[raindance]

Forgive me if this has already been posted, but I have built one of these and have some feedback on improvements to make it sound decent - any interest? It is a GREAT little amp for the money. In my system it sounds better than a far more expensive 300B amp I had previously.

http://www.ebay.com/itm/Douk-audio-5Z4P-6N1-6P3P-HIFI-Single-ended-class-A-tube-amp-amplifier-DIY-KIT-/251524477179

[raindance]

I didn't see any interest in my post, but anyway here are details on the circuit and the changes I did to make it work well. Pretty easy build with minimal quirks. It sounds a bit hard and bright with 6P3P output tubes, but EL34's sound SWEET. Very stable and low hum after the hum balance resistors are added to the EL34's to create a false grounded center tap. Green italics are the changes from the kit schematic.

[tull skull]

Thank you Raindance. I appreciate the time you invested to give us your help.

[raindance]

I have to recant the schematic I posted. The driver does not have enough gain to run the amp in triode unless you are driving headphones 

New schematic posted. It achieves the original goal of the designer but with a better method of strapping the output tube into pentode mode.

If you want to try triode mode, the feedback loop has to be completely removed. The transformers are not really good enough for this.

[PCKIDZ]

Thanks Raindance, for posting the mods. This is helpful as I am planning to replace my 6P3P with EL34. When I compare my stock schematic withyours, I see that grid bias is set at 4.7K in your mod. Did you measure the bias needed to arrive at this value? I have given my original schematic below for reference.

[undertowogt1]

These amps look like fun to build. Do they sound good with the stock parts? How are the instructions that come with the amp. I built a Bottlehead phono Preamp but the instructions were very well layed out, step by step, are these instructions step by step?

[PCKIDZ]

The amps are definitely fun to build. The instructions are quite basic - they give you a schematic and then a followup pictorial on demand. Without latter its difficult for a beginner to have a go, I think. The quality of the components is okay, though I would replace all the stock connectors. The sound has a lot of clarity compared to my Marantz and the Pioneer amps. Definitely worth a go.

Here is a picture of the underside after the build.


And here is the amp

[FullRangeMan]

An interesting option to this amp is 6L6 and KT66(very linear).

[raindance]

PCKIDZ - the 4k7 is not bias. It is a grid stopper required to prevent oscillation.

Just add the pin 1 jumper that I show on the socket and el34 should be good.

I think the 150 ohm cathode bias resistors could be a bit low. You'll need to measure to confirm.

[PCKIDZ]

Thanks fullrangeman, I already ordered the EL34 this round...so may be next round it will be KT66.

Raindance, thanks for your inputs - will try as soon as the EL34's arrive. I looked up grid stopper and understand the function now.

This circuit is supposed to be auto-bias. Does that really work? (I am new to tubes!).

[raindance]

It's cathode bias. I guess you could say auto bias as it doesn't need manual biasing, but you need to confirm current through the tube by measuring the voltage across the cathode resistor and then measure voltage across the anode and cathode if the el34 so you can check that you're under 25 watts of dissipation. There are some useful tutorials online about this.

[vshahid]

Thanks for the clarity and detail posted already. I bought this to help my son and myself get familiar with tube electronics. I was disappointed to find such simplistric circuit diagram that it appears that the diag is only for the right channel. I am not entirely sure of how to wire for the other channel and while I could mirror the diagram for the other channel, last I looked at the parts (about 6 months ago) it seemed that to mirror it, I'd be missing a capacitor. Can someone post some hints about what to do abou the other channel?
I plan to carefully review the 2 diagrams the original thread owner posted. If those address my question, please feel free to say so.
Thanks in advance.
Vic

[FullRangeMan]

Thanks fullrangeman, I already ordered the EL34 this round...so may be next round it will be KT66.

Raindance, thanks for your inputs - will try as soon as the EL34's arrive. I looked up grid stopper and understand the function now.

This circuit is supposed to be auto-bias. Does that really work? (I am new to tubes!).

If its autobias you cant adjust the bias to various tubes, you will have to check how much is it before rolling tubes.
The one channel schematic is usual in tube amps.

[raindance]

Thanks for the clarity and detail posted already. I bought this to help my son and myself get familiar with tube electronics. I was disappointed to find such simplistric circuit diagram that it appears that the diag is only for the right channel. I am not entirely sure of how to wire for the other channel and while I could mirror the diagram for the other channel, last I looked at the parts (about 6 months ago) it seemed that to mirror it, I'd be missing a capacitor. Can someone post some hints about what to do abou the other channel?
I plan to carefully review the 2 diagrams the original thread owner posted. If those address my question, please feel free to say so.
Thanks in advance.
Vic

You should have everything for the audio circuit duplicated. It is helpful to print the audio portion twice and then write the tube pin numbers on each one. The power supply circuit is common to both channels.

Note that the power transformer runs far too hot and I do not feel it is safe to leave this amp on unattended even with only 110 volts mains supply via a step down transformer.

I see some people claim to have gotten good results by decreasing or removing the global feedback - the output transformers are not even close to good enough to get hifi bandwidth by doing this, so please leave the feedback in place. Without it you will get more gain, but the frequency response will droop at both ends and the sound will be extremely load dependent.

[richp4003]

Great thread.. Ive always wondered about the quality of the stuff that Douk sells on eBay and Amazon.. they offer a lot of unique solutions that are priced right.