If you are buying new 6P1's I recommend the 6P1P-EV, the ones I got seem to be quite consistent, same for the one pair of 6N1P-EV I bought, they were reasonably close between all four sections. The 6N2's that came with my amp were all over the place. orpheus_2005, alexer1 and nixiestore are all good sellers with nice clean and well packaged product, as opposed to one seller who sent me some tubes, one tube had no getter, most had bent pins and all pins were heavily tarnished and difficult to clean up. It looks like its Svetlana or Shuguang/Beijing. For consistency, price and availablity, the Svetlana is the go IMO.
I got the LED's mounted nicely inside the case. I was originally sceptical as whenever I tried LED biasing on driver tubes I never liked it (on another amp). The LED's on the output tubes however really opened the sound up and the top end is crystal clear. This suggests to me that possibly the standard bypass capacitors are lacking, might be worth trying something local in there at least, for a comparison.
After reading the grounding schemes section at valvewizard I have realized the grounding scheme in this amp is all tits up, partly my fault from mods, partly designed in, I need to come up with a plan.
I measured the primary resistance across the OPT's, Gary, I get 104 ohms on one half and 130 ohms on the other half, total 234 ohms in circuit, same with both OPT's and the lesser resistance has the same wire colors, measured with 2 different meters. Very interesting, maybe not, I know little about transformers, but one would expect a centre-tap to be a centre-tap. 104/130 = 0.80 exact. Coincidence? Another Chinese amp I have uses 6L6 PP and it measures 50 ohms from anode to centre-tap, both sides of both OPT's.
Ian.
I got the LED's mounted nicely inside the case. I was originally sceptical as whenever I tried LED biasing on driver tubes I never liked it (on another amp). The LED's on the output tubes however really opened the sound up and the top end is crystal clear. This suggests to me that possibly the standard bypass capacitors are lacking, might be worth trying something local in there at least, for a comparison.
After reading the grounding schemes section at valvewizard I have realized the grounding scheme in this amp is all tits up, partly my fault from mods, partly designed in, I need to come up with a plan.
I measured the primary resistance across the OPT's, Gary, I get 104 ohms on one half and 130 ohms on the other half, total 234 ohms in circuit, same with both OPT's and the lesser resistance has the same wire colors, measured with 2 different meters. Very interesting, maybe not, I know little about transformers, but one would expect a centre-tap to be a centre-tap. 104/130 = 0.80 exact. Coincidence? Another Chinese amp I have uses 6L6 PP and it measures 50 ohms from anode to centre-tap, both sides of both OPT's.
Ian.
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Variations keep adding up. It seems I do not have the 51 Ohm droppers. That explains my 260 B+. But, I do seem to have a resistor in the screen of around 500 Ohms.
With 520 Ohms on the cathode, that means only 29 mA.
I measured 598 Ohms pin 1 to pin 1. So, that would also imply different iron.
I am begging to think these were actually built by TVR in Blackpool. No two are alike and neither matched the schematics. ( 1974 2500M )
With 520 Ohms on the cathode, that means only 29 mA.
I measured 598 Ohms pin 1 to pin 1. So, that would also imply different iron.
I am begging to think these were actually built by TVR in Blackpool. No two are alike and neither matched the schematics. ( 1974 2500M )
RLD amp did not have bypass caps. Glad you mentioned that. I will plan on a decent one there.
Ian, No grid stopper on the cathodyne and only 2K on the input?
With no feedback, would one not want to add a decoupling cap to the input cathode?
I question only for understanding, I am in no way trying to second guess anyone here!
Ian, No grid stopper on the cathodyne and only 2K on the input?
With no feedback, would one not want to add a decoupling cap to the input cathode?
I question only for understanding, I am in no way trying to second guess anyone here!
Yea, I have the small chassis. MENG 6P1 6AQ5 x4 Push-Pull Valve Integrated Tube Amp bk - eBay (item 250563427958 end time Jan-18-10 23:02:50 PST)
There is another variant, the larger chassis I believe you have.
MA 6p1 6AQ5 Valve Push-Pull Integrated Tube Amp BK hd - eBay (item 280445236433 end time Jan-18-10 21:20:19 PST)
It is claimed to be 15W. Mine 10, but of course, I get about 5.
There is another variant, the larger chassis I believe you have.
MA 6p1 6AQ5 Valve Push-Pull Integrated Tube Amp BK hd - eBay (item 280445236433 end time Jan-18-10 21:20:19 PST)
It is claimed to be 15W. Mine 10, but of course, I get about 5.
and yet a larger chassis, this one seems new.
Uround 6P1 Class A push-pull Hi-End tube amplifier - eBay (item 180447510576 end time Jan-19-10 09:24:52 PST)
Uround 6P1 Class A push-pull Hi-End tube amplifier - eBay (item 180447510576 end time Jan-19-10 09:24:52 PST)
Hey, and a big brother! same amp, 4 pairs outputs.
