Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dumbmonkey on July 31, 2025, 02:46:46 pm
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Hi all!
I am currently working on a 1965 Super Reverb that someone I met had sitting in a garage under a blanket. She tells me that it was previously in a shed, and it sure looks like it! Covered in dirt and rust. Anyway, she says it belonged to a family member she was close to, and would like to make it usable. Personally, I think she checked out what the prices are for used ones online, and might be looking to flip it, lol. I made her an offer on it as is
In any case, I've taken the project on and am in the process of cleaning, replacing and upgrading things. So far, I've replaced all the filter caps and dropping resistors, the entrire bias circuit, all the cathode bypass caps, screen and screen grid resistors, and the plate resistors. I've put protection diodes on the rectifier, and I'm definitely replacing the power cord with a grounded one.
My big question is, how far should I go with replacing reisistors? I'm measuring each one, and if they are way out of spec, I'm replacing with 1 watt carbon film, but is there a point where I shouldn't worry too much? For example, how important are the 68k resistors on the input jacks?
At this point, would it just make sense to replace all the resistors on the board?
Thanks in advance for your time!
cc
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If if were me, after doing the mains wiring safety ground, I’d replace the only the Electrolytic caps (including the bias supply cap) and then make sure none of the speaker coils are open, then check for bias voltage on the 6L6 grid pins, before popping them back in and switching it on with a current limiter. Don’t worry about replacing other stuff until you get it running. The existing resistors have extra mojo, as do the blue astrons (until you find out they are leaky).
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WHAT he said^^^^
did a repair on one, pretty sure it was a '65, you might look into "collectability" this guys amp at the time had a re-sell value in the many 1,000's
a scope n signal generator are great tools after the meter finds all the easy stuff.
the Bias voltage is critical BEFORE you put in the PA tubes with power, it needs to be stable over time AT the tube socket, no major AC riding on the DC
fwiw these are from my initial testing notes for reference.
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Hey there!
Thank you both for your replies! I think I got too caught up in the resistors, and forgot that this thing has to fire up first. I'll get on the mains and check that it powers up and measures and works correctly before diving in too much deeper.
For the record, this thing is pretty beat up, so I'm not too sure about collectability.
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I just recently went through a very similar experience. Mine was an early 67. Silverface cosmetics but completely AB763 inside. It was a gift, and had been home to many dirt daubers, scorpions, spiders, and a misguided master volume mod before it came into my possession.
Replacing out of spec resistors can very quickly spiral out of control and reduce some of the character that these amps take on as they age. Far too often people will shotgun replace plate and slope resistors because of Internet stories of noisy plate resistors. It would be best to know what the resistor is doing at a technical level and making a judgment call as to whether the drift works in your favor or against you.
For example, if a 100k plate resistor reads 112k, but it otherwise functional and not noisy, then that stage will have a slight bit more gain than stock. You may like this.
Take that same resistor scenario but as a slope resistor, and now you'll have a bit more high end at the expense of some lows and mids. You may not like that, so replacing the resistor is a good move.
The same applies for cathode resistors. Those drifting high will bias the stage moderately cooler. It can be a good thing, but in your tremolo oscillator or reverb driver it might result in weaker trem at low frequency or audible distortion in the reverb signal.
The only resistors I absolutely would change are dropping resistors in the HT, the dropper for the bias supply, and the 6L6 socket mounted resistors. Everything else should be evaluated after you've got the amp running again.
The other thing to consider on these barn find restorations is conductivity in the fiberboard. This is a challenge to (temporarily) root out, and is a losing battle in the long run as it will keep happening. Let me know when you get there and I can share some insights from my recent restoration (amongst several others for clients).
Finally, you'll want to clean the brass ground plate sandwiched between the chassis and pots on the front panel. Once that is clean and the pots are tightly cinched a lot of the background pops and clicks reminiscent of a record player spooling up will vanish.
Have fun with your project.
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The other thing to consider on these barn find restorations is conductivity in the fiberboard. This is a challenge to (temporarily) root out, and is a losing battle in the long run as it will keep happening. Let me know when you get there and I can share some insights from my recent restoration (amongst several others for clients).
Someone pointed me to a Lyle Caldwell video on this topic:
i=-Pfe_b5E_wqAw1WR
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The other thing to consider on these barn find restorations is conductivity in the fiberboard. This is a challenge to (temporarily) root out, and is a losing battle in the long run as it will keep happening. Let me know when you get there and I can share some insights from my recent restoration (amongst several others for clients).
Someone pointed me to a Lyle Caldwell video on this topic:
i=-Pfe_b5E_wqAw1WR
Thankfully no wax on a 65.
I tend to go a bit further with it. When I take the pots off the front panel to clean the brass plate I lift the board up to access the underside and hit with a flux brush and 99%IPA. The harder to reach areas I spray with alcohol.
I also take out the insulator board, clean it, and drench it in alcohol which I then dry with a heat gun. I subject it to 3 rounds of this.
On the top side I clean with a brush and alcohol, then cotton swabs and alcohol. I then pool alcohol around a target eyelet, use the iron to heat said eyelet and that'll boil off the alcohol, flux, and any crud built up over the years. I'm usually dabbing the area with an IPA soaked swab as this happens to catch the expelled contaminants.
Finally I'll power it up after reassembly and measure the board for DC. Any areas still leaking a bit get another drenching in alcohol and a targeted heat blast with an SMT hot air rework tool at 450°F. I'll let it sit on the board for about 10 seconds and make sure that I'm not heating insulation or sensitive components too much with the hot air.
