What Exactly Happens When You Pull Two Output Tubes On the Same Side In A Plexi ?

[Dai H.]

Asking based on two things I've experienced.. first was a guy who went by VHoholic on the old MetroAmp forum who used to run a Marshall 1959 circuit with only one 6CA7 output tube installed, and more recently Jim Gaustad has said he runs his custom-built EVH spec amp with V4 &V5 pulled.

I've always pulled either the two inner or out tubes to cut power down to 50W, but always been told never to pull tubes from the same side.

These guys claim this works with no harm to the amp - running them this way for decades.

hey, thanks for that!

(I saw the Gaustad stuff also, and) I was just thinking, "hmm... wasn't there someone doing this? Vintage Amps (Plexi Palace) forum(?) and some ensuing discussion?" Something I very vaguely remember (might be totally mis-remembering) some advice about keeping another tube in so there isn't unbalanced current in the OT.

Luckily his website has been archived in the wayback machine (archive.org) :

https://web.archive.org/web/20080430204 ... holic.com/

FWIW, Kevin O'Connor's view is that it's not a problem:

https://londonpower.com/tube-amps-faq/#Tube-Pulling

Also, Gaustad's view is that the AC voltage (using Variac) was reduced to something like 60VAC (he notes how the guitar isn't overpowering the drums and bass in Sunset Sound room mics recording and how the pilot light is off (pilot light barely going off is supposedly around 90VAC) when he sat in with the Letterman band), so seems like the lower voltage (Dave Friedman quoted 386V@90VAC--so even lower than that) would make it safer. Vaguely remembering quotes also about how EdVH wanted a balanced sound, and also Doug Messenger (he visited and was invited to try the amp) remarking that it was the "quietest Marshall" he ever heard (something like that) seem to fit.

Additionally, I also remembered some guitarist also doing this, which was apparently Paul Burlison.

[martin manning]

...in a Superlead circuit V4 and V5 draw double the current as V6 and V7 when the power amp is driven into full distortion.

Have you measured this? I don't see how it can happen.

[Purgasound]

Have you measured this? I don't see how it can happen.

Yes, this has been measured and observed. This is not exclusive to Superlead circuits.

Observe your current meter when biasing with probes connected to each output tube and you can also see this on your oscilloscope. When driven into distortion one side of the waveform is clipping irrespective to the other. This asymmetrical distortion is reflected by the imbalance in current draw. There is less energy required to produce a clean sine wave so if one side is squared off and closer to a DC constant, that side is going to draw more current. The meters will show this in real time.

This can be "corrected" to some degree by adjusting plate resistors the phase inverter but asymmetrical clipping occurs by design and is part of what gives those amps their character when driven hard.

Some power amplifiers like the Marshall Monobloc series have variable resistors in series with one of the PI plate resistors to balance them out. When set according to the manufacturer's specs those will output with even current draw across both halves of the output.

Cheers

Below is a picture I sent to a customer when measuring the output of a JCM900 SL-X 2100. Same behavior occurs when the power amp is clipping. The two meters show the current draw for one tube on each half of the output.
This one popped a brand new tube so the new ones were stress tested a bit more before being sent out the door.

20250519_192345(1).jpg

[Helmholtz]

Interesting.
So the culprit is the LTPI.
Your scope pic clearly shows a shifted duty cycle, so one tube conducts longer than the other.

[Roe]

how about 4 el34s with a very unmatched driver that redplates v4-v5, but not v6-v7 (a common scenario)

I'm not sure what you're getting at.

Yes, in a Superlead circuit V4 and V5 draw double the current as V6 and V7 when the power amp is driven into full distortion. The red plating is a sign something needs adjusting. If the amp is going to be driven that hard it's a good idea to increase the screen grid resistors to 2.2K and increase negative voltage for the bias. That should take care of any redplating. Cement wirewound for those resistors too. Those old shiny1K green ones and the black metal oxide ones can start to crack open when they get super hot.

the 2k2s change the attack too much. It is better to add resistance between the mains and screens filtering. An balancing the driver helps. you get a faster response with tighter bass then

[Dai H.]

(sorry, not super hard core, but bit of a VH nerd here) found this (from 15 years ago (in 2010)--seems a bit more possible evidence that pulling (one or possibly two) tubes was actually done) :

Bill New
15 years ago
I saw Van Halen play in front of the book store at Glendale College around 1975. After their set, I was checking out their "flash pots". Eddie walks up to me and gave me a tour of his setup. He showed me his one 100 watt Marshall and told me he pulled one of the power tubes out to give it more distortion. I asked if Anthony's SVTs would drown it out. He said no. He had a BLOND FENDER BANDMASTER amp for his backup then.

source:

https://www.vhnd.com/2010/07/30/the-fin ... vhi-setup/

https://www.reddit.com/r/vanhalen/comme ... community/

edit (adding this):

(luthier) Karl Sandoval's recollection (he can't recall the exact date, but says, "mid 70s maybe towards the latter 70s") here is (perhaps) an eyewitness account (without really understanding what was being done) of the tube pulling (I saw this before and assumed this was just with four but with the current info, possibly it wasn't). (From what I understand (Dave Friedman recollection on Youtube video), he was blowing up a lot of tubes because he wasn't using matched sets, but after Jose A. got him to use matched sets, the situation improved. Also, he apparently tweaked the bias to the hottest position, apparently having some tendency or belief that he could get more out of things by pushing the limits.) :

https://www.youtube.com/watch?v=piY_9RM4R88&t=532s

edit:

someone posted the actual quote (from Ted Templeman) in BAM (Bay Area Music) mag (Oct. 9, 1981) over on metroamp bbs :

“Van Halen wanted me to produce their albums because they liked the way Ronnie Montrose sounded on the two albums I did with him. I know how to pull the tubes out of an old Marshall amp and make it hurt, you know? To make guitars sound like chainsaws.”
(Ted Templeman: Super Producer David Gans, BAM, 1982)

https://www.ebay.com/itm/274289274955

https://i.ebayimg.com/images/g/FaMAAOSw ... l1600.webp

https://forum.metropoulos.net/viewtopic ... 49&t=48271

(bit confused since it sounds like Ted being the source of the tweak. So people perhaps came up with this independently to deal with volume issues or experimenting? I guess for the recording (push-pull disabling) two being pulled makes more sense in fitting Doug Messenger's description of an unusually quiet sounding Marshall since a 50W isn't significantly quieter.)

okay, so somebody already posted Templeman's quote on a Rig-Talk thread back in 2016 (also a link to the article by the original author--unfortunately unavailable even on archive.org), but apparently no one quite connected all the dots ('til now) :

https://www.rig-talk.com/forum/threads/ ... st-1977364