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    the safety precautions around high voltages.

Repairing Hi-End Preamplifier

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Like I said, my mod would install exactly where the original parts were and could be switched right back to stock in a very short time. No damage to the unit's value at all.

As for differences in sound, I'll have to test that. I do stock the other transistors.

-Chris
 
OK, one thing is clear: this design works close to the edge, which probably explains why it failed. <snip>
Enormous thanks for your time and help, I shall be doing something very similar to that you have suggested. I'm currently still going through the datasheets, I have spotted the Semitec S-562T which are 100v 5.6mA.

Yesterday I showed the circuit diagram to a tube amp designer/repairer who said he had never seen anything like it before, he said they should have had a 1M resistor across to the rails to allow the capacitors to discharge. He also suggested 10ma as the typical operating point.
 
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So I ordered the following 100v CCDs from Mouser
954-E-562 x8, 5.6ma
954-E-352 x2 3.5mA
954-E-452 x4 4.5mA
954-E-272 x2 2.7mA

I intend to use 2x5.6mA and one 4,5mA in parallel to give 15.7.mA for PS regulation and a 5.6mA and 4.5mA to give 10mA to supply the anode, that will leave 5.7 for the zener branch. Since postage was free over £33 I added a few others just in case more current or different values are required,

The CCLH100 series of constant current diodes looked idea until I realised I had to order £2,500 pound worth
 
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Back in the 90's I was repairing a unit for someone and the lead time on the diode that it needed was 6 months from all suppliers. I contacted the company directly and told them that I needed to purchase this diode. Their reply was " I'm sorry, but they will not be available for 6 months, but I can send you some free samples..."
 
So I ordered the following 100v CCDs from Mouser
954-E-562 x8, 5.6ma
954-E-352 x2 3.5mA
954-E-452 x4 4.5mA
954-E-272 x2 2.7mA

I intend to use 2x5.6mA and one 4,5mA in parallel to give 15.7.mA for PS regulation and a 5.6mA and 4.5mA to give 10mA to supply the anode, that will leave 5.7 for the zener branch. Since postage was free over £33 I added a few others just in case more current or different values are required,

The CCLH100 series of constant current diodes looked idea until I realised I had to order £2,500 pound worth

this schema is nothing special there, just a bunch of ccs diodes.
which does NOT give much high impedance to triode. and next stage will lower the loadline further.
in general, ccs diodes impedance is waay below what gyrator can achieve. and you will parallel few of them.

basic plate resistor is slightly worse in this respect.

i´d go with fet gyrator, which gives ~5 megohms for driver tube (small thd), and output resistance is much lower, not affecting the tube stage.
 
The parts arrived yesterday and fortunately they look identical to those being replaced. Before rushing ahead I decided to have a rethink. My tests showed the zeners used did not require much current to breakdown. I found these data sheets online 1N4125 pdf, 1N4125 description, 1N4125 datasheets, 1N4125 view ::: ALLDATASHEET ::: , which required only 1mA to be in the middle of their operating range.

This fitted in nicely with the original design that used only two CCD's, so I used two 5.6mA in the power section of the circuit and a 5.6 and 4.5 in the anode leaving 1.1mA for the zeners. This seemed like a risk worth taking since over current may damage the zeners.

So I rolled the dice and believe it or not it worked first time. First impressions are that it sounds as before but I'll shall reassess after some serious listening.

Again, thanks to the forum for their help - .time for a beer or two!

After comparing with another preamp the sound is very 3d and flowery but a bit distant. SO I suspect the bias spot is not quite on.

I measured the anode voltage and its 99v and stable once the power supply has warmed up. Looking at the bias curves I'm wondering if I need at least another 5mA on the anode?
 
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