Parallel single ended output load impedance - guitar amp

[Delta Sigma]

So, halve the load impedance when a second output tube is put in parallel. If one tube is not conducting, would I calculate the output impedance based on a single output tube?

I'm building a guitar amp based on one of the AX84 projects. I decided to do a switchable parallel single ended ouput. The idea is to have the second tube switched on and off by switching the screen from DC to ground. I'm laying out components and was deciding on switching transofmer taps via switch or simply plugging into a different jack. I then realized that I'd been assuming that a second output tube would only require a different load if conducting but I really don't know for sure.

 

[abbey road d enfer]

So, halve the load impedance when a second output tube is put in parallel. If one tube is not conducting, would I calculate the output impedance based on a single output tube?

Yes. Now, what do you expect from this arrangement?
Surely you don't need to get the max power in both cases. I'd rather think you're looking for different tone. I would make sure the dual-tube arrangement gets the max power, then the single-tube would run at reduced efficiency, but I wouldn't worry. Tubes are quite tolerant.
So you must make sure the bias does not change too much between single and dual tube. Is it fixed bias or cathode bias?

 

[Delta Sigma]

It's cathode biased, EL84s.

Yes, just looking for different tone. My single ended 6V6 (Tweed Princeton) gets the most use because it's quiet.

Parallel single ended doesn't get done very often, I'm sure for multiple good reasons, but I thought it would be fun.

 

[Delta Sigma]

Just realized I posted this in the Lab instead of Drawing Board. No idea how I did that.

 

[abbey road d enfer]

It's cathode biased, EL84s.

Yes, just looking for different tone. My single ended 6V6 (Tweed Princeton) gets the most use because it's quiet.

Parallel single ended doesn't get done very often, I'm sure for multiple good reasons, but I thought it would be fun.

I don't think you can expect much changes by running two tubes in parallels, only a change of max power.
Originally, SE was chosen for cost reasons, now for the sonic signature (2nd harmonics).
Using two tubes in parallels cost more than in push-pull (bigger transformer) and outputs less power. Which do you think the mktg guy would choose? 
If you want more power for a SE, use a bigger tube. Cost of wiring.

 

[Delta Sigma]

If you want more power for a SE, use a bigger tube. Cost of wiring.

Definitely makes the most sense. I just wanted the single ended tone and the low wattage for certain uses but then the ability to kick it up and push the speaker for recording.

I should post an update - just in case anyone comes across this in a search.

Using the screens to shut off a tube didn't work. When the second tube was off, it somehow loaded the single tube, causing a weakend grainy tone. I changed to opening the cathode and it worked but the single tube vs the parallel was too far out for biasing. I then cobbled some 5W resistors together to create a big passive sink for when the second tube wasn't drawing current. It worked great and I have an oversized chassis mount resistor on order.

It was a learning experienc and a fun build. It's surprisingly loud when parallel. Comparable to my Deluxe Reverb buid. It's off to my music partner's shop to have a cabinet built then I'll tweak the amp to work get the most out of the 10" Weber I bought.

 

[john12ax7]

It seems you've found a solution.  Just something to consider is that when paralleling tubes you can have each one have their own cathode bias resistor, that might make the switching easier.

 

[Heikki]

The most useful plan for parallel single ended guitar amps I have seen is to have separate master volumes for each power tube.

 

[Delta Sigma]

It seems you've found a solution.  Just something to consider is that when paralleling tubes you can have each one have their own cathode bias resistor, that might make the switching easier.

I started with separate cathode resistors; I had a feeling I might need the flexibility. I have a roomy chassis so the only issue was the cost of the extra cap.

The most useful plan for parallel single ended guitar amps I have seen is to have separate master volumes for each power tube.

The thought occured to me but I wanted experiment with the master volume in different ways. I set it up so that the master volume at minimum was 50% (1M pot on top of 1M resistor) and it only controls the input to the first tube. I never use master volumes and I thought this would give me a usable range with one output tube then a little blend control when parallel. It turned out quite well.

There are some amps that use a blend circuit for output tubes, EL84 and 6V6 blend, etc. That would be a fun build too!