Impedance in output transformer

[überfuzz]

I have a output transformer in an amplifier, that unfortunately not here with me. I plan to cut the amplifier and rebuild it during the summer vacation. Now as I try to put together somekind of schematics I can't find information on what output transformer a Carvin vintage 33 is packing. I need to know the impedance input to center tap. If anyone can help me..

Cheers!

[Jazz P Bass]

The Carvin Vintage 33 uses a quad of EL84 output tubes.

[Joe L]

I see from the schematic that the 33 uses 4 EL84s in push-pull, fixed bias at 360vDC on the plates. I would guess that a 4K primary would be appropriate. The HV does appear to be a bit high, so watch the idle dissipation.

[überfuzz]

I see from the schematic that the 33 uses 4 EL84s in push-pull, fixed bias at 360vDC on the plates. I would guess that a 4K primary would be appropriate. The HV does appear to be a bit high, so watch the idle dissipation.

Wow, the in transformator + rectifier and pumps ca 360VDC. With 4k the loadlines will be quite high... Is this common in EL84 quads?

[Gingertube]

The schematic says set bias for 75mA across standby switch. That means 75mA average DC power supply draw. Allowing say 5mA for preamp + phase splitter duty we are left with 70mA for 4 x EL84 (call it about 17 to 18 mA per tube). Even at B+ = 360V that is only 6.5 Watts anode + screen dissipation per EL84 and that is really cold bias point. Screens are protected by 350 Ohm resistors, I might be tempted to to take them up significantly, perhaps to 2K2, particularly if you run the amp hard pushing the output stage into overdrive. Note also that the power supply B+ tap for the output tubes has 2 x 22uF = 44uF, you are going to get a bit of sag at full drive. The other thing to note is that the additional dropping resistor (R53) gets teh screen voltages down to 5V below the anodes, that seems to be important for EL84 tone (5 to 10V less than the anode should be your aim).
Cheers,
Ian

[überfuzz]

Gingertube - Thanks for dishing out!


It seems I did the simulation with 2 EL84s, I guess that's why it was funky. I redid the simulation now with 2+2 EL84s. If I am to mark the maximum dissipation for each side.. Is it just 12*2 Watts per side?

[isaac]

Yea it makes sense to have a 4k primary. I know that many 36W Marshall transformers are 4k as compared to the 18W ones with 8k to accommodate two el84s instead of 4. So that makes sense. I'm not sure of what an AC30 might have, probably similar. As most of this is based off of tube data sheets. I looked on Carvin's website and I found that out[ut transformer for sale so you can replace yours if you had to, but there are no specifications on it. Chances are it is a 4k primary though...

[überfuzz]

Yea it makes sense to have a 4k primary. I know that many 36W Marshall transformers are 4k as compared to the 18W ones with 8k to accommodate two el84s instead of 4. So that makes sense. I'm not sure of what an AC30 might have, probably similar. As most of this is based off of tube data sheets. I looked on Carvin's website and I found that out[ut transformer for sale so you can replace yours if you had to, but there are no specifications on it. Chances are it is a 4k primary though...

Yeah what's up with the secrecy..? I guess you're right about the 4k impedance.

[Enzo]

It isn't secrecy, it is just information they don't need to include. If they sell a transformer as a replacement for model XYZ amp, then it is whatever impedance model XYZ needs. If they were selling it as a general part for the open market, that would be different.