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Luckily his website has been archived in the wayback machine (archive.org) :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

edit: (after a bit further thought and examination)

re: the eyewitness account from Karl Sandoval, he says it took place at the Pasadena Civic (around the 2:15 mark), and apparently this was not the nice-looking performance theatre with seating, but a rectangular Exhibition Hall (suitable for events like dances) immediately next to it, which looks about the same size, with similar capacity, according to Pasadena resident/musician (played in King Kobra (Carmine Appice), studio business guy (and also the source of bootlegs supplied to the band for reference in the latter period) Mike Wolf. :

https://www.youtube.com/watch?v=6JlopU1F8NQ&t=963s

https://thevendry.com/venue/261428/pasa ... ace/159640

so in that case, I'm thinking there was no need to take the volume down significantly, so less likelihood of the tube-pulling. (I'm guessing that) that was more about trying to find tubes that sounded better (since he wasn't apparently using matched tubes or biasing them properly). However, at the least it does seem to show no reservations or qualms about pulling tubes in and out.

Anyway, after listening to the Mike Wolf interview videos (he did speak to Ed early on, and apparently the supposed boiling of strings to stretch them out procedure was bogus), my sense is that he was upset or annoyed with guitarists copying him (IIRC there was an interview bit about one even copying his distinctive paint job) and thus was motivated to put out bits of disinformation such as the boiling of strings, not using fuses, boosting the voltage to 140VAC (I suppose more of a half-truth than a complete lie), and relating in interviews that Jose modded his amps (I personally believe this wasn't the case--or at least no secret suite of high gain Jose mods).

There is an account from Terry Kilgore (childhood friend, ex. DLR) about Ed blowing up the amp due to the use of tin foil around the fuse at or around 1971 at the Jewish Temple in Pasadena (Altadena) when he was 17 years old, so I think he probably had a good idea what might happen running an amp shorting out the fuse (so not using fuses and voltage boosting to 140VAC may possibly have been aimed at particular guitarists of great annoyance and not advice to cause problems for the general populace (I doubt he had any idea he would reach god-like levels of popularity in the early interviews)). Also, TK shared another recollection where they apparently had no clue back then about the need to use matched tubes and how it was considered a good idea to have another amp (Fenders are mentioned (and observed) in old gig pics) for backup when the Marshall gave up the ghost.

[Pulsonic]

forgot i had this in a yt file -
'Ol Sarge [Tube Tramp on the old PP forum]

puts the 6th to D, then C, then up to F - doing his thing =\

[68twelveUS7a]

he did speak to Ed early on, and apparently the supposed boiling of strings to stretch them out procedure was bogus

I used to boil my strings back in the day to rejuvenate them when I was piss poor and couldn't afford to keep buying new strings. I agreed with Ed and much preferred broken-in strings on electrics. Acoustics are the reverse. Boiling does remove crud & oils and relax tension in the strings from the manufacturing procedure, making them sound new but act more stable. I did all of Ed's tricks including preloading a twist for each wrap around the tuner peg to minimize tension and increase tuning fidelity with a straight open Fender non-locking whammy bar. Using pencil graphite & oil, filing the relief on the nut slots better, aggressively stretching strings before final tuning, these were all tricks of the trade. Randy once asked Eddie how he could stay in tune with his aggressive divebombs and Ed iced Randy and told him to basically work it out himself. So personally, I don''t regard "boiling strings" in the same category of Ed's misinformation & disinformation tidbits.

[Dai H.]

he did speak to Ed early on, and apparently the supposed boiling of strings to stretch them out procedure was bogus

I used to boil my strings back in the day to rejuvenate them when I was piss poor and couldn't afford to keep buying new strings. I agreed with Ed and much preferred broken-in strings on electrics. Acoustics are the reverse. Boiling does remove crud & oils and relax tension in the strings from the manufacturing procedure, making them sound new but act more stable. I did all of Ed's tricks including preloading a twist for each wrap around the tuner peg to minimize tension and increase tuning fidelity with a straight open Fender non-locking whammy bar. Using pencil graphite & oil, filing the relief on the nut slots better, aggressively stretching strings before final tuning, these were all tricks of the trade. Randy once asked Eddie how he could stay in tune with his aggressive divebombs and Ed iced Randy and told him to basically work it out himself. So personally, I don''t regard "boiling strings" in the same category of Ed's misinformation & disinformation tidbits.

Hi,

if memory serves (brain starting to get old so not feeling quite so reliable these days... ) I heard it from Alan Garber(sp?) who is one of the super knowledgeable hardcore guys (from a Youtube video/discussion), so it isn't first hand info, and I don't really have any knowledge about metallurgy, etc. to definitely say what boiling will (or won't) do to strings. I do remember trying that with a new set of strings a very long time ago, and I think I only did it the one time.

