Yaqin 300B re-biasing

[RGatherer]

Hello Tube loving people,

Having perused this forum on a number of occasions I hope that this is the place to find the answers!

I have recently bought a Yaqin 300B tube amplifier (my first foray into the world of audio valves). I am enjoying the sound very much but am already getting itchy fingers and am planning to switch out the stock 274B rectifier for a RCA NOS 5U4GB. I was initially under the impression that the amp was auto biasing however I have been told by the Chinese company I ordered the amp from that it is not and will need re-biasing. However unlike its sibling the MC10 it does not seem to possess nice easy access re-biasing ports.
Does anyone have any experience of re-biasing this amp and could tell me how to best go about it?

As an aside the motorized volume (as accessed via the remote) has never worked, is this likely to be a simple fix or should I resign myself to getting off the sofa if I want my music louder!

Thanks in advance

RG

 

[Rod Coleman]

I looked around for some specs or schematic on this model, with little success.

I gave up the search after reading the sales spec, which easily wins this year's nomination for auto-translated gobbledegook. Try this extract:

This machine use grade gallbladder *7 of 12As
not famous with behind grade use by gallbladder
the EL84s and top-grade English walnut side board
not wooden not real not famous.

 

[RGatherer]

Schematic found:

http://img529.imageshack.us/my.php?image=spyaqinms300bxo9.jpg

 

[Rod Coleman]

The schematic shows automatic (cathode resistor) biasing. It's R14 in the schematic. you can verify all this easily:

- Disconnect the mains plug & wait 30minutes
- Gently remove a 300B valve
- Use a DVM (any cheap one will do) to measure the resistance between either of the large diameter 300B pins to chassis. Use pointed probes to find the contact inside the socket - use a lamp to inspect the socket & see where the metal contacts are.

If you can measure a steady value (it will be low for a moment while the caps charge from the DVM) of 670 to 700 Ohms, you confirm an autobias design.

The RCA 5R4GY is very pleasant sounding in 300B-SE circuits, but there are some concerns about the way the 300B is operated in this circuit that would keep me from changing anything just yet:

- I wondered about their extravagant claim of 10W per channel from a 300B, and sure enough, the poor triode is being flogged with a 460V B+ and a 680 Ohm bias resistor - translating into an operating point of about 390V/100mA, 39W anode disspation! This is OK for KR300BXLS and other super 300Bs, but if you have WE, JJ or EH in there, they'll be running almost at the rated 40W dissipation & will have a short life. Meshies, like the TJ, will melt down in such a socket.

Soundwise the KR300BXLS will enjoy this operating point, but the normal 300B design likes being near the 360V/60mA suggested by WE. The circuit can be easily adapted to accommodate this -

Switch R14 from 680 to 1KOhm (50Watt rated) to reduce current;
- Replace the C14-C15 nasty electrolytic pair with a 1000V metallised polypropylene (I like the LCR brand here) value 0,68u or 1uF. BTW, the present C14/15 have no voltage balancing resistors present, which risks them going bang - taking your expensive rectifier along with them...

I assume you have, or will be fully trained in high voltage safety procedures, before deciding to mod the amp.

Modding invalidates the warranty, also.

I also note that the amp runs unfiltered rectified dc to the filaments. This will prevent any 300B from showing its true colours, and cheaper ones especially. Fix this with a current drive module from Tentlabs, or build/commission one like this

http://www.diyaudio.com/forums/showthread.php?s=&threadid=38248&perpage=30&highlight=&pagenumber=2

 

[RGatherer]

Rod:
Thanks for going to all that effort, its much appreciated.

I will test for the auto-biasing when I get home tonight. I am still waiting for the replacement rectifier (shipping from the US) so I will hold off on swapping tubes for the time being.

The mods you suggested sound interesting, they are currently out of the scope of my expertise, but that is not to say I'm not a willing student. Would you be open to the suggestion of me and my amp taking a trip up to Warwickshire some time in the future. You could have a look at the amp, perform the mods you have suggested and I can learn something in the process. I would of course pay for all parts, your time and refreshments! Sorry if you think this an impudent suggestion, but worth a try I thought.

Thanks again