (ECC83) is required. That old "Ampeg" tube probably has plenty of life left. it works. Just keep using it.

[Stevem]

That’s odd!
v7 is also part of the reverb circuit, so does the new tubes also produce less reverb then the ones you had in the Amp?

A indicator of how strong each to is can be seen by measuring the voltage across the 3.3K cathode resistor.
The stronger the tube the higher the voltage will be.

You might also want to jump in another 25uf bypass cap across the one that’s there.

If that’s original and has drifted down to less then 15uf then the gain of your new tubes maybe just below what’s needed to really drive the oscillator as it needs .

[Russ_L]

-Reverb is great with both Ampeg and JJ tubes.
-Clipping another 25uF cap across the existing bypass cap resulted in no change to the weak tremolo with the JJ tube.
-Voltage across the V7b cathode resistor is 0.8V for both Ampeg and JJ tubes (tremolo ON). In fact all the voltages from each V7 pin to ground are just about identical for the Ampeg and JJ tubes.

Thanks...

[Russ_L]

As a follow up, I replaced the JJ's with NOS GE long plate 12AX7's. Amp runs great, beautiful tremolo. Just a heads up if troubleshooting an Ampeg, don't assume a bug or circuit problem just because there are new tubes, JJ's in this case...