That 25k resistor builds out a fairly high impedance in parallel with the power tubes, that gets swamped after the switch is closed. At the instant the switch is closed, that higher static voltage prevents any current flow by debiasing the tubes, allowing silent switch-on. As soon as the tubes begin to flow current their lower impedance will draw down the DC voltage at the cathode and they will conduct as usual. Its actually brilliant, something I have not seen before, but WILL be installing my own scratch built amps that use cathode bias resistors/caps, and never had a 'standby'. Brilliant way to silence the turn on when the "standby" is done by ground side switching. Brilliant! I actually learned something new in tubes today👍🤣🤣🙋
Peter Flynn The 25k provide a weak connection from the plus to the cathodes, but its plenty to 'pull' the cathodes up towards positive, which puts them above the grid voltage that remains at the same potential. With the cathodes higher than the grids, no current at all can flow. Once the ground side switch is closed, the voltage divider between the cathode resistor and these 25k pulls down the voltage... as the tube conducts, its internal impedance is low, MUCH lower than the 25k in parallel with it. The 25k is going to pass a small amount of current that is just wasted, but that is a small sacrifice to make considering the huge benefit in creating silent switching and assuring the power tubes are truly off when in standby. Its SO simple, yet so clever. I been in hi-fi amp design for 50 years where we don't use 'standby', but have also serviced guitar amps along the way. I have never encountered this before, and am impressed with the original designer's solution to creating a safe ( not switching high voltage) and effective method of making a standby. Hope this helps... thanks for your interest. I love Uncle Doug's channel. He is a top level educator carrying forward knowledge to the next generations... 👍🙋
@@jenniferwhitewolf3784 But still, isn't the cathode resistor supposed to make the grid (noticeably) negative (in relation to the cathode); with cathodes at 90mA toghether and with 150R resistor this equals 13,5V; but with this voltage divider it is fixed at 1,8V? This approach kind of reminds me of tranistor biasing ...
I want to express my gratitude for not using ads. RU-vid has lately doubled the amount of video ads, and it drives me crazy. Clicking on your videos feels so relaxing in the midst of all this.
I have to say, I love videos like this. I'm trying as someone born into the digital age to learn and understand tube circuits and your videos are some of the most informative and direct to the point. I'm forever grateful that you're providing this wealth of knowledge to the world!
I love it when you run through the schematic and,back,and,forth to the chassis I am,trying to,absorb as,much of,your knowledge as I can and have learned a,great,deal allready very informative videos thanks so,much
@@UncleDoug I agree with Jeff. Your explanation between the schematic & the actual amp, along with _your_ experience, is a great way to learn how to understand tube amps. I've got an old amp that was _dropped_ by someone quite hard years (decades?) ago and then discarded. It's one of those combos with the casters under it due to its weight. I'll _slowly_ start diagnosing it using current limiters, Variacs, etc., until I can get it working well again. I intend to use what I've learned from your videos, and others too, to repair it. Hopefully I won't have to repair the woodwork too!
I'm the lucky owner of what appears to be an exact version of the Gibson GA-77 in your video (Just wish it was in as pristine condition as this one.) Even luckier, it came to me for FREE! My brother found it sitting aside a dumpster in an apartment complex. Don't know if someone was throwing out the goods of a departed spouse or what, but giving me this amp was probably the best thing my brother ever did for me. It's been a favorite for about 20 years! I like it as good or better than my 1953 Tweed Deluxe (and that's saying something.) However, my GA-77 is a bit different internally, as mine has all octal pre-amp tubes. Having owned more than a dozen Gibson/Epiphone amps of the 30s - late 60s variety, and working on my own amps for a few decades, I can confirm what you and other's have commented on; and what I've discussed with other's as Gibson's "amp of the day" building style. I learned some great stuff from your video. The part about the standby switch and +DC to the cathodes and eventual discovery that it aided in silencing the pop from standby switching. Great tip discovery there! BTW, some 60s Gibson amps had a "monitor out" that was simply a jack attached to the output transformer's speaker output, connected with an attenuating resistor to limit it. That way you get the driven sound of the power tubes as well. That's how my 1961 GA-19RVT is. Keep up the good work and playing. I especially liked what you played during the drone flight.
