Trying to make sense of the Sweet Peach

[S.Spielbergo]

I'm trying to make some sense out of my Sweet Peach SE amplifier, being a tube noob its coming along very slowly. I have a schematic off of the manufacturers web site but the values are almost impossible to read so I metered all the resistors and some of the caps to flesh out the schematic. I have a couple of burning questions I'm hoping someone can help me with. I see a 3k resistor between v1 (6n3p) and v2 (6n1p) as well as a 3k between 6n1p and the GU50, what purpose does this resistor serve? Is it there to artificialy lower the value of the coupling Caps (all are .1uf MKP's)? I'm not certain of why there are 47k resistors upstream of the V1, V2 anodes just there to knock the B+ down? Finally the 6n1p has a 1k cathode resistor which seems to be high, can I install ~200 ohm cathode resistor without changing any of the other components, was also thinking of substituting a 6n6p in place of the 6n1p (less gain, pleasant sound) but it also likes more current and normally uses about 200 ohm cathode resistor, can this be done without burning the house down?

[Gorgomat]

Hello,

the 3k resistors serve as gridstoppers and are used in order to prevent oscillation and stabilize the circuit.

George

[TheGimp]

The 47 K anode resistors are there as anode loads to develope the output voltage. As the grid voltage changes, the current through the tube changes. If you tied the anodes to B+ there would be no changing voltage output that could be coupled throgh the capacitors to the next stage. The changing current through the resistors produces a corrrosponding changing voltage which it the amplified signal.

No, you can't simply change the 1K resistor to a 200 Ohm resistor without upsetting the bias point of the tube.

On the other hand, if you change to 6N6P it won't sound as good as it will if you do change the cathode resistor from 1K to a lower value (I don't know if 200 Ohm is an optimum value or not).

I suggest you do some reading on tube theory to get a good basic understanding before making any changes to the circuit.

This is a good place to start:
On Line Tube Learning for newbies....

along with this for safety:
Safety Practices, General and Ultra-High Voltage

[Svein_B]

If you have any 6n6p you should be able to plug it right in without changing anything for a start. The 6n6 will draw a little more heater current, but should not matter that much since we only have one driver tube.

Ideally you might want to run it at a little higher current than the present 5mA which would require lower value cathode and anode resistors. This will draw more current from the B+ so you should check if you have any to spare. If the PT is not running very hot, some extra current may be OK, and you could reduce cathode and anode resistors for a hotter operating point, say 12mA?

Some care is required when choosing operating point for the driver since it must deliver 130V peak-to-peak for the GU50.

The reduction in gain will not be immediately apparent unless the global feedback is adjusted.

The gain seem to be on the high side in this amp, about 60x from input to speakers - with the global feedback. The gain could be reduced to about half by removing the cathode bypass on the input tube. The feedback may then have to be adjusted again - if you feel it is needed.

Svein

[S.Spielbergo]

Thank you all very much for the feedback, I have actually done a significant amount of reading over the past 2 years or so on basic electronics and more recently on valves I just need to see these principles in action, that and sometimes you just want answers! I have had the 6n6p's sitting in the closet waiting for a project, a cursory look at the data sheet suggests they can be used in this amp so I popped one in for a while and enjoyed the sound quality. My power trafo is nearly stone cold so I thought I should have some current to spare for the 6n6p.

[Bicolor]

Any comments on the sound you got with 6n6p instead of the 6n1p?
Just curious how the sound was affected.
I guess my amp is still at least a month away, and I am already visiting ebay checking for tubes for it.
Man I'm sick.

[S.Spielbergo]

I would say the stock chineese 6n1 is probably the weakest link, kinda harsh and shouty, the russian 6n1p-ev is significantly better sounding. The 6n6p sounds different, cybals and drums are more transparent and silky while the 6n1p has more kick (very subjective, I know)but this is with the stock circuit. The GU50's drop right in but all of mine did a lot of popping for the first minute or so of operation, they have worked fine since.

[Bicolor]

S.Spielbergo -> Previously I have used russian 6n1p and they didn't let me down. Beautiful to look at and good sound.
I will continue to follow this thread even more when my amp arrives. Guess I'll order some tubes in a week or two just to make sure I already have them when the amp arrives.

[S.Spielbergo]

I'm still trying to figure out how the 6n1p is being used at such low B+ voltages. I meter 200V above the 47k plate resistor and about 130v beneath it at the valve socket. I see that at 130v the 6n1p needs to idle at a super low current and the load line spans the most nonlinear portion of the curves at the very bottom. The 6n6p would be much happier with lower B+ voltages and seems to have better curves for this application.

[Svein_B]

Originally Posted by S.Spielbergo

I'm still trying to figure out how the 6n1p is being used at such low B+ voltages. I meter 200V above the 47k plate resistor and about 130v beneath it at the valve socket . . .

You are right, with 200V for the B+ supply to the driver tubes they will be starved.

From the pictures you posted earlier in: Any pretty good/bad chinese amps?
I see a RC stage from the 400V for additional smoothing of the supply to the driver board. The resistor (located on the edge of the PS board) looks to me as if it might be 2k or 2.7K. If it should happen to be 27K instead, it will drop the voltage to around 200V, which in my opinion is to low.

The estimates and suggestions I have made earlier are based on a supply voltage of 360-380V to the driver board.

Attached is a loadline for 6N1P with 380V supply and 47K anode load. It will be reasonably linear.
Imagine pushing that line down so it crosses 200V and idles at around 1.5mA and it will not perform well.

Svein