This helps a lot, but doesn't completely eliminate the problem. It's very time consuming, and I go through several fistfuls of cotton swabs.
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Use a light bulb limiter to fire up the amp for the 1st time and after power supply work, B+filter caps, screen grid R's, etc.
No larger than a 100w bulb for that amp, 2 x 6L6.
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Thankfully no wax on a 65.
When did Fender start with the wax? The board in my AA270 Bassman (https://el34world.com/Forum/index.php?topic=32844.0)doesn't look waxy - but this is the first one I've looked at so what do I know? It doesn't appear to have been stored anywhere too damp, so maybe I won't have a problem.
I haven't gotten very far on mine - just parts to fix the power supply and other known problems, and some basic cleaning - too many more urgent projects. Once I can power it up, I can start checking for stray voltages. Then I'll know how much more of your excellent advice I need to follow.
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Not sure when exactly they started with the wax. I'm sure there are others here that know. Without fail the later '70s stuff has it, so my guess is maybe '74?
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Thanks again for all your replies and advice everyone!
If anyone is curious, I’ve finished what I needed to replace on the board and installed the grounded power cord.
I’m wondering if someone could double check to make sure I didn’t miss anything with the power?
Here’s what I did:
Ground (green): to the chassis.
Hot (black): to hot of courtesy plug, then to center of fuse holder, then from side of fuse holder to one side of power switch.
Other side of power switch to one primary of transformer.
Neutral (white): to neutral of courtesy plug to other primary of transformer.
Was that correct?
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That'll work. My preferred (but I'm outnumbered by people with valid reasons) method is switch before fuse, but the way you've got it is likely more correct. My reasoning is that I want to have off mean off and eliminate voltage at the fuse.
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Hi again!
I have the amp up and running. Voltages look good so far, and I have the amp making sound. I did have to replace a section of questionable heater wiring, so now all the preamp tubes are lighting up correctly.
Quick question for anyone, I have channel one (Normal) working perfectly, but Channel 2 (Vibrato) has really low output. Does the AB763 circuit require that the reverb tank be connected for that channel to work fully, or should it work fine with or without the reverb tank? I can connect the reverb tank if so.
If something needs to be connected, can I jumper the reverb in and out with an RCA cable for testing and such?
Thanks again for everything!
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Last question first: No, you don't need the tank connected to get full volume. But it's detrimental to the reverb transformer's health to run it without a load for extended periods of time. Do not use an RCA cable to bridge the two jacks. This will give a boost effect that will sound unpleasant at best. If you're just taking measurements and such on the bench, then running without the tank connected is ok. But once you start running significant signal at higher volumes it's best to connect the tank.
As for why you have low signal, it really could be anything. Common causes are a busted solder connection at the grounds to the brass plate, broken connection between the input jacks, a failing component, a bad connection somewhere else in the signal path, loose pots on the front panel, etc.
You'll need to troubleshoot to find it.
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If it's not convenient to connect the reverb tank whilst testing the dry signal path / power amp etc, it's simple to just pull the valve in V3. That avoids stressing the reverb transformer if the secondary is left open circuit, or bothering with a dummy load for it etc
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Channel 2 (Vibrato) has really low output.
After you’ve tried a preamp tube swap for channel 2, test the coupling caps in channel 2 for DC leakage. But don’t just do a shotgun change out of all the coupling caps- test each one properly and only change it if it is actually leaking DC. (If DC gets onto the next stage’s grid, it will cause bias to change enough to reduce gain drastically).
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Hi all (again),
It looks like I have some troubleshooting ahead of me. Oh boy!
Tubeswell, can I check for DC leakage with just a multimeter, and pardoning my lack of experience, how should I go about doing that?
Thanks again for everything!
cc
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Hi
I checked for dc using my multimeter, and I found some at the .047 coupling cap that comes off of pin 7 of V1. Could that be causing an issue? All the tone stack caps look fine, but there is dc on the caps in the phase inverter section, should I be concerned about that too?
Thank again!
cc
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How much DC?
10mV, not a big problem
100mV, noticable and could cause issues
1V+, big problem
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Hi all (again),
It looks like I have some troubleshooting ahead of me. Oh boy!
Tubeswell, can I check for DC leakage with just a multimeter, and pardoning my lack of experience, how should I go about doing that?
Thanks again for everything!
cc
You need to unsolder the ‘ac lead’* and lift it out of the circuit, then clip a V-meter onto that (now floating) lead, switch the amp back on and watch the meter. If the cap is normal, you’ll see some changing DC voltage on the meter as the HT voltage comes up to normal power-on voltage, but then it should show 0VDC after everything has stabilised. (Remember that changing voltage will be seen across any coupling cap - but constant DC voltage should not be present). If there is any DC voltage present after the HT has stabilised, the cap will be bad.
*i.e., the other (dc) lead needs to stay connected to the plate of the preceding stage for this test to work.
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Hi again all,
I apologize for all my inexperience in this, and want to say thank you for all your time and patience with me.
In any case, I'm still plugging away at this vibrato channel issue. I've tried replacing a couple capacitors that I thought might be an issue, but apparently, they are just fine, so I put the original ones back in.
I'm at a loss as to what this issue could be with the lack of volume and response from this channel.
I went and measured voltages at the preamp tubes, and was hoping that if I posted them, someone might have some possible insights.
Here's the measured voltages followed by the voltage listed on the schematic, there's definitely something odd going on at V5...
