These amps were originally sold as A9 kits by a British company with an office in Taiwan way back in 2010. The case is unchanged. It is only in recent years that fully built amps became available from Boyuu Reisong in the guise of the A10. In 2018 SoundArtist had the 12ax7 version, long before Boyuu. I remember seeing an A9 kit demo'd here on TH-cam back in 2013 in this exact same case.
I have an A10 that uses the same chassis and internals (same voltages, and looks like the same OPT, so I am very exited to follow your rebuild if new OPT's will do the job
With relatively high transconductance and low drive requirements, 7591 were the mainstay of integrated tube amplifiers in my youth. I once converted a 7591 Eico ST0 to parallel-push-pull 6BX ()twin triode) using a 6H choke input to get the 6BX7 voltage right, replaced the input 12as with 6dj8 and direct coupled the6dj8 plate(as with stock 12ax) to the6sn7 inverter. That type can have equal value plate load resistors when the "tail" is very long via current source or negative rail and large Rk.It made a really good little triode amp. You've got the best bube channel in all of TH-cam.
The 6DJ8 is a great tube and as I mentioned, I've used both 6EJ7 and 6JC6A tubes triode strapped as drivers and they sound great. The Kegger/Blueglow KT88 amp runs a single 6DJ8 to drive both tubes so there is no reason it can't work here.
I am so glad you are going just rid it all out , and punt it out of the park . You tell them dear . I always knew great iron is need to make a great SET amp
hello , i appreciated your work on tube amplifier ... I am from France , i am 61 years now ...despite behing a PH in microelectronics , i have build more or less the same amplifier when i was 25 years old ... So long time ago .... this amplifier was with 250V/280V power suplly ( silicon ) and with 2.5K transformer from millerioux ( AH25BB as far I remember ) it has been working in my home for 10 years ..driver was the 12AX7 , parallell .... Then i go for 300B ... so for me you are fighthing against a transformer issue ... i was getting 6 Watts a 6 % distorsion .... and the sound was really good .... so good that my father ( electronic engineer in French Army ) decided to go back to tube .....sorry for my bad english ... this is not my native language ... currently i would like to buy high end hifi amplifier for my retirement but i find nothing to compare with a single end 300B with good output transformer , good power supply (polypropylene in oil ) , same for decoupling .... For me you have had a bad expeximent with a faulty product .... i do not say that all the chinese product are bad , i have myself been very disapointed with a line magnetic 845 and on the the other side been very happy with a low cost kit of 300B ..... i agree that to get the sound grall we have to upgrade them but it seems to me normal regarding the price pay ....FYI i build amplifier for 35 years ....I really appreciated your effort in our "tube lover" world ...
Just FYI, the 7591 tube is NOT a drop in replacement. The pinout is different and requires internal mods. My hope is it may require less internal mods than getting the EL34 to work correctly. Someone sent me a note that a 6V6 has a different pin out, not true in this use. The 6V6 ties the suppressor grid to the cathode internally, the amp does the same thing internally with the EL34 tube tying pin 1 and 8 together. It accomplishes the exact same thing and has nothing to do with how the tube performs.
Very good. I'm in for the ride and yes the chassis is nice so I would still use mine. I'm not against using 6V6 tubes if needed but I'm ok with 12AX7 because I have a ton of them. But where ever you go I'm good. Thank you.
They probably put the bobbin from a push pull amp onto a transformer with an air gap (if you're lucky) or they're just push pull transformers being used in a single ended amp. That would explain the extreme levels of distortion, it could be saturating. I'd be curious to see what the 6V6 tubes put out strapped to triode since the 6V6 UL tap should be 23% (as you know) and running it in triode might give 1.5 to 2W of fairly low distortion sound. Or you could nuke the amp from orbit. It's the only way to be sure. ;)
That is worth trying! As you said this 50% tap could be doing wonky stuff to this tube. I'm likely gonna nuke the inside of this thing next week lol. I do wanna see what dropping those 7591a tubes into the stock amp sound and measure like. It's sad my tiny little 6BM8 amp with 10W OT simply destroys this thing!