(edit--was going a bit zany wondering if actually remembered this correctly, but apparently I did..phew) :

FWIW, there was an interesting bit re: string stretching from Rob Cunningham (of D'Addario from a tone-talk episode) :

https://www.youtube.com/watch?v=ew1_8hDJGF0&t=1582s

[68twelveUS7a]

he did speak to Ed early on, and apparently the supposed boiling of strings to stretch them out procedure was bogus

I used to boil my strings back in the day to rejuvenate them when I was piss poor and couldn't afford to keep buying new strings. I agreed with Ed and much preferred broken-in strings on electrics. Acoustics are the reverse. Boiling does remove crud & oils and relax tension in the strings from the manufacturing procedure, making them sound new but act more stable. I did all of Ed's tricks including preloading a twist for each wrap around the tuner peg to minimize tension and increase tuning fidelity with a straight open Fender non-locking whammy bar. Using pencil graphite & oil, filing the relief on the nut slots better, aggressively stretching strings before final tuning, these were all tricks of the trade. Randy once asked Eddie how he could stay in tune with his aggressive divebombs and Ed iced Randy and told him to basically work it out himself. So personally, I don''t regard "boiling strings" in the same category of Ed's misinformation & disinformation tidbits.

Hi,

if memory serves (brain starting to get old so not feeling quite so reliable these days... ) I heard it from Alan Garber(sp?) who is one of the super knowledgeable hardcore guys (from a Youtube video/discussion), so it isn't first hand info, and I don't really have any knowledge about metallurgy, etc. to definitely say what boiling will (or won't) do to strings. I do remember trying that with a new set of strings a very long time ago, and I think I only did it the one time.

(edit--was going a bit zany wondering if actually remembered this correctly, but apparently I did..phew) :

FWIW, there was an interesting bit re: string stretching from Rob Cunningham (of D'Addario from a tone-talk episode) :

https://www.youtube.com/watch?v=ew1_8hDJGF0&t=1582s

If you were there back in the day, you probably did your share of MacGyver-ing I didn't have the scratch to own a lot of expensive gear. My philosophy was just like Ed's: you turn your lemons into lemonade. I spent a long time since as Tool & Die Maker and CAD/CAM Designer. Steels before the 21st Century all were cold or hot rolled from molten steel recipes. The process puts a directional grain into the steel much like wood has. This grain influences the stability & tensile strength of the steel. With steel wire, there's no telling what direction the grain is oriented in from the the consumer's eye and depending on the source of the steel (billets, ingots, pucks) and the extrusion process most likely reorients its own superseding directional, longitudinal grain - which is good. I'm sure that D'addario and Ernie Ball factor all of this into their products. Although these days they may be using powdered steel formulations as the costs have come down in the last thirty years. Impurities baked into the grain of the source steel could influence stability in the final extruded product, which is why powdered steels would be superior, effectively eliminating the legacy grain structure concerns. I mean they're 3D printing powdered steel now. Crazy. Anyways, Floyd Rose was a machinist and his double-locking design effectively mitigated the need to obsessively futz around with your strings. Of course, if you have a traditional Fender two or six point tremolo and open nut, well then these tricks are still as relevant today.

I apologize for getting too deep into the weeds. Most folks don't know that steel is also like a sponge and absorbs oils, particularly penetrating oils which also influence sound characteristics.

[Dai H.]

Well, as far as any MacGyver-type activities I would say I was less than clueless... Quite different from today, where you can access torrential amounts of infomation right from your fingertips.

FWIW (in case you haven't seen this), here's an eyewitness account of what EdwardVH apparently did to his nuts (hmm.. that sounds kinda weird, sorry... ) ... and also some discussion on how the stings were wound (I guess trying to reduce the break angle?) :

https://www.youtube.com/watch?v=SLG-LfxAjJg&t=984s

https://youtu.be/sg5uolcZ6dw?t=1345

This explains how he would throw in a wang bar lick to get the guitar back in tune:

https://www.youtube.com/watch?v=DScnML3sZzk&t=478s

(What's heard in this bootleg seemed to bear this out) :

https://www.youtube.com/watch?v=SJnDy5RgP4c&t=454s

[Dai H.]

FWIW, there were some interesting bits on the tone-talk Youtube channel (with David Friedman and Marc Huzansky) regarding a Steven Rosen interview with Jose Arredondo (from Steve Rosen's book).

source: https://youtu.be/yN2n53VsEMk?t=8679

(try to keep it short) :

- There were clipping diodes in the Marshall at one point (this may have been the "mystery knob" on the back panel (subject of much speculation)). (According to Friedman), they don't do much with the amp on 10, so (my speculation) if he always used it on 10 (to generate the required amount of distortion), maybe he didn't find it very useful and removed it at some point (the knob appears to be absent in later pictures). So perhaps an experiment that didn't prove to be fruitful.

- EdVH actually tried turning the voltage up with a Variac (and liked the sound), but with (the presumed) catastrophic results, it wasn't practical. However, turning the bias hotter in conjunction with lowering of the input voltage using the Variac was satisfactory tone wise (and of course would be safer), and seems to explain where he ended up (Variac down and cranked bias).

-The first stage (Bright channel) cathode resistor was apparently 2.7k (with 0.68uF), and not an 820 ohm resistor.

[Roe]

-The first stage (Bright channel) cathode resistor was apparently 2.7k (with 0.68uF), and not an 820 ohm resistor.

quite implausible, given other reports, original '68 specs, and sound.

[Dai H.]

Well that was kind of interesting to me personally, since (IIRC) John Suhr related that it was 820R years ago on alt.guitar.amps(?) newsgroup, so...??

[Roe]

Well that was kind of interesting to me personally, since (IIRC) John Suhr related that it was 820R years ago on alt.guitar.amps(?) newsgroup, so...??

The amp has been modded and reworked and restored several times. Original specs is 820R, and it certainly sounds like it. I've made 2k7 and 820r switchable on one of my amps for testing.