Amp Upgrade or not?

[xxJPMxx]

Here is one I found that is similar. A lot of the China amps are similar in topology, I always find different values because I think they just use whatever they can find at the lowest price.

 

Edited November 9, 2015 by xxJPMxx

[Jim]

JP nothing comes up when you click the link

[xxJPMxx]

If it was my amp I would try something like this and see how it sounds. I am trying to think of something useful for that extra triode for each channel instead of running them in parallel, I hate running tubes in parallel :/

 

 

Zero global feedback, 2.5% THD @ 4 watts out. Since it's a Chinese amp I thought this style front end is only fitting   

 

[Jim]

If it was my amp I would try something like this and see how it sounds. I am trying to think of something useful for that extra triode for each channel instead of running them in parallel, I hate running tubes in parallel :/

 

 

Zero global feedback, 2.5% THD @ 4 watts out. Since it's a Chinese amp I thought this style front end is only fitting   

 

 

It's all Chinese to me.

 

Only thing I think I can read is 7 resistors? 3 caps? 3 tubes? LOL

[xxJPMxx]

Not sure if anyone is still interested in this but Jim sent me the amp and I will share my progress.

 

Here is the actual schematic. I was fairly close with original schematic that I made from just looking at the pictures.

 

 

 

 

First on the bench I could only get about 2 watts out of it. EL34 Vp=270, much lower than I had thought, something must be wrong. With the rectifier out I get 317-0-317 w/ 125 ohms winding resistance end to end. I should have much higher B+. Swapped out the rectifier and sure enough the voltage came up to where it should be. Now measuring about 4.5 watts out @ 6% THD! Hooked it up to some speakers and the sound wasn't too bad, just not a lot of clarity and sound staging.  I now turn my attention to the why the heck they have the output stage wired like this, doesn't matter Jim has asked to go for Triode mode. I snipped the ultra linear taps and wired in a couple 150 ohm resistors from plate to grid 2. I had some overshooting during square wave testing which was helped via a lead comp cap. Power out is down to under 4 watts. I listened to it like this, and it was better. Now I have changed the feedback entirely, Power is down to 3.5 watts BUT THD is down to 1% at full clean output. The sound staging is good now and much better clarity, sounds like a much tighter amp.

 

Here is the current schematic. I plan to do a few "tweaks" on Monday and see if I can't tighten her up and get it even more accurate. I like accurate, when I got the amp it sounded colored and slightly boomy, not anymore.

 

Edited December 13, 2015 by xxJPMxx

[tube fanatic]

JP, I like what you have arranged there!  How does the 500 Hz square wave look?  It might be fun to try around .015 uf in series with the 220k fb resistor to create a filter with a corner frequency of around 48 Hz.  I'd be curious as to how it looks on the 'scope with that in place vs. the resistor only.  On the assumption that the output xfmrs aren't the best, a little decreased fb in the low bass region may yield some nice improvements.  At what frequency did you notice the overshoot on the square wave?

Maynard

[xxJPMxx]

Hello Maynard, 500Hz square wave looks great! I added a "trade secret" to help with the square wave response (It's just a 1nF 1kV cap from the EL34 plate to ground). I was going to hook it back up and do more testing so I will try and get some pics. I can never get good pictures of my analog scope with my phone, it's a tektronix 465. I am sure all analog scopes are the same, just like trying to take pictures of old TV sets and computer monitors.  I usually take snapshots using my digital scope on my computer and upload them that way but the digital filters cause ringing on square waves so it's pointless hooking it up the the computer for that. I will try and get some pics up next time it's on the bench. It shouldn't be too much more work to add the .015uF cap in series with the feedback resistor, if I have one....I might only have .01uF laying around. As far as the overshoot with the transformer in the feedback loop I believe it was at 200Hz square waves, this was days ago so don't hold me to that data.

[rebuy]

you can sell em cheap when shipping is $90

[Jim]

I can't read schematics, but from what I do see, it look like r8 was removed with wiring that was wired incorrectly to the powersource? And r9 was also moved?