@@SkunkieDesignsElectronics What's really sad are all the people who bought this thing thinking they were getting a six watt tube amp and instead got a chrome plated glowing effects box. I feel for the people that are defensive about buying the amp, no one likes to admit they made a mistake.
@@georgeswanson9483 Don't feel sorry. Most of them, like me, like to try stuff and I was aware of the fact that for € 310,00 delivered to the door, It wouldn't be top dollar. Even then, it sounds better than my Onkyo ss amp. I guess it helps that I have a powered sub and don't listen to anything below 180 hz via the tubes. I'll certainly take the gamble of trying the 6V6's and if it is going to be a total rebuild I won't make the same mistake again :=) and have a new hobby for the coming weeks...
@@theonoo I think the 6V6 idea is a good one. Also, since you don't listen to the tube amp below 60Hz you could make the coupling cap a much lower value and it would roll off the bass and cut down on the saturation of the transformer. Just an idea.
Wish you the best . My amp is the Audio Space AS3i with KT 77 power tube 2 12au7 1 12AX7 . Solid state regdifier . Wish you could do a review on the amp. It would be great to here about it. Could you please tell me your email and address soon
Since we suspect those opts are really push-pull, why not ditch the rectifier and place a L/R input tube into that socket and use both front and rear sockets for power tubes in a push-pull configuration?
Someone else suggested using a dual triode power tube to do the same. The problem is finding power tubes that are happy driving a 1.6K load. This stupid amp is making my head hurt, along with people arguing how great it actually is because some other influencers on TH-cam said it is! Time to go enjoy Thanksgiving :)
@@SkunkieDesignsElectronics On my similar A9 I got good results by: - biasing the Power tubes to 70mA with 270R resistors, with a B+ of 316v. -cascoding the 6SL7 input tubes with Broskie "Akido Mojo" -5AR4 rectifier - -3dB of GNFB taken off the 16 ohm tap of Hammond 1630sea OPTS (with hindsight, I should have got 5k but these 3.5k give good results) These mods have extended the frequency response - eliminating the LF roff-off and extending HF, flattering it's roll-off , producing clean, symmetrical output sine waves which match the input signal exactly on my oscilloscope from 20 to 20k. Mind, my PT is a normal center tap and runs at a constant 158°F and B+ is 316v. With the above mods my amp produces 3Wrms per channel on an 8ohm dummy load. Happy Thanksgiving!
@@DennisSantos All that sounds like a good plan. I just got an order in with some 250 and 300 ohm 10W resistors to play with on this rebuild. If I run into drive issues I might try a cascode to keep the 12AX7 tubes. And I think 3.5K OT work well with a EL34, a little bit of distortion but also a little more power. I also will probably play around with some local NFB once I get things behaving. I'm pretty sure I want to start with different OT that I know work well, build a good sounding amp, then toss the original OT in. If that kills it, we know they are junk. A pair of these 15W Edcors is around $150 shipped so not unreasonable. I'm sure people spend more than that rolling NOS ringer tubes in this thing.
@@SkunkieDesignsElectronics Changing the inputs from parallel to Cascode or anything else in series will flatten out the abysmal HF roll-off, for sure. All these SE amps seem to use the same schematic diagram, wiring the signal tubes in parallel. Only component values differ from model to model or even revision to revision (or whatever they had lying around) LOL BTW, swapping out the OEM rectifier with a 5AR4 increased systemwide voltages by about 20v. Voltage drop is an issue, more so with that smaller PT, I'm guessing.
@@DennisSantos Yeah I'm scratching my head why they needed to do that. A 12AX7 is just about like two 6SQ7 triodes in one tube, and one of those triodes drives a EL34 just fine. Why would they need to parallel it? If they wanted to increase the current for some reason (hard to drive tube or use a low value grid leak resistor), wouldn't you wire the second one as a cathode follower? I'm probably gonna just use one side of the 12AX7. I may try to keep the tube rectifier because so many people believe this make a huge impact on the sound, I'm not really feeling that, especially on a SE amp but... I will say if the PT start getting hot, it's gone lol.
