REALLY high voltage power transformer for tube amp

[Matthews Guitars]

A while ago I salvaged the transformers out of an already gutted EF Johnson Viking II HF radio transmitter. I have the full complement of transformers and chokes out of this monster.

As per the manual, the LOADED high voltage supply delivers between 620 and 660 volts. The transformer specs for the HV transformer indicate that the unloaded secondary voltages are 865 volts center tapped. (865-0-865)

That's a bit scary. This, coming from someone who's not very nervous working in a Fender 400 PS that nears 700 volts plate supply.

Has anybody ever built a guitar or bass amp using KT-120 or KT-120 tubes, or wants to, and needs a transformer like this?

The manual for this transmitter is found here:

https://www.google.com/url?sa=t&rct=j&q ... BNrFZ0f_Er

The transformer data is found on page 35.

If anybody wants ANY of these transformers or chokes, I'd be more than happy to donate them to you in exchange for two things:

1: Show me what you build with them!
2: Pay shipping costs.

I reserve the right to hold back any of these transformers for my own usage. The one with 298V secondaries looks interesting.

[Stevem]

I have a big ass high voltage transformer from a 70 volt pa amp that used two 5U4s and KT88 that I do plan on building a bass amp with.
The great thing about using a higher voltage trans then you need is how low you can get the 120 hz noise floor/ ac ripple down to zip!

You can drop the voltage by having two resistor and filter nodes even before you feed the output tube plates.

This way of building a power supply was done a lot in medical equipment where even smaller voltages then the weakest pickups you could imagine needed to be amplified with noise floors that where amazing.

I would have to go look , but I have at least one book where the amp uses stacked power supply and 1200 volts, I think it deals with British valves.

Just plan on the added heat from having heat sinked resistors on the chassis, or at least that would be the type I would use.

[Phil_S]

I'm a huge fan of recycling good iron.

No clear idea on what to do with 865-0-865. KT120 bass amp might work. I'm thinking it weighs in at around 8 pounds? It's only running 2x 6146, so maybe not so beefy and there is no filament supply.

Agreed, more interesting is the 298-0-298, which also has filament supplies. How to get a clear determination of mA capacity of the various secondaries?

Based on tube compliment, if I understand the block diagram, there are 3x 6AU6 and 1x 6AQ5, which makes for 1.35A @ 6.3V. Since there is no filament supply on the 865-0-865, I suppose there is another 2.5A for the pair of 6146's. Assuming it is a little bit over built, we have maybe 4A for filaments, so that doesn't look like a limiting factor.

For plate and screen supply, I think I'd figure 15mA for each of the 3 6AU6's and another 50mA for the 6AQ5. 95mA is enough for a low power amp and you might squeeze out a bit more. Even it if sags a bit, there may be some charm to it. Your typical PP 6V6 with two or three 12AX7's might be viable and I'd be inclined to give it a try. Remember, in a PP circuit, both power tubes aren't conducting at the same time. The B+ will be right.

If there is concern about not enough current, look at a pair of 6BM8's, which would ease the load a bit, or a pair of EL95's, which would be well within what this PT has to offer. In the past, I bought some very nice EL95's on eBay several years ago, however, looking today, while prices have increased considerably, they are still within reach of the ordinary person's budget.

There is more than enough to supply a 5V rectifier of your choice, particularly if you parallel the two 5V windings.

It would be nice to know if either of the larger chokes is beefy enough to use for a choke input circuit. The schematic shows them in C-L-C arrangement, which is still a good thing to have.

If I were in a building mood, I'd offer to take the 298-0-298 off your hands. But the build bug went to sleep on me a few years ago, so, I'm going to pass. I hope someone will run with this.

[Matthews Guitars]

One circuit I plan to build anyway is a noise figure test rig for 12AX7s and other similar preamp tubes. The circuit calls for a 300 volt supply.

I could make it serve double duty as a preamp circuit idea exploration rig.


I want to try a preamp topology that runs like this: Input, gain stage, cathode follower, gain stage, cold clipper, cathode follower, phase inverter, output to power stage.

With variable potentiometers placed between almost every stage that allow me to bleed off as much or as lttle signal as is needed.

This would be just to see what it does. No preconceived notions of what I'll end up with.

That transformer might be what I need.

Right now I'm not in any position to get started. I have to move my shop to a new location that hasn't been determined yet, so until that's done I'm afraid I'm having to put stuff in storage.

[sluckey]

It would be nice to know if either of the larger chokes is beefy enough to use for a choke input circuit. The schematic shows them in C-L-C arrangement, which is still a good thing to have.

Look at the schematic. The high voltage PT is feeding parallel 5R4s into a choke input filter.

[Phil_S]

Well, there are so many coils on that schematic, I missed it! Choke input and double tube recto likely explains how they tamed 865-0-865. Still, you'd need to add a filament tranny if that was your PT.

[sluckey]

Well, there are so many coils on that schematic, I missed it! Choke input and double tube recto likely explains how they tamed 865-0-865. Still, you'd need to add a filament tranny if that was your PT.

Yes, that's handled by T2. It was very common to see a separate PT dedicated to supplying high voltage to the final RF amp and modulator in high power RF transmitters. And another PT to handle the lower B+ and filaments for everything else.