Creative Bridging of Single Ended Tube Amplifiers

Author: Dmitry Nizhegorodov ( My other projects and articles


1.   In brief

With little imagination, a stereo Single Ended Tube amplifier can be bridged as a Parallel SE (aka PSE), push-pull (aka PP), and cascaded single ended with separate gain control and either odd or even harmonic flavor. The latter can be a fruitful tone generator for home/studio electric guitar studio and an idea for an electric guitar amp with interesting tone controls and colorful distortion. Low-watt amps are especially suitable because cranked up tube tine can be obtained at low volumes.

2.   PSE and PP from a Stereo SET

Consider a simple 2-stage No-feedback stereo SET amplifier similar to Decware Zen [1], or amps described in [2] or [3] on my site. With a few wires, connectors and very small number of passive parts, it can be PSE or PP


below we explain the options in some details

3.   Stereo SE or mono PSE

bridging in mono PSE mode can be done with parallel or sequential connection of OPTs. Regardless of which is used, it is imperative to make sure both channels deliver exactly the same output signal. One "creative" way of doing that is by bridge-balancing them. details/options TBD.

4.   Mono PP

The drawing describes the main idea. It has many variations. Among EL84/6BQ5/6p14p based SETs, most No-nfb audio amps run in triode mode, most guitar amps and NFB home amps run in pentode mode. Same can be said of 6gv8/6f5p amps, [2]. It is very easy to provide a triode/pentode switch thus elevating versatility of your amp. Another useful mod is a driver stage cathode tap, see the drawing. With these, a PP amp can be done either as a triode-mode No-NFB, or pentode mode NFB. The latter will deliver lesser odd harmonics. The amount of NFB and the way to what degree the PP amp will be symmetrical is easily adjustable by the volume pots. Unlike traditional PP amps, where the output stages share a single OPT, here we use two separate SE OPTs and hence there is no need to balance the halves. With freedom to mix-in any amount of inverted signal, many interesting flavors can be obtained.

5.   PP/PSE based on Mullard PP topology.

The driver arrangement in the so called "Mullard" 5-tube PP amplifier topology [5] allows to a very interesting, in my opinion, possibility to have a PP or PSE amp with a switch. Feedback, bypassing etc can be switched separately or not switched at all. The schematic uses 2 SE output transformers [6].

6.   Ideas for an electric guitar player

A stereo SET amp can be used to deliver a range of interesting tones for an electric guitar player. Low-watt amps are especially suitable because cranked up tube tone can be obtained at low volumes, see [4].

The main idea is to pass the signal from a guitar (preamplified to the levels of 50..500mV) to the left channel, then take the signal off OPT and pass to the right channel, attenuated. This loop offers some interesting tone control possibilities. The most interesting one is phase flipping. To do that you must use a SE amps that has floating OPT output taps.



The load of the left channel is 8ohm dummy load. It can be a simple resistor or a complex dummy load with inductive component. Even more interesting is to use a driver, and perform acoustical mix/match of the signals from the first and second stagers. The drivers can either in-phase and out-of-phase, which combined with the right channel phase flip and 3 volume controls offers endless variations of tone.

; here is actual configuration I used. The gain box is Daphon 9V micro amp.


6.1   Discussion, Further variations

More advanced configurations involve tube biasing adjustments.

Grid stopper resistors should be rather large.

7.   References


[2] 6f5p-set.htm

[3] 6p14p-set.sxml


Author: Dmitry Nizhegorodov ( My other projects and articles