Comments on this circuit

[georgehifi]

Comments on this circuit

Cheers George

[Zen Mod]

Velleman?
if yes-no ,tnx..........

[georgehifi]

Originally posted by Zen Mod
Velleman?
if yes-no ,tnx..........

No it's a Chinese one that my freind may buy two of, they are stereo amps and have separate 2x power transformer on each stereo chassis

[georgehifi]

Input tube is 12au7 and phase splitter is 12at7 price is very good at $453aus that's $348us here is a shot of the front

[bembel]

Hi, obviously these chinese amp ... good & ALMOST disheartening (having a hard life & no money)

PS: anyway can you give the references (I'm forevermore stupid)

[Zen Mod]

Originally posted by georgehifi
Input tube is 12au7 and phase splitter is 12at7 price is very good at $453aus that's $348us here is a shot of the front

with these toobz ,if OPTs are decent,then that can be good amp;

mebbe even with 12BH7 instead of 12AT7 as driver ....

[ray_moth]

A few things that occur to me:

It's a Williamson type design but with OP tubes switchable beween pentode-mode and triode-mode. I doubt that the same amount of NFB or the same OP impedance would suit both modes. so one mode probably works better than the other (or both could be bad!)

The NFB loop is incomplete as drawn: the other end of the OPT secondary should be grounded - probably just a drafting error.

The PS uses a voltage doubler for B+ with no choke, so regulation and hum could be problemmatical. There is a delayed turn-on of B+ using a 555 timer, which is a good idea as long as the relay contact is suitable for the job.

There is a mysterious half-wave rectified circuit connected to a winding to the right of the power transformer, under the bias supply. I can't see what it's supposed to be doing.

The fixed bias adjustment is not independent for each OP tube, so the output tubes need to be well-matched.

[Zen Mod]

how they do this.
and they do this well.

[georgehifi]

Originally posted by ray_moth
A few things that occur to me:

There is a mysterious half-wave rectified circuit connected to a winding to the right of the power transformer, under the bias supply. I can't see what it's supposed to be doing.

Could that be a separate heater for the input/splitter

[gingertube]

G'day georgehifi,

I've bought 2 of these Chinese amps in the past. Not this model but the EL34 Push Pull and the 845 SET (Music Angel brand name).
Both were OK out of the box and both are now significantly better after some tinkering.

Ray,georgehifi
That mystery half wave supply is the supply for J2 and J3 which are the TRIODE / Pentode switching relays for the left and right channels respectively. Those switch contacts you see doing the triode/pentode switching are actually the switch contacts of these relays.

This is a later model and I hope the bugs are now out of the product range BUT:
1) The 845 SET did not have a chassis earth connection. I added this immediately. In addition the main filter caps on the voltage doubler power supply protrude through holes in the top of the chassis. The top capacitor was of the type which has the can connected to the negative pin which meant that the body of the cap sat at +430V and relied upon the plastic sleeve to keep you from being fried.
2) The EL34 PP had a manufacturing fault - it used a paraphase splitter driver (6N8P = 6SN7). To balance these drivers one anode load resistor is just a little higher than the other - on one channel they'd managed to get these resistors reversed.

Both amps responded well to tube rolling and a few capacitor upgrades. The EL34 PP was eventually rebuilt to a new design.

So summary is they are good value for the dollars BUT its worth pulling the bottom off and having a good look round before powering up to check for basic stuff ups.

Cheers,
Ian