Over the years I've owned 5 Twin Reverbs.. In my humble view, the best guitar amp ever made whether turret or PCB circuit boards. As I got older , the weight became more of a problem, so I moved onto something lighter. Still trying to find 'that' sound, but so far it only seems available from the 'real' thing. Nice job Stuart and I did like your homebrew valve tester..
Happy days! I ws also interested in chemistry as a lad and used to regularly go down to our local chemist to buy ... chemicals! Seemed the logical place to me and they happily sold me various things like copper sulphate and suchlike!
Fender used two types of "hum-balance" circuit in various models; the commonest one, the one you'd expect to see, as shown here, uses a balancing potentiometer in the heater circuit for the tubes. However, some models of Fender amps had a pot which they termed "hum balance" but which was actually a "bias balance" adjustment which actually balance the current between the two (or the two pairs of) output tubes, and when the control was set so that audible hum was at its lowest, the current was roughly equal between the two tubes (or the opposing pairs), because the pushpull output circuit has an inherent hum-canceling function if the tubes are drawing equal current. The problem with this is that most of those amps with that type of output tube hum-balance circuit typically didn't actually include a proper up/down bias-adjustment pot; and when you consider the wide variability in characteristics and quality of today's modern Russian or Chinese amplifier tubes, having a proper up/down bias-adjust control is more important than being able to perfectly balance the tubes in their sockets. It is of course quite possible to convert the bias-balance circuit to a bias-adjust circuit; or add a second pot and configure the circuit so it can do both. Note that here in the States at least, some vintage tube hifi amps from the fifties and sixties (Scott, Fisher and the like) might incorporate either ---- or *both* ---- types of "hum balance" adjustment. Some circuits that were termed "hum balance" actually adjusted the linearity of the phase splitter in order to reduce distortion as well as hum!
That tool is most certainly a tool for a guitar, to remove the "big" nuts on the tuners on the front of the headstock (and then some). Should fit a pot on an amp too one would think. Love your videos, Stuart. They are very educational and pedagogical. That triggers my long lost electronics education from more than 40 years back, that I never followed up as a career. Amazing though how much is still laying around in the backroom of the brain and wakes up by your videos :) Physics doesn't change and what was the facts back then is mainly still the facts - if not all of it - and much of the components in today's valve amps are still the same too. Keep up the good work :)
I wonder if that crackling in the sockets won't return. When I find this problem I replace the sockets to prevent crackling again. But sometime the costumer doesn't want that. Then I clean them like you did, but warn the costumer it will return. As for the valves I use Tung-Sol 6550 I have my thoughts about JJ quality. The Russians are very good and stay within there voltage and current settings. Very stable, I can't say that from the JJ's. I'm glad I can set the negative voltage for all tubes separate. So a little unbalanced isn't an isue I readjust them in a later time. But those Tung-Sol they stay stable after a year. Well nice video of your work, keep it up👍
Hi yes I'm afraid my customers wouldn't want to pay for new sockets. Most of my customers do not have gig-critical gear, most just play at home to it's no real issue if something goes wrong again down the track. Agree about JJ, not very consistent quality. You'll be aware of the ddire shortage of valves at the moment!
Hey Stuart, Just found your channel..! Been following US techs for a while now, trying to learn as much as I can about valve amp repair etc..Great channel mate..Glad I found it. Interesting and great tips..I've subscribed...Keep up the great work..Ed..Herts..😁
Young Stuart, If you can find a good straw that fits the pot shaft, around the threads, maybe even some tubing if it doesn't melt, that might be a good way to get your cleaner into the pot from the shaft without getting it all over. Might be worth a try. Seems like I recall something like that was on the market years ago. Also, thanks for the new videos. It's like a desert out here now that I have binge watched everything.
If a socket doesn't fit the shaft nuts for the pots snugly and the socket tends to spin out and off the nut, the socket likely has too much bevel or chamfer on its inner edge. Try a different socket, or file the mouth of the socket flatter to reduce or nearly eliminate the inner bevel or chamfer. The shaft nuts used for potentiometers are thin and shallow, but most sockets are designed to fit the much thicker nuts found under the hood of your car!
Ah yes, reminds me of the time I'd liberally sprayed all the contacts in a pinball machine. I had my head right inside and turned it on..... KAWOOMPFFFF!
The standard 6.3 volt pilot lamp is a #47; there is a longer-life version of the #47, called an # 1830 lamp, with the same specs and the same brightness, but it has a heavier filament and additional support so that the filament is less likely to break from speaker vibration and transport shock.
@@stuartukguitarampguy5830 , I suppose you could use a modern LED bayonet lamp to replace the #47 pilot lamp but it would need to have a diode in the lamp base (as well as the usual dropping resistor) to run off of AC without flickering or having a shortened life. An LED could potentially introduce noise; also, the QC of modern LED bulbs leaves a lot to be desired and typically have the ground to the metal shell merely press-fit, not soldered (I've upgraded hundreds of old hifi units to LED'S and have seen most of the pitfalls at this point). Besides, everything now tends to be over-voltaged in these old amps, including the heater supply, and removing the current draw of an old- fashioned incandescent lamp would only tend to increase the heater voltage to the other tubes.
