'73 Super reverb breakup point

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'73 Super reverb breakup point

Postby RaistMagus on Sat Jul 22, 2017 8:12 pm

Hi all, I'm looking for some advice on my super reverb. It's a '73 model with the CTS Alnico speakers and a power tube bias mod. I play a strat with low output pickups through it. It has undergone a re-cap and general check-up and it should be in great playing condition.

I find that it starts distorting too early, after 3 on the volume dial. It is party because of the worn out speakers, but I found out that the signal is being distorted a lot after the PI tube when the volume goes above 3. I rarely go above 4 as its sound becomes too mushy. So on 1-2 it sounds lifeless because the tubes aren't working hard and after 4 it becomes too dirty and mushy. That's a narrow range of operation :(

At what point on the volume dial do your supers distort? Do you think mine is normal? Has any of you checked the signal with an oscilloscope after the PI tube?
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Re: '73 Super reverb breakup point

Postby Saturnine10 on Mon Jul 24, 2017 4:00 am

I have never owned or played a SR but I would think you'd be hitting natural tube distortion well above 3 on the vol... Like cranked at 7-10, not 3.. Something must be astray!
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Re: '73 Super reverb breakup point

Postby stratele52 on Mon Jul 24, 2017 11:12 am

Saturnne 10 write;
"....It has undergone a re-cap and general check-up and it should be in great playing condition. "


On both channel ?
Amp is not normal.
Are the check up is from a qualified tech ?
If yes, he should have a answer.

1- Tubes must be good ; tubes tester are not reliable
2- How are the voltages ?
3- Check Phase Inverter resistors
4- Output transformer could be bad.
5- Check with other speakers
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Re: '73 Super reverb breakup point

Postby RaistMagus on Thu Jul 27, 2017 12:19 pm

Thank you guys. I'll look into your suggestions!
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Re: '73 Super reverb breakup point

Postby tubeswell on Mon Aug 07, 2017 3:28 pm

I've recently serviced a few Super Reverbs including a 73 and a 67.

Break up point varied but was generally after about '7'. Factors affecting this were how worn the tubes were (I put a fresh set of 6L6s and a new GZ34 in the 67 and the B+ shot up from about 420 to about 470 with the tubes idling at 70%, and the power, bandwidth and cleans improved heaps).

The 73 was clean, the B+ was about 440, but the bias was very cold (18.5mA) with the stock SF AB270 bias circuit.

Both of them needed new filter caps and bias supply caps. They both needed new trem kits.

The 67 had a leaky coupling cap on the Vibrato channel at V1B that was leaking 1.6VDC and causing a feeble signal. Some dickhead had also subbed the 0.1uF coupling cap after the reverb mixing stage in the vibrato channel for a .002uF! This had the effect of killing the reverb when the channels were jumpered, because the signal from the normal channel was charging up this small cap to the point where it wouldn't pass a signal. Putting a 0.1uF back in cured that.

These amps do interesting things when you jumper the channels, because of the uneven number of stages in each channel, which introduces an element of out of phase signal-cancellation depending on the individual volume and tone settings in each channel. This adds to the palette of sounds.
He who dies with the most tubes... wins.
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