That’s like five Uncle Doug videos in the last couple of months! Fantastic content for a guitar nerd like me! Unbelievable craftsmanship. With very best wishes from the UK.
Saw the big A and the landscape and immediately recognized El Paso. I was stationed at Holloman AFB for almost 14 years in the 80’s and 90’s. I don’t play guitar, collect amps, or even do much electronics, aside from automotive, but I do appreciate watching people who have deep knowledge and skill on a subject. My Dad was in Signal Corps back in the mid-late 50’s and was an electronics instructor. He explained to me in my teens how vacuum tubes worked. I also took electronics courses in high school and a college level course for aircraft electronics, even though I was an Airframe Guy. I lost some of that knowledge over the years for lack of practice, and your videos help bring it back. Great channel, Doug!
I have the same Amp on my bench right now. With the same mods. You were a big help in getting me through this project. Together with Fan Tech comments this amp was a sweet rebuild... Love Your Videos
That big resistor looks like the anti thump when you switch on from hot standby ! It keeps a small voltage across the cathode capacitor . The 12k stops the open cathode floating up to the HT ( saving the 6L6 heaters ? ) .
@@misterhat5823 Right. If the screen is 300V, then a 25K/150 voltage divider takes it down to 1.8V. I think the purpose of the resistors is to keep from shocking the tubes when turned on. Otherwise, the first time a warmed up amp is taken out of standby, you've got a grounded cathode with no bias voltage until that cathode bypass cap charges up. If the two resistors charge the cap higher than the operating voltage, then you'll have a "soft start" when taken out of standby.
@@russellhltn1396 I agree. It does look like it's avoiding an overcurrent event when coming out of standby. Also, I'd add that it's not just the first time. Because the cathode resistor would discharge the bypass cap without the 25K, it would occur every time the amp comes out of standby.
Jack & Ollie’s technique has improved quite a bit over the months! ...They surely earned those treats. But I agree ; I think you’ve earned your cut. Thanks for your time & knowledge my friend... Very entertaining as usual.
Somehow you have the ability to make electronics repair entertaining and interesting. You have the patience to break down the more complex complex concepts. I always learn a few things.
Great vid, really nice sounding amp, and kudos especially to Jack and Kasey for an excellent selection of tunes, expertly rendered, to show off what this baby can do! Seems like they've been putting some time in in the woodshed.
I read all the great comments but I would just like to mention your drone fly over and music track. Its very nice. Thanks for all your videos I think I have watched most every one of them I have learned a ton. Best wishes to you and yours. Looking forward to the next one.
Not a lot of headroom in that little amp, but it has a really nice crunch to it. Great job sorting out the mess. Looks like you fixed Gibson's design flaws too. Kudos!
New subscriber here. I love your videos. I’m trying to learn as much as I can regarding tube amp repair. I’m working on designing/building my own tube amp, so you’ve become my reference canon for all tube amp designs, considerations, and repairs. Thanks a lot for taking the time to record, edit, and produce your videos. Not only are they very informative - they are entertaining!
I really like the sound of the single coils on this amp better than the humbuckers. This was as you say extremely cramped design and the working environment. Trying to get in deep with a soldering iron to work on some of those components is treacherous for the things you need to get past without damaging them. Back in the 60s when I was learning about tube circuitry and later in the 70s working on them we didn't have any tools like your plate monitoring current devices or digital capacitance meters. It was all trial and Terror that we had to work with. Nice job on this one again. There is no replacement for knowledge but more importantly on top of that practical experience.
Thanks so much, Mike. I agree that the single coils have more clarity and purity than the somewhat muddy HB's.....but for down and dirty music, there is something to be said for "muddy".
