Looks nice, can't wait to hear how it sounds.
Any 805A based amp owners?
- Thread starter Nixxuz
- Start date
Looks nice, can't wait to hear how it sounds.
Nixxuz
Super Member
Well it sounds great, although I have already planned some modifications. I already replaced the detented volume pot with a linear, and actually plan to get an ALPS RK27 in there by the weekend. I thought about going stepped attenuator, but I've found I like the feel and control of the ALPS in everything I've used them is so far. The power button is being replaced by one with an amber LED. I want to replace the white LEDs in the VU meter, but they are glued into recesses in a piece of 1/4" lexan, so it's going to be more of a project than I feel like attempting right at the moment. The stock Sprague caps will be swapped with a pair of Mundorf Supreme SGO, as I find the Sprague a little harsh right out of the gate, but they could perhaps mellow over time. Not my concern though as I know I love the sound of the Mundorf in everything I've tried them in.
I first tried KT77's in the secondary positions, and either my Kt77's are having problems, or this amp just doesn't like them. They caused some sort of grounding issue that gave me a slight headache, but putting the stock 5881's in their place seemed to resolve that.
I know I have some hours before things settle in to where they will stay, but right off the bat, there is a greater "immediacy" from this amp than either the Elekit 8200R, or my previous Yaqin MC-13S. The SE sound is there in gobs, but the extra power behind it makes it's presence known. There's a dynamism the lower powered 8200R just doesn't have, or at least not in the same role as the XinXin is in now. And yes, that's a pretty silly name for an amp, although it can mean dozens of things in Chinese. I guess I prefer "Core Truth", as it's a VERY unforgiving amp. Nothing is smoothed over when it comes to faults in a recording. I like that, although it might be a bit nagging when it comes to vinyl and surface noise.
I will say, firing up the amp gets a little getting used to. The 805A's come up to Edison lightbulb levels of glow in a second or so with a audible buzzing, but settle down within another 4-5 seconds.
All in all, there are quibbles and obvious slight flaws, but I knew there would be going in. I certainly can't recommend the XinXin for anyone who doesn't want to really "know", and service, their own amp.
But for us dabblers or DIY'ers, it's a heck of a nice jumping off point for Big Tube enjoyment.
Which I'm going to get back to while I still have a little time to do so!
I first tried KT77's in the secondary positions, and either my Kt77's are having problems, or this amp just doesn't like them. They caused some sort of grounding issue that gave me a slight headache, but putting the stock 5881's in their place seemed to resolve that.
I know I have some hours before things settle in to where they will stay, but right off the bat, there is a greater "immediacy" from this amp than either the Elekit 8200R, or my previous Yaqin MC-13S. The SE sound is there in gobs, but the extra power behind it makes it's presence known. There's a dynamism the lower powered 8200R just doesn't have, or at least not in the same role as the XinXin is in now. And yes, that's a pretty silly name for an amp, although it can mean dozens of things in Chinese. I guess I prefer "Core Truth", as it's a VERY unforgiving amp. Nothing is smoothed over when it comes to faults in a recording. I like that, although it might be a bit nagging when it comes to vinyl and surface noise.
I will say, firing up the amp gets a little getting used to. The 805A's come up to Edison lightbulb levels of glow in a second or so with a audible buzzing, but settle down within another 4-5 seconds.
All in all, there are quibbles and obvious slight flaws, but I knew there would be going in. I certainly can't recommend the XinXin for anyone who doesn't want to really "know", and service, their own amp.
But for us dabblers or DIY'ers, it's a heck of a nice jumping off point for Big Tube enjoyment.
Which I'm going to get back to while I still have a little time to do so!
Good to hear you got them working and like the sound. What speakers are you using with the 805 amp?
I am not surprised by your KT77 experience. The KT77 has nearly twice the transconductance and much less plate resistance than a 5881. You would have to modify the circuit (component value changes) to get the best performance from a KT77.
I am not surprised by your KT77 experience. The KT77 has nearly twice the transconductance and much less plate resistance than a 5881. You would have to modify the circuit (component value changes) to get the best performance from a KT77.
trainbuftony
Electron Herder
Out of curiosity i pulled the data sheet on this type
125w plate dissipation per tube, class b,
1500v b+
Building something that uses these tubes to their potential is going to cost thousands...
Excuse me while i check how many I own...
