Terry, I'll never do any of this work, but between you and Uncle Doug, I get my amp jollies satisfied. Great content and explanations, as always! Thumb is definitely up. [EDIT: the second tube in the final demo with the inverter tube (which I think was the original) has a better tone, in my opinion. It is right on the point of breaking up, and if you were to dig into the picking, I suspect it would go into a very nice distortion. Get Al to try it out]
The owner is going to be SO pleased with his amp now!!! Great mods and great sound...and "lack of sound" when it comes to the noise that was on it at the beginning of the video.
For all the cool amps of yours that Tony gets to play maybe he could help you out with a few lessons....😉. Thank you Terry for making a crazy 2020 much more tolerable with your great videos and tech tips. I have learned a great deal from you. Merry Christmas to you, Marcia, Emi, and your whole family.
This was an amazing video. Great catch on the volume wiring. And the 6ac7 I didn’t know you could sub that. The 6n7 hookup was great too. You rocked this one!!
Thanks for another excellent video Terry. I know you don’t play guitar, but if your mate could show you how to set up an open tuning to strum, I’m pretty sure we’d all see a smile grow upon your face as you hit a perfect G or E chord.
Terry, God Bless and Protect you and your family. Thank you for sharing your wisdom with your channels community. You produce amazing educational and inspirational videos. Bless you for sharing your life with us. We turn off the TV and turn on D-Lab Electronics!!!
As in life so are vintage amps......options! This is truly one of Gibson's best sounding amps of all time! It is clean and full sounding. In some ways the electro-dynamic speaker is better than more modern permanent magnet speakers. There is something about the choke being used as a speaker magnet and the power supply filter choke that gives the amp a cleaner, robust tone. There are great sounding PM speakers, but this one is hard to beat! Great video Terry! Merry Christmas to you and your family. Love from NW Colorado. Thanxz
Terry, just for testing purposes, you could ask your friend to do a short digital recording of straight in guitar playing so that you could play it back through the amp when you don't have a guitar player laying about the shop. It will give a better representation if how the amp will respond than a random strum.
Gibson made these amps for absolute clean at max volume. Overdrive at the output was like nails on a chalkboard to the old timers. These days, it’s blasphemy to not have overdrive. I do agree 100% with moving the volume pot location in the circuit. Gibson did the same thing a year or two later in a circuit revision in all of the instrument amplifiers. These amps had wax/paper coupling caps originally, by now, they’re all leaky. You will never hurt my feelings replacing those. Just don’t throw them in the trash.
After much contemplation I wonder... given the age of this amp the microphone input was probably designed around being used with a crystal mic. These are drastically different than a modern electro-magnetic system of transducer, so it would have been interesting to test the stock configuration with an antique piezo crystal microphone.
I guess you could use a 220k anode resistor to even get a little more gain as well as bootstrapping the cathodyne phase inverter. But really nice you fixed that noise issue!
You are correct sir. But, going that route may be too much gain to bring on distortion. In the day, every thing was geared to clean output. Good catch though.
@@kengevers8738 True, it will also change the sound slightly. But it was nothing more than just a little idea if people are looking for a bit more gain.
Seems like the big issue here is a misunderstanding of the design. The control is clearly labeled "microphone". Apparently it was expected that the guitarist would use the controls on the guitar to control their volume. The pot was just to strike a balance between mic and guitar. I did notice that your redesign changes the input impedance - which could affect sound.
Great video. Adding flexibility is always cool. The 6N7 did perhaps have a bit better sound. My only question is: in your experience, are either of the two inverters more robust than the other?
My first assumption (assuming proper design), would be anything AFTER the volume control! can see why so many techs went down a rabbit-hole with this amp!
I have a GIBSON RVT 35 it has 2 of the 7591 Power Tubes and 6EU7's in the pre-amplifer stage. I built a new Cabinet 5ply birch and a new speaker. It squeals and has to much treble I hope there is cure for it. At 35 Watts is a powerhouse amp.
Would you happen to have a indept video on the conversion of a Tube radio to a guitar amp? Im looking at getting a Zenith C724P (The Super Caroline) and was wanting to know how difficult this project would be.