V1
1:280 (270)
3:1.922 (2.1)
6: 284.5 (270)
8: 2.154 (2.1)
V2
1:255.2 (270)
3: 2.047 (2.1)
6: 274.5 (270)
8: 2.18 (2.1)
V3
1: 459.2 (450)
3: 4.56 (8.4)
8: 4.56 (8.4)
V4
1: 281 (270)
3: 2.18 (2.0)
6: 287.8 (270)
8: 2.18 (2.0)
V5
1: 452 (280)
3: 0 (2.5)
6: 403.2 (390)
8: 0 (170)
V6
1: 298.4 (280)
3: 67.1 (106)
6: 305.1 (230)
8: 67.1 (106)
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V1
1:280 (270)
3:1.922 (2.1)
6: 284.5 (270)
8: 2.154 (2.1)
V2
1:255.2 (270)
3: 2.047 (2.1)
6: 274.5 (270)
8: 2.18 (2.1)
V3
1: 459.2 (450)
3: 4.56 (8.4)
8: 4.56 (8.4)
V4
1: 281 (270)
3: 2.18 (2.0)
6: 287.8 (270)
8: 2.18 (2.0)
V5
1: 452 (280)
3: 0 (2.5)
6: 403.2 (390)
8: 0 (170)
V6
1: 298.4 (280)
3: 67.1 (106)
6: 305.1 (230)
8: 67.1 (106)
I have a feeling if you hit the button on your vibrato footswitch your voltages on V5 will come into alignment. That tube is the tremolo oscillator and driver for the bulb. Unless your issue gets better/worse with the Intensity knob, then I suspect it's not the issue.
Nothing in your DC voltages are pointing to an obvious problem with your vibrato channel.
I think your issue is in the signal path between the triodes. If you don't have a scope I'd recommend getting the Fender Super Reverb reissue schematic. It has AC voltages at each stage. You can make a reasonable attempt at replicating those measurements with a DMM capable of VAC RMS measurements. You'll need a signal generator. I use a phone app and an 1/8" to 1/4" cable. You can even use alligator cables in a pinch.
This will allow you to track the change in signal amplitude as it travels through the amp. If it drops where it shouldn't--or doesn't rise as much as it should, then we focus on the associated circuitry.
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^What Stratocaster said^
Also
...test the coupling caps in channel 2 for DC leakage. But don’t just do a shotgun change out of all the coupling caps- test each one properly and only change it if it is actually leaking DC. (If DC gets onto the next stage’s grid, it will cause bias to change enough to reduce gain drastically).
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Thank you both Stratomaster and Tubeswell!
I printed out a 65 reissue schematic in a nice 11x17" size (hooray for free school copiers!) and I've marked off all the test points for the vibrato channel preamp signal. (unless you think I should check out the phase inverter section as well?) I have a signal generator on my phone, so the 1k sine wave shouldn't be any issue.
Thanks for your help, I'll keep y'all posted.
cc
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Cool. Regarding the PI, we'll burn that bridge when we get to it. The fact that your issue is isolated to a single channel rules the PI out. That doesn't mean it's 100% healthy--just that it's not the immediate concern.
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... I have a signal generator on my phone, so the 1k sine wave shouldn't be any issue.
A potential divider between the phone output and amp input might be helpful in achieving the specified 20mV signal voltage at the amp input.
Be aware your meter might not be good enough to accurately measure up at 1kHz, especially at only 20mV.
But the signal voltages noted can be used as a guideline to estimate the difference in signal levels at the different test points at lower frequencies.
200Hz is often within the capabilities of most DMMs. But I haven't checked this kinda thing for a while, so the spec of ghe typical DMM might have got better.
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Hi again!
I'm sorry, but I'm going to off on a tangent here. I haven't gotten around to checking with the signal generator, but as I was trying the channel out again with a guitar last night, I noted that as I increased the level on the Reverb control (no tank connected yet), the volume of the channel increased significantly. Is this normal, or could it be the indication of something I should be checking out? Maybe something that narrows my search for what might be the issue in the channel?
Thanks in advance!
On a side note, how crucial is a 12AT7 in the Reverb circuit? Will subbing a 12AX7A cause any problems?
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When troubleshooting it is best to eliminate variables. Do not deviate from the print, especially if you're unable to evaluate the impact of those deviations for yourself. If you absolutely must substitute a tube, then put at 12AX7 in for the PI and use a 12AT7 in the reverb.
Good observation with the reverb, but please do not try to short cut the troubleshooting strategy. Systematically and methodically evaluating your amp vs the print is a skill unto itself, eliminates variables, and builds familiarity with the circuit.
There's a good chance the reverb circuit was modified to operate as a lead boost at some point in this amp's existence. Take a look at the RCA jacks and measure resistance tip to tip with the tank disconnected. It should be open circuit. If it's low 100s ohms then someone likely modified the stage to act as a boost of 330Hz+
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Wow, this thread is really getting ridiculously long and I'm so sorry that my inexperience is taking up so much space.
Thank you all for putting up with me!
It seems the sine wave plan is going to be a bust as I don't have a the right means to measure the testing points, I only have standard multimeters. If anyone can recommend one I should have, let me know and I'll order one. I probably ought to have an oscilliscope, but I wouldn't even know where to start from there, lol.
Anyway, I'm still at a loss, but now I'm noticing that the phase inverter section is noisy and when I chopstick it it makes all sorts of banging noises, which seem to go away as the amp warms up. I've tried a couple different tubes in there but it still happens. Any thoughts on that, or as mentioned before, is that a separate issue.
