One half push pull?

[xtian]

Buddy just texted me to say he was running his push pull amp with only one power tube and it sounded good. I thought this was a bad idea but couldn't say why.

Why?

[Blackburn]

How can you run an amp 'push pull' with a single power tube? Sounds a little contradictory to me.

[Reeltarded]

It works though. You can't coun't the cycles and off is never off. It works.

[Blackburn]

Really??? I had no idea. What goes on with the OT's primary impedance when you do this?

[JazzGuitarGimp]

I suspect the bigger issue here is in the DC current through the OT primary. With both tubes in the circuit, the DC current through both halves of the OT primary are flowing in opposite directions and therefore cancel each other for a (near) net of zero DC current flowing in the primary. Because of this principal, PP OT's don't need to be as large as SE OT's. if the amp is used for extended periods with just one tube, the OT will likely overheat and eventually short. I wouldn't reccommend this.

[darryl_h]

There is an additional problem if the amplifier is cathode biased with a single bias resistor common to both output valves. With one output valve removed, the remaining valve will be seriously under-biased.

[Leo_Gnardo]

There is that dusty old legend about country music guitarist Paul Burlison accidentally having one of the 6V6's in his Deluxe fall out, thereby creating distortion that later became his trademark. So the legend goes, Paul returned the errant tube to its socket at another session but the engineer or producer remarked he didn't have that "cutting" or "fuzzy" tone they used to good effect on the previous recording. So Paul just popped the tube out of its socket & after that he was the guy with the special sound.

Having a peek at some of the articles Google led me to, apparently there's some dispute about how distorted tones on some recordings involving Paul Burlison were made. I'll leave it to those interested to find & read. And you can choose to believe the legend, or not.

Granted, one tube missing would force more current thru the remaining tube but that would be limited by the cathode resistor. And yes, the output transformer might object by overheating. But if it makes hit record$ then keep a supply of OT's on hand and carry on.

Maybe this is one of the first stories about "laming" an amp for a special tone. People used to cut the cones too, for a distorted sound.

[pdf64]

Maybe xtian's buddy hasn't had any hit records recently and would rather not risk wearing tubes out etc':D'
My feeling is that this should be avoided if cathode biased (unless modded for individual cathode resistors).
If the amp is fixed bias and is biased close to class A then it may not sound too bad; when a tube has gone bad in an amp of mine (lost vacuum, effectively wasn't there) the amp sounded terrible.

With both tubes in the circuit, the DC current through both halves of the OT primary are flowing in opposite directions and therefore cancel each other for a (near) net of zero DC current flowing in the primary. Because of this principal, PP OT's don't need to be as large as SE OT's

My understanding is that it's the magnetic fields created by the static currents that cancel out (thereby avoiding magnetic saturation / allowing less iron in p-p); with current in one leg only, there would be half the heating, so I don't perceive that there would be any additional stress on the OT (in fixed bias).
A similar effect to removing a tube could be obtained by putting a switch in series with the coupling cap to one of the output tubes, or otherwise reducing/removing the signal fed to it.
Pete

[Leo_Gnardo]

A similar effect to removing a tube could be obtained by putting a switch in series with the coupling cap to one of the output tubes, or otherwise reducing/removing the signal fed to it.
Pete

FWIW Peavey has had a made-in-China amp out for a couple years (sorry can't remember the name now) that does exactly this, with a pot on the back panel so you can reduce or cut off the drive to only one of the output tubes. So you can dial in degree-of-funkiness if you look at it this way, or degree-of-lameness if you look at it the other way.

[Colossal]

First saw this PP/SE mod on 18watt.com on Mark Durham's (RIP) 18W Vajra amp. A pot is subbed for one of the grid leaks in the power section, in parallel with a PPIMV such that the total resistance is about 500k, keeping with the 1974 circuit's 470k/470k traditional values.

It's come up in discussion before...search for a post by Dartanion. I believe his comments were favorable, IIRC.

[renshen1957]

A similar effect to removing a tube could be obtained by putting a switch in series with the coupling cap to one of the output tubes, or otherwise reducing/removing the signal fed to it.
Pete

FWIW Peavey has had a made-in-China amp out for a couple years (sorry can't remember the name now) that does exactly this, with a pot on the back panel so you can reduce or cut off the drive to only one of the output tubes. So you can dial in degree-of-funkiness if you look at it this way, or degree-of-lameness if you look at it the other way.

Hi,

It sounds like you are describing the Texture Control in Peavey's Valve King.

Peavey stole the idea from London Power amps which developed and built the circuit from prior to 1990's and still build the circuit. The Ultimate Tone Vol 2 which dates from the same time described the circuit, Peavey just reversed the Pot and wired it backward.

Peavey had the gall to apply and received from the USPTO a US Patent in 2004. (Then again, the US Patent Office granted a number patents to a Guitar Amp manufacturer for Public Domain Circuits lifted from the RCA tube manual which had expired circuits in 1955, I won't mention the company by name but's its initials are Mesa Boogie.)

Its a tradition in the Musical Instrument industry to clone other companies' circuits.

I've built the "body" control of Kevin O'Connor into one amp (London Power) and paid a $5.00 royalty (even though Kevin said I was covered by a book purchase), just in case I ever chose to sell the amp. The effect throws PP between the two sides of the power amp out of balance while still maintaining the current balance through the transformer. You can dial out the one tube entirely for a single end effect.


Best regards,

Steve

[Leo_Gnardo]

A similar effect to removing a tube could be obtained by putting a switch in series with the coupling cap to one of the output tubes, or otherwise reducing/removing the signal fed to it.
Pete

FWIW Peavey has had a made-in-China amp out for a couple years (sorry can't remember the name now) that does exactly this, with a pot on the back panel so you can reduce or cut off the drive to only one of the output tubes. So you can dial in degree-of-funkiness if you look at it this way, or degree-of-lameness if you look at it the other way.

Hi,

It sounds like you are describing the Texture Control in Peavey's Valve King.

Peavey stole the idea from London Power amps which developed and built the circuit from prior to 1990's and still build the circuit. The Ultimate Tone Vol 2 which dates from the same time described the circuit, Peavey just reversed the Pot and wired it backward.

Peavey had the gall to apply and received from the USPTO a US Patent in 2004. (Then again, the US Patent Office granted a number patents to a Guitar Amp manufacturer for Public Domain Circuits lifted from the RCA tube manual which had expired circuits in 1955, I won't mention the company by name but's its initials are Mesa Boogie.)

Its a tradition in the Musical Instrument industry to clone other companies' circuits.

Best regards,

Steve

Valve King is it, thanks Steve!

And I'll keep my copy of the RCA manual around in case Randall's legal team ever gives me a hard time, I'll remind 'em of "Prior Art." Cheeky so & so... Heck that stuff was all old hat in '55!

[Structo]

I did that by accident one time with my 50w D'lite.

For some reason I had taken one 6L6 out and forgot to put it back.

Turned the amp on and it sounded different, not really bad, but different.

So I take a glance in the back and see only one power tube glowing......

I was kind of curious what kind of damage it would cause if allowed to operate like that for a while?

[pdf64]

I can't think of any reason the risk of damage would be increased by removing an output tube in a regular fixed bias amp.
Pete

[Roe]

one el34 failed in my express once. to my surprise it worked seemingly fine, although output was greatly reduced