High voltage Hiwattish build progress

[cbass]

Can't he just use a fwb rectifier and connect the ct to the middle of his series's filter caps and get screen voltage from there .I've never tried it so maybe I'm not even close here.

[jazbo8]

However I am wondering what the bias should be for KT88s running with 700v plates and 350v screens.

the idle current could be estimated by 0.7 x (42/700) = 42mA, but you still need to know the OPT impedance to see if the load line strays too much above the max Pda limit. Do you know the KMD OPT's spec? Is it 6.6k (for most 6L6 amps) or something else?

[lord preset]

However I am wondering what the bias should be for KT88s running with 700v plates and 350v screens.

the idle current could be estimated by 0.7 x (42/700) = 42mA, but you still need to know the OPT impedance to see if the load line strays too much above the max Pda limit. Do you know the KMD OPT's spec? Is it 6.6k (for most 6L6 amps) or something else?

The only info I have on the OT is from the schematic. The KMD ran at the same 700/350 voltages and used 6L6GCs, but that's all I know. KMD sourced the transformers from Tranilamp UK which still exists but has never been known for audio transformers.

[jazbo8]

The only info I have on the OT is from the schematic. The KMD ran at the same 700/350 voltages and used 6L6GCs, but that's all I know. KMD sourced the transformers from Tranilamp UK which still exists but has never been known for audio transformers.

I found a possible reference to the OPT here. Assuming the KMD OPT has the same specs, then the load line is pretty steep if you run 4.7k:8, which will stress both the PT and the OPT... Instead you may want to run it at 9.4k:8 (8R speaker on the 4R tap) just to be safe.

The 42mA idle current is calculated by biasing the tube at 70% of the 42W maximum plate dissipation at 700V.

[ToneMerc]

The only info I have on the OT is from the schematic. The KMD ran at the same 700/350 voltages and used 6L6GCs, but that's all I know. KMD sourced the transformers from Tranilamp UK which still exists but has never been known for audio transformers.

I found a possible reference to the OPT here. Assuming the KMD OPT has the same specs, then the load line is pretty steep if you run 4.7k:8, which will stress both the PT and the OPT... Instead you may want to run it at 9.4k:8 (8R speaker on the 4R tap) just to be safe.

The 42mA idle current is calculated by biasing the tube at 70% of the 42W maximum plate dissipation at 700V.

q

If he uses the stock KMD OT transformer for EL34's or 6L6's he will be fine. I would have speced a transformer for the KT88's though. However, I do think the Heyboer PT he had wound is terribly underrated even for the smaller bottles and not to mention the KT88 tubes.

TM

[jazbo8]

If he uses the stock KMD OT transformer for EL34's or 6L6's he will be fine. I would have speced a transformer for the KT88's though. However, I do think the Heyboer PT he had wound is terribly underrated even for the smaller bottles and not to mention the KT88 tubes

I Agree, it would be safer with the smaller tubes.

[lord preset]

If he uses the stock KMD OT transformer for EL34's or 6L6's he will be fine. I would have speced a transformer for the KT88's though. However, I do think the Heyboer PT he had wound is terribly underrated even for the smaller bottles and not to mention the KT88 tubes.

TM

If you were to spec an OT for KT88s running at 700v what parameters would you give the manufacturer? The only specs I have seen for OTs are primary impedance, secondary impedance and wattage. The KMD OT is massive and was used in an amp that used transistor driven cathodes to get 100 watts from 2x6l6GC. For 2xKT88s run conventionally I was guessing roughly comparable wattage. Is the primary impedance required for KT88s that different? I have no problem running EL34s but I'd like to understand. Thx

[jazbo8]

Is the primary impedance required for KT88s that different? I have no problem running EL34s but I'd like to understand. Thx

Yes, they are somewhat different, check the datasheets and you will see the differences when used under "typical operations". You can also do a simple test to verify the spec of the KMD OPT - apply a small AC voltage to the secondary winding of the OPT and measure the AC voltage across the primary winding, to determine its winding ratio.

[Meat&Beer]

You can also do a simple test to verify the spec of the KMD OPT - apply a small AC voltage to the secondary winding of the OPT and measure the AC voltage across the primary winding, to determine its winding ratio.

Here's a good video that shows you how to do just that, in case you'd like some added visual and commentary.

https://www.youtube.com/watch?v=QQNVyYEzcJ8

[Structo]

Gerald Weber's books were my first tube amp books I bought after a long
spell of not doing any electronic work for many years.

For that they are good but there are better resources for tube knowledge nowadays.

Check out the Reading Material on Steroids sticky, in the technical section.

https://tubeamparchive.com/viewtopic.php?t=9023

[lord preset]

I finally got around to building a head cab for this beast. Now I can get some real playing time on this before I do any tonal tweaks and put this on the block. I starting this amp think it would be a keeper but my affection for big stupid amps has waned. Actually I'm pushing to complete some other builds to raise cash for one of Henry's Redplate combos.

[Structo]

That looks very nice!

Somebody will love it!