Super Reverb funny business

[phydauex]

Hey All.

currently working on a terribly maintained SF Super Reverb that I've noticed some strange values in the reverb circuit.
While doing some inspection and replacing broken wires and burnt resistors, I came across a funny resistor on the reverb driver tube. It had a 470 ohm resistor on the cathode, instead of the usual 2.2k. It caught my eye first because it was cracked, but then I realized it was 470. It started looking at all my Fender schematics and everything had 2.2k. So I replaced it and stuck a new tube in there and watched the tube start to replate after a few moments so I shut it down. I was measuring about 65 volts on the cathode as I was turning it off.

Now, after doing some more looking, I'm seeing some of the master volume models used a 470 resistor with no cap bypass. I was a little confused at first, but Fender obviously used a different transformer on these amps? I happen to have a replacement reverb trans and it measures @1k on the primary. The one in the Super measures 1.9k.

Has anyone run into this before? I'm assuming that Fender used whatever they could get without keeping true to the schematics.

I've already redone the bias balance to a proper bias setup, changed the PI to 100k/82k and 220k on the bias feed. New 1.5k grids and 470's on the screens. The amp sounds real good in the Normal channel, but distorted and weak in the reverb, as discussed above. Also snipped the snubbers on the power tubes also.
Matt

[Chuck H]

Are you saying the reverb is weak, or that the reverb channel is weak?

Is the reverb driver cathode resistor bypassed with a cap or not? If yes I suspect the cap is shorted.

I don't know about the 1k vs. 1.9k measurements. Are these DC readings?

The redplate tube is obviously too little bias voltage. The question is why. With a 2.2k resistor on the cathode you should have enough bias to keep the tube from red plating unless the impedance on the reverb transformer is tragically low. This is why I susect a shoted bypass cap or a miswire.

[phydauex]

Hey Chuck,

Let me back up: I had the tank unplugged and was working on the power section. Got that looking good and started to inspect the front end. I plugged into the normal channel and it sounded great, then moved on to the reverb channel. Before I plugged the tank in, I spied the cathode resistor that was cracked. I pulled it and was scratching my head because it was a 470 ohm, not a 2.2k. I also looked at the driver tube and it was a 12AX7, so I put in a 12AT and then replaced the broken resistor with a 2.2k. and then plugged in the tank. Turned it on, and tube started to redplate. I also heard something loose in the tank bag and opened it up to find one of the springs had broken off both ends and was lying up against the tank housing. When measuring the resistance on the driver transformer, nothing appears to be shorted, and the primary measures about double from the replacement one I have lying around. I measured the trans disconnected from the circuit.

Like I said this thing was a mess. I think it supposed to have a 470 in there as it looked like the stock part, it hadn't been resoldered and the board looked to be all original. So I'm going to take the 2.2k out and put a 470 in as it appears to have come from Fender that way. I just haven't seen one before with that value on the driver tube, but that wouldn't be the first time.

The resistor was also not bypassed with a cap. So the bad cap idea isn't applicable. I didn't know Fender used that setup before I was digging into late model SF schematics this evening and came across some that show a 470 with no bypass cap on the reverb driver. So this one must have been a transitional build.

Matt

[Chuck H]

So... Is the reverb channel weak, or just the reverb?

Perhaps the tank is shorted, presenting a heavy load on the tube. Perhaps the reverb tranny is bad, presenting a heavy load on the tube. Perhaps the tube is bad, so any load seems like a heavy one. In any case, reducing the value of the cathode resistor is only going to decrease the bias voltage. For where your at right now that won't fix anything.

Try replacing the tube first. That's the easiest thing to do. Measuring ohms on the tank is probably a fruitless effort. I don't if the Accutronics site (now run by Beldon) still shows the resistances of different tanks, but you could check. Testing the reverb transformer would be like testing any OT. You'd need a low voltage AC source. Like a wall wart or a burly signal generator.

[Enzo]

Or RG's transformer tester, simple to make and use. Go to Geofex.com.

If you think they used a different transformer, look on the schematic. Aren;t the transformer numbers on it?

That reverb drive circuit will sit there running all day without a reverb pan attached.

If the tube is drawing excess current, make sure there is no DC on its grids from leaky coupling caps from prior stages. And make sure the cathode is not shorted to some other element, like a plate. Inside the tube, or on the tube socket.

65v across 470 ohms is 138ma.

Find and use the proper schematic for YOUR amp. Just about every Fender schematic is floating around, so there is no reason to look at other Fender drawings and try to equate them. It isn;t about the parts, it is about the circuit. We can look at one resistor and note it is different from some other amp, but what about the rest of the context it is in?

[g1]

Does it have the cap across the primary of the reverb transformer? (500pf 5KV?) Quite rare but they can short and cause problems.

[phydauex]

Well, looks like it was a bum tube. I just grabbed a used one off my bench to put in there it was toast. I popped in a fresh JAN 12AT and the cathode sits at 9.4v, which gives it (9.4v/2200=4ma). looks good and stable. That will teach me to keep bad tubes I pull out of repairs. The voltage on the plate is a little hot (445v), but other than that it's working alright.

Enzo --- This amp is either a AA1069 or AA270, but since the tube chart was ripped I couldn't tell which other than matching up parts/layout etc.. What threw me a bit was seeing an un-bypassed 470 on the cathode of the driver. Because that doesn't match up with any of the AA series schematics, But does show up on the master volume series, which was next after this one. Whatever, it runs good with the 2.2k in there instead and I think a little cooler which is fine.

G-one - - - It doesn't have the 500pf across the primary, but it does have the .002 cap on the secondary of the reverb tra. That's ok.

[Jazz P Bass]

That 470 on the cathode was used on the Mid '70's Twin Reverb.
The 100 watt amp with the Master Volume.

[Ronsonic]

The theory is that the guys at Fender figured they could use a 470R and get the same gain as the 2K2 and save the pennies that a bypass cap would cost. It works but it beats up the driver tube pretty good. I generally replace it with the larger resistor and cap.

[Chuck H]

Right. I was going to suggest adding the cap too. More gain even though the bias remains the same. This provided the reverb doesn't distort. Since the reverb tranny reads different on DCR there may be other differences too. But, reverb pans usually sound best when pushed so the bypass cap is worth a try. Nothing to lose.