Old Geloso 35W OT to be rewound for 2 x 6V6 + 2 x EL84 tubes in mixed PP

[kagliostro]

I've an old Geloso 35W 5411R transformer who originally was used for a PP of 6CA7 tubes

I would like to use it for a mixed PP with 2 x 6V6 and 2 x EL84 tubes

the PT is a 280v-0-280v AC and the PS uses a GZ34 rectifier

I would like to have a good match of the OT with all the tubes so I'll ask to rewound the

transformer with a winding that has a common CT with separated intakes for 6V6 and EL84 (something that resemble  an UL OT)

Usually a PP of 4 x EL84 uses an impedance of 4K, but which will be the better value for the 6V6 intakes ?

I can use the 4 tubes in PP with a 4k transformer, but as I've this iron to be rewound and a friend that will do the job, why not take advantage ?

I need only the right values to give him

Many Thanks

K

[HotBluePlates]

I've an old Geloso 35W 5411R transformer who originally was used for a PP of 6CA7 tubes. I would like to use it for a mixed PP with 2 x 6V6 and 2 x EL84 tubes.

...

I would like to have a good match of the OT with all the tubes so I'll ask to rewound the transformer ...

Don't rewind.

4x EL84's, 4x 6V6's and 2x EL34's all typically operate into the same load impedance, so your transformer is fine.

... I'll ask to rewound the transformer with a winding that has a common CT with separated intakes for 6V6 and EL84 (something that resemble  an UL OT) ...

Why?

Place 1x EL84 and 1x 6V6 on each side of the primary. Now you're done.

If you use cathode bias with shared bias resistors, have the EL84's share a single resistor and the 6V6's share a single resistor (because they'll need different values for correct bias with the same B+).

You may choose to apply the phase inverter signals to EL84's and 6V6's separately, because the EL84's won't need as much drive signal and will distort first. But this will look something like 2 post-phase inverter master volume controls (1 dual-pot per output tube type).

[kagliostro]

The transformer must be rewound because it is broken, I got it at a flea market, it was very cheap, and the weight in iron worth the money

I've a friend that can rewound it, so the idea to experiment something of particular

at the begin I've think to two separated windings giving the ability to have different B+ levels on each pair of tubes

then the idea to semplify and to have intakes of different impedance on the same winding to match at the best the two different tubes

4 el84 want 4K but I'm not not so confident with 6v6 and I was thinking at something in the order of 6k or 6.6k

Now I've re read the a 6v6 datasheet and seems that at 315v an 8.5k is used for two tubes, so 4K (as you told) for the mix of tubes may be OK

Class A Amplifier
Source ........................................         RCA RC-29 - 1975
Plate Voltage .................................         315 V
Grid No. 2 Voltage ............................        225 V
Grid No. 1 Voltage ............................       -13 V
Peak Grid No. 1 Voltage........................      13 V
Plate Resistance (approx) .....................      77K Ω
Transconductance ..............................      3750 µ
Plate Current (Zero Signal) ...................      34 mA
Plate Current (Maximum Signal) ................   35 mA
Grid No. 2 Current (Zero Signal) ..............     2.2 mA
Grid No. 2 Current (Maximum Signal) ...........  6 mA
Load Resistance ...............................        8.5K Ω
Power Output (approx) .........................     5.5 W
Total Harmonic Distortion .....................      12 %

about the drive of the final tubes I think to arrange the PI for the el84 6V6 value and use a resistor voltage divider to tame the signal for the grids of the 6v6 el84 tube

K

[HotBluePlates]

... arrange the PI for the el84 value and use a resistor voltage divider to tame the signal for the grids of the 6v6 tube

6V6's have a larger bias voltage, and therefore need more drive signal than EL84's.

Other than that, you are correct. You set up the amp to deliver all the signal the 6V6's need, then knock it down if desired for the EL84's.

[kagliostro]

I wrote the exact contrary of what is to do

at the moment I wrote I swapped the more sensible tube for the less sensible one (why ???)

my real intention was to push the 6v6 to their max and to use standard PI values used for this tubes

the voltage divider was intended to tame the signal to the more sensible tube, the el84 tube

o picciotto a sape a lezione è che se confonne ......

I apologize, be patient HBP

K

[PRR]

> read the a 6v6 datasheet ..an 8.5k is used for two tubes

You are looking at the SINGLE-ended data. (Clue: two 6V6 can do 10W to 14W; this data shows 5.5W.)

Two 6V6 normally run 10KCT load for lower current(*), 8KCT load for maximum power. In fix-bias with high supply voltage they can pull 6K6CT loading.

EL84 is very much the same "size" of a tube in a different bottle and higher gain (it is newer and they learned to do things a little better). 8K loading, maybe 6K or 10K.

(*)6V6 and the small 6AQ5 were often used in car radios where B+ current is precious.

For four tubes, use half the impedance.

So 3K, 4K, or 5K.

I'd just use 4K for everything.

If you insist on taps, wind 3K 4K and 5K, or any two in the same range.

[kagliostro]

As from HotBluePlates council and your clarification

Two 6V6 normally run 10KCT load for lower current(*), 8KCT load for maximum power

I'll go with 4k CT OT winding

Thanks PRR

K