Super Reverb troubleshooting...weak power output

[TubeGeek]

Hello, it has been a while since I have posted.  I still pop by now and then to check out what's happening. 


I have a super reverb on the bench that I am struggling with.  I could write a book on what I have done so far in troubleshooting.


The problem is the amp does not have the expected power.  Everything seems to be working as expected but the output sounds weak, under-powered and thin.  Normally on a super reverb I expect to get clean tones up until 4 or 5 on the vol pot.  After that overdrive sets in...By the time I am at 7 or 8, the amp is pretty loud.  This amp barely has any clean headroom.  It breaks up after 3 and does not have the loudness and substance it should.


The amp came to me from another tech that gave up on fixing it.  I accepted it because I was trying to help my other tech buddy out.  The first thing I noticed is that it had a replacement Hammond 274X power transformer.  The previous tech had replaced all the filter caps with F&T caps.  Everything looks good.


I measured voltages and they were around 50-60 volts higher than I expected.  Upon looking into it further, I noticed the replacement PT did have a bit higher secondary voltage than it should.  The plate voltage was 507 VDC. 


While looking at the PT specs I figured that the weak, compressed output power could be due to the hammond PT being limited in the mA it could provide being only 120mA.


I recommended to the owner to let me order a more correct PT and a set of power tubes. The tubes in it were mis matched and not functioning at all.  I ordered the parts...few weeks later they show up and I install them.


I plug back in and the amp still sounds the same.  Not enough power.  I'd say it sounds like a 20 watt amp is all. 


The voltages are now spot on.  Bias is good.  Speakers are good.  I wired up another harness so I could connect to another 4x10" cab...still sounds like it is half powered.


I swear I have checked everything I can think of.


I luckily had another OT for a super reverb on the shelf.  I substituted that OT in place of the original, still sounded the same. Unless my spare is faulty too? 


I replaced all the tubes, still the same.


Replaced screen grid resistors. No improvement.


I could sure use a hand with this one.  Any ideas?

[shooter]

I just sent one out the door.  had other issues, but once addressed I found the same "lack of power".  I installed 1ohm R's on the cathodes and found the tubes were only putting out ~ 8W, bumped bias til they were ~ 12W each and got a BIG sound bump.  I left it, talked with customer and he was happy, even said, "Man, that's loud"  I smiled.  I left it still cool biased because there was no tube history.
anyway
fwiw

[pdf64]

The rather low V3 cathode voltage may be hamstringing the vib channel somewhat. Id investigate that, eg bad cathode resistor or bypass cap?
But the normal channel should be unaffected by that, may be noticeably beefier at high volume settings?
Have you got a dummy load etc that you can use to check the actual output power with?

[sluckey]

The rather low V3 cathode voltage may be hamstringing the vib channel somewhat. Id investigate that, eg bad cathode resistor or bypass cap?

V3 is the reverb driver. Later SRs used much smaller cathode resistors so the voltage would run lower. At any rate, even if you pull the reverb driver tube the vib channel would not be hamstrung.

[TubeGeek]

The low cathode voltage there alarmed me too when I took the measurement.  Turns out that a 12ax7 is installed in that position.  As soon as I swap to a 12at7, the voltages come within expectations.  You can see the adjustment on the voltage sheet I posted (8.31).


At this point I feel it is a process of elimination.


First up...replacing the PI coupling caps.

[SILVERGUN]

Don't forget about the possibility of conductive board syndrome.
Drove me to grown man tears once...I'll never let it get me again.


If all DC voltages are correct and then you go through and inject an AC signal at each stage you can eventually rule it in or out.

[John]

Have you tried measured the NF resistor? Maybe try taking it out of circuit, see what happens.


Dunno much about bias, but is it possible the filter cap on the - supply is leaky? (no idea if that would matter anyway)

[TubeGeek]

Using the crude wattage check from Gerald Weber's way...I measure 2 volts AC across the speaker terminal while playing my guitar into channel 2 which is at 10 for volume.


