Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: flacman on September 14, 2023, 11:35:27 am
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I recently purchased a 1966 Fender Super Reverb. The chassis appears to be 99%. As far as I can tell there has been one resistor that has been changed.
I have some limited experience but I have A LOT to learn. I’m waiting for some electrolytic caps and trying to find some NOS Allen Bradley’s for the cathode resistors but they are hard to find.
The first part of my plan is to install new power filter caps and resistors, electrolytic caps and resistors, and the bias cap and check the resistor and diode. Also, to install a 3 prong grounded power cord.
I measured all of the resistors on the preamp board but they were all in circuit.
A few questions:
1)Can you get an accurate reading from resistors in circuit?
2) If they are out of spec, is there a typical out of range that determines when to replace them?
3) Has anyone had any luck hunting down NOS Allen Bradley’s?
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I meant to say the chassis seems to be 99% original.
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A few questions:
1)Can you get an accurate reading from resistors in circuit?
2) If they are out of spec, is there a typical out of range that determines when to replace them?
3) Has anyone had any luck hunting down NOS Allen Bradley’s?
1 - Most of them yes, if they are indeed all original the ones that can't be read accurately in circuit will likely be way out in a way that is unusual. Note them on the layout / schematic and when you're ready to work on the amp just lift a leg of those suspect resistors and re-test with the MM to confirm.
2 - Up to you. Easy standard is if they're out of tolerance, so some resistors require 5% tolerance and imo those should be swapped if they are outside that range. The rest should 10%ers but I typically replace if they're more like 20% out.
3 - Ebay might have some
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> Can you get an accurate reading from resistors in circuit?
In general: it depends. There are often other resistors in parallel.
In Tube Amps: 9/10ths of resistors only go to a tube or cap, a cap or a cold tube is non-conducting, so do not affect the reading.
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1)Can you get an accurate reading from resistors in circuit?
2) If they are out of spec, is there a typical out of range that determines when to replace them?
3) Has anyone had any luck hunting down NOS Allen Bradley’s?
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1- No
Sometime yes, you should look where the are connected. In series on parallel with other parts
As you are a beginner better to me to say NO
2- 3 - Most resistors on these old amps are good . I mean, good enough to keep the vintage sound and preserve the amp's personality.
It would be a mistake to replace them.
As beginner in the field, it's a pity that you're learning and experimenting on such a good amp at the risk of devaluing it, if not damaging it.
You have to study vintage amp before, know to do and what not to do
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My take is somewhat different than Latole's.
The best answer is always the correct answer. On metering resistors in circuit, PRR has the right answer. If you get a reading that is way off, the resistor is either fried or is in parallel with other resistor(s) and must be measure out of circuit.
How well resistors have held up over the years, depends on how the amp has been treated. But Fenders of that era are pretty robust.
Replacing out of tolerance resistors is not always a mistake. It depends on what your intentions are. Do you intend to play the amp, gig with it, preserve it as part of your retirement plan? Tube amps were meant to be maintained. I maintain and play mine - that means I replace components as needed, saving the removed parts for posterity. As others here, if resistors are over 20% out, and the sound has deteriorated, I replace them.
And don't expect NOS resistors to be on-spec. They drift, even when in storage.
BTW - I see that you have done some reading and have a plan - and you are asking questions - all good!
Best of luck with a cool amp. Go slow and be careful.
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One thing to consider when measuring resistors. If it measures open or extremely high, it's always a failed resistor, bad connection, or bad meter reading. Paralleled components will never read high when measured from leg to leg
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I never say to not replace bad resistor ( or capacitor) .
I'm just worried that this exceptional amp will have its value and sound diminished.
Apart from replacing the filter capacitors with F&T or Sprague, I avoid Asians in vintage, check the 470 ohm resistors on the 6L6s and replace the defective tubes while checking the bias on the 6L6s, I wouldn't touch anything else if the amp is working well.
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I recently purchased a 1966 Fender Super Reverb. The chassis appears to be 99%. ...
The first part of my plan is to install new power filter caps and resistors, electrolytic caps and resistors, and the bias cap and check the resistor and diode. Also, to install a 3 prong grounded power cord. ...
I have to ask "Why?" for all the bolded items.
- If the amp sounds good, strongly consider leaving it alone. Don't go fixing things that aren't broken.
- It is wise to replace the electrolytic capacitor in the bias supply.
- It could be smart to replace the power supply filter caps. However, I have the test gear needed to honestly check their performance, and have encountered a case where the 1964 filter cap tested as-good or better than a modern capacitor that might have taken its place.
- The advice to "simply swap filter caps after ____ years" comes from service technicians whose customers won't pay for the extra time needed to actually test the caps. Additionally, those techs don't need a griping customer when a cap that "works fine today" fails tomorrow.
- The preamp cathode bypass caps could fail as a dead-short and probably not damage anything, due to the current-limiting provided by the plate-load resistors (the resulting ~1w through a 1/2w resistor might not even burn it). So there is little-risk in leaving them alone. I have found over the years that I prefer the sound of blackface amp where the cathode bypass caps have aged and fallen in value from their original 25µF to 1-2µF. This barely-shaves the level of the bottom 3-5 notes on the Low E. I also find I need to turn the Bass control down to 1-2 to balance the Treble at 4-5 and Middle up near max (the blackface circuit has way too much low end, especially if you push the amp towards distortion).
1)Can you get an accurate reading from resistors in circuit?
2) If they are out of spec, is there a typical out of range that determines when to replace them?
Most resistors in the amp could have an actual value double its marked value, and you might not even notice. Certainly, the amp will almost always "work" and could even sound quite good despite being "out of spec."
When I started out, I assumed there was some magic in "the spec" and I didn't know enough about electronics to know what might alter an amp's performance. So all I could think to do was try to be "on spec" so the amp might work "as designed."
I recommend you do nothing more than replace that bias filter cap, and the power supply filter caps. Then go ahead and measure all the parts, and annotate their values on a schematic/layout. Then listen to the amp and live with it for a while. Get to know how it sounds & what it does before rushing in to "restore" stuff.
Because there may not be anything that needs fixing here...
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I agree only replace if broken.
I have a Silvertone 1484 that has been discussed in this forum. Sounds amazing.
https://el34world.com/Forum/index.php?topic=26406.msg310044#msg310044
photo essay:
https://stillampd.com/silvertone-1484-restoration
I even left the Filter Caps (beautiful Planet reds) because they were not noisy,
although being able to leave orig filter caps seems rare.
Make it safe, Add 3 prong, move fuse & switch to line side (many vintage have on neutral)
if present remove death cap (or relace with safety cap), Then fix things that are broken.
Leave the rest alone.
Always be careful inside, know where live voltages are
and when off, bleed down caps before touching.