Nothing more gratifying than finding a design flaw and fixing it. In addition: Installing an electrolytic 50uF at 50VDC across the 200 ohm common cathode resistor of the output tubes should increase the gain a little more. Also, a slight mistake in your new schematic for the 6AC7 tube. The location of the suppressor grid pin 3, which is grounded, should be swapped with the screen grid pin 6. It should be plate then suppressor grid pin 3 then screen grid pin 6. Compare it to the 6AC7 tube schematic. Good job though.
Terry, next time you have your guitar player friend over for a demo record the guitar part. Save it until you need to do something like this. Then replay it through whatever you are testing.
very good, I am Brazilian and I follow all your amazing videos ... Do you have any video or tutorial explaining how to put Send / retourn on the Fenser Frontman 25r amplifier? Thank you very much
If the first stage is grid-leak biased, doesn't it also need a coupling cap at the input, like in the original schematic (.01uf)? Your schematic is missing it.
@@d-labelectronics I've read that it is to prevent grid current from leaking thru the input resistors and that it would help keep the grid at negative potential to help bias the tube. Interesting it still works without it. Maybe it's a cautionary addition to grid leak bias circuits. All amps with it have that cap.
Yes, I was also wondering how the tube would properly bias without the grid leak bias cap there to charge up from grid current and bring it negative (hence the massive grid leak to hold charge in the cap). converting to cathode bias would be a more stable configuration anyways.
Hey Terry, great video. In your new schematic, shouldn't the Suppressor Grid be pin 3 and the Screen Grid be pin 6? Also, in later Gibson amplifiers late '50s early '60s, they started putting caps from cathode and screen to ground and started using grounding inputs on the instrument jacks. This amp could really benefit from those two mods as well. Gibson just loved pentode preamp tubes for some reason. I'm not really a fan.
According to both the 6SJ7 and 6AC7 datasheets, the suppressor is on pin 3 and the screen is on pin 6. Having your suppressor grid tied directly to ground, doesn't seem right to me.
@@d-labelectronics Not arguing that. I just find it strange that it would have a voltage on the screen and then have the suppressor tied to the same point as the cathode. Kind of defeating the purpose of the pentode design.
Hi Terry! I get into these old Gibson amps here and there and enjoy seeing your solutions! I have a question about the final schematic. The 6AC7 seems to have the screen grounded and 25 volts on the suppressor grid through the very large resistors. Doesn't grounding the screen turn off the tube? I know I'm missing something here.
@@MrCarlsonsLab Hi Paul! I love that you watch the other TH-camrs like the folks that watch your channel. You all have a unique way of conveying knowledge and guys like me are grateful for guys like you and Terry that are willing to share.
Gibson amps are like Fender acoustic guitars. A nice thought. When Gibson made a good amp it was a direct copy of a Fender amp. When Fender builds a good acoustic... it's because they own Guild
hey D-lab im assuming you gave that 6sj7/6ac7 spot a cathode resistor? cuz you took out the grid leak bias circuit. so if theres no cathode bias added the tube should be able to run away right? your drawn schematic doesn't show a cathode resistor and it made me wounder if its miss drawn?
I'd image that 1) the amp was always running wide open and 2) the pot was in the most sensitive part of the circuit (the input) and acting like an antenna.
@@d-labelectronics Any specific reason not to put them in series? That way you will have plenty of gain and/or more freedom to add some additional things.
I did not want to modify the circuit. This method allows use of the stock tube as well as the 6N7. Im sure there are many improvements that could be incorporated, however not the intent of this video
Pardon my intrusion, but, twisting tubes into place to find the socket groove is a NO-NO..!!!!! You are depositing the metal from the tube pins to the socket, allowing metal to span from socket hole to socket hole... Especially nice when installing 6L6-EL34 type tubes, then shorting plate to filament!! NO, you cannot remove/clean ANY of that metal as it is ingrained into the socket material, so the socket MUST be changed out.. I do a brisk business replacing sockets and tubes, if the fuse(s) has blown in time and you haven't shorted the output trafo..!! LOL! Go, boob toobers go.. I love it..!!! LOL!
@@kornami8678 After more than 40+ years as a successful tube amp tech, I think I have seen a LOT more than you.. BUT, hopefully (can I pray?), many boob toobers will read this and decide that your and D-labs method is better and makes more sense than mine.. NEXT..!!! LOL! boob toobers, ya have to luv 'em.....!