P=V2/R


I have 2 watts according to this.


Oh yeah, replaced the PI coupling caps...didn't help.  It actually created a bit of a headache as it made things worse until I realized that my guitar was the issue.  The battery in my clapton strat died and wasn't working.  The amp gods are mad at me on this job.

[TubeGeek]

Have you tried measured the NF resistor? Maybe try taking it out of circuit, see what happens.


Dunno much about bias, but is it possible the filter cap on the - supply is leaky? (no idea if that would matter anyway)

Yes, I thought of this and did swap the resistor to confirm.  It wasn't the culprit.

[TubeGeek]

Don't forget about the possibility of conductive board syndrome.
Drove me to grown man tears once...I'll never let it get me again.


If all DC voltages are correct and then you go through and inject an AC signal at each stage you can eventually rule it in or out.

I've been there before too.  Long time ago though.


I measure 0.2 VDC from the board to ground.  Probably not the issue.

[SILVERGUN]

Don't forget about the possibility of conductive board syndrome.
Drove me to grown man tears once...I'll never let it get me again.


If all DC voltages are correct and then you go through and inject an AC signal at each stage you can eventually rule it in or out.

I've been there before too.  Long time ago though.


I measure 0.2 VDC from the board to ground.  Probably not the issue.

My board wasnt showing the stray voltage everywhere.
There were only certain spots on the board that would give me a reading.
Not to say that this is definitely your problem, just to give you a heads up.


Good luck to ya!

[shooter]

I have 2 watts according to this.

so put 1ohmers in, set / redo bias based on current/dissipation and see, you can always take them  back out if it's an aesthetic thing

 

[pdf64]

The rather low V3 cathode voltage may be hamstringing the vib channel somewhat. Id investigate that, eg bad cathode resistor or bypass cap?

V3 is the reverb driver. Later SRs used much smaller cathode resistors so the voltage would run lower. At any rate, even if you pull the reverb driver tube the vib channel would not be hamstrung.

My findings are that generally, with the 2 channel BF reverb amps, given almost any reasonable control settings, the V3 reverb driver cathode voltage sets the Vib channel clipping point, for mid and high frequency ranges at least.
As the signal level rises from 0, the 1st point to clip in the Vib channel is generally V3 grid, because when the instantaneous signal level positive peak at the 2nd stage plate gets close to the V3 cathode voltage, the forward biased V3 g-k diode clips the positive going waveform at that level.
The power amp will already have reached its clipping level.
But if the V3 reverb driver cathode voltage is ~half its normal value, the Vib channel may clip earlier than the power amp, hence the amp's power output may seem lower than normal.

The SF amps with a master volume removed the V3 cathode bypass (and hence moved the clipping point to a higher signal level, and also made it a more rounded clip) presumably because this mid and high frequency asymmetrical clipping sounds unpleasant. The reduction in cathode resistor value then presumably being made in order to increase the stage gain closer to its bypassed level.

[sluckey]

My findings are that generally, with the 2 channel BF reverb amps, given almost any reasonable control settings, the V3 reverb driver cathode voltage sets the Vib channel clipping point, for mid and high frequency ranges at least.

That's interesting. But even if the gk became a dead short that would just be the equivalent of putting a 500pF across the signal path. Probably knock some zing off the top end but hardly what I'd call hamstringing. But it's easy enough to test that theory. Just pull V3 . I really don't expect the volume to jump up. And what about the normal channel that would be unaffected by a hamstrung vib channel?

Didn't TubeGeek fix it by replacing the battery in his guitar?

[TubeGeek]

I have 2 watts according to this.

so put 1ohmers in, set / redo bias based on current/dissipation and see, you can always take them  back out if it's an aesthetic thing

Bias is sitting at 38-40mA per tube.  That puts them at around 17w per tube.


