50 Watt Amp From 100 Watt Transformers?

[wattsup]

I know this has probably been asked before but can you build a 50 watt amp from 100 watt transformers? Would it hurt the transformers or would they be ok?

[DonMoose]

The OT will handle to power just fine - the one thing there that you've doubled the impedance the primary faces (half the tubes), so you need to use the next step lower on the speaker side (plug an 8-ohm load into the 4-ohm tap).

The PT will also be fine, but your voltages may be a bit high as they're less loaded. Expect a bit of fiddling in the dropping string, and make sure the first couple of stages of filter caps can handle the extra voltage.

When you go to bias the thing, measure tube CURRENT (by way of voltage across a 1-ohm Rk), and start with the bias at the most negative voltage you can get (coldest).

Hope this helps!

[Reeltarded]

Moose!!!

[matt h]

(deleted)

[wattsup]

The PT will also be fine, but your voltages may be a bit high as they're less loaded. Expect a bit of fiddling in the dropping string, and make sure the first couple of stages of filter caps can handle the extra voltage

I'm guessing here... So the difference between switching two tubes on a half watt switch to make 50 watts and simply not having the tubes there is just the load on the transformer?

[Stevem]

Yes, that's the deal!
The only other issue is how much higher the filament voltage will be with 2 output tubes out as it could get high enough to shorten tube life and or make the preamp tubes sound brittle on the top end!
If it where me I would leave in or get 2 weak output tubes and disconnect there plate wire from the OT at the socket and heat shrink those 2wires off, in this way the filaments are happy and you have the load your looking for on the OT!

[DonMoose]

Yes, that's the deal!
The only other issue is how much higher the filament voltage will be with 2 output tubes out as it could get high enough to shorten tube life and or make the preamp tubes sound brittle on the top end!
If it where me I would leave in or get 2 weak output tubes and disconnect there plate wire from the OT at the socket and heat shrink those 2wires off, in this way the filaments are happy and you have the load your looking for on the OT!

I'd think about a couple of resistors to simulate the missing heater load - maybe thermally bonded to chassis. R = 6.3V/Ih; power rating at least 2*(6.3*Ih).

[xtian]

I recently had to do this. I put 0.5R resistance in series with EACH leg of the heater winding, and got exactly 1vac reduction (from 7.3 to 6.3vac). This was in my Eico 377 build...only two tubes in that thing.

[Gaz]

Using bigger iron with the wrong primary impedance and switching taps to compensate will reduce bandwidth. I'd love to tell you it's not noticeable, but it is!

[Reeltarded]

It sounds different but what doesn't?



Big PT but change the OT. There.

[Gaz]

[quote="Reeltarded"]It sounds different but what doesn't?



Big PT but change the OT. There.[/]

Yeah, sometime you just think bigger OT = bigger sound, when it's actually reducing bandwidth. I also don't like overkill PTS either unless the High voltage is conservative, otherwise the delicate PT sag/tube self-destruct balance can be upset.

[M Fowler]

Makes one think especially when Trainwreck transformers are 300mA 70w rating for low plate voltage and high z OT similar to early Marshall 50w amps that had low plate voltage and high z OT but mA was probably 150 to 200mA.

You would be able to keep the amp clean without any squash using 100w PT/OT and watch OT impedance.

[Reeltarded]

Yeah good for balls out clean.. if that is the kind of clean you are looking for. Strident.

[Gaz]

I should've asked, why not just build a 100W? What kind of amp are you planning?

[norburybrook]

I should've asked, why not just build a 100W? What kind of amp are you planning?

yep a 100w amp is only 3db louder than a 50w.

If it's a master volume amp then it would make very little difference , just give you a little more weight at the same volume level I'd say.


Marcus