Ebay Tube Amps - Page 5

Any updated opinions of these amps? Duggeh what's your overall viewpoint?

Okay, so I gave fran's schematic to my Dad. He is an electronics guru and part-time audiophile. He had a look at it, and made a few corrections based on looking at the pictures and from a few educated guesses. He also had some observations on the circuit. I have taken these from the email he sent me.

Here is the schematic he drew.



Any opinions?

Stew

P.s. Oh, and our NOS valves STILL haven't arrived yet... grrr....

Ok, I'll have to check in more detail later but:

1. double triode tube is 6N1 rather than 6P1

2. The trim pots for grounding the heaters was 50R, but I have replaced these with 2 x 100R to ground on all tubes (although I might just go to DC anyway)

3. The headphones output is taken before the output transformer through a 4K7 resistor. eh, I think anyway!

4. I have added in another 2 caps 220uF/200V in parallel with each of C1 and C2

Could you ask your Dad are the 2 other pots for biasing the output tubes? They read in the order of 50K (from memory) and would he have any idea of what they should be at, or how to adjust them (eg what do I measure whiole adjusting the pot?)

SWT61 - sound quality is bloody good - beats my LD2 and zhaolu upgraded amp. However, you most likely would have to deal with the hum as I have had and maybe buy some decent tubes. Depends on whether you're happy to tinker or not.
Fran

OK,

An update, put in the extra caps (220uF/400V) for the filter. Amp again is a bit quieter, I can't measure the hum anymore on the headphones out, and I'm down to <1mV on one side and 1.2mV on the loudspeaker outs. I tried changing the heaters to AC but it made no difference. So to summarise, the 4 things worth doing:

1. Safety first, earth the chassis
2. Centre tap the heaters with 100R resistors to ground
3. Add in bigger smoothing caps (to the ones rated 100uF/200V)
4. Add in bigger filter caps (to the ones rated 120uF/400V)









Stewtheking: The pot is 100K (not sure if its log or linear), see the resistors right above the 6N1 tube, labelled 100K, they are 200K. And where he has the 250R coming from the 6P1 tubes, that should be 180R. The unknown resistor below C14 should be 860K. Primary on the px is 230V, secondaries are 140V/6.3V/6.3V.

As for the output tx I haven't measured it, and in any case I would only be able to do the most basic of measurements and its a bit of hassle desoldering the leads. So unless I really need to I ain't gonna bother.


Fran

The reply from my Dad...

1Oops of course it is, I'll change the schematic!

2OK I'll add the 50 Ohm value to the schematic (which represents the
original amp rather than any upgraded version). The idea of the trim pot is
of course to balance the heaters as ± 3V rather than a single ended 6V to
cancel the hum. Replacing the pots with fixed resistors removes the ability
to trim this to compensate for any imbalance of pickup from the 2 ends of
the heater to the cathodes. However the original arrangement seems dangerous
to me if there are really 2 pots on a single heater supply, as if one is
adjusted to one end and the other to the other end the heater supply would
be shorted out probably resulting in burning out the pots.

3The headphone output seems rather surprising! 4k7 is a rather high
value for connection to the speaker output in the way I thought it was but I
can't see a sensible place in the circuit to connect it on the high voltage
side of the output and it would need a capacitor in series too to avoid
being dangerous. If you can work out any more details I'll update my
schematic.

4 I agree that the total power supply reservoir capacity seems a bit
low for a hi-fi amplifier, I would be inclined to add extra capacitance in
parallel with the 120µF 400V capacitors and the 33µF on the triode anode
supply rather than adding to the 100µF voltage doubler capacitors due to the
increased surge load that extra capacitance here places on the rectifier
diodes, particularly at switch on.

5I'm not a valve amp expert but as far as I can see the second pot is
intended to balance the drive to the 2 output valves. I guess that the
attenuation produced by the pot and fixed resistor should be equal to the
gain of the second triode so that the signals at the grids of the 2 output
valves would be equal and opposite. If I am correct then setting up would
require a signal generator and scope or a.c. voltmeter. Actually thinking a
little further I guess it is not the grid voltage which would be matched but
the a.c. anode currents of the 2 output valves so the best place to measure
might be across the un-bypassed part of the cathode resistors for the 2
output valves. So I'd put a sine wave signal in and measure the amplitudes
at the top of the 250 Ohm resistors (or for that matter on the cathodes of
the 2 output valves) then adjust the pot to make them equal.


And his updated schematic....

I'm pretty sure I wouldn't have any caps rated at 400V to parallel to the 33uF, but if I can lay hands on some then I'll certainly try it.

Sound is just lovely, I can hear the hum now very low with my HD650s, even less with some PX100s and not at all with AKG501s. Pretty happy with this amp now, so if the new tubes make a positive impact then that would be icing on the cake.

Many thanks to your Dad for making up the drawing and his ideas. They clarify a lot. I know very little about these valve amps (though I will know more, my 2 morgan jones books arrived today!). Its a pity about the set up of those 2 pots being so complicated. I know I fiddled with mine, and they are both now at 40.9K. I can only bow to your superior knowledge of this! I don't know what they should be at.

So basically the amp is now "good" and I am content with it. I will try to lay hands on some 400V caps, what would you think would be appropriate? - another 33uF?

Fran

I received the NOS tubes in the post yesterday. (They had been sitting in my building's reception since the 7th, and I had asked every day, makes me very angry) It really hammers it home how crappy the stock tubes were. At switch-on there is a very low hum, which builds as the valves warm up, and then dies away, taking the original hum away with it, and after about a minute there's no audible hum with the k1000's in the open position.

This is a splendid amp with the k1000's. It drives them magnificently and that added warmth complements the magnificent detail of these things.

I am certainly very glad I took the plunge.

Apart from them lying in reception (I know how you feel!) that would mean delivery from the ebay seller of <1 week. Not bad!

Fran

Any update on this? Have you got your new baby yet, and is it nice?

mmmm interesting, it looks very similar to the PCB of the 6P1 amp that myself and stewtheking have, but with some different components. I wonder if its some kind of generic board thats then stuffed in different ways depending on the tubes used?

Don't throw away those chinese tubes yet either, although to my ears, and only after very little burn in and listening time the russian milspec ones are better, its not by very much.

Fran

EDIT: Duggeh, it looks like your new amp doesn't have a safety earth either. The quick fix is to bring an earth wire from the IEC socket to one of the screws holding down a transformer. Make sure you scrape the paint off to give good electrical contact. You don't want to risk not doing it!

That new amp looks pretty sweet Duggeh, I like the all-wood front, and the switched inputs are a nice feature. It looks like you've been putting it through its Mike Oldfield paces as well, judging by the CD collection behind it.

Where is the headphone jack

Hi Duggeh,

I've been looking at these for a week or so now for driving my Grado125s and have been puzzled as I cannot see any headphone jack on the ebay pictures.

Can you confirm if there is one and where it is ?

many thanks
Neil.