custom built 200watt bass amp transformer

[blindboybenton]

I have a custom built 200 watt bass head using 4 x 6550. I have not got a schematic for it. and i believe its made in poland

4 x 6550 and 4 x 12AX7

The power tx is gone. Ive got my transformer make to make me a new one. He has asked me a question and i want to be sure ive got it right
He asked about the 250v-0v-250v asking what current is required. I normally do some searching on the net and look up but i cant find any info that i can cross reference. I know the voltages is correct as there are making on the tx, but nothing about current draw

Is this ok ?
250v-0v-250v - 0.25amp
heaters 10.7 amp

Many thanks

Bassman

[J M Fahey]

Stop the order momentarily.
Those specs look wrong to me.

Please draw a schematic yourself, we know nothing about your amp, whether it's ultralinear or not, whether it's dual rail with very different voltages for plates and screens, kind of biasing, tube or SS rectification, "full wave" or bridge, stacked caps or not.

Also the famous question: *where* are you?
Guess not in USA, definitely not Australia, unlikely in UK, since amp is made in Poland and homebuilt to boot, it can be (or already is) a veritable Pandora's box.

Hand draw not the full amp, here preamp is not that important, but power tubes, OT, PI, power supply, not forgetting biasing circuit.

Then recheck it before posting and add a couple pictures.
No sensible answer is possible before that, and you will easily burn an expensive transformer, an expensive amp and lots of money if you get it wrong.
Thanks.

EDIT: just one reason to be worried about your data: 250-0-250VAC if full wave rectified means a meager 350V +B , totally unsuitable for am 200W amplifier.
On the contrary, if bridge rectified means scary chilling 700V +B .

More suited for a 200W amp for sure but screens should use a midpoint rail to survive, not sure you have them, and many other doubts remain.

[R.G.]

What JM said, but more so, and from another perspective.

250-0-250 can be rectified as noted to about 350Vdc or 700Vdc. If you're going for the max, 700Vdc with a transformer current rating of 0.25A is only going to produce 175W of DC at best, and not all of that will come out of the OT, a significant amount being eaten up as heat in the tubes and supporting circuitry. It won't make 200W, even if the original did. It's worse if the 0.25A rating is rms transformer current. Bridge rectifiers cause transformer currents of 1.6 to 1.8 times the DC current. That means it would only produce about 109W of DC for the output stage to use.

A much better approach is as suggested: draw out the circuit, then look at the specifications for the 6550 tube and from the tube data pick the right plate and screen voltages for the 6550 tubes to operate at a conservative maximum power. Then live with what you get with four 6550s.

[MarkusBass]

I have a custom built 200 watt bass head using 4 x 6550. I have not got a schematic for it. and i believe its made in poland

4 x 6550 and 4 x 12AX7

Is this ok ?
250v-0v-250v - 0.25amp
heaters 10.7 amp

Can you post a photo of the amp? Does it have any name? I know few guys who manufacture tube amps in Poland. Maybe I could just ask them for the transformer specification?
It seems that bot voltages and currents that you specified are incorrect. For example, 4x6550 require 6.4A for heaters. If you add 1.2A for 4x12AX7 you get 7.6A. How did you calculate 10.7A for heaters?

Mark

[TigerAmps]

Agreed: You would need 7.6A of filament current

Finding one transformer to fit the bill might be difficult.

Option 1: a Mercury Magnetics transformer for a Marshall Major (you would still be short 0.3A of filament but it might work)
Option 2: use two more "generic transformers a Hammond 378CX (which would give your about 560V on the plates) and
a Hammond 266S6 (which can provide 6.3V at 10A). Both are setup for 120/220 primaries. Use the center tap of the 378CX's 6.3V
for the 12AX7's and the 266S6 for the 6550 filaments.

Finding one transformer to meet the exact specs will be a challenge. There may be other options as well.
Either way, it will be an expensive proposition; if you want the WATTS, your going to pay!!!!.

Then comes the question of: Have you tested the output transformer? If it's no good, then you are looking at even more money.
Output transformers of that size are NOT cheap!

[blindboybenton]

Thanks for the replies. Ive decided to send the tx to the tx maker for him to rewind or make a new one on the spec of the old one, I was just hoping i could do it without having to send it too him

Bassman

[blindboybenton]

to answer your question... Unlikely in the uk..... Im in the uk

[blindboybenton]

Hi Bassman

As you stated you are in the uk.. Check out Transformer equipment ltd in kent.. Ive used them many times and they are very helpful

BBB

[J M Fahey]

to answer your question... Unlikely in the uk..... Im in the uk

Ok, one answered, what about the others, which are more important to solve this problem?
Thanks.

a Mercury Magnetics transformer for a Marshall Major

We have no clue yet about the schematic, even less about it being a Marshall Major clone.

I can name half a dozen amps using 4xKT88 (or 6550 but which will accept both), all different.

And 250-0-250 @ 250mA was specifically mentioned , that's not a Marshall Major transformer by any means.
Think some 100V less +B and 800mA and we are talking.
But anyway it stays unconfirmed.

[TigerAmps]

Juan:

Sorry - I made a few assumptions:
1) It really was a 200Watt amp (which we are are unsure of)
2) The original poster had an understanding of amp, power supplies, output stages, bias circuits, etc.
If someone was going to attempt to work on a 200+ watt amp (Marshall Major, Fender PS300 or PS400 for that matter), they knew what they were doing.
Amps with that kind of power are not a project for the novice. They could easily kill them someone if they are not extremely careful.
3) The 250-0-250 @ 250ma was just not possible if it were a really a 200 watt amp. I think a 200 watt setup with a voltage doubler would be hard to find.

Lastly - After thinking about your post, there are way too many unknowns (especially without pictures) to make any determinations. Higher power amps like the Major
(of which I own two) have the shielded wires running from the ohm selector switch to the output transformer for a reason. In short, the higher power can cause
problems you just don't normally see in 50 or 100 watt amps. Pictures would definitely provide some valuable clues.

Perhaps it is worth fixing, but it might be more cost effective to scale down the power (after someone figures out the schematic and other details).
It's like a race car - the more power you want the more expensive it gets!