KT88 vs EL34 vs 6L6 Substitution In Vacuum Tube Amplifier.wmv

This video was inspired by a question I was asked about swapping KT88's in place of EL34/6CA7's. I have used a couple of amps in the video and made cathode current and power output measurements on three types of tubes -- 6L6GC, KT88 and EL34. This video is not meant to encourage anyone to swap the output tubes in their amplifier out for other types but in the case of musicians, I know they are always looking for a unique sound that they may be able to get through vacuum tube substitution. Remember also, swapping around the 12AX7, with a 12AU7 or 12AT7, etc. may also change the sound and no over current conditions are going to exist with these tubes. In rare cases I have seen instabilities introduced by substituting these tubes resulting in oscillations. In any case, I hope the video does provide some (safe) ideas on tube substitution and helps prevent destruction of equipment.

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Автор Mike Muratagic (1 месяц)
question, when your figuring out the wattage output of an amp, i know you
use voltage squared divided by the ohms gives you the wattage, but when
your checking the voltage do you have to have to use a true rms meter, or
is there a way to do it with a regular multi meter?also, do you know of any
cheap devices that I can get to verify the actual output of an amp before
clipping? or do you basically have to have a osciloscope, frequency driver,
rms volt meter, resistive load(to emulate speaker), and distortion
analyzer? or can i not use an osciliscope and just get away using
a distortion analyzer?(somehow i have a feeling a dist analyzer is more
expensive then a scope)

sorry for my igrorance but i'm relatively new at electronics repair and am
trying to set myself up for working on tube/ss power amps. 

Автор Jean Luc De Marc (2 дня)
A person with a lot of knowledge.. Do you by chance have any schematic for
tube amp with gu-81m or 833a/c or tb3/100 or gu-48 in SE

Автор Vaff angool (4 месяца)
*Pardon my ignorance,* but I'm confused about the way you're reading your
digital multimeter—is it set to display 10x? Or is that just a *thing* you
need to know about multimeters?

*For example,* at 08:48 the display reads *0.684*, which you say indicates
68 milliamps. Since there are 1000 mA to the ampere, I would have assumed
it was 684 mA, but throughout the video you are consistent about reading it
as though it overstates the true value by a factor of ten.

*I could see* how the limited range of a mechanical needle might require
the user to know beforehand the reasonable range of results in order to
properly interpret its readings, but why would a digital unit display the
wrong order of magnitude when it could just move the decimal point over?

Автор Mike Muratagic (1 месяц)
Thankyou soo much for your input on that, as that is one of the most common
questions people ask me when buying used amps is how can they make sure
they are getting what they are paying for, or if there is less output then
what the amp should be putting out.. In your experiences have you ever come
across amps that should put out more power, but for some reason don't? Or
are all manufacturers honost when they publish rms power output? (I know
the peak power output of some amps are a farce,) haha I've seen computer
speakers that say 1500watts pmpo output! hahaha... Maybe they are like
10watts at most probably less i'm guessing and yet no one does anything
about them putting those crazy numbers as far as i'm concerned they should
get sued for putting that crap.. - I've bookmarked you and subscribed, you
are very knowledgable in your field as I can see and your definately my new
mentor, please keep making your videos they are very informative. If you
wouldn't mind I would like to keep in contact and probe you for knowledge
now and then if thats okay with you..? I think i'm going to start with tube
amps, as their circuts seem so simplistic compared to solid state.
Always look forward to your reply. - Mike.