@@pjf7943 Hi PJF. Can you explain how metal is deposited on the phenolic or ceramic socket when the center index pin (Guide pin) is longer than the tubes electrical contact pins, "in all applications?" How can you deposit metal, when no metal touches until the index (guide pin) allows the tube to drop in square? The solder on the tube base connections doesn't rub off and leave a track either. Where do people come up with this stuff?
Terry, I'll never do any of this work, but between you and Uncle Doug, I get my amp jollies satisfied. Great content and explanations, as always! Thumb is definitely up. [EDIT: the second tube in the final demo with the inverter tube (which I think was the original) has a better tone, in my opinion. It is right on the point of breaking up, and if you were to dig into the picking, I suspect it would go into a very nice distortion. Get Al to try it out]
The owner is going to be SO pleased with his amp now!!! Great mods and great sound...and "lack of sound" when it comes to the noise that was on it at the beginning of the video.
For all the cool amps of yours that Tony gets to play maybe he could help you out with a few lessons....😉. Thank you Terry for making a crazy 2020 much more tolerable with your great videos and tech tips. I have learned a great deal from you. Merry Christmas to you, Marcia, Emi, and your whole family.
Ditto!
This was an amazing video. Great catch on the volume wiring. And the 6ac7 I didn’t know you could sub that. The 6n7 hookup was great too. You rocked this one!!
Thanks for another excellent video Terry. I know you don’t play guitar, but if your mate could show you how to set up an open tuning to strum, I’m pretty sure we’d all see a smile grow upon your face as you hit a perfect G or E chord.
Terry, God Bless and Protect you and your family.
Thank you for sharing your wisdom with your channels community.
You produce amazing educational and inspirational videos. Bless you for sharing your life with us.
We turn off the TV and turn on D-Lab Electronics!!!
As in life so are vintage amps......options! This is truly one of Gibson's best sounding amps of all time! It is clean and full sounding. In some ways the electro-dynamic speaker is better than more modern permanent magnet speakers. There is something about the choke being used as a speaker magnet and the power supply filter choke that gives the amp a cleaner, robust tone. There are great sounding PM speakers, but this one is hard to beat! Great video Terry! Merry Christmas to you and your family. Love from NW Colorado. Thanxz
Thank you for your kind support. Hey, my daughter lives in Windsor
That's over by Denver and I am on the western slope, but what the hey, it's still Colorful Colorado! Thanxz
Terry, just for testing purposes, you could ask your friend to do a short digital recording of straight in guitar playing so that you could play it back through the amp when you don't have a guitar player laying about the shop. It will give a better representation if how the amp will respond than a random strum.
I agree with Mouse on gain boost. Good job Terry.
Gibson made these amps for absolute clean at max volume. Overdrive at the output was like nails on a chalkboard to the old timers.
These days, it’s blasphemy to not have overdrive. I do agree 100% with moving the volume pot location in the circuit. Gibson did the same thing a year or two later in a circuit revision in all of the instrument amplifiers.
These amps had wax/paper coupling caps originally, by now, they’re all leaky. You will never hurt my feelings replacing those. Just don’t throw them in the trash.
After much contemplation I wonder... given the age of this amp the microphone input was probably designed around being used with a crystal mic. These are drastically different than a modern electro-magnetic system of transducer, so it would have been interesting to test the stock configuration with an antique piezo crystal microphone.
Good stuff. You do know how to improve the amplification factor using some tube knowledge Terry!
I guess you could use a 220k anode resistor to even get a little more gain as well as bootstrapping the cathodyne phase inverter.
But really nice you fixed that noise issue!
You are correct sir. But, going that route may be too much gain to bring on distortion. In the day, every thing was geared to clean output. Good catch though.
@@kengevers8738 True, it will also change the sound slightly.
But it was nothing more than just a little idea if people are looking for a bit more gain.
Seems like the big issue here is a misunderstanding of the design. The control is clearly labeled "microphone". Apparently it was expected that the guitarist would use the controls on the guitar to control their volume. The pot was just to strike a balance between mic and guitar. I did notice that your redesign changes the input impedance - which could affect sound.