Should be plenty loud but it's not near what it should be.

[TubeGeek]

My findings are that generally, with the 2 channel BF reverb amps, given almost any reasonable control settings, the V3 reverb driver cathode voltage sets the Vib channel clipping point, for mid and high frequency ranges at least.

That's interesting. But even if the gk became a dead short that would just be the equivalent of putting a 500pF across the signal path. Probably knock some zing off the top end but hardly what I'd call hamstringing. But it's easy enough to test that theory. Just pull V3 . I really don't expect the volume to jump up. And what about the normal channel that would be unaffected by a hamstrung vib channel?

Didn't TubeGeek fix it by replacing the battery in his guitar?

Removing v3 doesn't change the low power.


I am now using a passive guitar (tele) to troubleshoot.  The battery in my strat was just another issue that happened while troubleshooting.  I have eliminated that variable now.

[Deric]

Is the power supply stable when a signal is applied ?

[TubeGeek]

Is the power supply stable when a signal is applied ?

Yeah, seems to be fine.

[shooter]

I've been there before too.

yup, is your board a "double" layer, I had one that once I spaced the 2 layers weirdness was gone.

sounds like you're ready for drastic, you got an amp with a pre out, or line out, temp it into the PI .001 and eliminate 1/2 the amp....maybe

[John]

With everything else you've tried, it sure sounds like audio signal is getting bled to ground somewhere, which would explain the ~2W at the speaker. I bet you've tried this too, but bypass all the tone/Vol pots straight to PI?
Do you have a pure AC meter where you can measure the signal going into and out of each tube?

[PRR]

Have you replaced the "decoupling" caps in the B+ filter chain?

I got lost in some woods and noticed that an effect of too-tiny decoupling caps in a many-stage amplifier is overall degeneration, more loss of lows than highs. This could sound like "weak, under-powered and thin".

[TubeGeek]

Have you replaced the "decoupling" caps in the B+ filter chain?

I got lost in some woods and noticed that an effect of too-tiny decoupling caps in a many-stage amplifier is overall degeneration, more loss of lows than highs. This could sound like "weak, under-powered and thin".

Yes, just did and no improvement

[TubeGeek]

With everything else you've tried, it sure sounds like audio signal is getting bled to ground somewhere, which would explain the ~2W at the speaker. I bet you've tried this too, but bypass all the tone/Vol pots straight to PI?
Do you have a pure AC meter where you can measure the signal going into and out of each tube?

I'm gonna have to think through how to bypass the tone/vol pots...never tried that before that I can remember.


Also I have not measured AC in out before.

[TubeGeek]

I am suspecting something might be going on under the main board. 


A lot of the eyelets have a huge mound of solder on them, way too much for reason.  I also notice that some extra holes have been drilled in the board where the first mounting screw is.  Maybe there were some mods or something done that I just can't see under the board.  Maybe not mods but poor work or some excess solder has formed a jumper to ground or something like that.


I honestly want to give up and move on from this one.  Part of me wants to stick with it too. 


This will be the last amplifier I work on for a while since I need to take care of my hands. I have carpal tunnel syndrome and surgery is scheduled for August.  Working on amps is hard on my hands.

[Colas LeGrippa]

INPUT JACK may be def. , grid stoppers too high. Measure the preamp k voltages and if too weak investigate for defective grid leaks resistors. The k voltages are critical. If the original resistor value of a given tube has drifted away too much, you ll get a weak power , maybe no sound at all. I think your issue is in the preamp.

[SILVERGUN]

...Also I have not measured AC in out before.

This is where a dual trace scope (or just 2 scopes, or actually any scope) comes in very handy.

I tend to work backwards from the power tubes.
With a signal generator (put down the guitar) and a dummy load you can evaluate each stage and eventually SEE where you're losing it.

Takes all of the guesswork out of finding a signal loss issue.

[mresistor]

or actually any scope) comes in very handy.

