Tube Rolling on the Mr Liang 845

[shahsy]

Has anyone had any joy tube rolling with a Mr Liang 845?

I have recently bought the non-300b version and was wondering if anyone had any success with rolling the pre valves?

I have the stock Shuguang 845 and 845b power valves.

Ta

s

[Ali Tait]

Yes,it's worthwhile changing the front end valves.For the inputs,look on the bay for 6ac7's.These are cheap as chips and you can get '40's NOS for a few quid.Buy a few pairs as the sound can be a bit variable.NOTE-If you use these,bear in mind that they are metal-bodied valves,so you need to confirm that pin 1 is grounded to the chassis for safety!!

For the driver valves,you have the choice of KT66,KT88 or 6L6/5881.All of these will swap straight in.I found Russian Reflectors were very good at a reasonable price.

It's also very worthwhile changing the coupling caps on the front end stages.

[shahsy]

Hi Ali,
Thanks for responding.......

How do check that Pin 1 is grounded on the metal base 6ac7's?

Do I look up the datasheets for the valve or is it the sockets on the amp that I check?

Ta

s

[Ali Tait]

Use a multimeter to check between pin 1 and the chassis using the ohms setting.

[shahsy]

Ouch!!!
I don't fancy poking around the innards of this whilst it's powered up and upside down, mate!

I wouldn't even know what I was looking for with regards to an Ohm reading on the multimeter.:doh:

Cheers

s

[Ali Tait]

No,switch the amp off,let the valves cool down and then remove the input valves-the two in the middle at the front.Use a multimeter set to ohms or the "buzzer" setting,which makes the meter sound a beep when the probes are touched together.Put one probe in the pin 1 hole of the valve base,and the other on the chassis of the amp.If they are connected,the buzzer will sound,or,if you have ohms selected,you will get a reading of less than 1 ohm.See this diagram for pin 1 of the valve base-

http://www.r-type.org/static/baseio.htm

DO NOT go inside this amp if you're not sure what you are doing.This amp operates at 1000 volts AND IT WILL KILL YOU!

[Ali Tait]

Whereabouts are you located?

[shahsy]

I'm in London.

I checked Pin 1 against the hole for the valve cage and the resistance was high.

Does that mean that Pin 1 is NOT grounded?

Cheers

Syed

[Marco]

Hi Shahsy,

Is Syed your proper first name? :)

Marco.

[Ali Tait]

It should buzz anywhere you are touching the metal of the chassis.This needs to be grounded,as a short could leave the valve live at 250v.Also,with the amp off,check that there is continuity between the earth pin of the iec and the amp chassis.Some of these amps came with no chassis earth!

[swampy]

They come without an earth on the chassis as standard !!

First job is to fit an earth wire from iec earth pin to chassis