Also, does anyone think there might me something with the pots on this channel? They all seem to work, but the bright switch doesn't although both the switch and the bright cap measure fine. Actually, pretty much all the components measure within specs.
I'm getting frustrated with my lack of knowledge and experience, and I'm almost thinking about shotgunning the parts to find the culprit (don't like that idea tho...)
Any advice is welcome.
cc
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What meter do you have? Most are capable of mV AC measurements. You don't necessarily need an oscope if you're not chasing a waveform issue. A DMM will be just fine for amplitude tracking.
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Hi again!
I'm sorry, but I'm going to off on a tangent here. I haven't gotten around to checking with the signal generator, but as I was trying the channel out again with a guitar last night, I noted that as I increased the level on the Reverb control (no tank connected yet), the volume of the channel increased significantly. Is this normal, or could it be the indication of something I should be checking out? Maybe something that narrows my search for what might be the issue in the channel?
Thanks in advance!
On a side note, how crucial is a 12AT7 in the Reverb circuit? Will subbing a 12AX7A cause any problems?
You should not be getting this. Check the resistances in and around the reverb level pot against the schematic. Include checking the 3M3/470k divider in the dry signal path and the 220k from the reverb pot wiper to ground.
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Hi again Stratomaster!
I have regular multimeters, so I'll try those again. Sadly, I don't have a dummy load, so it got pretty loud down there...
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Hi Tubeswell,
I had check that section earlier and replaced the 470k and 220k. The 3.3meg seemed within spec. I did notice that when I measure the 220k in circuit, it measures in the 400K+ range. Does that seem right to you?
cc
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Hi again Stratomaster!
I have regular multimeters, so I'll try those again. Sadly, I don't have a dummy load, so it got pretty loud down there...
Pull the power tubes or temporarily short across the power tube grids. You're just signal tracing right now. The later will be equivalent to turning a cross-line master volume down to zero. It will mess with your PI voltages, but you'll be able to signal trace in silence.
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Pulling power tubes will be ok? I'll give that a whirl.
cc
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Yes. Your DC and filament voltages across the board will go up. So it's not the best thing to do long term. But will be fine for troubleshooting purposes.
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Cool, thanks! I'll report back.
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Well that was interesting.
Pulled the power tubes and ran the test tone in. I didn’t get very far with my measurements as the 1st testing point (supposed to be 20mv) showed no voltage. I tried a couple other points and got either 0 mv, or really low readings.
I’m thinking that maybe the input packs need to be replaced or rewired?
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Check the solder connection between the two jacks. It tends to crack and causes the jack to lose ground reference. You'll often hear it snap into place if you reflow that connection. Tighten the jacks to the chassis with a socket wrench before reflowing.
You'll also need to make sure you're putting the 20mV signal into the amp. Confirm that your signal generator is outputting that amplitude. You can do this by measuring tip to ground outside of the amp before connecting to the input jack.
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Hi Tubeswell,
I had check that section earlier and replaced the 470k and 220k. The 3.3meg seemed within spec. I did notice that when I measure the 220k in circuit, it measures in the 400K+ range. Does that seem right to you?
cc
The 220k is wired between the pot wiper and ground so it will not measure as 220k in circuit - rather it will measure as being in parallel with other resistance, which will vary depending on the pot rotation. At which points is the R-meter clipped when you measure 400k-ish? That doesn’t seem right.
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Hi Tubeswell,
I had check that section earlier and replaced the 470k and 220k. The 3.3meg seemed within spec. I did notice that when I measure the 220k in circuit, it measures in the 400K+ range. Does that seem right to you?
cc
Hi,
I was measuring the 220K right at each end of the resistor. I'm not 100% sure about the measurement I was getting, but I'll check it when I get home and report back.
Thanks for your time!
The 220k is wired between the pot wiper and ground so it will not measure as 220k in circuit - rather it will measure as being in parallel with other resistance, which will vary depending on the pot rotation. At which points is the R-meter clipped when you measure 400k-ish? That doesn’t seem right.
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Hi Tubeswell,
I measured that 220k resistor with the reverb level all the way down across the actual resistor and got 481k.
Man, I wish I could figure out what the heck is going on here...
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You should be measuring 470k||220k when the reverb level pot is cut. If something is funny with one of the coupling caps in the reverb driver circuit or the vibrato channel, you might get a funny resistance reading.
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Tubeswell,
So 480k with the reverb cut is ok then? If so, then that section seems good.
Any other troubleshooting ideas?
I’m starting to think I’ve gotten too far over my head with this…
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No it should be 470k||220k with the pot cut (as this puts the resistors in parallel). However, if there is a leak in say the coupling cap after the second stage of the Vibrato channel, this could put other resistors (the 100k plate resistor on the 2nd stage in the vibrato channel and the 3m3) into the equation. Or if the 500pF cap going to the reverb driver was leaking DC, this could put the 3m3 and the 1M in the equation.
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Maybe it's as simple as the 220K is open or has a bad connection. I would disconnect one end of the 220K to measure its resistance.
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Hi,
The 220k in question is one that I replaced. I did remove it and double check it and it does measure correctly out of circuit. I'm assuming that something else is amiss there. That right in an area where I replaced cathode bypass caps and resistors, so I'm going to remove those and clean and re install those to make sure they are correct as well.
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Test the 20nF coupling cap in the vibrato channel for DC leakage.