@@stuartukguitarampguy5830 , the lamps are cheap, readily available, and the heat they generate isn't an issue. In vintage hifi gear, however, the heat of multiple dial lamps and function-indicator lamps causes fading, discoloration or even melted plastic bezels, and in some cases require major disassembly of the unit in order to replace the incandescent lamps. LED'S, done right, are long-lived and generate almost no heat, but getting a good color match and even dispersion of light which still looks "vintage" can be tricky. The hardest part is invariably replacing a miniature light bulb that is built into the base of the dial pointer, while it is still attached to the dial string.
Purists might cringe, but it is quite possible to drill a small pinhole in the circuit board next to or in-between the solder connections for each potentiometer, so that you can squirt contact cleaner through the newly made holes in the board and into the open slot at the bottom of each pot (preferably with a syringe). I've done this many times over the years on vintage 70's/80's analog hifi receivers, Pioneer and Kenwood and the like, where the volume/tone/function-switching board is hardwired, tethered in place with so many stiff solid-core wires that it is virtually impossible and certainly impractical to remove it for access to the pots and switches (the wires fatigue and break off from even minor flexing). This does require a Dremel tool, a tiny carbide bit, and a very steady hand, and is not for the faint of heart (I have been using a Dremel tool regularly for over 40 years!); and If the traces are as tiny and and closely spaced as they are here on this Fender amp, you might drill through one of the traces accidentally, or perhaps on purpose, and subsequently have to replace the broken section of trace with a short jumper wire. On the negative side, you can do a lot of damage if you're not sufficiently skilled (though I haven't damaged a pot or screwed up a circuit board yet!); but on the plus side, you'll never have to remove that circuit board in order to clean the pots ever again. (If it's a unit that cannot be operated and tested while apart, It truly sucks to clean all the pots, and then put it back together to test it, only to discover that one or more pots or controls are still noisy and you must disassemble the unit a second time in order to clean the pots again). If the amp gets gigged with a lot, or goes on the road with a band that can afford a proper guitar/amp tech, the road crew will appreciate the newly simplified maintenance of the modified circuit board...
@@stuartukguitarampguy5830 , so far, I've not dropped any bits of fiberglass circuit board inside the pots, but like I said, I use a very small carbide bit, and I might actually break through the other side of the board, or ream the tiny hole out slightly larger, by hand, with a carbide bit chucked into the handle of an ExActO knife. Often I will find that the limited physical access to the circuit board while it is in place inside the amp requires me to drill at a bit of an angle through the board anyway. Also, If you prop the amplifier up on its side, gravity will tend to pull the chips ( swarf!) sideways and down, away from the opening in the underside of the potentiometer. I've also drilled into "sealed", pushbutton, plastic-body, signal-carrying switches in old hifi gear so that I can get cleaner into them. That can be trickier than drilling through a PC board to clean the pots.
Thanks for this nice video. Your explanation helped me to get my Twin back with no cracking. Which bias meter is used in this video? Any recommendations what to get?
I love watching your work Stuart. I'm curious to know what you think about lightly coating the valve pins with dielectric grease. Do you think it might help protect against oxidation? I've not tried it.
@@stuartukguitarampguy5830 It's a silicone dielectric grease some times called bulb grease. I've found small tubes of it at automotive supply stores. I use it on outside light bulb bases to prevent them getting frozen in the bulb sockets. I don't know if it helps, but I've been using it on flashlight batteries too hoping to prevent corrosion if the batteries start leaking.
Thanks for your expertise Stuart! I have two of these in for repair and one has intermittent loud cracking sound on the output; just wondering if the Panel board connectors give contact problems. I used to buy all valves from Watford Valves, I used to live just outside Watford. Unfortunately although the valves are cheaper than in Brazil, they want £100 shipping which is so much that it's not worth it. I have found a supplier of NOS valves here like RCA, Phillips, Reflector and GE; I am aware that the older valves last longer, what are your experiences? P.S. I would never buy Chinese!
Hi. Its very hard to say if older valves last longer. I doubt anyone has done the test? Crackling can be SO many things I'm affraid. It's a case of just going through everything systematically. Sorry I can;t be more help!
@@stuartukguitarampguy5830OK thanks anyway, don't like the look of the connectors so I'm going to put a dab of hot melt on them and put contact cleaner in the sockets (although the amp looks new), no effects loop return jack to play up on this one!
@@stuartukguitarampguy5830 Hi Stewart, I don't want to keep bothering you as like me I know you're busy. The homemade tube tester you have; is it the same design as the one from Valve Heaven? A 30v tapped transformer with a voltage quadroupler diode/capacitor circuit to bring up the HT voltage?