I would go with new output transformers, no point in gutting the internals if ultimately the original Tx's are not up to the job. With my A10, I've tried Voskhod 6AX7's, JJ ECC803's and EVH 12AX7's (rebadged JJ's) in place of the 6N2J's. Replaced the rectified with a JJ GZ34. For the output I've tried EH EL34's, GL KT77's and now EVH EL34's. I really like the amp with the EVH 12AX7's and EL34's. It drives my RP-600M's with plenty of bass (though a lot could be down to room reinforcement due to the fact this system is in my condo). Almost as much money in tubes as I have in the amp but I bought this as a project piece to learn tube electronics because when I did my training as an electronics engineer tubes where ready history. I find the amp to sound good, drives the Klipsch's well. Sound stage is ok ish... If I cover the flat screen that is between the speakers the sound stage improves significantly. It's not a patch on my previous systems (when I was young dum and full of cum, Roksan Caspian with Triangle speakers/ Gryphon Callisto 2200 and Art Audio Impression speakers) but then I have in this system about what I paid for just the speaker cable on the Gryphon system. I find it interesting that you prefer the 6V6 sound. I've heard it said that when referenced to guitar amps 6V6 is the American sound and EL34 is the British sound (I'm British). Looking forward to your further modifications but I'll be sticking with the EL34's 😊
I think it's more than just the OT, the operating point is WAY off for what a class A amp should be running. Replacing the OT isn't going to fix that. I don't think replacing any one thing is going to fix it. And like I said, there is no reason to parallel a 12AX7 to drive an EL34 or any tube in this class. And I think you are talking about the 6CA7 vs EL34. Those are both 25W pentode tubes, one is US and the other Europe. And yeah they do sound very different, I prefer the El34. The 6V6 is a different class, much lower power pentode tube, that when put into the stock amp is running closer to a Class A operating point. That is why the distortion drops away, the stock setup is like trying to run a one side of a class AB push pull amp.
Glad I never purchased this amplifier, based upon the yet to be determined amount of money that will be added to make it decent. I wish you the best with this project nevertheless.
Actually if you listen to most of those reviews, they never say "It's great". They say things like "For someone interested in trying a tube amp, this is a way to do that for $500". And I can't help what I am seeing. 7% distortion at 1W and the electrical inability of this amp to produce over 3W, which is 1/2 of it's claimed performance, is never going to be "Great". That's like saying a car that actually hesitates and stumbles, and then finally makes 1/2 the horsepower the manufacturer claims is great/fast, just because the paint is shiny and it's cheap. I suppose compared to not having a car that would seem great? I'm sorry people have purchased this amp and are now defending their purchase choice based off some influencers praise, that too doesn't change the facts. If this was sold in an environment where people could A-B listen to various amplifiers before the purchase, I doubt many people would be buying this. And again, I'm sorry if this has upset some people, but I'm never going to candy coat the facts about this stuff and I don't care what "Many other people say". Many of those same people are trying to sell you $1500 speaker cables...
Love your rants. They have always a solid foundation
These amps were originally sold as A9 kits by a British company with an office in Taiwan way back in 2010. The case is unchanged. It is only in recent years that fully built amps became available from Boyuu Reisong in the guise of the A10. In 2018 SoundArtist had the 12ax7 version, long before Boyuu. I remember seeing an A9 kit demo'd here on TH-cam back in 2013 in this exact same case.
I'm so glad I found your channel, keep doing what you do and thank you!