Additionally,  a listening amp is also very useful in tracing signal.  It's at the bottom of the link..  Mine has been very handy to have since I don't currently own an O'scope. It's at the end of the link


https://el34world.com/Hoffman/tools.htm

[TubeGeek]

Troubleshooting still continues.


The amp really sounds great, just like it should, only at 2 watts.


There was an excess of solder on most of the eyelets throughout the main board.  Way too much.  I took a dentists mirror and propped up the main board so i could shine a flashlight under the board to have a look.  There were literally standoffs under the eyelets of solder.  So I cleaned up all the eyelets of the excess solder, very tedious btw.  Ugh.  I'm a bit grumpy.  There is still a ton of leftover flux residue there.  Don't tell me I have to clean that too.


In 15 years I haven't had my butt kicked with a fender amp.  I've worked on dozens and dozens, probably in the hundreds now.


Now that the solder is sorted out and things are looking better...power on...still have 2 watts. WTF.


I sorta have a love hate relationship with this amp now.  I hate that it's taking so much time but I also want to figure out what the hell is going on.


I am now back to thinking it is the OT.


Is it possible for an OT to only put out 2 watts like this?  Wouldn't it be a pass/fail thing, no middle ground? 


I do have another vintage super reverb OT on my shelf.  I did wire it in place to try and rule out a bad OT...same low power happened though.  I don't know if that spare OT is good or not though, it was given to me in a box of parts. The primary windings measure 44 ohms from CT to plate lead, red or blue.  The OT installed in the amp measures the same too.


So....I do have a few other OT's sitting around that I am thinking of wiring up again, just to make sure 100% I am not chasing my tail and it is indeed a shit OT that is the problem.


I have a mercury magnetics MAR100-OM sitting around not being used. It's a replacement for a Marshall DSL 100 or TSL 100w amps.  I also have a classic tone 20W deluxe reverb style OT, 40-18087.


I am leaning to trying the marshall style OT first.  Any reasons why I shouldn't do this? I realize the impedances of the marshall style OT are different but I just want to see if I get more power briefly.  I am concerned the deluxe reverb OT is not sufficient.


Looks like a super reverb OT is around 4k primary impedance...my classic tone deluxe reverb OT is 6.6k.  Even if I temporarily wire this one in and connect to another speaker cab for testing so the impedance matches, I will then rule out the OT as faulty or not. The marshall will be around 2k primary impedance.


I'm grasping at straws here.

[sluckey]

Have you examined the speaker jacks closely? You're not plugged into the wrong jack are you?

[TubeGeek]

Have you examined the speaker jacks closely? You're not plugged into the wrong jack are you?

Yeah, thought of that before too.  When I first got this amp, I noticed the speaker out jack was toast. So I replaced it.


I am sure I am plugged into the right output.

[TubeGeek]

I do have a 5f6-a of my own here.  I guess I could just wire that OT in temporarily to confirm 100%, it is the same impedance so that wouldn't be an issue.


I'll try this next and stop farting around with other ideas.

[pdf64]

In cases like this, a suspect OT can be easily assessed by comparing the idle and full load power tube cathode currents. If the output is weak but the current increases significantly then that indicates that the OT is probably wonky, eg shorted turns across its primary.
With a SR, I’d expect cathode current to rise from ~30mA at idle to ~200mA at full load.

Such shorts may need HT / some degree of signal to manifest, so may not be apparent when checking with a DMM.

[SILVERGUN]

How are those speaker jacks connected?
This is what I see when I blow the pic up.
Is there a link between the 2 jacks?
I see black going to one and green going to the other but then ? 

[TubeGeek]

In cases like this, a suspect OT can be easily assessed by comparing the idle and full load power tube cathode currents. If the output is weak but the current increases significantly then that indicates that the OT is probably wonky, eg shorted turns across its primary.
With a SR, I’d expect cathode current to rise from ~30mA at idle to ~200mA at full load.

