Transformers lying around - ideas for amp builds

[Roe]

I got the following transformers lying around and would appreciate suggestions on possible applications:

-100W plexi OT 1k7 primary, 8 and 16 ohms secondary
-80W twin OT 2k/4ohms [or 4k/8ohms or 8k/16ohms]
-40W universal OT 4k/2,4,8ohms [or 8k/4,8,16 ohms]
-20H, 320ohm ac30 choke
-8h plexi choke
-4h fenderish choke
- unidentified BIG PT. Secondaries: 900VAC w/center tap, 6.3V & 5V (came out of 50w amp w/el34s with ca 600VDC)

I'm considering the following
- using the big PT with the twin OT into a 16ohms load for a 8k primary impedance with a pair of el34s/6550s with around 400-450v at the screens at 600v at the plates

- a 20+W build with the 40W OT running 6v6s/5881s/kt66s at 8k into 16ohms load

- a 100w vox ac100 or plexi hotrod with the plexi 100w OT

Any ideas or suggestions?

[kdmay]

[quote="Roe"]
- using the big PT with the twin OT into a 16ohms load for a 8k primary impedance with a pair of el34s/6550s with around 400-450v at the screens at 600v at the plates
[/quote]

Looking at doing something similar with a pair of KT88s, so will be curious to hear your results.

Out of interest, why such a high primary? Still getting my head around load lines and the like.

How do you plan to drop the screens? Big resistor in series with choke, large screen resistors, or UL OT?

[Roe]

...Looking at doing something similar with a pair of KT88s, ...why such a high primary? Still getting my head around load lines and the like.

How do you plan to drop the screens? Big resistor in series with choke, large screen resistors, or UL OT?

Hi, I may need to use kt88s as well, esp if the screen voltage is high. the 88s can probably handle a lower primary (e.g. 4k5) but the el34s would need a higher primary at this voltage, wouldn't they? Also, I thought I could just use the twin OT I have lying around for 8k/16ohms.

I plan to drop the screens by using two different plate electrolytics in series (e.g. 220u and 100uf), paralleled with different resistors (e.g. 220k and 100k). I simply plan to connect the choke and screens to the junction of the caps and resistors - hoping it will give me 400+V

[Aurora]

Load impedance for a given setup of tubes is generally the same, irrespectively of anode voltage....
or you "could" sell that 40W to me..... I'm in old Noreg, too...

[Roe]

No, you need to increase load impedance with high voltages like these, otherwise you'll get redplating (cf. loadlines). As an example you can use Music man iron with 700v on the plates of the el34s and a 5k5 load for a quad (marshall uses 1.7k)

PM sent on the OT

[kdmay]

[quote="Roe"][quote="kdmay"]...Looking at doing something similar with a pair of KT88s, ...why such a high primary? Still getting my head around load lines and the like.

How do you plan to drop the screens? Big resistor in series with choke, large screen resistors, or UL OT?[/quote]

Hi, I may need to use kt88s as well, esp if the screen voltage is high. the 88s can probably handle a lower primary (e.g. 4k5) but the el34s would need a higher primary at this voltage, wouldn't they? Also, I thought I could just use the twin OT I have lying around for 8k/16ohms.

I plan to drop the screens by using two different plate electrolytics in series (e.g. 220u and 100uf), paralleled with different resistors (e.g. 220k and 100k). I simply plan to connect the choke and screens to the junction of the caps and resistors - hoping it will give me 400+V[/quote]

The other options is using a VVR just for the screens. I am exploring this also.

My plan for the KT88 build is to run a JTM45/100 OT, for 4k & 100w. I have a spare Hammond 1750N lying around, but it's only 3k4 and 50w and I'm sure the KT88s won't be too happy with that at 530vdc.

[Roe]

could 807s work with 600v?

[kdmay]

[quote="Roe"]could 807s work with 600v?[/quote]

807s?

[Roe]

could 807s work with 600v?

807s?

yes old 807 tubes

[kdmay]

[quote="Roe"][quote="kdmay"][quote="Roe"]could 807s work with 600v?[/quote]

807s?[/quote]

yes old 807 tubes[/quote]

Hmmm, not sure, never had any experience with them

[M Fowler]

Here is an old weber design they no longer offer using 807 tubes.

[Prairie Dawg]

The original Standel 25L15 used 807s. No reason not to use them, and they can often be found for modest sums.