I never will buy an A12 tube amp
I have an A10 that uses the same chassis and internals (same voltages, and looks like the same OPT, so I am very exited to follow your rebuild if new OPT's will do the job
With relatively high transconductance and low drive requirements, 7591 were the mainstay of integrated tube amplifiers in my youth. I once converted a 7591 Eico ST0 to parallel-push-pull 6BX ()twin triode) using a 6H choke input to get the 6BX7 voltage right, replaced the input 12as with 6dj8 and direct coupled the6dj8 plate(as with stock 12ax) to the6sn7 inverter. That type can have equal value plate load resistors when the "tail" is very long via current source or negative rail and large Rk.It made a really good little triode amp. You've got the best bube channel in all of TH-cam.
The 6DJ8 is a great tube and as I mentioned, I've used both 6EJ7 and 6JC6A tubes triode strapped as drivers and they sound great. The Kegger/Blueglow KT88 amp runs a single 6DJ8 to drive both tubes so there is no reason it can't work here.
I am so glad you are going just rid it all out , and punt it out of the park . You tell them dear . I always knew great iron is need to make a great SET amp
Great I found you! Love what you're doing and how! Godspeed
hello , i appreciated your work on tube amplifier ... I am from France , i am 61 years now ...despite behing a PH in microelectronics , i have build more or less the same amplifier when i was 25 years old ... So long time ago .... this amplifier was with 250V/280V power suplly ( silicon ) and with 2.5K transformer from millerioux ( AH25BB as far I remember ) it has been working in my home for 10 years ..driver was the 12AX7 , parallell .... Then i go for 300B ... so for me you are fighthing against a transformer issue ... i was getting 6 Watts a 6 % distorsion .... and the sound was really good .... so good that my father ( electronic engineer in French Army ) decided to go back to tube .....sorry for my bad english ... this is not my native language ... currently i would like to buy high end hifi amplifier for my retirement but i find nothing to compare with a single end 300B with good output transformer , good power supply (polypropylene in oil ) , same for decoupling ....
For me you have had a bad expeximent with a faulty product .... i do not say that all the chinese product are bad , i have myself been very disapointed with a line magnetic 845 and on the the other side been very happy with a low cost kit of 300B ..... i agree that to get the sound grall we have to upgrade them but it seems to me normal regarding the price pay ....FYI i build amplifier for 35 years ....I really appreciated your effort in our "tube lover" world ...
Turns out the output transformers are OK, I did some adjustments to the operating points and got plenty of power :)
You nailed it !!!!!!!!!!!!!!! Made a similar comment on another video ..................
Just FYI, the 7591 tube is NOT a drop in replacement. The pinout is different and requires internal mods. My hope is it may require less internal mods than getting the EL34 to work correctly. Someone sent me a note that a 6V6 has a different pin out, not true in this use. The 6V6 ties the suppressor grid to the cathode internally, the amp does the same thing internally with the EL34 tube tying pin 1 and 8 together. It accomplishes the exact same thing and has nothing to do with how the tube performs.
It is deer hunting season after all. Outstanding series of videos !!
Very good. I'm in for the ride and yes the chassis is nice so I would still use mine. I'm not against using 6V6 tubes if needed but I'm ok with 12AX7 because I have a ton of them. But where ever you go I'm good. Thank you.
Powered sub with 6V6 tubes is the simple fix lol.
This is great 😊
Well Done, You Star...
They probably put the bobbin from a push pull amp onto a transformer with an air gap (if you're lucky) or they're just push pull transformers being used in a single ended amp. That would explain the extreme levels of distortion, it could be saturating. I'd be curious to see what the 6V6 tubes put out strapped to triode since the 6V6 UL tap should be 23% (as you know) and running it in triode might give 1.5 to 2W of fairly low distortion sound.
Or you could nuke the amp from orbit. It's the only way to be sure. ;)
That is worth trying! As you said this 50% tap could be doing wonky stuff to this tube. I'm likely gonna nuke the inside of this thing next week lol. I do wanna see what dropping those 7591a tubes into the stock amp sound and measure like. It's sad my tiny little 6BM8 amp with 10W OT simply destroys this thing!
@@SkunkieDesignsElectronics What's really sad are all the people who bought this thing thinking they were getting a six watt tube amp and instead got a chrome plated glowing effects box. I feel for the people that are defensive about buying the amp, no one likes to admit they made a mistake.