The final tube configuration sounded the best. Ditch the stock preamp tube and inverter for the 'hot rod' version! LoL
Great video. Adding flexibility is always cool. The 6N7 did perhaps have a bit better sound. My only question is: in your experience, are either of the two inverters more robust than the other?
They are both vintage metal tubes, still surviving. Both are robust
My first assumption (assuming proper design), would be anything AFTER the volume control! can see why so many techs went down a rabbit-hole with this amp!
I have a GIBSON RVT 35 it has 2 of the 7591 Power Tubes and 6EU7's in the pre-amplifer stage. I built a new Cabinet 5ply birch and a new speaker. It squeals and has to much treble I hope there is cure for it. At 35 Watts is a powerhouse amp.
Would you happen to have a indept video on the conversion of a Tube radio to a guitar amp?
Im looking at getting a Zenith C724P (The Super Caroline) and was wanting to know how difficult this project would be.
Nothing more gratifying than finding a design flaw and fixing it. In addition: Installing an electrolytic 50uF at 50VDC across the 200 ohm common cathode resistor of the output tubes should increase the gain a little more. Also, a slight mistake in your new schematic for the 6AC7 tube. The location of the suppressor grid pin 3, which is grounded, should be swapped with the screen grid pin 6. It should be plate then suppressor grid pin 3 then screen grid pin 6. Compare it to the 6AC7 tube schematic. Good job though.
you sir are certainly an amp wizard!
Terry, next time you have your guitar player friend over for a demo record the guitar part. Save it until you need to do something like this. Then replay it through whatever you are testing.
very good, I am Brazilian and I follow all your amazing videos ... Do you have any video or tutorial explaining how to put Send / retourn on the Fenser Frontman 25r amplifier? Thank you very much
Sorry, never worked on that model
What is the can-thing on the side of the inside of the chassis that's opposite to the controls?
That was the old filter capacitor mount
If the first stage is grid-leak biased, doesn't it also need a coupling cap at the input, like in the original schematic (.01uf)? Your schematic is missing it.
Hello, Good observation. I added it as a test, no difference in gain, Thanks for the input, I would assume it could be configured either way
@@d-labelectronics I've read that it is to prevent grid current from leaking thru the input resistors and that it would help keep the grid at negative potential to help bias the tube. Interesting it still works without it. Maybe it's a cautionary addition to grid leak bias circuits. All amps with it have that cap.
Yes, I agree and installed it. Will update the schematic
Yes, I was also wondering how the tube would properly bias without the grid leak bias cap there to charge up from grid current and bring it negative (hence the massive grid leak to hold charge in the cap). converting to cathode bias would be a more stable configuration anyways.
Hey Terry, great video. In your new schematic, shouldn't the Suppressor Grid be pin 3 and the Screen Grid be pin 6? Also, in later Gibson amplifiers late '50s early '60s, they started putting caps from cathode and screen to ground and started using grounding inputs on the instrument jacks. This amp could really benefit from those two mods as well. Gibson just loved pentode preamp tubes for some reason. I'm not really a fan.
Hello, I did not change that socket wiring. It is as shown on the schematic, right/wrong? Cannot say
According to both the 6SJ7 and 6AC7 datasheets, the suppressor is on pin 3 and the screen is on pin 6. Having your suppressor grid tied directly to ground, doesn't seem right to me.
Yep, Thats how it was factory wired, The 1st one I had here last year was the same. Dont know what to say : )
@@d-labelectronics Not arguing that. I just find it strange that it would have a voltage on the screen and then have the suppressor tied to the same point as the cathode. Kind of defeating the purpose of the pentode design.
Hi Terry! I get into these old Gibson amps here and there and enjoy seeing your solutions! I have a question about the final schematic. The 6AC7 seems to have the screen grounded and 25 volts on the suppressor grid through the very large resistors. Doesn't grounding the screen turn off the tube? I know I'm missing something here.
Hello, The actual base conf was not changed from the 6SJ7 when using the 6AC7, see the tube data charts for more info on that screen
@@d-labelectronics Thanks much!
Did you ask the customer if the amp always had this particular noise issue?
Yes, it had been at a few shops before mine
Thank you for your videos
Glad you like them!
Ther does not seem to be any bias on the input tube
It's grid leak biased
@@drteming70 The actual term is "Initial velocity bias." Grid leak bias is a term for a different application.