MENG X1 6P1 6AQ5 x8pcs Vaccum Integrated Amplifier NEW - eBay (item 260537789583 end time Jan-19-10 23:02:27 PST)
MENG X1 6P1 6AQ5 x8pcs Vaccum Integrated Amplifier NEW - eBay (item 260537789583 end time Jan-19-10 23:02:27 PST)
My last measurement quoted was when hot. Just measured again, very consistent between channels: 173 one side of OPT to CT and 167 ohms the other. Significantly lower ohms means either less turns or heavier wire used. It may or may not mean an impedance difference, only a turns ratio check will tell you that, and then ONLY if you know the impedance of one side to start with.
I took the transformer cover off for the first time yesterday (it's stainless steel I think) to drill a couple of small holes for ventilation in the top of the rear side. Fairly good sized iron stack on the PT, but the OPTs are definitely only made for a small amp. I'd say no more than 10W rated.
Gary
If you are working out your plate current by 15v across 520 ohms at the cathode, remember that cathode current includes both plate and screen current, so your actual plate current will be lower.
I still can't reconcile the 1v drop you measured across the OPT with 598/2 ohms. That would mean 3.3mA plate current.
Gary
Yes that's the one I have, but without the 😱 blue LEDs. Mine didn't have any LEDs under the tubes, but mine was not new so the previous owner may have removed them.
Both our models seem to be superceded now by the 8-tube job, and a revamp of your chassis with 2 EL34s in SE. That 8 tube one has to run hot I think, as the tubes are all clustered together on a small chassis, and I shudder to think of the dissipation of the transformers under the closed covers.
I think you may have your terms mixed up there. Grid stoppers go in series with the grid to stop oscillation. The 2k is serving that purpose. Remember I said about the 10k grid stoppers in the output stages. They are actually there on all the tubes in my amp.
You may mean grid leak (as in from grid to ground or cathode). The volume control is serving that function, but often another resistor is placed in parallel with the pot (in mine there is 100k in parallel) to preserve bias in case the pot fails.
When I first put Ian's cathodyne in my amp I was still using the 6N2 and bypassing was needed to get the gain up, but the 6N1P has much more gain and the bypass isn't needed. I tried it with and without, and no audible difference with the 6N1P.
OK, got distracted for a bit. Had to figure out what failed in my good old no-longer-trusty NAD 7225. Failure of the positive regulator, transistors obsolete, thermister unknown. Usual. Anyway, it is on the shelf to order some parts. I may do something exotic with it like a real regulated supply.
Anyway. I just happen to have a DC amp-clamp. It says 39 mA. plate current.
Again, I measure 260 on the plate, 262 on the screen, 15.5 on the cathode.
It does not add up. I will go unsolder the OT and measure it's resistance where the PS can't mess up the readings.
More like it, 105 and 117 Ohms. But still something is not right here.
467 Ohm resister feeding both screens.
So, 29 mA on the cathode,
39 mA on the plate,
means 10 mA on the screen.
Can't be right, as the cathode current should be the SUM of plate and screen currents. I'll wager that your clamp meter is leading you astray. Most of them are not accurate at low current levels. Or your output stage is oscillating (grid stoppers?)
10mA screen current is over the limits for 6P1.
Measure the voltage across the screen resistor, and with power off, measure the screen resistor value. 467 is not a standard marking (470 nominal marking). Voltage and resistance will give you actual screen current.
You need to get the plate voltage down to 250v (either bias for more idle current or add series resistance to B+).
I see the 2K on the input stage. Smaller than usual, but 2K bigger than they bothered to use in my amp! I guess that it is a fraction of an inch from pin 1 to pin 7, a resistor does not do much.
Hew, whadda expect from a $30 Harbor Freight amp clamp! At least I have a decent True RMS 4 1/2 bench meter.
OK across 467 Ohms, current to both screens ( only one r4esistor) is 1.56 V 1.7 mA per screen is more like it. I don't see a max spec for this. If that is high, than I would need to increase the resistor I guess. So that puts the plate at more like 31 mA. If I can cross between Chineese and Russian, it is rated at 70. So in the ball park I guess.
A lot of red ink on my drawing. The "as publicized" seems closer to within spec.
Guess I should knock it off and order some parts.
Reconfigure for cathodyne, trippler etc.
RLD style CVS
Add droppers to get b+ within range
Fast diodes (while I am there)
Add the chassis ground
Alps pot, an a second input.
P-P the heater wiring at least, shield the pot and feedback ( if I keep feedback)
Brass hardware to lift the transformers off the steel deck
and of course, some new tubes.
Which means, the chassis and transformers will be all that is left! Call this a "radiator cap restoration". That is when you lift the radiator cap and drive a new car under it.
OK across 467 Ohms, current to both screens ( only one r4esistor) is 1.56 V 1.7 mA per screen is more like it. I don't see a max spec for this. If that is high, than I would need to increase the resistor I guess. So that puts the plate at more like 31 mA. If I can cross between Chineese and Russian, it is rated at 70. So in the ball park I guess.
