Bridging a tube stereo power

[coolhandjjl]

Hi,

In addition to hi-fi, I play bass guitar and am into separates, preamps, power amps.

I'm looking at some tube power amps for my bass guitar. There are a few tube power amps out there, Mesa Boogie Strategy 400 Stereo, Peavey Classic 120/120, a couple more. These are typically push/pull designs with either 6L6 or 6550 power tubes. Only the Peavey has a built in feature to bridge for mono output, so I know tube amps can be bridged.

So if I have a two channel power amp, 100 watts a channel, and I want to get one mono output of 200 watts going to one speaker, is that parallel bridging? How difficult or expensive would it be to design bridge circuitry for one of the others that is only available as two channel output? Is this Frankenstein stuff, or too expensive to even consider.

Is it as simple as paralleling the inputs and outputs together as was once recommended for the Dynaco ST70?

[Geek]

Paralleling channels for mono procedure:
- All tubes should be matched.
- Tie inputs together.
- For 8 ohm speakers, tie the 16 ohm outputs together and run one
8-ohm speaker off that 16 ohm position. (If you have 4 ohm speakers,
tie the 8 ohm outputs together and run one speaker channel off that 8
ohm output and if you have a 2 ohm cab [common for bass], tie the 4 ohms together and run off that).

The last step is required for proper paralleling for increased power.
Each set of output tubes sees 1/2 the anode resistance, but that's OK,
since you're paralelling them.

If people are just paralelling the 8 ohm speaker outputs and adding 8
ohm speakers to that output, the amp will see the same as if you ran
it stereo with 16 ohm speakers.... power will go down and THD will do
weired things.

To bridge, you run the inputs 180 degrees out of phase and perfectly equal (good job for a little Edcor line transformer).
- Use the same impedance selections, but don't parallel the outputs.... you place the speakers across the channels and they must float.

Tube amps parallel much nicer than they bridge, IME.

[coolhandjjl]

Thanks. The amps I'm looking at are in the 120~200wpc range. Would excessive heat and/or distortion enter into the picture using the Dynaco method?

If I decide to bridge, how do I make two input signals, one being 180 out of phase? I guess I didn't undersand the Edcor mention. Is this something I have to make?

[Geek]

If you have a dual channel oscilloscope, you can use an opamp to split phase too.

You need the scope to get **perfect** phase amplitude balance.

[coolhandjjl]

Some say parallel adds heat and distortion. It's just going be a bass guitar amp, not hi-fi, so parallel seems easier and I don't have to buy any parts.

[coolhandjjl]

I ended up hooking it up via parallel method. Sounds very robust. The background noise level is up, but seeing it is a stage amp, that's okay. I've only run it for a few minutes this way. The amp is fan cooled, so hopefully heat will not be an issue.

Thanks for all the great suggestions!