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See Fig 4-4 on P57
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Tubeswell,
Thank you so very much!!! That will be so very helpful!
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Get the entire book...
https://trinityamps.com/ForumGallery/trinity/resources/Jack_Darr_Handbook.pdf
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Get the entire book...
https://trinityamps.com/ForumGallery/trinity/resources/Jack_Darr_Handbook.pdf
^^^ What he said. Jack Darr Handbook is a treasure of tips and tricks for the new builder.
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Hi all!
I'm gonna bet that anyone following this thread is getting pretty sick of my postings.
Tubeswell, thank you for the info, it was really enlightening and will be a good guide for me moving forward. I had checked that .022 coupling cap before (it's a ceramic disc) and tried replacing it with an Orange Drop, but that didn't change anything. I checked the 500pf in series with that and got a few milivolts of DC so I replaced that with a SM that I had here. Still no changes.
Could the issue possibly be someplace in the pots? I cleaned them all out with Deoxit, but I didn't pull them and clean the brass plate. I'm going to try that and see if that does anything. I have some new shunting jacks, resistors and bright switch on the way, should I try redoing the inputs as well?
I have to keep reminding myself that the amp is 60 years old, and was in a shed, so I shouldn't be surprised with all the issues.
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Get the entire book...
https://trinityamps.com/ForumGallery/trinity/resources/Jack_Darr_Handbook.pdf
Wow! Great! Thank you!!!
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Hi all!
I'm gonna bet that anyone following this thread is getting pretty sick of my postings.
Tubeswell, thank you for the info, it was really enlightening and will be a good guide for me moving forward. I had checked that .022 coupling cap before (it's a ceramic disc) and tried replacing it with an Orange Drop, but that didn't change anything. I checked the 500pf in series with that and got a few milivolts of DC so I replaced that with a SM that I had here. Still no changes.
Could the issue possibly be someplace in the pots? I cleaned them all out with Deoxit, but I didn't pull them and clean the brass plate. I'm going to try that and see if that does anything. I have some new shunting jacks, resistors and bright switch on the way, should I try redoing the inputs as well?
I have to keep reminding myself that the amp is 60 years old, and was in a shed, so I shouldn't be surprised with all the issues.
It could be a lot of things. Guessing and replacing parts is like playing Battleship.
If you stuck with the systematic signal tracing I suggested you'd have zeroed in on at least the problematic section of the circuit by now.
I'm imploring you to resume signal tracing.
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Stratomaster
I totally understand, and I don't want to keep piecemealing it like I have been; it makes me feel like a total hack. I'll definitely give the signal tracing another try.
A couple questions about signal tracing:
wouldn't it be easier with an oscilloscope?
how hard is it to do basic signal tracing with an oscilliscope (ie, for a newbie)?
what affordable oscilloscope would anyone recommend for a complete beginner?
As always, thanks for all your time!
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wouldn't it be easier with an oscilloscope?
Not easier, but definitely more thorough and better for your skillset long term. I recommended the multimeter because the mechanics of taking AC voltage measurements are identical to the DC ones you're already taking.
how hard is it to do basic signal tracing with an oscilliscope (ie, for a newbie)?
getting familiar with the scope would be the challenge. It's worth the effort in my opinion, but as a beginner who might not take it much further that fixing an amp or two or a kit build, it may not be worth it. There are concepts like triggering, DC vs AC coupling, peak to peak vs RMS, having to keep track of probe multipliers, etc that can trip people up until they get used to them. If you're wanting to learn, then there were several good instructionals and explainers on YouTube. And with the flood of inexpensive and feature rich scopes on the market the barrier to entry has never been lower.
what affordable oscilloscope would anyone recommend for a complete beginner?
Any scope you can afford will be able to do amp work. Our frequencies are so low that any single channel scope with a 10x probe will to most anything you would need. It's nice to have a second channel, and newer digital scopes have the ability to do much of the math for you on the waveforms, so if you're not used to working with a calculator by your side, that's a plus. I recently bought a $35 FNISRI (or something like that) scope on Amazon just out of curiosity. It would easily do the signal tracing we're discussing, but the UI is a but clumsy for someone like me used to knobs and switches.
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Hi Stratomaster!
And I bet you thought you were rid of me! Not so much!
I tried again with the signal tracing utilizing the Super Reverb Reissue schematic that you recommended. I purchased a $40 oscilliscope from Amazon and used the signal generator as the input. Unfortunately, I could only get the output of the generator down to about 30 mvAC, but I figured that could at least get me in the ballpark. In any case I tried some measurements. V2 seemed to be ok or at least close with the schematic, but V3 was way off. I had pins that were not showing any voltage when they should have been. Pins 1/6 and 2/7 were showing no voltage at all, and the schematic has voltage listed. Do you think the problems I'm having with this channel have something to do with the Reverb driver, and if so, what do you think I should be checking out?
Thanks again for everything!
cc
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I don't think the reverb driver is the issue, and I doubt there's actually 0v on those pins. I think measurement error on a new/unfamiliar instrument is more likely. You can spot check your oscope measurements with the multimeter. What I think is more likely to have happened is the input AC decoupling capacitor in your new scope got nuked by high voltage exposure, and it lasted long enough to make it to the third tube.
As far as next steps, check if you can get a larger input. If you're using a phone app to a headphone cable you should be able to get 200mV or more. This leads me to believe your 10x setting on your scope was likely incorrect or you have a problem right at the input jack like we talked about a few posts back.
