Four output 6L6 tubes two 6.6k output transformers - how to connect ?

[kagliostro]

A friend is asking about how to connect two 6.6k / 4-8-16ohm output transformers to four 6L6 tubes

he has only one 16ohm speaker cabinet

My council is to connect two tubes to each transformer and to connect the two OT secondaries in parallel

He doubt that this is feasible because of the NFB connection and I'm not able to give him a sure answer

What do you think ?

Thanks

Franco

[PRR]

In parallel, he should use the 32 Ohm taps. (Which he does not have.)

The 8 Ohm windings can be series to give a combined 16r output. But I would really rather work parallel to ensure equal voltage on both windings.

I am assuming the B+ and bias are suitable for 6.6K/pair loading. If B+ is low, like 400V, then put 16r load on parallel "16r" windings to throw ~~3.3K to each pair.

[kagliostro]

Ciao PRR

I didn't know the B+, I'll ask to my friend

---

My friend posted this schematics and asked which one to use with two 6.6K / 4-8-16ohm 50W OT he has



I suggested him to use the schematic that is on the bottom at left (this)



so he can use the 6.6K OT (2 tubes for each OT) with the secondaries connected in parallel without the need to connect a 4ohm speaker to the 8ohm speaker to halve the the reflected impedance (necessary if he connects the two primaries in parallel to the whole 4 output tubes)

like in the schematic that is on top left (this)



---

In parallel, he should use the 32 Ohm taps.

Excuse me, but I'm not able to follow you, can you explain me ?

I was considering that if I parallel the primaries and the secondaries, the turn ratio is always the same, so there is no change in impedance and to be able to use two 6.6K with 4 6L6 connected I must connect a 4ohm speaker to the 8ohm intake as to halve the reflected impedance to the tubes (3.3K for 4 tubes PP)

I think to a pair of OT with primaries in parallel and secondaries in parallel as a single bigger OT, someting like an OT wired with paralleled double wires, turn ratio is the same between primary and secondary, current abilities are doubled because of the doubled section of the windings and the doubled core section

---

I was thinking also that using a switch to parallel the secondaries it will be possible to use two distinct speaker cabinets (that can aford each 1/2 of the total power of the amp)

But I've a problem in my mind .... the NFB .... can it be connected only to one of the secondaries as to avoid to have them in parallel ?

I know ... a lot of questions, I apologize, but I and my friend we are not able to give an answer to this thing

Thanks

Franco

[Ritchie200]

Franco,


This is an interesting topic.  I think the early 100 watt Marshall prototypes used two 50 watt output transformers - obviously they didn't sell it like that. 


I have to ask...  Could he just sell those two transformers and buy one properly suited for this application?  More space on the chassis.


Jim

[Ed_Chambley]

Does his speaker cabinet have more than 1 speaker?  I actually really like a build I made with 2 Output Transformers and 4 output tubes.  Both 10K primaries hefty transformers.  I have it in a 2, 10" cabinet running 2 8 ohm speakers.  It has the ability to run 2 EL84's or 2 6x6 one speaker at a time.  You can also run both.  I am using 2 10" Celestion golds.


Neat thing about it is playing low volumes with all 4 tubes and both speakers in play I have a very nice Fendery clean, but turn it up and the EL84's distort fairly quickly.  I am very happy with the results and have considered doing this with 6L6 and EL34's.


I thought I would mention this as you friend may want to consider this option if he has more than one speaker in his cabinet.