125w plate dissipation per tube, class b,
1500v b+
Building something that uses these tubes to their potential is going to cost thousands...
Excuse me while i check how many I own...
Nixxuz
Super Member
Well, for some reason the store listing, (manual is photocopied and entirely in Chinese), says it can use EL34's 6L6's 5881's etc. I've never seen a situation where KT77's don't fall into that bunch.
Using DIY speakers, but the only drivers being fed by the 805 are Lii Audio Fast10S.
I'm assuming the driver tube (EL34, 5881, etc.) is used in triode mode. Having built a similar 845 amp with a triode-wired power tube as driver, it's pretty easy to substitute any of the common pentodes or beam tubes in that psition, with only minopr variations in bias/plate voltage. The only hitch is that an EL34 or KT77 is going to have more gain in triode connection than a 6L6 variant (11 as opposed to 8), so it will change the sensitivity of the amp. Just something to bear in mind in terms of the difference one might hear.
The buzzing at startup is filament noise from the 805s until the plate voltage kicks in. Nothing to worry about, IMO.
BLAH BLAH
Super Member
I still reckon you ought to buy one...and yes I agree totally black is a nice look for a valve amp would match my Sansuis..It looks far nicer totally black rather than with the silver front shown on their site. Fantastic that it sounds good too.... I'd still go with a resistor based attenuation but that's just me
PakProtector
AK member
The 5881 is rigged as a cathode follower in this circuit. The gain is not going to exceed 1 in this case, though the EL34/KT77 will likely come closer than a 5881/6L6 variant. Something like .8 for a 5881 and .85 for a KT77 I would estimate.
cheers,
Douglas
If the 5881 is a cathode follower- I wonder if the problem with the KT77 might be related to the heater-cathode voltage. A cathode follower sometimes has the cathode a hundred or more volts over ground- if the heaters aren't also DC biased to a similar voltage, you may be getting heater-cathode leakage. That could explain the hum.
It may just be that the particular 5881s you have, are more resistant to heater-cathode leakage, than the KT77s...
Regards,
Gordon.
It may just be that the particular 5881s you have, are more resistant to heater-cathode leakage, than the KT77s...
Regards,
Gordon.
Ah, so it is, I didn't see the schematic. So yes, little change in gain.
PakProtector
AK member
Gordon, check the schematic, the grid is the biasing element. As in the cathode is connected to the grid and nothing else. This puts the 805 tube well positively biased. The driver tube is no doubt having some difficulty telling it is not a standard cathode bias affair...
grid of the driven tube instead of a resistor.cheers,
Douglas
Gordon, check the schematic, the grid is the biasing element. As in the cathode is connected to the grid and nothing else. This puts the 805 tube well positively biased. The driver tube is no doubt having some difficulty telling it is not a standard cathode bias affair...
cheers,
Douglas
Man- that looks like an invitation for disaster, depending on the transconductance of whatever is plugged in as the cathode follower. There's literally nothing regulating what grid voltage, and more importantly- how much grid current is going into the 805.
That may be why the EL34/KT77 is having trouble, whereas the 5881/6L6 is not- the lower plate resistance and higher transconductance of the KT77, is probably allowing more grid current into the 805- which may be over-biasing it. I can't imagine how that could work consistently- the bias to the 805 is completely dependent on what tube is plugged into the cathode follower position, with no provision for bias adjustment back into an optimal range, should it not be there inherently.
EDIT: Looking at the schematic- one other issue with the KT77/EL34 type tube, is that it looks like pin 1 isn't connected to anything. That means the suppressor grid isn't connected to anything. It may be the "tetrode kink"- due to running the KT77 as a tetrode, with the suppressor grid disconnected- that's causing the issue with the sound- that would cause quite odd distortion, under some conditions...
Regards,
Gordon.
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Good catch, Gordon.
Nixxuz
Super Member
Quick question, just for those that are better reading schematics than I am;I'm going to replace the (3) white LEDs that have been glued into the VU meter with yellow ones. I'm guessing this is literally going to be a straightforward swap process...
BUT, I don't know if the voltage going to those LEDs, through resistance of course, is 6.3V or 12.6V, as I can't seem to get a grip on that side of the schematic.
Worst case scenario is I end up with a non-lit VU meter, which will at some point become lit again through trial and error.