If I'm wrong and all the equipment is working and you really can only get 30mV in, then you need to scale the measurements by the multiple factor vs the schematic input voltage. I don't remember what it was, but let's say it was 150mV, so you'd want to scale by a factor of 5 to do a direct comparison with the schematic. What I'm really more interested in, however, is not how closely the signal amplitude matches but how it changes through the circuit. I would like to see 35-60X multiples between the plate and grid, and losses where expected (voltage dividers, tonestack losses, etc).
The best way to convey this information is a clear and consise voltage table showing the DC and AC voltage at each pin through the preamp (except the filament pins). Leave the tremolo oscillator and PI alone for the time being. The PI seems to be healthy since the normal channel sounds good. And the oscillator is highly unlikely to be the issue unless the signal attention varies with the Intensity control.
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Hi again
I should have been more specific and I’m sorry for any confusion. I was under the impression that I had to have a 1k sine wave going in at 20mv. Using that info I used my new $40 oscilloscope signal generator to produce the correct sine wave but. Could only get it as low as 32mv, so that’s what I input to the amp. I then used my multimeter to read mvac at the tube pins according to the Super Reverb reissue schematic. I double checked the measurements on V3 and still did not get readings on those pins.
Arrrrrrrgh….
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Hi again
I should have been more specific and I’m sorry for any confusion. I was under the impression that I had to have a 1k sine wave going in at 20mv. Using that info I used my new $40 oscilloscope signal generator to produce the correct sine wave but. Could only get it as low as 32mv, so that’s what I input to the amp. I then used my multimeter to read mvac at the tube pins according to the Super Reverb reissue schematic. I double checked the measurements on V3 and still did not get readings on those pins.
Arrrrrrrgh….
Cool. That is a much better situation to be in than I had thought. V3 isn't the issue for losing signal. Please make the voltage chart I suggested. It'll really come in handy. You can even include a column that gives schematic reference points for AC and DC.
We'll deal with the V3 issue soon enough, but first let's get a baseline.
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Ok will do voltage chart. Should I measure all voltages (DC and AC) with the sine wave going in, and all controls as specified by the Super Reverb reissue schematic?
Thx again!
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That would be good. I don't remember if you pulled the PI or power tubes to get it to be silent while measuring, but the DC will be elevated with the power tubes removed.
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I built a 4ohm dummy load.
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Hi, and here we go again.
On the advice of Stratomaster, I have created a chart of voltages to compare the DC voltages and the AC voltages. I used the original 65 Super Reverb schematic and layout for the DC voltages, and the Reissue schematic for the AC voltages. I'm not seeing too much off with the DC voltages except for V5, but there are big differences with the AC voltages.
For testing, I followed the instructions on the Reissue schematic. The unit was plugged into a 4ohm dummy load, reverb tank connected and a 1K test tone was input on each channel. Unfortunately, the lowest I could be the voltage on the sine wave was about 32 mVAC, so I went with that.
Anyway here is the chart in PDF format. If anyone can use the data to help shine some light on this conundrum, I would really be interested to finally find a way to get this thing off my bench, lol.
As always, thank again for everything!
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except for V5
Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments
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except for V5
Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments
The test points for that tube require the tremolo to be switched on with the knobs at a specific point. I asked him to ignore V5 as it's not contributing to the problem he's having.
It looks like V1 isn't as healthy as we thought. The plate voltages are very low compared to V2. Both are supposed to be connected to the same HT node, so the plate voltages should be relatively similar. Check the values of the plate and cathode resistors on V1. Of particular concern is V1b, the amplification factor (Plate AC voltage/Grid AC voltage) is way off.
Also, the phase inverter isn't as healthy as we thought either. The DC measurements other than the plates here aren't all that useful, but your AC voltages are off by an order of magnitude. Check the plate resistors here too.
If the plates are healthy, then it's time to look at the sockets and the tubes. An old amp will likely need cleaning and tensioning of the sockets as a matter of course, then you can try a different tube in that spot to see if the amplification factor comes more in line with the expected behavior.
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Hi!
Just checking back in for a thank you to both Stratomaster and Tubeswell for all their efforts. The plate resistors on V1-3 were replaced back when I did the filter caps, and cathode bypass stuff. I changed out the 100k on V5 then as well. I will change out the plate resistors on the phase inverter, and do a bit of tube socket tightening before trying those voltages again. I was also using most of the tubes that came with the amp, so for the next foray, I'll try to load it up with tubes I have that I know are good. I have a bunch of new JJ's and Sovteks that I can try.
Should I consider changing out the phase inverter caps and the 220ks there as well?
Thanks again!
cc
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Don't replace anything that measures good. There is no benefit and it only increases the likelihood of collateral damage and increases variables.
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Another update:
As mentioned previously, I had replaced the plate resistors on V1-3 when I did my first round of stuff to this amp (filter caps and dropping resistors, cathode bypass caps and some resistors, etc). I checked the 100k on V5 pin 3 that Tubeswell suggested and that measures fine. Following stratomaster's advice to not replace anything that measures good, I measured the plate resistors on the phase inverter and grid leaks on the phase inverter (removing on leg of each from circuit) and here is what I got:
Plate resistors:
100k: measures 101.9 (seem alright?)
82k: measures 100.1 (doesn't seem right to me)
Grid Leaks
220k: measures 232.7
220k: measures 238
Those both seem to be within 10% so should I not be concerned with them?
As always, thank you for the advice.
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The schematic calls for 5% tolerance for the bias leaks and PI plate resistors. Your call if you want to change them, but they aren't the root of the problem.
