Modifying the Lowther Club HK Jazz 6L6 ($449 SET amp)

[Doglover]

I recently purchased a Jazz 6L6 direct from Lowther Club HK and have been quite impressed with this little amp especially given its modest price.
Even unmodded and with the Chinese valves it arrived with it delivers very smooth, relaxing sound especially at the low volume levels I tend to do most of my listening at.

I'm hoping to improve its sonics even further and have ordered an assortment of NOS tubes which I'm sure will do the trick, along with some Obbligato premium gold film caps to replace the 0.33uF WIMA coupling caps, because I've never been a big fan of WIMAs and find them somewhat veiled.

I've worked with valve circuits before but my knowledge of electronics is quite rudimentary so I mainly limit myself to simple mods like replacing caps/resistors etc and simple circuit changes.
Lowther Club HK were kind enough to send me a schematic of the amp and I'd love it if some of the more knowledgeable folk on the forum could take a look and gauge if there are any fairly simple changes I can make to improve things even more.

Cheers, thanks for reading guys.



Here's a photo of the amp:

An externally hosted image should be here but it was not working when we last tested it.

The specs and further details are listed here:

Lowther Club Home


Here's the schematic:

An externally hosted image should be here but it was not working when we last tested it.

 

[julcat]

any news about you mod??

 

[DF96]

SET usually means single-ended triode, but here it seems to mean single-ended tetrode - otherwise known as pentode mode.

If changing the coupling cap then keep the same value, to avoid LF loop stability problems.

 

[speakerfritz]

hows the mod project. with 90db S/N this amp is right up there with the cary amps which come in at 90db as well. I'm thinking of ordering one as well.

 

[tubeworld]

Man... the price is very attractive.

Here are some that I can propose:
- replace 240k 6L6G grid leak resistor with grid choke
- change 100k pot to a stepped attenuator
- bypass 680u/50v with good quality film cap, or change to BlackGate
- bypass 22u/400V and 220u/450v with good quality film cap
- change output transformer.
- change 5H choke to amorphous choke
- regulate the 6L6 grid supply. Zener used tends to sound noisy. YMMV.
- change to Mundorf SIO or Gold-IO perhaps for coupling?
- change to interstage transformer coupling

 

[speakerfritz]

Man... the price is very attractive.

Here are some that I can propose:
- replace 240k 6L6G grid leak resistor with grid choke
- change 100k pot to a stepped attenuator
- bypass 680u/50v with good quality film cap, or change to BlackGate
- bypass 22u/400V and 220u/450v with good quality film cap
- change output transformer.
- change 5H choke to amorphous choke
- regulate the 6L6 grid supply. Zener used tends to sound noisy. YMMV.
- change to Mundorf SIO or Gold-IO perhaps for coupling?
- change to interstage transformer coupling

These three seem low effort and are great ideas....but can you use a grid choke on a 6l6....I normally see them on input tubes mainly due to their very low current capacity....any grid choke recomendations? thanks

- replace 240k 6L6G grid leak resistor with grid choke
- bypass 680u/50v with good quality film cap, or change to BlackGate
- bypass 22u/400V and 220u/450v with good quality film cap

 

[tubeworld]

It's better to use grid choke than grid leak resistor on output tube. Maybe you mixed up with plate load choke. Grid choke does not carry DC but only AC. It is good for the driver/input tube where it sees higher impedance -> easier to drive, it also present a low DCR for the output tube grid.

There are several vendors that has grid choke. Check the vendors below the line:

 

[speakerfritz]

yes...I am confused...thought that grid and anode chokes only pass DC...thus the benifet since it wont effect audio signal which is AC.

so I need to do more homework on this.

AudioKarma.org Home Audio Stereo Discussion Forums

 

[artosalo]

Here are some that I can propose:
- replace 240k 6L6G grid leak resistor with grid choke
- change 100k pot to a stepped attenuator
- bypass 680u/50v with good quality film cap, or change to BlackGate
- bypass 22u/400V and 220u/450v with good quality film cap
- change output transformer.
- change 5H choke to amorphous choke
- regulate the 6L6 grid supply. Zener used tends to sound noisy. YMMV.
- change to Mundorf SIO or Gold-IO perhaps for coupling?
- change to interstage transformer coupling

...then make some measurements and notice that you have ruined frequency response, got higer THD and have less money .

 

[tubeworld]

speakerfriz - that pic you got is spot on.

artosalo - that's where all the fun is, no?

 

[DavesNotHere]

wow that is too much negative feedback. I'm sure this circuit could sound better if someone like me works out the apparent design flaws.

negative feedback should be used as gain control and the last resort. and not to patch up the design flaws.

in a perfectly designed amp, removing the global negative feedback resistor doesn't change the noise floor.

 

[Airhead]

wow that is too much negative feedback. I'm sure this circuit could sound better if someone like me works out the apparent design flaws.

negative feedback should be used as gain control and the last resort. and not to patch up the design flaws.

in a perfectly designed amp, removing the global negative feedback resistor doesn't change the noise floor.

But if you remove feedback loop completely from this amp you should connect output tube as triode. Otherwise frequency response will suffer a lot.

 

[DavesNotHere]

response has nothing to do with the bias operation.

the quality and the amount of saturation the output transformer does (in se designs) effect response.

 

[artosalo]

response has nothing to do with the bias operation.

What ?
Do you refer to Airhead's comment about connecting output tubes as triodes ?

 

[jazbo8]

Man... the price is very attractive.

Here are some that I can propose:
- replace 240k 6L6G grid leak resistor with grid choke
- change 100k pot to a stepped attenuator
- bypass 680u/50v with good quality film cap, or change to BlackGate
- bypass 22u/400V and 220u/450v with good quality film cap
- change output transformer.
- change 5H choke to amorphous choke
- regulate the 6L6 grid supply. Zener used tends to sound noisy. YMMV.
- change to Mundorf SIO or Gold-IO perhaps for coupling?
- change to interstage transformer coupling

Nothing like spending the other's money, eh?! Most of these are "audiophile" mods, they hardly address the amp's "design".

 

[painted]

Correct, before fiddling with components you'd better start with a sound design. SE penthode would have high uneven order harmonics compared to SE triode, which have to be compensated by the driver stage or through overall feedback or both. Any known measurements ?