@@georgeswanson9483 Don't feel sorry. Most of them, like me, like to try stuff and I was aware of the fact that for € 310,00 delivered to the door, It wouldn't be top dollar. Even then, it sounds better than my Onkyo ss amp. I guess it helps that I have a powered sub and don't listen to anything below 180 hz via the tubes. I'll certainly take the gamble of trying the 6V6's and if it is going to be a total rebuild I won't make the same mistake again :=) and have a new hobby for the coming weeks...
@@theonoo I think the 6V6 idea is a good one. Also, since you don't listen to the tube amp below 60Hz you could make the coupling cap a much lower value and it would roll off the bass and cut down on the saturation of the transformer. Just an idea.
@@georgeswanson9483 The coupling cap being the 0.22uF orange one? What would be a realistic value?
Hi Skunkie.Loving your videos.I own a Boyuurange Reisong A10 Tube Amp,do you think this amp will have the same problems as the A12.Many thanks
I believe so but don't have one to test.
Maybe tried to design a solution around parts they couldn't move or got cheap
You are wonderful Darling❤
Wish you the best . My amp is the Audio Space AS3i with KT 77 power tube 2 12au7 1 12AX7 . Solid state regdifier .
Wish you could do a review on the amp. It would be great to here about it. Could you please tell me your email and address soon
Since we suspect those opts are really push-pull, why not ditch the rectifier and place a L/R input tube into that socket and use both front and rear sockets for power tubes in a push-pull configuration?
Someone else suggested using a dual triode power tube to do the same. The problem is finding power tubes that are happy driving a 1.6K load. This stupid amp is making my head hurt, along with people arguing how great it actually is because some other influencers on TH-cam said it is! Time to go enjoy Thanksgiving :)
@@SkunkieDesignsElectronics
On my similar A9 I got good results by:
- biasing the Power tubes to 70mA with 270R resistors, with a B+ of 316v.
-cascoding the 6SL7 input tubes with Broskie "Akido Mojo"
-5AR4 rectifier
- -3dB of GNFB taken off the 16 ohm tap of Hammond 1630sea OPTS
(with hindsight, I should have got 5k but these 3.5k give good results)
These mods have extended the frequency response - eliminating the LF roff-off and extending HF, flattering it's roll-off , producing clean, symmetrical output sine waves which match the input signal exactly on my oscilloscope from 20 to 20k.
Mind, my PT is a normal center tap and runs at a constant 158°F and B+ is 316v.
With the above mods my amp produces 3Wrms per channel on an 8ohm dummy load.
Happy Thanksgiving!
@@DennisSantos All that sounds like a good plan. I just got an order in with some 250 and 300 ohm 10W resistors to play with on this rebuild. If I run into drive issues I might try a cascode to keep the 12AX7 tubes. And I think 3.5K OT work well with a EL34, a little bit of distortion but also a little more power. I also will probably play around with some local NFB once I get things behaving. I'm pretty sure I want to start with different OT that I know work well, build a good sounding amp, then toss the original OT in. If that kills it, we know they are junk. A pair of these 15W Edcors is around $150 shipped so not unreasonable. I'm sure people spend more than that rolling NOS ringer tubes in this thing.
@@SkunkieDesignsElectronics Changing the inputs from parallel to Cascode or anything else in series will flatten out the abysmal HF roll-off, for sure.
All these SE amps seem to use the same schematic diagram, wiring the signal tubes in parallel. Only component values differ from model to model or even revision to revision (or whatever they had lying around) LOL
BTW, swapping out the OEM rectifier with a 5AR4 increased systemwide voltages by about 20v.
Voltage drop is an issue, more so with that smaller PT, I'm guessing.