Such shorts may need HT / some degree of signal to manifest, so may not be apparent when checking with a DMM.

This is what I see with this amp.  35mA idle...play an e chord with volume on 10 and the current immediately jumps to ~180mA.

[TubeGeek]

How are those speaker jacks connected?
This is what I see when I blow the pic up.
Is there a link between the 2 jacks?
I see black going to one and green going to the other but then ? 

[SILVERGUN]

That looks good but I'd have to try jumpering the black wire directly to the speaker jack on the right just to cross it off the list

[TubeGeek]

That looks good but I'd have to try jumpering the black wire directly to the speaker jack on the right just to cross it off the list

Checked off the list.  Didn't make any difference.


Side note...when I replaced the speaker out jack, it was because the switch terminal was bent and there was a little piece of cardboard stuck in between the switch terminal and the tip terminal.  Hmmmm. Perhaps the amp was powered on without a speaker load, with a signal and it fried the OT and power tubes???

[SILVERGUN]

I checked one of those 12A switching jacks here and the TIP pin is the one in the middle of the three.
If that is the case with your jack then you have it jumpered to ground.

[SILVERGUN]

I compared an actual Switchcraft 12A to an aftermarket and they are different.
If yours was a Switchcraft, then you would be fine.
If it's one of the others then you are hosed.

[TubeGeek]

I compared an actual Switchcraft 12A to an aftermarket and they are different.
If yours was a Switchcraft, then you would be fine.
If it's one of the others then you are hosed.

I'll check this again when I get back to it tonight. Wouldn't that be funny if this is the problem! 

[SILVERGUN]

Yeah...real funny 

It was the light color of the insulating material that made me look. I have some of those here and I can't remember where I got them....maybe Radio Shack, maybe 

I am shocked that they are different, but I don't know why I'm shocked.

Added a pic for reference:

[sluckey]

I checked one of those 12A switching jacks here and the TIP pin is the one in the middle of the three.
If that is the case with your jack then you have it jumpered to ground.

The pin you circled in the pic is the switch! Not the tip. That jack is wired correctly if it is a Switchcraft 12A.

[2deaf]

Switchcraft uses the lighter-colored wafers on their more recent jacks.  The word "Switchcraft" stamped into the jack in question would seem to eliminate all doubt about the brand.

[SILVERGUN]

I checked one of those 12A switching jacks here and the TIP pin is the one in the middle of the three.
If that is the case with your jack then you have it jumpered to ground.

The pin you circled in the pic is the switch! Not the tip. That jack is wired correctly if it is a Switchcraft 12A.

I covered this in reply #38
Good thing 2deaf came along...I was about to bet you a cheesesteak.

Good eyes 2deaf, I missed that...although I'm secretly hoping that it says "Beijingcraft"

Sorry TG, my intentions were obviously honorable 

[sluckey]

Hey, we're all just trying to find some cheap quirk to fix.  James should know by now if it is the OT.

[John]

So, any luck yet?

[TubeGeek]

Well it indeed was the output transformer.


I proved it by connecting my tweed bassman OT to the super reverb.


I was away camping, didn't have inet access to update you.


I am just glad to get this thing finished. 

[John]

Good to hear! And I learned something new, I always figured if a transformer went bad, it didn't work at all, so this is good to know!

[shooter]

And I learned something new

that's what makes it fun, we usually do
fwiw, I've fixed 2 amps with bad OT's, neither was "Dead", one had 1/2 the primary partially shorted, the other had the secondary partially shorted...they were "tricky" proving it before I spent other ppls money 

[TubeGeek]

It threw me off that the OT still functioned and sounded good at 2w.


I assumed it was a go/no-go with transformers too.


And that the spare OT I had on the shelf displayed the exact same issue also confused me.  Well now I know why that spare OT was in a box with a bunch of other parts was given to me.


Thanks to all for chiming in on this effort.