A lot of red ink on my drawing. The "as publicized" seems closer to within spec.
Guess I should knock it off and order some parts.
Reconfigure for cathodyne, trippler etc.
RLD style CVS
Add droppers to get b+ within range
Fast diodes (while I am there)
Add the chassis ground
Alps pot, an a second input.
P-P the heater wiring at least, shield the pot and feedback ( if I keep feedback)
Brass hardware to lift the transformers off the steel deck
and of course, some new tubes.
Which means, the chassis and transformers will be all that is left! Call this a "radiator cap restoration". That is when you lift the radiator cap and drive a new car under it.
That's more like it. Max for the 6P1 is 7mA screen current.
Plate power dissipation is the main killer of tubes, and with a higher than spec plate voltage it's easy to run over the limit. I make your plate current to be 28mA if your 520 cathode resistance is correct (once again thats a non-standard value but who knows what the Chinese use). Mine was 540 (2x270).
Either way you are safe at about 7W dissipation. I'm running 244 on the plate at about 35mA, or 9W, and the tubes are happy enough. The max spec is 12W but I wouldn't try that with the Chinese 6P1s.
B+ needs to come down a little, all the same.
BTW, the chassis ground is the FIRST thing to do with these amps.
tvr, when I changed to the LED's without bypass capacitor (as in RLD) the sound quality improved significantly, therefore I thought it possible that the standard bypass caps I was using with the LM317's weren't that good, and that it might be a good idea for anyone with this amp with standard cathode resistors or LM317 to try different bypass caps, just to see if some significant improvement might be made. The ones in my amp were Sanyo 220uF 63V. Also the LED's need to be chosen, not just any LED's, (hint) try TLUR5400 Farnell P/N 1045373 or read the thread.
As far as I understand, no bypass caps in the driver stage or output stage increases local feedback, so for me running no global feedback, using no bypass caps may help.
The specs for 6P1 and 6P1P that I read say 12W anode and 2.5W screen.
For the 6P1P-EV it is 12W and 1.3W. The -EV has a higher plate resistance so the gain is maybe higher.
So it seems our output transformers are custom made for each individual amp, 3 different owners have 600, 340 and 234 ohms from anode to anode. Well I dunno 🙂 That just throws any credibility right out the window...I suppose if 2 "identical" heater windings on my power tranny are incapable of putting out the same voltage at the same current, then why should I be surprised?
Here is a design from an Australian amp in 1956, a similar cct to what I am running, that put out around 8W class A. I drew that schem a year ago and forgot about it until today, I still have that amp. Seems reminiscent of some designs 50 years later...
The way I look at it, LEDs provide the cathode with an ideal constant voltage load without the need for a bypass capacitor because of the LED's inherently low impedance. So, you gain two ways: totally predictable bias and excellent cathode feedback. Must sound better.
It's just a shame that so many are needed because of the current in the output stage.
Here's a couple of pics of mine.(6n1/6p14)
Sorry the quality isn't great.
Been following this thread for a while but been busy with other amps.
Time to start thinking about a hardwired rebuild.
Sorry the quality isn't great.
Been following this thread for a while but been busy with other amps.
Time to start thinking about a hardwired rebuild.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Good, someone with the EL84/6P14 version of the Mini.
Pic quality is fine.
Layout quite different to the 6P1 model in all areas including the PCB power supply sections.
Transformers are different too (there goes my idea of turning the 6P1 model into EL84)
Gary
"Also the LED's need to be chosen, not just any LED's, (hint) try TLUR5400 Farnell P/N 1045373 or read the thread."
Bummer. I was out searching for a grab bag. " 100 for $4" kind of thing. If they cost real money, it changes the viability of the project. 76 of them a least. 10 cents is about tops. Any old will do I think for me, as ultimate sound is not really the goal, it is learning about it. This has been a lot of fun. I am surprised how listenable the actually are, but again, while I am fiddling with it, I plugged in one of my spare Hafler HD120's. Not a bad amp at all. Maybe I won't fix my NAD power supply. I need to get back to my next speaker design and I need to learn Perl. Gad does time fly. Maybe I will win the lottery and not have to work, then I could catch up with my hobbies.
Into 7 ohm resistive load, I clip at about 5.6 W. The splitter is clipping first I believe.
Bummer. I was out searching for a grab bag. " 100 for $4" kind of thing. If they cost real money, it changes the viability of the project. 76 of them a least. 10 cents is about tops. Any old will do I think for me, as ultimate sound is not really the goal, it is learning about it. This has been a lot of fun. I am surprised how listenable the actually are, but again, while I am fiddling with it, I plugged in one of my spare Hafler HD120's. Not a bad amp at all. Maybe I won't fix my NAD power supply. I need to get back to my next speaker design and I need to learn Perl. Gad does time fly. Maybe I will win the lottery and not have to work, then I could catch up with my hobbies.
Into 7 ohm resistive load, I clip at about 5.6 W. The splitter is clipping first I believe.
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