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Old 12th July 2014, 09:57 PM   #1
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Default Bridging stereo tube amp to one more powerful mono output

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 combine the channels for one mono output equal to the combined watts of each channel, 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 if I buy an amp that is only available as two channel output?

Is it as simple as paralleling the inputs and outputs together as was once recommended for the Dynaco ST70?
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Old 12th July 2014, 11:34 PM   #2
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The Dynaco method SUCKS. Most of the time, you get bad sound and lots of heat.

The fool proof method is to drive the 2 sections with 180o out of phase signals and connect the voice coil across the "hot" taps. A 16 Ω voice coil connects to 8 Ω taps and an 8 Ω voice coil connects to 4 Ω taps. Unless very unusual O/P "iron", with 2 Ω taps, is present, a 4 Ω voice coil may not be used.

What is happening is that the voltage swings of the 2 sections get summed in the load. Each section "sees" only 1/2 of the load, which is why things get wired up, as described.
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Old 13th July 2014, 12:40 AM   #3
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Thanks Eli.

I also asked on DIYTube, and a membed offered both methods, but stated the paralell method better for tubes than bridge. Oh well!

I am looking at two channel tube power amps in the 120~200wpc range if that helps. All the designs have massive output transformers, these amps weigh in from 50~75 lbs.

What is the preferred way to take my my mono input signal and make two input signals, one being 180 degrees out of phase? Put two 1:1 transformers on each input, but one has output leads reversed?

Last edited by coolhandjjl; 13th July 2014 at 12:53 AM.
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Old 13th July 2014, 01:59 AM   #4
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Don't forget that transformers have to be driven by low impedance circuitry.

Sowter's model 8920 is quite suitable.

A differential gain block is another way to go from "single ended" to "balanced". The Schmitt implementation might be a good choice, as it does not need a B- rail. RK1 provides bias. Replacing RK2 with a constant current sink (CCS) will force symmetry between the 2 triodes. Use a high gm/low RP type like the ECC99 or 6H30П (6n30p).
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Old 13th July 2014, 02:29 AM   #5
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Quote:
Originally Posted by coolhandjjl View Post
I am looking at two channel tube power amps in the 120~200wpc range if that helps. All the designs have massive output transformers, these amps weigh in from 50~75 lbs.
That's why amps in this class are usually "mono-blocks".

jeff
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Old 13th July 2014, 03:15 AM   #6
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Quote:
Originally Posted by Eli Duttman View Post
Don't forget that transformers have to be driven by low impedance circuitry.

Sowter's model 8920 is quite suitable.
Output impedence of my bass pre amp is approx 1 k ohms or greater with a 0 dB signal, often referred to as Hi Z.

The Mesa Power amp has input impedence of 75 k ohms.

Where does that put me if I just use the Sowter?

Last edited by coolhandjjl; 13th July 2014 at 03:31 AM.
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Old 13th July 2014, 06:40 AM   #7
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Quote:
Originally Posted by coolhandjjl View Post
I am looking at two channel tube power amps in the 120~200wpc range if that helps.
Why don't you just look for a good used Peavey Classic 120, seriously doubt you can build one better yourself...
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Old 13th July 2014, 09:45 AM   #8
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If you do try the parallel method, don't forget the impedances work the other way: e.g. 16 ohm output taps on the amps (in parallel) should drive an 8 ohm speaker.
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Old 13th July 2014, 07:37 PM   #9
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Yep, got it, thanks.
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Old 14th July 2014, 12:41 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

If you bridge you need a speaker double the nominal impedance.
Paralleling is not bridging and you need to add current sharing
resistors, say 0.2R to each output before commoning them.
Parallel will drive halve the nominal speaker impedance.

rgds, sreten.
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