5F2A / 5F1 build w/ old OT and speakers, need advice

[boroman]

I plan to make myself an amp from parts of a 1950s tube radio that is incomplete and not working but still have its original small speaker and output transformer. So, the question is, can I build (from scratch) 5F2A circuit (let's say 5F1 with tone control), get a 5F1 power transformer and use just that OT and speaker from the radio? This would be my first built, want to try something from scratch, and just wondering - if OT is not blown, and usually it corresponds well with the speaker = that I'd need just to PT and other parts.

Forget sound quality, it would be an amazing test. Just need to know if the 5F1 PT will hook up good to those random OTs

[Chuck H]

Need more info. If the chassis is still intact and there are any markings can you determine the radios original tube compliment? And what does "small speaker" mean exactly? 4"? 6"?

The power transformer you propose could probably be wired somehow to work just fine regardless since it doesn't technically "hook up to" the OT. It hooks up to the rectifier and HV rail and THAT hooks up to the OT. Probably more PT than you need. But the voltage and current specs for the PT in and of itself isn't the issue...

The issue is whether or not the OT and speaker can handle the WATTAGE produced by a class A 6V6 tube AT the voltage supplied by a Champ power transformer. If it's a table radio, so, really small even compared to a Champ, it may have had something like a 6ak6 running relatively low voltage only putting out a watt or two. If the OT and speaker correspond to this without adequate over spec then a 6V6 putting out four or five watts might blow things up. A PT with a lower secondary voltage might be more suitable in that case.

[boroman]

The issue is whether or not the OT and speaker can handle the WATTAGE produced by a class A 6V6 tube AT the voltage supplied by a Champ power transformer. If it's a table radio, so, really small even compared to a Champ, it may have had something like a 6ak6 running relatively low voltage only putting out a watt or two. If the OT and speaker correspond to this without adequate over spec then a 6V6 putting out four or five watts might blow things up. A PT with a lower secondary voltage might be more suitable in that case.

Thank you. Yes I am a bit aware of that... Thought about it of course.
These is antique bang olufsen radiio 1940-1950 era. pretty large stuff, like 60cm width. I found schematic below, is it possible to tell +/- how many watts it could handle?
They all from that era were pretty same sizes and around same wattage...

[Helmholtz]

In typical circuits the UL41 produced 4W into a 3k load.

[Chuck H]

Looks to be an unusual OT (for guitar amp considerations anyway) with a tapped primary and dual secondary. To be honest I'm having some trouble interpreting aspects of the drawing. The UL41 seems to be a relatively low voltage tube so since the OT looks to be purpose made for the unit I might be concerned about running Champ PT voltages through it on the off chance that the magnet wire insulation isn't up to it. That seems doubtful, but it's a consideration. Otherwise we can guess from the UL41 data that the OT has a primary impedance of 3-4k when used with whatever the intended load is. Which at face value is ok for Champ. But what about the size of the OT, magnet wire gauge, etc. I'd say if the OT looks more burly than a typical Champ OT (wouldn't take much) then I might risk it if I were also willing to lose the OT with a shrug.

As to the speaker, the schematic tells me nothing I can decipher. So nothing I can say about it's suitability.

[Helmholtz]

Don't know the impedance of the original Fender OT.

But the 6V6 datasheet suggests a primary impedance of 8.5k.

[stokes]

Dont remember where I read it, but I think Fender ran the 6V6 at 6.5k into the 4ohm load.

[g1]

Dont remember where I read it, but I think Fender ran the 6V6 at 6.5k into the 4ohm load.

Thereabouts I think. Replacement OT's for tweed Champ range from mojotone 5K to TAD 8.2K.
Hammond 1750C offers choice of 5K and 8K primary taps.

[Helmholtz]

Hammond 1750C offers choice of 5K and 8K primary taps.

Yes, I saw.
But they specify a speaker impedance of 3.2 Ohm.
So with a 4 Ohm speaker the 5k primary actually becomes 6.25k.

Generally the higher B+, the larger the optimum load impedance.

[loudthud]

The radio seems to be line powered with just some diodes in series at the upper right of the schematic (selenium ?). What is the intended mains Voltage ?

[g1]

Yes, I saw.
But they specify a speaker impedance of 3.2 Ohm.
So with a 4 Ohm speaker the 5k primary actually becomes 6.25k.

I have no idea what the impedance of the speaker in this radio is. I was going to suggest he use a speaker double the impedance of the radio speaker but I think he wants to use the original radio speaker so that would not be an option.
If the existing OT primary is around 3K like you implied, then doubling the load impedance would have reflected back around 6K primary, which would have been fairly suitable for a 6V6.

[Chuck H]

I was also going to suggest doubling the speaker impedance to match the pri Z of the existing OT. Except that it may depend on whether the OT in question is even up to that kind of voltage and wattage.?. In which case, if a lower B+ is used (for a lower output power) then the stock 3-4k primary Z may be more efficient and appropriate. It all depends.

[boroman]

Thanks guys, so I think I have few ways to build it:

1. Use Champ-specs OT with exisiting speaker; are there any exact specs I'm looking for? I may find something local (EU), cheaper than Mojotones etc.
2. Use other low wattage build like 1-3W with, like above, dedicated OT's...; are there any fine sounding & simple builds I can look for?

[g1]

1. Use Champ-specs OT with exisiting speaker;

What is the impedance of the speaker that came with the radio?

[Chuck H]

It sounds like the only things you'll be salvaging from the old radio then would be the speaker and maybe the tube sockets? Unless you're planning to use the radio console cabinet.?.

I'd be inclined to use a low voltage PT and a 6V6 or 6K6 and the original OT. Then the speaker, cabinet and OT should be right at home. If it only puts out a watt and a half that's still plenty for playing at home. It was never going to be a gigging amp anyway and nor would any Champ be even at four watts.