375-0-375 for a KT66 build?

[soma_hero]

I have an Eico ST-40 Stereo power transformer thats a huge chunk of metal. I want to build a KT66 rocket variant (so many good discussions on this!) and I'm wondering if this thing will be too much voltage.

The PT is :
375-0-375 (no idea on current measurements for any windings)
2X 6.3V
5V

I know Markusv used a 325-0325, and the KT66 can take over 400V on the plates right? Could I use this if I severely dropped the B+ after tube rectification? How would I go about calculating a resistor value since I don't know the HT current? Should I just try to find a more suitable transformer?

Thanks all, can't wait to get started on this thing, got the preamp board all wired up

[Blind Lemon]

A 375-0-375 will put you up around 500V, roughly 1.4(GZ34) X 375 = 525vdc unloaded. Loaded you would be down around 500vdc. You'll have to look at the specs for the KT66 and decide. Components for the lower voltages are more readily available and cheaper. JMTC

BL

[Markusv]

Soma

Welcome to the Rocket addiction!

My voltages were as you state
I use a 5U4 rectifier though which will help you drop about 40- 50 volts I think.

So 375 x 1.4= 525

Minus 50 V = 375

Perhaps you should have a look at max screen voltages. I think those are probably the critical ones- adjust dropping resistors if needed.
The plates should take the voltage no problem

No doubt you have seen the other extensive thread going on in a similar direction

Perhaps others who are better informed than me, will comment
I built my rocket based on the very generous contributions from RJ, Zippy and others

Markus V

[ontariomaximus]

You'd be better off doing a quick mockup PS with a simulated load. My guess is you'd have around 440VDC. I don't think the ST40 had anywhere near 500VDC as the output tubes were 7591's. The caps were rated at 500vdc at the first stage and subsequent preamp caps were rated for 350vdc. It's a nice piece of iron.

[M Fowler]

soma_hero

The PT is :
375-0-375 (no idea on current measurements for any windings)
2X 6.3V
5V

The 5U4GB has a 50v drop and 5V4 has 25v drop and 5AR4 has 17. However, the 5U4GB is used in 3A applications. Which you don't have the specs on your transformer which you should get.

The KT66 maximum plate voltage ?450v. But if you have 500v to the tubes and want to drop the voltage use a reverse zener diode the AK type bolted directly to the chassis off the CT. The NTE5275AK zener is used for a 50v drop and there are many others that can be used depending on the voltage drop needed.

But why would the 375v tranny you have right now work for this amp?

Mark

[soma_hero]

Well the data sheets say in Tetrode Push-Pull the typical Plate is 450V and Screen is 415V. In Triode, typical Plate is 440V, screen is 400V.

There is good dsicussion about how this tube is actually running in the Rocket circuit, but either way the maximums are about 500V. I'll try and model the power supply soon.

[soma_hero]

But why would the 375v tranny you have right now work for this amp?

Mark

I just have it here and thought I'd make use of it if possible. I don't know if I want to go through the trouble of setting up a special voltage dropping configuration, I may just buy a new tranny. Wanted to keep the out of pocket cost lower on this than on my Liverpool.

[M Fowler]

That should have read: Why wouldn't the 375v tanny work for this amp build?

Sorry about that.

Mark

[soma_hero]

Actually I goofed up the numbers. It's 357-0-357. Just a little bit more manageable. Think I'll go for it

[M Fowler]

I would go with it and let us know how your doing all this is way too much fun.

Mark

[Markusv]

My voltages were as you state
I use a 5U4 rectifier though which will help you drop about 40- 50 volts I think.

So 375 x 1.4= 525

Minus 50 V = 375

Sorry
That should read Minus 50V = 475V

Looks like M Fowler put you on the right track though?

Markus V

[Zippy]

If/when you do go for it, adjust your dropping string to get appropriate voltages in the preamp.

[RJ Guitars]

Soma,

Let us know how that goes... I expect you will achieve a really loud beautiful clean tone... then if your ears can take it, it should eventually break up for you.

I agree with Zippy, you probably want to match up your preamp voltages to something similar to the other Rockets if you want to maintain that Rocketish character of the preamp.

Are you going to use the Rocket schematic layout similar to the Tweedeluxe, MarkUSV, or myself used?

It will be good to add a data point on the higher voltage end of the spectrum to the existing collection.

rj

[Markusv]

Hi
I still owe everyone here a voltage breakdown and will do so as soon as possible

However, perhaps worth mentioning that with my 320-0-320 tranny and 5u4the amp stays cleanish up until about 10'oclock. By then it is Very Loud! The breakup occurs after that with some perceptible volume increase

You can refer to ealier in the thread to look at the posted rough voltages

The current plate voltage is around 341V
Cathode voltage is around 26 V
I achieve about 2 volts drop acrossthe screen resistor giving me about 338V

Currently I run a 220R 25 watt cathode resistor (Aluminum screw- down type)

I have a variable bias pot in there, but it is currently not really used as it is set at minimum. I believe the tubes can perhaps be run a bit hotter, but the sound right now is already very satisfying.

BTW
Anyone aware of well priced 10" speakers? Well priced as in cheap, but not cheap sounding
I may post in the trade section because I have a Hammond 272JX tranny for trade (Brand new)

I decided on a 2x10 combo and the cabinet is under construction
Quarter sawn pine with Imbuia accents. Finished in hand rubbed Tung oil or perhaps my simple old stand-by: Boiled Linseed oil mixed with turpentine. (multiple layers give a nicely aged yellowing effect)

Markus V

[RJ Guitars]

... I still owe everyone here a voltage breakdown and will do so as soon as possible...

I am curious to see it...

BTW
Anyone aware of well priced 10" speakers? Well priced as in cheap, but not cheap sounding...
Markus V

I understand that there is now a Greenback in 10" and the reviews are all good... I've never heard that they are cheap though.

rj