I know this might not help, but the factory pic HiFi Exquis has of the interior seems to show a pair of 75, or 76, ohm resistors going to those LEDs.
I'll probably pop the hood and get in there to check things out in any case, but I figured I'd ask and check if anyone is seeing anything I'm not.
Also, it sounds like I should have the option to go with 6L6 tubes instead of 5881's? I've seen them to be considered direct replacements, with the 5881 being "ruggedized". But then I've also seen people say their parameters started to deviate at some point in production, so they may function similarly, but not exactly the same. Considering my snafu with the KT77's, and the indication that the behavior of the 805A's are directly impacted by the "smaller" tubes, I don't want to mess things up, or waste money on a pair of tubes that don't do what they need to do.
These tubes can provide 200W from a pair in class B push-pull.
In SET the best is likely 50W, but will need special transformer with feedback winding. Without that and using relatively low plate voltage (800 volts) you can get 32-37W at 5% THD in midrange. Lower, of cause, at bass or treble. That is what Line Magnetic 508 provides.
I have two amps - one with 845 tubes, A1 mode (no grid current) with 950 volts at plate. That one delivers 15W at 1.5% THD, but clips quickly above that. Second uses 805 tube, A2 mode (significant grid current) with 800 volts at plate. It delivers 33W at 5% THD and clips at 38W. But distortion spectrum is not as clear like in A1 mode.
One that OP has likely can do 8-10W with acceptable distortion level. Keep in mind that for almost all SET amplifiers power rating is given at 10% distortion, when it outputs almost square wave.
Nixxuz
Super Member
Well, I'm liking it a LOT more now. The seller HiFi Exquis, apparently had a shipping error, and my 805A tubes were not delivered with the amp. I emailed them and they admitted a mistake, and said they would rush ship them to me as fast as they could. Not having any 805A tubes, I didn't want to sit and stare at my amp. I had also figured it would be a good idea to get my hands on a replacement set, since, at the time, I didn't think you could get capless 805A's without waiting for shipping from China. After a little research I found you actually can get them from ONE NA based seller. So I ordered a pair from The Tube Store, (you may recognize from the manny sponsor banners at the top of AK!), and paid for next day shipping.
I got the tubes, they fired up and I thought all was mostly well.
It was not. Huge imbalance to one side. I checked everything as far as interconnects, speaker wires, source volume and balance. Couldn't figure it out.
Well I went so far as swapping the potentiometer, to an ALPS RK27, which I had planned on anyways. Got it installed, and the imbalance was still there.
Except it was now on the other side.
My brain kicked in and I shut it down, waited for the 805A's to cool, and swapped them. There it was. It was following a tube.
Got out the IR thermometer and found out the weak tube had a plate temp of 450F, whereas the strong tube measured 550F.
Luckily, the tubes from China showed up yesterday and I swapped them in. Everything was perfect. No imbalance.
Did some listening and now I can honestly say it's about the best sound I've personally heard, but I don't go to audio shows and my circle of friends isn't really into audio stuff, so take it for what it's worth.
Pair of Mundorf SGO coming in Thursday to swap in for the Sprauge Vitamin Q's. Hoping for an improvement I can't actually imagine as of yet.
I got the tubes, they fired up and I thought all was mostly well.
It was not. Huge imbalance to one side. I checked everything as far as interconnects, speaker wires, source volume and balance. Couldn't figure it out.
Well I went so far as swapping the potentiometer, to an ALPS RK27, which I had planned on anyways. Got it installed, and the imbalance was still there.
Except it was now on the other side.
My brain kicked in and I shut it down, waited for the 805A's to cool, and swapped them. There it was. It was following a tube.
Got out the IR thermometer and found out the weak tube had a plate temp of 450F, whereas the strong tube measured 550F.
Luckily, the tubes from China showed up yesterday and I swapped them in. Everything was perfect. No imbalance.
Did some listening and now I can honestly say it's about the best sound I've personally heard, but I don't go to audio shows and my circle of friends isn't really into audio stuff, so take it for what it's worth.
Pair of Mundorf SGO coming in Thursday to swap in for the Sprauge Vitamin Q's. Hoping for an improvement I can't actually imagine as of yet.
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kevin thomas
New Member
I just saw your post and I know its ancient but I have a pair of budelman 133a push pull 805's with altec outputs eager to gain knowledge so I can get these jewels up and running