Hi Stuart... thanks for your wonderful videos!! I would really like to see a video on your homemade valve tester. Besides the three DC supplies (Plate, Bias, Heater), what types of components and connections are there in the valve box. Thank you in advance.
Hi Thomas. Well the story is this. I was walking past a skip full of rubbish and on top was the piece of gear you see in the background! It's a valve power supply with heater, Bias and HT outputs!! All I did was hook it up to some sockets. The only components are a 1 phm resistor in the cathode, across which I meaure the bias current (voltage), there is also a 470 ohm 2W screen resistor - and that's it!
@@stuartukguitarampguy5830 Thank you.... I have a Heathkit Sp-2717A Power Supply. So for the High Voltage source, direct connection to the plate, but a 470 ohm resistor to the screen. I will add the 1 ohm resistor to the cathode. I will give it a try to a test board before building the box. So I'll give it a try. Cheers !!!
@@ThomasACarlos Yes. 2W on that screen resistor. You'll also need about -50V maximum for the grid of course. About 350V is ok for the plate. Adjust grid slowly up in value (more positive) whilst monitoring the cathode current across the 1 ohm resistor. Aim for about 35mA (=35mV).
@@stuartukguitarampguy5830 I finally completed my test jig - thank you!! I realize that you are choosing arbitrary voltages here, just to get a current reading and then comparing valves. Based on the 6L6GC and voltages you chose, it seems like 17 ma is in the "cold" region. I would have expected that a good test would be an HT setting of 360vdc, running the valve (by adjusting the Bias voltage) to a target of 50%, approx 50 ma (if simulating a Class A circuit), and then comparing other valves to that. This way, we are testing the valve in a more realistic environment. Does that make sense?
Thanks for putting together this video Stuart! I'm actually just after picking up a new Twin Reverb myself. There is a slight hiss and hum when the amp is fired up (nothing I can't live with) but it goes extreme when I engage the reverb via the foot switch. The noise is akin to having unplugged the lead from my guitar if that makes sense. Could this just be grounding issue? I moved the amp around the house but it doesn't seem to make any odds
Hi Fran. Hmm, tricky to diagnose over the internet. It could be the leads down to the reverb pan. If you have a spare pair of twin phono leads, maybe try swapping them out?
Hi Stuart! Thanks for the reply and for the suggestions, it's very much appreciated! I don't have a spare of leads but will try and pick up some and give that a go. The amp is actually brand new and still within the return period but I had it shipped from Germany as they are very hard to come by at the minute. I think I'm going to take a chance and hold onto it so hopefully it's nothing serious. If I lived in the UK (I'm in Ireland) i'd gladly have you look at it :-)
Hi Phil I don;t really have one but it's quite easy. Just take the -ve bias to the grid, the HT to the anode, ground the cathode via a 1 ohm 1W rresistor and have a 470ohm 2W resistor from anode to the screen. Measure in mV across the 1 ohm. I have three DVMs. One for HT, One for bias volts and one for the mV across the 1 ohm, which gives you the current of course. Hope that helps.
I STUART thanks for the info .have you got the model number for the psu. I have been makeing tube amp out of old junk for year. now I am no longer working and have a lot more time nice to chat PHIL.
@@stuartukguitarampguy5830 I STUART thanks for comeing to me. yes i got you now.my old power supply has ajust H.T. and 6.3v but i will need to fit a L.T. ajust supply for the Bias 0--30v all some think like that. I looking to fill sum of my time. so this will help.thank you and if you like to chat sum time on the net give us call. keep the tube amp working PHIL MANNING.
@@philmanning6488 Hi Phil Make the bias supply 0-50 rather than 0-30 as that won't give you enough negaitive bias. Should be fairly easy with an old transformer with the right voltage (say 48V) and a bridge rectifier and a smoothing cap. Don;t forget to connect the right way! E.g positive to GROUND and negative to your -50V bias pot.
Hello, I have a 1964 Super Reverb and it makes the same loud "boom" noise when I flip the standby switch on. Is that normal? What do you recommend checking if I get a lot of hiss when turning the Treble knob up?
Yes I seem to recall that's normal. If you get excessive hiss it might be a noisy valve. Try swapping out the ECC83 (12AX7) one at a time and see i that helps.
@@stuartukguitarampguy5830 Thanks! Do you have any ways to fix or check when the Volume and Treble knobs having a lot of noise when turning up?? (In the Vibrato Channel) Any steps to check it?
@@stuartukguitarampguy5830 when the amp is turned on, if I'm turning the volume or the Treble knobs up I get a loud white noise even if the amp is on 3 on the volume. I've sprayed the pots, and it's not the problem. Can you tell me what should I check and in which order?
@@stuartukguitarampguy5830 i would rather pay the extra than go through the hassle of caps failing. Older IC caps are good quality as they are in my 1978 and 1983 Peavey Classics. They never fail.
I think it's supposed to be a multi tool for jacks and pots. Pretty useless really. I only bought it because someone told me off for using an adjustable spanner!