@@DennisSantos Yeah I'm scratching my head why they needed to do that. A 12AX7 is just about like two 6SQ7 triodes in one tube, and one of those triodes drives a EL34 just fine. Why would they need to parallel it? If they wanted to increase the current for some reason (hard to drive tube or use a low value grid leak resistor), wouldn't you wire the second one as a cathode follower? I'm probably gonna just use one side of the 12AX7. I may try to keep the tube rectifier because so many people believe this make a huge impact on the sound, I'm not really feeling that, especially on a SE amp but... I will say if the PT start getting hot, it's gone lol.
You can blame Kevin Deal for the bias that a lot of us have for believing that tube rectification is the best way to go. That's who I heard that from.
Dolly sighting 0:15 #needmoreDolly :)
Haha the dog knows that, after the famous "hey yall", it's no more attention given.
Have a Happy Thanksgiving Skunkie, don’t eat to much turkey. lol
I would go with new output transformers, no point in gutting the internals if ultimately the original Tx's are not up to the job. With my A10, I've tried Voskhod 6AX7's, JJ ECC803's and EVH 12AX7's (rebadged JJ's) in place of the 6N2J's. Replaced the rectified with a JJ GZ34. For the output I've tried EH EL34's, GL KT77's and now EVH EL34's. I really like the amp with the EVH 12AX7's and EL34's. It drives my RP-600M's with plenty of bass (though a lot could be down to room reinforcement due to the fact this system is in my condo). Almost as much money in tubes as I have in the amp but I bought this as a project piece to learn tube electronics because when I did my training as an electronics engineer tubes where ready history.
I find the amp to sound good, drives the Klipsch's well. Sound stage is ok ish... If I cover the flat screen that is between the speakers the sound stage improves significantly. It's not a patch on my previous systems (when I was young dum and full of cum, Roksan Caspian with Triangle speakers/ Gryphon Callisto 2200 and Art Audio Impression speakers) but then I have in this system about what I paid for just the speaker cable on the Gryphon system.
I find it interesting that you prefer the 6V6 sound. I've heard it said that when referenced to guitar amps 6V6 is the American sound and EL34 is the British sound (I'm British). Looking forward to your further modifications but I'll be sticking with the EL34's 😊
I think it's more than just the OT, the operating point is WAY off for what a class A amp should be running. Replacing the OT isn't going to fix that. I don't think replacing any one thing is going to fix it. And like I said, there is no reason to parallel a 12AX7 to drive an EL34 or any tube in this class. And I think you are talking about the 6CA7 vs EL34. Those are both 25W pentode tubes, one is US and the other Europe. And yeah they do sound very different, I prefer the El34. The 6V6 is a different class, much lower power pentode tube, that when put into the stock amp is running closer to a Class A operating point. That is why the distortion drops away, the stock setup is like trying to run a one side of a class AB push pull amp.
Glad I never purchased this amplifier, based upon the yet to be determined amount of money that will be added to make it decent. I wish you the best with this project nevertheless.
Yeah, I can't recommend anyone do this unless you are into it for DIY reasons, and only then if the transformers turn out to be decent.
Many top rated reviewers claim that it's great! Many...
Actually if you listen to most of those reviews, they never say "It's great". They say things like "For someone interested in trying a tube amp, this is a way to do that for $500". And I can't help what I am seeing. 7% distortion at 1W and the electrical inability of this amp to produce over 3W, which is 1/2 of it's claimed performance, is never going to be "Great". That's like saying a car that actually hesitates and stumbles, and then finally makes 1/2 the horsepower the manufacturer claims is great/fast, just because the paint is shiny and it's cheap. I suppose compared to not having a car that would seem great?
I'm sorry people have purchased this amp and are now defending their purchase choice based off some influencers praise, that too doesn't change the facts. If this was sold in an environment where people could A-B listen to various amplifiers before the purchase, I doubt many people would be buying this. And again, I'm sorry if this has upset some people, but I'm never going to candy coat the facts about this stuff and I don't care what "Many other people say". Many of those same people are trying to sell you $1500 speaker cables...
Suspence is killing me..... :=)
1st 🤗
Even a 5751 can drive an EL34..............