Measure the plate resistors you replaced anyway for V1-3 and also the cathode resistors. The more info, the better.
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Well, if that's the case, I'm not going to replace any of them. I would really love for all of this back and forth to shine some light on the root of the problem, but I've gotten really frustrated with this whole excursion, and I'm thinking I've just gotten in too far over my head with this. I think it's past the time that I should throw in the towel, and let this woman know that I've done all I could.
Thanks everyone for all their time. I appreciate all the hand holding for the newbie.
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That's disappointing, but I get it. This type of work requires a level of Zen that escapes most, myself included at times.
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you're holding your own pretty well, lets me get more farm-boy time in :icon_biggrin:
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I checked the 100k on V5 pin 3 that Tubeswell suggested and that measures fine.
Use your R-meter to check for DC-continuity between the ground leg of that resistor and the actual ground return.
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Tubeswell, thanks I will definitely do that.
So, apparently I am either too stupid or too stubborn to give up just yet (even thought this thing has been sitting on my bench since June, lol).
Stratomaster, I measured all the plate and cathode resistors and they all measure fine. I pulled all the old tubes, flipped the thing over and tried retensioning the tube sockets. I've gone through all the sockets again with contact cleaner and deoxit, and I'm going to load it up with tubes I'm pretty confident about and try it out again. New voltage chart to follow.
If that doesn't provide any light, I'm going to try reflowing solder into every solder joint I can. Maybe there's a cold or broken joint somewhere.
Any other suggestions?
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Just the voltage chart. No need to shotgun when a precision strike will do. It's easy to flow too much solder into these eyelets and cause puddles under the top board. Plus the boards themselves can become conductive over time, no need to help them along. And believe it or not, some people put value into the spherical shape of the original joints. :dontknow:
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except for V5
Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments
Hi Tubeswell. I'm confused. My layout has the cathode resistor on pin 3 of V5 as a 2.7k. The 100k is on pin 8. (Which is what is going on in the amp) Am I missing something?
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except for V5
Check you have the rightsized cathode resistor for Pin 3 of V5 - should be 100k with a 10M plate resistor - see attachments
Hi Tubeswell. I'm confused. My layout has the cathode resistor on pin 3 of V5 as a 2.7k. The 100k is on pin 8. (Which is what is going on in the amp) Am I missing something?
No that's what they should be.
My question related to whether or not the lead on the ground-side of the 100k resistor has DC-continuity with the rest of the ground in the amp? (In case you've forgotten to complete a wiring connection somewhere?) I can't seen any photos of your actual amp, so ...
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Hi Tubeswell,
I checked. There is continuity between both the cathode resistors on V5 and the amp ground (chassis).
I took some pics tonight and I’ll post them as soon as I can resize them appropriately.
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I'm now wondering about your trem roach. When did you last replace it?
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The only way the tremolo can affect volume is if the LDR in the roach is low resistance. And you'll be able to control this with the Intensity control. There's no point in changing it if the Intensity control doesn't affect the low volume.
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The only way the tremolo can affect volume is if the LDR in the roach is low resistance.
Edit And it will remain low if the roach bulb isn't going off
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Other way around. They go low resistance with exposure to light.
So either the bulb will have to get stuck on, or the LDR would have to fail in a way that lowers resistance.
In both cases the Intensity control would affect the volume essentially acting as a pre phase inverter master volume in reverse.
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Other way around. They go low resistance with exposure to light.
You caught me in the act of waking up LoL - never mind.
It's got to be somewhere in the 'vibrato' channel/circuit there's a problem resulting in signal loss. If he's tested all the coupling caps for leakage, it can't be that. If he's replaced all the tubes with good ones, it can't be that. If all the pots are good, it can't be that. Must be an unintended signal load somewhere.
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Hi!
I've never replaced the optocoupler, it's original to the amp. Could that cause the problem I'm having? It does light up and change speeds, although it seems to stick midway through travel. How could I test that?
Should I consider the pots as a potential issue? They all seem to measure as they are supposed to, and even though the output is anemic, they all seem responsive. The bright switch doesn't seem to do anything, but I'm not concerned about that at this point.
I really appreciate all the attention that you are giving to this, it's really enlightening.
I'm going to try and post some photos.
cc
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Here's some photos that I took of what I've done so far. Hopefully it doesn't look too amateurish...
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Voltage chart with the new tubes in place after the new tubes and socket maintenance please.
Could it be any of the things you listed? Maybe, but most likely not. Measuring and troubleshooting is how we'll find it. Not replacing on a guess. The goal is to minimize variables and rule things out first. Then only replace what is necessary.
It looks like some pretty extensive work had been done, so that's lots of new variables. However the photos are too compressed to see values, connection integrity, etc.
There are image hosting sites that others have used to enable you to post high resolution images. I'm not familiar with how it works, but I've seen it done and I'm sure there's an instructional on how if you search the forum.
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Updated voltage chart with new (ish) tubes.
I used new JJ's in V1 and V2. I didn't have a newer 12AT7, so I ended up using Sovtek LPS in all the other tube positions. I'm sure that affected the voltages on V3, but that's all I have available now.
Let me know your thoughts, and as always, thank you!
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Is the oscillator working normally?
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Hi Tubeswell, the oscillator seems to be working fine, although with the really low output on the channel, it's hard to know for sure.
I did notice that as I turn up the reverb control, the channel volume increases to what seems to be the correct levels. Then, the oscillator works just fine, but the reverberation itself is kind of low.