I like that trial and terror description. I almost had an early chance to get a leg in the door working on guitar amps, with a friend that had a Bass model I forget now what it was. But notably it had the red plate disease and he (and the dealer) wanted me to fix it. The only thing I really knew about that situation was that there was dangerous voltages about and only a fool would charge in there without an insurance policy. I had to decline the opportunity due to a decided lack of any further information than I already had, which was next to nothing and I'm just not that guy.
@@leebarnes655 Lee, you just described the exact situation I saw my dad in when I was 10 years old. Except he did charge in and almost get electrocuted. I watched him fly across the room. It was then I decided that there should be some learning about things like that and that set me off on a path to learn about electronics. While I was still in high school I started studying so that by the time I got to college I knew some stuff. A little ahead of the game I like to be because I didn't like what I saw flying across the room and hitting the wall.
How simple is that! For some one like me, struggling to understand this circuitry stuff, seeing the channels on the face plate laid out that way on this Gibson made the schematic navigable for me. I finally had a understanding of a simple little circuit like this. And you know Doug I've been watching this channel for a long time. I'll get it. I'll get it. thanks UD. Meow
Thanks, Doug. Watched this one late as I have had net problems. Great video. What a great amp. Hope the cats enjoyed those treat as much as I did you working on this Gibson :-)
Hey Uncle Doug, thank you so much for what you do. You sir are an excellent teacher and I have learned a lot from your videos. Please keep doing what you are doing. Thank You and God Bless.
Claustrophobic? You weren't kidding. Hey that 25K forms a voltage divider with the 4K to set the 6L6GA's screen voltage (which is referenced to the cathode). The 150 cathode bias resistor completes the circuit and references both the cathode and the screen voltages to GND when the standby switch is in on position. 10u cap on the screens is a bypass cap which reduces the negative feedback which would occur between the fluctuating screen voltage being injected into the cathode voltage. So it is a biasing resistor but not for the cathode but establishes the screen voltage. Well this is my first pass reply, I'll probably come back and fix this later when something else pops into my head. Thanks for the video.
Yes, but is this actually necessary? I imagine it serves to reduce the shock to the HV circuit when the cathodes are connected.......but how is this any different than the shock encountered when a normal Standby switch is used.
@@UncleDoug My back of the napkin calcs estimate that the 25K in circuit sets the 4K/25K/150 current about 10mA (with a Vb = 300V). I estimate that the screen current is 1/15 * plateCurrent 43mA (you measured 43mA, 1/15 came from datasheet calc). This all combined together, with 25K in circuit sets the screen voltage at 247V, without the 25K in circuit it raises to 288V. Without it, the bias point is now different, which you compensated for by increasing the cathode resistor value (nice touch here BTW). So what, you all ask? I suspect the amp designer used a 6L6GA datasheet with the plate characteristic and design values showing Screen Voltage at 250V. That's the main reason why the 25K is in the circuit. Its all good, amp sounds great, let's all rock on with those giggin' cats.
@@CraigHollabaugh Furthermore, with 10mA current, there would be only 1.5V from ground across the 150Ω resistor, the 25kΩ would have 257V across and the 4kΩ would have 41V across. Simple voltage divider math.
@@zaraak323i Keep in mind a few items (you may already understand these). When datasheets list voltages, like the plate and screen voltages, these voltages are referenced to the cathode not the circuit GND. I estimated the screen current from the GE or RCA datasheet. Your simple voltage divider math holds true, keep in mind that there is a superposition current addition when calculating the 4K resistor voltage. I also estimated the circuit 300V Vb. Even with these estimations, my assumption about the 250V screen voltage being based on the datasheet design center values still holds. Amp designers had to start biasing calculations from somewhere, choosing the screen voltage is the most likely place. With that voltage established, the datasheet values lead the way to the biasing component value selection. Sorry for being such a talker here, hope some designers can gleam a little more knowledge when biasing pentodes from our discussion.
Another day, another outstanding video from our very own Uncle Doug! Thank you for the look at that magnificent amp. She was a beauty for sure. Very impressed by the tone. The early Gibson’s sure were nice! Also, that song at the end was great! I’ve watched pretty much all your videos and your playing has greatly improved over the years. Keep em coming Uncle D! Yer the man!