@@MrCarlsonsLab Hi Paul! I love that you watch the other TH-camrs like the folks that watch your channel. You all have a unique way of conveying knowledge and guys like me are grateful for guys like you and Terry that are willing to share.
@@MrCarlsonsLab you’ve just shattered everything I’ve known to be true. 🤪
@@MrCarlsonsLabSo much to learn. Thanks!
Gibson amps are like Fender acoustic guitars. A nice thought. When Gibson made a good amp it was a direct copy of a Fender amp. When Fender builds a good acoustic... it's because they own Guild
hey D-lab im assuming you gave that 6sj7/6ac7 spot a cathode resistor? cuz you took out the grid leak bias circuit. so if theres no cathode bias added the tube should be able to run away right? your drawn schematic doesn't show a cathode resistor and it made me wounder if its miss drawn?
Grid leak cap is installed. I will publish that in the new schematic rev on my website
@@d-labelectronics oh i see, i thought you took the grid cap out. it makes more sense now lol
would a 20 watt push pull work with this amp?
How come at the end of the video you were plugged into "Microphone" ?
all 3 input jacks had the 68k on them is that why it didn't matter?
The Mic jack has the shorting contact, it must be used 1st, like Channel 1 on a Fender
Where are those schematics posted? I'm trying to modify a Maas Carillon amp for guitar.
d-labelectronics.com/tek tips tab I have not posted the 2010 schematic yet. E-mail if yo want that one
D labs, I'm confused why the variable pot BEFORE the tube caused noises and hissing issues, any reasons why?
I'd image that 1) the amp was always running wide open and 2) the pot was in the most sensitive part of the circuit (the input) and acting like an antenna.
I hear quite a bit more detail with the 6N7...
Yes, I noticed that the 6N7 was a bit fuller sounding. By the way, this is a dual triode tube, strapped in parallel
@@d-labelectronics Any specific reason not to put them in series?
That way you will have plenty of gain and/or more freedom to add some additional things.
I did not want to modify the circuit. This method allows use of the stock tube as well as the 6N7. Im sure there are many improvements that could be incorporated, however not the intent of this video
👍🏻👍🏻👍🏻👍🏻👍🏻👍🏻👍🏻
Wow! That's really poor design! Were proper designs scarce back in (what appears to be) the 1940?
Terry, tune that guitar to an open chord!
Totally! His buddy could do it or, you guessed it: A TH-cam tutorial! lol
@@johntilson2535 He could crack open a new bottle of wine and become a slide master.
Why not have Tony tune your guitar to a chord? That'll make it more pleasing to the ears for us. You might even use a slide....
Pardon my intrusion, but, twisting tubes into place to find the socket groove is a NO-NO..!!!!! You are depositing the metal from the tube pins to the socket, allowing metal to span from socket hole to socket hole... Especially nice when installing 6L6-EL34 type tubes, then shorting plate to filament!! NO, you cannot remove/clean ANY of that metal as it is ingrained into the socket material, so the socket MUST be changed out.. I do a brisk business replacing sockets and tubes, if the fuse(s) has blown in time and you haven't shorted the output trafo..!! LOL! Go, boob toobers go.. I love it..!!! LOL!
I agree, just showing that I am human
The guide pin structure is longer than the tube pins. As long as Terry keeps the tube level and plumb, the tube's pins should never touch the socket.
@@kornami8678 After more than 40+ years as a successful tube amp tech, I think I have seen a LOT more than you.. BUT, hopefully (can I pray?), many boob toobers will read this and decide that your and D-labs method is better and makes more sense than mine.. NEXT..!!! LOL! boob toobers, ya have to luv 'em.....!
@@pjf7943 Hi PJF. Can you explain how metal is deposited on the phenolic or ceramic socket when the center index pin (Guide pin) is longer than the tubes electrical contact pins, "in all applications?" How can you deposit metal, when no metal touches until the index (guide pin) allows the tube to drop in square? The solder on the tube base connections doesn't rub off and leave a track either. Where do people come up with this stuff?
@@MrCarlsonsLab 'Mr. Carlson's Lab = Sleepy-time videos.... It's called experience and a lot more than you are old......!!
Great Tech. fix!