Thorsten Phono Amp problem.

[Tweeky]

A good year ago I purchased the components to build Thorsten L's 83/88 phono amp. Today, being stuck indoors full of cold, I built one channel and connected it up in place of the right channel of my WAD, and nothing! . I have the heaters glowing in both valves, but no output. The bit that's really confusing me is that with it connected, there's no output from the untouched WAD left channel either. I have built it without mods to the standard design, other than star earthing. I have a DMM, but no scope. I have checked and rechecked the wiring to the point where I wouldn't spot a cock-up now anyway. I have checked the values of all the components, and all are ok, apart from the 1nF cap, which is measuring 2nF in circuit. I would appreciate some help or pointers from those of you far more knowledgable than me. Even if one of you could give some values I could check at various points in the circuit, it would help.

[Andrew]

Hi,

I would start by checking the voltages on the anodes and cathodes of each valve as well as confirming the values on the heater pins. Let us know what you find.

cheers,

-- Andrew

[Tweeky]

Hi Andrew,
Appreciate you taking the trouble to reply.
I have taken some measurements this morning and they are as follows:-
I will give the pin numbers of the valves as well, in case I have made a basis error.
Pin 5 on both is the +heater connection, with pin 9 on the ecc83 being the 0V heater connection. I have connected pin 8 of the ecc88 to the 0V heater connection and this is not connected to the star earth. the rail voltage is 282V, and the heater supply is 6V.
On the ecc83, pin 6 (anode?) is 72.9V, pin 7 (grid) is -1.7mV, pin 8 (cathode) is 0.466V.
On the ecc88, pin 6 (anode?) is 282V, pin 7 (grid) is 0.61V, pin 8 (cathode) is 42.8V, and pin 9 is 0.5mV.
You have probably guessed by now that I know just enough to be dangerous! Looking at those figures, I am confused by 42.8V at pin 8 of the ecc88 when I have it connected to the 0V heater connection.

Many thanks

Steve

[Andrew]

Hi Steve,

The 88 looks very wrong, so let's start there.

Pin 8 should be ground (0v).

The anode of the 88 (1 & 6) should be about 75 volts, the cathodes (pin 3 & 8) should be 0v and the grid (2 & 7) should be below the cahodes by a bit, say -0.5v. Don't forget to look up from underneath when numbering.

Which way around the heaters are wired for DC on the 88 is not important, just make one positrive and the other the return, somewhere in the PSU I would have thought the return should be at ground. Check this with your meter set to Omhs put one end on the heater 0v and the other where you know the ground is.

cheers,

-- Andrew

[Tweeky]

Andrew,
Just to clarify as I'm not totally clear. The psu being used is the WAD unit in totally standard format. I am, for the purposes of checking the tl circuit, using that to power the left channel of the wad phono and the right (only) channel of the tl. The pin numbers I gave you are as viewed from underneath counting clockwise. Please could you confirm the heater connection pin numbers for the ecc88 as surely I shouldn't be grounding the heater through the cathode?

Thanks

Steve

[Andrew]

Hi Steve,

I had forgotten about the WAD PSU having raised heater supply, it looks to me like you've connected pin 8 to the WAD PSU heater supply. Pin 8 is the cathode and for T's circuit should be at ground.

To recap, the ECC88 from underneath clockwise...

1 = a'
2 = g'
3 = k'
4 = h (go to heater plus which should be about 46v)
5 = h (go to heater neg which should be about 40v)
6 = a (to the 12k)
7 = g (100R grid stopper from the 10nF cap)
8 = k (ground this it should be 0v)
9 = N/C (or perhaps shield, I would ground 0V this)

I you google "duncan amps" then look on his site he has all the data sheets for valves.

cheers,

-- Andrew

[Tweeky]

Hi Andrew,
I have rewired the ecc88 as follows:-
pin 4 is heater positive,
pin5 is heater negative(0V)
pin6 unchanged, was already going to 12K
pin 7 unchanged was already going to 100R
pin 8 & 9 to star earth.

I no longer have the ecc88 heaters glowing, voltages are now:-
pin 4 3V
pin 5 -2.9V
pin 6 282V
pin 7 40mV
pin 8 0V
pin 9 0V

Thanks

Steve

[Tweeky]

Andrew,
Addition to last post.
You mentioned raised heater output from the WAD PSUII, but the heater voltage is 6.3V from this PSU.

Thanks

Steve

[NickG]

Its 6.3v between the heater pins, but the entire thing is raised 40v above signal 0v.

I suspect you are in danger of blowing up the power supply if you are not carefull.

Can you draw out what you have, and how its connecetd into the wad supply, that should give a better idea whats going on. If you can add any voltages you have measured to the drawing that would help as well.

[Andrew]

Hi Steve,

Still something wrong here...hmmm

The WAD uses a raised heater supply since the cathode of the last valve will be at quite high voltage relative to a heater supply of 0v to 6.3v; this is typical for a cathode follower. WAD did this to reduce humm and prevent the insulation between the caothode and heater on the last cathode follower valve going pop. I think they raised it by 40volts so 40v and 46.3v relative to ground.

Pin 6 is way too high, the anode should be 75v on the 88.

Pin 8 and 9 are definitely now correct.

The difference between pin 4 and pin 5 is just about right ~ 6volts, its just not correct relative to anything else.

Pin5 should just be connect to the heater- and Pin 4 to the heater+, it would be best make sure there's nothing else connecting these pins. Let's assume if the heaters worked in the WAD they will work in this steup. I suspect you've got the heater- connected to ground, which would be incorrect.

cheers,

-- Andrew