@@UncleDoug I have a very similar circuit design, with an additional bias circuit for the erase head of a tape recorder as the BSR (Elizabethan) Deck was designed to work as a cheap deck on its own or with an integral amp/speaker.
Wow I thought I was listening to Jimmy and George. I guess its a senior moment. Great video. And now we have some Morrison. Flash back. Maybe not so bad getting old. ZZ. Wow.
Sunday Service - Amen to that! I was thinking of putting a rotary standyby switch on a 300B valve hifi amp, which don't usually have one being tube rectified. I was toying with a massive resevoir capacitor and running on a valve rectifier but starting up on a silicon rectifier to allow the large in-rush of current. Anyway, maybe the weird connections where to do with that rotary switch - I need to look at the schematic at the start of the video again. Great work Doug, always a total pleasure! :o)
UD.....another great vid. The instrumental at the end of the vid was so fitting and pleasing with the flight over the area......it would not surprise me if one of these days, some of your fans will go on top of one of those hills and put the letters " U D " up there ! P.S. - It finally cooled off up here in Rhode Island into the 60's. Finally shut the fans off in the house and basement....someone mentioned the four letter word "S N O W" this week as they seem to want to curse the northern part of the New England area !
Tomk1tl, the weather here in Southern New England has been glorious for the last couple days---- but we're in a moderate drought and had better get some rain or precipitation of some kind soon. At least one well has gone dry that I know of in my small town. Good thing my pump is 300 feet down!
I applaud you for your usual skill and wisdom in the amp repair and continue the applause standing up for your patience and judicious use of copy and paste in the comments. ;)
Probably the cleanest Gibson I've seen! I think it's also the only amp I can recall that came stock with a 5V4. Definitely my second favorite tube rectifier.
Nice job Doug..Fab looking amp. Love the way you methodically sort out it's issues and recommend alternatives..! Educational to say the least.Thank you for sharing your expert knowledge as always..Ed..Uk.
Pretty good trick producing tunes with the Off/Standby/On switch in the Off posit. Unless the pilot light is out it seems to confirm that Uncle Dug has figured a way around the physics we all have to live by.
Making a separate digital track for the audio demo creates some special challenges, one of which is the need for a matching video track showing all the settings.....which is recorded after the fact and then synchronized with the digital video. No need to run the amp during the video creation.....it's just to show the settings.
Fantastic little amp fix Uncle Doug. Seems it was ahead of it's time. Don't trust those resistors with the round ends, they either go open or change their value significantly.
Doug, great videos that help amplify our common knowledge and provide resistance to electronic stupidity & the discharge of faulty and biased information! With the intro of this amp's standby circuit, it would be cool if you did a short video about the stand-by switching used in musical tube amps. There seems to be huge controversy about them on the internet; why they only seem to be on musical amps and not all tube equipment, what they are really for, how to best use them, or if they are even needed at all. thx
To be honest, other than in high-wattage amps with diode rectification, I feel that standby switches cause more harm than good......so a video showing their attributes is not likely, Ned. I've never understood how warming up the tubes a bit by energizing their filaments and then slamming them with high voltage is a good idea :) Tube rectification, particularly with an indirectly-heated-cathode rectifier, like a GZ34, is by far a better solution, providing a slow, gentle startup.
@@UncleDoug yeah, I have HIWATTs & SVTs, with 100+ watts. The HIWATT DR103 has Diode rectifiers (BYX94), with 4-EL34's on output. Standby looks like on secondary of xfromer & cuts the 460-470 voltage, but does not touch the 38v Bias. SVT is 330watts with 6550 tubes, 8 diodes for rectifier and standby is pre primary, with secondary voltages tapped from 330v to 600 volts. Most people seem say to just leave the standby on and turn the vol down when taking a break.