 

 

Is that the secret trade secret?

[xxJPMxx]

Hey Jim!   Yes R8 was the original feedback resistor which included the output transformer in the loop (not power transformer). And yes R9 was also removed. In the revised schematic the new feedback resistor is the R9, 220K. I guess we can call it "Inverse-Voltage Feedback for adjustment of tube impedance". Strapping the EL34 as a triode lowers it's plate resistance to about 950, adding the amount of feedback I did lowers it further to 500. Not too shabby

 

 

The "trade secret" is a capacitor that isn't in the schematic.

Edited December 14, 2015 by xxJPMxx

[xxJPMxx]

I am very surprised nobody picked up on no grid stoppers on the schematic.........they are on the tube socket just not in the schematic   I used as big as I felt comfortable on the EL34 to try and cut down on grid current during overloading. Also it's a high transconductance tube. 

[tube fanatic]

The trade secret is finally revealed (JP, did you happen to notice the drone hanging around your shop window with a letter "M" on the fuselage??? ).  We may have to put JP on double secret probation!!!!!

 

https://www.youtube.com/watch?v=hostgKc7qV4

 

 

Maynard

 

 

 

 

[xxJPMxx]

[Jim]

Hey Jim!   Yes R8 was the original feedback resistor which included the output transformer in the loop (not power transformer). And yes R9 was also removed. In the revised schematic the new feedback resistor is the R9, 220K. I guess we can call it "Inverse-Voltage Feedback for adjustment of tube impedance". Strapping the EL34 as a triode lowers it's plate resistance to about 950, adding the amount of feedback I did lowers it further to 500. Not too shabby

 

 

The "trade secret" is a capacitor that isn't in the schematic.

 

Don't know what anything you said means, but I figured I'd comment on what it looked like to me. This way at least I can say I was involved in things,lol.

[CECAA850]

Great thread!

[Jim]

I'm patiently awaiting for this to get back to me to see how nice the upgrades are going to sound. I am being teased daily in a good way with the updates.

[xxJPMxx]

Here it is, I installed plutonium grade capacitors and Helium 3 resistors that were used in the apollo missions They only cost $1k a piece.........the sound is out of this world!!

 

 

 

JK lol!!

 

Since there is no global feedback I replaced the two cathode resistors with just on on the preamp tube. Global feedback switched to "Schade" feedback. Amps was rewired from ultralinear to triode mode. The old GE 5U4G was very weak so that was replaced with a NOS Sylvania, I am now getting 40+ more B+. I rearranged the two stages AC coupling for a smaller time constant and added a big fat grid stopper to help with recovery time. I also added a 1kV 1nF cap shunting the EL34 plate to ground to help response. Cheap Chinese coupling caps were replaced with real Wima MKP10 polypropylene's. The Bypass caps were upgraded from cheap Chinese brands to Elna Silmic II's. There was a safety concern where the output transformer wires were being chaffed against the hole in chassis, you don't want 310V shorting to the chassis so I fixed that.  I am probably forgetting some stuff.

 

Impressions: Much better clarity and soundstaging. I know there was talk about splitting the power supply rails but I felt that the -46db channel separation was adequate. Don't let the price tag fool you, this amp is heavy and the output iron is better than expected. Really deep tight bass!! The response will better what most of the Klipsch speakers can go down to. My honest opinion is that Jim hit a home run with this amp! I really can't wait for him to listen to it! One word; WOW!!

 

Here is the grizzly detail.

 

[xxJPMxx]

The other components are not that hard to replace with better components.

 

That is after you scrape the white silicone crud off. It took longer to clean that stuff of then it did to de-solder and re-solder.

[Jim]

I just wish that thesloth was still around to give his input on things. I'm sure though he'd agree with you JP on some of the parts you used.

 

He and Maynard were/are always welcome replies with their input on things.

[xxJPMxx]

I have read some posts from this sloth guy.......I agree with some of his views but he seemed like a jerk about it.  He does use the same simulation software as me so he can't be all that bad.