Whydoesn't my Super Reverb clone sound like a Super

[Andy Le Blanc]

With a long tail you cant set AC and DC balance at the same time. Push pull
needs the AC balance, get two meters, measure to ground. With DC you can
measure between the plates and adjust for "0".

Use the tone from a turner, scopes give you something to look at but can
lead to self deception if you don't know what your looking for, the reverb out
wont be a clean sin.

Try and correlate to your tube data by doing the math finding plate dissipation in watts.
tubes are a 20% device, just because one super sounds god-like at
one ma. setting .........

Make sure you get the power side of the unit sorted before you focus on the pre.
Most fellows (me too) learn amp design and repair in reverse.

35ma at say around 465v is only 16w, if your measuring for a pair of 6l6
thats really cold.

[Firestorm]

Does the problem occur both on the Normal channel and the Vibrato channel? A stock Normal channel shouldn't have enough signal to overdrive either the PI or the output tubes. But if the Vibrato channel is not wired stock, you can develop way too much gain in the reverb mix stage. There's a 470K in series to the reverb pot wiper AND a 220K in parallel to ground (effective R of about 150K). Together with the 3M3, these form a voltage divider that reduces the gain of the reverb mix amp almost to unity. If there's a mistake with the wiring or values here, the gain of that third stage will be plenty to overdrive the PI. Some people do that on purpose, by disconnecting the reverb pot wiper for example. Check that part of the circuit to make sure everything's Kosher.

[Bob-I]

Does the problem occur both on the Normal channel and the Vibrato channel? A stock Normal channel shouldn't have enough signal to overdrive either the PI or the output tubes. But if the Vibrato channel is not wired stock, you can develop way too much gain in the reverb mix stage. There's a 470K in series to the reverb pot wiper AND a 220K in parallel to ground (effective R of about 150K). Together with the 3M3, these form a voltage divider that reduces the gain of the reverb mix amp almost to unity. If there's a mistake with the wiring or values here, the gain of that third stage will be plenty to overdrive the PI. Some people do that on purpose, by disconnecting the reverb pot wiper for example. Check that part of the circuit to make sure everything's Kosher.

You may be onto something here. It does appear that this additional stage is the culprit. I'll pull it apart and check the wiring and values. I'll bet it's an OFU (operator f**k up)by the builder (me).

[Bob-I]

Does the problem occur both on the Normal channel and the Vibrato channel?

I forgot to answer. The answer is not really. The normal channel has far more bottom end and it's quite a bit cleaner.

[Firestorm]

Ahah (maybe). There should be a lot of bottom on the Normalk channel (if you like that sort of thing). The tone stack has 250pF, .1uF and .022uF, which makes it very bassy. Compare the other AB763 stacks (Twin, Deluxe, Pro) where the stack is 250pF, .1uF and .047uF. And the Normal channel couples through a .047 cap so there's plenty of bass response. Fender marketed this amp as a modern version of the 4x10 Bassman (fat chance) so they goosed the bass a bit. (Plus the 10" Utahs didn't have that much bottom to them to begin with).

[Bob-I]

Hokay......

I trhink I found it thx to everyone's responses.

1) The bias was way too low. Kicked it up to about 65% and it cleaned right up. I'm so used to working with Dumble's where low bias is the norm. Typically I run them at 33ma.

2) The reverb bypass resistor was 2.2M not 3.3M. Added a 1M in series and the whole charactor of the vibrato channel changed. Lots more bottom end, a lot more clean headroom and that classic Super Reverb tone. It starts to break up at about #4 on the volume control, and it's damn loud.

My guess is the 2.2M was what I had on hand when I built this amp almost 5 years ago. Since I haven't really had a use for this amp it's just been my practice amp, never turned above #1.

So another mod I just did was to lower the bypass cap on V2A. The 25uF was just too much bottom end so I dropped it to .68uF. Really tightened up the amp.

[Bob-I]

A few more things for information.

I googled and found a sine wave generator on the interweb. I also found a schematic with AC voltages on it beginning with a 20mVAC input so I was able to check the various stages. It comes out very close, close enough to blame discrepencies on tolerances. For example PI output shows 5.5VAC and 5.9VAC on the 2 sides. I'm showing just under 5 on both.

Calculating output power, I'm getting 35.9Watts before clipping. I'm happy with that!!

This has been both fun and frustrating. Amazing how simple the fix really was (crossing fingers)

[Firestorm]

Interesting. With a 2M2 in there instead of the 3M3 (the AA763 circuit used 4M7!), the signal into the third stage goes up by close to 50%. High frequencies are also affected by the relationship between that resistor and the 10pF bypass cap around it.

[Bob-I]

Interesting. With a 2M2 in there instead of the 3M3 (the AA763 circuit used 4M7!), the signal into the third stage goes up by close to 50%. High frequencies are also affected by the relationship between that resistor and the 10pF bypass cap around it.

It was that difference that made me think the 2.2M wasn't an issue.

The amp's sounding GREAT now.