No light ON or switch to ON during sound test??Huh?? That really IS maJic............ Nice work and vid as usual Uncle Doug and a nice bit of music during the flight too very soothing, which I needed for my OCD about the Light and switch situation. : )
Making a digitally recorded audio track creates special challenges, GK, including the need for a separate, after-the-fact video track which is purely for showing the settings.
I noticed at the third test... a telepathic guitar, the evovution there is just defying my understanding of everything, I'll go hide under the bed for the remainder of this demonstration... with a yellow tin hat - Justin Case.
I hope the customer is happy. That is one of the best sounding amps that you have worked on. I need to find one. BTW in the small town where my father went to high school, they pained the graduation class year on the side of the mountain. He graduated in 1925. He had a rather short life and my family repainted the "25" in 1975. Now my brothers and I are going to repaint the "25" in five years. We are way too old to do it, but we will figure it out somehow.
Hi uncle Doug, glad to see more vids, which are always educational, and like the wonderful vids of scenery at the end as well. Take care and stay safe.
@@UncleDoug Mr. Franklin, my high school physics teacher (2 years worth for me! Physics 1 and Physics 2.) made us aware of why parsecs are important. And it also made us, back in the halcyon days of the late seventies, very appreciative of how "primitive" scientists (hah!) were doing their best to measure astronomical distances. I think it was a German guy (Bessel?) in the early 19th century who first started trying to use trigonometry to determine the distance of some stars. But that is boring old science. Your videos always make my day.
@@kendavis8046 Thanks for the historical review, Ken. One of my favorites is how the the speed of light was measured by Ole Rømer using the eclipses of Io, way back in the 1670's.
@@UncleDoug I remember that fellow, now that you mention it. First, it was brilliant that he was able to prove that light was not "instantaneous", that is, had a speed. But given the tech of the day, he was remarkably close to what came later, and those findings ultimately led to some fellow in a patent office figuring out some very interesting things! All that said, I loved science in high school, but actually did economics and sociology (hey, so did Reagan!) in college, then did an unrelated career in IT. BONUS - the way-better half still puts up with me even approaching a 40-year mark!
Part two was visually and audibly beautiful. It felt drenched in history and memories and your tune was so beautiful. Thank you Uncle Doug for sharing the story and doing that flight for us (Does Edward Scissorhands ever come down from the mountain for visits these days? I bet you could make him new hands) What a tightly packed amp and full of circuit diagram quirks. Your detailed analysis and repair was a pleasure to watch. You would need to travel many parsecs to find another wonder that is Uncle Doug.
25:45 you may got confused a bit because this circuit is a bit unusual, but it was just fine, see you got about 450v B+ goes through a 29kΩ resistor to an 150Ω which would give about 2.3V. this is done to have a bit of cathode bias when the switch is back to ON, rather than 0V (no bias at all due to the fully discharged bypass capacitor) which will be more prone to cause a "crackle". simplifying the voltage divider, the only thing that can emit electrons in the tube is the cathode, with it disconected the screen and anode can pass no current, the capacitors in DC are to be ignored. and in this way you can just ignore everything and just look at resistors. so what this circuit does is a softer start from stand-by.
LOL.. kerosene and sawdust... I think some of my amps were prepared with that option.!! . The El Paso mountains sure look fantastic.!. Nice job on this tight GA-77 Uncle Doug..!!! Thanks
The flammable materials were in gallon tin cans (from the school cafeteria), so the fire (generally) didn't spread to the mountainside ;) Thanks so much, Mr. G.
Thanks, Jeremiah. Making a separate digital track for the audio demo creates some special challenges, one of which is the need for a matching video track showing all the settings.....which is recorded after the fact and then synchronized with the digital video. No need to run the amp during the video creation.....it's just to show the settings.
@@kolinevans9127 I agree completely, Kolin, and the fact that the channel with a single tone control had more "guts and bite" than the one with separate TC's merely emphasizes the fact.
@@UncleDoug I already know; if you just freeze-frame at the end of one of your drone sequences, then zoom way in, you realize your face is 3 or 4 beige pixels...