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as I turn up the reverb control, the channel volume increases to what seems to be the correct levels.
There shouldn't be much/any difference in the vibrato channel volume between the reverb level being cut or being maxed out. The reverb effect should be fairly independent of the vibrato channel volume. This is because with the reverb level cut, the grid leak resistance for the V4 mixing stage's grid is the parallel combination of 470k||220k being ~150k, which forms part of a 3M3/150k voltage divider for the dry signal coming out of the vibrato channel tone stack recovery stage. In comparison, with the reverb level maxed out, we're seeing the bottom leg of the divider being 570k/220k being ~158k (ignoring the AC-load component on the other side of the 0.003uF coupling cap). The difference in the vibrato channel dry/wet signal output is minimal.
If you are noticing a significant volume difference, then something must be wired wrong in the vicinity of the reverb level pot and the associated 470k and 220k resistors,
or the LFO lamp is stuck 'on' (thereby increasing the signal impedance* after the reverb dry/wet mixing stage).
*i.e., 'more signal impedance' = decreased signal
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Updated voltage chart with new (ish) tubes.
I used new JJ's in V1 and V2. I didn't have a newer 12AT7, so I ended up using Sovtek LPS in all the other tube positions. I'm sure that affected the voltages on V3, but that's all I have available now.
Let me know your thoughts, and as always, thank you!
Did you redo the AC voltages too? This looks like a mix of new and old measurements along with tube substitutions. If it's incomplete/inaccurate its worse than useless because we'll start making recommendations based on bad data.
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Stratomaster,
You told me to redo the voltage chart with tube socket maintenance, cleaning and new tubes. Those are the voltages as I measured them after all that.
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V2b jumps out as the area of concern based on that chart. There's a factor of 10 difference in the amplification vs the schematic reference.
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Stratomaster,
Thank you for reviewing those for me. I can't express my thanks for all your time with this!
Now, that we have a possible area of concern, should I be focusing on the circuit going into V2b or exiting (or both?).
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Both. My primary suspect would be the cathode network and a potentially defective cathode bypass cap. Then looking at unintentional signal loading (board conductivity, unintentional connection, tin whiskers, suspect solder joint, wire broken in the insulation--rare but happens, etc).
Take high res photos of the area and we can help you look.
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That's great! Thank you!
I'll start with the cathodes caps and reistors since those were replaced.
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Test and measure. Don't replace.
A quick test would be to disconnect the cathode cap at one end and check the AC at the grid and plate. If the cap is defective there will be little to no change in the amplification between the bypassed and unbypassed cathodes.
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Hi again,
Well, I had high hopes for the bypass capacitor on V2b. I lifted a leg and got the same readings as before, so I assumed the capacitor was defective. Replaced the capacitor, and I'm still getting the same mVac readings on pins 6 and 7.
I guess the bypass cap wasn't the issue. I did double check the resistor and is indeed 820ohm as per the schematic and layout.
I'm going to attempt to put up a photo of that section of the board. Hopefully it wont be too big to attach.
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Photo for your perusal.
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A few things:
The first being most likely the problem:
The resistor that comes off the Reverb knob wiper is supposed to be a 470k. You put in a 470Ω. You'll get a good bit of gain back and you'll load down the V2b stage less.
Second: those PR01 Vishays are rated to only 350V. It may never be an issue, but something to keep in mind.
Third: I see some FOD (pieces of components, solder balls, etc). It would be good practice to take care of those as you work as they're easy to lose track of later, and they may cause issues.
Recheck your voltages after you put in the proper resistor. I think they'll come more in line.
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The resistor that comes off the Reverb knob wiper is supposed to be a 470k. You put in a 470R
If you are noticing a significant volume difference, then something must be wired wrong in the vicinity of the reverb level pot and the associated 470k and 220k resistors, ...
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Holy crap.
I am soooo freaking embarrassed.
Thank you both so much! I am ordering resistors tomorrow. I’ll change out those plate resistors as well.
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Confusing 470k with 470R, and visa versa, seems to be a fairly common mistake, eg at the screen grid, LTP bias, and this.
I wonder whether the brown / yellow of the multiplier band might be hard to differentiate if the builder has very slight colour blindness?
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yep, American "red" n ccp "red" are NOT the same color to most eyes.
the fix is easy; Ohm every R BEFORE you install. takes 13 minutes longer initially, saves hours later.
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Hi again!
Ok, so my shipment of stuff came in, and wow, that 470K did the trick as far as the volume of the vibrato channel is concerned.(that'll teach me to pay more attention to the multimeter...) I'm going to deal with the plate resistors, and then remeasure all voltages to post an updated chart. Hopefully, I'm on the way to getting this thing healthy and out of my basement.
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Hi All!
Hopefully, this is going to be my last post on this topic.
Now that my bout with the Dunning Kruger effect has been addressed, and the correct resistor (470k vs 470 ohm) has been dealt with, the volume issue on the vibrato channel is no more. I replaced the problematic plate resistors with more reliable ones (PR02), and cleaned up some sloppy potentiometer wiring. All seems to be working as it should be. I went through and checked DC voltages, and they all seem to be within specified ranges.
My next steps are to roll some tubes with the old tubes it arrived with, get it back into the cabinet, and get it out of my sight. Not that I have anything against Super Reverbs, but as this isn't going to be mine, I'd rather it go back to it's rightful owner and I can have my bench back, lol.
Thanks for all your time and assistance with this, I don't think I could have done this without other eyes and knowledge!