Parallel Secondaries

[Xander8280]

What are the ground rules fore say?

I understand multiple cabs is a good idea but, if someone was to build a 2203/2204 with twin power amps with parallel secondaries is there damage that would be caused? It's not bridging power amps but it's weird. The goal would be blending the two power amps to taste on one mono signal. Mixing a quad of valves would be easy this would allow blending night and day....

5E3 power amp(paraphase->6V6->OT)
-5Y3 saggy supply, cathode biased, maybe 6L6 triode mode upgrade.

2203/2204 power amp(long tail->tetrode->OT)
-ss rect rigid supply, fixed bias

With parallel secondaries on a common load.


My main question concern...
Would the 50-100W amplifier cook the smaller 5E3 OT?
What are the "rules" that are the norm for mixing transformers?

When lectures are back in session I plan to ask some of my professors about transformer mixing circuits. You guys here are the tube wizards though.
I have Merlin and O'Connor's books, if you want to reference anything for me to double check in there I can.


Thanks!

[Malcolm Irving]

The problem is that transformers are a ‘two way street’. The output of amp A would force voltage and current back onto the output tubes of amp B.

Might just about be OK with two identical amps, with the speaker impedance halved for proper matching.

With one amp being more powerful, I could imagine that ‘blowing’ the output tubes of the weaker amp.

Overall, I wouldn’t like to try it myself.

[Xander8280]

The problem is that transformers are a ‘two way street’. The output of amp A would force voltage and current back onto the output tubes of amp B.

Might just about be OK with two identical amps, with the speaker impedance halved for proper matching.

With one amp being more powerful, I could imagine that ‘blowing’ the output tubes of the weaker amp.

Overall, I wouldn’t like to try it myself.

Yes, that was my main worry. I guess a single 2203 style OT for a 2204 shared with a 5881/6L6 triode pair would be the realistic alternative for the "2204/5E3" power amp. Impedances would be decent and the tweed/brit blend copped.

[Leo_Gnardo]

Would the 50-100W amplifier cook the smaller 5E3 OT?
What are the "rules" that are the norm for mixing transformers?

Cook? Well something like that.

I've seen some stereo tube amps where paralleling output taps is allowed, very closely matched amps are necessary to keep the game afloat. McIntosh 240 for instance, can be run as a mono 80W amp this way, likely other old Macs too. There's also the mighty Bogen MO200A, a behemoth of a PA amp, which is essentially made of two MO100's on the same chassis, with common input & feedback circuits, and paralleled output transformer secondaries.

In the event of the power amps going out of match with the other, I'm sure the results aren't pretty. Note this sort of thing is definitely verboten with solid state amps.

In any case, I'd avoid paralleling the outputs of amps in all cases. Parallel inputs all you want. Run separate speaker cables & connectors if necessary. Parallel outputs is kind of a tightrope walk, OK when things are going right but just one misstep and splat.

[pdf64]

I remember reading that the early Marshall 100 watters have 2 x 50 watt OTs wired in parallel.

[Stevem]

A far better thing is to wire for instance a 4 -12 cab so one amp is powering two of the drivers only.
Just note that the jack plate on the cab needs to be plastic so the grounds of both amps are not landed together.

[Leo_Gnardo]

I remember reading that the early Marshall 100 watters have 2 x 50 watt OTs wired in parallel.

RadioSpares apparently didn't have OT's bigger than 50W so Ken Bran cobbled 'em that way. Funny t'ing IIRC a couple years ago there was a reissue version done the same way, y'know for "authenticity's sake."

[David Root]

From what I've gathered, both here and at the EL34 forum, yes it can be done but it's safer and also better sounding to split the two output tube pairs to their own OTs and then feed each OT to its own 1x12 cabinet.

I'm hatching something along those lines because I have a pair of old NOS Stancor A-3852s and a pair of '61 Rola alnico plug speakers, I'm thinking maybe with two pairs of 6K6GTs for 20W or so.

So in that event what is the best way to prevent trouble from mixing OT grounds, and also from frying an OT by not having a speaker connected?