Автор Juan Vargas (4 месяца)
hello sir this video is really teaching a lot and it does help me a lot I
still reading and comprehending more about tube amplifiers, I have a
question if you don't mind, I still in the process of making a KT88 push
pull ultra linear hi fi amp "not for guitar" home use on the cathode of the
KT88 what resistor should I use 10R 3W is fine or should I get a 5W ? many
people have use 5W and 3W what is correct ?

the B+ will be around 460V DC from a Edcor transformer model XPWR247-120
330 - 0 - 330 you mention already "long time ago" that I should use 4.2K
output transformer one more question about output transformer, should I get
one that can go up to 100W or one that supply about 60W ? I was thinking
about the B+ and I'm not sure what transformer should I get because of the
460V DC B+ I'm a bit confuse to be honest


Автор Hector J (1 месяц)

Автор Jyme Bale (9 месяцев)
Yes, Tim, your question will work with a JTM 45, you can use a dozen
different tubes, you do have to change the bias resistor though.

Автор Steve R (4 месяца)

Автор Nikos Damigos (6 месяцев)
Hi my friend thanks for your videos
i have one old 70's Sound City-50plus amp, with JJ EL34 power tubes
I want to make his headroom bigger and stop the break up with bigger
volumes (i have allready check my preamps and i find the right for me)
could i change them with KT 88. KT66 or other powertubes.
How this could change the sound with the KT88 or other set?.
Thanks a lot again

Автор Sin sin sinner (10 месяцев)
KT66 Should have been mentioned

Автор Tim Stanley (10 месяцев)
Do you think this will work with any Kt88-6550 Amp? I have a 50 watt Ampeg
VL502 and I wanna try a set of 5881"s or Kt66"s. I cant get a good yes or
no from any one... the amp has the switch to run either EL34"s or KT88"s

Автор Alexander Shibilski (1 год)
What is that piece of equipment that tells you the voltage and percentage
of distortion?
I've got a scope and would love something like that!!

Автор AeroKitsune (1 год)
So you could potentially run a pair of 6L6GC's with a pair of KT88's? Or am
I mistaken?

Автор Mark70609 (1 год)
Could you list the equipment you used?

Автор Bobby Dhal (1 год)
you are awesome !!!!!! thank you thank you 

Автор Tomii S Sonic (1 год)
This was a great video, articulated very well! Thank you!

Автор orange70383 (9 месяцев)
Tubes are great, there's honesty in them, you get what you get.

Автор munja2111 (1 год)
Great video Sir! Please advise me, I have HiFi amplifier Cayin A55T which
have 4 KT88 in output in push pull configuration. Anode voltage B+ is about
400 V, Cathode is connected to 10 ohm resistor to GND. Bias is adjusted by
negative voltage on grid and can go from -70V to -10V. I biased KT88 of
about 45 mA (0.45 V across cathode resistor). Can I put EL34 valve in my
amp and rebias it to about 40mA cathode current (voltage on grid can go up
to -10 V)? Pin 1 and 8 are connected together in valve socket. I know that
I will lose some power but I love EL34 tube a lot. Thanks :)

Автор jamesterry51 (11 месяцев)
thats what is guitar players love is the distortion on a tube amp when you
crank it

Автор Bernard Mahoney (11 месяцев)
i had an 80 watt valve amc power amplifier using kt88 it was awesome a
great sound for the money .

Автор lukefenderrhodes (1 год)
hello boss on a line balance potentiometer i dont have a 27 ohm resistor
but i hve a thirty ohm resistor, how would that effect the line balance on
the plate, or could i take a twenty ohm and a 8 ohm put them in parrallel
together? thanks for listing to my nonsense

Автор wqpeb (1 год)
Wonder what this MK III would actually sound like? 

Автор Wang Tat Wu (1 год)

Автор marmuguk (2 года)
Hi, a 1965/7 amplifier 4 octal and five 9 pins, I think it showed 800V on
the plates, blew a capacitor and stopped fiddling! I am guessing a KT88
amp. I have EL34's but they quickly glowed very orange! G2's go through 1
KOhm 5W resistors to the PT output, and G1's go through 55KOhm 1W resistors
to the preamp section. Kathodes are tied to pin1's (G3) could this be the
main cause of the orange glow along with 650V on g2 ? Is it safe to check
delivered voltage with no power tubes or speaker? cut@1

Автор Peter Swartz (3 года)
WHich tubes sound best?

