PP amp has become SE?

[Dougie]

Hello,
I'm a newbie rank tube wannabe. I built a Real McTube FX box as a first project...success! Then hit my local auction house for some practise material. Came home with a Bogen M330A PA amp and a Stromberg-Carlson 209 Amp with a Stromberg-Carlson 207-M Power Supply, 6VDC to 400VDC tube inverter, more on this later, all for $20.00 Canadian that is.

I got the M330A schematic from a post on this forum by Zeke (thank you Zeke). It uses two 7868 output tubes and a push pull transformer.
I pulled out the tubes, plugged it in and tested the HV power supply. All was in spec. Being impatient I put the tubes back in, hooked up a speaker and fired it up. One of the output tubes was getting hot. Shut it down and checked further. The fault lies here;
On the primary of the output transformer (reading resistance from the centre tap) I found that one side of the primary is an open circuit. The other side reads 250ohms. After resoldering any suspect wires pulling the output tubes, checking again. I confirmed this.
I ran the amp with the tube on the "bad" side of the output tranny removed and it works, lots of volume, no distortion. Swapped the other tube into the "good" side and it still works.
So here are my questions. If this has become a Single Ended Amp why does it still work with the phase-inverter in place? Will I destroy it running it this way? Could I mod it to be SE?

Thanks in Advance
Dougie

[analog_sa]

So here are my questions. If this has become a Single Ended Amp why does it still work with the phase-inverter in place? Will I destroy it running it this way? Could I mod it to be SE?

As only one tube works at output the phase-inverter is just an extra gain stage. Running it like this will not harm it in any way. You should still check that the anode voltage does not rise too high with only a single tube loading the PS.

Push-pull output transformers may or may not like unbalanced current through the primary. I have used a vintage WE PP (all E on one side, all I on the other - that's how they stacked it) with up to 65mA in SE mode with no ill effects. If there is no bass or it distorts than the current is too high.

[ray_moth]

An OPT designed for PP expects a balance between the currents in the two halves of the primary, effectively canceling the DC. The OPT probably has no air-gap in the core, which would allow it to tolerate DC, and so it won't be very happy with only one half of the primary passing current. This will cause magnetic saturation of the core, which will lower its inductance. From your description, apparently this works but I wouldn't expect there to be much power and bass would be lacking. I once had a similar situation, when one of my OP tubes stopped conducting. There was still sound from the speaker but at a low level and it sounded anaemic, like a table-top radio from the 1950s.

As to whether it will cause damage to run it this way, that will depend on the circuit. Tube amps can be very forgiving but it's better to be safe than sorry. Looks like you need to replace the OPT but you must also try to figure out how the primary came to be damaged in the first place. Maybe the tube using the blown winding was drawing excessive current, possibly through a bias problem?

[CLS]

I have a pair of Altec 1568A, 6CG7 drive 6CA7 PP.

One of the 6CA7 died, so that mono block became SE. Funny enough, I didn't even know exactly when it began working that way. It sounded normally. (Maybe it's also because I seldem run it hard. At that time they drove the high effecient mid & hi.... )

The OPT was cool as usual, not much warmer (they have stickers of Peerless on them).

Before that, I had even raised their bias current somewhat. Very forgiving indeed.

[SY]

You've got a dead output transformer- why worry about "damaging" anything in the circuit?

[PRR]

> If this has become a Single Ended Amp why does it still work with the phase-inverter in place?

Phase-splitter just makes two drive signals. It is fine to use just one, though it is a waste of a tube.

All push-pull amps will "run" single-ended with one tube. Maximum power will be typically 1/4 of the 2-tube p-p condition: half because you only have half the total tubes, and half again because p-p amps usually swing tubes very hard, relying on push-pull action to mask the distortion as each tube hits cut-off.

> Will I destroy it running it this way?

If you run one tube on a cathode-bias resistor computed for two tubes, that one tube will cook. Double the value of the cathode resistor.

> Could I mod it to be SE?

Obviously you did. No changes needed except re-biasing for one tube instead of two.

BUT: the output transformer design is based on push-pull, with DC cancelled. Now that you can not cancel DC, it is working "soaked" in DC current. Inductance is low, bass response is poor. You could do a little better by prying all the laminations out, swapping them so they butt together without overlap. Even that is far from optimum: a proper SE transformer for one tube of a 2-tube push-pull amp would have a bigger core and more turns, with a painstakingly computed air gap.

The easy answer is to get a good transformer. Either p-p like before, or (if it will fit) an SE transformer selected to use both output tubes in parallel.

[Dougie]

Gentlemen,
I thank you very much. I will try this both ways re-bias/SE tranny and replace the PP tranny. Should be a learning/listening experiance and may even stop my head reeling from all this "virtual paper theory". Just what I needed a reason to finally get out the wire cutters and soldering iron.

much appreciated

Dougie