Автор hifijohn (2 года)
classic calculator im sorry i gave mine away all those years ago.

Автор Dean H (2 года)
the marshall jtm 45 head is basically the same as the fender bassman, which
is the original westinghouse design that fender bought the license to back
in the 1950's. marshall used the kt-88 in this amp for a time. all 3 tubes
have been used in various models of the same circuit. the typical marshall
tube today is assumed the el 34, but the kt-88 was in the marshall amp eric
clapton used way back. but best to have a pro set it up.

Автор ElPaso TubeAmps (2 года)
Yes, 420 - 450 is a healthy plate voltage for 6L6's although 490 or so is
not uncommon and I believe that is what the Dynaco and MA230 cloan is
running at in this video.

Автор inventhead (3 года)
those dynaco amps work best with 6550 tubes and each amp is mono 2 for
stereo if u want a lil more push u can add a preamp or use a stronger
preamp tube and plate resistor of 100k to a 90 k or a 500k to 490k that
should bring you about 60 watts and a lil bit of distortion but still give
a nice warm tone 1 fact in mind is wat class amp it is such as A AB C
pushpull class A is pro the other 2 are mid to hi grade home stereo use and
have hi distortion not so good when u want quality sound

Автор ElPaso TubeAmps (3 года)
You should be able to plug your KT88's into your 6CA7 amp (if and only if)
you have enough range in your bias adjustment to lower the cathode currents
in the KT88's to 70 mA per tube. The socket as wired for the 6CA7 should be
OK for the KT88 and as I mentioned below, the worst case would be bias
voltage on the metal ring of the KT88's. Check your socket. If pin one is
grounded there will be no bias voltage on the metal ring. Work safely with
High Voltage.

Автор ElPaso TubeAmps (2 года)
KT88's draw 1.6 amps per tube filament current to a 6L6 of only 0.9 amps
per tube so, that is one concern. The bias on the KT88 will need to be made
considerable more negative to reduce the cathode current to an acceptable
value. There is likely not enough adjustment in the bias pot to accomplish
this. You will need to change bias divider resistors and even then, you may
not be able to attain a proper negative voltage setting for your KT88's.
You may find more help in some of my other videos.

Автор Vaes Joren (3 года)
Howcome you dant even gegt 50W with the pair of 6l6 in push pull, while 6l6
in push pull is supposed to do 60?

Автор ronnieparfait (1 год)
Great Video, you must be an O'l TV repair man with all the knowledge you
have, thanks for taking the time to educate some of the students of
electronics out there., and please keep up the good work, I myself is can't
thank you enough for the free teachings that I'm sure you had to pay for
through out life. Thanks again and people out there if you don't like the
video don't watch it,, it's that simple.

Автор ElPaso TubeAmps (2 года)
Remember that power is voltage squared divided by resistance - so, for an
example, 20 volts across 8 ohms is 20 squared (400) divided by 8 which is
50 watts. For 50 watts across 4 ohms it would be only 14.14 volts. Across 2
ohms it would be 10 volts for 50 watts. Hope this helps.

Автор rw5791 (2 года)
kt88's suck .I will take NOS mullard el34's over new kt88's all day long. I
don't have any of this fancy test gear....all I got is my two ears.

Автор fiream773 (2 года)
I am brand new to the world of tube amps. I have a '55 AMi G-120 Jukebox
with a mono 6L6 amp that a knowledgble friend helped me rebuild. I've been
bugging him about modifying the amp to clean up the sound and improve the
headroom with 6550's. He's an excellent engineer, but not good at
explaining why I can't just drop in 6550's and make 1 billion gigawatts.
This video showed very clearly the pitfalls of tuning an amp "by ear". Best
15 minutes I've ever spent on YT. Thank you 4 posting!

Автор ElPaso TubeAmps (3 года)
And lastly, remember that these "experiments" in swapping tubes around are
not meant to be permament changes. KT88's are more powerful tubes than the
6L6 or 6CA7 and will require more energy to be delivered by the power
supply. Ultimately, my goal was to answer a client's question and my last
and final recommendation is to Not swap around your tubes unless you are
knowledgable and experienced with working with vacuum tube amps and are
willing to accept the risks to yourself and your equipment.

Автор Lefty McStinky (1 год)
The chassis rusted very easily so you would have had to store in inert dry
gas. Cosmetics are everything because the orientals set the market and they
are buying a totem. Someone is selling repro chassis complete with original
silkscreen and Mc refuses to do anything which will bite them in the ass
when repro xfmrs become available.

Автор Fendernirvana (2 года)
I would like to try EL34s in my Scott 296. Can it be done, replacing
GE7581A(KT66) with 6CA7/EL34s? The 7581 is similar to 6L6GC with more
output 35w vs 30w respectively.

Автор ElPaso TubeAmps (2 года)
If you check plate voltage with no tubes the power supply will be operating
with no load and it will be abnormally high and not the voltage that will
be on the plates of the tubes when they are plugged in. The bias voltage
will not be affected with or without the tubes being in.

Автор BixLives32 (2 года)
The extra power sure helps with bass and transients. Frequencies below
200Hz use most of your current. You speaker is 8 Ohms @ 1000Hz (NOMINAL
impedance). At 80 Hz where the deep bass of the kick drum or string bass
resides, most speakers go down to 1-2 Ohms -or less! -That's where all that
great KT88 current goes! Now they even make a KT120. But if you get up to
that power range, Why not experiement with 811s or 572Bs (they're not
expensive) -You might like class A at 50 watts!

Автор waterkeeper03 (3 года)
How long did you play on them before you noticed? if you been listening for
a while and didn't notice until you looked.. I see no reason to be pissed.

Автор ossopalatino (3 года)
I think that bias adjutaments are related with plate voltage.

Автор Noillacspart (2 года)
Hi, i have an Engl 930/60 watt power amp and was looking into changing the
6L6's inside to KT88's. Will i be able to do this or will they have to be
biased for them to work?

Автор catdumpling (1 год)
those KT120's are impressive. they look like Mason jars compared to a lot
of other tubes!

Автор ElPaso TubeAmps (1 год)
I have only experienced the KT120 in some Dynaco spin-offs - can't remember
the brand - impressive tube, very healthy bottle. I think their cathode
current is a bit higher than the KT88 and KT90 so. some care should be
taken before switching them.

Автор Noillacspart (2 года)
Well my ENGL's 60 watt's per side and spec's say that each power amp tube
gives has a fuse of 160mAM, is that enough to power it? Will i have to look
at other 6L6GC's?

Автор IdimmuXul (3 года)
I'm not as knowledgeable on the subject, but shouldnt you always bias your
new tubes no matter what you had before?? or am i misunderstanding what you
mean by Direct Replacement?

Автор Dean H (2 года)
ok, so all three tubes are pinned the same then? i'm no expert and i
suspected as much. but i don't want to blow anything up. 2 6l6's get around
35 watts, i take 'clean' because all class ab amps that are touted as 50
watts use two el34's or 6l6's. people tell me i need to tube my 50 w.
marshall head with kt 88's. will all i need is have the bias set? btw, i
suspect the bias adjust on my marshall head is NOT very wide at all

Автор ElPaso TubeAmps (3 года)
The Fender Bassman 300 I have a video on has around 750 volts on the plates
of the 6550's. Certainly no place to plug in a 6L6 or 6CA7. 450 volts on
the plates of the 6L6 is not uncommon and is typical in the old McIntosh
amps. I agree with you that for a typical 6L6 style amp, 6.5Kohm output
transformer performs well - much better than a 5Kohm. Thanks again.

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